1 of the mössbauer effect in chemistry 1 and solid-state

TECHNICAL REPORTS SERIES No. 50

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Applications 1

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f J ^ J INTERNATIONAL ATOMIC ENERGY AGENCY,VIENNA, 1966

APPLICATIONS OF THE MÖSSBAUER E F F E C T IN

CHEMISTRY AND SOLID-STATE PHYSICS

The following States are Members of the International Atomic Energy Agency:

AFGHANISTAN FEDERAL REPUBLIC OF NICARAGUA ALBANIA GERMANY NIGERIA ALGERIA GABON NORWAY ARGENTINA GHANA PAKISTAN AUSTRALIA GREECE PARAGUAY AUSTRIA GUATEMALA PERU BELGIUM HAITI PHILIPPINES BOLIVIA HOLY SEE POLAND BRAZIL HONDURAS PORTUGAL BULGARIA HUNGARY ROMANIA BURMA ICELAND SAUDI ARABIA BYELORUSSIAN SOVIET INDIA SENEGAL

SOCIALIST REPUBLIC INDONESIA SOUTH AFRICA CAMBODIA •IRAN SPAIN CAMEROON IRAQ SUDAN CANADA ISRAEL SWEDEN CEYLON ITALY ' SWITZERLAND CHILE IVORY COAST SYRIA CHINA JAMAICA THAILAND COLOMBIA JAPAN TUNISIA CONGO, DEMOCRATIC KENYA TURKEY

REPUBLIC OF REPUBLIC OF KOREA UKRAINIAN SOVIET SOCIALIST COSTA RICA KUWAIT REPUBLIC CUBA LEBANON UNION OF SOVIET SOCIALIST CYPRUS LIBERIA REPUBLICS CZECHOSLOVAK SOCIALIST LIBYA UNITED ARAB REPUBLIC

REPUBLIC LUXEMBOURG UNITED KINGDOM OF GREAT DENMARK MADAGASCAR BRITAIN AND NORTHERN DOMINICAN REPUBLIC MALI IRELAND ECUADOR MEXICO UNITED STATES OF AMERICA EL SALVADOR MONACO URUGUAY ETHIOPIA MOROCCO VENEZUELA FINLAND NETHERLANDS VIET-NAM FRANCE NEW ZEALAND YUGOSLAVIA

The Agency's Statute was approved on 23 October 1956 by the Conference on the Statute of the IAEA held at United Nations Headquarters, New York; it entered Into force on 29 July 1957. The Headquarters of the Agency are situated in Vienna. Its principal objective Is "to accelerate and enlarge the contribution of a tomic energy to peace , health and prosperity throughout the world".

© IAEA. 1966

Permission to reproduce or translate the information contained in this publication m a y b e obtained by writing to the International Atomic Energy Agency, Kärntner Ring 11, Vienna I, Austria.

Printed by the IAEA in Austria March 1966

TECHNICAL REPORTS SERIES No. 50

APPLICATIONS

OF THE MÖSSBAUER EFFECT

IN CHEMISTRY

AND SOLID-STATE PHYSICS

REPORT OF A PANEL ON APPLICATIONS OF THE MÖSSBAUER EFFECT IN CHEMISTRY AND

SOLID-STATE PHYSICS HELD IN VIENNA, 26-30 APRIL 1965

INTERNATIONAL ATOMIC ENERGY AGENCY VIENNA, 1966

International Atomic Energy Agency. Applications of the Mössbauer effect in

chemistry and sol id-state physics . Report of a panel . . . held in Vienna, 26 - 30 April 1965. Vienna, the Agency, 1966.

267 p_. (IAEA, Technical reports s e r i e s no. 50)

539 .172.3:539. 2 543.42

APPLICATIONS OF THE MÖSSBAUER EFFECT IN CHEMISTRY AND SOLID-STATE PHYSICS

IAEA, VIENNA, 1966 STI/DOC/10/50

FOREWORD

The recently discovered Mössbauer effect is an example of a pheno-menon whose uses are being developed for a wide range of research appli-cations. A Panel was convened by the Agency on 26-30 April 1965, to review the present situation, particularly in chemistry and solid-state physics, and to discuss possible future developments.

The contributions by the Panel members range from applications in nuclear physics to those in biochemistry. Although no claim is made for a complete coverage of this rapidly expanding subject, the papers give a fairly representative picture of the present state of Mössbauer effect studies, and together with the discussions and recommendations they point to many possible new lines of research that might be well worth following.

CONTENTS

THEORY (Session 1)

Nuclear hyperfine structure and the solid state and chemical appli-cations of the Mössbauer effect 1 G.K. Wertheim

Discussion 16 Анализ изменения зарядового радиуса ядра Sn119 при его возбуждении

на основе мессбауэровских спектров 17 И. Б. Берсукер, В. И. Гольданский и Е. Ф. Макаров

Discussion 25 Some new applications of the Mössbauer effect in chemistry 26

R. M. Golding Discussion 35

APPARATUS AND TECHNIQUES (Session 2)

Mössbauer effect at high pressures 37 H. Frauenfelder and R. Ingalls

Discussion 47 The Bell Laboratories Mössbauer effect Doppler modulator 48

G. K. Wertheim and R. L. Cohen The National Bureau of Standards Mössbauer spectrometer 53

J.J. Spijkerman, F.C. Ruegg and J.R. DeVoe Discussion 58 Mössbauer scattering 58

P. Debrunner and H. Frauenfelder Discussion 72 Surface studies by means of the Mössbauer effect 73

J. W. Burton, H. Frauenfelder and R. P. Godwin Discussion 87

STRUCTURAL INVESTIGATIONS (Session 3)

Mössbauer effect and chemical bonding in transition metal com-plexes 89 J. Danon

Discussion 109 Intermolecular bonding effects in 119Snm Mössbauer spectroscopy . . 110

R.H. Herber and H.A. Stöckler Discussion 120 Mössbauer parameters for metal-organic (Fe, Sn) compounds 121

R.H. Herber Applications of Mössbauer spectroscopy to chemistry of iron and

to biochemical problems 134 E. Fluck

Discussion 140

STRUCTURAL INVESTIGATIONS (continued) (Session 4)

Mössbauer spectra of some iron compounds 143 T. C. Gibb and N. N. Greenwood

Discussion 155 Chemical aspects of isomer shifts and quadrupole splittings in tin

compounds 156 M. Cordey-Hayes

Discussion 163 Mössbauer spectra of some organo-tin compounds 163

T. C. Öibb and N. N. Greenwood Discussion 168 Изучение оловоорганических производных баренов методом

мессбауэровской спектроскопии 168 А.Ю. Александров, В.И. Брегадзе, В.И. Гольданский, Л. И. Захаркин, О. Ю. Охлобыстин и В.В. Храпов

Discussion 172 Isomer shifts in rare earths 173

W. Henning, S. Hüfner, P. Kienle, D. Quitmann and E. Steichele

Discussion 177

STRUCTURAL INVESTIGATIONS (continued) (Session 5)

Mössbauer effect studies with actinides 179 J.A. Stone

Discussion I 199 Isomer shift of the 81-keV gamma line of 133Cs 200

W. Henning, S. Hüfner, P. Kienle, D. Quitmann and E. Steichele

Discussion 204 The Mössbauer effect in the noble gases. A review 205

P. Hi 11m an Discussion 212 Unconventional Mössbauer studies 213

M. Atac, H. Frauenfelder and M. Garrell Discussion 222

KINETIC INVESTIGATIONS (Session 6, Part 1)

The influence of the dipolar aprotic solvents on the quadrupole splitting of Mössbauer spectral lines of dibutyltin dichloride . . . . 223 V.l. Goldanskii, O. Yu. Okhlobystín, V. Ya. Rochev and V. V. Khrapov

Discussion 223

RECOIL STUDIES (Session 6, Part 2)

Limit on the lifetime of non-equilibrium Fe3+ ions in some ionic solids from the Mössbauer effect : 225 G. K. Wertheim and H. J. Guggenheim

Discussion л.. 225

Mössbauer effect study of the isomeric de-excitation in H9Snm . . . . 226 R.H. Herber and H.A. Stöckler

Discussion 227 Recoilless gamma-emission after neutron capture reactions . . . . . . 227

J. Fink and P. Kienle Discussion 232 Recoilless gamma-emission from exited atomic states in europium 233

E. Steichele, W. Henning, S. Hüfner and P. Kienle Discussion 236

MAGNETISM (Session 7, Part 1)

Some recent contributions to the study of the magnetism of metals, alloys and intermetallic compounds 237 G.K. Wertheim

Discussion 248 Суперобменное индуцирование магнитных полей на ядрах

немагнитных атомов 249 В. И. Гольдаиский, В. А. Трухтанов, М.Н. Девишева и В.Ф. Белов

Discussion 250

ANALYTICAL MEASUREMENTS AND STANDARDS (Session 7, Part 2)

Mössbauer resonance absorption study on photolysis and radiolysis of ferric oxalate 251 N. S ai to, H. Sano, T. Tominaga and F. Ambe

Discussion 253 A standard reference material for Mössbauer spectrometry of iron

and its compounds 254 J.J. Spijkerman, F.C. Ruegg and J.R. DeVoe

Discussion 258

RECOMMENDATIONS OF THE PANEL 261

List of Chairmen and Rapporteurs of Sessions and Secretariat of Symposium 265

List of participants 266

THEORY

(Sess ion 1)

NUCLEAR HYPERFINE STRUCTURE AND THE SOLID STATE AND CHEMICAL APPLICATIONS OF

THE MÖSSBAUER E F F E C T *

G. K . WERTHEIM

BELL TELEPHONE LABORATORIES

M U R R A Y H I L L , N . J . , U N I T E D STATES OF A M E R I C A

Many of the chemical and solid-state applications of the Mössbauer effect depend on the fact that line-widths are encountered which are small compared to the characteristic energy of interaction of nuclei with their surrounding electrons, e. g. those which arise from the coupling of the nuclear magnetic dipole moment with the magnetic electrons of the atom. Before Mossbauer's discovery, the possibility of being able to resolve such hyperfine structure by observing the transitions of -y-rays between nuclear levels had been uni-versally discounted, although a less direct measure of quadrupole and magnetic coupling could be obtained from y-y directional correlations and from nuclear polarization or alignment.

The fact that hyperfine structure, could, in principle, be studied with his technique was recognized by Mössbauer, but he was not the first to carry out such an experiment. The first hyperfine structure obtained and analysed was that of 57 Fe in metallic iron reported by Hanna and his collaborators [1] early in 1960. The experiment gave results of significance to both nuclear and solid-state physics. This interdisciplinary aspect of Mössbauer experiments will be in evidence throughout much of this discussion.

A. ISOMER SHIFT

The Mössbauer effect makes it possible to compare with high precision the nuclear transition energies in two materials. At first thought this does not appear to be a particularly valuable accomplishment, unless the levels are split, because one tends to believe that the nuclear levels are them-selves fixed in position. This view overlooks the fact that the nucleus is surrounded and penetrated by electronic charge with which it interacts electrostatically. The energy of interaction can be computed classically by considering a uniformly charged spherical nucleus imbedded in its s-electron charge clöud. A change in the s-electron density such as might arise from

* This section is a condensation of Chapters V, VI and VII of Mössbauer Effect, Principles and Applications, Academic Press, New York, 1964.

1

a change in valence will result in a change in the Coulombic interaction which manifests itself as a shift of the nuclear levels. This effect is properly con-sidered a part of the e lectr ic hyperfine structure and could be called the "electric monopole shift'1 in analogy with the electric quadrupole splitting. However, the term "isomer shift" has been almost uniformly adopted be-cause the effect depends on the difference in the nuclear radii of the ground and isomeric, excited states. The term chemical shift has also been used [2].

There is a second, distinct mechanism which can change the energy of a nuclear 7-ray by an amount of comparable magnitude. It is a relativistic effect arising from the mean square thermal motion of the atoms, generally called the thermal red-shift [3]. It is usually difficult to separate a truly temperature-dependent isomer shift from the relativistic shift.

The electrostatic shift of a nuclear level is readily computed from the following model: the nucleus is assumed to be a uniformly charged sphere whose radius, R, is given by the empirical radius formula; and the electronic charge density, p, is assumed to be uniform over nuclear dimensions. To simplify the calculation, the difference between the electrostatic interaction of a hypothetical point nucleus and one of actual radius R, both having the same charge, is computed. For the point nucleus the electrostatic potential, Vpt, is Ze/r; for the finite one the potential V is Ze/R(3/2-r 2/2R2) for r SR and Ze/r for г Ш R. The energy difference 6E, see Fig. la, is given by the integral

* E . / „ V - V » Л 5 ) „ „ , „

О О

= - - ^ Z e p R 2 = ^Ze 2R 2 | (¿ / (0) | 2 (2)

where -e|^/(0)|2 is an alternate expression for the electronic charge density.

_/_AbJ±* EXCITED. STATE

G R O U N D S T A T E

(a)

SEgd * S E e x

FIG. la . The origin of the isomer shift

This expression relates the electrostatic energy of the nucleus to its radius, which will in general be different for each nuclear state of excitation or energy level. Observations, however, are made not on the location of individual nuclear levels but on 7-rays resulting from transitions between two such levels. The energy of the 7-ray represents the difference in

2

electrostatic energy of the nucleus in two different states of excitation which, in the present model, differ only in nuclear radius. The expression for the change in the energy of the 7-ray due to the nuclear electrostatic interaction is therefore the difference of two terms like Eq. 2, written for the nucleus in the ground (gd) and excited (ex) states:

5 E e x - 6 Egd = f Z e 2 ( R e x - R f d > l*<°> I2 ( 3 )

(At this step the contribution to 6E from the point nucleus drops out. ) The shift relative to some standard substance, obtained by taking the

difference of Eq. 3 written both for a standard source and an absorber (abs. ), is the isomer shift (I. S. ),

I . S . I2 - H ( 0 ) I 2 } (4) 5 V ex gay L a b s ^ source J

or

I . S . = ^ Ze2R2 Щ { I f(0) I2 - I * (0 ) I2} (5) 3 v. abs . source J

nuc lear atomic

where 6R= R e x - Rgd [4]. Figure lb gives an example of the i somer shift of 57Fe in a metal-organic compound relative to 57 Fe in a metallic host.1

1 ° 1-IL p ОС 2

О CO л а * <

z " S • а ш 10

FERRICINIUM BROMIDE AT 20 • К VS с г : " с о

. j u X w í - v ' • f- • •

( b )

-О A -0.3 -0.2 -0.1 О 0.1 0.2 VELOCITY ( C M / S E C )

0.3 0 . 4

FIG. lb . Isomer shift in ferricinium bromide

This equation consists of two factors: the f irst contains only nuclear parameters, in particular the difference between the radius of the isomeric, excited state and that of the ground state; the second contains the electronic charge density at the nucleus, which is basically an atomic or chemical pa-rameter since it i s affected by the electronic configuration of the atom. The electronic charge density is determined mainly by the value of the atom and by the details of the bonding. However, it i s also sensitive to pressure and

The isomer shift may also be expressed in terms of the mean square charge radius by making the substitution R2 = 5 /3 < R2 > .

3

temperature. It has been shown for metallic iron that the s -e lectron charge density increases with pressure as though it were simply a volumetric effect [5] . The effect of thermal expansion which should be of s imi lar origin has not been separated f rom the thermal red-shif t .

For realizable changes in |^(0)|2 the energy shift of Eq. 5 is vanishingly small when considered in terms of the precision of instruments which are capable of giving an absolute measure of energy such as a bent -crysta l spectrometer . It b e c o m e s measurable in a Mössbauer experiment, because we compare the nuclear transition energy in a source with that in an absorber. By this method, in which we adopt a convenient substance as a standard, we can measure small differences in the energy of 7 - rays without actually having to know its magnitude; with 5 7Fe, for example, the energy of the y-ray is known to be 1.436 X10 4 eV with an uncertainty of ± 1 0 eV [6], but we can readily measure d i f ferences in the energy as smal l as 10"i° eV.

A ser ie s of Mössbauer experiments done with common salts of iron were the f irst to show systematic behaviour with respect to the i somer shift [4] . Characteristic values of the shift were found for ionic salts of both divalent, 3d6 , and trivalent, 3d5 , iron. At f i r s t sight it may s e e m strange that d i -and trivalent salts show a different isomer shift since their atomic configura-tions differ only by a d electron which does not itself contribute to the charge density |^(0) I2. The effect ar ises indirectly via the 3s electrons which spend a fraction of their t ime further from the nucleus than the 3d electrons. The electrostatic potential which they experience there depends on the screening ef fects of 3d e lectrons . Thus, the addition of a d electron reduces the attractive Coulomb potential and causes the 3 s - e l e c t r o n wave function to expand, reducing its charge density at the nucleus. In this indirect way, the removal of the 6 th 3d electron in going f rom Fe2 + to Fe3+ increases the charge density at the nucleus and produces a s izable i s o m e r shift . This shift does not suffice to determine the two unknowns in Eq. 4, however, since the difference in the electronic charge density and the change in the nuclear radius occur as factors. If we make the additional assumption that the e lec -tronic charge density in an ionic salt i s the same as in a f ree ion, then the results of Hartree-Fock calculations done for the various configurations of multiple-ionized free iron ions can be used to supply values for the chemical factor. It should be noted that we do not require that the Hartree-Fock cal -culations give the magnitude of the charge density accurately, but only that they reproduce the e f fec t of the addition of a 3d e lectron with reasonable accuracy.

This procedure makes it poss ible to establ ish the sign and magnitude of the nuclear factor in Eq. 5, so that it can then be considered as a means for measur ing the e lectronic charge density, F ig . 2. In this procedure it was assumed that the ions in ionic compounds are electronically equivalent to f ree ions . The covalent compounds present a problem, s ince their c o -valency cannot be adequately represented by a single parameter. We should take into account the re la t ive importance of s, p and d wave functions in the bonding, as well as the extent to which these wave functions are changed by admixture of higher ones. The effect of d -e l ec tron contribution i s a l -ready apparent from the Hartree-Fock charge densities: increasing the d -electron density decreases the electronic charge density at the nucleus and

4

FIG. 2. Isomer shift diagram for "Fe from Walker et al. , Ref. [4] . The correlation between the Hartree-Fock charge densities on the left ordinate and the isomer shifts on the right ordinate is made through the compounds FeF2, KFeF3, FeS04 • 7H20 and Fe2(S04)3 • 6H20. The slopes of the solid lines are determined by the Fermi-Segré-Goudsmit formula

results in a larger, positive isomer shift. The effects of s electrons may be obtained from the Fermi-Segré-Goudsmit formula [7] , which provides a means of obtaining | 0) |2 from the term values. Adding s electrons in-creases the electronic charge density, thus having an effect opposite to that of adding d electrons. The contribution due to p electrons should be small. The range of isomer shift values encountered in 51 Fe is illustrated in Fig. 3.

The isomer shift has been observed for a number of other isotopes. An equally extensive exploration has been made for n^Sn, where the behaviour is also well understood. The shifts for stannous (Sn2+) and stannic (Sn4+) compounds, as well as for a number of tin-intermetallic and tin-organic compounds, have been systematized [8] . Very interesting results have been obtained with №1, one of the few non-metallic elements suitable for Mössbauer effect studies [9] . With la7Au and 151Eu, less complete studies have been made [10, 1,1] largely because of the smaller number of compounds available. In many of the other suitable isotopes the isomer shift has not been detected, mainly because of the greater line-widths characteristic of them.

5

METAL -ORGANIC COMPOUNDS

INTERMETALLIC COMPOUNDS

Fe IN DILUTE SOLUTION IN METALS

F e e -Fe<+

Fe 3 +

Fe2*

-0 .8 -0.6 -0.4 - 0 . 2 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 2.0

ISOMER SHIFT AT 7B°K IN м м / SEC

FIG. 3. S1 Fe isomer shifts in a variety of hosts and compounds relative to 57Co in metal l ic chromium at room temperature.

B. QUADRUPOLE COUPLING

It is important to realize that the isomer shift equation was derived as -suming the nucleus to be spherical and the charge density to be uniform. If these conditions are relaxed, other effects appear which are in fact higher-order terms in the multipole expansion of the electrostatic interaction. These terms do not shift the nuclear levels , they split them, i. e. they lift all or part of their 21+1 fold degeneracy (I is the nuclear-spin quantum number).

The second non-vanishing term of the electrostatic interaction of a nucleus with its surrounding electronic charge is the quadrupole coupling [12] . This is the result of the interaction of the nuclear quadrupole moment, Q, with the gradient of the electric field due to other charges in the crystal. The nuclear quadrupole moment reflects the deviation of the nucleus from spherical symmetry. An oblate (flattened) nucleus has a negative quadrupole moment while a prolate (elongated) one has a positive moment. Nuclei whose spin is 0 to 1/2 are spherically symmetric and have a zero quadrupole mo-ment; thus the ground state of 57xre, with 1= 1/2, cannot exhibit quadrupole splitting. The field gradient i s obtained by applying the gradient operator to the three components of the electric field, which is itself a vector. The field gradient is consequently a 3 by 3 tensor, which can, however, be re -duced to diagonal form so that it can be completely specified by three com-ponents: d2 V/dx2 , d2 V/dy2 , d 2 V/dz 2 (geñerally abbreviated Vxx, Vyy and

(COT)Fe (СО)Э F e ( C 0 ) 5 / Fej(CO), FERRiciNiuM

/ BROMIDE | FERROCENE

0 IDCI F e A l Р е с а з F,e2,RE) / « s i / FeGe2 1 F e A b 1 ' ^ ' ^ N i F e G e

t WrrY^eS»* 310 a*. STAINLESS „, Be P t ! Щц !/ j

I CR N T ^ Y ^ C U Y A U 1 1 1 1 »'И' \ л 1

(Fè«+) к 2 FeO<

i

( F e « ) SrFeOj

( C H j ^ N F e p u

^ ' Y IG ОСТ Y I G t < ? J ^ F e C l j

M « - - " '

T"T 1 1 1 1 1

Fe(CN)„4- F e S 0 4 - 7 H 2 0 ¡ F e o , M g o ( F e ) - f ® ( N H « ) t (S0 4) 2-6H 20 l ERROCENE p C H , ) 4 N ] F e C l X P f l . - H r f

1 ^ 1 1 1 I4* / 1 1 { 1 1 'l 1 t ! т ' т ' У ч V !

6

Vz z). These three components are not independent, however, since they must obey the Laplace equation ina region where the charge density vanishes. (The charge density of the s electrons of the atom at whose nuclear site the field gradient is being evaluated does not vanish, but since the s electrons have a spherically symmetric distribution they do not contribute to the field gradient. ) As a result there remain only two independent components, usu-ally chosen as Vzz, often denoted eq, and 17, the asymmetry parameter, de-fined by

Vx Vyy V,, (6)

Many of the properties of the electric field gradient (EFG) tensor can be deduced from the symmetry properties of the crystal. For example, if the crystal has a fourfold axis (rotation by one-fourth of a circle leaves the crystal in a state indistinguishable from the original one), we choose this axis as the z-direction of the EFG tensor. Since a rotation by 90° produces no change in the crystal and can therefore produce none in the EFG tensor, it follows that the components Vxx and Vyy must be equal, so that the asymmetry parameter is identically zero. Under these circumstances we speak of an axially symmetric field gradient which can be completely specified by its z-component. It can be readily shown that a threefold axis (120° rotation) also suffices to ensure an axially symmetric field gradient, and that two mutually perpendicular axes of threefold or highèr symmetry result in a vanishing field gradient.

The interaction between the nuclear electric quadrupole moment, Q, and the gradient of the e lectr ic field i s expressed by the Hamiltonian

w -H - 4 1 ( 2 1 - 1 ) 31; 1 ( 1 + 1 ) + g ( l 2 + I 2 ' ( 7 )

where I+ and I. are raising and lowering operators. The eigenvalues of Eq. 7 for particular values of I are:

for 1= 1: E 0 = je2qQ(l±rj); - | e 2 q Q ( 8 )

and

for 1= 3/2: E q = ± \ eSqQll + TjS/S)* ( 9 )

If the field gradient has axial symmetry the eigenvalues are given by:

E r e 2 q Q

41(21-1) 3m¡ -1(1+ 1) (10)

for any I. This expression contains only the second power of the magnetic quantum number mj , which means that states whose m¡ differ only in sign remain degenerate, Fig. 4. For 1=3/2 the sign of the quadrupole splitting

7

3/2

1 / 2

Eq(±X3/2)

- i 3 / 2

A EQ

- i l / 2

í 1/2 I S O M E R Q U A D R U P O L E S H I F T S P L I T T I N G

FIG. 4. Quadrupole splitting in 57Fe

or of the quadrupole moment cannot be obtained from a pure quadrupole hyperfine structure.

Measurements of quadrupole coupling unfortunately give only the product of the nuclear moment and the field gradient at the nucleus. To obtain a value for the moment requires an independent evaluation of the EFG tensor. Note that this problem parallels that encountered in the treatment of the iso-mer shift where the electronic charge density and the nuclear radius change appeared as products.

The examination of the origin of the EFG is a problem of solid-state and atomic physics. The two fundamental sources are the charges on distant ions and the electrons in incompletely filled shells of the atom itself. Distant ions contribute, provided their symmetry is lower than cubic. If the crystal structure is known to high precision and if an ionic charge can be assigned to the lattice sites, then the value of the EFG at the atomic site can be ob-tained from a straightforward electrostatic calculation. This is not, how-ever, the EFG at the nuclear site. The latter is usually greatly modified by the atom's own electrons, whose wave functions are distorted by inter-action with the external EFG, and as a result create an EFG of their own [13] . Usually it serves to amplify the EFG due to the distant charges, a pheno-menon which carries the name antishielding and is amenable to calculation. The EFG due to partially filled, non-spherical shells tend to be larger than that due to distant charges. This field gradient is also subject to antishield-ing corrections.

With the 57ре, two different approaches have been taken to determine the quadrupole moment of the first excited state. The first is based on measure-ments of ionic divalent iron salts [14], many of which show a similar low-temperature quadrupole splitting. The electronic configuration of Fe2+ in a weak crystal field where Hund's rule is obeyed is 5D4, i. e. there is one d electron outside a half-filled shell with spherical symmetry. (When the crystal field splitting is weak compared to the exchange interaction, the d shell f i l ls with spins parallel for the first five electrons. Up to five additional electrons pair, spin antiparallel, with the original five. SD4 is defined by 2 s + iLj with S = 2 due to four unpaired electrons; L= 2, denoted by D in the usual spectroscopic notation, due to the sixth electron (the first five make up a half-filled shell with vanishing orbital angular momentum)

8

and Т=3+ С. ) The entire field gradient then arises from this 6th 3d electron assumed to occupy a definite crystal field state [15]. The radial part of the 3d wave function has been obtained by Hartree-Fock calculations, and the field gradient e < r _ 3 > 4 / 7 may be calculated. This result is then combined with the computed antishielding factors for the inner shells, including the half-filled 3d shell, to obtain a value for the quadrupole moment of the excited state.

The distinguishing feature of this approach is that the field gradient is assumed to depend only on the electrons of the atomic shell of the nucleus under consideration; the role of the lattice is simply to provide a crystal field to lift the degeneracy of the d orbitale of the free ion. (In the absence of the crystal field the d electron would divide its time among five orbitale in such a way that it would have spherical symmetry, which of course, would preclude the existence of an electric field gradient.) Where the EFG arises from electrons associated with the atom itself, the temperature dependence of the quadrupole splitting is often very pronounced. The origin of the effect here is the change in population of the d-electron orbitals, which have been split by the crystal field, and spin-orbit coupling [15].

The second case is illustrated by ionic trivalent iron (3d5) where smaller quadrupole couplings are found. In a weak crystal field the con-figuration of the outer shell of the iron atom is 6S5/2. The five 3d electrons are in a half-filled shell with spherical symmetry which does not contribute to the field gradient, so that the observed quadrupole splitting must arise from the field gradient due to other ions in the crystal. The EFG tensor components due to distant point charges are most conveniently expressed as the z-component and the asymmetry parameter

where r ¡ , Git and qj i are the spherical co-ordinates of the charge e ¡ . The indicated summations can be made numerically when the positions

of the ions in the crystal are known with precision from X-ray structure analysis. Computer programmes for this so-called lattice sum have been developed. Difficulties arise from two sources: (1) if the crystal cannot be assumed to be strictly ionic so that it is not clear what charge to associate with a lattice point; and (2) if the lattice sum turns out to be very sensitive to small changes in the position of the ions, changes which fall within the limits of error of the structure determination.

With 57Fe, the quadrupole splitting has been measured in two oxide com-pounds, o—Fe203 and yttrium iron garnet, for which sufficient structure in-formation is available to make the lattice sum with confidence [16] . When

2,

( I D

( 1 2 )

9

these results are combined with the antishielding effects of the inner electronic shells of the iron atom, a value of 0.28 b is obtained for the quadrupole moment.

With the trivalent iron salts, the quadrupole splitting has generally been found to vary only slightly with temperature. This is not surprising, since the changes depend only on the thermal expansion of the crystal. It is, of course, possible to imagine a situation where an accidental cancellation of the contributions to the field gradient occurs at some temperature, the EFG changing sign át that point so that large changes may be observed. A simi-lar cancellation between the field gradient due to distant charges and that due to 4f electrons has been observed in ТтгОз at elevated temperature [Д7] •

There is a third source of quadrupole splitting which appears to violate one of the rules mentioned above, namely, that the EFG vanishes in a cubic crystal. For example, quadrupole splitting has been observed for161Dy in dysprosium iron garnet [18] and 169Tm in TmFe2 [19], where the r a r e -earth sites are cubic. The explanation of this effect is to be found in the fact that the compounds in question are ferrimagnets which are cubic only when their magnetic properties are neglected. When classified in the ex-tended crystallographic system which takes spin into account, their sym-metry is reduced. The effect arises from the magnetic interaction which gives rise to a splitting of the 4f-electron wave functions, resulting in a non-cubic electron charge distribution. This in turn creates the EFG. The temperature dependence of the quadrupole splitting is similar to that of the magnetization, M, which is fundamentally a weighted average over the Jz states, i. e. a Brilloqin function:

+ J ^ 6 ß H J z \

For the field gradient the analogous form is:

+ J

- J(J+1)

/ S p H J j

V E T ^

V z z / Vz z ^ ° )

- J

J ( 2 J

+ J / s ß H J z X

. x ) £

(14)

- J

This expression has been tested for wiDy in dysprosium iron garnet where it results in a value for the quadrupole moment of the excited state which is in agreement with independent determination.

1 0

So far we have been concerned only with the position of the quadrupolar lines and have ignored their intensity which contains additional information. With pure quadrupole splitting of an 1= 3/2 level the +3/2 and - 3/2 magnetic substates remain degenerate, as do the + 1/2 and -1 /2 states. The relative transition probabilities and angular intensity dependences of the two quadru-polar lines for 1= 3/2 to 1= 1/2 transition then are:

Relative transition Angular probability dependence

±3/2 - ± 1 / 2 1 3/2(1 + cos2 0) ( 1 5 )

± 1/2 - ± 1/2 1 1 + 3/2 sin20

where 0 is the angle from the axis of symmetry. Note: (1) that there is no angular position where either line vanishes; (2) that the maximum difference in intensity occurs where 6=0, where the ratio of intensities is 3 (at 90° the ratio is 3/5); and (3) that intensity averaged over a sphere is the same for both lines. (This is equivalent to saying that the intensities would be equal in a polycrystalline specimen: cos20= 1/3; sin20 = 2/3. )

The last statement is subject to modification if the recoil-free fraction is anisotropic. The angular dependence of the intensity measured in a single crystal is then the product of the expressions above and angular dependence of the recoil-free fraction [20] . The latter is in general also a function of the angle 0 measured from crystallographic axis of highest symmetry. If the resulting expressions are averaged over a sphere different values will in general be obtained for the two transitions. This is equivalent to saying that in a polycrystalline sample the two quadrupolar lines are generally not of equal intensity. This effect offers a simple way to demonstrate the aniso-tropy of the recoil-free fraction without requiring single crystals, but does not, of course, permit a determination of the angular dependence of f.

Care must be taken to distinguish this effect from similar manifest actions produced by preferentially aligned crystalline absorbers and from relaxation effects [21].

C. MAGNETIC HYPERFINE STRUCTURE

The most familiar part of the hyperfine structure is without doubt the magnetic part arising from the interaction of the nuclear magnetic dipole moment with a magnetic field, H, due to the atom's own electrons. (Magnetic hfs is always absent for nuclear levels whose spin is zero, since their mag-netic moment is identicaUy zero. ) The Hamiltonian of the interaction is

" m = = - S ^ 1 - 1 1 ( 1 6 )

and the energy levels which are obtained are

E m = - M - H m j A = - g M ^ H m j (17)

ULj. = I, 1-1, ... , -I

11

w h e r e ц п is the n u c l e a r magne ton and g the n u c l e a r g - f a c t o r . A c c o r d i n g to Eq. 17 t he r e a r e 21+ 1 equally spaced levels ; the splitt ing between adjacent l e v e l s i s gßn H and the sp l i t t i ng be tween the lowes t and the h ighes t l e v e l s i s 2g/ jnHI. T h i s equa t ion i s appl ied to 5 1 F e in F i g . 5.

m ,

I S O M E R S H I F T

M A G N E T I C D I P O L E

S P L I T T I N G

FIG. 5. Magne t i c hfs in " F e . is negat ive

T h e ground-state magne t i c m o m e n t is posi t ive, the e x c i t e d - s t a t e m o m e n t

In convent ional n u c l e a r magne t ic r e s o n a n c e e x p e r i m e n t s , d i r ec t ob -s e r v a t i o n s a r e m a d e of t r a n s i t i o n s be tween a d j a c e n t m a g n e t i c s u b l e v e l s ; tha t i s t r a n s i t i o n s in which the magne t ic quantum number changes by 1 ( Д т = ± 1 ) . In the M ö s s b a u e r e f fec t , however , what i s o b s e r v e d a r e -y-ray t r a n s i t i o n s be tween two n u c l e a r l eve l s , which in gene ra l both exhibit m a g -netic hfs . The Y~ray co r r e sponds to a t rans i t ion f r o m a pa r t i cu la r magnetic sublevel of an exci ted n u c l e a r s ta te to a sub leve l of the ground s ta te . The se lec t ion ru l e depends on the mul t ipo la r i ty of the r ad ia t ion .

In the p r e s e n c e of magnet ic hfs, the individual t r ans i t ions between m a g -net ic sub leve l s may be r e s o l v e d , making it of i n t e r e s t to compute the ind i -vidual t r ans i t i on p robab i l i t i e s . These a r e given by the s q u a r e s of C lebsch-Gordon c o e f f i c i e n t . F o r 5 7 F e the r e l a t i v e p r o b a b i l i t i e s a r e given be low:

T r a n s i t i o n s Д ш • T o t a l A n g u l a r dependence

3 3 / 2 ( 1 + c o s 2 e ) 3 / 2 1/2 - 1

- 3 / 2 - - 1 / 2 +1

1/2 - 1/2 0 (18)

- 1 / 2 -» - 1 / 2 0 2 2 S Í n 2 0

-1 /2 1/2 +1

1 / 2 - - 1 / 2 - 1 1/2(1+ cos 2 6)

1 2

In addition to the total integrated intensity associated with these tran-sitions the angular dependence is also of interest. Thus, for L= 1, the Д т = 0 transitions have a "radiation pattern" given by sin2 0, which is just that of a classical dipole, whereas that of the Д т = 1 transitions is l+cos 2 0. Note (1) that the intensity of the Д т = 0 transition vanishes for 0 = 0, i. e. along the magnetic axis; and (2) that the sum of the three angular functions is independent of angle, i. e. that the total radiation emitted between the 1=3/2 and 1=1/2 state has spherical symmetry.

The magnetic hyperfine interaction, Eq. 17, contains a nuclear para-meter, gMnI = /u, and an atomic parameter, H, which cannot be separated experimentally. This is a dilemma analogous to those met in the discussion of the isomer shift and the quadrupole splitting. The situation is more favourable here, however, because one can apply an external magnetic field and measure the resultant splitting, whereas it is not possible to apply an external electric field gradient in the quadrupolar case. This experiment has been done satisfactorily only for a few isotopes because fields of sufficient strength to yield well-re solved hyperfine structure are not available for most of them.

In practice this problem does arise since the magnetic moments of the ground states of stable isotopes are known from conventional microwave resonance experiments. The known ground-state moment and the measured Mössbauer hyperfine structure then determine both /uex and Heff. Isotopes with 1=0, e. g. 166Er, are an exception which require another approach [22]. Fields sufficiently large to produce well-resolved hyperfine structure are obtained by using ferro-, ferri- or antiferromagnetic materials. The first successful experiment was done with 57 pe in metallic iron [1, 23]. This approach has been followed in the determination of the effective field at the nucleus and the excited state moment for a number of isotopes. The hfs of 119Sn has been obtained in the ferromagnetic, intermetallic compound MngSn [24] and álso for tin dilutely dissolved in iron, cobalt, and nickel. The latter approach has also been successfully used for 197Au [25] . With ísiDy [26], ferrimagnetic dysprosium iron garnet was used to provide the magnetic environment. Other isotopes which have yielded well-resolved hyperfine structure include i^Trn [27], l66Er [22, 28] and i5iEu [29] .

In metallic iron the hyperfine field is dominantly the result of the Fermi contact interaction with polarized s electrons, including both those in inner, filled s shells and conduction electrons. The s-electron polarization in turn arises from the exchange interaction with the electrons in partially-filled inner shells, e. g. the 3d shell. In salts of iron there may also be contri-butions to the hyperfine field from the orbital electronic moment and from dipolar interaction. A particularly simple case is provided by trivalent iron. In a weak crystal field Fe3+ has five 3d electrons with spin parallël resulting in a half-filled 3d shell. There is no orbital angular momentum associated with a half-filled shell, i, e. it is in an S state. Moreover, such salts are insulators, so that there are no conduction electrons to contribute to the contact interaction. Below the Néel temperature the atomic magnetic mo-ments are coupled via the exchange interaction in such a way that each atom has a definite temperature dependent, time-averaged component of magne-tization along the magnetic axis. The dipolar field due to the five Bohr mag-netons in the d shell is many orders of magnitude smaller at the nucleus than

13

the fields which are actually observed. These arise almost entirely via the Fermi contact interaction from the d-shell spin moment. With 57Fe, fields from 200-250 kOe per spin have been observed in various compounds of trivalent iron. This behaviour parallels that previously demonstrated for divalent manganese [30] which is electronically equivalent to trivalent iron.

It should be pointed out that most, but not all, Mössbauer-effect magnetic hyperfine structures have been obtained in magnetically ordered materials. There is no reason why hf structure cannot be obtained in paramagnetic ma-terial, but several conditions must be met which make Mössbauer experi-ments more difficult. Let us compare F e 2 0 3 and Fe3+ in A1203 . In both substances the iron atoms have a spin S= 5/2. In Fe2Û3 the exchange inter-action has split the crystal field states in such a way that one, ms = 5/2, lies much lower than the rest and is exclusively populated at low temperature.

' ' ' ' i i i I i i -J i i i -1.2 -0.8 -0.4 О 0.4 0.8 1.2

DOPPLER VELOCITY (CM/SEC)

FIG. 6. Magnetic hfs of 57Fe in А1гОэ single crystal. The direction of the y-rays is along the c-axis of the absorber. The magnetic field is also applied in the с direction

The Mössbauer absorption spectrum at low temperature consists of a single hf pattern, corresponding to a field of 548 kOe. Iron, in sufficient dilution, in A1203 does not have an exchange interaction. The iron atoms are para-magnetic impurities with crystal field states split by only 1°K. At tempera-tures above a few degrees all the states m = ±1/2 , ±3 /2 and ±5 /2 will

14

be populated, and each will give rise to its own exchange polarization and hence hyperfine structure. These hfs are observable only if the relaxation time of the atomic spin is large compared to Ti/ßH. This condition is met for Al203:Fe3+ provided the iron is sufficiently dilute so that spin-spin inter-action remains unimportant. The spin-lattice relaxation time is inherently long because Fe3+ is an S-state ion, i. e. it has no orbital angular momentum. The complicated hfs due to the superposition of m s = 5/2, 3/2 and 1/2 is shown in Fig. 6.

Paramagnetic hyperfine structure has also been found in concentrated salts of some rare-earth isotopes [32]. It has been shown that the long re-laxation time in these substances is the result of highly anisotropic electronic g factors [33].

SUMMARY

The three aspects of the nuclear hyperfine structure, the isomer shift, quadrupole coupling, and magnetic dipole coupling make important contri-butions to the study of solids and to chemistry. The isomer shift provides a unique tool for the measurement of the electronic charge density at the nucleus which has immediate application to chemical bonding. The quadru-pole coupling is sensitive to the symmetry of the atomic site and provides information on crystal field and spin orbit coupling. The magnetic dipole interaction provides a technique for the study of magnetically ordered ma-terials . Sublattice magnetization, Curie or Néel temperature, and atomic magnetic moments can be deduced from the magnetic hyperfine field. Re-cently it has been shown that the magnetic hyperfine structure in paramagne-tic materials provides an opportunity for the study of the spin relaxation time and the g tensor.

R E F E R E N C E S

[1] HANNA, S.S. e t al. , Phys. Rev. Lett. 4 (1960) 177. [2] This effect was first observed in a Mössbauer exper iment by KISTNER, O. C . , SUNYAR, A. W. , Phys.

Rev. Lett. 4 (1960) 412. It had been previously detected in optical spectroscopy where a shift in spectral lines was found when the nucleus is in an isomeric, excited state; see MELISSINOS, A . C . , DAVIS, S. P. , Phys. Rev. И 5 (1959) 130-, see also WEINER, R. , Nuovo Cim. ± (1956) 1587 , Phys. Rev. 114 (1959)256.

[3] POUND, R.V. , REBKA, G. A. , Jr. , Phys. Rev. Lett. 4 (1960) 274.

[4] de BENEDETTI, S., LANG, G., INGALLS, R., Phys. Rev. Lett. 6 (1961) 60-, WALKER, L.R., WERTHEIM, G.K., IACCARINO, V. , Phys. Rev. Lett. 6 (1961) 98-, SHIRLEY. D.A. , Rev. mod. Phys. 36 (1964) 339.

[5] POUND, R.V. , BENEDEK, G. B. , DREVER, R. , Phys. Rev. Lett. 7.(1961) 405. [6 ] The wavelength of this y-ray has recently been measured to be 858. 453 ± 0. 003 xu, BEARDEN, I. A. ,

Bull. Am. phys. Soc. j) (1964) 387. [ 7 ] FERMI, E. , SEGRÉ, E. , Z . Phys. 82 (1933) 729; GOUDSMIT, S .A . , Phys. Rev. 4 3 (1933) 636. [8] KISTNER, o . e . , JACCARINO, V. , WALKER, L. R. , in Mössbauer Effect, Proc. 2nd Int. Conf. on the

Mössbauer Effect, Saclay, France, 1961, John Wiley and Sons, New York (1962) p. 264; SPINEL, V. S. , BRUYKHANOV, V.A. , DELYAGIN, N.N. , Zh. éksp. teor. Fiz. 41 (1961) 1767; Soviet Phys. JETP 14 (1962) 1256; BOYLE, A.J .F . , BUNBURY, D. St. P. , EDWARDS, C. , Proc. phys. Soc. Lond. 76 (1962) 416; GOLDANSKII, V . l . e t al. , Dokl Acad. Nauk SSSR 147 (1962) 127; BUKAREV, V.A. , Zh. éksp. teor. Fiz. 44 (1963) 852; Soviet Phys. JETP 17 (1963) 579.

15

[9] DeWAARD, H . , DePASQUALI, G. , HAFEMEISTER, D. , Phys. Lett. 5 (1963) 217; HAFEMEISTER, D.W. ,

DePASQUALI, G. , DeWAARD, H. , Phys. Rev. ¿35 (1964) B1089.

[10] SHIRLEY, D.A. , Phys. Rev. 124 (1961) 354.

[11] BARRETT, P. H. , SHIRLEY, D.A. , Phys. Rev. 131 (1963) 123; WICKMAN, H.H. , SHIRLEY, D.A. ,

to be published.

[12] For more detailed discussion see COHEN, M. H. , REIF, F . , in Solid State Physics (SEITZ, F . ,

TURNBULL, D. , Eds) Academic Press, Inc. , New York 5 (1957) p. 321.

[13 ] STERNHEIM ER, R.M. , Phys. Rev. 80 (1950) 102; 84 (1951) 244; 86 (1952) 316; 95 (1954) 736; 105

(1957) 158; STERNHEIM ER, R.M. , FOLEY, H.M. , ibid. 92_ (1953) 1460.

[14 ] DeBENEDETTI, S. , LANG, G. , INGALLS, R. I. , Phys. Rev. Lett. 6 . (1961) 60; LANG, L. G. ,

DeBENEDETTI, S. , INGALLS, R. I. , in Mössbauer Effect, Proc. 2nd Int. Conf. on the Mössbauer Effect,

Saclay, France, 1961, John Wiley and Sons, New York (1962) p. 168; ABRAGAM, A. , BOUTRON, F. ,

Compte rendue 252 (1961) 2404; JOHNSON, С. E. , MARSHALL, W. , PERLOW, G.J. , Phys. Rev. 126 (1962) 1503; INGALLS, R. I. , Phys. Rev. 128 (1962) 1155.

[15] For a treatment including the effects of spin-orbit coupling see INGALLS, R. I. , Phys. Rev. 133 (1964) A787.

[16] BURNS, G. , Phys. Rev. 124 (1961) 524; STERNHEIMER, Phys. Rev. 130 (1963) 1423.

[171 MÖSSBAUER, R.L. , Revs. mod. Phys. 36 (1964) 369. [18] О FER, S. , et al. , Phys. Rev. Lett. _5_ (1960) 177; Phys. Rev. 120 (1960) 406. [19] COHEN, R.L. , Phys. Rev. 134 (1964) A94. [20] GOLDANSKII, V . l . , MAKAROV, E. F. , KHRAPOV, V. V. , Zh. 'éksp. teor. Fiz. 44 (1963) 752; Soviet

Phys. JETP 17 (1963) 508; see also Phys. Lett. 3 (1963) 344. [21] BLUME, M. , Phys. Rev. Lett. 14 (1965) 96. [22] See for example COHEN, R.L. , WERNICK, J .H. , Phys. Rev. 134 (1964) B503. [23] PRESTON, R.S. et al. , Phys. Rev. 128 (1962) 2207. [24] MEYER-SCHÜTZMEISTER, L. , PRESTON, R. S. , HANNA, S.S. , Phys. Rev. 122 (1961) 1717. [25] SHIRLEY, D.A. , KAPLAN, M. , AXEL, P. , Phys. Rev. 123 (1961) 816; ROBERTS, I . D . , THOMSON, J.Q,

Phys. Rev. 129 (1963) 664. [26] OFER, S. et al. , Phys. Rev. 120 (1960) 406; the hfs of i«Dy in Dy metal was reported by BOYLE, A.J. F.

et al. , in Proc. 2nd Int. Conf. on the Mössbauer Effect, Saclay, France, 1961, John Wiley and Sons (1962) p. 182.

[27] KALVIUS, M. et al. , Z. Phys. 172 (1963) 231; COHEN, R.L. , Phys. Lett. 5 (1963) 177; Phys. Rev. 134 (1964) A94.

[28] EICHER, H. , Z . Phys. 169 (1962) 178. [29] BARRET, P. H. , SHIRLEY, D.A. , Phys. Rev. Ш . (1963) 123. [30] Van WIERINGEN, J. S . , Discuss. Faraday Soc. 19 (1955) 118. [31] WERTHEIM, G.K. , REMEIKA, J .P . , Phys. Lett, j j ) (1964) 14; Proc. XIII Colloque Ampère, to be

published.

[32] DOBLER, H. e t al. , Phys. Lett 10 (1964) 319; OFER, S. et al. , ibid U. (1964) 205.

[33] NO WIK, I. , Phys. Lett, (in press).

DISC USSION

R. H. HERBER pointed out that the term "valence", which did not carry a clear meaning to a chemist, should be avoided and suggested that one should specify the "formal oxidation state" and "co-ordination number".

N. N. GREENWOOD voiced general agreement with this suggestion and added that the terms "high spin" and "low spin" were less ambiguous than other terms which had been used to indicate the extent of spin-pairing of non-bonding electrons.

P. KIENLE pointed out that recent work on isomer shifts in Eu isotopes suggested that the simple electrostatic picture might not be adequate.

16

J. DANON (Chairman) asked whether these effects were small. P. KIENLE said that they were large. He then commented on the pos-

sible application of the Mössbauer effect to paramagnetic hyperfine structure in cases where e. p. r. was not applicable.

АНАЛИЗ ИЗМЕНЕНИЯ ЗАРЯДОВОГО РАДИУСА Я Д Р А Sn1 1 9 ПРИ Е Г О ВОЗБУЖДЕНИИ НА ОСНОВЕ

МЕССБАУЭРОВСКИХ СПЕКТРОВ

И.Б.БЕРСУКЕР, В.И.ГОЛЬДАНСКИЙ и Е.Ф.МАКАРОВ ИНСТИТУТ ХИМИЧЕСКОЙ ФИЗИКИ

АН ССР, МОСКВА СССР

ABSTRACT

ANALYSIS OF THE CHANGE IN THE CHARGE RADIUS OF THE 1,9Sn NUCLEUS FOLLOWING EXCITATION. ON THE BASIS OF THE MÖSSBAUER SPECTRA. It has hitherto been assumed that the charge radius of the II9Sn nucleus in an excited state at the 23.8-keV level is greater than in the ground state R/R>0). This deduction from data for the chemical shift of lines in the Mössbauer spectra of tin compounds was based on a very approxi-mate conception of the structure of the electron shell of these compounds, and their interaction with the nucleus. The authors' analysis has produced a general formula for the dependence of the chemical shift on the parameters of the molecular orbits and those of the inner shells, estimation of which for tetrahalogenides of tin leads to the conclusion thatAR/R<0. These results are important for the interpretation of many of the experimental data obtained by Mössbauer spectroscopy of tin compounds.

1 . ВВЕДЕНИЕ

Относительное изменение зарядового радиуса ядра AR/R при его воз-буждении является существенной характеристикой его контактного взаимо-действия с электронной оболочкой. Определение этой величины встречает известные трудности. Некоторые возможности в этом отношении представ-ляют спектры Мессбауэра, а именно зависимость химического сдвига цент-ра тяжести спектра от изменения электронной плотности на ядре, в кото-рую AR/R входит явным образом:

6 = const ^ [ \ф'(0) I 2 - \ф(0) I 2 } (1)

Здесь \ф(0) I2 и |i//'(0) I2 — полные электронные плотности на ядре в источ-нике у - квантов и поглотителе соответственно.

Для ядра SnH9 (в этом случае const = 1,97-Ю~22 мм/сек см3) впервые Шпинель, Брюханов и Делягин [1] использовали формулу (1) для определе-ния знака AR/R ПО измеренным химическим сдвигам в спектре поглощения у -квантов с энергией 23,8 кэв и пришли к выводу о том, что A R / R > 0 .

17

Бойль и др. [2] более полно рассмотрели вопрос и оценили абсолютное зна-чение этого отношения AR/R = +1,1-10"4 . Эти результаты основаны на довольно грубой апроксимации значений |i//(0)|2, для которой использовались следующие основные допущения: 1) для электронной плотности на ядре су-щественен только вклад внешних 5s - электронов; 2) этот вклад определя-ется только числом 5s - электронов, которое было принято равным нулю для так называемых "ионных" соединений четырехвалентного олова (пример -SnF4) и двум - для подобных соединений двухвалентного олова (пример -SnCl.2); 3) вклад каждого 5s - электрона можно определить по формуле Ферми-Сегре [3, 4] .

Полученное в этих предположениях значение отношения A R / R ( H даже знак его) легко подвергнуть сомнению в связи с несостоятельностью пер-вых двух допущений. Действительно, прямыми наблюдениями спектров ядерного квадрупольного резонанса (ЯКР) на галогенах достаточно убеди-тельно показано (см., например, [5]), что доля электронного заряда, пере-тягиваемого на йод, бром и хлор в соединениях типа SnX4 (т.н. ионность связи S n - X , см. ниже), не превышает 0 , 3 - 0 , 4 . Отсюда вытекает необхо-димость более точного определения как доли электронного облака 5s - элек-тронов, вносящих непосредственный вклад в li (O) I2 , так и 5р - электронов, которые, экранируя s - электроны, уменьшают этот вклад*.

В настоящем сообщении проведен более полный учет распределения электронного облака в соединениях олова и его влияния на электронную плотност"ЕГ на ядре. При этом получена общая формула зависимости сдвига б от параметров молекулярных орбит в этих соединениях. Оценки этих параметров для тетрагалогенидов олова позволяют сделать вывод о том, что AR/R <0, в противоположность приведенному выше результату работ [ 1 , 2 ] . Предположение об отрицательном знаке AR/R было впервые выдви-нуто двумя из нас [ 6] . Отрицательный знак этого отношения для Sn119 /и зна-чение 10~4/ были получены впоследствии и В .А .Беляковым [7] в результате расчета изменения зарядового распределения в ядре Sn119

при его возбуждении в приближении теории конечных ферми-систем.

2. ОСНОВНЫЕ ФОРМУЛЫ

Рассмотрим молекулярный комплекс, состоящий из атома олова и ато-мов (молекул, ионов, радикалов) первой координационной сферы, и пред-положим, что его внешние одноэлектронные состояния могут быть описаны некоторым набором молекулярных орбит типа:

* k = N k ^ K + ^ X j (2)

N* = [1 +X?H-2XkSkr1 (3)

S k = J ^ X k d r (4)

* Собственный вклад p - э л е к т р о н о в (в. состоянии р ) в величину lt//(0)p не превышает в случае олова 4,5% и поэтому здесь и далее не учитывается.

18

где фк — линейная комбинация атомных функций атома олова (р. :

Фк ^ (5) Хь — соответствующая линейная комбинация волновых функций лигандов, a S|, — интеграл перекрывания.

Полная электронная плотность в нуле, фигурирующая в формуле хими-ческого сдвига (1), может быть приближенно записана в виде:

4

\ф(0) I2 = [ C k N f o 2 12 ^ 2 1 ^ ( 0 ) I2 ( 6 )

k n = l

При этом предполагается, что в принятом одноэлектронном приближении вклад в электронную плотность в нуле вносят только S - электроны, причем из них в образовании молекулярных орбит (2) участвует только 5s - орбита олова (очевидно, что (6) не зависит от симметризации одноэлектронных состояний). Константа характеризует заселенность соответствующих молекулярных орбит и может принимать значения 2, 1 и 0, а вторая сумма в (6) дает вклады внутренних заполненных орбит олова.

Определив по (6) \ф'[0)\2 и 10(0) I2 и подставив их выражения в (1), можно получить

5s k j •

где А = 1,97-Ю-22 мм/сек.смЗ x ( 0 ) |2 , a ß характеризует относи-тельный вклад изменений внутренних оболочек в электронную плотность на ядре при переходе от источника к поглотителю

(3 = ( l < P 5 s ( 0 ) l 2 ) - 1 |<Рп . (0) | 2 - l < p n s ( 0 ) ! 2 ] ( 8 ) П = 1

Заметим, что, учитывая в этих формулах отличие (0) I 2 от \(р (0) I 2 , мы принимаем во внимание не только перенормировку волновых функций валентных электронов в результате образования ими молекулярных орбит, но и перераспределение электронной плотности в атомных состояниях при переходе от одного соединения к другому.

Для оценки отношения l<p5's (0)l2/|<p5s (0) I2 можно воспользоваться формулой Ферми-Сегре [3, 4] . Принимая во внимание, что фигурирующее в ней эффективное квантовое число п^ф является слабой функцией эффек-тивного заряда Z, можно непосредственно получить из этой формулы:

К(°Л 8 . Z'2 ... \ 9 Ы ( 0 ) | 2 ~ Z 2 ( 9 )

где Z' и Z - эффективные заряды для 5s - электронов в атоме олова, нахо-дящегося в поглотителе и источнике соответственно. Их значения опреде-

19

ляются как зарядом атомного остова Z0, так и экранирующим влиянием остальных валентных электронов, которое зависит от параметров соответ-ствующих молекулярных орбит . Обозначив параметр экранирования 5s -электрона другим 5s - или 5р - электроном (как электроны одного слоя, по-следние экранируют примерно одинаково) посредством а, можно записать:

Z' = Zq — а(Е e¿N¿2- l)

Z = Z 0 - o ( E e ' N j 2 - 1) ( 1 0 )

f = l + a ( E e ¿ N k 2 - £ e ¡ N ? ) ¿j it j

где введен параметр источника

Z 0 - « ( E e j N 2 - l ( I D J

i 12 Оценку относительного изменения |<pns(0)| для внутренних электронов

по (8) можно получить, используя результаты Кроуфорда и Шавлова [8] ( полученные ими при оценке влияния экранирования внутренних электронов внешними на величину изотопического сдвига. Полагая, что основной вклад в экранирование вносится 5s - электронами, имеем

2 • Упз(0) I2 2

fns = E N ^ Pn2 (r^drj P | (r )dr-E (r^dri/P5

2 (r)dr (13)

где (r) = rRns(r), Rns(r) - радиальная часть волновой функции. Отличие функций P5's и P5s в (13) становится все менее существенным

по мере увеличения оЬласти интегрирования и, следовательно, может ока-заться актуальным лишь для глубоких ns - состояний. Но для них член fns и даваемый им вклад в ß очень мал [8] . Поэтому можно пренебречь этим различием при оценке ß. Тогда имеем:

ß = ß0(E€, N f a f , - E e ^ a L ) 2 (14) J k

где l < p n s ( 0 ) I

I ( 0 ) I2

P„s P5s2(r)dr (15)

2 0

Такой же поправочный член можно написать и для экранирующего влияния внешних р - электронов.

Подставляя (9), (10).и (14) в (7), мы получим общую формулу химичес-кого сдвига мессбауэровского спектра ядра Sn119 в соединении поглоти-теля относительно соединения источника через параметры молекулярных орбит этих двух соединений:

6 = А{ ( [ l + a q ^ N ^ -E^N. 2 ) ] 2 — ß0 ) X £ e a' N^2 — (1 — ß0)£ e.'a^N.2) (16)

Без ограничения общности можно упростить эту формулу, выбрав в качестве источника у - квантов тетраэдрическое - серое олово (a — S n). Для него в хорошем приближении можно полагать, что каждый атом олова тетраэдрически связан с окружающими его атомами олова посредством четырех гибридных sp3-орбит. В этом случае легко получить: ají. = 1 / 4 , е = 2 , Xj = 1 ; 2N? = (1 + S j = 1, 2, 3, 4 и формула (16) принимает вид:

б = а { ( [ l + а ( ^ N Ù 2 - ¡ i ^ ) ] ' ^ ) ^ - ( l - ft) ( l + S ) 1 } k k (17)

a a~ Zq — <x(3 +4S) (18>

По этой формуле можно в принципе рассчитать величину 6, если из-вестны параметры молекулярных орбит и AR/R. Наоборот, при известном б и ÄR/R ее можно использовать для определения параметров молекуляр-ных орбит. Ниже уравнение (17) используется для оценки величины ÄR/R, входящей в А.

Отметим здесь, что из приведенных выше трех основных допущений, которые использовались ранее при интерпретации данных по химическому сдвигу в спектрах Мессбауэра, в формуле (16) и (17) используется лишь одно - возможность определения |<^(0)|2 по формуле Ферми-Сегре [3] . Можно надеяться, что после расширения границ применимости последней, проведенного Фолди [4], это предположение не является сильно ограничи-вающим. Кроме того, остаются, конечно, все допущения, связанные с при-менимостью метода молекулярных орбит.

3- ОЦЕНКИ И ВЫВОДЫ.

Для определения по (17) AR/R необходимо знание параметров молеку-лярных орбит соединений олова, для которых измерено б. Обратимся к простейшему классу этих соединений, а именно к тетраэдрическим молеку-

лам типа SnR^ где R-одновалентный атом или радикал. Для них так же, как и для тетраэдрического олова, связь осуществляется через четыре гибридные sp3 - орбиты, e¿ = 2, ak

2 = 1 /4, к = 1,2, 3, 4, но связь Sn - R в от-личие от Sn - Sn может быть полярной, так что X¡, = X Очевидно, что

2 1

X зависит от акцепторных способностей R, которые в ряде случаев (напри-мер, для галогенов) могут быть оценены по экспериментальным данным, полученным методом ЯКР [5] . Введем параметр у, удовлетворяющий со-отношению

. 2N'2>P = 1 +у (19)

Так как 2N,2X2 равно доле электронного заряда, смещенной к лиганду на двухэлектронной связи Sn - R , то у характеризует; очевидно, изменение заряда на R при образовании им связи. Заметим, что в неполярной связи, т .е . при X =1, Y = - S'fl + S')"1, т .е . отлично от нуля. Отсюда следует, что хотя величина у и играет примерно ту же роль, что и так называемая сте-пень ионности связи, она не совпадает с последней, будучи меньше нее на величину порядка интеграда перекрывания S'. Из уравнений (3) и (19), пре-небрегая S zno сравнению с 1, можно получить: •

2N*2 = 1 — у — 2SVl - у 2 (20)

С учетом этого соотношения формула (17) для тетраэдрических моле-кул принимает вид:

б = А{ [1 + 4a (y -S +?S'J1 -Y2)] (1 - y - 2 S V l - Y2) + ßo(Y ~ S+2S'>/1 -72) - 1 + S)

(21)

В этой формуле константы А, а и ßG могут рассматриваться как пара-метры (интегралы перекрывания S и S' легко вычисляются), и если измере-но 6 для трех тетраэдрических соединений олова, для которых известно у, то определение А и, следовательно, отношения AR/R, не представляет особых трудностей. Как уже отмечалось, параметры у для тетрагалоге-нидов олова SnX4, X = Cl , Br, J определяются по спектру ЯКР на галоге-нах. К сожалению, получаемые таким образом значения у зависят от до-полнительных факторов, главным образом от возможности образования du _Ptr связей S n - X , которая в значительной степени неопределенна [5] . Однако, ход изменения у вдоль серии различных X, т .е . разности уС1 -"уВг>

— Yj и т .д . оказываются почти не зависящими от этих дополнительных факторов. На рисунке приведены экспериментальные кривые зависимости б от Y для тетрагалогенидов олова: (А) в случае, когда у определяется без учета d - Rir связей [9] и (В) — с поправкой на образование таких связей по [5] . Из рисунка видно, что для малых у экспериментальная за-висимость 6 от т линейна. Это подтверждает наш результат по формуле (21). Действительно, при малых у, отбрасывая у2 по сравнению с едини-цей, легко получить.

6 =А(8а+ ß 0 - l ) Y +А(8а+ ß0 -1)(2S' - S ) (22)

Сравнивая эту формулу с экспериментальной кривой, мы легко нахо-дим по наклону (производной) при у =0, что

А(8а + ß0 - 1) = - 5,6 мм/сек (23)

22

а для точки пересечения с ординатой 60

6о =A(8a + ß0 -1 ) (2S' - S ) (24)

Из (23) и (24) мы прежде всего получаем сразу, что б0 = - 5,6(2S' - S). Это позволяет непосредственно решить вопрос об абсолютных значениях "ион-ностей" у в рассматриваемых соединениях, и путем их сравнения со значени-ями, полученными по данным ядерного квадрупольного резонанса, определить долю d„ — р„ — связей в этих соединениях. Заметим, что никакими другими экспериментальными данными эта величина не определяется . Последствия этого результата будут обсуждаться подробнее в другой публикации.

Интересующее нас здесь отношение AR/R МОЖНО определить из уравне-ния (23), если известны параметры а и ß0 . Попытаемся получить их при-ближенные оценки.

Константа а существенно зависит от параметра экранирования 5s -электрона другим электроном того же слоя —а. Заметим, что по смыслу, вытекающему из вывода формулы Ферми-Сегре 14], Zo означает эффек-тивный заряд остова на больших-расстояниях. Отсюда следует, что при оценке Z0 необходимо учитывать возможно большую долю экранирования другими электронами, т . е . достаточно большие значения а. Поэтому, мож-но утверждать, что а > а0 =0,35, где а0 — слэтеровское значение этого па-раметра, удовлетворительно описывающее среднее экранирование при вы-числении энергии и радиусов орбит. Из формулы (18) видно, что если вме-сто а подставить а 0 , то мы уменьшим значение параметра а. Положив S = 0,1 (обычное значение; а слабо зависит от S), легко получить: 2а ^ 0,25. Если же использовать более точные данные для Z0 и а, полученные Урусо-вым [12], то получаем 2а ^ 0,5. В любом из этих случаев, учитывая, что ß0 > О, можно получить из (23), что А < 0 и, следовательно, A R / R < 0 .

Для определения ß0 по (15) необходимо знать атомные волновые функ-ции одноэлектронных состояний, которые для олова неизвестны. Такая же величина, вычисленная Кроуфордом и Шавловым [8] для иона ртути (Hg*), равна 0,16. Так как 5s - электрон олова экранирует сильнее 6 s - электрона ртути, естественно считать, что искомое ß0 > 0,16. Грубую оценку можно получить, определяя отношение l<pns(0) Г2 / l<p5s(0) I2 по формуле Ферми-Сег-

2 3

ре и используя для оценки интеграла в (15) аналогичные величины, вычис-ленные для ртути в [8] , с пересчетом их для олова, в предположении их увеличения, пропорционального отношению l<P5s(0) I2 / Itp (0) |2 . При этом получается ß0 ~0,2. Несколько большее значение ß0 получается при его оценке по сдвигам рентгеновских К-, L-, М-термов, полученным в [10, 11].

Очевидно, что полученные численные оценки не позволяют сколько-ни-будь надежно вычислить абсолютное значение AR/R. Используя значение |<p5s(0)|2 = 1,56-1026см"3 [2] и значения 2а ^ 0,5, ß0 * 0,2, можно получить UR/R I s 1,6- 10"Ч Можно надеяться, что исследование других соедине-ний олова на основе общей формулы (16) или (17) позволит получить допол-нительные данные как для определения абсолютного значения AR/R, так и для параметров электронных оболочек этих соединений. Но уже полу-ченные здесь оценки параметров на примере тетрагалогенидов (в частно-сти, отрицательный знак для отношения AR/R) должны быть приняты во внимание при интерпретации экспериментальных данных по спектрам Мес-сбауэра в соединениях олова.

Заметим, что изменение отношения (Z0)norjI /(Z0)HCT в функции ион-ности связи (при а 0,8) близко к аналогичному изменению отношения 1 " и с т / г п о г л г полученному методом Сандерсена [13] . Это позволяет пред-ставить величину химического сдвига через отношение ионных радиусов, как и было сделано двумя из нас [6] . В результате удалось получить не-которые наглядные соотношения, удовлетворительно объясняющие ряд на-блюдаемых экспериментальных фактов. Так, в ряду тетраэдрических со-единений олова величина гист / г п о г л непрерывно возрастает при увеличении ионности связи. Учет этого обстоятельства при анализе данных о хими-ческих сдвигах приводит к выводу об отрицательном значении AR/R. Про-должив эту аналогию, легко качественно видеть, что, например, при AR/R <0 химический сдвиг белого олова (ß-Sn) должен быть положитель-ным по отношению к » - Sn (что в действительности имеет место), так как средний атомный радиус олова в (ß - Sn) больше атомного радиуса олова в a - S n . Положительный знак химического сдвига, по-видимому, можно бу-дет получить и для соединений двухвалентного олова, так как в последних, как это показано в работе [14], ковалентный радиус существенно больше ковалентного радиуса в a - Sn. Наконец, как это было показано в работе [6], оперируя атомными радиусами олова, внедренными в различные метал-лические матрицы, при отрицательном значении AR/R можно получить хо-рошее согласие расчетов химических сдвигов с экспериментом [15] .

Естественно, что для количественных расчетов химических сдвигов в соединениях олова с более сложными, чем тетраэдрическая, структура-ми необходимо провести строгий анализ на основе формулы (17).

Этим вопросам, а также другим следствиям полученных в данной рабо-те соотношений будут посвящены наши следующие публикации.

Л И Т Е Р А Т У Р А

[1] ШПИНЕЛЬ B . C . , Б Р Ю Х А Н О В В . А . , Д Е Л Я Г И Н H . H . , ЖЭТФ, 4 1 , 1 7 6 7 (1961) . [2] B O Y L E A. G . F . , B U N B U R Y D . S . , E D W A R D S С . , P r o c . P h y s . S o c . 7 9 , 416 ( 1 9 6 2 ) . [3) F E R M I E . , S E G R É E . , Z s . P h y s . 82, 729 (1933); GÖUDSMIT S . , P h y s . R e v . 43, 636

(1933). [4) F O L D Y L . L . , P h y s . R e v . Щ , 1093 (1958).

2 4

[5] W H I T E H E A D S. M. A . , Y A F F E H. H . , T h e o r e t . C h i m . A c t a I, 210 (1963). [6] GOLDANSKY V . l . , MAKAROV E . F . , P h y s . L e t t . 14, 2 (1965). [7] BELYAKOV V . A . , P h y s . L e t t . 16, (1965). [8] CRAWFORD M . F . , SCHAWLOV A. L . , P h y s . R e v . 76 ,1310 (1949). [9] ГОЛЬДАНСКИЙ В . И . , Г О Р О Д И Н С К И Й Г . М . , и д р . ДАН С С С Р , 147, 127 (1962) .

[10] С У М Б А Е В О . И . , М Е З Е Н Ц Е В A . B . , ЖЭТФ , 48 , 445 ( 1965) . [11] NORD LING С . , A r k . F u r F y s . 15, 241 (1959). [12] У Р У С О В B . C . , Журнал с т р у к т у р н о й химии , 3 , 437 (1962) . [13] SANDERSON В . , J . I n o r g . Nuc l . C h e m . 7, 288 (1958). [14] RUNDLE R. E . , OLSON О. H . , I n o r g . C h e m . 3j_ 596 (1964). [15] БРЮХАНОВ В . А . Д Е Л Я Г И Н H . H . , ШПИНЕЛЬ B . C . , ЖЭТФ, 47 , 80 (1964) .

D I S C U S S I O N

N.N. GREENWOOD pointed out the relation of the treatment just given to the nephelauxetic effect, i . e . the radial expansion of the wave functions by shielding.

V.l . GOLDANSKII added that pressure experiments with different tin-containing compounds would be valuable to confirm the conclusions reached in his paper and to obtain some quantitative results on AR/R values.

M. CORDEY-HAYES pointed out that there should be a relationship be-tween the isomer shift and the Knight shift, and said that such experiments were being undertaken at his Laboratory.

J. DANON (Chairman), P. KIENLE and V.I. GOLDANSKII pointed out. various difficulties which might arise in such an experiment since the isomer shift measured total s-electron density whereas the Knight shift measured the unpaired s-electron density. These need not be simply related.

N. N. GREENWOOD commented on the quadrupole splitting in Sn11 cofai-pounds, pointing out that it was dangerous to speak of ionic stannous com-pounds since no compound was known which contained the Sn2+ ion. The normal configuration for SnH was a pyramidal arrangement of three covalent bonds.

H. FRAUENFELDER pointed out that theoretical calculation of AR/R using nuclear models could be in error by factors as large as 100 in some cases.

P. KIENLE disagreed, pointing out that in some regions, i . e . for de-formed nuclei with rotational states, theories gave good predictions of ex-cited-state moments and should be reasonably reliable for calculations of AR/R.

V. l . GOLDANSKII suggested that experiments be done on compounds like Snl4 or Rj Sn^-j where both the H9Sn and the *29i Mössbauer effect iso-mer shifts could be studied.

J. DANON (Chairman) suggested that atomic wave-functions, recently computed, be used instead of the Fermi-Segré formula.

P. KIENLE pointed out that shielding parameters could be obtained from the isotope effect.

J. DANON (Chairman) suggested that muonic atoms provided another means of studying AR/R.

V. l . GOLDANSKII agreed.

2 5

SOME NEW APPLICATIONS OF THE MÖSSBAUER E F F E C T IN CHEMISTRY

R.M. GOLDING DEPARTMENT OF SCIENTIFIC AND INDUSTRIAL RESEARCH

PETONE, NEW ZEALAND

INTRODUCTION

The Mössbauer spectrum arises from transitions between ground and first excited states of the nucleus, the energy differences between these levels being governed mainly by the properties of the nucleus. However the energy differences are also dependent on the environment of the nucleus and it is these small effects due to interactions with the surroundings that are of principal importance to the chemist. In Mössbauer spectroscopy these perturbations are reflected in the differences in б and AEq values (for the definition of Ô and AEQ see Fig. 1). This paper outlines how AEQ depends on the electron wave-functions in some iron complexes and discusses the relationship between Mössbauer spectroscopy and other forms of spectro-scopy such as electron-spin resonance (e. s. r.) and nuclear magnetic r e -sonance (n. m. г.).

к A EQ

K s — t

- 0 - 4 - 0 - 2 О

VELOCITY ( c m / s e c )

» 0 2

FIG. 1. The Mössbauer spec t rum of F e S 0 4 7 H 2 0 . Exper imen ta l pa rame te r s 6 and AE q are de f i ned

26

A. , MÖSSBAUER SPECTRA

Mössbauer spectra can be interpreted from the experimental Hamiltonian

= -KyH-I + P[(3I2 -1(1+ 1)) + § (I2 + I?)]

where H is the observed magnetic field at the nucleus, y is the nuclear gyro-magnetic ratio and i) is the asymmetric parameter of the field gradient tensor. P is a nuclear constant depending on its environment. When the ob-served magnetic field is zero the Hamiltonian contains only the quadrupöle term, i, e.

= P [ 3 I 2 - 1(1+1)]

(taking »}= 0 for simplicity). . For 57Fe this gives the following nuclear energy level diagram.

E

T 6 P

fx IX iX

Experimentally AEQ is readily evaluated as the doublet separation and in our notation AEQ = 6P.

B. THE QUADRUPOLE INTERACTION

This section deals with the term P in molecules and with the calculation of AEQ for several iron complexes from the d electron wave-functions. The interaction between an electron and the nucleus can be written as a sum of a set of multipole interactions. Here the important term is the quadrupole term and this can be represented by the Hamiltonian,

1 ( 1 + 1 ) 3 ( r j - 1 ) 2 1 ' r 3 r 5

i i -1

Qi = Q( l -T^) where Q is the nuclear quadrupole moment; ( l ~ 7 œ ) the Sternheimer screening factor; is the average value of r"3; and the

e Q V ^ 2 1 ( 2 1 - 1 ) L

27

distance between the i-th electron and the nucleus i s r¡ . With equivalent operators the Hamiltonian (1) may be written for d electrons as

, = e 2 Q K r ' 3 ;

7 1(21-1) z I) - 21(1+ 1)

We shall now discuss briefly the expected AEQ results for iron com-pounds where the iron atom is in: (a) an octahedral crystal field; and (2) a field of D4 symmetry.

1. Fe111 case

The electronic ground state for a d5 ioii in a strong crystal field is 2 T 2 . Spin-orbit coupling partially lifts the degeneracy of this ground state and if spin-orbit mixing of the excited states is negligible the electron energy level is as shown:

3Ç/2

If the ion is in a crystal field of D4 symmetry this may be represented by an octahedral crystal field modified by a distortion interaction, eL*. This gives the Hamiltonian

• I 71(21- 1) ( V I ) 2 } From this Hamiltonian are calculated the energy levels for various ratios of e/Ç (Fig. 2) and the corresponding EQ values where

= А . е ^ ^ г - з ) Г з туг2 — T(T + 1 ) 1 «5 2 8 1 ( 2 1 - 1 ) l d M

The value of A¡ is given for the three electron energy levels, E i ( in Fig. 3. The average EQ value is given by a summation over all states. Therefore,

2 w ,3ч EA¡exp-E¿/kT e28I(2i r- l) [ 3 M 2 - I ( I + D ] ' £ e x p _ E ¡ / k T

2 8

e/r

FIG. 2. The splitting of the ground term of the low-spin ferric ion in a pseudo-axial field. £ is the spin-orbit

coupling constant and e the distortion parameter

This equation gives the temperature dependence of <(EQ^>. Figure 4 is a plot of

Ç A i e x p ( - E i / k T ) . ! .

Eexp(- EJ/kT)

over a range of kT/g values when e/Ç= 1. 0. This figure shows that for F e m

compounds, AEQ should be temperature independent or show a slight decrease at higher temperatures. This is observed.

To illustrate the range of AEQ values expected for F e m compounds we shall consider three cases:

(a) When e = 0 (octahedral case) AI = 0, A2 = 2, A3= -2 and E 2 = E3 (see Figs. 2 and 3).

Therefore <E 0 > = 0 (2)

2 9

e/5

FIG. 3. The A¡ values for the three energy levels, E¡ (Fig. 2) from which the e lect r ic field gradient at the

nucleus can be calculated using equation

for any e/C value

(b) When e is positive and very large

If we consider just the electronic ground state

< E q ) = e7

2 ^ < _ r j )3 > [ 3 M 2 - I ( I + 1 ) ] ( 3 )

(c) When e is negative and very large If we consider just the electronic ground state

< E Q > = - M Q i 1 ( 2 i r - ^ [ 3 M 2 - 1 ( 1 + 1 ) 1 <4>

3 0

vol 1 1 1

О 1 0 2-0 3-0

¥ FIG. 4. This figure illustrates the temperature dependence of <EQ > for Fe111 compounds when e/C = 1. 0

Ç A j e x p ( - E¿/kT)

Y ~ Z e x p ( - E ¡ / k T ) i

EJ and AJ values are read from Figs. 2 and 3 respectively

2. Fe2* case

In the d6 configuration the ground state for the weak field case is 5T2 . The calculations here give the results:

(a) When e = 0 (octahedral case)

< E Q > = .0 (5)

(b) When e is positive and very large

< E q > = - e ; 4Q

Ii < I

r : \ > [ 3 M 2 - Ц 1 + 1 ) ] ( 6 )

(c) When e is negative and very large

m i - i ) ) | 3 M 2 ' I ( m i 1 ( 7 )

3 1

3. Fe3* and Fe11 cases

The ground states for d5 and d6 ions in weak and strong octahedral crys-tal fields are 6Aj and ¿Aj respectively. Hence we expect all Fe3+ and Fen complexes to have zero AEQ values.

4. Comparison of AEQ values for iron complexes

From Eqs. (2-7) and from section (B. 3) we can obtain the relative AEQ values to be expected for iron compounds, as shown in Table I.

The agreement between the calculated relative AEQ values and the ex-perimental results clearly indicate that the theoretical treatment is adequate to explain the variations of AEQ for iron compounds.

TABLE I

A E q VALUES FOR IRON COMPOUNDS

Calculated relat ive

range of AEQ values

Experimental range of

JAEQI values (cm/s)

2 + Fe - 1 to 2 0 to 0. 370

FEIII - 1 to 2 0 to 0.170

F e 3 + 0 0 to 0. 065

F e " 0 0 to 0. 080

C. SINGLE CRYSTAL STUDIES

The intensities of the y-ray transitions depend on the angle 0between the direction of the Y_ray and the axis of quantization. For the 51 Fe com-pounds ( I e = 3/2 and Ig = 1/2) the intensity ratio of the two arms of the doublet is 3(l + cos20) : ( 5 - 3 cos20), the corresponding transitions being | ± 3 / 2 > e - |± 1/2 >g and |± l / 2 > e - | ± l / 2 > g , where 'e' and 'g' refer to the excited and ground nuclear states respectively. In a powdered sample cos26 is averaged over all angles and the two lines are of equal intensity. However, this is not the case for single crystals. From single crystal studies we are able to determine experimentally the order of the energies of the |± 3/2 J>e

and |± 1/2 > e nuclear levels. At present we are investigating the Mössbauer spectra of several single crystals in order to determine not only the sign of the AEQ but the direction of the electric field gradient in the crystal. In Fein complexes, for example, we should readily be able to determine the type of distortion from octahedral symmetry in the molecule (i. e. e positive or negative).

3 2

D. MÖSSBAUER SPECTROSCOPY AND ITS RELATION TO OTHER FORMS OF SPECTROSCOPY

The electronic wave-functions and the energy levels of the d electrons in iron complexes not only determine AEQ but are also involved in other physical phenomena such as u. v. absorption (in which we are primarily in-terested in d-d transitions), e. s. r. and n. m. r. spectroscopy, and magnetic susceptibility measurements. For example, for an Fein ion in an octa-hedral crystal field AEQ = 0 (section B. 1)

.ml 3Ç

3x+ 8 - 8 exp(- 3x/2) 1+2 exp(-3x/2)

where x= Ç/kT, x is the magnetic susceptibility and

ДН = - ah/3 H 18fgNßN

Зх+ 16 - 16 exp(- 3x/2) 1+ 2 exp(-3x/2)

(8)

(9)

where AH (measured by n. m. r. spectroscopy) is the magnetic field at the nucleus arising from the unpaired d electron. The parameter 'a1 depends on the s-electron density at the nucleus. It-follows from these three related equations that if a model can be found to explain AEQ, the same model should also explain x and ДН.

This can be further illustrated by comparing the calculated and the ex-perimental AEQ, the average magnetic moment /U and the three principal g values for the ferricyanide ion, assuming e/Ç = 1/6 (see Table II).

TABLE II

VALUES FOR FERRICYANIDE ION

Calcula ted Experimental Remarks

AEQ

S„

Sx

2. 9 x 10"4 cm" 1

2 . 4 8 B.M.

1. 00

2 . 4 1

1 . 2 x l 0 " 4 c m " 1

2 . 3 5 B.M.

0 . 9 1 5

g x = 2 . 3 5 g„ * 2. 10

powdered sample

powdered sample

K 3Fe(CN) 6 di luted in cobal t fer r icyanide , Bleaney, O'Brien, Proc. phys. Soc. 69 В (1956) 1216.

Relationship between magnetic moment and AEQ

It follows from the above that in a series of Fe111 compounds, the mag-netic moment, ¡л, will be related to AEQ. For an octahedral Fein complex AEQ = 0 but experimentally the AEQ values lie between 0 and 0. 170 cm/s .

33

These non-zero AEQ values can be explained (see section B. 1) in terms of the crystal field environment of the iron atom. The magnetic moment at room temperature of an octahedral iron complex is about 2. 48 Bohr magne-tons (from Eq. 8). However, an Fein ion in a distorted octahedral crystal field has a lower magnetic moment approaching 1. 73 Bohr magnetons (the spin only value) for large distortions. Hence a relationship between AEQ and ß would be expected. This is illustrated in Table III.

TABLE III

C O M P A R I S O N O F C A L C U L A T E D A N D E X P E R I M E N T A L R E S U L T S

Calculated results

AEQ ß

Experimental results •

AEQ ß Compounds

Fe111 (octahedral) 0 2 .48 0. 000 2. 35 (a) Fe1111 (distorted) 2 1 .73 0 .142 1 .99 (b)

(a) Na 3 Fe(CN) 6 H z O (b) Fe(l : 10 phenanthroline) (CN)2 N0 3 4 H 2 0

E. ISOMERIC SHIFT

The isomeric shift is given by

б = F ( Z ) ^ - ( I 0,(0)1= -I 0,(0)1!)

where |0S(O)IA AN(L |0S(O)|E refer to the s-electron densities of the absorber and the emitter respectively. As б reflects the s-electron density at the nucleus it must depend on the electronic configuration in the molecule since the electron-nucleus interaction (represented by the Fermi contact term) can arise in three ways: (i) from mixing of the excited electronic states con-taining unpaired s electrons with the ground state; (ii) by a spin polarization effect due to spin exchange interactions, and (iii) by ligand orbital mixing of the appropriate electronic configuration with the ground state. It is there-fore not surprising that for iron compounds ô l ies over a range of values and that AEQ may be correlated with 6.

The Fermi contact term also accounts for the isotropic hyperfine inter-actions in e. s. r. spectroscopy and the magnitude of temperature-dependent shifts in n. m. r. spectra of paramagnetic complexes (see Eq. (9) for the t | case) and hence we have another related experimental parameter.

3 4

F. CONCLUSIONS

This paper illustrates the variations of AEQ arising from the electronic configuration in some iron compounds and briefly discusses the isomeric shift. The paper also discusses how the Mössbauer effect is complementary to other forms of spectroscopy. This can be further illustrated by con-sidering the experimental Hamiltonian for electron-nuclear systems

gßH • S + D SI - | S ( S + 1 ) + E(S2-S2) + ASZ-IZ

+ B(Sx-Ix + Sy -I y)+P[3I | - I ( I+ l ) + rj(l2-l2)] -f iTH-I + JI-I

The terms that arise in e. s. г . , n. m. г. and Mössbauer spectroscopy are indicated below

e. s. r. n. m. r. Mössbauer

g, D, E, A, В, P -fty, J, (A+2B) P, -Tiy, (A+2B)

Mössbauer spectroscopy thus supplements other forms of spectroscopy and is another powerful technique in investigating electronic configurations in molecules.

D I S C U S S I O N

N. N. GREENWOOD asked how other contributions to the quadrupole splitting could be sorted out.

P. HILLMAN asked about the temperature dependence of the splitting in Fe2+ .

R. M. GOLDING said it was similar to the example shown. V.l. GOLDANSKII asked whether he had considered the case of ferro-

cene and related compounds. R. M. GOLDING said that they had not, but thought it would require a

change in spin-orbit splitting from the 400 cm"1 assumed in his calculations to 250 cm"1.

J. DANON (Chairman) pointed out that the crystal field splitting of the lower triplet in the ferricyanide complex had been measured to be ~ 100 cm"1

in disagreement with the value of 23 cm"1 reported as fitting e. p. r. and Mössbauer data.

R.M. GOLDING allowed that the 23 cm"1 value was the result of a one-parameter fit; if the other parameters were allowed to vary other results would be obtained, suggesting that the 23 cm"1 value was not definitive.

P. KIENLE pointed out that shielding effects could produce major changes in the results reported.

R. M. GOLDING answered that shielding effects were assumed to be the same in all compounds.

3 5

APPARATUS AND TECHNIQUES

(Sess ion 2)

MÖSSBAUER E F F E C T AT HIGH PRESSURES*

H. FRAUENFELDER AND R. INGALLS UNIVERSITY OF ILLINOIS

URBANA, ILL. , UNITED STATES OF AMERICA

A. INTRODUCTION

The physics and chemistry of the past century have, to a very large degree, been sciences "as a function of temperature". The reason is ob-vious: it is easy to put a Bunsen burner under the material to be studied and it is not too difficult to drop it into liquid helium. It is considerably more difficult to apply extreme pressures and make useful measurements. High-pressure studies hence developed only slowly up to about 1950 and, in particular, very few "microscopic" studies had been made at high pres-sures. In the past ten years, the situation has changed rapidly. Through modification of Bridgman's technique and through the use of shock-waves, the pressure range has been extended considerably and it reaches now to about 500 kilobars under static and to a few megabars under pulsed con-ditions. In addition, "microscopic" experiments, such as magnetic reso-nance, X-ray and Mössbauer work, have become possible. The present situation has been reviewed in a number of publications [1-3].

In the present report, we discuss the Mössbauer effect under very high pressure. The number of publications in this field is still small; a few experimental [4-8] and two theoretical papers [9, 10] have appeared.

The quantities that can be determined in Mössbauer experiments, namely the recoil less fraction, electric field gradients and magnetic fields at the nuclear site, and isomer shift, give valuable information if studied as a function of pressure: (a) They are of interest in studying the electronic structure of solids. In

particular, if a theory predicts the behaviour of one of the Mössbauer quantities mentioned above as a function of volume, then the pressure measurement may be converted to volume by the formula

* Supported by the US Off ice of Naval Research under contract 1834(05) and by the US Atomic Energy Commission under contract AT(11-1U198.

(1)

3 7

(b) The pressure dependence of the various quantities X is necessary for correcting the temperature dependence at constant pressure to constant volume for comparison with theory:

(c) The quantities may shed light on the nature of some interesting phase changes that occur at high pressure.

(d) The recoilless fraction may in some cases be greatly enhanced at high pressures. The Mössbauer effect has some advantages and disadvantages as a tool

to study the pressure dependence of properties. Among the advantages we mention the high sensitivity with which shift and splitting of the 7-ray spec-trum can be obtained, the smallness of the required sources, the absence of leads to the sample, and the fact that no external fields must be applied in most experiments. The main disadvantages are the fact that suitable Mössbauer isotopes exist only for a few elements and that a window must be provided to transmit the 7-rays.

B. THEORY

Of the various quantities studied in Mössbauer experiments perhaps the one most accessible to simple theory is the dependence of the recoilless fraction f upon pressure. The most straightforward way is to use the Debye model and calculate the volume dependence of the Debye temperature either by means of the Grüneisen relation [11, 12]:

or the Lindemann relation [13-16]

Here, 7 is the Grüneisen constant, 01( Tml, and 02, Tn2 are the Debye tem-peratures and melting points at the specific volumes Vi and respectively. The • *lting change in the recoil less franction may then be determined by inserts 4, the calculated Debye temperature into the expression for the Debye-Waller factor.

To show what Debye temperatures can be expected at the pressures that are available tod4y, we plot in Fig. 1 the Debye temperature, calculated from Eqs. ( l ) and (3), versus pressure for a number of substances. It is clear from Fig. 1 that for "softer" materials, f will depend dramatically on the external pressure. Stone et al. [5] have shown that f of ißiDy in Gd metal increased by an order of magnitude in going from atmospheric pressure to 100 kb.

38

ш ОС

о 100 200 300

PRESSURE (KB)

FIG. 1. Debye temperatures as a function of pressure calculated by using the Grüneisen relation [Eq. (3)} together with published data for the volume as a function of pressure. In obtaining these curves the Griineisen parameter for each material was assumed constant over the entire pressure range

Dlouhá [10] has derived a more general relation between the volume and temperature dependence of the recoilless fraction; it is also based on the Grüneisen approximation but does not use the Debye model. Her relation for the Debye-Waller factor W (f = e"2W) is

Some quantitative work has also been done that can be related to the variation of the electric field gradient (EFG) with pressure. In particular, one can show that for axial ionic crystals under isotropic compression, the EFG depends upon axial crystalline field splitting [16] which in turn varies as 1/V. For axial metals, calculations exist [17] that show the dependence of the EFG upon с/a, the volume dependence of which may be determined from high pressure X-ray studies.

Thé isomer shift varies with pressure through changes in ¥2(0). Thus when compressing a metal ¥2(0) may vary by simply a scaling of the valence electron density at the nucleus or it may change because of a transfer of electrons to or from the s-like energy bands. Earlier work on the isomer shift [18] in 5,,Fe enables one to interpret high-pressure results in terms of a decrease or increase in the effective number of 4s electrons on the Fe atom.

The pressure dependence of the Internal magnetic field is much more difficult to calculate because magnetism is not fully understood and because

(5)

39

the relative magnitudes of the many contributions to the internal field are not accurately known. The best approach at present is to refer to the Bethe-Slater curve [19] of exchange integrals versus inter-atomic spacing for a crude qualitative explanation of the experimental results. The situa-tion is somewhat better understood with ionic materials.

C. EXPERIMENTAL ARRANGEMENT

Experimental techniques developed to study the Mössbauer effect under pressure are described by Pound, Benedek and Drever [4], Stone et al. [5], Nicol and Jura [6], Pipkorn et al.[7], Herber and Spijkerman [20] and Ingalls [21], We restrict our discussion to the system developed by Drickamer and co-workers [22, 23]; further details are given in the Refs. [7] and [21].

The main features of the experimental layout are shown in Fig. 2. Pres-sure is applied to the Mössbauer source in the pressure cell by a hydraulic press. Detection of the 7-rays and motion of the absorber can be made with any conventional set-up. The novel problems introduced by the high-pressure aspects are: (1) the production of the high pressure, (2) preparation of suit-able Mössbauer sources, (3) the calibration of the pressure system.

1. The pressure cell

We show in Fig. 3 a cross-section through the major components of the pressure cell. As indicated, machined pellets of pyrophyllite fit the tapered parts of two work-hardened, sintered tungsten carbide (grade 999 Carboloy) pistons which ride in a hardenened-steel cylinder containing a window slot. Between the two pellets is a thin wafer of LiH, of the same diameter1, con-taining the Mössbauer source at its centre. This wafer, which serves as an inner window for the emitted radiation, is shown in detail in Fig. 4, and described in the next section. Shims between the cylinder and jacket of the bottom piston permit adjustment of the wafer so that it is directly opposite the window slot of the cylinder. Pressure is applied to the entire piston and cylinder assembly by means of a hydraulic press .

For short periods of time, pressures up to about 500 kb can be reached. Unfortunately, a typical run to get reliable data takes of the order of many hours or days. For this reason and the fact that the piston flats bow-in at higher pressures and cut off the radiation, such runs allow pressures only up to about 300 kb.

2. The Mössbauer source

The source may be in either the form of a foil or a powder. In the former case it is bent to make a cylindrical surface (see Fig. 4) and located next to a small pyrophyllite disc at the centre of the LiH wafer. In the latter case a segment of the disc is sanded away and replaced by a mixture of the powdered source and epoxy resin; this mixture and partial disc are moulded

1 The dimensions of the cylinder and the d iameters of the pistons and piston flats vary with app l i -cat ion. Specif ic details a re given in Ref. [23] .

4 0

Connection to pump and gauge

Hydraulic -ptess

Proportional counter

Pressure cell containing

Mössbauer source

Variable speed dr ive

Absorber

FIG. 2. The experimental layout showing (left to right): hydraulic press containing the pressure cell; moving absorber; proportional counter; cam drive and gears for moving the absorber. (After Pipkorn et al. , Ref. [7])

Pyrophyllite - ^ r - - - » - C ^ ^ ^ l pellets Li H and В Wafer-

U—^ containing source

Threaded - steel insert J

Hardened-steel cylinder

¡i Z N -

M M .

-jj

Щ -Window

slot

FIG. 3. Exploded cross-section of the high pressure cell

in a hole drilled in a sheet of Teflon2 . Surrounding the small disc 'and source is. a pressure-transmitting pellet composed of a mixture of 85% В and 15% LiH. The pellet is in turn surrounded by pure LiH which forms the main body of the wafer. The entire wafer is fabricated, using flat pistons, in the same steel cylinder which forms the body of the high-près sure cell. The method consists in fusing a wafer of LiH (at ~ 3 . 6 kb) and drilling a hole in its centre. Into the hole is placed the B-LiH pellet (fused earlier at ~ 14 kb in a separate cylinder) and the two are pressed together (at ~ 9 kb).

2 The method of loading powder samples has been developed mainly by C. Coston.

4 1

Pyrophyll i te disc

В I

Mössbauer source

_ 8 5 8 5 % В - 15 % Li H Pellet

LiH Wafer 0 015" T h i c k n e s s

FIG. 4. Detail of the LiH disc containing the source. (After Pipkorn et al. , Ref. [7])

The small pyrophyllite disc and source are then pressed (at ~ 1 7 kb) into a hole drilled in the centre of the wafer. The source is of course placed so that it will be between the small disc and the window slot of the main cy-linder. The tapered pistons are used to seat the tapered pyrophyllite pellets against the assembled wafer (at ~ 6 kb). After aligning the wafer with the window of the cylindèr the cell is ready for use.

3. Pressure calibration

Before the actual loading, each pair of pistons must be calibrated to-gether with a pair of pellets machined from the same piece of pyrophyllite and tapered at the same angle as those to be used with the Mössbauer source. The calibration is made by measuring the electrical resistance versus applied pressure of bismuth, loaded in the manner described in section C. 2. The pistons themselves serve as the leads, with one piston wrapped with mica for insulation and used with a slightly larger threaded-steel insert (see Fig. 3). The bismuth phase change at 87 kb shows up as a discontinuity in the electrical resistance at a certain applied pressure. This calibration point is then compared with X-ray data from many pairs of identical pistons, and for which a family of curves of actual pressure versus applied pressure can be obtained up to 500 kb. These curves are not linear in applied pres-sure and also depend upon the angle of taper and quality of pyrophyllite used. From the bismuth transition point, one therefore determines which calibra-tion curve to use for the particular pistons and pellet combination over the entire range of applied pressure. Calibration points at higher pressures (e. g. 133 kb for iron or 161 kb for lead) are not as convenient for this me-

4 2

thod because the resistance discontinuities are much smaller and also be-cause the pistons have greater tendency to deform.

4. Experimental problems

The geometrical arrangement and physical s i ze of the cel l put some constraints on its versatility. One of the most serious is background radia-

tion from higher-energy 7 -rays which scatter off the various parts of the cel l and which are more easi ly transmitted through the support material than the Mössbauer 7 -ray . Coupled with the relatively large background is an extremely small sample 1017 atoms) sind also a small solid angle of emission. Therefore, high specif ic activity (with the associated need for rather delicate source preparation) is required to get a satisfactory counting rate. Even in favourable cases a run may last more than a week, so that there is also a high stability requirement on the entire experimental system.

The problems outlined above make use of such a cell for studying ab-sorbers l e s s practical than for sources.

D. RESULTS

We present here a selection of some recent results. These examples indicate the power of the approach and they show that we can expect many interesting resul ts in the future. The discuss ion i s res tr ic ted to 5 7Fe.

1. Isomer shift of 57Fe in iron

At low pressures , iron has a bcc structure and is ferromagnetic. At about 130 kb occurs a f irst-order phase transition to the nonferromagnetic hep structure. Figure 5 shows the i somer shift for 57Fe in iron [7]. To compare the observed shift with theory, we assume that the increase in s -electron density at the nucleus sca les with the decrease in volume. The relation between the s-e lectron density and the isomer shift i s taken from the semi-empirical relationship of Walker, Wertheim and Jaccarino [18]a. The isomer shift e then should change with volume according to

0 . 1 4 С Ш / 3 ( 6 )

The relation i s indicated by the solid line in Fig. 5. The behaviour in the bcc phase is well approximated by the assumption made above. The change at the phase transition and the small dependence of the isomer shift on vo-lume in the hep phase indicate a change in the s character of the valence electrons.

3 Note, however, tha t the revised relationship put forward by Danon wil l require a reinvestigation of Eq. (6) (see Danon, J . , "Mössbauer e f fec t and chemica l bonding in transition me ta l complexes" , this Report. )

4 3

V/V„

FIG. 5. Isomer shift for 5 ,Fe in iron as a function of volume. (After Pipkom et al. , Ref. [7] )

V/V,

FIG. 6. Isomer shifts for 57Fe in nickel, copper and palladium as a function of volume. (After Edge et al. , Ref. [ 8 ] , and Drickamer et al. , Ref. [24])

2. Isomer shift of 57Fe in nickel, copper and palladium

Figure 6 shows the isomer shift as a function of volume for 57Fe em-bedded in nickel, copper and palladium [8], which have the fee structure. The i somer shift i s considerably l e s s than predicted by Eq. (6). Similar i somer- sh i f t data have been found for 5 7Fe in hep phases in titanium [8], cobalt [24] and iron [7]. It appears as if the common behaviour is caused by a similar decrease in s-electron character of the valence electrons. The energy of the 4s band seems to increase with decreasing volume more ra-pidly than that of the 3d electrons.

Measurements have also been made on other systems and it is clear that such experiments will be very helpful in understanding the behaviour of the s and d bands with changes in volume.

4 4

P(KB)

FIG. 7. Quadrupole splitting of 57Fe in hep cobalt as a function of pressure. (After Drickamer et al. , Ref. [24])

P ( K B )

FIG. 8. Quadrupole splitting of 57Fe in C0CI2 as a function of pressure. (After Drickamer et al. , Ref. [24])

3. Quadrupole splitting

Figure 7 shows the quadrupole splitting for 57 Fe embedded in hep cobalt [24], and Fig. 8 displays the quadrupole splitting for &7Fe in CoCl2 [24, 25]. The latter has a rhombohedral structure. The shape of the curve in Fig. 8 closely resembles a calculation of Ingalls [16] in which the quadrupole split-ting increases with increasing axial crystal field component. For isotropic changes in the lattice parameter this component varies as 1/R3, where R is the ion ligand distance. Thus one predicts and observes an increase in quadrupole splitting with decreasing R.

4. Internal magnetic field

The magnetic field at the 57Fe nucleus in iron [7], nickel and cobalt [24] is'shown in Fig. 9. The decrease in internal field with pressure for iron agrees very well with NMR measurements bf Litster and Benedek [26]. The 57Fe internal field also decreases in nickel, but at a smaller rate. In cobalt

the 5TFe internal field appears to rise somewhat and then decrease with in-creasing pressure . The only high pressure NMR results available here for comparison are for 59Co in both cobalt and nickel [27] and 61Ni in nickel

4 5

P(KB)

FIG. 9. Internal magnetic field of 5 ,Fe in iron, cobalt and nickel as a function of pressure compared With nmr results of Litster and Benedek, Ref. [26]. (After Pipkorn et al. , Ref. [7] , and Drickamer et al. , Ref.[24])

[28]. In each of these three systems, the internal field shows a rapid in-crease with pressure in the range below 10 kb. On the other hand it is noted that the magnetization in iron, cobalt and nickel [19] decreases with pressure in the same pressure interval. As yet, there is no satisfactory explanation of these results.

5. Outlook

It is clear that the Mössbauer effect is a very powerful tool to study the electronic structure of solids and compounds as a function of pressure, to investigate phase transitions, and to study lattice dynamics problems. It i s also clear that the work will never be of a routine nature since each run must be very carefully prepared, each source must be constructed and inserted, and each run will take of the order of days.

R E F E R E N C E S

[1] Solids Under Pressure ( Paul, W. and Warschauer, D . M . , Eds. ) McGraw-Hill, New York (1963). [2] BENEDEK, G. B., in Magnetic Resonance at High Pressure, Interscience Publishers Inc., New York (1963). [3] DRICKAMER, H. G., Scientific Research at Very High Pressure, Phys. Today, 17 .11 (1964) 59. [4] POUND, R.V., BENEDEK. G. B., DREVER, R., Phys. Rev. Lett 7 (1961) 405. [5] STONE, J. A. , NICOL, M., JURA, G., RASMUSSEN, J .O . , University of California, Lawrence Radiation

Laboratory Rpt, UCRL-10630 Rev., April 1963. [6] NICOL, M., JURA, G. .Science 141 (1963) 1035. [7] PIPKORN, D. N. , EDGE, C. K. , DEBRUNNER, P. , DePASQUALI, G. , DRICKAMER, H. G. , FRAUENFELDER, H.,

Phys. Rev. 135 (1964) A1604. [8] EDGE, C . K . , INGALLS, R., DEBRUNNER, P., DRICKAMER, H. G., FRAUENFELDER, H. , Phys. Rev.

(in press). [9] HANKS, R.V. , Phys. Rev. 124 (1961) 1319. [10] DLOUH / , J . , Czech. J. Phys. В 14 (1964) 570. [11] MONTROLL, E. W. , in Handbook of Physics (Condon, E. U. and Odishaw, H. , Eds. ) McGraw-Hill,

New York (1958) sect. 5-150. [12] BLACKMAN, M. , in Encyclopedia in Physics, Springer-Verlag, Berlin, Vol. VII /1 (1955). [13] LINDEMANN. F. A . , Phys. Z. 11 (1910) 609.

46

[14] BUNDY, F. P. , STRONG, H. M. , in Solid State Physics (Seitz, F. and Turnbull, D . , Eds. ) Academic Press, New York and London 13 (1962) 118.

[15] KAUFMAN, L., CLOUGHERTY, E . V . , WEISS, R .J . , Acta metaU. 11 (1963) 323. [16] INGALLS, R., Fhys. Rev. 133 (1964) A787. [17] DAS, T. P. , POMERANTZ, M. , Phys. Rev. 123 (1961) 2070. [18] WALKER, L. R., WERTHEIM, G. К . , JACCARINO, V . , Phys. Rev. Lett. 6 (1961) 98. [19] KOUVEL. J. S. . in Ref. [1] p. 277. [20] HERBER, R. H. , SPIJKERMAN, J. , (in press). [21] INGALLS, R., to be published in Advances in Mössbauer Effect Methodology. [22] DRICKAMER, H. G., BALCHAN, A. S . , in Modern Very High Pressure Techniques, ÍWentorf, R. H., J r . ,

Ed.) Butterworth Scientific Publishers Inc. , Washington f 1962) 25. [23] PEREZ-ALBUERNE, E. A. , FORSGREN, К. H. , DRICKAMER, H. G. , Rev. scient. Instrum. 35 (1964) 29. [24] DRICKAMER, H. G. , INGALLS, R. L., COSTON, C . J . , to be published. [25] COSTON, C . J . , private communication. [26] LITSTER, J . D . , BENEDEK, G. B., J. appl. Phys. 34 Ц963) 688. [27] BENNETT, L. H. , Decennial Conference on Magnetism and Magnetic Materials, Minneapolis, Nov. 1964. [28] KUSHIDA, T . , quoted in Ref. [2] p. 23.

DISCUSSION

R. H. HERBER asked whether the pressure change was isothermal or not, and whether the pressure could be lowered again to give a replicate result at lower pressures after the sample has been compressed.

H. FRAUENFELDER stated that the process was isothermal, but pres-sure could not be lowered in a reproducible manner.

E. FLUCK pointed out that the method should be useful to study chemical reactions that occur in the solid state.

P. HILLMANN asked the reason for the sluggishness of the transition in iron at 130 kb.

J. SPIJKERMAN thought it could be a nucleation phenomenon. R.M.GOLDING and V.l. GOLDANSKII wondered whether one could uniquely

distinguish between a change in the value of R, a change in the d wave-functions, and a change in shielding. No definite answer to this question was given. Golding remarked that the connection with isomeric shift cal-culations could be helpful.

V. I. GOLDANSKII asked whether asymmetries in the quadrupole split lines had been observed.

R. H. HERBER said that he had seen asymmetry of the intensity of the two peaks in stannic oxide under pressure.

R.M.GOLDINGpointed out that asymmetries were to be expected if pres-sure was anisotropic.

R. H. HERBER remarked that anisotropies could occur even with iso-tropic pressure. In a non-cubic bonding environment, the electron distribu-tion could become anisotropic on the application of Isotropic compression.

4 7

THE B E L L LABORATORIES MÖSSBAUER E F F E C T D O P P L E R MODULATOR

G.K. WERTHEIM AND R.L. COHEN BELL TELEPHONE LABORATORIES. MURRAY HILL, N .J . , UNITED STATES OF AMERICA

The purpose of the modulator is to provide a means of shifting the energy of a nuclear Y-ray by a precisely controlled amount, making it possible to examine a region in the vicinity of the nuclear transition energy. The shift required to observe hyperfine coupling energies is only a small fraction, e.g. 10"10 , of the y-ray energy.

By far the most convenient modulation technique is based on the Doppler effect. The principle was introduced by Mössbauer and has found almost universal acceptance, although the mechanisms now used bear no resem-blance to the one originally used. Other methods, using for example the thermal red-shift or ultrasonically produced side-bands, have been pro-posed for special applications, but have not been much used in actual experiments.

The velocity, v, required to shift the energy of a 7-ray of energy, E, by an amount, 6E, is v= с 6E/E where с is the velocity of light. Velocities as high as 100 cm/s have been used in experiments on some rare-earth iso-topes while shifts as small as 100 M/S have been measured with precision in others. For work with 57Fe, 1 cm/s is required and measurements are generally reported to a precision of ±0. 002 to ±0. 0002 cm/s. Spectrometer calibrations are usually based on the well-known hyperfine spectrum of metal-lic iron, whose outer lines are separated by 1. 065 cm/s.

The essential element of the Doppler modulation technique is a mechani-cal motion with precisely controlled velocity; (The emphasis must be on velocity rather than displacement, since the energy shift is a linear function of velocity). Mechanical motion devices based on cams have met with only limited success, since control of velocity requires control of the angular derivative of the cam radius. Wear and bearing noise tend to cause pro-blems in systems based on mechanical linkages and cams. Many successful devices have been based on velocity-controlled electromechanical feed-back systems which can be constructed with relative ease.

Various types of motion have been used, including constant velocity (saw-tooth motion), sinusoidal velocity, and constant acceleration (parabolic motion). Desirable properties include a linear energy scale and a spectrum which is accurately flat in the absence of absorption. The simplest way to achieve this is to count for a fixed time at equally-spaced velocity values, by using either a constant-velocity saw-tooth motion with the retrace portion eliminated from the counting, or else a symmetrical saw-tooth motion in which positive and negative velocity counts are stored separately. The utility of such systems is usually limited by the stability of the counting system and by the difficulty of realizing the ideal motion. The large forces acting for short periods which are required. Figs, la and b, put extreme require-ments on the amplitude and frequency response of the feed-back system.

Both of these difficulties can be overcome by using a motion of lower harmonic content which sweeps repeatedly through the range of velocities

4 8

d+

d2x

dx d t

d x г

( а ) CONSTANT VELOCITY ( b )

- Ч У Ч У

d t 2 f (с) ¥ (d)

CONSTANT ACCELERATION

FIG. 1. Displacement , velocity and acceleration in various Doppler modulation schemes

of interest, while synchronously sorting the counting information in a large number of scalers, i . e . into a multichannel analyser. The simplest possible motion, sinusoidal motion, can be obtained with greatest ease. Excellent motion can be produced by driving a high-Q spring-mass system with an amplitude-sensing feed-back arrangement. Such devices have been widely used, but they do not satisfy the criteria of providing both a linear energy scale and a flat no-absorption spectrum. The usual arrangement has been to use a linear-velocity scale and to normalize the experimental curve by dividing it by an experimental no-absorption curve. This relatively clumsy procedure can be justified only where very high velocities are needed, i. e. greater than 100 cm/s . y

A linear-velocity scale coupled with a flat no-absorption spectrum can be realized at only a slight cost in harmonic content. These requirements mean that equal lengths of time must be spent in equal velocity increments. This is equivalent to requiring motion with constant acceleration, which of course implies a motion which is parabolic in time, Figs, lc and d. The optimum choice of displacement as a function of time consists of segments of parabolas of positive and negative acceleration, joined so as to give a wave shape resembling a sinusoid, Fig. Id.

The harmonic content of this double parabolic motion is much lower than that of the symmetrical saw-tooth. Both motions contain the odd cosines cos (2n- l)ut, but with amplitude factors (2n - l)-2 for the symmetrical saw-tooth and (2n- 1)-з for the parabola. Thus the amplitude of the third term in parabolic motion is only 1/125 that of the first. From a physical point of view, the advantage of double parabolic motion is equally apparent. It has a continuous first derivative (velocity), so that it can be realized without impulsive forces.

The problem of storing the counting information in a large number of channels is solved by using a multichannel analyser. The problem of con-

4 9

verting velocity into channel number has been met in two ways. In the first, the instantaneous velocity information is obtained from a (coil and magnet) velocity transducer; the resulting voltage is used to code the velocity in-formation into the amplitude of the y-ray counts, which can then be sorted by a conventional pulse-height analyser i. The chief disadvantage here is that any non-linearity in the velocity or ADC appear as a modulation of the no-absorption spectrum, making it difficult to detect weak broad absorptions. The second scheme uses the multichannel analyser as a time analyser, i. e. it is allowed to step at a clock-controlled rate through its channels. Syn-chronism between the mechanical motion and the analyser is maintained by triggering the analyser once per cycle [ l ] , or by using a voltage from the analyser as the input to the motion generator [2]. Inhomogeneities in the motion/now appear in the velocity scale, but these are generally smaller than the resolution of the analyser which is set by the number of channels.

A block diagram of a system currently in use at the Bell Telephone Laboratories, which uses double parabolic motion, is shown in Fig. 2. The wave-form generator consists of a symmetrical saw-tooth function generator, Fig. 3, and an electromechanical feed-back system, Fig. 4.

The saw-tooth generator is entirely d. c. coupled and produces a saw-tooth whose peak amplitudes are determined by Zener diodes. The period is determined by the RC of the integrator. The bistable comparator com-pares the integrator output voltage with that of one of the Zener diodes. When the magnitudes of the two voltages become equal, the diode network is driven to the other Zener diode. This has the effect of reversing the input voltage of the integrator. The amplitude stability of the output depends on the tem-perature coefficients of the Zener diodes and of the resistors in the com-parison network. The frequency stability depends only on the temperature coefficient of the RC combination. With reasonable care, frequency stability of ±1 part in 104 is readily obtained without resorting to temperature con-trol. The electro-meahanical transducer and velocity sensor contain the magnets and voice coils of two high compliance loud-speakers. The voice coils are mechanically coupled by a low-inertia system. (Similar systems comprising one loud-speaker ahd one LVsyn (Sanborn Company), the latter used as a velocity transducer, are being used with equal success UJ). On the other hand, it is not advisable to use the two electrically independent voice coils which are available on these speakers as driver and sensor be-cause the inductive coupling falsifies the velocity signal.

The feed-back system. Fig. 4, subtracts the standard saw-tooth signal from the velocity signal, amplifies the difference in the first amplifier, in-tegrates the resulting signal and applies it to the driver voice coil through a unity voltage gain current amplifier. The integration is required to com-pensate for the differentiation produced by the electro-mechanical assembly2. Note that the system is a. c. coupled between the two amplifiers. This is necessary since the integrator would drift excessively if a small offset were

1 Many pulse-height analysers have provisions for a velocity-signal input which obviates the pulse-height coding step.

2 At frequencies well below the mechanica l resonant frequency of the speaker voice-coi l assembly the displacement is proportional to the input voltage. The output voltage, on the other hand, is proportional to the velocity of this assembly. As a result the output voltage is the derivative of the input.

5 0

WAVE FORM

GENERATOR

SYNCHRONIZING SIGNAL TIME

ANALYSER

ELECTRO-MECHANICAL TRANSDUCER AND-VELOCITY SENSOR

SOURCE'

SCINTILLATION SPECTROMETER

. ABSORBER IN DEWAR

FIG. 2. Block diagram of t ime-synchronized parabolic motion spectrometer

3 0 K * I х М Л ^

M / W ~ 1 5 v 1K I N 4 1 5 4

В С D E T E R M I N E S F R E Q U E N C Y R = H 2 0 K , C = 0 . 4 / ¿ f — » . 6 C P S

» L O W T E M P E R A T U R E C O E F F I C I E N T SYNCH. T A K E - O F F

FIG. 3. Symmetrical saw-tooth function generator. The operational amplif iers labelled P2 and P65 are made

by George A. Philbrick Researches, Dedham, Mass. , USA

a p p l i e d to i t s inpu t . The a . c . coup l ing d o e s not , h o w e v e r , c a u s e a d i s t o r t i o n of t he m o t i o n of t he m a g n i t u d e e x p e c t e d f r o m the RC p r o d u c t . (The e f f e c t i v e t i m e c o n s t a n t i s ob ta ined by m u l t i p l y i n g RC by the l o w - f r e q u e n c y loop ga in of t h e s y s t e m ) .

It shou ld be no ted tha t the s p e c t r u m obta ined in the a b s e n c e of a b s o r p t i o n w i t h any of t he v e l o c i t y - s c a n n i n g s y s t e m s i s f l a t on ly if t h e s o u r c e - d e t e c t o r d i s t a n c e r e m a i n s c o n s t a n t , i . e . p r o v i d e d the m o t i o n i s app l i ed to the a b s o r b e r r a t h e r t h a n t o t h e s o u r c e . T h e e q u a t i o n of t he n o - a b s o r p t i o n s p e c t r u m , f o r s m a l l s o l i d a n g l e wi th p a r a b o l i c m o t i o n a p p l i e d to t h e s o u r c e , i s

N ( v ) =

шах

- i т гА ( i А) J s max

51

1 / ¿ F 0.05/&F i ALF

TWO M E C H A N I C A L L Y C O U P L E D L O U D S P E A K E R

V O I C E C O I L S 2 N I 2 9 3

FIG. 4. Electromechanical feed-back system designed for operation at 6 c/s . The loud-speakers are modified

University C8HC high compliance woofers. From Wertheim, G. К. , Mössbauer Effect: Principles and Ap-plications, Academic Press, N.Y. (1964)

w h e r e A i s t h e a m p l i t u d e of t h e m o t i o n , d t h e s o u r c e - d e t e c t o r d i s t a n c e ; v t h e - v e l o c i t y and v m a x - 8Ay i t s p e a k v a l u e . The m a x i m u m d i s t o r t i o n i s p r o -p o r t i o n a l to A / d .

If t h e two h a l v e s of t he s p e c t r u m w h i c h a r e ob ta ined a r e c o m b i n e d , t h e d i s t o r t i o n i s r e d u c e d to a s e c o n d - o r d e r e f f e c t in A / d . T h e e q u a t i o n of t h e n o - a b s o r p t i o n s p e c t r u m t h e n i s

N ( v ) » 1 + f l -d ¿ V v c

max

T h i s e q u a t i o n a l s o a p p l i e s to the p u l s e - h e i g h t m o d u l a t i o n s c h e m e d e s c r i b e d a b o v e .

R E F E R E N C E S

[ 1 ] COHEN, R .L . , McMULLIN, P . G . , WERTHEIM, G . K . , Rev. scient, lnstrum. 34 (1963) 671. [ 2 ] KANKELELT, E . , Rev. scient. Instrum. 35(1964) 194.

5 2

THE NATIONAL BUREAU OF STANDARDS MÖSSBAUER SPECTROMETER

J.J. SPIJKERMAN, F.C. RUEGG AND J. R, DeVOE NATIONAL BUREAU OF STANDARDS

WASHINGTON DC, UNITED STATES OF AMERICA

A d r i f t - f r e e p r e c i s i o n D o p p l e r s p e c t r o m e t e r wi th a wide v e l o c i t y r a n g e a n d h igh r e s o l u t i o n w a s d e s i g n e d f o r M ö s s b a u e r s p e c t r o s c o p y . M ö s s b a u e r s p e c t r a c a n b e a c c u m u l a t e d by u s i n g a s p e c t r o m e t e r in two m o d e s : c o n s t a n t a c c e l e r a t i o n a n d c o n s t a n t v e l o c i t y . In t h e c o n s t a n t a c c e l e r a t i o n m o d e , a m u l t i c h a n n e l a n a l y s e r i s u s e d t o s o r t and s t o r e t h e d a t a by a s s i g n i n g a n i n c r e m e n t of v e l o c i t y t o e a c h c h a n n e l , and by hav ing tha t channe l a c c u m u l a t e c o u n t s on ly w h e n t h e s p e c t r o m e t e r i s m o v i n g a t t he c o r r e c t v e l o c i t y . T h i s v e l o c i t y i s d e t e r m i n e d by the m u l t i c h a n n e l a n a l y s e r which g e n e r a t e s the input s i g n a l f o r t h e l o u d s p e a k e r d r i v e . A s i n g l e s c a l e r i s u s e d t o s t o r e d a t a i n t h e c o n s t a n t v e l o c i t y m o d e . C o u n t s a r e a c c u m u l a t e d f o r e a c h s e t v e l o c i t y a n d t h e n t h e v e l o c i t y i s c h a n g e d , a n d t h e c y c l e i s r e p e a t e d u n t i l a n e n t i r e s p e c t r u m i s o b t a i n e d .

A M ö s s b a u e r d r i v e h a s b e e n d e v e l o p e d [1] wh ich o v e r c o m e s one of t he l a r g e s t d e f e c t s t h a t h a s b e e n e n c o u n t e r e d in e l e c t r o m e c h a n i c a l M ö s s b a u e r s p e c t r o m e t e r s . T h e d r i v e h a s b e e n d e s i g n e d s o t h a t t h e s p e a k e r c o i l and t h e m e c h a n i c a l l y c o u p l e d l i n e a r v e l o c i t y t r a n s d u c e r ( L . V . Syn) [2] c a n n o t d r i f t out of t h e l i n e a r r e g i o n of o p e r a t i o n b e c a u s e of l o n g - t e r m i n t e g r a t i o n of s e c o n d - o r d e r v e l o c i t y e r r o r s . T h i s h a s b e e n a c h i e v e d by s y n c h r o n i z i n g t h e inpu t s i g n a l t o p u l s e s f r o m t h e p o s i t i o n - s e n s i t i v e p h o t o c e l l s l i t s y s t e m . T h i s s y s t e m s t a r t s e a c h c y c l e i n t h e s a m e p o s i t i o n , a n d h e n c e , p o s i t i o n d r i f t s h a v e b e e n e l i m i n a t e d .

D I G I T A L L Y - C O N T R O L L E D C O N S T A N T A C C E L E R A T I O N

A m u l t i c h a n n e l a n a l y s e r c o u p l e d wi th a D o p p l e r s p e c t r o m e t e r p r e s e n t s t w o m e t h o d s of c o n v e n i e n t l y o b t a i n i n g M ö s s b a u e r e f f e c t d a t a in t he c o n s t a n t a c c e l e r a t i o n m o d e . T h e f i r s t i s t h e m o d u l a t i o n of t h e p u l s e - h e i g h t by t h e d r i v i n g w a v e f o r m , and w i t h t h e a n a l y s e r in t h e p u l s e - h e i g h t a n a l y s i s m o d e . T h i s m e t h o d wi l l g ive a good M ö s s b a u e r s p e c t r u m if the a n a l o g u e - t o - d i g i t a l c o n v e r t e r (ADC) i s l i n e a r , but t he c o u n t - r a t e i s r e s t r i c t e d b e c a u s e of the long d e a d - t i m e r e q u i r e d to a n a l y s e and a d d r e s s e a c h p u l s e in to the m e m o r y . T h e s e c o n d m e t h o d i s t o r u n t h e a n a l y s e r in a m u l t i s c a l e r m o d e a n d d e r i v e a c o n s t a n t i n c r e m e n t of v e l o c i t y f o r e a c h c h a n n e l . T h i s m e t h o d i s f a r b e t t e r t h a n t h e f i r s t b e c a u s e t h e d e a d - t i m e i s no l o n g e r a f a c t o r r e l a t e d t o c h a n n e l n u m b e r , a n d t h e l i n e a r i t y r e q u i r e m e n t of t h e A D C i s e l i m i n a t e d , bu t t h e a c c u r a c y of t h e s p e c t r u m p r o d u c e d by t h i s m e t h o d i s o n l y a s g o o d a s t h e s y n c h r o n i z a t i o n b e t w e e n t h e a n a l y s e r and t h e s p e c t r o m e t e r .

In the t i m e m o d e , t h e a n a l o g u e s i g n a l of t he a d d r e s s s c a l e r s h a s a s a w -t o o t h v o l t a g e d e p e n d e n c e on t i m e , wh ich i s t he r e q u i r e d input s i g n a l f o r c o n s t a n t a c c e l e r a t i o n , w i t h an e l e c t r o m e c h a n i c a l v e l o c i t y t r a n s d u c e r . S ince

53

t h e a n a l o g u e s i g n a l i s d e r i v e d f r o m t h e a d d r e s s s c a l e r s , i t i s a d i r e c t f u n c t i o n of t h e c h a n n e l n u m b e r , a n d p r o v i d e s d i r e c t s y n c h r o n i z a t i o n b e -t w e e n t h e v e l o c i t y i n c r e m e n t a n d c h a n n e l n u m b e r .

C O N S T A N T V E L O C I T Y

F o r p r e c i s e s t e p - w i s e s c a n n i n g of a M ö s s b a u e r s p e c t r u m , a c o n s t a n t -v e l o c i t y d r i v e h a s a d v a n t a g e s o v e r c o n s t a n t a c c e l e r a t i o n , s i n c e t h e r e g i o n of i n t e r e s t c a n b e e m p h a s i z e d . To o b t a i n a c o n s t a n t v e l o c i t y , t he r e q u i r e d i n p u t s i g n a l f o r t h e d r i v e i s a r e c t a n g u l a r w a v e - f u n c t i o n . T h e d r i v e u n i t c a n g e n e r a t e t h i s f u n c t i o n f r o m a p h o t o c e l l wh ich t r i g g e r s a b i s t a b l e c i r c u i t . T h i s c i r c u i t w a s d e s i g n e d s o t h a t the s y m m e t r y of t he r e c t a n g u l a r wave could b e c h a n g e d , m a k i n g i t p o s s i b l e t o o b t a i n d i f f e r e n t v e l o c i t i e s i n t h e f o r w a r d and b a c k w a r d d i r e c t i o n s , a n d t h e r e f o r e , n o n - s y m m e t r i c s p e c t r a c a n be s t u d i e d w i t h g r e a t e r e a s e .

T h e p h o t o c e l l m e t h o d of g e n e r a t i n g t h e r e c t a n g u l a r w a v e s e l i m i n a t e s a l l p o s i t i o n d r i f t s , s i n c e t h e o s c i l l a t i o n s of t h e s p e c t r o m e t e r a r e l i m i t e d a n d i n i t i a t e d b y t h e p h o t o c e l l s l i t s y s t e m .

M E C H A N I C A L D E S I G N

A d r a w i n g of t h e d r i v e i s s h o w n in F i g . 1. T h e m o t i o n i s g e n e r a t e d b y a U n i v e r s i t y C 8 H C l o u d s p e a k e r (1), t h e c o i l (7) w h i c h i s s u p p o r t e d b y t w o f l e c t u r e p l a t e s ( 2 ) . T h e f l e c t u r e p l a t e s ( F i g . 2) a r e p h o t o - e t c h e d f r o m 0 . 0 0 5 - i n . b e r y l i u m - c o p p e r a n d p r o v i d e a l i n e a r d i s p l a c e m e n t of 1 i n . A 6 L V 2 l i n e a r v e l o c i t y t r a n s d u c e r (4) i s m o u n t e d d i r e c t l y b e h i n g t h e l o u d -s p e a k e r and c a n b e a d j u s t e d f o r a l i g n m e n t by e igh t s e t s c r e w s in t h e s u p p o r t (3) . T h e 6 L V 2 c o r e i s c o n n e c t e d on one s i d e t o t h e l o u d s p e a k e r co i l by t h e a b s o r b e r o r s o u r c e h o l d e r (11) , and t h e o t h e r s i d e c o n n e c t s t o t h e s l i t s y s t e m (8) . A N o . 222 m i n i a t u r e 1 .3 -V l a m p (6) a n d a C L 6 0 3 p h o t o c e l l (5) a r e p l a c e d on o p p o s i t e s i d e s of t h e s l i t s y s t e m and a l i g n e d f o r a z e r o p o s i t i o n h a l f w a y b e t w e e n t h e t w o s l i t s . T h e d r i v e un i t i s m o u n t e d t o t w o a l u m i n i u m end p l a t e s (10) w h i c h a r e s e p a r a t e d by a h e a v y w a l l e d t u b e (9) t o a s s u r e p r o p e r a l i g n m e n t of t h e flecture p l a t e s .

E L E C T R O N I C D E S I G N

T h e s c h e m a t i c f o r t h e s p e c t r o m e t e r i s s h o w n in F i g . 3 . F o r c o n s t a n t a c c e l e r a t i o n , t h e a n a l o g u e s i g n a l of t h e a d d r e s s s c a l e r s i s f e d t o a n o p e r -a t i o n a l a m p l i f i e r t o i s o l a t e t h e d r i v e e l e c t r o n i c s f r o m t h e m u l t i c h a n n e l a n a l y s e r , and to p r o v i d e a b i a s v o l t a g e t o ob t a in t h e r e q u i r e d v e l o c i t y r a n g e . F o r c o n s t a n t v e l o c i t y t h e d r i v e i npu t s i g n a l i s g e n e r a t e d b y t h e a m p l i f i e r d r i v e r w h i c h i s t r i g g e r e d b y t h e p h o t o c e l l p u l s e g e n e r a t o r . T h e s e l e c t o r s w i t c h , SW-3, c o n n e c t s e i t h e r input t o t he d r i v e a m p l i f i e r with the a t t e n u a t o r Rg and RJO, to c o n t r o l t he v e l o c i t y r a n g e .

54

FIG. 1. Mechanical construction of the drive

V E L O C I T Y AND L I N E A R I T Y CALIBRATION

T o d e t e r m i n e t h e p r e c i s i o n of t h e v e l o c i t y of t h e s p e c t r o m e t e r , a v o l t a g e - t o - f r e q u e n c y c o n v e r t e r and a m u l t i c h a n n e l a n a l y s e r a r e u s e d . T h e ou tpu t of t h e m o n i t o r i n g L . V. Syn co i l i s i m p r e s s e d on t h e input of a 100 kHz v o l t a g e - t o - f r e q u e n c y c o n v e r t e r w h i c h h a s a l i n e a r i t y of 0 .03%, a n d w h o s e o u t p u t p u l s e s a r e c o u n t e d b y t h e m u l t i s c a l e r s of t h e a n a l y s e r .

In t h e c o n s t a n t a c c e l e r a t i o n m o d e , t h e a n a l y s e r i s d r i v i n g t h e s p e c t r o -m e t e r whi le i t i s a c c u m u l a t i n g the coun ts f r o m the v o l t a g e - t o - f r e q u e n c y c o n -

55

FIG. 2. Flecture plate

v e r t e r . T h i s m e t h o d s h o u l d p r o d u c e a V - s h a p e d s p e c t r u m in t h e c o n s t a n t a c c e l e r a t i o n m o d e a n d o u r i n s t r u m e n t p r o d u c e d t h i s s p e c t r u m w i t h a c a l -c u l a t e d e r r o r of ±0.1% of t h e s l o p e wi th in a p p r o x . 0 . 0 0 1 m m / s and 20 c m / s . I m p r o v e m e n t s i n t h e e l e c t r o m e c h a n i c a l s y s t e m w o u l d e x t e n d t h e s e l i m i t s .

In t h e c o n s t a n t v e l o c i t y m o d e , t h e a n a l y s e r i s s y n c h r o n i z e d t o c o u n t on ly when the d r i v e i s m o v i n g in a g iven d i r e c t i o n . T h i s m e t h o d should p r o -d u c e a s t r a i g h t l i n e , a n d o u r i n s t r u m e n t d o e s t h i s w i t h a n e r r o r of ±0 .1% w i t h i n v e l o c i t y l i m i t s s i m i l a r t o t h a t of c o n s t a n t a c c e l e r a t i o n .

T h i s m e t h o d i s a f a s t w a y t o t e s t , w i t h n u m e r i c a l o u t p u t , t h e l i n e a r i t y of t h e e n t i r e s y s t e m , b e c a u s e t he c o m p o n e n t s a r e b e i n g u s e d a s if a n a c t u a l M ö s s b a u e r s p e c t r u m w e r e b e i n g a c c u m u l a t e d w h e r e t h e c o u n t i npu t i s d e -r i v e d f r o m a n d i s a f u n c t i o n of t h e v e l o c i t y .

A n a b s o l u t e m e a s u r e m e n t of t he v e l o c i t y c a n be o b t a i n e d by i n c o r p o r a t i n g a M i c h e l s o n i n t e r f e r o m e t e r w i t h t h e d r i v e . If o n e r e f l e c t o r of t h e i n t e r -f e r o m e t e r i s m o u n t e d on t h e m o v i n g p a r t of t h e s p e c t r o m e t e r , and t h e s e -cond r e f l e c t o r i s s t a t i o n a r y , t h e l igh t i n t e n s i t y w i l l b e a f u n c t i o n of t h e v e l o c i t y . T h e c u r r e n t p r o d u c e d by t h e p h o t o m u l t i p l i e r

I = I D + I 0 c o s ^ 27rt

w h e r e Ip i s t he d a r k c u r r e n t , and I 0 t h e m a x i m u m photo c u r r e n t . The m o d u -l a t i o n f r e q u e n c y , f , i s d i r e c t l y p r o p o r t i o n a l t o t h e v e l o c i t y , o r v = X f / 2 .

5 6

FIG. 3. Block diagram of dr i f t - f ree Mössbauer spectrometer

SLIT

FREQUENCY f- - у

FIG. 4. Optical interferometer for the measurement of absolute velocity

T h e s e f r e q u e n c y p u l s e s a r e c o u n t e d b y t h e m u l t i s c a l e r s , a<nd p r o v i d e a n a c c u r a t e m e a s u r e m e n t of t h e v e l o c i t y p r o d u c e d by t h e s p e c t r o m e t e r a n d a p r e c i s e m e a s u r e m e n t of i t s l i n e a r i t y . T h e a l i g n m e n t of t h e i n t e r f e r o m e t e r c a n b e s i m p l i f i e d c o n s i d e r a b l y by u s i n g c o r n e r cube r e f l e c t o r s ( s e e F i g . 4 ) .

57

R E F E R E N C E S [1] RUEGG, F. С . , SPIJKERMAN, J . J . , DeVOE, J. R., Rev. scient. Instrum. 36 (1965) 356. [2] COHEN. R.L. , McMULLIN, P. G. , WERTHEIM, G.K. , Rev. scient. Instrum. 34 (1963) 671.

. D I S C U S S I O N

(on t h e f o r e g o i n g t w o p a p e r s )

P . H I L L M A N s t a t e d t h a t , f o r o b s e r v i n g s m a l l e f f e c t s , t h e t i m e m o d e a p p e a r e d t o b e b e s t w h e r e a s f o r d e t e r m i n i n g a c c u r a t e v e l o c i t i e s t h e p u l s e -h e i g h t m o d e a p p e a r e d t o b e p r e f e r a b l e .

G . K . W E R T H E I M , P . K I E N L E and J . S P I J K E R M A N a l l p o i n t e d ou t t h a t t h e y u s e d t h e t i m e m o d e w i t h v e r y good s u c c e s s f o r a l l p u r p o s e s .

P . H I L L M A N b r i e f l y d e s c r i b e d h i s t e c h n i q u e f o r i n v e s t i g a t i n g s m a l l m o t i o n s i n t h e e a r . T h e a b s o r b e r w a s d r i v e n s i n u s o i d a l l y ; t h e s i n e w a v e w a s d i v i d e d i n t o s i x p a r t s . T h e c o u n t s i n t h e s e s i x p a r t s w e r e f a n n e d o u t i n t o s i x s c a l e r s .

MÖSSBAUER SCATTERING*

P. DEBRUNNER AND H. FRAUENFELDER UNIVERSITY OF ILLINOIS

• URBANA, ILL., UNITED STATES OF AMERICA

A . I N T R O D U C T I O N

M o s t e x p e r i m e n t s i n v o l v i n g t h e M ö s s b a u e r e f f e c t h a v e s o f a r b e e n d o n e in a t r a n s m i s s i o n g e o m e t r y . T h e b e a u t i f u l s i m p l i c i t y of s u c h an a r r a n g e -m e n t , l a r g e e f f e c t , e a s y e v a l u a t i o n and l a r g e c o u n t i n g r a t e s a r e s o m e r e a -s o n s f o r p r e f e r r i n g t r a n s m i s s i o n e x p e r i m e n t s . S c a t t e r i n g e x p e r i m e n t s h a v e b e e n u s e d f o r s p e c i a l p u r p o s e s , s u c h a s t h e s t u d y of t h e r e c o i l l e s s f r a c t i o n of t h e s c a t t e r e d r a d i a t i o n Ц , 2] , t h e o b s e r v a t i o n of R a y l e i g h s c a t t e r i n g [3], o r t h e i n v e s t i g a t i o n of t h e i n t e r f e r e n c e b e t w e e n M ö s s b a u e r a n d R a y l e i g h s c a t t e r i n g [ 4 - 8 ] . In a d d i t i o n , s e c o n d a r y c o n v e r s i o n e l e c t r o n [9] and X - r a y s [10] h a v e b e e n d e t e c t e d .

S c a t t e r i n g e x p e r i m e n t s c a n , h o w e v e r , a l s o b e u s e d f o r s y s t e m a t i c s t u d i e s of " h i g h - e n e r g y M ö s s b a u e r " t r a n s i t i o n s . T h e f r a c t i o n of 7 - r a y s e m i t t e d w i t h o u t r e c o i l e n e r g y l o s s d e c r e a s e s r a p i d l y w i t h i n c r e a s i n g t r a n s -i t i o n e n e r g y . T h e e f f e c t b e c o m e s s o o n s o s m a l l t h a t i t s d e t e c t i o n in t r a n s -m i s s i o n b e c o m e s v e r y d i f f i c u l t . At s u c h e n e r g i e s , s c a t t e r i n g e x p e r i m e n t s

* Supported by the US Office of Naval Research under contract 1834(05).

58

b e c o m e a v e r y u s e f u l t o o l . In t h e p r e s e n t r e p o r t w e d i s c u s s t h e b a s i c a s -p e c t s of s c a t t e r i n g e x p e r i m e n t s a n d i n d i c a t e h o w t h e y c a n b e u s e d t o s t u d y " h i g h - e n e r g y M ö s s b a u e r " t r a n s i t i o n s e f f i c i e n t l y . At t h e p r e s e n t t i m e , t h e h i g h e s t y - r a y e n e r g y w i t h w h i c h t h e M ö s s b a u e r e f f e c t h a s b e e n s e e n i s 155 k e V [11] . We h o p e , h o w e v e r , t h a t t h e l i m i t c a n b e p u s h e d h i g h e r .

B . L I M I T A T I O N S O F T H E S C A T T E R I N G M E T H O D

W e f i r s t d i s c u s s t h e c o n d i t i o n s u n d e r w h i c h a s c a t t e r i n g e x p e r i m e n t i s f e a s i b l e . C o n s i d e r a n a r r a n g e m e n t a s s h o w n in F i g . 1.

TRANSMISSION

SOURCE MOVING AT [ t ó -VELOCITY V"" ' •fl "0

RESONANT ABSORBER

SCATTERING

SOURCE MOVING AT f ^ . VELOCITY V

" E F F E C T " - ^ / B A C K G R O U N D S (RECOILLESS-

(RECOILLESS PART PLUS NON-RESONANTLY REC01LLESS ABSORBED) PART)

DETECTOR

" E F F E C T ' ^ "BACKGROUND" (RESONANTLY (DUE TO NON-SCATTERED RËSONANT INTENSITY) PROCESSES)

SOURCE VELOCITY V SOURCE VELOCITY V

' FIG. 1. Comparison of transmission and scattering geometry

T o f i n d t h e m a x i m u m e n e r g y a t w h i c h M ö s s b a u e r e f f e c t c a n s t i l l b e o b -s e r v e d , w e c o n s i d e r t h e c a s e w h e r e s o u r c e a n d s c a t t e r e r a r e a t l o w t e m -p e r a t u r e s . T h e D e b y e - W a l l e r f a c t o r i s t h e n g i v e n b y

f ( T = 0) = e x p [ - 3 E 2 / 4 M c 2 k 6 ] (1)

w h e r e f i s t h e f r a c t i o n of 7 - r a y s e m i t t e d w i t h o u t r e c o i l e n e r g y l o s s , E i s t h e t r a n s i t i o n e n e r g y , M i s t h e m a s s of t h e d e c a y i n g a t o m , and 0 i s t h e Debye t e m p e r a t u r e . F o r t h e s a k e of s i m p l i c i t y w e c o n s i d e r t h e c a s e of an i s o t o p e w i t h a s i n g l e 7 - t r a n s i t i o n a n d an u n s p l i t M ö s s b a u e r s p e c t r u m . P o s t p o n i n g t h e d i s c u s s i o n of t h e p e a k - t o - b a c k g r o u n d r a t i o w e a s s u m e f o r t h e m o m e n t t h a t t h e M ö s s b a u e r e f f e c t i s o b s e r v a b l e w h e n e v e r t h e r e c o i l l é s s f r a c t i o n i s l a r g e r t h a n a c r i t i c a l v a l u e f c r i t . M ö s s b a u e r s c a t t e r i n g h a s b e e n o b s e r v e d w i t h r e c o i l l e s s f r a c t i o n s a s s m a l l a s 5 X 1 0 - 3 [ 12 ] a n d i t s e e m s f e a s i b l e t o l o w e r t h e l i m i t e v e n f u r t h e r .

F o r a g i v e n f c r i t , a c r i t i c a l y - r a y e n e r g y E c r i t c a n b e c a l c u l a t e d i m -m e d i a t e l y f r o m E q . (1 ) :

E c r i t = [ | м с 2 к 0 1 п ( 1 / i « * ) ] 1 ( 2 )

59

F o r t h e v a l u e f c r i t = 0 . 005 g i v e n a b o v e , w e g e t f o r E

E c r i t ( f = 0 . 0 0 5 ) = 0 . 7 6 [ A 0 ] i (E in k e V , в i n °K) (3)

A p l o t of t h e c r i t i c a l e n e r g y E c r i t v e r s u s m a s s n u m b e r A f o r v a r i o u s v a l u e s of в i s s h o w n i n F i g . 2 . L i k e l y c a n d i d a t e s f o r M ö s s b a u e r s c a t t e r i n g e x -p e r i m e n t s a r e g i v e n i n T a b l e I .

FIG. 2. The critical energy ECrit defined in Eq. (2) as a function of mass number A for f c r ¡ t - 0. 005 and Debye

temperatures в between 100'K and 400"K. The curves А, В and С represent ratios M/R (Eq. (4)) of 1, 0.1 and 0.01, respectively

C . C O M P A R I S O N O F T R A N S M I S S I O N A N D S C A T T E R I N G

In e v a l u a t i n g t h e r e l a t i v e m e r i t s of a t r a n s m i s s i o n a n d of a s c a t t e r i n g g e o m e t r y t h e r a t i o of e f f e c t t o b a c k g r o u n d m u s t b e c o n s i d e r e d . As i n d i c a t e d in F i g . 1 t h e e f f e c t i s h e r e d e f i n e d a s t h e d i f f e r e n c e in c o u n t i n g r a t e on and off r e s o n a n c e ; t h e b a c k g r o u n d i s d e f i n e d a s t h e c o u n t i n g r a t e off r e s o n a n c e . In a w e l l - d e s i g n e d s c a t t e r i n g e x p e r i m e n t t h e r a t i o of e f f e c t t o b a c k g r o u n d c a n be m u c h l a r g e r t h a n in t r a n s m i s s i o n , and, a s a c o n s e q u e n c e , t h e n u m b e r of c o u n t s r e q u i r e d f o r a g i v e n a c c u r a c y i s m u c h s m a l l e r t h a n in t r a n s m i s s i o n .

T o d i s c u s s t h e i n h e r e n t l i m i t a t i o n s of t h e s c a t t e r i n g a n d of t h e t r a n s -m i s s i o n m e t h o d w e c o n s i d e r a h y p o t h e t i c a l e x p e r i m e n t w i t h n o i n t e r f e r i n g b a c k g r o u n d . We c a n t h e n d e r i v e an u p p e r l i m i t f o r t h e e f f e c t - t o - b a c k g r o u n d r a t i o f o r t h e two g e o m e t r i e s .

In an i d e a l i z e d t r a n s m i s s i o n e x p e r i m e n t , a t r e s o n a n c e , a l l t h e r e c o i l -l e s s 7 - r a y s of t h e i n c i d e n t b e a m , i . e . a f r a c t i o n f , w i l l b e a b s o r b e d , a n d

6 0

TABLE I

M Ö S S B A U E R N U C L E I S U I T A B L E F O R S C A T T E R I N G E X P E R I M E N T S

Isotope E

(keV) T i I (ns)

Abundance

(1°) Parent T i 2

Typica l 0 D

(°K) f (T = 0)

5 'Fe 136 8 . 8 2 .17 " C o 270 d 400 (Fe) 5 . 4 X 1 0 " 4

101 Ru 127 1 . 4 16 .98 101Rh 5 yr 400 (Ru) 2. 5 X 1 0 " 2

123 T e 159 0 . 1 9 0 . 8 9 123j 13 h 280 (NajHjIOj) 1 . 1 X 1 0 " 3

139 La 163 1 . 7 9 9 . 9 1 1 3 9Ce 140 d 270 (Ce0 2 ) 1. 5 X 1 0 " 3

Lu 114 0 . 0 8 97. 5 175Hf 70 d 240 (Hf) 5 . 7 x 1 0 - 2

ISO Os 187 0 . 3 5 2 6 . 4 190t 12 d 315 (Ir) 4. 8 X 1 0 " 3

Os 206 0 . 2 8 41. 0 19Zb 74 d 315 (Ir) 1. 6X10~3

only a f r a c t i o n 1 - f wi l l b e t r a n s m i t t e d . A c c o r d i n g to o u r de f in i t ion t he e f f e c t -t o - b a c k g r o u n d r a t i o i s t h e n g i v e n b y t h e s o u r c e r e c o i l l e s s f r a c t i o n f . T h i s i d e a l s i t u a t i o n c a n a t b e s t b e a p p r o x i m a t e d b y u s i n g a " b l a c k " a b s o r b e r and a d e t e c t o r of s u f f i c i e n t l y h i g h r e s o l u t i o n t o e l i m i n a t e any b a c k g r o u n d c o n t r i -b u t i o n d u e t o o t h e r y - t r a n s i t i o n s , b u t in a l l p r a c t i c a l c a s e s t h e s i g n a l - t o -n o i s e r a t i o f o r t r a n s m i s s i o n i s s m a l l e r t h a n f .

In an i d e a l i z e d s c a t t e r i n g e x p e r i m e n t t h e d e t e c t o r d o e s no t s e e any d i r e c t y - r a y s , b u t o n l y t h e r a d i a t i o n o r i g i n a t i n g f r o m t h e s c a t t e r e r . T h e s c a t t e r e d r a d i a t i o n c o n s i s t s of M ö s s b a u e r , R a y l e i g h and Compton s c a t t e r i n g . T h e C o m p t o n s c a t t e r e d y - r a y s h a v e an e n e r g y w h i c h , f o r b a c k s c a t t e r i n g , i s c o n s i d e r a b l y l o w e r t h a n t h e i n c i d e n t e n e r g y , a n d t h e y c a n t h e r e f o r e b e r e j e c t e d by p u l s e - h e i g h t s e l e c t i o n . On t h e o t h e r h a n d , b o t h t h e M ö s s b a u e r . and t h e R a y l e i g h s c a t t e r e d y - r a y s w i l l b e a c c e p t e d s i n c e t h e y h a v e t h e f u l l e n e r g y . T h e e f f e c t - t o - b a c k g r o u n d r a t i o in t h i s t h e o r e t i c a l l i m i t i s g i v e n b y t h e i n t e n s i t y r a t i o of t h e M ö s s b a u e r t o t h e R a y l e i g h s c a t t e r i n g . F o r a t h i n s c a t t e r e r t h e r a t i o i s [13, 14]

M / R = 4 . 25 X1010 P ( 0 , j J f ^ (4.)

H e r e P ( 0 ) i s a f a c t o r of o r d e r u n i t y d e p e n d i n g on t h e s p i n s of t h e n u c l e a r l e v e l s i n v o l v e d a n d on t h e s c a t t e r i n g a n g l e 0, a i s t h e a b u n d a n c e of t h e M ö s s b a u e r i s o t o p e , f and f ' a r e t h e r e c o i l l e s s f r a c t i o n s of s o u r c e and s c a t -t e r e r r e s p e c t i v e l y , a i s t h e t o t a l c o n v e r s i o n c o e f f i c i e n t , E / m g c ' ^ i s t h e y -r a y e n e r g y in u n i t s of t h e e l e c t r o n r e s t m a s s , and Z i s t h e a t o m i c n u m b e r . T o s e e t h e l i m i t a t i o n s i m p o s e d on t h e s c a t t e r i n g m e t h o d by t h e c o m p e t i t i o n

61

f r o m R a y l e i g h s c a t t e r i n g , w e h a v e c a l c u l a t e d t h r e e c u r v e s , M / R = 1, 0 . 1 and 0 . 0 1 r e s p e c t i v e l y . T h e s e c u r v e s a r e s h o w n i n F i g . 2; t h e y a r e v a l i d f o r Б 2 t r a n s i t i o n s , ff« =f c

2r i t = 2 . 5 X 1 0 " 5 a n d Р ( в ) = 1.

In p r a c t i c e t h e e f f e c t - t o - b a c k g r o u n d r a t i o in s c a t t e r i n g i s a l w a y s s m a l l e r t h a n t h e t h e o r e t i c a l l i m i t , s i n c e o t h e r p r o c e s s e s c o n t r i b u t e t o t h e b a c k -g r o u n d l e v e l a t t h e e n e r g y of i n t e r e s t . M o s t s e r i o u s a r e h i g h e r - e n e r g y 7 -r a y s w h i c h m a y g i v e r i s e t o a f i n i t e C o m p t o n b a c k g r o u n d a t t h e e n e r g y of t h e M ö s s b a u e r l i n e . S i n c e t h e b a c k g r o u n d i s u s u a l l y c o n t i n u o u s r a t h e r t h a n d i s c r e t e , a h i g h r e s o l u t i o n c o u n t e r w i l l i m p r o v e t h e s i g n a l - t o - n o i s e r a t i o . F o r s o u r c e s w i t h n o h i g h e r e n e r g y 7 - t r a n s i t i o n s , t h e s i g n a l - t o - n o i s e r a t i o h a s b e e n f o u n d t o b e u p t o 10 t i m e s l a r g e r t h a n t h e r e c o i l l e s s f r a c t i o n [12] . I n s u c h a c a s e , s c a t t e r i n g i s o b v i o u s l y f a r s u p e r i o r c o m p a r e d w i t h t r a n s -m i s s i o n , p r o v i d e d t h a t s t r o n g e n o u g h s o u r c e s a r e a v a i l a b l e .

D . E X P E R I M E N T A L A R R A N G E M E N T

We d i s c u s s an e x p e r i m e n t e d a r r a n g e m e n t t h a t w a s s p e c i f i c a l l y b u i l t f o r t h e o b s e r v a t i o n of M ö s s b a u e r t r a n s i t i o n s w i t h s m a l l r e c o i l l e s s f r a c t i o n [11]. T h e s y s t e m o p e r a t e s a t a t e m p e r a t u r e of 10 t o 3 0 ° K a n d i s c h a r a c t e r i z e d b y a r e l a t i v e l y l a r g e p r o d u c t s o l i d a n g l e of u p t o 1. 5 X 1 0 " 3 and b y an e f f e c t -t o - b a c k g r o u n d r a t i o t h a t c a n b e u p t o 10 t i m e s l a r g e r t h a n t h e r e c o i l l e s s f r a c t i o n .

T h e m o s t c r i t i c a l p a r t of t h e w h o l e s y s t e m i s t h e s o u r c e - s c a t t e r e r a s -s e m b l y s h o w n i n F i g . 3 . A n a x i a l l y s y m m e t r i c a r r a n g e m e n t i s u s e d w i t h t h e s o u r c e r e s t i n g on a t u n g s t e n b l o c k f a c i n g t h e c o n e - s h a p e d s c a t t e r e r . T h e s c a t t e r i n g m a t e r i a l i s g l u e d t o a 6 - m i l a l u m i n i u m c o n e . T h e d e t e c t o r s i t t i n g b e l o w t h e t u n g s t e n b l o c k i s s h i e l d e d f r o m t h e d i r e c t s o u r c e r a d i a t i o n . T h e s h i e l d i n g r e q u i r e m e n t s d e p e n d on t h e 7 - r a y e n e r g i e s of t h e s o u r c e .

T h e m o s t a b u n d a n t s o u r c e of b a c k g r o u n d i s C o m p t o n s c a t t e r i n g . S i n c e t h e s c a t t e r i n g a n g l e i s l a r g e , t h e C o m p t o n p e a k due t o t h e M ö s s b a u e r t r a n s -i t i o n i s s h i f t e d t o e n e r g i e s b e l o w t h e 7 - r a y w i n d o w . T h e w a l l s of t h e s c a t -t e r i n g c h a m b e r a r e l i n e d w i t h l e a d t o m i n i m i z e t h e i r c o n t r i b u t i o n t o t h e C o m p t o n b a c k g r o u n d . F o r h i g h Z e l e m e n t s t h e p h o t o e l e c t r i c c r o s s - s e c t i o n i s m u c h l a r g e r t h a n t h e C o m p t o n c r o s s - s e c t i o n a n d t h e r e f o r e m o s t of t h e 7 - r a y s h i t t i n g t h e w a l l s g i v e r i s e t o X - r a y s o n l y .

If a / З - e m i t t e r i s u s e d a s t h e s o u r c e t h e j 3 - p a r t i c l e s m u s t no t b e a l l o w e d t o h i t t h e s c a t t e r e r s i n c e o t h e r w i s e t h e y w i l l g i v e r i s e t o a c o n t i n u u m of e x -t e r n a l B r e m s s t r a h l u n g .

A s m e n t i o n e d b e f o r e , t h e r a t i o of e f f e c t t o b a c k g r o u n d i m p r o v e s w i t h i m p r o v i n g e n e r g y r e s o l u t i o n of t h e d e t e c t o r . I f , h o w e v e r , a h i g h e f f i c i e n c y a n d l a r g e d e t e c t o r a r e a a r e r e q u i r e d a s i s t h e c a s e h e r e , a s o d i u m i o d i d e s c i n t i l l a t i o n c r y s t a l m a y b e p r e f e r a b l e .

I n F i g . 4 t h e c o m p l e t e s y s t e m i s s h o w n , c o n s i s t i n g of a n a l l - m e t a l h e l i u m c r y o s t a t , t h e s o u r c e s c a t t e r e r a s s e m b l y , t h e d e t e c t o r and t h e m a g -n e t i c d r i v e . The - s c a t t e r e r i s t h e r m a l l y c o n n e c t e d t h r o u g h a n a l u m i n i u m r o d and a s o f t c o p p e r s p r i n g t o t h e h e l i u m c o n t a i n e r . It i s c o u p l e d by a l o n g r o d t o t h e m a g n e t d r i v e o n t o p of t h e s y s t e m . T h e v o l t a g e of t h e p i c k - u p c o i l i s u s e d t o m o d u l a t e t h e p u l s e s f r o m a s i n g l e - c h a n n e l a n a l y s e r a n d t h e r e s u l t i n g s p e c t r u m i s s t o r e d in a m u l t i c h a n n e l a n a l y s e r .

62

Aluminium Rod

Ci CO FIG. 3. The axially symmetric scattering geometry

0 2 4

FIG. 4. Experimental arrangement for low temperature back-scattering

E . D A T A A N A L Y S I S

1. Total resonance scattering

T h e a n a l y s i s of t h e d a t a o b t a i n e d i n s c a t t e r i n g i s v e r y s i m i l a r t o t h e a n a l y s i s of t r a n s m i s s i o n d a t a . In s c a t t e r i n g , h o w e v e r , i t i s g e n e r a l l y n e -c e s s a r y t o u s e a v e r y l a r g e s o l i d a n g l e and , a s a c o n s e q u e n c e , t h e r a w d a t a h a v e to b e c o r r e c t e d f o r t h e r e s u l t i n g v e l o c i t y s p r e a d . F o r a 7 - r a y e m i t t e d a t an a n g l e ф w i t h r e s p e c t t o t h e d i r e c t i o n of m o t i o n of t h e s c a t t e r e r t h e e f -f e c t i v e v e l o c i t y i s v c o s ^ / , w h e r e v i s t h e v e l o c i t y of t h e s c a t t e r e r . If t h e s o l i d a n g l e s u b t e n d e d b y t h e s c a t t e r e r i s l a r g e , t h e r e s u l t i n g v e l o c i t y s p r e a d

• m a y b r o a d e n t h e l i n e c o n s i d e r a b l y . F o r t h e a x i a l l y s y m m e t r i c a r r a n g e m e n t d e s c r i b e d a b o v e a n d f o r L o r e n t z i a n e m i s s i o n a n d a b s o r p t i o n l i n e s of e q u a l w i d t h Г , t h e o b s e r v e d l i n e - s h a p e c a n b e v e r y w e l l a p p r o x i m a t e d b y t h e e x p r e s s i o n

î = cos i>l

A =cosd y(v ) = L

1 + ( V C 0 S & - yres Ч Г/2

2 1

3 = 0050!

/ d c o s ^

J v ( B - А ) ( Г / 2 ) = t tv (B - A) t g _ 1 ( Г / 2 ) 2 + ( B v - v r e s ) (Av - v r e s ) ( 5 )

H e r e A a n d В a r e t h e c o s i n e s of t h e l a r g e s t a n d s m a l l e s t p o l a r a n g l e s ф d e f i n e d in F i g . 3, a n d v r e s i s t h e D o p p l e r v e l o c i t y a t r e s o n a n c e . F i t t i n g t h e e x p e r i m e n t a l c u r v e s w i t h t h i s f u n c t i o n y i e l d s t h e w i d t h Г, t h e r e s o n a n c e v e l o c i t y v r e s a n d t h e i n t e n s i t y of t h e l i n e i n a s t r a i g h t f o r w a r d w a y .

R e s o n a n c e s c a t t e r i n g i s n o t i s o t r o p i c in g e n e r a l . T h e n u c l e a r r e s o n a n c e s c a t t e r i n g p r o c e s s i s f o r m a l l y e q u i v a l e n t t o t h e e m i s s i o n of t w o s u c c e s s i v e Y - r a y s . O n e c a n t h e r e f o r e a p p l y t h e f o r m a l i s m d e s c r i b i n g t h e a n g u l a r c o r -r e l a t i o n of s u c c e s s i v e 7 - r a y s . A m e a s u r e m e n t of t h e s i ngu l a r d i s t r i b u t i o n of t h e s c a t t e r e d r a d i a t i o n y i e l d s i n f o r m a t i o n a b o u t t h e a n g u l a r m o m e n t a of t h e n u c l e a r l e v e l s a n d t h e r a d i a t i o n i n v o l v e d a n d , i n t h e p r e s e n c e of i n t e r n a l o r e x t e r n a l f i e l d s , a l s o a b o u t t h e i n t e r a c t i o n of t h e n u c l e u s w i t h t h e s e f i e l d s [15] . If s o u r c e o r s c a t t e r e r s p e c t r a o r b o t h a r e s p l i t , t h e a n g u l a r d i s t r i b u -t i o n i s q u i t e c o m p l i c a t e d .

F r o m t h e i n t e n s i t y of t h e r e s o n a n c e l i n e c o r r e c t e d f o r f i n i t e s o l i d a n g l e a n d a n g u l a r c o r r e l a t i o n e f f e c t s o n e c a n d e r i v e t h e r e c o i l l e s s f r a c t i o n s f o r s o u r c e a n d a b s o r b e r . T h e r e s o n a n t l y s c a t t e r e d r a d i a t i o n c o n s i s t s of a r e c o i l l e s s p a r t a n d a n o n - r e c o i l l e s s p a r t N j " i r . F o r L o r e n t z i a n s i n g l e -l i n e s o u r c e a n d s c a t t e r e r , a t r e s o n a n c e , t h e i n t e n s i t i e s of t h e t w o p a r t s [ 1 3 ] a r e g i v e n b y :

r d u d u , w ( 0 ) F Í X L A ) R 1 +a J (1 + s i n y 1 c o s e c 7 2 )

6 4

N ' fNR (7)

F o r s m a l l r e c o i l l e s s f r a c t i o n s t h e t o t a l s c a t t e r e d i n t e n s i t y i s g i v e n by

N 1 = N 1 + N1 s т о т R NR r ñ J d " d u ' W ( 0 ) F ( X 2

R , A ) (8)

H e r e Nn i s t h e s o u r c e i n t e n s i t y f o r t h e t r a n s i t i o n of i n t e r e s t ; f a n d f 1 a r e . the s o u r c e a n d a b s o r b e r r e c o i l l e s s f r a c t i o n s r e s p e c t i v e l y ; a i s t h e t o t a l c o n v e r s i o n c o e f f i c i e n t . W(0) i s t h e a n g u l a r d i s t r i b u t i o n f u n c t i o n a n d F i s a s c a t t e r i n g f u n c t i o n d e f i n e d in F i g . 5. T h e a n g l e s Y p y 2 , 0 a n d t h e i n c r e -m e n t s of f r a c t i o n a l s o l i d a n g l e du and du1 a r e i n d i c a t e d in F i g . 5. F o r e q u a l l i n e - w i d t h Г a n d Г1 f o r s o u r c e and a b s o r b e r , t h e s c a t t e r i n g f u n c t i o n F d e p e n d s on t h e p a r a m e t e r s A and X2 d e f i n e d in F i g . 5. In F i g s . 6 and 7 t h e s c a t t e r i n g f u n c t i o n a t r e s o n a n c e i s p l o t t e d f o r s e v e r a l v a l u e s of t h e p a r a m e t e r s A and X2 .

SCATTERING F U N C T I O N F I X , A ) AT RESONANCE FOR L O R E N T Z I A N SOURCE AND S C A T T E R E R

OF EQUAL L I N E W I D T H x2 + l

J ( l * v 2 ) U a (l + y ) ( X +y )

NON-RECOILLESS PART F f X ^ A )

RECOILLESS PART A)

> DETECTOR

A = / i e t (cosec cosec y2 )

,Лъ \ г A XZR = /¿et(cosec /¡ «-cosec

+ / i n t cosec yt

г A X N R = ( / i e t / i n ) t ( c o s e c x | + c o s e c y 2 )

f ' R E C O I L L E S S FRACTION OF S C A T T E R E R

n NUMBER OF M Ö S S B A U E R N U C L E l / u N I T VOLUME

cr M A X I M U M RESONANCE C R O S S - S E C T I O N

FIG. 5. D e f i n i t i o n of t h e s c a t t e r i n g f u n c t i o n F ( \ 2 , A) and o f t h e p a r a m e t e r s a n d A in t e r m s o f t h e s c a t t e r i n g

g e o m e t r y a n d of t h e l i n e a r e l e c t r o n i c and n u c l e a r a b s o r p t i o n c o e f f i c i e n t s f i e a n d p n , r e s p e c t i v e l y

If t h e s c a t t e r e r i s t h i n t h e s c a t t e r e d i n t e n s i t i e s [13] a r e :

1 + « Г ' + Г dudu)1 ß n t с о в е е т ^ © ) O )

6 5

x2

FIG. 6. T h e scat ter ing funct ion F(X2, A) def ined in Fig. 5 as a funct ion of X2 for var ious va lues of A

NNR = N ° l + " gf , ) Р Т Г / d w d u ' M n t c o s e c T i W ( e ) (10)

Mn = f ' n a 0 (11)

H e r e n i s t h e n u m b e r of M ö s s b a u e r a t o m s p e r uni t v o l u m e and a0 i s t he m a x i -m u m r e s o n a n c e c r o s s - s e c t i o n .

T h e l i n e - s h a p e o b s e r v e d f o r a s i n g l e l i n e s o u r c e and a s i n g l e l i n e s c a t -t e r e r i s a l m o s t L o r e n t z i a n , a p a r t f r o m t h e v e l o c i t y s p r e a d due t o f i n i t e so l id a n g l e . A s i n t h e c a s e of t r a n s m i s s i o n , f o r a t h i n s c a t t e r e r t h e l i n e - w i d t h i s t h e s u m of t h e s o u r c e and s c a t t e r e r width Г + Г 1 , w h e r e a s f o r a t h i ck s c a t -t e r e r t h e l i n e - w i d t h i s ХГ1 + Г.

2. Analysis of resonantly scattered radiation [13]

A n i n t e r e s t i n g a p p l i c a t i o n of t h e s c a t t e r i n g t e c h n i q u e o b t a i n s f o r t r a n s i t i o n s wi th l a r g e r e c o i l l e s s f r a c t i o n s . If t h e r e s o n a n t l y s c a t t e r e d r a d i a -t i o n i s a n a l y s e d b y a r e s o n a n t a b s o r b e r t h e n u m b e r s NJ a n d N J ^ of r e c o i l -l e s s l y and n o n - r e c o i l l e s s l y s c a t t e r e d т - r a y s c a n b e d e t e r m i n e d . A t y p i c a l a r r a n g e m e n t f o r s u c h a r h e a s u r e m e n t and t h e d a t a ob t a ined wi th i t a r e shown

6 6

FIG. 7. T h e sca t te r ing funct ion F(X2, A) def ined in Fig. 5 as a funct ion of X2

A . —7777-ГГТТ7-ГГ7Т7Т7Л b>

SCATTERER

, < • ABSORBER, VELOCITY V

B=COUNT-RATE W I T H SOURCE AND SCATTERER OFF RESONANCE

TRANSMISSION THROUGH MOVING ABSORBER WITH SOURCE AND SCATTERER ON RESONANCE

ABSORBER V E L O C I T Y v

FIG. 8. Analysis of resonantly scat tered rad ia t ion by a mov ing absorber , s c h e m a t i c

s c h e m a t i c a l l y in F i g . 8. Wi th s o u r c e and s c a t t e r e r off r e s o n a n c e t h e b a c k -g r o u n d l e v e l В i s m e a s u r e d . If s o u r c e and s c a t t e r e r a r e on r e s o n a n c e t h e

67

t r a n s m i s s i o n t h r o u g h a b l a c k a b s o r b e r o n a n d off r e s o n a n c e i s g i v e n b y

T ( 0 ) = B + N ^ . T(OO) = В + + NJJ ( 1 2 )

F o r a t h i n s c a t t e r e r o n e o b t a i n s , a c c o r d i n g t o E q s . (8 , 9 , 10) , t h e s i m p l e r e l a t i o n

f• / ( 1 - f ' ) = (T(oo) - T ( 0 ) ) / ( T ( 0 ) - B) (13)

If t h e s c a t t e r e r i s n o t t h i n a n d if t h e a b s o r b e r i s n o t b l a c k , o n e c a n e a s i l y c a l c u l a t e t h e n e c e s s a r y c o r r e c t i o n s . T h e r e s u l t , h o w e v e r , i s i n s e n s i t i v e t o t h e a s s u m p t i o n s u n d e r l y i n g t h e e v a l u a t i o n of t h e c o r r e c t i o n s .

F . S O M E T Y P I C A L R E S U L T S

1. Analysis of the scattered radiation

T h e m e t h o d o u t l i n e d in s e c t i o n E . 2. w a s u s e d t o a n a l y s e t h e 1 4 . 4 - k e V 57Fe t r a n s i t i o n , r e s o n a n t l y s c a t t e r e d b y a s t a i n l e s s - s t e e l a b s o r b e r . T h e f r a c t i o n of Y - r a y s s c a t t e r e d r e c o i l l e s s l y w a s m e a s u r e d a s a f u n c t i o n of t h e s c a t t e r e r t e m p e r a t u r e f r o m 20 t o 300°C [2]. T o k e e p s o u r c e and s c a t t e r e r o n r e s o n a n c e t h e s o u r c e w a s D o p p l e r s h i f t e d b y a c o n s t a n t v e l o c i t y d r i v e t o c o m p e n s a t e f o r t h e t e m p e r a t u r e s h i f t . A t y p i c a l s e t of t h e c u r v e s o b -t a i n e d i s s h o w n in F i g . 9. F r o m a l e a s t - s q u a r e f i t to t h e s e c u r v e s t h e r a t i o of t h e r e c o i l l e s s t o t h e n o n - r e c o i l l e s s s c a t t e r i n g w a s d e t e r m i n e d , and f r o m t h i s r a t i o t h e r e c o i l l e s s f r a c t i o n w a s o b t a i n e d . T h e r e c o i l l e s s f r a c t i o n w a s found to v a r y l i n e a r l y wi th t e m p e r a t u r e f r o m 0. 6 9 ± 0. 02 at 20°C to 0.51 ± 0.03 at 300°C.

2. 145-keV transition in шрг [12]

T h e d e c a y of 141Ce t o w i p r i s i d e a l l y s u i t e d f o r a b a c k - s c a t t e r i n g e x p e r i -m e n t s i n c e t h e r e a r e no o t h e r Y - r a y s p r e s e n t b e s i d e s t h e 1 4 5 - k e V t r a n s i t i o n . 1 4 1 C e i s e a s i l y p r o d u c e d b y n e u t r o n c a p t u r e , a n d t h e n a t u r a l a b u n d a n c e of t h e s c a t t e r e r i s o t o p e i 4 i P r i s 100%.

S o u r c e s of СеОг and C e F 3 w e r e m e a s u r e d wi th a s c a t t e r e r of 0. 45 g / c m 2

of Р г 6 О и . T h e s p e c t r u m o b t a i n e d w i t h t h e C e 0 2 s o u r c e i s s h o w n in F i g . 10. T h e s i z e of t h e M ö s s b a u e r p e a k w a s r o u g h l y t h e s a m e f o r b o t h s o u r c e s . T h e l i n e - w i d t h s o b s e r v e d w e r e t h r e e a n d t w o t i m e s l a r g e r t h a n t h e m i n i m u m w i d t h c a l c u l a t e d o n t h e b a s i s of t h e k n o w n h a l f - l i f e of 2 n s of t h e 1 4 5 - k e V l e v e l . T h e p r o d u c t of t h e r e c o i l l e s s f r a c t i o n s f o r t h e С е О г s o u r c e a n d t h e P r 6 O n s c a t t e r e r w a s f f ' = (2 . 4 ± 1. 2 )10-5 . T h e m e a n v a l u e of t h e r e c o i l l e s s f r a c t i o n , f = (4. 9 ± 1. 2) 1 0 - 3 ^ c o r r e s p o n d s t o t h e v e r y h i g h D e b y e t e m p e r a t u r e of e D = ( 2 7 0 ± 2 0 ) ° K .

68

SOURCE M O V I N G SCATTERING A N A L Y S I S OF SCATTERED RADIATION

100

• •

100- — . •*• « y r M

8 0 • • • 8 0 3 0 0 ° с

6 0 ч 6 0

4 0

100 Ь

100 / 2 0 0 ° С

8 0 • 8 0 •

6 0 t * •

s

100 • • 100

8 0

• •

8 0

1 0 0 e c у

6 0 • • * • V

100 • - 100

— - V w .

8 0 * •

4 •

8 0 -

•

• 2 0 ° С *

6 0 • Л ч 6 0 -

4 0 1 1 1 —

1 1 1 \ 1 I CHANNEL

1 1 1 4 0 1 0 • 2 ° , 3 0 4 0 5 9 6 0 10 2 0 3 0

l i l i 4 0 5 0 6 0 1 1 1 1

- . 1 О .1 - . 1 О .1 c m / s e c

FIG. 9. Typical velocity spectra obtained with the arrangement of Fig. 8 for the 14.4-keV transition of 5 7Fe. Source, scatterer and absorber are made of stainless steel and show a broadened single-line spectrum. Results are shown for several different scatterer temperatures. On the left hand side the counting rate is plotted versus source velocity with the absorber moving at high velocity approximating the off-resonance condition. Olí» the right hand side the counting rate is plotted versus absorber velocity with the source and scatterer on resonance.

3. 97-keV and 103-keV transition in 153Eu

153Eu i s a p a r t i c u l a r l y i n t e r e s t i n g i s o t o p e b e c a u s e i t h a s t w o l e v e l s , t h a t c a n b e s t u d i e d b y M ö s s b a u e r t e c h n i q u e s . B o t h l e v e l s a r e p o p u l a t e d i n t h e К c a p t u r e of iS3Qd. In t h e /3-de с a y of i53Sm o n l y t h e 1 0 3 - k e V s t a t e i s o b -t a i n e d . A f e w t y p i c a l s p e c t r a a r e s h o w n in F i g s . 1 1 - 1 3 .

T h e N i l s s o n m o d e l d e s c r i b e s t h e two l e v e l s a s i n t r i n s i c e x c i t a t i o n s wi th d i f f e r e n t a s y m p t o t i c q u a n t u m n u m b e r s and d i f f e r e n t d e f o r m a t i o n s . T h e p r e -d i c t i o n s of t h i s m o d e l c o n c e r n i n g t h e m a g n e t i c m o m e n t s and t h e m e a n s q u a r e c h a r g e r a d i i c a n b e c h e c k e d e x p e r i m e n t a l l y by M ö s s b a u e r e x p e r i m e n t s . We h a v e m e a s u r e d t h e m a g n e t i c m o m e n t s of b o t h l e v e l s u s i n g an u n s p l i t s o u r c e and a f e r r i m a g n e t i c e u r o p i u m i r o n g a r n e t s c a t t e r e r ,

69

о cm/sec

FIG. 10. The scattering count-ra te versus scatterer velocity for the 145-keV transition of 141Pr observed with a source of 141Ce in C e 0 2 at (15 ±3)° К and 0 .45 g / c m 2 scatterer of Pr6Ou a t (17 i 2)° К

F I G . l l . The scattering count-ra te versus scatterer velocity for the 103-keV transition of 153Eu observed with a source of 1 5 3 sm in Sm 2 0 3 a t ( 3 0 i l 0 ) ° K and A 0 . 3 g / c m 2 scatterer of natural EU203 a t (30±5)°K

STABLE ^Eu

(i1 Vf

Al SCATTERING Ш SCATTERI N6 CONE—s Ж MATER IА1-7

SOURCE-,

TEMPERATURE OF SOURCE IS a l?»*

TEMPERATURE OF SCATTERER IS 16±2eK

IM Wfi î I I i 1

c m / sec

FIG. 12. The scattering count-rate versus scatterer velocity for the 103-keV transition of 153Eu observed with a source of is'Sm in Lu203 at (8 ±2)°K and a 0. 35 g/c'm2 scatterer of natural Eu metal at (16 ±2)°K

1 i 1 1 1 1 i i i I i i i 6 5 « 3 2 1 0 - 1 - 2 - 3 - < - 5 - 6

c m / sec

FIG. 13,. The scattering count-rate versus scatterer velocity for the 97-keV transition of K3Eu observed with a source of 153Gd in G d 2 0 3 at (12 ±2)°K and a 0. 45 g / cm z scatterer of natural Eu meta l at (23±2)"K. The Mössbauer peak due to the 103-keV transition is at least ten times smaller than the peak due to the 97-keV transition

71

T h e r e s u l t s a r e s h o w n in T a b l e II, t o g e t h e r wi th t h e p r e d i c t i o n s of t h e N i l s s o n m o d e l .

T h e t h e o r e t i c a l v a l u e s s h o w n in T a b l e II a r e c a l c u l a t e d f o r t h e p a r a -m e t e r s r? = 3. 75 , g f f f / g s = 0. 61 d e d u c e d f r o m t h e m a g n e t i c m o m e n t of t h e g r o u n d s t a t e [16] .

TABLE II

M A G N E T I C M O M E N T S O F T H E 9 7 - k e V A N D 1 0 3 - k e V S T A T E S O F 1 5 3 E u

Energy Spin, par . Nilsson

assignment H a l f - l i f e

Magne t ic m o m e n t measured

Nilsson mode l

97 keV 5/2, - [532] > 0, 2 ns 2. 5 ± 0. 25 2. 67

103 keV 3/2. + [411] 3 . 4 ns 1. 9 ± 0 . 2 1. 75

T h e r e s u l t s of t h e i s o m e r - s h i i ' t m e a s u r e m e n t s a r e s t i l l p u z z l i n g , s i n c e t h e y c a n n o t b e f i t t e d with a s i m p l e l i n e a r e x p r e s s i o n f o r the i s o m e r sh i f t [17].

T h e s u p e r i o r i t y of t h e s c a t t e r i n g t e c h n i q u e f o r t h e s e m e a s u r e m e n t s i s d e m o n s t r a t e d b y t h e f a c t t h a t M ö s s b a u e r p e a k s of u p t o 120% a r e o b s e r v e d w i t h 1 5 3 Gd.

R E F E R E N C E S

[1] MAJOR, J. K., Nucl. Phys. 33 (1962) 323. [2] DEBRUNNER, P . , MORRISON, R.J . , Rev. mod. Phys. 36 (1964) 463. [3] TZARA, С . , BARLOUTAUD,. R., Phys. Rev. Lett. 4 (1960) 405. [4] BLACK, P . J . , EVANS, D . E . , O'CONNOR, D. A. , Proc. roy. Soc. A270 (1962) 168. [5] BLACK, P . J . , LONGWORTH, G.', O'CONNOR, D . A . , Proc. phys. Soc. 83 (1964) 939. [6] MILLER, J . , MOINE, J . J . , Phys. Lett. 2 (1962) 50. [7] BERNSTEIN, S . , CAMPBELL, E . C . , Phys. Rev. 132 (1963) 1625. [8] O'CONNOR, D . A . , BUTT, N . M . , Phys. Lett. 7 (1963) 233. [9] KANKELEIT, E., Z. Physik 164 (1961) 442.

[10] FRAUENFELDER, H. , COCHRAN, D .R .F . , NAGLE, D . E . , TAYLOR, R.D. , Nuovo Cim. 19 (1961) 183. [11] MORRISON, R.J . , A TAC, M . , DEBRUNNER, P . , FRAUENFELDER, H. , Phys. Lett. 12 (1964) 35. [12] MORRISON, R.J . , Ph.D. thesis, University of Illinois, Sept. 1964. [13] DEBRUNNER, P . , MORRISON, R.J . , Rev. Scient. Instrum. 36 (1965) 145. [14] METZGER, F. R., Prog. nucl. Phys. 7 (1959) 54. [15] WITTMANN, F , Z. Naturf. 19a (1964) 1409. [16] deBOER, J . , ROGERS, J. D . , Phys. Lett. 3,(1963) 304. [17] ATZMONY, U. , OFER, S. . Phys. Lett. 14(1965) 284.

D I S C U S S I O N

P . KIEN L E a s k e d w h y " v e r y h i g h " e n e r g i e s w e r e u s e d and w h e t h e r one s t i l l go t u s e f u l i n f o r m a t i o n .

72

H. F R A U E N F E L D E R a n s w e r e d t h a t , f i r s t of a l l , i t w a s j u s t t h e f u n of s e e i n g s u c h t r a n s i t i o n s t h a t m a d e t h e e x p e r i m e n t s a t t r a c t i v e . In a d d i t i o n , h o w e v e r , u s e f u l i n f o r m a t i o n on n u c l e a r and s o l i d - s t a t e p r o p e r t i e s cou ld s t i l l b e o b t a i n e d .

J . A . S T O N E s t a t e d t h a t F i g . 2 s e e m e d t o i n d i c a t e t h a t l i g h t e r e l e m e n t s w e r e b e s t s u i t e d t o s c a t t e r i n g t e c h n i q u e .

H. F R A U E N F E L D E R r e p l i e d t h a t in s o m e s e n s e t h i s w a s t r u e . As Eq . (4) i n d i c a t e d , t h e r a t i o M / R w a s m o s t f a v o u r a b l e a t low Z . H o w e v e r , t h e m a x i -m u m e n e r g y t h a t c o u l d b e r e a c h e d f o r a g i v e n D e b y e t e m p e r a t u r e w a s s m a l l e r a t l o w A .

SURFACE STUDIES BY MEANS OF THE MÖSSBAUER EFFECT*

J.W. BURTON**, H. FRAUENFELDER AND R.P. GODWIN UNIVERSITY OF ILLINOIS, URBANA. ILL. , UNITED STATES OF AMERICA


T h e M ö s s b a u e r e f f e c t , b e c a u s e of i t s s e n s i t i v i t y t o s t r e n g t h a n d a n g u l a r d i s t r i b u t i o n of b i n d i n g , t o m a g n e t i c a n d e l e c t r i c f i e l d s , a n d t o t h e d e n s i t y of s e l e c t r o n s a t t h e n u c l e u s , c a n be u s e d a s a t o o l t o s t u d y s u r f a c e s . Such s t u d i e s a r e e x p e r i m e n t a l l y e x t r e m e l y d i f f i c u l t , b u t t h e y p r o m i s e t o y i e l d i n f o r m a t i o n t ha t m a y be e v e n h a r d e r t o o b t a i n i n o t h e r w a y s . T h e a d v a n t a g e s a r e e a s y t o p o i n t o u t . T h e f r a c t i o n of y - r a y s e m i t t e d w i t h o u t l o s s of r e c o i l e n e r g y d e p e n d s o n t h e m e a n s q u a r e d i s p l a c e m e n t of t h e e m i t t i n g n u c l e u s f r o m i t s e q u i l i b r i u m p o s i t i o n . T h e d i s p l a c e m e n t i s e x p e c t e d t o be d i f f e r e n t on s u r f a c e s t h a n i n t h e b u l k of a c r y s t a l ; m o r e o v e r , i t s h o u l d d e p e n d on t h e d i r e c t i o n of t h e 7 - r a y e m i s s i o n . T h e l i n e s t r u c t u r e w i l l d e p e n d on t h e m a g -n e t i c f i e l d s and on t h e e l e c t r i c f i e l d g r a d i e n t s a t t h e s u r f a c e . In p a r t i c u l a r , i t i s e x p e c t e d t h a t t h e v a r i o u s c o m p o n e n t s of a l i n e s p l i t b y a q u a d r u p o l e i n t e r a c t i o n w i l l h a v e d i f f e r e n t d i r e c t i o n a l d i s t r i b u t i o n s w i t h r e s p e c t t o t h e a x i s of t h e f i e l d . A n a n i s o t r o p y i n t h e b i n d i n g a t t h e s u r f a c e m a y l e a d t o a n i s o m e r i c s h i f t . A d i f f e r e n c e of t h e m e a n s q u a r e v e l o c i t y b e t w e e n s u r f a c e a n d b u l k a t o m s w i l l r e s u l t i n a s e c o n d - o r d e r D o p p l e r e f f e c t .

A l t h o u g h t h e a d v a n t a g e s of s u c h M ö s s b a u e r e x p e r i m e n t s a r e e a s y t o e n u m e r a t e , t h e d i f f i c u l t i e s a r e a l s o e a s y t o p o i n t out bu t h a r d to o v e r c o m e . A l l e x p e r i m e n t s m u s t b e m a d e i n u l t r a - h i g h v a c u u m ; t h e s u r f a c e m u s t b e

* Supported by the US Air Force Off ice of Sc ien t i f ic Research under cont rac t AF49 (638) - 1 0 4 8 . * * Present address: Physics Depar tment , Carson-Newman Col lege, Jefferson Ci ty , T e n n . , United

States of Amer ica .

7 3

w e l l c l e a n e d a n d t h e r a d i o a c t i v e m a t e r i a l m u s t b e d e p o s i t e d i n l e s s t h a n a m o n o l a y e r . F i n a l l y , t h e t e m p e r a t u r e of t h e s u r f a c e s h o u l d g e n e r a l l y b e h e l d w e l l b e l o w r o o m t e m p e r a t u r e ; o t h e r w i s e , t h e t r a c e r a t o m s m a y r a p i d l y d i f f u s e t o a c r a c k o r a s u r f a c e i m p e r f e c t i o n a n d h e n c e f a l s i f y t h e r e s u l t s .

T h e d i f f i c u l t i e s a s s o c i a t e d w i t h t h e e x p e r i m e n t a l t e c h n i q u e s m a k e i t o b v i o u s w h y o n l y v e r y f e w e x p e r i m e n t s h a v e b e e n c a r r i e d ou t [ 1 - 3 ] . H o w -e v e r , e x t e n s i v e t h e o r e t i c a l s t u d i e s r e l a t e d t o t h e M ö s s b a u e r e f f e c t o n s u r -f a c e s h a v e b e e n m a d e [ 4 - 1 2 J ,

In t h e p r e s e n t r e p o r t , we s k e t c h s o m e of t h e b a s i c a s p e c t s of t h e t h e o r y , r e v i e w t h e e x p e r i m e n t a l a p p r o a c h , a n d d i s c u s s s o m e e x p e r i m e n t a l r e s u l t s .

B . T H E O R Y

1. The fraction f

T h e f r a c t i o n f of y - r a y s e m i t t e d w i t h o u t r e c o i l i s g i v e n b y

f - е х р ( - < х ^ 2 > / * 2 ) (1)

w h e r e < ^ 2 ) > i s t h e m e a n s q u a r e d i s p l a c e m e n t of t h e e m i t t i n g n u c l e u s a l o n g t h e d i r e c t i o n of t h e e m i t t e d 7 - r a y a n d X i s t h e r e d u c e d w a v e l e n g t h of t h e 7 - r a y . Now c o n s i d e r t h e t h r e e e x t r e m e c a s e s s h o w n i n F i g . 1. E q u a t i o n (1) s h o w s t h a t t h e f r a c t i o n f i s s m a l l e s t i n t h e d i r e c t i o n of l a r g e s t e x c u r s i o n

FIG. 1. Three typical positions of a nucleus undergoing a Mössbauer transition

f o r t h e e m i t t i n g n u c l e u s . W e t h e n e x p e c t , i n a c u b i c c r y s t a l , t h a t f w i l l b e i s o t r o p i c f o r t h e n u c l e u s A s i t t i n g i n t h e b u l k . N u c l e u s B, s i t t i n g m t h e s u r f a c e , w i l l s h o w a s m a l l e r f a l o n g t h e n o r m a l rî t h a n p e r p e n d i c u l a r t o i t ,

w h e r e a s n u c l e u s C, s i t t i n g on t h e s u r f a c e , s h o u l d d i s p l a y i t s l a r g e s t f a l o n g

t h e n o r m a l ñ*. F u r t h e r m o r e , we e x p e c t fA t o be l a r g e r t h a n e i t h e r fg o r f c .

7 4

W e w i l l now put t h i s i n t u i t i v e d i s c u s s i o n in to a m o r e q u a n t i t a t i v e f o r m . F o r a p e r i o d i c l a t t i c e i n t h e h i g h t e m p e r a t u r e l i m i t * t h e m e a n s q u a r e d i s -p l a c e m e n t i s g i v e n by [13]

2 y

Y к «шша2 <2>

w h e r e a i n d i c a t e s b o t h t h e w a v e v e c t o r a n d p o l a r i z a t i o n f o r t h e l a t t i c e v i -b r a t i o n s , К i s B o l t z m a n n ' s c o n s t a n t , and the o t h e r s y m b o l s h a v e t h e i r c o n -v e n t i o n a l m e a n i n g s . If we t r a n s f o r m t h e s u m to an i n t e g r a l we f ind

<x 2> OC Y oC u)2duu = / du) (3) Y a m

T h u s a l l f r e q u e n c i e s e n t e r t he c a l c u l a t i o n of f w i th e q u a l w e i g h t . T h i s r e -su l t a l l o w s u s t o c h o o s e a s i n g l e t y p i c a l f r e q u e n c y tha t c a n be u s e d to d i s c u s s t h e r e c o i l l e s s f r a c t i o n f ; h e n c e t h e E i n s t e i n s o l i d i s a u s e f u l m o d e l f o r d i s c u s s i n g t h e r e c o i l l e s s f r a c t i o n . V i s s c h e r [14] h a s v e r i f i e d t he u s e f u l n e s s of the E i n s t e i n m o d e l n u m e r i c a l l y . P r o p e r l y a v e r a g i n g ы"2 s h o w s we should c h o o s e

•" typical " j - , <«max . ( 4 )

o r e q u i v a l e n t l y © ( E i n s t e i n ) = ( 1 / \ / з ) © ( D e b y e ) . S i n c e w e a r e a b l e t o u s e a s i n g l e f r e q u e n c y we a p p r o x i m a t e a n u c l e u s i n a c r y s t a l by a s i n g l e p a r t i c l e of m a s s m l o c a t e d in an a n i s o t r o p i c h a r m o n i c o s c i l l a t o r p o t e n t i a l w i t h H o o k e ' s l aw f o r c e c o n s t a n t s k j , кг and к з . In t he high t e m p e r a t u r e r eg ion th i s s i m p l e m o d e l y i e l d s f o r the r e c o i l l e s s f r a c t i o n :

2 2 f = exp -[- - ^ - т Г s i n Seos q> + —ñ s i n 0 s i n ¥ + ~~4 c ° s 2 e ] | (5)

L К Л L ®2 ®з J J

w h e r e f o r a p p l i c a t i o n t o s u r f a c e s we a s s u m e 9 t o be t h e a n g l e b e t w e e n t h e s u r f a c e n o r m a l and t h e d i r e c t i o n of t h e y - r a y e m i s s i o n . @¡ i s a n E i n s t e i n t e m p e r a t u r e d e f i n e d b y

W e n o t e t h a t

/ 2> 1 1 <x > oc - j ce Г { 7 )

' ® у Y .

w h e r e ky i s t h e e f f e c t i v e f o r c e c o n s t a n t in t he d i r e c t i o n of t he e m i t t e d Y - r a y .

1 For brevity only the high temperature limit is discussed in this paper.

7 5

T o a p p l y t h i s m o d e l , w e a s s u m e t h a t t h e e f f e c t i v e f o r c e c o n s t a n t s c a n b e o b t a i n e d b y c o u n t i n g t h e e f f e c t i v e b o n d s o p e r a t i n g i n t h e v a r i o u s d i r e c t i o n s . A s a n e x a m p l e , c o n s i d e r a s i m p l e - c u b i c l a t t i c e . F o r s i m p l i c i t y , a s s u m e t h a t n e a r e s t a n d n e x t - n e a r e s t n e i g h b o u r s c o n t r i b u t e e q u a l l y t o b i n d i n g b u t t h a t b o n d s p e r p e n d i c u l a r t o t h e d i r e c t i o n of y - e m i s s i o n h a v e n o e f f e c t ( s e e F i g . 2) . W e f i n d b y t h i s " b o n d c o u n t i n g " a p p r o a c h t h a t t h e m e a n s q u a r e d i s -p l a c e m e n t of a s u r f a c e a t o m i s a n i s o t r o p i c a n d i s g r e a t e r p e r p e n d i c u l a r t h a n p a r a l l e l t o t h e s u r f a c e f o r a n a t o m s i t u a t e d i n t h e s u r f a c e , w h e r e a s i t i s s m a l l e r p e r p e n d i c u l a r t h a n p a r a l l e l t o t h e s u r f a c e f o r a n a t o m s i t u a t e d o n t h e s u r f a c e — j u s t a s w e i n d i c a t e d e a r l i e r b y a n i n t u i t i v e a r g u m e n t ( s e e F i g s . 2 a n d 3 ) . I n t h e s e c a s e s t h e r a t i o of t h e s u r f a c e a t o m t o b u l k a t o m m e a n s q u a r e d i s p l a c e m e n t s v a r i e s f r o m 1. 2 5 t o 5, d e p e n d i n g o n t h e g e o -m e t r y of t h e s i t u a t i o n .

ASSUME к CC NUMBER OF NEAREST AND NEXT-NEAREST NEIGHBOUR BONDS NOT AT RIGHT ANGLES TO y - DIRECTION.

Y

к rr "SURFACE X + 4

In t h e m o d e l w e h a v e b e e n d i s c u s s i n g t h e a t o m s i n t h e s e c o n d l a y e r of a c r y s t a l w o u l d b e h a v e e s s e n t i a l l y a s b u l k a t o m s . W e w i l l n o w i n d i c a t e , b y a s i m p l e a r g u m e n t d u e t o C l e m [ 1 5 j , h o w r a p i d l y s u r f a c e e f f e c t s d e c a y i n t o t h e l a t t i c e . W h e n f r e e s u r f a c e s a r e i n t r o d u c e d i n t o t h e p r o b l e m ( i . e .

7 6

POSITION В POSITION С

FIG. 3. Ratio of perpendicular to paral le l mean square displacement for atoms in and on the surface of a

s imple-cubic la t t ice

when p e r i o d i c b o u n d a r y c o n d i t i o n s a r e r e m o v e d ) E q . (2) b e c o m e s m o r e c o m -p l i c a t e d . F o r h igh t e m p e r a t u r e s we h a v e in g e n e r a l

by2** I 3 B a 2 , i W mui a a

w h e r e B a i ¡ i s an e i g e n v e c t o r of t h e t h r e e - d i m e n s i o n a l n o r m a l m o d e s p r o b -l e m a n d i s , a t b e s t , d i f f i c u l t t o f i n d . F o r a o n e - d i m e n s i o n a l l a t t i c e w i t h f r e e e n d s c o m p o s e d of N a t o m s w i t h e q u i l i b r i u m s p a c i n g [11 ] a

В q i " ( f J ' c « C < i - * £>qa] ( 9 )

w h e r e q = 2?r/X = (j - l ) ) r / N a , j = 2, 3, . . . N. We a s s u m e a l i n e a r d i s p e r s i o n r e l a t i o n s o t h a t u q <x q. To e s t i m a t e wha t wi l l happen in a t h r e e - d i m e n s i o n a l c r y s t a l w e u s e t h e o n e - d i m e n s i o n a l B q i a s a p r o t o t y p e f o r t h e t h r e e -d i m e n s i o n a l p r o b l e m . W i t h t h i s a s s u m p t i o n we f i n d

1/2

< XY î i > I ~ V c o s 2 ^ i * 2 > 4 a ] (10)

S i n c e a l l f r e q u e n c i e s c o n t r i b u t e e q u a l l y t o t h e m e a n s q u a r e d i s p l a c e m e n t , w e a g a i n d i s c u s s t h e m e a n s q u a r e d i s p l a c e m e n t w i t h a t y p i c a l f r e q u e n c y . T h e t y p i c a l f r e q u e n c y ( E q . (4)) c o r r e s p o n d s t o a w a v e l e n g t h

2TT 2 / 3 TT „ г . ,

7 7

w h e r e q m = тг/а i s g i v e n by t h e b o u n d a r y of t h e f i r s t B r i l l o u i n z o n e . N o w c o n s i d e r E q . (10) . F o r a n e n d a t o m (i = 1) e a c h m o d e c o n t r i b u t e s a t e r m of o r d e r u n i t y s i n c e t h e c o s i n e s q u a r e d f a c t o r i s abou t one f o r m o s t m o d e s . F o r a t o m s f a r f r o m a s u r f a c e ( i » l ) , h o w e v e r , t h e v a r i o u s f r e q u e n c i e s p r e s e n t c a u s e t h e f a c t o r c o s 2 [ ( i - i ) q a ] t o o s c i l l a t e b e t w e e n z e r o a n d o n e . A f t e r a t y p i c a l w a v e l e n g t h ( i . e . f o r i > 3 ) we w o u l d e x p e c t t h i s o s c i l l a t i o n t o g i v e a n a v e r a g e c o n t r i b u t i o n of o n e - h a l f . T h u s i t s e e m s

< 0 W ) 2 > „ 2 ( 1 2 )

< S , i > 3 > 2 >

T h e s i z e of t h i s r a t i o w i l l v a r y w i t h d i r e c t i o n and t h e d e t a i l s of t he c r y s t a l m o d e l a s s u m e d , bu t i n a l l c a s e s t h e q u a l i t a t i v e b e h a v i o u r of t h e d e c a y to t h e b u l k / v a l u e i s g i v e n c o r r e c t l y by t h e a r g u m e n t j u s t p r e s e n t e d .

W e n o w i n d i c a t e t h e m o d e l s u s e d i n a n u m b e r of d e t a i l e d c a l c u l a t i o n s of the e f f e c t s of f r e e c r y s t a l s u r f a c e s on a t o m i c m e a n s q u a r e d i s p l a c e m e n t s . R i c h [7] h a s c o m p u t e d s u r f a c e e f f e c t s on t h e m e a n s q u a r e d i s p l a c e m e n t s a s a f u n c t i o n of t e m p e r a t u r e f o r a n a t o m in a s e m i - i n f i n i t e i s o t r o p i c c u b i c l a t t i c e w i t h n e a r e s t n e i g h b o u r i n t e r a c t i o n s . C o r c i o v e i and B e r i n d e [8] and Ce ly [9] e x a m i n e d m o d e l s s i m i l a r to tha t of R ich . M a r a d u d i n and M e l n g a i l i s [10] u s e d a doub le t i m e G r e e n ' s f u n c t i o n a p p r o a c h to ob ta in the m e a n s q u a r e d i s p l a c e m e n t s of t he a t o m s in a s e m i - i n f i n i t e cub ic l a t t i c e wi th n e x t - n e a r e s t n e i g h b o u r a s w e l l a s n e a r e s t n e i g h b o u r i n t e r a c t i o n s in t he h igh t e m p e r a t u r e l i m i t . C l e m a n d G o d w i n [11] h a v e o b t a i n e d a n a l y t i c r e s u l t s f o r t h e m e a n

• s q u a r e d i s p l a c e m e n t s of t he a t o m s of a l i n e a r l a t t i c e wi th f r e e e n d s f o r both h igh t e m p e r a t u r e s and T = 0°K. C l a r k , H e r m a n and W a l l i s [12] used a m a t r i x f o r m a l i s m d u e t o B o r n t o c a l c u l a t e t h e m e a n s q u a r e d i s p l a c e m e n t s i n t h e h i g h t e m p e r a t u r e l i m i t f o r a n a t o m in a f a c e - c e n t r e d c u b i c c r y s t a l w i t h n e a r e s t n e i g h b o u r i n t e r a c t i o n s f o r v a r i o u s f r e e c r y s t a l f a c e s . T h e i r c a l -c u l a t i o n i s t h e f i r s t m a d e on a r e a l i s t i c c r y s t a l m o d e l . T h e d e t a i l e d t h r e e -d i m e n s i o n a l c a l c u l a t i o n s a g r e e q u a l i t a t i v e l y w i t h t h e s i m p l e a r g u m e n t s we p r e s e n t e d a b o v e , i . e . t h e r a t i o of s u r f a c e a t o m to b u l k a t o m m e a n s q u a r e d i s p l a c e m e n t i n t h e h i g h t e m p e r a t u r e l i m i t i s a p p r o x . 2 a n d v a r i e s wi th d i -r e c t i o n a n d c r y s t a l m o d e l i n a m a n n e r c o n s i s t e n t w i t h o u r s i m p l e " b o u n d c o u n t i n g " p i c t u r e . In a d d i t i o n , a l l t he t h r e e - d i m e n s i o n a l c a l c u l a t i o n s show t h a t s u r f a c e e f f e c t s on t h e m e a n s q u a r e d i s p l a c e m e n t b e c o m e v e r y s m a l l f o r a t o m s d e e p e r t h a n f i v e a t o m i c s p a c i n g s .

2. Isomer shift and second-order Doppler shift

T h e i s o m e r s h i f t i s g i v e n a p p r o x i m a t e l y by

S E I = ! ^ 2 £ < r B2 > - < r A

2 > ] { K o ) a | 2 - K o ) e | 2 } ( 1 3 )

w h e r e Z e i s t h e n u c l e a r c h a r g e , < | гв 2 ^ a n d <^гд2)> a r e t h e r m s r a d i i of t h e n u c l e a r e x c i t e d a n d g r o u n d s t a t e s , a n d ¥(о)а a n d ¥(о)е a r e t h e e l e c t r o n i c w a v e - f u n c t i o n s a t t h e n u c l e u s f o r t h e a b s o r b e r a n d e m i t t e r r e s p e c t i v e l y .

7 8

A g i v e n M ö s s b a u e r t r a n s i t i o n i n v e s t i g a t e d i n d i f f è r e n t e n v i r o n m e n t s ( f o r e x a m p l e , on s u r f a c e s and in t he bulk of a c r y s t a l ) c a n y ie ld i n f o r m a t i o n about t h e e l e c t r o n i c w a v e - f u n c t i o n s in t he v a r i o u s s i t u a t i o n s . We know of no t h e o -r e t i c a l p r e d i c t i o n s of t h i s e f f e c t f o r s u r f a c e s , b u t ' a s M a r a d u d i n [ 4 j h a s p o i n t e d out we would e x p e c t i t to be a p p r o x i m a t e l y t e m p e r a t u r e i n d e p e n d e n t u n l e s s a c r y s t a l p h a s e t r a n s i t i o n o c c u r s .

T h e s e c o n d - o r d e r D o p p l e r s h i f t 6 E D i s g i v e n by

5 E d = - \ <V2> (14) 2c

w h e r e E i s t h e 7 - r a y e n e r g y a n d <(v2 )> i s t h e m e a n s q u a r e v e l o c i t y of t h e e m i t t i n g n u c l e u s . T o c a l c u l a t e t h e m e a n s q u a r e v e l o c i t y , we r e t u r n to t h e a n i s o t r o p i c h a r m o n i c o s c i l l a t o r m o d e l d i s c u s s e d i n t h e p r e v i o u s s e c t i o n a n d f i n d i n t h e h i g h t e m p e r a t u r e l i m i t

2 ( v

2 ) = Ж + (k +k -He ) (15) 12KTm 1 J

I n s e r t i n g < v 2 > in E q . (14) and l e t t i n g k i + к г + к з = k eff we ge t

5 E P = _3KT + ft2 . ^ e f f (16) E 2mc2 24(me)2 K T

In o u r " b o n d c o u n t i n g " m o d e l we f i n d k eff f o r a t o m s in t h e b u l k i n t h e s u r f a c e and on the s u r f a c e of a s i m p l e - c u b i c l a t t i c e to be in the a p p r o x i m a t e r a t i o 3 : 2 : 1 . W a l l i s and G a z i s L6J a n d M a r a d u d i n a n d M e l n g a i l i s LI0J h a v e s h o w n t h a t d e t a i l e d c a l c u l a t i o n s v e r i f y E q . (16) .

T h e s e c o n d t e r m in E q . (16) i s t h e q u a n t u m c o r r e c t i o n to t h e c l a s s i c a l e x p r e s s i o n ; it i s i n t e r e s t i n g b e c a u s e i t i s d i r e c t l y r e l a t e d to the " d y n a m i c a l m a t r i x " f r o m w h i c h the e q u a t i o n s of m o t i o n f o r a t o m s in r e a l c r y s t a l s could be w r i t t e n d o w n . T h i s t e r m i s e x p e c t e d t o b e c o m e c o m p a r a b l e t o t h e f i r s t one a t t e m p e r a t u r e s of t h e o r d e r of 1 / 3 t h e D e b y e t e m p e r a t u r e .

T h e c o m b i n e d l i n e s h i f t 6E i s

SE - SEj + 5Ed (17)

If we a s s u m e óE¡ t o b e i n d e p e n d e n t of t e m p e r a t u r e and p lo t

3EKT 1 .„ 2 2mc

SE g" v s - T ( 1 8 )

t h e s l ope ( see E q . (16)) should give u s i n f o r m a t i o n about the d e t a i l s of s u r f a c e

l a t t i c e b ind ing .

3. Magnetic and electric effects

T h e Z e e m a n s p l i t t i n g of t h e n u c l e a r e n e r g y l e v e l s m a k e s i t p o s s i b l e t o s t u d y m a g n e t i c f i e l d s a t n u c l e i l o c a t e d on s o l i d s u r f a c e s . S ince t he p h y s i c s

79

i n v o l v e d i s n o d i f f e r e n t f r o m t h a t f o r ' a n u c l e u s i n t h e b u l k , w e w i l l no t d i s -c u s s i t f u r t h e r .

T h e e l e c t r i c q u a d r u p o l e s p l i t t i n g of t h e n u c l e a r e n e r g y l e v e l s a l l o w s u s t o s t u d y e l e c t r i c f i e l d g r a d i e n t s a t s u r f a c e s . F o r a p u r e q u a d r u p o l e s p e c -t r u m w i t h a x i a l l y s y m m e t r i c f i e l d g r a d i e n t s t h e q u a d r u p o l e s h i f t 6 E m f o r t h e s u b s t a t e s of a n u c l e a r s p i n I s t a t e i s g i v e n b y

2 SEm = 6 У [ 4 1 ( 2 1 - 1 ) ] 0 2 - K I + U ] (19)

w h e r e m i s t h e m a g n e t i c q u a n t u m n u m b e r , Q t h e n u c l e a r q u a d r u p o l e m o m e n t , a n d 9 2 V / 9 Z 2 t h e e l e c t r i c f i e l d g r a d i e n t . C o n s i d e r t h e M ö s s b a u e r i s o t o p e 5 7 F e . T h e e x c i t e d s t a t e ( 1 = 3 / 2 ) s p l i t s i n t o t w o l e v e l s ( | m | = 1 / 2 , 3 / 2 ) w h i l e t h e g r o u n d s t a t e (1= 1 / 2 ) i s u n s p l i t . T h e n f o r 5 7 F e E q . (19) g i v e s

6EQ = ÔE 3 / 2 - 6E 1 / 2 = I E Q ( 2 0 ) OZ

T h e a c t u a l s p l i t t i n g i s l a r g e r b e c a u s e of t h e p o l a r i z a t i o n of t h e d e l e c -t r o n s b y t h e e x t e r n a l f i e l d g r a d i e n t . T o a c c o u n t f o r t h i s p o l a r i z a t i o n , t h e s p l i t t i n g E q . (20) i s m u l t i p l i e d b y t h e S t e r n h e i m e r a n t i s h i e l d i n g f a c t o r .

T o e s t i m a t e t h e o r d e r of m a g n i t u d e of t h e f i e l d g r a d i e n t o n a s u r f a c e , w e s u p p o s e t h a t a c h a r g e - e i s p l a c e d a t a t y p i c a l l a t t i c e s p a c i n g a a b o v e t h e s u r f a c e , s i m u l a t i n g a m i s s i n g p o s i t i v e l y c h a r g e d i o n . W e t h e n g e t

4 = = Ц . (21) èZ^ a 3

A s s u m i n g a d i s t a n c e a of a b o u t 3 X 1 0 " 8 c m , a S t e r n h e i m e r a n t i s h i e l d i n g

f a c t o r of a b o u t 15, a n d a q u a d r u p o l e m o m e n t of a b o u t 0. 2 b , w e f i n d

5E q ~ 10" 8 eV (22)

S i n c e t h e n a t u r a l w i d t h of t h e M ö s s b a u e r l i n e i n 5 7 F e i s a b o u t 10"8 eV, s u r -

f a c e f i e l d g r a d i e n t s m a y be o b s e r v a b l e .

T h e f i e l d g r a d i e n t a t a s u r f a c e i s e x p e c t e d t o h a v e a w e l l - d e f i n e d a x i s .

W e t h e n e x p e c t t h a t t h e t w o c o m p o n e n t s t h a t r e s u l t f r o m t h e s p l i t t i n g of t h e 5 7 F e 1 4 - k e V l i n e b y q u a d r u p o l e i n t e r a c t i o n w i l l s h o w d i f f e r e n t d i r e c t i o n a l

b e h a v i o u r . T h e m a g n e t i c d i p o l e t r a n s i t i o n s f r o m t h e 1 = 3 / 2 s t a t e to t h e l = 1/2

g r o u n d s t a t e i n 57Fe o c c u r w i t h A m = ± 1 f r o m t h e l e v e l s w i t h m = ± 3 / 2 , a n d

w i t h Д т = 0, ± 1 f r o m t h e l e v e l s w i t h m = ± l / 2 . T h e d i r e c t i o n a l d e p e n d e n c e of t h e i n t e n s i t i e s of t h e t w o l i n e s i s

3 / 2 2 F ' (0) = 1 + cos e and

(23)

F 1 / 2 ( 6 ) = 5 /3 - c o s 2 e

8 0

w h e r e в i s t h e a n g l e m e a s u r e d f r o m t h e a x i s of t h e f i e l d g r a d i e n t . T h u s a s t u d y of t h e a n g u l a r d e p e n d e n c e of t h e i n t e n s i t i e s of a q u a d r u p o l e s p l i t M ö s s b a u e r l i n e c a n g i v e u s t h e d i r e c t i o n of t h e f i e l d g r a d i e n t .

C . A N E X P E R I M E N T

1. Experimental problems

A n u m b e r of d i f f i c u l t t e c h n i c a l p r o b l e m s m u s t b e s o l v e d t o p e r f o r m a s u r f a c e M ö s s b a u e r e x p e r i m e n t t h a t y i e l d s q u a n t i t a t i v e i n f o r m a t i o n . F i r s t we m e e t t h e p r o b l e m , c o m m o n t o a l l s u r f a c e e x p e r i m e n t s , of ob t a in ing and m a i n t a i n i n g a n a t o m i c a l l y c l e a n s u r f a c e . R e c e n t a d v a n c e s i n u l t r a - h i g h v a c u u m t e c h n o l o g y a n d m e t h o d s f o r c l e a n i n g a n d o b s e r v i n g s u r f a c e s ( f o r e x a m p l e , in ion s p u t t e r i n g , c r y s t a l c l e a v i n g i n v a c u u m , f i e l d e m i s s i o n a n d f i e l d - i o n e m i s s i o n m i c r o s c o p e s , and l o w - e n e r g y e l e c t r o n d i f f r a c t i o n ) b r i n g u s n e a r e r a s o l u t i o n t o t h i s p r o b l e m but i t w i l l n e v e r be a s i m p l e o n e . T o o b t a i n i n f o r m a t i o n t h a t c a n e a s i l y b e i n t e r p r e t e d t h e o r e t i c a l l y , s u r f a c e e x -p e r i m e n t s , l i k e b u l k e x p e r i m e n t s , m u s t be m a d e on s i n g l e c r y s t a l s . T o avo id m o d i f y i n g o u r s u b s t r a t e by t he p r e s e n c e of d e p o s i t e d M ö s s b a u e r a t o m s we m u s t w o r k a t low s u r f a c e c o v e r a g e s ( < 0 . l m o n o l a y e r o r ~1013 a t o m s / c m 2 ) . T h e d e p o s i t i o n of t h e a c t i v e i s o t o p e p o s e s a p r o b l e m s i n c e i t i s d i f f i c u l t t o d e p o s i t m a t e r i a l on s u r f a c e s , e v e n in u l t r a - h i g h v a c u u m , wi thout i n t r o d u c i n g u n d e s i r a b l e s u r f a c e c o n t a m i n a t i o n . O n e - t e n t h of a m o n o l a y e r of p u r e 51 Co d e p o s i t e d on a l - c m 2 s u b s t r a t e c o r r e s p o n d s t o a b o u t 0. 1 m C i of a c t i v i t y . T h i s a c t i v i t y c o m b i n e d w i t h t h e l o w t r a n s m i s s i o n of t h e 1 4 - k e V M ö s s b a u e r y - r a y r e q u i r e s l o n g c o u n t i n g t i m e s t o a c h i e v e s t a t i s t i c a l l y m e a n i n g f u l M ö s s b a u e r s p e c t r a . T o m i n i m i z e s u r f a c e a n d b u l k d i f f u s i o n a n d i s l a n d g r o w t h t h e s u b s t r a t e shou ld be c o o l e d . P r o v i s i o n shou ld be m a d e f o r v a r i a -t i o n a n d c a r e f u l m o n i t o r i n g of t h e s a m p l e t e m p e r a t u r e . A n g u l a r v a r i a t i o n of t h e s u r f a c e s a m p l e m u s t be i n c l u d e d in an e x p e r i m e n t if one h o p e s to s tudy t h e a n g u l a r d e p e n d e n c e of t h e r e c o i l l e s s f r a c t i o n f a n d of t h e i n t e n s i t i e s of a l i n e s p l i t ' b y s u r f a c e e l e c t r i c f i e l d g r a d i e n t s .

2. Apparatus and procedure

A b r i e f d e s c r i p t i o n i s g i v e n of a n e x p e r i m e n t m a d e a t t h e U n i v e r s i t y of I l l i n o i s [ 3 ] . T h e i n v e s t i g a t i o n w a s c a r r i e d ou t i n a b a k e a b l e g l a s s a n d s t a i n l e s s - s t e e l u l t r a - h i g h v a c u u m s y s t e m w h i c h c o u l d b e e v a c u a t e d t o 10 -ю T o r r . T h e p u m p i n g e q u i p m e n t c o n s i s t e d of two m o l e c u l a r s i e v e r o u g h -i n g p u m p s , t w o i o n i z a t i o n p u m p s f o r c o n t i n u o u s o p e r a t i o n , a n d a w a t e r -c o o l e d t i t a n i u m s u b l i m a t i o n p u m p f o r h a n d l i n g p r e s s u r e b u r s t s o c c u r r i n g d u r i n g e v a p o r a t i o n s .

A d o u b l e e v a p o r a t i o n p r o c e s s w a s u s e d t o o b t a i n a d e p o s i t i o n of a b o u t one t e n t h m o n o l a y e r of " c l e a n " 5 7 Co on t h e f i n a l t a r g e t . T h e f i r s t e v a p o r -a t i o n w a s c a r r i e d ou t a t p o s i t i o n A ( s e e F i g . 4) . T h e i n i t i a l e v a p o r a t i o n a s -s e m b l y c o n s i s t e d of an o h m i c a l l y h e a t e d t u n g s t e n c r u c i b l e m o u n t e d on a r e -m o v a b l e v a c u u m f l a n g e t h a t a l l owed in se r t i on ' o f the r a d i o a c t i v e s o u r c e . A f t e r s a m p l e i n s e r t i o n t h e t r a n s f e r v e h i c l e (which c o n s i s t e d of a t u n g s t e n f o i l t a r g e t

81

FIG. 4. Surface Mössbauer apparatus

m o u n t e d on a f r a m e a t t a c h e d t o a Ni c y l i n d e r ) w a s p l a c e d o v e r t h e c r u c i b l e c o n t a i n i n g t h e 5 1 Co. T h e t r a n s f e r t a r g e t w a s t h e n c l e a n e d by hea t i ng a t about 2 0 0 0 ° C w i t h e l e c t r o n b o m b a r d m e n t w h i l e t h e t u n g s t e n c r u c i b l e c o n t a i n i n g t h e a c t i v i t y w a s h e a t e d t o 1200°C, i . e . t o a t e m p e r a t u r e j u s t below tha t w h e r e c o b a l t e v a p o r a t e d . T h e t r a n s f e r t a r g e t w a s t h e n c o o l e d a n d t h e c r u c i b l e f l a s h e d t o e v a p o r a t e t h e a c t i v i t y o n t o t h e t r a n s f e r t a r g e t .

T h e t r a n s f e r t a r g e t w a s t h e n m o v e d , by m e a n s of a m a g n e t , i n t o p o s i t i o n i n f r o n t of t h e f i n a l t a r g e t f o r h e a t i n g b y e l e c t r o n b o m b a r d m e n t t o d e p o s i t t h e a c t i v i t y o n t o t h e f i n a l t a r g e t ( s e e F i g . 4) . T h e f i n a l t a r g e t in t h e e x p e r i -m e n t t h a t w e d i s c u s s h e r e w a s a l i n X l i n s h e e t of p o l y c r y s t a l l i n e t u n g s t e n t h a t h a d b e e n e l e c t r o - p o l i s h e d . L a t e r a s m a l l e r s i n g l e - c r y s t a l w e d g e w a s u s e d . T h e f i n a l t a r g e t w a s t h e r m a l l y c l e a n e d b y e l e c t r o n b o m b a r d m e n t f o r m a n y h o u r s a t a b o u t 1500°C.

T h e t a r g e t w a s m o u n t e d be low a n i c k e l b a r a t t a c h e d to a r o t a t a b l e c o p p e r s h a f t a s s h o w n in F i g . 5. T h e t o p of t h e t a r g e t m o u n t i n g a s s e m b l y s e r v e d a s a t e m p e r a t u r e b a t h f o r c o o l i n g w i t h l i q u i d n i t r o g e n o r o t h e r a g e n t s a n d f o r h e a t i n g e l e c t r i c a l l y . T h e r o t a t a b l e s h a f t c o u l d be c l a m p e d f o r r i g i d i t y a n d good t h e r m a l c o n t a c t w i t h t h e t e m p e r a t u r e b a t h by m e a n s of a s m a l l b e l l o w s . T h e c l a m p i n g a r r a n g e m e n t a l l o w e d b o t h c o o l i n g and a n g u l a r v a r i a t i o n of t h e s a m p l e . A t u n g s t e n - n i c k e l t h e r m o c o u p l e c o u l d be a t t a c h e d to t h e t a r g e t f o r t e m p e r a t u r e m e a s u r e m e n t . A t h i n n e d g l a s s w i n d o w i n f r o n t of t h e t a r g e t g r e a t l y i n c r e a s e d t h e t r a n s m i s s i o n of 1 4 - k e V y - r a y s .

T h e M ö s s b a u e r a n a l y s e r c o n s i s t e d of a s i n g l e l ine 57Fe e n r i c h e d s t a i n l e s s -s t e e l a b s o r b e r d r i v e n by a s p e a k e r . T h e 1 4 - k e V 7 - p a r t i c l e s w e r e a n a l y s e d a c c o r d i n g t o v e l o c i t y b y a m u l t i c h a n n e l a n a l y s e r . T h e v e l o c i t y w a s c a l i -b r a t e d b y m e a s u r i n g a M ö s s b a u e r s p e c t r u m of 5 i C o i n i r o n .

82

FIG. 5. Rotatable target mounting with temperature control

3. Results

T h e M ö s s b a u e r e f f e c t of 5 7 F e on t h e s u r f a c e of p o l y c r y s t a l l i n e t u n g s t e n h a s b e e n m e a s u r e d a t s e v e r a l t e m p e r a t u r e s i n t h e r a n g e of 100-500°K, and a t a n g l e s of 0° and 60° wi th r e s p e c t t o t he s u r f a c e n o r m a l . A l e a s t - s q u a r e s f i t c o m p u t e r a n a l y s i s i n d i c a t e d t h a t t h e M ö s s b a u e r s p e c t r a c o n s i s t e d of t h r e e u n r e s o l v e d l i n e s ( s ee F i g . 6), t h e m i d d l e l ine a g r e e i n g wi th m e a s u r e d s p e c t r a of 5 7 F e in t h e bu lk of t u n g s t e n , and t h e o u t e r two b e i n g a t t r i b u t e d to s u r f a c e e f f e c t s . A c u r s o r y e x p e r i m e n t m a d e on a s i n g l e c r y s t a l s u r f a c e s h o w e d a l m o s t n o e v i d e n c e of t h e c e n t r a l p e a k , t h u s s t r e n g t h e n i n g t h e a s s u m p t i o n t h a t i t w a s due t o d i f f u s i o n of 5 7 F e i n t o t he bu lk .

T h e a s y m m e t r y of t h e s u r f a c e s p e c t r a , t h e a n g u l a r d e p e n d e n c e of t h e s h a p e , a n d t h e m a g n i t u d e of t h e s p l i t t i n g of t h e t w o l i n e s a r e a t t r i b u t e d t o s u r f a c e q u a d r u p o l e s p l i t t i n g . T h e y i n d i c a t e an e l e c t r i c f i e l d g r a d i e n t a t t he n u c l e u s of - (3 . 6 ± 0. 1) X10 1 1 V / c m 2 p e r p e n d i c u l a r to the s u r f a c e if we a s s u m e 0. 2 b f o r t h e q u a d r u p o l e m o m e n t of t h e 3 / 2 s t a t e i n 5 7 F e . If w e f u r t h e r

83

FIG. 6. Mössbauer spectra on polycrystalline W at 100°K (Burton experiment)

a s s u m e a n a n t i s h i e l d i n g f a c t o r of 15, t h e f i e l d g r a d i e n t d u e t o t h e s u r f a c e i s a b o u t - 2 X l O 1 6 V / c m 2 . T h e a v e r a g e r a t i o s of t h e i n t e n s i t i e s of t h e t w o l i n e s a r e 2. 1 ± 0 . 1 a t 0° and 1 . 4 ± 0 . 1 a t 60° . The e x p e c t e d r a t i o s wi th c o r -r e c t i o n s f o r e x p e r i m e n t a l e f f e c t s a r e b e t w e e n 2. 5 and 1. 9 a t 0° and b e t w e e n 0. 9 and 1. 0 at 60° . The d i s c r e p a n c y at 60° m a y be a r e s u l t of the d i f f i cu l t i e s of a n a l y s i n g t h r e e u n r e s o l v e d l i n e s .

T h e i n t e n s i t y of t h e c e n t r a l c o m p o n e n t of t h e u n r e s o l v e d l i n e e x h i b i t e d a t e m p e r a t u r e d e p e n d e n c e c o r r e s p o n d i n g to a De bye t e m p e r a t u r e o f 4 0 3 ± 7 0 ° K , w h e r e a s a n i n d e p e n d e n t m e a s u r e m e n t on 5 7 C o d i f f u s e d i n t o p o l y c r y s t a l l i n e t u n g s t e n g a v e a r e s u l t of 4 0 6 ± 1 2 ° K . T h e p o s i t i o n of t h e c e n t r a l c o m -p o n e n t a g r e e d w i t h t h a t of t h e b u l k m e a s u r e m e n t . T h e c o m b i n e d i n t e n s i t y of t h e o u t e r c o m p o n e n t s of t h e l i n e y i e l d e d a D e b y e t e m p e r a t u r e of 3 4 0 ± 3 0 ° K p e r p e n d i c u l a r t o t h e s u r f a c e , and 2 7 3 ± 3 0 ° K a t 60° f r o m the n o r m a l . Taking the s o l i d a n g l e in to a c c o u n t and e x t r a p o l a t i n g to a n g l e s of 0° and 90° we f ind

84

t he e f f e c t i v e Debye t e m p e r a t u r e t o be 3 5 4 ± 3 0 ° K p e r p e n d i c u l a r t o t he s u r f a c e and 2 5 5 ± 3 0 ° K p a r a l l e l t o t h e s u r f a c e . T h e s e r e s u l t s c o r r e s p o n d to t he f o l -l o w i n g m e a n s q u a r e d i s p l a c e m e n t r a t i o s ( r e c a l l t h a t t h e m e a n s q u a r e d i s -p l a c e m e n t in t he h igh t e m p e r a t u r e l i m i t i s i n v e r s e l y p r o p o r t i o n a l to t he s q u a r e of t h e e f f e c t i v e D e b y e t e m p e r a t u r e )

1 .9 + .4

<*2« > = 1 .3 + .2 a n d — = - 2 — » 2 . 5 + .5

(24)

< x bu lk* < xbulk>

An e x t e n s i o n of t h e " b o n d c o u n t i n g " a n a l y s i s t o a n a t o m on t h e s u r f a c e of a b o d y - c e n t r e d c u b i c l a t t i c e s u c h a s t u n g s t e n y i e l d s

< » y >

< * i >

< * 2 j _ >

s 1.2

» 2 and

(25)

<*2,l >

< x b u l k )

2 . 5

M o r e s o p h i s t i c a t e d a s s u m p t i o n s about e f f e c t i v e bonding in v a r i o u s d i r e c t i o n s i m p r o v e t h e a g r e e m e n t wi th e x p e r i m e n t .

T h e i s o m e r s h i f t w i th r e s p e c t t o i r o n a t 300°K i s 0. 0151 ± 0. 0002 c m / s in bulk, and 0. 018 ± 0 . 002 c m / s on the s u r f a c e . T h e t e m p e r a t u r e dependence of t he s h i f t w a s not d e t e r m i n e d wi th s u f f i c i e n t a c c u r a c y to d e t e c t any s u r f a c e e f f e c t s on t h e s e c o n d - o r d e r D o p p l e r e f f e c t .

D. DISCUSSION

Up t o t h e p r e s e n t t i m e t h e s t u d y of s u r f a c e s w i t h t h e M ö s s b a u e r e f f e c t h a s y i e l d e d m e a g r e i n f o r m a t i o n . F l i n n e t a l . [ l j c a r r i e d out an e x p e r i m e n t t h a t d e m o n s t r a t e d t h e p o s s i b i l i t y of s t udy ing s o l i d s u r f a c e s , but t he i n v e s t i -ga t i on w a s not m a d e u n d e r u l t r a - h i g h v a c u u m c o n d i t i o n s . Al len [2j h a s done s u r f a c e e x p e r i m e n t s i n u l t r a - h i g h v a c u u m w i t h 5 7 F e on t h e r m a l l y - c l e a n e d s i n g l e c r y s t a l s i l i c o n a t r o o m t e m p e r a t u r e a n d a b o v e . H e f i n d s a D e b y e t e m p e r a t u r e o n l y s l i g h t l y l e s s f o r s u r f a c e a t o m s t h a n f o r b u l k a t o m s (@s= 555°K, ©b = 588°K) a n d q u a d r u p o l e s p l i t l i n e s w h i c h s h o w n o a n g u l a r v a r i a t i o n . In c o n t r a s t t o h i s r e s u l t , t h e e x p e r i m e n t d i s c u s s e d h e r e s e e m s t o be e x p l a i n e d by s i m p l e m o d e l s in w h i c h t h e o n l y d i f f e r e n c e b e t w e e n s u r -f a c e and bu lk a t o m s i s t h e c u t t i n g of b o n d s a t t h e s u r f a c e . If one c o n s i d e r s t h e l o w - e n e r g y e l e c t r o n d i f f r a c t i o n ( L E E D ) s t u d i e s of c l e a n s u r f a c e s w e

85

f i nd a s i m i l a r s i t u a t i o n . C l e a n m e t a l s d i s p l a y d i f f r a c t i o n p a t t e r n s in L E E D t h a t a r e c h a r a c t e r i s t i c of s u r f a c e s t r u c t u r e s o b t a i n e d b y m e r e l y c u t t i n g a b u l k c r y s t a l w i t h l i t t l e o r n o a t o m i c r e a r r a n g e m e n t . O n t h e o t h e r h a n d , c e r t a i n s u b s t a n c e s ( f o r e x a m p l e , g e r m a n i u m a n d s i l i c o n a s w e l l a s s o m e c o n t a m i n a t e d m e t a l s ) g i v e e v i d e n c e of r a t h e r d r a s t i c s t r u c t u r a l c h a n g e s a t f r e e s u r f a c e s . T h e r e l a t i v e i n t e n s i t y of s c a t t e r e d e l e c t r o n s i n L E E D i s g i v e n by t h e s a m e f a c t o r f t h a t a p p e a r s i n t h e M ö s s b a u e r e f f e c t . M a c R a e and G e r m e r [17, 18] h a v e d o n e L E E D e x p e r i m e n t s on c l e a n s i n g l e - c r y s t a l n i c k e l t o o b s e r v e s u r f a c e a t o m m e a n s q u a r e d i s p l a c e m e n t s . O n e c a n h o p e t h a t t h e M ö s s b a u e r e f f e c t , L E E D , a n d o t h e r e x p e r i m e n t a l t o o l s w i l l b r i n g a b o u t a n u n d e r s t a n d i n g of w h e n a n d h o w t h e v e r y i n t e r e s t i n g p h e n o m e n o n of a t o m i c r e a r r a n g e m e n t a t s u r f a c e s o c c u r s .

T h e e l e c t r o n i c a n d l a t t i c e v i b r a t i o n a l p r o p e r t i e s of s u r f a c e a t o m s a r e i m p o r t a n t in m a n y p r a c t i c a l s i t u a t i o n s . T h e c h e m i c a l i n d u s t r y m a k e s p r o -f i t a b l e u s e of c a t a l y s i s on s u r f a c e s but c a t a l y s i s i s p o o r l y u n d e r s t o o d . Sol id-s t a t e e l e c t r o n i c d e v i c e s , p a r t i c u l a r l y of the t h in f i l m v a r i e t y , a r e p r o f o u n d l y a f f e c t e d by s u r f a c e p r o p e r t i e s . E v e n a d s o r p t i o n , d i f f u s i o n , a n d c o r r o s i o n ( f o r e x a m p l e , o x i d a t i o n ) on c r y s t a l s u r f a c e s , w h i c h a t a g l a n c e m a y s e e m r a t h e r s i m p l e , a r e l i t t l e u n d e r s t o o d d e s p i t e t h e i r i m p o r t a n c e .

T h e M ö s s b a u e r e f f e c t w i t h i t s a b i l i t y t o m i c r o s c o p i c a l l y s e n s e e l e c t r i c and m a g n e t i c f i e l d s , e l e c t r o n i c c h a r g e d e n s i t i e s , and d e t a i l s of l a t t i c e bonds a p p e a r s t o b e a t o o l w h i c h s h o u l d b e f u r t h e r e x p l o i t e d i n s u r f a c e s t u d i e s . W e h o p e t h a t , d e s p i t e t h e t e c h n i c a l d i f f i c u l t y of s u r f a c e M ö s s b a u e r e x p e r i -m e n t s , a s y s t e m a t i c s t u d y of v a r i o u s t y p e s of c r y s t a l s u r f a c e s w i l l be m a d e .

' R E F E R E N C E S

[ 1 ] FL1NN, P . A . , RUBY, S . L . , KEHL, W . L . , "Mössbauer effect for surface atoms". Science 143 (1964) 1434.

[ 2 ] ALLEN, F . G . , "Mössbauer e f fec t from C o n on a clean silicon surface", Bull. Amer . phys. Soc. 9_ (1964) 296.

[ 3 ] BURTON, J . W . , "The Mössbauer effect of Fesi on the surface of tungsten", Ph.D. thesis. University of Illinois, February 1965.

[ 4 ] MARADUDIN, A. A . , "Lattice dynamical aspects of the resonance absorption of gamma rays by nuclei bound in a crystal", Rev. mod. Phys. 36 (1964) 417.(review paper) .

[ 5 ] WALUS, R . F . , "Surface effects on lat t ice vibrations", Surface Science 2 (1964) 146, Proc. Int. Conf. on the Physics and Chemistry of Solid Surfaces (review paper) .

[ 6 ] WALLIS, R . F . , GAZIS, D . C . , "Surface ef fec ts in the second-oider Doppler shift of the Mössbauer resonance", Phys. Rev. 128 (1962) 106.

[ 7 ] RICH, M . , "Resonant g a m m a absorption near crystal surfaces", Phys. Lett . 4 ( 1 9 6 3 ) 153. [ 8 ] CORCIOVEI, A . , BERINDE, A . , "Sur l ' e f fe t Mössbauer dans les couches minces", J. Phys. Radium,

Paris 24(1963) 89. [ 9 ] CELY, I . , "Der Debye-Waller-Faktor für die Diffraktion langsamer Elektronen", Phys. Stat . Sol. 4

(1964) 521. [10] MARADUDIN, A . A . , MELNGAILIS, J . , "Some dynamical properties of surface atoms". Phys. Rev. 133

(1964) A1188. [11] CLEM, J .R . , GODWIN, R .P . , "Dynamical properties of a one-dimensional 'crystal ' with free ends",

to be published. [12] CLARK, B.C. , HERMAN, R., WALLIS, R.F. , "Theoretical mean-square displacements for surface atoms

in nickel single crystals". Bull. Amer. phys. Soc. 9 (1964) 624; private communication. [13] PINES, D . . Elementary Excitations in Solids, Chap.2, W. A. Benjamin Inc . , New York (1963).

86

[14] VISSCHER, W.M. , Phys. Rev. 129(1963) 28. [15] CLEM, J .R . , private communication. [16] See for example, "Atomic structure of crystal surfaces", Surface Science 2 (1964) 465-566. [17] MacRAE, A . U . , GERMER, L .H. , "Thermal vibrations of surface atoms", Phys. Rev. Lett. 8 (1962) 489. [18] MacRAE, A . U . , "Surface atom vibrations", Surface Science 2 (1964) 522.

D I S C U S S I O N

G. K. W E R T H E I M r e p o r t e d t h a t A l l e n h a d found ou t t h a t t he e v a p o r a t e d m a t e r i a l in h i s e x p e r i m e n t s c o n s i s t e d of m a n y m o n o l a y e r s . He a s k e d w h e t h e r t h e e x p e r i m e n t i n d e e d u s e d l e s s t h a n a m o n o l a y e r .

H. F R A U E N F E L D E R a n s w e r e d t h a t t h i s q u e s t i o n co u l d not be a n s w e r e d u n a m b i g u o u s l y . H o w e v e r , t h e o b s e r v e d a n g u l a r d i s t r i b u t i o n and t h e c h a n g e s t h a t o c c u r r e d w h e n t h e s u r f a c e w a s h e a t e d w e r e a t l e a s t s o m e e v i d e n c e t h a t a t r u e s u r f a c e e f f e c t h a d b e e n s e e n .

J . D A N O N r e p o r t e d t h a t h e r e p e a t e d M ö s s b a u e r m e a s u r e m e n t s w i t h 5 1 F e o n A I 2 O 3 bu t o n e c o u l d no t b e c e r t a i n t h a t t h e i r o n w a s r e a l l y a t t h e a l u m i n a s u r f a c e .

M. C O R D E Y - H A Y E S p o i n t e d out t h a t t h e c a l c u l a t i o n s m a d e f o r s u r f a c e c o n d i t i o n s m i g h t a l s o b e u s e f u l f o r a p p l i c a t i o n s t o l o n g l i n e a r m o l e c u l e s .

P . K I E N L E s u g g e s t e d t h a t s u r f a c e s m i g h t a l s o b e s t u d i e d b y u s i n g s c a t t e r i n g e x p e r i m e n t s of t h e t y p e c a r r i e d ou t by B e r n s t e i n a n d C a m p b e l l a t O a k R i d g e .

V . l . G O L D A N S K I I a s k e d w h e t h e r d i f f u s i o n i n t o t h e b u l k a n d o x i d a t i o n h a d b e e n o b s e r v e d .

H . F R A U E N F E L D E R a n s w e r e d t h a t d i f f u s i o n i n t o t h e b u l k h a d i n d e e d b e e n s e e n bu t t h a t o x i d a t i o n h a d no t b e e n l o o k e d a t .

G. K. W E R T H E I M s u g g e s t e d c o m b i n i n g M ö s s b a u e r e f f e c t and l o w - e n e r g y e l e c t r o n d i f f r a c t i o n to s t u d y the s a m e s u r f a c e .

87

STRUCTURAL INVESTIGATIONS

(Sess ion 3)

MÖSSBAUER E F F E C T AND CHEMICAL BONDING IN TRANSITION METAL COMPLEXES

J. DANON

CENTRO BRASILEIRO DE PESQUISAS FISICAS RIO DE JANEIRO, BRAZIL

T h e M ö s s b a u e r e f f e c t i s t h e r e c o i l l e s s e m i s s i o n and r e s o n a n t a b s o r p t i o n of 7 - r a d i a t i o n . T h i s p h e n o m e n o n i s b a s e d o n t h e f a c t t h a t a l a r g e f r a c t i o n of 7 - r a d i a t i o n i s e m i t t e d w i t h o u t i n d i v i d u a l r e c o i l f r o m n u c l e i bound in s o l i d s and i n c o n s e q u e n c e t h e p h o t o n s a r e i n a c o n d i t i o n t o b e r e s o n a n t l y s c a t t e r e d b y n u c l e i i n t h e g r o u n d s t a t e . B e c a u s e of t h e n a r r o w l i n e - w i d t h of t h e n u c l e a r t r a n s i t i o n s i n v o l v e d in t h e M ö s s b a u e r e f f e c t ( abou t 10"s eV) , t h e r e -s o n a n t a b s o r p t i o n i s e x t r e m e l y s e n s i t i v e t o e n e r g y v a r i a t i o n s of t h e i n c i d e n t 7 - r a d i a t i o n . F o r t h i s r e a s o n s m a l l i n t e r a c t i o n s b e t w e e n t h e n u c l e u s and t h e o r b i t a l e l e c t r o n s , w h i c h a r e d i f f i c u l t t o o b s e r v e b y u s u a l m e t h o d s , m a n i f e s t t h e m s e l v e s v e r y m a r k e d l y in t h e M ö s s b a u e r e f f e c t [ 1 - 4 ] .

I t i s t h u s t h e i n f l u e n c e of t h e c h e m i c a l e n v i r o n m e n t o n t h e 7 - r a d i a t i o n e m i s s i o n and a b s o r p t i o n n u c l e a r t r a n s i t i o n t h a t c o n s t i t u t e s t h e b a s i s of a p p l i -c a t i o n s of t h e M ö s s b a u e r e f f e c t t o c h e m i c a l i n v e s t i g a t i o n s [5, 6 ] . '

A l t h o u g h t h e M ö s s b a u e r e f f e c t h a s b e e n o b s é r v e d i n m o r e t h a n 30 d i f -f e r e n t n u c l i d e s , i t s u s e in ch femica l i n v e s t i g a t i o n s h a s b e e n l i m i t e d e s s e n t i a l l y t o i r o n a n d t i n a s a c o n s e q u e n c e of t h e f a v o u r a b l e n u c l e a r p r o p e r t i e s e x h i b i t e d by the a v a i l a b l e i s o t o p e s of t h e s e e l e m e n t s .

T o i n v e s t i g a t e t h e p o s s i b i l i t i e s of t h e M ö s s b a u e r e f f e c t a s a t o o l in c h e -m i c a l s p e c t r o s c o p y i t i s n e c e s s a r y t o c o m p a r e t h e i n f o r m a t i o n t h a t c a n b e

' d e r i v e d b y t h i s m e t h o d w i t h t h a t o b t a i n e d b y m o r e e s t a b l i s h e d o n e s . T h e c o v a l e n t c o m p l e x e s of t r a n s i t i o n m e t a l s a p p e a r t o b e t h e s y s t e m s f o r w h i c h t h i s c o m p a r i s o n c a n b e m a d e i n m o r e f a v o u r a b l e c o n d i t i o n s . O n t h e o n e h a n d , t h e b o n d i n g i n t h e s e c o m p l e x e s h a s b e e n t h e o b j e c t of m u c h i n v e s t i -g a t i o n b y s t r u c t u r a l , o p t i c a l a n d m a g n e t i c m e t h o d s [ 7 - 1 1 ] . O n t h e o t h e r h a n d , t h e M ö s s b a u e r e f f e c t w i t h c o v a l e n t c o m p l e x e s of i r o n e x i b i t s a n u m b e r of f e a t u r e s w h i c h i n d i c a t e a m a r k e d i n t e r a c t i o n of t h e c h e m i c a l e n v i r o n m e n t w i t h t h e n u c l e a r t r a n s i t i o n r e s p o n s i b l e f o r t h e r e c o i l l e s s 7 - r a y e m i s s i o n .

T H E H Y P E R F I N E I N T E R A C T I O N S IN T H E M Ö S S B A U E R E F F E C T

F o r o u r p u r p o s e t h e m o s t i m p o r t a n t h y p e r f i n e i n t e r a c t i o n s w h i c h m a n i -f e s t t h e m s e l v e s i n t h e M ö s s b a u e r e f f e c t a r e t h e n u c l e a r i s o m e r s h i f t a [12] a n d t h e n u c l e a r q u a d r u p o l e c o u p l i n g Д Е [13 ] .

89

T h e i s o m e r s h i f t , w h i c h m a n i f e s t s i t s e l f a s a s h i f t f r o m z e r o v e l o c i t y of t h e c e n t r o i d of t h e r e s o n a n c e s p e c t r u m , i s due t o t h e e l e c t r o s t a t i c i n t e r -a c t i o n b e t w e e n the c h a r g e d i s t r i b u t i o n of t h e n u c l e u s and the e l e c t r o n s wh ich h a v e a f i n i t e p r o b a b i l i t y a t t h e r e g i o n of t h e n u c l e u s .

T h e n u c l e a r i s o m e r s h i f t i s the ' n u c l e a r s h i f t a n a l o g o u s t o t h e t w o w e l l - k n o w n h y p e r f i n e i n t e r a c t i o n s o b s e r v e d i n t h e o p t i c a l s p e c t r u m : t h e i s o t o p e s h i f t a n d t h e i s o m e r s h i f t [14] . T h e s e e f f e c t s a r e c h a n g e s i n w a v e -l e n g t h of s p e c t r a l t e r m s of d i f f e r e n t i s o t o p e s and i n i s o m e r i c s t a t e s of one i s o t o p e .

T h e s e p h e n o m e n a a r e d u e t o c h a n g e s in n u c l e a r d i m e n s i o n s wi th a c o n -s e q u e n t a l t e r a t i o n of t h e e l e c t r o s t a t i c i n t e r a c t i o n w i t h t h e e l e c t r o n i c c l o u d a t t h e n u c l e a r r e g i o n . Only s e l e c t r o n s h a v e a f i n i t e p r o b a b i l i t y a t t he r e g i o n of t h e n u c l e u s , a l t h o u g h r e l a t i v i s t i c c o r r e c t i o n s i n t r o d u c e a p j e l e c t r o n d e n s i t y a t h e a v y n u c l e i .

T h e n u c l e a r i s o m e r s h i f t a s m e a s u r e d b y t h e M ö s s b a u e r e f f e c t c a n b e e x p r e s s e d a s a D o p p l e r e n e r g y :

a = F ( Z ) [ k s ( 0 ) | f - ^ s ( 0 ) | 2 ] (1)

w h e r e F ( Z ) and A R / R d e p e n d on c h a r g e a n d r a d i u s of t h e g r o u n d and e x c i t e d n u c l e a r s t a t e s r e s p e c t i v e l y . T h e d i f f e r e n c e in p r o b a b i l i t i e s i n b r a c k e t s a l l o w s t h e m e a s u r e m e n t of t h e t o t a l s - e l e c t r o n d e n s i t y of a g i v e n a b s o r b e r r e l a t i v e t o t h a t of a g i v e n s o u r c e .

It m u s t b e p o i n t e d out t h a t ф£(0) m e a s u r e s t h e t o t a l c h a r g e d e n s i t y and i s no t t o b e c o n f u s e d wi th t h e m o r e f r e q u e n t m e a s u r e m e n t s of s p i n d e n s i t y . A p a r t f r o m t h e t r i v i a l c a s e s i t i s not a n e a s y m a t t e r t o c o r r e l a t e t h e s e t w o t y p e s of m e a s u r e m e n t s .

T h e n u c l e a r q u a d r u p o l e i n t e r a c t i o n i s due t o t h e c o u p l i n g of t h e q u a d r u p o l e m o m e n t Q of t h e n u c l e u s wi th an e l e c t r i c f i e l d g r a d i e n t q a t t he r e g i o n of t he n u c l e u s . It m a n i f e s t s i t s e l f a s t h e s p l i t t i n g of t h e r e s o n a n c e s p e c t r u m i n two o r m o r e t r a n s i t i o n s a r i s i n g f r o m t h e l i f t i n g of t h e d e g e n e r a c y of t h e n u c l e a r - s p i n e x c i t e d and g r o u n d s t a t e s . F o r t r a n s i t i o n s f r o m 1 = 3 / 2 t o 1 / 2 , s u c h a s wi th 5 7 F e and 1 1 9 S n m , t h e s p e c t r u m s h o w s t w o r e s o n a n c e p e a k s . T h e v e l o c i t y i n t e r v a l b e t w e e n t h e s e p e a k s c o r r e s p o n d s t o a D o p p l e r e n e r g y w h o s e e x p r e s s i o n i s

Д Е = | e 2 q Q [1 + i n 2 l i (2)

w h e r e 17 i s t h e a s y m m e t r y p a r a m e t e r of t h e f i e l d g r a d i e n t d e f i n e d a s t h e r a t i o of t h e t e n s o r c o m p o n e n t s 17 = ( V x x ~ Vyy ) / V Z 2 .

T h e m a g n i t u d e and s i g n of t h e f i e l d g r a d i e n t a and i t s a s y m m e t r y p a r a -m e t e r 17 c a n b e r e l a t e d t o t h e e l e c t r o n i c s t r u c t u r e of t h e e n v i r o n m e n t of t h e n u c l e u s . T o d e r i v e t h i s i n f o r m a t i o n f r o m t h e M ö s s b a u e r e f f e c t t h e » m e a s u r e m e n t s m u s t be e f f e c t e d with s i n g l e c r y s t a l s of known s t r u c t u r e . T h e m o r e f r e q u e n t s p e c t r a o b t a i n e d w i t h p o l y c r y s t a l l i n e s a m p l e s o n l y g i v e t h e a b s o l u t e v a l u e of t h e q u a d r u p o l e i n t e r a c t i o n .

T h e g r a d i e n t i s a m e a s u r e of t h e d e v i a t i o n f r o m s p h e r i c a l o r c u b i c s y m m e t r y of t h e c h a r g e s e x t e r n a l t o t he n u c l e u s . C o n t r i b u t i o n s to t h e f i e ld

90

g r a d i e n t c o m e f r o m i n n e r e l e c t r o n s h e l l s w h i c h a r e p a r t l y f i l l e d and f r o m e x t e r n a l c h a r g e s of t h e l a t t i c e .

F i l l e d o r h a l f - f i l l e d s h e l l s h a v e s p h e r i c a l s y m m e t r y a n d g i v e no c o n t r i -bu t ion t o t h e f i e l d g r a d i e n t ; f o r t h e s e c a s e s the m a i n c o n t r i b u t i o n c o m e s f r o m c h a r g e s i n t h e l a t t i c e . If t h e p a r t l y f i l l e d s h e l l d o e s not p o s s e s s s p h e r i c a l s y m m e t r y t h e n it g i v e s t h e m a i n c o n t r i b u t i o n t o t h e f i e l d g r à d i e n t . T h i s i s t h e m o s t i n t e r e s t i n g s i t u a t i o n s i n c e a d i r e c t c o r r e l a t i o n e x i s t s b e t w e e n t h e f i e l d g r a d i e n t and t h e e l e c t r o n i c s t r u c t u r e of t h e p a r t l y f i l l e d s h e l l .

P e r t u r b a t i o n of t h e i n n e r e l e c t r o n s h e l l s ' s p h e r i c a l s y m m e t r y by t h e f i e l d g r a d i e n t i t s e l f c a n be t a k e n in to a c c o u n t t h r o u g h the S t e r n h e i m e r a n t i -s h i e l d i n g f a c t o r s [13] .

C H E M I C A L BONDING IN TRANSITION M E T A L C O M P L E X E S

T o i n v e s t i g a t e t he c o n n e c t i o n s b e t w e e n t h e h y p e r f i n e i n t e r a c t i o n s of t h e M ö s s b a u e r e f f e c t a n d t h e b o n d i n g i n c o m p l e x e s of t r a n s i t i o n m e t a l i o n s w e s h a l l c o n s i d e r s o m e a s p e c t s a n d r e s u l t s of. t h e m a i n t h e o r e t i c a l d e s c r i p -t i o n s of c h e m i c a l b o n d i n g i n t h e s e c o m p l e x e s .

I t i s k n o w n t h a t t h e v a l e n c e b o n d t h e o r y g e n e r a l l y p r e d i c t s c o r r e c t l y t h e g e o m e t r i c a l s t r u c t u r e and m a g n e t i c p r o p e r t i e s of c o m p l e x e s . T h e s p d h y b r i d i z a t i o n s c h e m e h a s b e e n s e v e r e l y c r i t i c i z e d , h o w e v e r , m a i n l y b y s p e c t r o s c o p i s t s , i n v i e w of t h e d i f f i c u l t i e s e n c o u n t e r e d in e x p l a i n i n g e l e c t r o n i c t r a n s i t i o n s i n t h e s e c o m p l e x e s L7 ,10J . T h e r e i s , h o w e v e r , o n e a s p e c t t o t h e v a l e n c e b o n d t h e o r y w h i c h c a n b e u s e f u l f o r t h e p r e s e n t i n v e s t i g a t i o n : i t i s t he d i s t i n c t i o n b e t w e e n o u t e r - s h e l l h y b r i d i z a t i o n 4 s 4 p 3 4 d 2 , wh ich would o c c u r i n " i o n i c " c o m p l e x e s , a n d i n n e r - s h e l l h y b r i d i z a t i o n 3 d 2 4 s 4 p 3 w i t h the " c o v a l e n t " c o m p l e x e s . A l though t h i s d e s c r i p t i o n can be b e t t e r e x p r e s s e d i n a n o t h e r w a y , i t h a s t h e m e r i t of s t r e s s i n g t h e d i f f e r e n c e s b e t w e e n t h e t ype of d o r b i t a l e u s e d f o r b o n d i n g . T h i s c a n h e l p in u n d e r s t a n d i n g t h e d i f -f e r e n t m e c h a n i s m s of s h i e l d i n g of t he i n n e r e l e c t r o n i c s h e l l s of t he t r a n s i t i o n ion .

C r y s t a l f i e l d t h e o r y i n t r o d u c e d , a s a f u n d a m e n t a l c o n t r i b u t i o n , t he d i s -t i n c t i o n b e t w e e n t w o g r o u p s of o r b i t a l e b y l i f t i n g t h e d e g e n e r a c y of t h e 3d O r b i t a l s in a f i e l d of c u b i c s y m m e t r y : l g o r dy(d x2.yi , d z s ) and t2g o r d e (d X y , d x z , dyZ) . T h e p a r a m e t e r 10 Dq m e a s u r e s t h e s e p a r a t i o n in e n e r g y b e t w e e n t h e s e two t y p e s of o r b i t a l s .

T h e lg o r b i t a l s a r e of c r - type a n d t^g of 7r - type . C o m p a r i s o n b e t w e e n t h e v a l u e of 10 Dq and e x c h a n g e i n t e r a c t i o n s of t h e e l e c t r o n s a l l o w s the d i s -t i n c t i o n of t he t w o l i m i t i n g c a s e s : h i g h - s p i n c o n f i g u r a t i o n w h e n t h e s p i n a r r a n g e m e n t of t he 3d e l e c t r o n s f o l l o w s H u n d ' s r u l e of m a x i m u m mul t ip l i c i t y , and l o w - s p i n c o n f i g u r a t i o n w h e n t h e v a l u e of 10 Dq s u r p a s s e s t he m a g n i t u d e of e x c h a n g e i n t e r a c t i o n s ' a n d t h e 3d o r b i t a l s a r e f i l l e d w i t h p a i r s of e l e c -t r o n s w i t h o p p o s e d s p i n s .

F o r f i e l d s w i t h s y m m e t r y l o w e r t h a n c u b i c t h e r e i s a f u r t h e r l i f t i n g of d e g e n e r a c y of t h e l o w e r t r i p l e t and u p p e r d o u b l e t . T h e d e s c r i p t i o n of t h e a r r a n g e m e n t of t h e e l e c t r o n s i n t h e d i f f e r e n t 3d l e v e l s t h u s d e p e n d s on d e -t a i l e d k n o w l e d g e of t h e s y m m e t r y of t h e e l e c t r i c c h a r g e s s u r r o u n d i n g t h e t r a n s i t i o n i o n . It i s p o s s i b l e , a s we s h a l l s e e b e l o w , t o i n t e r p r e t s e v e r a l r e s u l t s f r o m M ö s s b a u e r h y p e r f i n e i n t e r a c t i o n s i n t he l ight of t h e s e s c h e m e s .

91

It i s , h o w e v e r , t h e f u n d a m e n t a l c o n c e p t of c o v a l e n c y w h i c h m u s t b e i n t r o d u c e d f o r a m o r e c o m p l e t e d e s c r i p t i o n of t h e c h a n g e s in i s o m e r s h i f t a n d t h e q u a d r u p o l e c o u p l i n g in c o m p l e x e s . T h e m o l e c u l a r o r b i t a l (MO) t h e o r y [15] d e a l s w i t h t h i s c o n c e p t i n a n a p p r o p r i a t e w a y .

T h e m o l e c u l a r o r b i t a l e f o r a m e t a l c o m p l e x h a v e t h e g e n e r a l f o r m

ф(МО) = С м Ф ( М ) + С ь Ф ( Ь ) (3)

w h e r e t h e v a l u e s of C M and Q, a r e s u b j e c t e d t o n o r m a l i z a t i o n and o r t h o g o -n a l i t y c o n d i t i o n s .

F o r t h e f i r s t s e r i e s of t r a n s i t i o n m e t a l s t h e v a l e n c e o r b i t a l s Ф(М) a r e t h e 3d , 4 s and 4p a n d Ф(Ь) l i g a n d o r b i t a l c o m b i n a t i o n s w h i c h p o s s e s s t h e a p p r o p r i a t e s y m m e t r y f o r c o m b i n i n g w i t h t h e m e t a l o r b i t a l .

F i g u r e 1 s h o w s a c o n v e n i e n t c o - o r d i n a t e s y s t e m f o r a and ж b o n d i n g i n a n M L g o c t a h e d r a l c o m p l e x .

T ä b l e I g i v e s t h e p r o p e r m e t a l and l i g a n d o r b i t a l c o m b i n a t i o n s f o r s u c h t y p e s of c o m p l e x e s .

F i g u r e 2 r e p r o d u c e s t h e M O e n e r g y l e v e l s s c h e m e p r o p o s e d f o r a n i o n i c c o m p l e x [15] of t he t y p e of F e F ¿ 3 . T h e bond ing o r b i t a l s a r e f i l l ed wi th p a i r s of e l e c t r o n s . T h e v a l u e of 10 Dq i s g i v e n by t h e d i f f e r e n c e in e n e r g y b e t w e e n t h e l g (c r*) a n t i b o n d i n g and t2g(Tr*) a n t i b o n d i n g .

FIG. 1. Co-ordination system for a and тг bonding in an ML6 octahedral complex

It i s now i m p o r t a n t t o c o n s i d e r t h o s e MOs which , b e s i d e s the m i x i n g wi th f i l l e d l i gand o r b i t a l s , i nc lude m i x i n g of m e t a l o r b i t a l s with e m p t y l igand 7г o r b i t a l s . T h e CN" l i g a n d i s a t y p i c a l e x a m p l e , and F i g . 3 s h o w s d i a g r a -m a t i c a l l y t h e o r b i t a l s of t h i s l i g a n d a v a i l a b l e f o r b o n d i n g . I t c a n b e s e e n t h a t t h e тг* a n t i b o n d i n g of CN" i s e m p t y a n d c a n o v e r l a p w i t h o n e of t h e t2g of t h e m e t a l .

T h i s p a r t i c i p a t i o n of e m p t y l i gand o r b i t a l s in t he тг MO l o c a l i z e d e s s e n -t i a l l y on t h e m e t a l i s c a l l e d b a c k - d o n a t i o n . T h i s i m p o r t a n t c o n c e p t h a s f r e q u e n t l y b e e n u s e d f o r t h e i n t e r p r e t a t i o n of p r o p e r t i e s of l o w - s p i n t r a n -s i t i o n m e t a l c o m p l e x e s [16 , 17] .

T h e i m p l i c a t i o n s of b a c k - d o n a t i o n f o r d i f f e r e n t p r o p e r t i e s of c y a n i d e c o m p l e x e s of t r a n s i t i o n m e t a l s w e r e r e c e n t l y i n v e s t i g a t e d b y S h u l m a n a n d S u g a n o [18] .

92

TABLE I

P R O P E R M E T A L AND LIGAND O R B I T A L COMBINATIONS F O R O C T A H E D R A L C O M P L E X E S

Representations Metal orbital:

Ligand orbital combinations

»lg M

, e g (o )

tiu(o,ir)

l 2g M

t2u С )

t l g W

u22

dx2-y2

Px

Py

P?

d

xz

'dyz dxy

— (0l+ 02+ О3+ O4+ O5+ Oj)

~j r (2o s + 2o6- o¡ - o¡- a 3 - a4)

í ( O r O2+O3-O4)

"/2 i ( V ÏÏX4_ V

72(°г-°4). i "yj" V V

•^(05-Об), г СУ1+ %2" v

W

i c w V V

i ( V V V ^ P

i C y r W ' y P i ( W W i ( "уГ l r X 2 " i ï X 3 + V

i í V V V y í

i W W

F r o m t h e i r f i n d i n g s we c o n s i d e r h e r e t h e t h r e e M O s w h i c h a r e f o r m e d f r o m d x y , d X z . d y z e l e c t r o n s and t h e тг b o n d i n g a n d ж* a n t i b o n d i n g o r b i t a l e f r o m C N - .

With t he u s u a l a p p r o x i m a t i o n s of the L C A O t h e o r y we can w r i t e f o r t h e s e o r b i t a l e :

tf'l = N - t ( X i + T i $

ф 2 = N - H < P ~ \ X ! +72X2) (4)

ф3 = N -Цх 2 -Х 2 <р)

93

METAL MOLECULAR ORBITALS ORBITALS ORBITALS

'kl'*, «*>

/ »t, I«'' \ / /

/ í » / n l ' 'H ( (n.ll« «,!"•! ,11

^ \\-1 : 1 ty f >2."l''lli»' V .I») t V

к 4Vi - f u \ ' U «

4 t *

% . . . . . .y

FIG. 2. Molecular orbital energy level diagram for octahedral meta l complexes

F I G . 3 . V a c a n t ( т г * 2 р ) O r b i t a l s a v a i l a b l e f o r b a c k - d o n a t i o n o f d e e l e c t r o n s f r o m t h e m e t a l

w h e r e x-| and x ? a r e l i n e a r c o m b i n a t i o n s of тг and n ^ c y a n i d e o r b i t a l s , and (p i s t h e p r o p e r m e t a l d f u n c t i o n (d€ o r b i t a l ) . T h e f o l l o w i n g r e l a t i o n s a r e r e q u i r e d f o r m u t u a l o r t h o g o n a l i t y of t h e o r b i t a l s :

X I = 7 J + S I (5)

X2= 72+ S 2 (6)

w h e r e S a r e o v e r l a p i n t e g r a l s b e t w e e n <p and x o r b i t a l s . S h u l m a n a n d S u g a n o [18] h a v e s h o w n t h a t t h e b a c k - d o n a t i o n a l t e r s t h e

p a r a m e t e r 10 Dq b y a q u a n t i t y Д e w h i c h c a n b e e x p r e s s e d i n f i r s t a p p r o x i -m a t i o n a s :

Д е = W t l b l x d - s L 6 t ] * ( 7 )

w h e r e фх. a n d e t r e p r e s e n t t h e a n t i b o n d i n g t fg o r b i t a l a n d i t s e n e r g y i n t h e a b s e n c e of b a c k - d o n a t i o n ; e * t h e o r b i t a l e n e r g y of IT* a n t i b o n d i n g a n d ; h a H a r t r e e - F o c k H a m i l t o n i a n . S i n c e e f l i e s a b o v e e t , Д е i s p o s i t i v e . I n

94

c o n s e q u e n c e t h e b a c k - d o n a t i o n s t a b i l i z e s t h e t ^ o r b i t a l a n d i n c r e a s e s t h e v a l u e of 10 D q .

T h i s r e s u l t i s i n a g r e e m e n t w i t h t h e MO d i a g r a m p r o p o s e d b y G r a y and B e a c h [19] f o r t h e h e x a c y a n o c o m p l e x e s of t r a n s i t i o n m e t a l s ( F i g . 4 ) .

OBBIWLS MOLECULAR ORBITALS ORBITALS

•ijlo"! '2.

i \ ll4 ) <1Ч-1И .У ['<«2.>2.

ЧУ Vv,

"Л j \

i \ ->4 V

t*

1 ' V

У S

Mi1

V 'i.'*"'. 'Ii'*'1

-V >

№ t„i*»> in 4 H

Mi,

Ai ом.

FIG. 4. Molecular orbital energy level diagram from octahedral meta l complexes containing ligands which have тгЬ and relatively stable IT * orbitals

T h e p u r e ж m o l e c u l a r o r b i t a l s a r e f o r m e d b y n d x y , n d X Z ) n d y z m e t a l o r b i t a l s and the t 2 g c o m b i n a t i o n s of and ir* l i g a n d m o l e c u l a r o r b i t a l s . T h e i n s t a b i l i t y o r d e r of t h e c o m b i n a t i o n ж o r b i t a l s i s a l w a y s 7rb(L) < n d u < jr*(L) . T h u s t h e r e a r e f o r m e d t h r e e d e g e n e r a t e s t r o n g l y b o n d i n g ж M O s m a i n l y l o c a l i z e d on t h e CN~; t h r e e v i r t u a l l y n o n - b o n d i n g M O s m a i n l y l o c a l i z e d o n t h e m e t a l ; and t h r e e s t r o n g l y a n t i - b o n d i n g M O s m a i n l y l o c a l i z e d on t h e CN".

A s h a s b e e n p o i n t e d out b y B a l l h a u s e n and G r a y [20], t he m a i n p r o b l e m f o r M O s of d i s t o r t e d o c t a h e d r a l c o m p l e x e s MX5Y i s d e s c r i b i n g how t h e t h r e e d £ o r b i t a l s of ж s y m m e t r y a r e d i s t r i b u t e d a m o n g the l i g a n d s .

T h e M O s f o r d i s t o r t e d o c t a h e d r a l c o m p l e x e s w i t h t e t r a g o n a l s y m m e t r y w e r e e s t a b l i s h e d o n t h e b a s i s of a r u l e a c c o r d i n g t o w h i c h n e a r l y a l l t h e ж b o n d i n g i s a x i a l l y d i r e c t e d a n d i n v o l v e s t h e m e t a l d x z a n d dyZ o r b i t a l s . T h e s t r o n g e r a x i a l ж b o n d i n g m a y b e e i t h e r M - X o r M - L , d e p e n d i n g on w h e t h e r t h e ж-orbital e n e r g i e s of X o r L m o r e c l o s e l y a p p r o x i m a t e t h e m e t a l d - o r -b i t a l e n e r g i e s . It i s a good a p p r o x i m a t i o n t o n e g l e c t p l a n a r ж b o n d i n g and a p p r o x i m a t e t h e m e t a l d x y o r b i t a l a s n o n - b o n d i n g .

T h e MO d i a g r a m p r o p o s e d by G r a y , B e r n a i and B i l l i ng [21] f o r c a r b o n y l a n d n i t r o s y l c o m p l e x e s i s r e p r o d u c e d i n F i g . 5 . I t s m a i n f e a t u r e s a r e : a s t r o n g a bond ( a i ) b e t w e e n t h e N ( o r C) 2s o r b i t a l and t h e ndZ2 m e t a l o r b i t a l ; t w o s t r o n g ж b o n d s (e) i n v o l v i n g t h e m e t a l n d x z a n d n d y z o r b i t a l s a n d t h e NO(CO) jT(2px, 2py) a n t i b o n d i n g o r b i t a l s ; a a b o n d (bi) b e t w e e n t h e m e t a l ndx 2 .y2 o r b i t a l a n d t h e {01-02 + 03-04) l i g a n d o r b i t a l c o m b i n a t i o n ; t h e n d x y

95

METAL ORBITALS

LIGAND ORBITALS

ANTIBONDING a LEVELS

( n . l i p / !

(n+ l ï s } 1 »i(o*"l [ n< j , l ] U / / M / / b o**) [ndx2_y2 ) • I > V ' \ » /

»*(NO.CO, « t e l

F b.Ind,

Ц V \ .

\\\ 4 s N « ' ( N O , CO, « te l l>

' Л 0-INO, CO. «tel / / <

1 \ «HT'l |NO,CO| / / / 1 ^ ' // BONDING в LEVELS

FIG. 5. Molecular orbital energy level scheme for ML5CO or ML5NO complexes

m e t a l o r b i t a l i s n o n - b o n d i n g if m e t a l L ж b o n d i n g i s i g n o r e d ; f i n a l l y , t h e m e t a l (n + l ) s and (n + l ) p o r b i t a l s f o r m f o u r m o r e cr b o n d s ( a j , a 1 ; e) w i t h t h e l i g a n d o r b i t a l s .

T h e r e i s r e a s o n a b l e e v i d e n c e s u p p o r t i n g t h e o r d e r i n g of M O s o f t h i s b o n d i n g s c h e m e , p a r t i c u l a r l y i n r e s p e c t of t h e t f g s h e l l . S e v e r a l r e s u l t s c o n f i r m t h e e x i s t e n c e of u n u s u a l l y s t r o n g ж b o n d i n g b e t w e e n t h e m e t a l and t h e n i t r o s y l l i g a n d [22] . T h e o r d e r of t h e a n t i b o n d i n g o r b i t a l s , h o w e v e r , h a s b e e n t h e o b j e c t of d i s c u s s i o n [23 , 24] .

O t h e r a p p r o a c h e s t o c o v a l e n t bonding , s u c h a s t h e n e p h e l a u x e t i c e f f e c t [25] , e m p h a s i z e t h e a s p e c t s of t h e b o n d i n g c o n n e c t e d w i t h t h e a v e r a g e s p r e a d i n g of t h e d s h e l l . W h e r e a s t he MO t r e a t m e n t d e a l s wi th t he e x t e r n a l p r o p e r t i e s of t he w a v e - f u n c t i o n s (on t h e r e g i o n of o v e r l a p be tween t h e m ) ; the n e p h e l a u x e t i c e f f e c t c a n b e c o r r e l a t e d w i t h s o m e i n n e r p r o p e r t i e s of t h e s e f u n c t i o n s , w h i c h a r e of c a p i t a l i m p o r t a n c e f o r t h e h y p e r f i n e i n t e r a c t i o n s b e t w e e n e l e c t r o n s a n d n u c l e u s .

9 6

T h e n e p h e l a u x e t i c e f f e c t i s t h e d e c r e a s e of t h e p a r a m e t e r s of i n t e r -e l e c t r o n i c r e p u l s i o n of t h e p a r t l y f i l l e d s h e l l of t h e c o m p l e x i n r e s p e c t t o t h e f r e e i o n . C . K . J ^ r g e n s e n d i s t i n g u i s h e s t w o c a u s e s f o r t h e n e p h e l a u x e t i c e f f e c t : t h e c e n t r a l f i e l d c o v a l e n c y a n d t h e s y m m e t r y r e s t r i c t e d c o v a l e n c y . I n c e n t r a l f i e l d c o v a l e n c y t h e r a d i a l p a r t of t he p a r t l y f i l l e d s h e l l i s o n l y c h a n g e d b y t h e p r e s e n c e of m o r e n e g a t i v e c h a r g e , s c r e e n i n g t h e n u c l e u s f r o m the p a r t l y f i l l e d s h e l l , in t h e c o m p l e x ion t h a n i n t he g a s e o u s i on . T h e s y m m e t r y r e s t r i c t e d c o v a l e n c y c o r r e s p o n d s t o t h e d e r e a l i z a t i o n of e l e c - . t r o n s of t h e p a r t l y f i l l e d s h e l l in M O s by t h e i r i n t e r m i x i n g wi th l i g a n d o r b i t a l s .

O n t h e b a s i s of t h e n e p h e l a u x e t i c e f f e c t i t i s p o s s i b l e t o e s t i m a t e t h e f r a c t i o n a l c h a r g e s o n t h e c e n t r a l i o n a n d t h e a v e r a g e r a d i u s of t h e p a r t l y f i l l e d s h e l l f o r d i f f e r e n t c o m p l e x e s . A s we s h a l l s e e , ' t h e s e r e s u l t s c a n b e p r o f i t a b l y u s e d f o r t h e u n d e r s t a n d i n g of the r e l a t i o n b e t w e e n c h e m i c a l bonding and h y p e r f i n e i n t e r a c t i o n s .

N U C L E A R ISOMER SHIFT AND C H E M I C A L BONDING

A s m e n t i o n e d b e f o r e , t h e e l e c t r o n i c c o n t r i b u t i o n t o t he i s o m e r s h i f t i s g i v e n by t h e t o t a l s - e l e c t r o n d e n s i t y a t the n u c l e u s .

F o r a n e l e m e n t s u c h a s i r o n t h e c o n t r i b u t i o n f r o m t h e b o n d i n g i s o n l y a v e r y s m a l l p a r t of t h e t o t a l s - e l e c t r o n d e n s i t y , t h e m a i n c o n t r i b u t i o n c o m i n g f r o m t h e f i l l e d i n n e r s - e l e c t r o n o r b i t a l s .

L e t u s f i r s t a n a l y s e t h e i n f o r m a t i o n w h i c h c a n b e d e r i v e d f o r t h e f r e e i r o n i o n t h a t h a s f i l l e d I s + 2 s + 3 s o r b i t a l s w i t h p a i r s of e l e c t r o n s .

T a b l e II g i v e s t h e v a l u e s a t t h e n u c l e u s of I s , 2s and 3 s r e s t r i c t e d H a r t r e e - F o c k w a v e - f u n c t i o n s c a l c u l a t e d b y W a t s o n 126, 27] f o r d i f f e r e n t d c o n f i g u r a t i o n s . It c a n b e s e e n t h a t t h e t o t a l s - e l e c t r o n d e n s i t y d i f f e r s f o r t h e v a r i o u s . d n c o n f i g u r a t i o n s ; t h e d i f f e r e n c e , h o w e v e r , i s e s s e n t i a l l y d u e t o c h a n g e s in 3s d e n s i t y , w h e r e a s t h e l s e 2 s r e m a i n p r a c t i c a l l y c o n s t a n t f o r a l l c o n f i g u r a t i o n s .

TABLE II

R E S U L T S F O R D I F F E R E N T d n C O N F I G U R A T I O N S

Fel(d 8 ) FeII(d 2) FeIU(d 6) F eIV(d5 ) FeV(d*) FeVI(d3 )

Is +269.9656 259.9656 259.9621 259.9572 259.9504 259.9423

2s -78.78302 -78.77200 -78.77200 -78.77020 -78.77805 -78.79529

3s +29.10285 +29.16182 29.26054 29.49729 29.94355 80.040612

T h i s i m p o r t a n t r e s u l t h a s b e e n i n t e r p r e t e d by W a l k e r , W e r t h e i m a n d J a c c a r i n o [28] a s a c o n s e q u e n c e of t h e s h i e l d i n g of t h e 3 s o r b i t a l s b y t h e 3d o r b i t a l s , s i n c e t h e r e m o v a l of a 3d e l e c t r o n i n c r e a s e s t h e 3s d e n s i t y a t t h e n u c l e u s .

97

T h e r a d i a l p a r t of t he 3s w a v e - f u n c t i o n in a m u l t i - e l e c t r o n i c ion d e p e n d s o n t h e e f f e c t i v e n u c l e a r c h a r g e Z eff w h i c h i n t e r a c t s w i t h t h e 3 s e l e c t r o n . Now, t h e r a d i a l 3d w a v e - f u n c t i o n h a s a p a r t w h i c h i s c l o s e r t o t h e n u c l e u s t h a n t h e 3s w a v e - f u n c t i o n . T h e 3 s e l e c t r o n s i n t e r a c t w i t h a n e f f e c t i v e n u c l e a r c h a r g e w h i c h i s l e s s t h a n t h e a c t u a l Z v a l u e a n d m a y b e e x p r e s s e d

r i Zeff =-Z - y V | , ( r ) d r (8)

о

w h e r e r : i s t h e l i m i t of t h e i n n e r p a r t of t h e 3d w a v e - f u n c t i o n .

T h e s u m ^ (0) , c a l c u l a t e d f r o m t h e W a t s o n w a v e - f u n c t i o n , g i v e s l

t h e t o t a l s - e l e c t r o n d e n s i t y a t t h e i r o n n u c l e u s f o r d i f f e r e n t c o n f i g u r a t i o n s of t h e f r e e i o n . T h i s c a l c u l a t i o n i n c l u d e s t he s h i e l d i n g e f f e c t of the 3d w a v e -f u n c t i o n of t he f r e e ion on t h e t o t a l s d e n s i t y .

T h e e l e c t r o n n u c l e u s i n t e r a c t i o n s i n t h e f r e e i o n c a n b e m o d i f i e d b y t h e b o n d i n g t h r o u g h t h e a l g , e g a n d t2g o r b i t r a l s w h i c h i n v o l v e t h e 4 s a n d 3d m e t a l o r b i t a l s , a s c a n b e s e e n f r o m t h e m o l e c u l a r o r b i t a l s l i s t e d in T a b l e I .

T h e p o s s i b l e m e c h a n i s m s t h a t c a n m o d i f y t h e i s o m e r s h i f t h a v e b e e n s t a t e d by S h u l m a n and Sugano [18], s u m m a r i z i n g t h e c o n c l u s i o n s of p r e v i o u s a u t h o r s : .

(a) C o n t r i b u t i o n f r o m 4 s b o n d i n g . (b) I n d i r e c t c o n t r i b u t i o n f r o m 3 d b o n d i n g w h i c h c a n b e a s f o l l o w s :

(1) C o v a l e n c y e f f e c t s b e t w e e n d e l e c t r o n s a n d f i l l e d l i g a n d o r b i t a l s wh ich wi l l i n c r e a s e t h e n u m b e r of d e l e c t r o n s on the m e t a l . T h i s i n c r e a s e i n d - e l e c t r o n d e n s i t y c a n d e c r e a s e t h e s - e l e ç t r o n d e n s i t y a t t h e n u c l e u s b y t h e s a m e s h i e l d i n g m e c h a n i s m o b s e r v e d wi th t he f r e e i on . T h i s e f f e c t would t e n d to c o m p e n s a t e t he e f f e c t of s bonding upon the i s o m e r s h i f t .

(2) T h e b o n d i n g of d e l e c t r o n s wi th e m p t y l i g a n d o r b i t a l s wi l l d e c r e a s e t h e d - e l e c t r o n d e n s i t y a t t he m e t a l ion b e c a u s e of b a c k - d o n a t i o n . T h i s t e n d s t o d e c r e a s e t h e i s o m e r s h i f t (IS) by i n c r e a s i n g t h e s - e l e c t r o n d e n s i t y a t t h e n u c l e u s .

T o a c c o u n t f o r t h e 4 s c o n t r i b u t i o n . W a l k e r e t a l . [28] a d d a f r a c t i o n of 4 s d e n s i t y , x ф | s ( 0 ) , t o t h e c o n t r i b u t i o n of t h e i n n e r s o r b i t a l s .

It i s i m p o r t a n t t o o b s e r v e t h a t t h e s i m p l e a d d i t i o n of a v a r i a b l e 4 s d e n -s i t y t o t h e i n n e r s - e l e c t r o n c o n t r i b u t i o n of t h e f r e e ion d o e s not a c c o u n t f o r t h e p o s s i b l e s h i e l d i n g of t h e i n n e r s e l e c t r o n s by the 4 s . T h i s i s , h o w e v e r , j u s t i f i e d by t h e r e s u l t s of W a t s o n [27] w h i c h show a n e g l i g i b l e d i f f e r e n c e of H a r t r e e - F o c k w a v e - f u n c t i o n s f o r t h e 3 d n 4 s 2 and 3d11 c o n f i g u r a t i o n s .

V a l u e s of 0 | s (0) have b e e n c a l c u l a t e d by W a l k e r et a l . [28] with t he F e r m i -S e g r é - G o u d s m i t h f o r m u l a f o r h y p e r f i n e c o n t a c t , which i s known to be a good a p p r o x i m a t i o n f o r a s i n g l e o u t e r s e l e c t r o n [14] .

S i n c e o n l y r e l a t i v e v a l u e s of (0) c a n b e d e r i v e d f r o m t h e m e a s u r e d i s o m e r s h i f t s , i t i s n e c e s s a r y t o c a l i b r a t e t h e c a l c u l a t e d t o t a l s - e l e c t r o n d e n s i t y w i t h m e a s u r e d i s o i r i e r s h i f t s . T h i s c a l i b r a t i o n h a s b e e n m a d e [28]

98

a s s u m i n g t h e m e a s u r e d v a l u e s f o r t h e i s o m e r s h i f t of t h e m o r e i o n i c Fe 1 1

and F e U I c o m p l e x e s ( e . g . f l u o r i d e s and a q u o c o m p l e x e s ) a r e t h e s a m e a s t h o s e f o r t h e f r e e i o n s . In o t h e r w o r d s , f o r t h i s c a l i b r a t i o n , o n e a c c e p t s t h e h y p o t h e s i s t h a t i t i s p o s s i b l e t o n e g l e c t t h e 4 s - o r b i t a l o c c u p a t i o n i n t h e m o r e i o n i c c o m p l e x e s of Fe 1 1 and F e n l .

W e h a v e p o i n t e d o u t i n p r e v i o u s p a p e r s [29 , 30] t h a t a l t h o u g h t h i s a p p r o x i m a t i o n c a n b e , a c c e p t e d f o r Fe1 1 i t i s not t r u e f o r Fe111 . T h i s c a n b e s e e n f r o m o p t i c a l d a t a , e l e c t r o n e g a t i v i t y and m o l e c u l a r o r b i t a l c a l c u l a t i o n s . G o l d a n s k i i [5] r e a c h e d t h e s a m e c o n c l u s i o n on the b a s i s of e f f e c t i v e c h a r g e m e a s u r e m e n t s in c o m p l e x e s by X - r a y a b s o r p t i o n s p e c t r a .

T a b l e III g i v e s t h e v a l u e s of e n e r g y d i f f e r e n c e s b e t w e e n t h e b a r i c e n t r e s of t h e 6 S - 4 G c o n f i g u r a t i o n s of g a s e o u s i o n s a n d of h i g h s p i n Mn11 and Fe1 1 1

c o m p l e x e s [10] .

TABLE Ш

V A R I O U S V A L U E S O F E N E R G Y D I F F E R E N C E

6 S - 4 G ( c m _ 1 )

Mn + + 3d 5 .26.846

M n " F 2 23.300

MnII(H2 0)1+ 25.000

Fe+++3d5 32.800

Fel l lFS- 25.350

FeIH(H20)?+ 24.450

J ^ r g e n s e n [25] c a l c u l a t e s f r o m t h e s e d a t a v a l u e s f o r t he e f f e c t i v e c h a r g e s of t h e c e n t r a l ion i n t h e s e c o m p l e x e s , w h i c h t u r n out t o b e a b o u t 1 .8 i n t h e +2 and 1 .7 in t h e +3 o n e s .

It i s d i f f i cu l t to a c c e p t t h a t t h i s s t r o n g p e r t u r b a t i o n of t he F e m ion d o e s no t i m p l y a n i m p o r t a n t i n v a s i o n of t h e c e n t r a l i o n 4 s o r b i t a l . A s h a s b e e n no ted b y S y r k i n [31] ( t h e " t r i p l e c h a r g e d " ion m a y a c t to a f a r g r e a t e r ex t en t a s an a c c e p t o r i n to i t s v a c a n t o r b i t a l s in r e l a t i o n t o t h e l i g a n d s . T h e r e f o r e t h e l o n e e l e c t r o n p a i r s of t h e l i g a n d a r e v e r y g r e a t l y d i s p l a c e d t o w a r d s t h e c e n t r a l a t o m , t h u s c o n s i d e r a b l y d i m i n i s h i n g i t s e f f e c t i v e c h a r g e .

By u s i n g t h e m e t h o d s of e f f e c t i v e e l e c t r o n e g a t i v i t y e q u a l i z a t i o n of a t o m s in m o l e c u l e s [32] we h a v e b e e n ab l e t o d e r i v e an o r d e r of m a g n i t u d e f o r t h e 4 s - o r b i t a l o c c u p a t i o n i n t h e s e c o m p l e x e s . F o r t h e Fe 1 1 1 F | ~ t h i s v a l u e i s a b o u t 0 . 3 6 [30] .

R e c e n t l y a m o d i f i c a t i o n of t h e W o l f s b e r g H e l m h o l z m e t h o d h a s b e e n p r o p o s e d a n d s e l f - c o n s i s t e n t c h a r g e a n d c o n f i g u r a t i o n M O c a l c u l a t i o n s h a v e b e e n r e p o r t e d f o r m a n y o c t a h e d r a l a n d t e t r a h e d r a l t r a n s i t i o n - m e t a l c o m p l e x e s [33] . T h e t h e o r e t i c a l v a l u e f o r 10 Dq = 15 .4 c a l c u l a t e d f o r Fe1 1 1

99

F I " c o m p a r e s w e l l w i t h 1 4 . 0 o b s e r v e d e x p e r i m e n t a l l y . T h e 4 s p o p u l a t i o n i s found t o be 0 . 3 2 , w h i c h i s in good a g r e e m e n t w i t h t h e v a l u e e s t i m a t e d b y u s w i t h m o r e c r u d e a p p r o x i m a t i o n s .

T a b l e IV g i v e s v a l u e s of i s o m e r s h i f t a id q u a d r u p o l e c o u p l i n g of i r o n c o m p o u n d s . T h e i s o m e r s h i f t f o r t h e Fe1 1 1 F | " c o m p l e x e s i s +0 .055 c m / s e c r e l a t i v e t o a s t a i n l e s s - s t e e l s o u r c e . If t h i s c o r r e s p o n d s t o a 3d 5 4s 0 - 3 2 c o n -f i g u r a t i o n i t i s p o s s i b l e t o u s e t h e W a l k e r et a l . [28] i n t e r p r e t a t i o n and f ind t h e i s o m e r s h i f t c o r r e s p o n d i n g t o t h e f r e e F e 3 + i on r e l a t i v e to t h e s a m e s o u r c e b y e x t r a p o l a t i o n t o 4 s = 0 . T h e v a l u e t h u s o b t a i n e d i s +0 .10 c m / s e c . T h e i s o m e r s h i f t f o r t h e m o r e i o n i c F e 1 1 c o m p l e x e s i s +0 .14 c m / s e c , a n d o n e c o u l d a s s u m e t h a t t h i s v a l u e c o r r e s p o n d s t o t h e . F e 2 + i o n . W i t h t h e s e t w o v a l u e s i t i s p o s s i b l e to r e c a l i b r a t e t h e t o t a l s - e l e c t r o n d e n s i t y d i a g r a m i n t e r m s of m e a s u r e d i s o m e r s h i f t s . T h e r e s u l t s o b t a i n e d a r e s h o w n i n F i g . 6 .

TABLE IV

V A L U E S O F I S O M E R S H I F T A N D Q U A D R U P O L E C O U P L I N G O F I R O N C O M P O U N D S a

Complexes o ( c m / s e c ) AE(cm/sec ) Ref.

FeUlFj +0.055 0.000 [29]

FeIIIci3 +0.040 0.000 [29]

FcIIlBr3 +0.030 0.000 [24]

Na3[FeIIlF6] +0.055 0.000 [33]

FeIlF2 +0.140 0.270 [28]

FeIIS0 4 - 7 H 2 0 +0.140 0.320 [13]

K2 F e V I o 4 -0.08 0.00 [34]

SrFeIVo 3 +0.01 - [36].

R [ F e I I I c l 4 ] +0.040 0.030 [29]

K 2 [FeII(CN) 6 ] +0.008 0.000 [39]

K3[FeIII<CN)6] 0.000 0.030 [39]

Na2 [ F e n ( C N ) 5 N O ] -0 .012 0.185 [39]

N a 3 [ F e I I ( C N ) 5 H 2 0 ] +0.015 0.080 [39]

Na 2 [Fe m (CN) 5 H 2 C>3 0.000 0.182 [39]

a Measurements were m a d e at room t empera tu re with a 5 7Co stainless-s teel source.

T h i s i n t e r p r e t a t i o n of t h e i s o m e r s h i f t n e g l e c t s a n y c o n t r i b u t i o n f r o m c o v a l e n c y e f f e c t s b e t w e e n d e l e c t r o n s and f i l l e d l i gand O r b i t a l s . T h i s c o n t r i -b u t i o n h a s b e e n a s s u m e d t o b e of c o n s i d e r a b l e i m p o r t a n c e a n d d e v i a t i o n s

100

i f

"Us

/ / -~F.m С 1 /

/ - F i n V s У

-

/ / / r « D S -

/ / -

-

4 4 . . F . n F2

F e I r S 0 , . 7 H , 0

1

« и)

E 0 z

à Ul 1— СЛ to UJ _l z

-0.05 < -0.03 Ul

-0.01 e 0.00 Ш 0.01 >

< 0.03

Ul 0.05

a: 6

0.07 u. 0

0.09 u

0.11 z

^ 0.13 u.

z 0.15 (S>

0 20 <0 60 во 100 120 * ä < s ELECTRON CONTRIBUTION IN»/o

FIG. 6. Total s-electron density as a function of the percentage of 4s character for various d-electron con-figurations. The constant с = 11873 a^3 . The outer scale on the right is the original isomer shift calibration from Ref. [28] and the inner scale is that obtained from the calibration of the present work

f r o m t h e W a l k e r e t a l . [28] i n t e r p r e t a t i o n h a v e b e e n a t t r i b u t e d to t h e a u g m e n -t a t i o n of 3d o r b i t a l s a n d c o n s e q u e n t d e c r e a s e of t h e 3 s d e n s i t y [ 3 4 , 3 5 ] .

I t i s now p o s s i b l e t o s h o w t h a t m o s t of t h e d i s c r e p a n c i e s r e p o r t e d o n t h e a p p l i c a t i o n of t h e W a l k e r e t a l . [28] i n t e r p r e t a t i o n a r e r e m o v e d w i t h t h e p r o p o s e d c a l i b r a t i o n of t h e d i a g r a m in F i g . 6 , w h i c h d i s r e g a r d s t h e e f f e c t s of 3d a u g m e n t a t i o n b y b o n d i n g .

T h e m e a s u r e d s h i f t : i n K 2 F e V I 0 4 ( - 0 . 0 8 0 c m / s e c ) c o r r e s p o n d s , a c c o r d i n g t o t h e W a l k e r , W e r t h e i m and J a c c a r i n o c a l i b r a t i o n , to a 3 d 4 . 3 c o n f i g u r a t i o n w h i c h i s f a r f r o m t h e 3d 2 c o n f i g u r a t i o n p r o p o s e d f o r F e V I i n t h i s c o m p l e x . F r o m t h e d i a g r a m r e p r o d u c e d i n F i g . 6 o n e f i n d s t h e u p p e r l i m i t < 3 d 3 f o r t h i s c o m p l e x .

T h e M ö s s b a u e r s p e c t r u m of SrFe I VC>3 i n d i c a t e s t h a t t h e i s o m e r s h i f t f o r i r o n in t h i s c o m p o u n d i s a b o u t +0 .01 c m / s e c [36] . A c c o r d i n g t o F i g . 6, t h i s c o r r e s p o n d s t o 3d44s°-0 s c o n f i g u r a t i o n in a g r e e m e n t wi th t h e a s s i g n m e n t of Fe iv . f o r t h i s c o m p l e x .

F o r t e t r a h e d r a l Fe1 1 1 CI41 c o m p l e x t h è v a l u e of i s o m e r s h i f t + 0 . 0 4 c m / s e c c o r r e s p o n d s t o a 3d 5 4s 0 - 4 0 c o n f i g u r a t i o n w h i c h c o m p a r e s w e l l w i t h 4s p o p u -l a t i o n of 0 .41 c a l c u l a t e d f r o m m o l e c u l a r o r b i t a l t h e o r y [33, 37] .

It i s a l s o p o s s i b l e t o s h o w t h a t t h e p r e s e n t c a l i b r a t i o n p r e d i c t s v a l u e s f o r t h e c o n f i g u r a t i o n a n d u n p a i r e d s p i n d e n s i t y in m e t a l l i c i r o n , w h i c h a r e i n a g r e e m e n t w i t h e x p e r i m e n t a l d a t a a n d t h e o r e t i c a l c a l c u l a t i o n s [38] .

101

T h e s e r e s u l t s s u g g e s t t h a t t h e c o n t r i b u t i o n of t h e b o n d i n g t o t h e i s o m ë r s h i f t i n h i g h s p i n c o m p l e x e s i s due t o 4 s - o r b i t a l o c c u p a t i o n ; t h e r e i s no e v i d e n c e f o r a c o n t r i b u t i o n a r i s i n g f r o m 3 d - o r b i t a l a u g m e n t a t i o n d u e t o b o n d i n g w i t h f i l l e d l i g a n d o r b i t a l s .

T h i s c o u l d m e a n t h a t b o n d i n g w i t h f i l l e d l i g a n d o r b i t a l s d o e s no t a l t e r t h e s h i e l d i n g of t h e 3 s b y t h e 3d r e p o r t e d f o r t h e f r e e - i o n c o n f i g u r a t i o n . I t i s i n t e r e s t i n g t o o b s e r v e t h a t t h e o u t e r h y b r i d i z a t i o n c o n c e p t i n v o l v i n g m o r e e x t e n d e d o r b i t a l s of t h e m e t a l i o n a d e q u a t e l y d e s c r i b e s t h i s s i t u a t i o n .

O n e c a n c o n v e n i e n t l y m a k e u s e of t h e n e p h e l a u x e t i c e f f e c t f o r i n t e r -p r e t i n g t h i s r e s u l t . A s a c o n s e q u e n c e of t h e 3 d - o r b i t a l a u g m e n t a t i o n f r o m b o n d i n g , t h e a v e r a g e r a d i u s of t h e 3d s h e l l i n c r e a s e s . T h e i n c r e a s e i n 3 s s h i e l d i n g a s a r e s u l t of t h i s i n c r e a s e i n 3d d e n s i t y m a y b e c o m p e n s a t e d b y t h e e x p a n s i o n of t h i s s h e l l . It h a s b e e n f o u n d t h a t s p i n - f o r b i d d e n i n t r a -s u b s h e l l t r a n s i t i o n s i n СгШ c o m p l e x e s h a v e a m u c h l e s s d r a s t i c a l l y v a r y i n g n e p h e l a u x e t i c e f f e c t t h a n t h e s p i n - a l l o w e d o n e s . T h i s i s e v i d e n c e f o r a m o r e e x p a n d e d r a d i a l f u n c t i o n of t h e l gcr a n t i b o n d i n g o r b i t a l s , w h i c h a r e i n v o l v e d i n t h e 3d d e n s i t y a u g m e n t a t i o n , a s c o m p a r e d w i t h t h e t2g jt a n t i b o n d i n g [ 2 5 ] .

In c o n c l u s i o n t h e 3 s - 3 d s h i e l d i n g m e c h a n i s m d o e s no t a p p e a r t o b e a l t e r e d b y b o n d i n g i n h i g h - s p i n c o m p l e x e s . T h e 3d s h e l l m a i n t a i n s i t s ; f r e e -i o n b e h a v i o u r f r o m t h i s po in t of v i e w . T h i s cou ld m e a n t h a t a l l m o d i f i c a t i o n s i n t h e 3 d - o r b i t a l d e n s i t y a r e t o b e f o u n d o n l y i n t h e o u t e r p a r t of t h e w a v e -f U n c t i o n , t h e i n n e r p a r t r e m a i n i n g e s s e n t i a l l y u n a l t e r e d .

T h e p r e s e n c e of b a c k - d o n a t i o n i n c o m p l e x e s w i t h e m p t y v * l i g a n d o r -b i t a l s c h a n g e s t h i s s i t u a t i o n r a d i c a l l y . S t a r t i n g f r o m t h e o b s e r v a t i o n t h a t t h e d i f f e r e n c e of i s o m e r s h i f t b e t w e e n F e u ( C N ) | " and F e ® (CN)g_ i s e s s e n t i a l l y n e g l i g i b l e w h e r e a s i t i s a b o u t +0 .10 c m / s e c f o r t h e h i g h - s p i n F e n a n d F e m s t a t e s , S h u l m a n a n d S u g a n o i n v e s t i g a t e d t h e c o n t r i b u t i o n of b a c k - d o n a t i o n t o t h e i s o m e r s h i f t of i r o n .

T h e i r b a s i c i d e a i s t o c a l c u l a t e t h e e f f e c t i v e n u m b e r of d e l e c t r o n s t h a t r e m a i n i n t h e c e n t r a l i o r i * a f t e r b o n d i n g . T h i s n u m b e r c a n b e e x p r e s s e d a s :

neff = n 2 - n 3 (9)

w h e r e 1 n^ i s t h e n u m b e r of d e l e c t r o n s in t h e l i m i t i n g i o n i c c a s e , n 2 t h e e l e c t r o n s w h i c h c a m e f r o m l i g a n d - t o - m e t a l b o n d i n g and П3 t h e e l e c t r o n s r e -m o v e d f r o m t h e m e t a l b a c k - d o n a t i o n .

T o c a l c u l a t e t h e s e n u m b e r s S h u l m a n a n d S u g a n o [18] d e r i v e a n a l y t i c a l e x p r e s s i o n s f o r t h e o r b i t a l a n g u l a r m o n i e n t u m r e d u c t i o n f a c t o r k . T h i s c o v a l e n c y p a r a m e t e r c a n b e e x p r e s s e d a s a f u n c t i o n of t h e c o e f f i c i e n t s f r o m Xi a n d x 2 f r o m E q . (4) w h i c h r e p r e s e n t l i n e a r c o m b i n a t i o n s of ir b o n d i n g a n d jr* a n t i b o n d i n g l i g a n d o r b i t a l s .

k « l - | ( X f + r | ) (10)

If n e a n d n t r e p r e s e n t t h e n u m b e r of h o l e s i n e | a n d t^ g a n t i b o n d i n g o r b i t a l s r e s p e c t i v e l y , w e h a v e

Пг= ne-yi+ntY? (11)

w h e r e y & a n d Y t a r e t h e c o e f f i c i e n t s of 3d o r b i t a l s i n b o n d i n g e g a n d t2g

r e s p e c t i v e l y .

102

T h e n u m b e r of e l e c t r o n s w h i c h m o v e t o t h e l i g a n d s п з i s g i v e n by

n 3 = (6 - n t ) y | (12)

w h e r e i s d e f i n e d in E q . ( 4 ) . T h e o b s e r v e d v a l u e s f o r к a r é

(13) к = 0 .87 f o r F e « 1 ( C N ) 6

' к = 0 . 7 4 f o r Mn1 1 (CN)g

w h i c h b y E q . ( 1 0 ) g i v e

T l ) 2 += ( X i + t | ) 3 + + 0 . 2 6 (14)

S i n c e i t i s p o s s i b l e t o a s s u m e i n g e n e r a l t h a t

w e f i n d a f t e r s u b s t i t u t i n g i n E q s . ( 1 5 ) and (12) :

n 3 ( M n 2 t ) - n 3 ( F e 3 + ) ~ 1 .3 (16)

T h e s a m e r e s u l t i s o b t a i n e d b y a s s u m i n g Ai s ; 72 f o r bo th 2+ and 3+ c o m -p l e x e s , a n d t h a t | У ! | 2 + IS t h e s a m e f o r M n " a n d F e 1 1 . F r o m E q . ( 1 0 ) w e h a v e ( 7 g) = 0 . 1 3 a n d ( 7 | ) 2 + = 0 . 2 6 , w h i c h g i v e s

n 3 ( F e ? + ) - n 3 ( F e 3 + ) = 0 . 9 ( 1 7 )

T h i s s h o w s q u i t e g e n e r a l l y t h a t we h a v e a d i f f e r e n c e of a p p r o x i m a t e l y o n e e l e c t r o n b e t w e e n t h e b a c k ^ d o n a t i o n of F e 2 + a n d F e 3 + .

S i n c e t h e v a l u e s of n 2 i n F e 2 + a n d F e 3 + m u s t b e s i m i l a r , w e f i n d f r o m E q , ( 9 )

ne ff ( F e 2 + ) ~ n e f f ( F e 3 4 (18)

T h e i r o n i o n i s a p p r o x i m a t e l y i s O e l e c t r o n i c i n t h e s e c o m p l e x e s and t h e d i f f e r e n c e s of e l e c t r o n d e n s i t y w i l l b e o b s e r v e d on t h e l i g a n d s .

T h i s r e s u l t e x p l a i n s w h y n o d i f f e r e n c e in i s o m e r s h i f t i s o b s e r v e d b e -t w e e n f e r r o - a n d f e r r i h é x à c y a n i d e s . T h i s c o n c l u s i o n a s s u m e s t h a t t h e 4 s d e n s i t y i s a p p r o x i m a t e l y t h e s a m e f o r t h e s e c o m p l e x e s . If w e n e g l e c t a n y c o n t r i b u t i o n f r o m b o n d i n g w i t h f i l l e d l i g a n d O r b i t a l s ( n 2 = 0 ) , E q . ( 9 ) b e c o m e s :

n eff = " i - ( 6 - n t ) 7 ¡ (19)

S u b s t i t u t i n g w i t h t h e v a l u e s f o r n t a n d y | g i v e n b e f o r e , one f i n d s :

( n e f f W i ( C N ) 6 = 4 . 4 4 ( 2 0 )

( n e f f b e l l i ( C N ) e = 4 . 3 5 (21)

103

With t h e c a l i b r a t i o n of i s o m e r s h i f t in F i g . 5 one f i n d s by i n t e r p o l a t i o n 4 s =0 .30 f o r both f e r r o - and f e r r i c y a n i d e c o m p l e x e s .

T h e g e n e r a l c o n c l u s i o n i s t h a t t h e v a r i a t i o n of i s o m e r s h i f t in h i g h -sp in c o m p l e x e s i s due to d i f f e r e n t d e g r e e s of occupa t i on of t h e 4 s o r b i t a l , w h e r e a s in l o w - s p i n c o m p l e x e s i t i s e s s e n t i a l to a c c o u n t f o r d i f f e r e n c e s in 3s dens i t y due to back -dona t ion to vacant я-* ant ibonding o r b i t a l s of the l igand.

T h e d i f f e r e n c e b e t w e e n t h e s e two m e c h a n i s m s i s c l e a r l y i l l u s t r a t e d by the i s o m e r s h i f t s of i r o n c o m p l e x e s with two s e r i e s of i s o e l e c t r o n i c l i gands , a s a f u n c t i o n of l igand e l e c t r o n e g a t i v i t y :

Fe111 В г з < Fe111 СД3 < Fe111 F 3

i s o m e r sh i f t

» e l e c t r o n e g a t i v i t y < i s o m e r sh i f t

Fe11 (CN)5 NO < Fe11 (CN) 5 CO < F e " ( C N ) 6

< e l e c t r o n e g a t i v i t y

With t h e h i g h - s p i n c o m p l e x e s the s - e l e c t r o n d e n s i t y i n c r e a s e s wi th i n -c r e a s i n g t e n d e n c y of t h e l i g a n d to g ive e l e c t r o n s t o the m e t a l ion ; wi th the l o w - s p i n c o m p l e x e s it i n c r e a s e s wi th the i n c r e a s i n g t e n d e n c y of t h e l i gand t o w i t h d r a w e l e c t r o n s f r o m the m e t a l i on .

T h e r e s u l t s w i t h t h e l o w e r s y m m e t r y c o m p l e x e s w i t h s t r o n g тг-bond l i g a n d s s u c h a s F e n ( C N ) s N 0 2 " a r e v e r y i n t e r e s t i n g and can a l s o be t r e a t e d wi th t h e s a m e a s s u m p t i o n u s e d f o r t he h e x a c y a n o c o m p l e x e s of i r o n .

N U C L E A R Q U A D R U P O L E C O U P L I N G AND C H E M I C A L BONDING

A s we have a l r e a d y men t ioned , the quadrupo le coupling with ^ F e nucleus s p l i t s t h e M ö s s b a u e r s p e c t r u m in to a d o u b l e t . T h e d i s t a n c e Д Е b e t w e e n t h e t w o p e a k s i s p r o p o r t i o n a l t o the p r o d u c t of t he n u c l e a r q u a d r u p o l e m o -m e n t by the e l e c t r i c f i e l d g r a d i e n t a t t he n u c l e u s .

F r o m t h e v e r y b e g i n n i n g . a r e m a r k a b l e d i f f e r e n c e h a s b e e n o b s e r v e d i n E b e t w e e n t h e t w o m o r e c o m m o n o x i d a t i o n s t a t e s of i r o n . T h e v a l u e s of ДЕ f o r s p i n - f r e e Fell g 0 f r o m 0.15 to .0 .36 c m / s e c and f o r s p i n - f r e e Fein f r o m 0 .03 t o 0.07 c m / s e c .

T h e s e r e s u l t s h a v e b e e n i n t e r p r e t e d i n t e r m s of c r y s t a l f i e l d t h e o r y . In an e l e c t r i c f i e ld p r o d u c e d by a s l i g h t l y d i s t o r t e d o c t a h e d r o n of nega t ive

c h a r g e s a r o u n d t h e t r a n s i t i o n i o n , t h e 3d w a v e - f u n c t i o n g i v e s t h e fo l l owing f i e l d g r a d i e n t s and s y m m e t r y f a c t o r s a t t h e n u c l e u s :

O r b i t a l q < r -3>" 1 r¡

d Г dX2-y2 + 4 / 7 0

M d z 2 - 4 / 7 0

(22)

0

+3

- 3

+4 /7

- 2 / 7

- 2 / 7

104

T h e h i g h - s p i n o u t e r e l e c t r o n c o n f i g u r a t i o n 3 d 5 i s a tíS5/2 i o n i c s t a t e . T h e h a l f - f i l l e d s h e l l h a s s p h e r i c a l s y m m e t r y a n d g i v e s 110 c o n t r i b u t i o n t o the f i e l d g r a d i e n t . T h e s m a l l v a l u e s f o r the i n t e r a c t i o n o b s e r v e d can be' r e -l a t e d t o t he g r a d i e n t p r o d u c e d by ion i c c h a r g e s f r o m the l a t t i c e .

T h e f r e e F e 2 + i o n h a s a n o u t e r e l e c t r o n c o n f i g u r a t i o n of 3d6 a n d i s i n a s t a t e . T h e m a i n c o n t r i b u t i o n t o t he f i e l d g r a d i e n t c o m e s f r o m a s i n g l e e l e c t r o n wi th s p i n a n t i p a r a l l e l t o the f i ve o t h e r s [13].

T h e o r b i t a l i n w h i c h t h i s e l e c t r o n i s t o b e p l a c e d d e p e n d s on t h e d i s -t o r t i o n f r o m c u b i c s y m m e t r y of t h e c r y s t a l f i e l d . A x i a l and r h o m b i c c o m -p o n e n t s l i f t t h e d e g e n e r a c y of t h e d e s h e l l . F u r t h e r s p l i t t i n g of t h e e n e r g y l e v e l s o c c u r s by s p i n - o r b i t coup l i ng . T h e r e l a t i v e p o p u l a t i o n of t h e s e l e v e l s a c c o u n t s f o r t he t e m p e r a t u r e d e p e n d e n c e of t he q u a d r u p o l e s p l i t t i n g .

C o v a l e n t b o n d i n g c o n t r i b u t e s t o t he d e c r e a s e of t h e q u a d r u p o l e s p l i t t i n g s i n c e i t e x p a n d s t h e r a d i a l p a r t of t h e 3d w a v e - f u n c t i o n . R e a s o n a b l e e s t i -m a t e s of e n e r g y s p l i t t i n g s of t he de o r b i t a l s in s e v e r a l Fe11 c o m p o u n d s h a v e b e e n ob ta ined f r o m M ö s s b a u e r m e a s u r e m e n t s [13].

T h e f i r s t m e a s u r e m e n t s m a d e on the c y a n i d e c o m p l e x e s of i r o n r e v e a l e d i n t e r e s t i n g d i f f e r e n c e s f r o m the h i g h - s p i n c o m p l e x e s . A h i g h e r q u a d r u p o l e s p l i t t i n g , w h i c h r e a c h e s t h e v a l u e s of h i g h - s p i n f e r r o u s s a l t s , i s o b s e r v e d i n f e r r i c c y a n i d e c o m p l e x e s w i t h a 3 d | l o w - s p i n c o n f i g u r a t i o n of i r o n . T h e i n t e r a c t i o n i s s m a l l e r i n t h e f e r r o u s c o m p l e x e s w i t h a 3d® s p i n p a i r e d c o n f i g u r a t i o n .

In a f i r s t a t t e m p t t o u n d e r s t a n d th i s r e s u l t we u s e d a c r y s t a l f ie ld s c h e m e [39] . T h e s i x p a i r e d e l e c t r o n s i n t h e u n p e r t u r b e d 3df s h e l l p o s s e s s c u b i c s y m m e t r y and g ive no c o n t r i b u t i o n t o the f i e l d g r a d i e n t a t t h e n u c l e u s . F o r s p i n - p a i r e d 3 d | t h e r e i s a h o l e i n t h e d e s h e l l . T h e m a g n i t u d e of t h e f i e l d g r a d i e n t p r o d u c e d by t h e ho l e i s t h e s a m e a s t h a t of a s i n g l e u n p a i r e d e l e c t r o n , w h i c h a c c o u n t s f o r s o m e r e s u l t s of t h e f e r r i c y a n i d e c o m p l e x e s . Wi th K 3 [ F e i H (CN)6 ], h o w e v e r , t h e q u a d r u p o l e i n t e r a c t i o n i s m u c h s m a l l e r t h a n t h a t e x p e c t e d f r o m t h e p r e s e n c e of a h o l e in t he d 6 s h e l l . We h a v e i n t e r p r e t e d t h i s r e s u l t a s s u m i n g tha t t he t h r e e d e l e v e l s a r e c l o s e in e n e r g y , and i n c o n s e q u e n c e t h e y a r e a l m o s t e q u a l l y p o p u l a t e d a t r o o m t e m p e r a t u r e . T h u s , t h e a v e r a g e f i e l d e x p e r i e n c e d b y t h e n u c l e u s a p p r o a c h e s c u b i c s y m m e t r y .

R e c e n t m e a s u r e m e n t s of m a g n e t i c s u s c e p t i b i l i t y w i t h K 3 [ F e l n ( C N ) g ] c o n f i r m t h i s i n t e r p r e t a t i o n . T h e b e h a v i o u r of t h e s u s c e p t i b i l i t y and a n i s o t r o p y w i t h t e m p e r a t u r e h a s b e e n i n t e r p r e t e d i n t e r m s of t h e 2T2g t e r m s p l i t b y a n a x i a l l i g a n d f i e l d c o m p o n e n t of about 100 c m " 1 , wh ich i m p l i e s an a p p r e -c i a b l e p o p u l a t i o n of u p p e r l e v e l o r b i t a l s a t room t e m p e r a t u r e [40] .

A m o r e c o m p l e t e i n t e r p r e t a t i o n of the q u a d r u p o l e sp l i t t i ng in Fe111 (CN)3" c o m p l e x e s in t e r m s of e l e c t r o n d e r e a l i z a t i o n and m o l e c u l a r o r b i t a l s i s u n d e r i n v e s t i g a t i o n .

S o d i u m n i t r o p r u s s i d e h a s p r o v e d i t s e l f a s y s t e m in w h i c h t h e c o m -p a r i s o n b e t w e e n t h e M ö s s b a u e r f i e l d g r a d i e n t d e t e r m i n a t i o n a n d the M O b o n d i n g s c h e m e i s m o r e e a s i l y m a d e [41] . T h e s a m e Д Е i s o b s e r v e d w i t h s i n g l e c r y s t a l s a n d i n f r o z e n s o l u t i o n s wi th t h e c o m p l e x a t l i q u i d n i t r o g e n t e m p e r a t u r e s , s h o w i n g no i n f l u e n c e of t h e c r y s t a l l a t t i c e o r m i x i n g of c o n f i g u r a t i o n s on t h e f i e l d g r a d i e n t .

Wi th s i n g l e c r y s t a l s of N a 2 [ F e u (CN)5 NO] 2 H 2 0 i t h a s b e e n p o s s i b l e t o d e t e r m i n e t h e m a g n i t u d e a n d s i g n of the . f i e l d g r a d i e n t a n d i t s a s y m m e t r y .

105

f a c t o r a t t h e i r o n n u c l e u s . T h e s e d e t e r m i n a t i o n s a r e b a s e d u p o n t h e d i f f e r e n c e i n p o l a r i z a t i o n of t h e 7 - r a d i a t i o n e m i t t e d i n t h e t w o t r a n s i t i o n s t o t h e g r o u n d s t a t e . T h e M - l t r a n s i t i o n f r o m ± 3 / 2 t e r m i s f u l l y c i r c u l a r l y p o l a r i z e d w h e r e a s t h e ± 1 / 2 t r a n s i t i o n t o t h e g r o u n d s t a t e i s 2 / 3 l i n e a r l y p o l a r i z e d a n d 1 / 2 c i r c u l a r l y p o l a r i z e d . In c o n s e q u e n c e , t h e p r o b a b i l i t y of a b s o r p t i o n of t h e r a d i a t i o n f r o m bo th h y p e r f i n e c o m p o n e n t s i s d i f f e r e n t wi th r e s p e c t t o t h e d i r e c t i o n of t h e f i e l d g r a d i e n t a x i s .

F o r a n a x i a l l y s y m m e t r i c f i e l d g r a d i e n t (17 = 0) a t a n a n g l e в w i t h t h e i n c i d e n t 7 - r a d i a t i o n , t h e r a t i o of i n t e n s i t i e s of t h e h i g h e r e n e r g y t e r m to t h e l o w e r e n e r g y t e r m f o r a p o s i t i v e i n t e r a c t i o n i s :

I ± 3 / 2 1 + cos 2 (9 I ± 1/2 " f - cos2в (¿á)

T h e X - r a y s t r u c t u r e of s o d i u m n i t r o p r u s s i d e h a s b e e n r e c e n t l y d e -t e r m i n e d [42] a n d s h o w s f o u r m o l e c u l e s i n e a c h un i t c e l l f o r m i n g two e q u i -v a l e n t F e - N O d i r e c t i o n s .

G e n e r a l e q u a t i o n s f o r t h e q u a d r u p o l e i n t e r a c t i o n in s i n g l e c r y s t a l s w i th m o r e t h a n o n e d i r e c t i o n f o r t h e p r i n c i p a l f i e l d g r a d i e n t h a v e b e e n d e r i v e d b y Z o r y [43] . F o r s o d i u m n i t r o p r u s s i d e t h e s e e q u a t i o n s r e d u c e t o É q . ( 2 3 ) f o r p a r t i c u l a r d i r e c t i o n s of t h e i n c i d e n t 7 - r a d i a t i o n .

D e t e r m i n a t i o n s of t h e q u a d r u p o l e i n t e r a c t i o n f o r a l l c r y s t a l l o g r a p h i c p l a n e s of a s i n g l e c r y s t a l of s o d i u m n i t r o p r u s s i d e a r e u n d e r i n v e s t i g a t i o n [44], and p r e l i m i n a r y r e s u l t s g a v e good a g r e e m e n t wi th E q . ( 2 3 ) . T h i s s h o w s tha t t h e f i e l d g r a d i e n t a t t h e i r o n n u c l e u s i n t h i s c o m p l e x i s p o s i t i v e a n d a x i a l l y s y m m e t r i c [41] .

T o i n t e r p r e t t h i s r e s u l t w e a s s u m e t h a t t h e f i e l d g r a d i e n t i s due t o t h e a s y m m e t r i c e x p a n s i o n of t h e f i l l e d 3d£ s h e l l t o w a r d s t h e l i g a n d s . A c c o r d i n g t o t h e MO bond ing s c h e i n e i n F i g . 2 d i f f e r e n t c o v a l e n c y p a r a m e t e r s c h a r a c t e r i z e t h e d £ s h e l l i n t h e n i t r o s y l c o m p l e x e s : k i f o r t h e p a i r s of e l e c t r o n s i n t h e d x z , d y 2 l o w e r d o u b l e t w h i c h i s i n v o l v e d i n t h e s t r o n g ir b o n d s t o NO, a n d k 2 f o r d x v e l e c t r o n s .

F r o m T a b l e III one c a l c u l a t e s f o r t h e c o v a l e n c y c o n t r i b u t i o n t o t h e fiield g r a d i e n t

q = 2 ( k 2 - k x ) - q ' , n = 0 (24)

w h e r e q1 = 4 / 7 < r " 3 > .

S i n c e t h e dXy o r b i t a l i s a s s u m e d t o b e n o n - b o n d i n g , a n d t h e d X 2 , d y z i s i n v o l v e d i n s t r o n g ж b o n d s , w e h a v e n e c e s s a r i l y k¿> k j a n d i n c o n s e q u e n c e q > 0 , a s o b s e r v e d e x p e r i m e n t a l l y .

I t i s p o s s i b l e t o u s e E q . ( 2 4 ) f o r e s t i m a t i n g t h e c o v a l e n c y p a r a m e t e r s k . T h e q u a d r u p o l e i n t e r a c t i o n i n F e ( C N ) 5 N 0 2 " c o r r e s p o n d s t o 4 3 . 1 M c .

T a k i n g Q = + 0 . 1 5 b o n e f i n d s f o r t h e e l e c t r i c f i e l d g r a d i e n t t h e v a l u e q =+1 .212 a . u . T h e a v e r a g e of t h e i n v e r s e c u b i c of t h e d i s t a n c e b e t w e e n a 3d e l e c t r o n and t h e n u c l e u s < r - 3 > s7Fe h a s b e e n e s t i m a t e d by Ok i j i and K a n a m o r i [45] t o be 4 .4 a . u . S u b s t i t u t i n g in E q . ( 2 4 ) g i v e s :

( k 2 - k x ) = 0 . 2 4

If t h e dXy o r b i t a l i s a s s u m e d to be non-bo i id ing ( n e g l e c t i n g d e r e a l i z a t i o n .to t h e c y a n i d e l i g a n d s ) k 2 = 1 and one f i n d s k i = 0 .76 .

106

I t h a s r e c e n t l y b e e n s h o w n b y F o r t m a n a n d H a y e s [46] t h a t t h i s i s i n a g r e e m e n t w i t h t h e r e s u l t s o b t a i n e d b y q u a d r u p o l e c o u p l i n g s p l i t t i n g of t h e e l e c t r o n s p i n r e s o n a n c e s p e c t r u m of M n H ( C N ) 5 N 0 2 " . T h i s s p e c t r u m h a s b e e n i n v e s t i g a t e d in s o l u t i o n and i n s i n g l e c r y s t a l e n v i r o n m e n t [46, 47 ] . S a t e l l i t e l i n e s of low i n t e n s i t y w e r e o b s e r v e d b e t w e e n t h e Д т = 0 a b s o r p t i o n s . T h e s e Д т - ± 1 t r a n s i t i o n s w e r e i n t e r p r e t e d a s a r i s i n g f r o m q u a d r u p o l e c o u p l i n g w i t h t h e 55Mn n u c l e u s .

F r o m t h e q u a d r u p o l e c o u p l i n g c o n s t a n t o b t a i n e d , - 6 2 . 4 M c , a n d t h e q u a d r u p o l e m o m e n t of 5 5Mn = 0 .35 b , one o b t a i n s a f i e l d g r a d i e n t o f - 0 . 7 5 4 a . u .

F r o m t h e a n i s o t r o p i c coup l ing c o n s t a n t of t h e h y p e r f i n e i n t e r a c t i o n wi th 5 5 Mn one f i n d s < г - з > 55мп = 3 .10 a . u . [46] .

A c c o r d i n g to t h e MO s c h e m e f r o m F i g . 5 t h e g r o u n d s t a t e c o n f i g u r a t i o n f o r t he m a n g a n e s e ion in t h i s c o m p l e x i s (d xz)2 < V 2 (^xy)1 > a n d o n e h a s t h e

fo l l owing c o n t r i b u t i o n s t o t h e f i e l d g r a d i e n t :

1 = - q ^ - q ' k x + q ' k 2 = q ' ( k 2 - 2к г) (25)

S u b s t i t u t i n g f o r t h e v a l u e s of q a n d < r " 3 > g i v e n b e f o r e , o n e f i n d s

( k j - 2 k j ) = 1 .43 (26)

M a k i n g k 2 = 1 o n e f i n d s k i = 0 . 7 2 . T h i s v a l u e c o m p a r e s w e l l w i t h t h a t o b t a i n e d f o r t h e n i t r o p r u s s i d e f r o m M ö s s b a u e r s p e c t r o s c o p y a n d i n d i c a t e s a s t r o n g d e r e a l i z a t i o n of t h e d x z , d y z o r b i t a l s i n t h e p e n t a c y a n o n i t r o s y l s , i n a g r e e m e n t w i t h t h e t h e o r e t i c a l p r e d i c t i o n s .

C O N C L U S I O N S

T h e a i m of t h e p r e s e n t w o r k h a s b e e n t o c o m p a r e t h e i n f o r m a t i o n d e -r i v e d f r o m t h e M ö s s b a u e r e f f e c t w i t h t h a t p r o v i d e d b y o t h e r m e t h o d s .

T h e m o s t i n t e r e s t i n g r e s u l t s w e r e o b t a i n e d f r o m t h e c o m p a r i s o n wi th , e l e c t r o n s p i n r e s o n a n c e d a t a . T h i s m e t h o d of i n v e s t i g a t i o n g i v e s t h e m o s t d e t a i l e d i n f o r m a t i o n on c o v a l e n t d e r e a l i z a t i o n f r o m h y p e r f i n e i n t e r a c t i o n s of u n p a i r e d e l e c t r o n s w i t h n u c l e i . T h e s a m e c o v a l e n c y f a c t o r s a r e of f i r s t i m p o r t a n c e f o r t h e h y p e r f i n e i n t e r a c t i o n s of t h e M ö s s b a u e r e f f e c t in t r a n -s i t i o n m e t a l c o m p l e x e s .

T h e m a i n c o n t r i b u t i o n of t he bond ing to t he n u c l e a r i s o m e r s h i f t in h i g h -s p i n c o m p l e x e s c o m e s f r o m t h e 4 s c o v a l e n c y . I n f o r m a t i o n on t h i s t y p e of c o v a l e n c y i s s c a r c e , b u t f r o m o p t i c a l d a t a and m o l e c u l a r o r b i t a l c a l c u l a t i o n s i t i s p o s s i b l e t o g ive a c o h e r e n t p i c t u r e of t he n u c l e a r i s o m e r s h i f t m e a s u r e -m e n t s on t h e b a s i s of t h e W a l k e r e t a l . i n t e r p r e t a t i o n [28] . T h e M ö s s b a u e r e f f e c t c o n s t i t u t e s , t h r o u g h t h e s e m e a s u r e m e n t s , a un ique m e t h o d f o r e s t i m a t i n g s - e l e c t r o n c o v a l e n c y in t r a n s i t i o n m e t a l c o m p l e x e s .

I t i s c u s t o m a r y t o p o s t u l a t e dir b o n d i n g b e t w e e n m e t a l s a n d c e r t a i n l i g a n d s t o e x p l a i n m a n y p r o p e r t i e s of l o w - s p i n c o m p l e x e s . A s h a s b e e n o b s e r v e d b y N y h o l m [47], i t i s s u r p r i s i n g how f e w u n a m b i g o u s e x p e r i m e n t a l r e s u l t s a r e a v a i l a b l e t o s u p p o r t t h i s h y p o t h e s i s . T h e M ö s s b a u e r e f f e c t i s One of t h e b e s t p i e c e s of e v i d e n c e f o r t h e i m p o r t a n c e of b a c k - d o n a t i o n i n

107

l o w - s p i n c o m p l e x e s , s i n c e t h e m e c h a n i s m of n u c l e a r i s o m e r s h i f t v a r i a t i o n i n t h e s e c o m p l e x e s i s d i r e c t l y c o n n e c t e d wi th t h e s p r e a d i n g of t h e m e t a l dff

e l e c t r o n s in v a c a n t l i g a n d o r b i t a l e [41] . O n t h e b a s i s of t h e c o n c e p t of b a c k - d o n a t i o n , S h u l m a n a n d S u g a n o [18]

h a v e s h o w n t h a t s e m i - q u a n t i t a t i v e c o r r e l a t i o n s c a n be m a d e b e t w e e n c o v a l e n c y f a c t o r s d e r i v e d f r o m e l e c t r o n - s p i n r e s o n a n c e and n u c l e a r i s o m e r - s h i f t d a t a .

. E l e c t r i c f i e l d g r a d i e n t s d e r i v e d f r o m q u a d r u p o l e coup l ing m e a s u r e m e n t s of t h e M ö s s b a u e r e f f e c t h a v e b e e n s u c c e s s f u l l y c o r r e l a t e d w i t h t h e e n e r g y l e v e l d i s t r i b u t i o n of d O r b i t a l s in h i g h - s p i n c o m p l e x e s [13] .

T h e e l e c t r i c f i e l d g r a d i e n t i n l o w - s p i n c o m p l e x e s o r i g i n a t e s f r o m t h e u n b a l a n c e d d e l e c t r o n s d u e t o t h e a s y m m e t r i c e x p a n s i o n of t h e d s h e l l i n b o n d i n g . O n t h i s b a s i s i t i s p o s s i b l e t o e s t i m a t e v a l u e s f o r t h e d - e l e c t r o n c o v a l e n c y w h i c h c o m p a r e we l l wi th t h o s e o b t a i n e d f r o m e l e c t r o n - s p i n r e -s o n a n c e m e a s u r e m e n t s .

T h e i n v e s t i g a t i o n of c h e m i c a l b o n d i n g i n t r a n s i t i o n m e t a l c o m p l e x e s s e e k s t o g ive a q u a n t i t a t i v e a n s w e r to the q u e s t i o n of e l e c t r o n d e r e a l i z a t i o n . U n t i l n o w we h a v e b e e n r e s t r i c t e d t o m e t h o d s w h i c h m a k e u s e of m a g n e t i c p r o p e r t i e s of u n p a i r e d e l e c t r o n s . T h e M ö s s b a u e r e f f e c t , by c o m b i n i n g t w o h y p e r f i n e i n t e r a c t i o n s of e l e c t r o s t a t i c o r i g i n , i s not s u b j e c t to t h e s e r e s t r i c -t i o n s , a n d i n c o n s e q u e n c e a p p e a r s t o b e a m o s t p r o m i s i n g m e t h o d f o r d e -t e r m i n i n g t h e p a r a m e t e r s of t h e e l e c t r o n i c s t r u c t u r e of a g e n e r a l c l a s s of m e t a l c o m p l e x e s .

R E F E R E N C E S

[1] ABRAGAM, A. , L'Effet Mössbauer et ses Applications à l 'é tude des Champs Internes, Gordon and Breach, New York, London (1964).

[2] BOYLE, A.J. F . , HALL, H. E., "The Mössbauer effect",Rep. Progr. Phys. 25 (1962)441. 13] FRAUENFELDER, H . , The Mössbauer Effect, W. A. Benjamin Inc . , New York (1962). L4] WERTHEIM, G . K . , Mössbauer Effect, Academic Press, New York (1964). [5] GOLDANSKII, V. I . , G. E. C. Atom. Energy Rev. 1 (1963) 3. [6] FLUCK, E., KERLER, W., NEUWIRTH, W. , Angew. Chem. (Int. ed. ) 2 (1963) 277. [7] COTTON, F. A . , WILKINSON, G . , Advanced Inorganic Chemis t ry , Interscience Publishers (1962). [§] GRIFFITH, J. S . , Theory of Transition Metal Ion, Cambr idge University Press (1961). [9] ORGEL. L . , Introduction to Transition Metal Chemistry, Methuen, London (1960).

[10] J0RGENSEN, C. K. , Absorption Spectra and Chemical Bonding in Complexes, Pergamon Press Ltd., London (1962).

[11] LEWIS, J . , WILKINS, R. G., Modem Coordination Chemistry, Interscience Publishers (1960). [12] SHIRLEY, D.A. , Rev. mod. Phys. 36 (1964) 339. [13] INGALLS, R. E., Tech. Rpt Nos. 2 and 3, Carnegie Institute of Technology, Pittsburgh (1963). [14] KOPFERMANN, H. , Nuclear Moments, Academic Press, New York (1958). [15] BALLHAUSEN, C . J . , Introduction to Ligand Field Theory, McGraw Hill Book Co. , Inc. (1962);

GRAY, H. B. , J. chem. Educ. 41 (1964) 1. [16] GRIFFITH, W. P . , Q. Rev. chem. Soc. 16 (1962) 188. [17] NAIMAN, C . S . , J. chem. Phys. 39 (1963) 1900. [18] SHULMAN, R. G. , SUGANO, S . , J. chem. Phys. 42 (1965) 39. [19] GRAY, H . B . , BEACH, N. A . , J. Amer. chem. Soc. 85 (1963) 2922. [20] BALLHAUSEN, C . J . , GRAY. H . B . , J. inorg. Chem. 2 (1963) 246. [21] GRAY, H .B . , BERNAL, I . , BILLING, E . , J. Amer. chem. Soc. 84 (1962) 3404. [22] TOSI, L . , DANON, J . . J. inorg. Chem. 3 (1964) 150.

108

123] DANON, J . , MUNIZ, R .P .A . , PANEPUCCI, H. , J. chem. Phys. 41 (1964) 3651. [24] RAYNOR, J . B . , Nature, Lond. 201 (1964) 1216. [25] J0RGENSEN, C. K. , The Nephelauxetic Series in Inorganic Chemistry 4, Interscience Publishers (1962) 73. [26] WATSON, R. E., Solid State and Molecular Theory Group, Tech. Rpt No. 12, M. I. T. (1959). [27] WATSON, R. E., Phys. Rev. 119 (1960) 1934. [28] WALKER, L. R,, WERTHEIM, G. К . , JACCARINO, V. , Phys. Rev. Lett. 6 (1961) 98. [29] DANON, J . , Rev. mod. Phys. 36 (1964) 459. [30] DANON, J . , Proc. 8th International Conference on Coordination Chemistry, Springer Verlag, Vienna

(1964). [31] SYRKIN, Y. K . , Usp. Khim. 31 (1962) 397. [32] FERREIRA, R., Trans. Faraday Soc. 59 (1963) 1064. [33] VISTE, A . , GRAY, H. B., J. inorg. Chem. 3 (1964) 1113. [34] WERTHEIM, G. K . , HERBER, R. H. , J. chem. Phys. 36 (1962) 2497. [35] JACCARINO, V . , WERTHEIM, G. K. , The Mössbauer Effec't, Proc. Second International Conference,

Saclay (1961), John Wiley and Sons, New York, London (1961). [36] SHIRANE, G., COX, D. E., RUBY, S. L., Phys. Rev. 125 (1962) 1158. [37] ZASLOW, B., RUNDLE, R. E., J. phys. Chem. 61 (1957) 490. [38] DANON, J . , to be published. [39] COSTA, N. L., DANON, J . , ZAVIER, R. M. , Physics Chem. Solids 23 (1962) 1783. [40] FIGGIS, B. N . , LEWIS, J . , in Progress in Inorganic Chemistry 6, Interscience Publishers (1964) 172. [41] DANON. J . , J. chem. Phys. 41 (1964) 3378. [42] MANOHARAM, P. R., HAMILTON, W. C . , J. inorg. Chem. 2 (1964) 1043. [43] ZORY, P.S. , J r . , Tech. Rpt No.4, Carnegie Institute of Technology, Pittsburgh (1964). [44] DANON, J . . LANARELLA, L., to be published. [45] ОКЩ,«А., KAN A MORI, J . , J. phys. Soc. Japan 19 (1964)'908. [46] FQRTMAN, J . J . , HAYES, R. G. , personal communication (March 1965). [47] NYHOLM, R. S . , Electron Configuration and Structure of Transition -metal Complexes, Tilden Lecture,

Proc. chem. Soc. (1961) 273.

R . M . G O L D I N G o u t l i n e d a d i f f e r e n t a p p r o a c h t o n i t r o s y l p r u s s i d e by c o n -s i d e r i n g the q u a d r u p o l e s p l i t t i n g of t h i s c o m p o u n d f r o m the l i gand f i e l d p i c -t u r e . F o r t h e d 6 i r o n a t o m in a s t r o n g c r y s t a l f i e l d t h e g r o u n d s t a t e w a s r e p r e s e n t e d by ^ w h e r e t h e c o m p l e x d w a v e - f u n c t i o n w a s

He f u r t h e r s t a t e d t h a t , a s s h o w n in h i s p a p e r , <EQ> = 0 , f o r a c o m p l e x wi th a i A j g r o u n d s t a t e w h e t h e r t h e i r o n w a s in a c r y s t a l f i e l d of o c t a h e d r a l o r t e t r a g o n a l s y m m e t r y . H o w e v e r , if t he b o n d i n g b e t w e e n t h e d o r b i t a l s a n d t h e l i g a n d o r b i t a l s w a s c o n s i d e r e d , t he w a v e - f u n c t i o n m i g h t b e r e -p r e s e n t e d a s :

D I S C U S S I O N

1AX= I Ф^ Ф|Ф§>

1 0 9

w h e r e

Ф1 = k j | 1> + l i g a n d o r b i t a l s

Ф 2 = k 2 I - l )> + l i g a n d o r b i t a l e

Ф3 = к з I > + i i g a n d O r b i t a l s

T h i s e x p r e s s i o n l e d t o t h e s a m e AEQ v a l u e a s g i v e n b y D a n o n w h e n к х = k 2 .

INTERMOLECULAR BONDING EFFECTS IN 119Snm MÖSSBAUER SPECTROSCOPY

R.H. HERBER* AND H.A. STÖCKLER THE STATE UNIVERSITY. NEW BRUNSWICK, N.J .

UNITED STATES OF AMERICA

I N T R O D U C T I O N

T h e M ö s s b a u e r s p e c t r a of a n u m b e r of i n o r g a n i c a n d m e t a l - o r g a n i c t i n c o m p o u n d s h a v e b e e n e x a m i n e d b y G o l d a n s k i i a n d h i s c o - w o r k e r s [1] , b y C o r d e y - H a y e s [2] , b y t h e p r e s e n t a u t h o r s [ 3 , 4] a n d o t h e r s [ 5 - 7]. In t h e s t u d y of m o s t o r g a n o m e t a l l i c t i n c o m p o u n d s i t i s n e c e s s a r y t o c o o l t h e a b -s o r b e r t o l i q u i d n i t r o g e n t e m p e r a t u r e s (77°K) b e f o r e a p p r e c i a b l y l a r g e r e s o -n a n c e e f f e c t s c a n b e o b s e r v e d . T h i s s i t u a t i o n i s i n c o n t r a s t t o t h a t o b s e r v e d f o r i n o r g a n i c c o m p o u n d s s u c h a s S n C l é . 5 Н г О , SnOz K.2SnF6 a n d S n F 4 , a l l of w h i c h s h o w m e a s u r a b l y l a r g e r e s o n a n t a b s o r p t i o n e f f e c t s a t r o o m t e m p e r a t u r e ( ~ 3 0 0 ° K ) . T h i s c o r r e l a t i o n w a s f i r s t n o t e d b y G o l d a n s k i i e t a l . [8] w h o c a l l e d a t t e n t i o n t o t h e f a c t t h a t a l l of t h e t i n c o m p o u n d s t h a t s h o w e d m e a s u r a b l y l a r g e r e a s o n a n c e e f f e c t s a t r o o m t e m p e r a t u r e i n v o l v e d 6 -c o o r d i n a t e t i n a t o m s . T h i s c o r r e l a t i o n — i n f o r m a l l y c a l l e d " G o l d a n s k i i ' s R u l e " - w a s i n t e r p r e t e d t o p r o v i d e a d i a g n o s t i c p a r a m e t e r i n d i c a t i v e of 6 -c o o r d i n a t e t i n . S u b s e q u e n t s t u d i e s h a v e s h o w n h o w e v e r , t h a t 6 - c o o r d i n a t i o n i s n e i t h e r a n e c e s s a r y n o r a s u f f i c i e n t c o n d i t i o n f o r t h e o b s e r v a t i o n of a m e a s u r a b l y l a r g e M ö s s b a u e r r e s o n a n c e e f f e c t a t r o o m t e m p e r a t u r e .

T h e q u e s t i o n of wha t c o n s t i t u t e s s u c h a m e a s u r a b l e e f f e c t i s , of c o u r s e , d e p e n d e n t on t h é d e t a i l s of t h e e x p e r i m e n t a l m e t h o d and the n u m b e r of e v e n t s

* N.S .F . Senior Post Doctoral Fellow and Research Council Fellow.

110

w h i c h a r e s c a l e d a t e a c h D o p p l e r v e l o c i t y p o i n t of t h e s p e c t r u m . F o r t h e p u r p o s e s of t h e p r e s e n t d i s c u s s i o n a p a r a m e t e r , R , w i l l b e d e f i n e d in w h i c h

_ e 3 0 0 - K . . R " e 78°K ( 1 )

w h e r e e i s t h e r e s o n a n c e e f f e c t d e f i n e d b y t h e u s u a l q u a n t i t y

00

a n d t h e s u b s c r i p t s i n E q . (1) r e f e r t o r o o m a n d l i q u i d n i t r o g e n t e m p e r a t u r e , r e s p e c t i v e l y .

I n a u s u a l M ö s s b a u e r e x p e r i m e n t w i t h 119SnmC>2 o r M g 1 1 9 S n m s o u r c e s , t h e r e s o n a n c e e f f e c t a t 78°K w i l l b e t y p i c a l l y of t h e o r d e r of 0 . 1 0 a n d t h e n u m b e r of c o u n t s s c a l e d a t e a c h v e l o c i t y p o i n t i s of t h e o r d e r of 1 X 1 0 5 . U n d e r t h e s e c o n d i t i o n s a n o b s e r v a b l e r e s o n a n c e e f f e c t a t r o o m t e m p e r a t u r e w o u l d h a v e t o b e a t l e a s t

. s t a n d a r d d e v i a t i o n e ^ ~ t o t a l c o u n t s

* ^ 3 . 1 X 1 0 - 3

3 1X 10"3

s o t h a t R m u s t b e e q u a l o r l a r g e r t h a n ~ у щ - 1 ~ 0 .031 b e f o r e t h e p r e s e n c e

of a r e s o n a n c e e f f e c t a t r o o m t e m p e r a t u r e c a n b e e s t a b l i s h e d w i t h a p r o b a -b i l i t y of 0 . 6 6 .

T H E O R Y

T h e m a g n i t u d e of t h e r e s o n a n c e e f f e c t , e , i s d i r e c t l y r e l a t e d t o t h e f r a c t i o n of r e c o i l - f r e e a b s o r p t i o n e v e n t s i n t h e a b s o r b e r . T h i s f r a c t i o n i s d e f i n e d b y t h e r e l a t i o n s h i p

f1 = e" 2 W (3)

i n w h i c h 2W i s t h e D e b y e - W a l l e r f a c t o r f o r t h e s o l i d . A s W e r t h e i m [9] h a s s h o w n , t h e t e r m 2W i n E q . (3) c a n ' b e e x p a n d e d i n t e r m s of a n i n t e g r a l o v e r t h e d e n s i t y of v i b r a t i o n a l s t a t e s s u c h t h a t

w m a x

f 1 = e - c o n s t - f ф (u) du (4) Л

w h e r e ф (и) i s a f u n c t i o n r e l a t e d t o t h e l e v e l s p a c i n g s b e t w e e n a d j a c e n t v i -b r a t i o n a l e n e r g y l e v e l s .

Ill

F o r a m o n a t o m i c s o l i d t h i s f u n c t i o n c a n b e e v a l u a t e d f r o m t h e D e b y e m o d e l of s o l i d l a t t i c e s i n w h i c h t h e a t o m s a r e t r e a t e d a s h a r m o n i c o s c i l -l a t o r s w i t h l e v e l s p a c i n g s g i v e n b y

h к m (5 )

i n w h i c h к i s t h e r e s t o r i n g f o r c e c o n s t a n t f o r t h e h a r m o n i c o s c i l l a t o r a n d m i s t h e a t o m i c m a s s . A s s u m i n g o n l y t h e f i r s t f e w l e v e l s of a q u a n t i z e d o s c i l l a t o r p o t e n t i a l t o b e p o p u l a t e d , t h e d e n s i t y of s t a t e s i s g i v e n t o a g o o d a p p r o x i m a t i o n b y t h e h a r m o n i c o s c i l l a t o r d e n s i t y of s t a t e s

E_ E 2 w [ " m l » (6) e h L к J

w h e r e E i s t h e t o t a l l a t t i c e e n e r g y of t h e s o l i d . F o r a m e t a l l i c l a t t i c e of t i n a t o m s , [ m / k ] i i s s u f f i c i e n t l y l a r g e s o t h a t

t h e n u m b e r of v i b r a t i o n a l l e v e l s w h i c h a r e a p p r e c i a b l y p o p u l a t e d a t r o o m t e m p e r a t u r e i s no t n e g l i g i b l e , a n d r e s o n a n c e e f f e c t s a t t a i n e d u s i n g m e t a l l i c t i n a b s o r b e r s b e c o m e a p p r e c i a b l e on ly a t m o d e s t l y l ow t e m p e r a t u r e s .

I n t r o d u c i n g t h e t i n a s i m p u r i t y a t o m s i n t o a h e a v y - m e t a l l a t t i c e h a s t h e e f f e c t of i n c r e a s i n g t h e f o r c e c o n s t a n t , k , a n d h e n c e d i l u t e s o l u t i o n s of t i n in h e a v y m e t a l m a t r i c e s s u c h a s Au,. P t , P d and Ag c a n g ive r i s e to l a r g e r e c o i l - f r e e f r a c t i o n s a t r o o m t e m p e r a t u r e [10] w h e n t h e s e a l l o y s a r e u s e d a s a b s o r b e r s .

W h e n t h e M ö s s b a u e r a t o m i s p a r t of a l a r g e r m o l e c u l a r un i t - f o r e x a m p l e (CH 3 )4Sn - t h e m o l e c u l e a s a w h o l e m u s t b e c o n s i d e r e d a s t he h a r -m o n i c o s c i l l a t o r s o t h a t

e i = — 2v (7) m 1 v '

w h e r e k 1 i s now t h e f o r c e c o n s t a n t which d e s c r i b e s t h e i n t e r m o l e c u l a r f o r c e s b e t w e e n (CH3>4Sn m o l e c u l e s , a n d M i s t h e m o l e c u l a r w e i g h t of ( С Н з ^ Б п . T h e j u s t i f i c a t i o n f o r t h i s m o d e l i s t h a t t h e c o u p l i n g b e t w e e n t h e l a t t i c e v i -b r a t i o n a l m o d e s of t h e m o l e c u l a r u n i t s a n d t h e o p t i c a l i n t r a - m o l e c u l a r v i -b r a t i o n s - i . e . f o r e x a m p l e , of t h e S n - C b o n d s - i s v e r y w e a k . T h u s , t h e o p t i c a l m o d e s w h i c h l i e a t m u c h h i g h e r f r e q u e n c i e s t h a n t h e p h o n o n m o d e s of t h e c r y s t a l l a t t i c e a r e n o t a p p r e c i a b l y p o p u l a t e d , a n d n e e d no t b e c o n -s i d e r e d i n t h e i n t e g r a t i o n o v e r v i b r a t i o n a l s t a t e s .

F r o m a c o n s i d e r a t i o n of m e l t i n g p o i n t s a n d h e a t s of p h a s e t r a n s i t i o n f o r o r g a n o m e t a l l i c s o l i d s a s c o m p a r e d t o p u r e m e t a l s , i t i s s e e n t h a t t h e i n t e r m o l e c u l a r f o r c e s b e t w e e n m o l e c u l a r u n i t s i s m u c h s m a l l e r t h a n t h e i n t e r a t o m i c f o r c e s in a m e t a l l i c l a t t i c e . M o r e o v e r , t h e m o l e c u l a r w e i g h t

112

of t h e c o m p o u n d i s i n s u f f i c i e n t l y l a r g e t o r e d u c e t h e r e c o i l e n e r g y to v a l u e s l e s s t h a n t h e n a t u r a l l i n e - w i d t h of t h e M ö s s b a u e r n u c l i d e . 1

S i n c e k*<k and M > m , e, and t h e n u m b e r of l a t t i c e v i b r a t i o n a l s t a t e s w h i c h c a n b e p o p u l a t e d a t a g i v e n e x p e r i m e n t a l t e m p e r a t u r e i s l a r g e r t h a n it i s in t h e m e t a l l i c l a t t i c e . F o r t h i s r e a s o n , e v e n l o w e r t e m p e r a t u r e s a r e r e q u i r e d t o o b s e r v e m e a s u r a b l y l a r g e r e s o n a n c e e f f e c t s in m o l e c u l a r s o l i d o r g a n o - t i n a b s o r b e r s t h a n i s r e q u i r e d of t h e s i m p l e m e t a l l i c t i n l a t t i c e .

O n t h e o t h e r h a n d , t h e p r e s e n c e of s t r o n g i n t e r m o l e c u l a r b o n d s w i l l r e d u c e t h e d e n s i t y of v i b r a t i o n a l s t a t e s a n d t h u s i n c r e a s e t h e r e c o i l - f r e e f r a c t i o n o b s e r v e d a t a g i v e n t e m p e r a t u r e . T h i s c a n b e i l l u s t r a t e d b y r e f e r e n c e t o ( C H e b S n F . C l a r k e t a l . [11] h a v e s h o w n t h a t in t h i s c o m p o u n d in t h e s o l i d s t a t e , t h e ( С Н з ) з З п m o i e t y i s n e a r l y p l a n a r a n d a d j a c e n t t r i -m e t h y l t i n u n i t s a r e l i n k e d t h r o u g h f l u o r i n e b r i d g e s . T h e r e s u l t i n g s p 3 d t i n bond i s a p p r e c i a b l y s t r o n g and s e r v e s t o c o u p l e t he l a t t i c e v i b r a t i o n a l m o d e s of a d j a c e n t m o l e c u l a r u n i t s . U n d e r t h e s e c o n d i t i o n s t h e l e v e l s p a c i n g s of t h e h a r m o n i c o s c i l l a t o r b e c o m e

2 ir 'is11 ' M

i (8)

i n w h i c h n i s t h e e f f e c t i v e r e d u c e d m a s s of t h e c o u p l e d m o l e c u l a r u n i t s and k 1 1 i s t h e f o r c e c o n s t a n t w h i c h d e s c r i b e s t h e coup l ing of a d j a c e n t m o l e c u l a r u n i t s . S i n c e t h e f o r m u l a w e i g h t s of (СНз)4 Sn and (CH3)3SnF a r e n e a r l y t h e s a m e ,

ц = M (9)

H o w e v e r , к 1 1 > к > к 1 s o t h a t e 1 1 > e > e 1 , a n d h e n c ë f o r t h e c o u p l e d o s c i l -l a t o r s , t h e d e n s i t y of v i b r a t i o n a l s t a t e s in t h e t e m p e r a t u r e i n t e r v a l 0-» T i s v e r y m u c h r e d u c e d . In s u c h c o u p l e d s y s t e m s , t h e n t h e R v a l u e s s h o u l d a p p r o a c h u n i t y a s k 1 1 i n c r e a s e s .

T h i s m o d e l h a s b e e n t e s t e d i n t h e p r e s e n t s t u d y b y d e t e r m i n i n g t h e m a g n i t u d e of t h e M ö s s b a u e r r e s o n a n c e e f f e c t , e , f o r a n u m b e r of i n o r g a n i c and m e t a l - o r g a n i c t i n c o m p o u n d s a t r o o m t e m p e r a t u r e a n d a t 77°K.

E X P E R I M E N T A L

T h e d e t a i l s of t h e p a r a b o l i c m o t i o n M ö s s b a u e r s p e c t r o m e t e r u s e d i n t h i s s t u d y h a v e b e e n d e s c r i b e d p r e v i o u s l y . T h e o r g a n o - t i n c o m p o u n d s w e r e u s e d a s r e c e i v e d w i t h o u t f u r t h e r p u r i f i c a t i o n . D a t a a c c u m u l a t i o n a t r o o m t e m p e r a t u r e and at l iqu id n i t r o g e n t e m p e r a t u r e w a s e f f e c t e d u n d e r cond i t ions of c o n s t a n t g e o m e t r y a n d w i t h t h e s a m e s o u r c e - a b s o r b e r p a i r . T h e s p e c -t r u m p a r a m e t e r s a r e t h u s d i r e c t l y c o m p a r a b l e . T h e M ö s s b a u e r s o u r c e

1 The necessity for reducing the recoi l energy to a value smal l compared to the natural l ine-width has been discussed by many authors. Among the best introductory mate r ia l in this field are: Frauenfelder, H . , The Mössbauer Effect, Benjamin, W . A . , New York (1962); Goldanskii, V . l . , The Mössbauer Effect and its Application in Chemistry, Consultant's Bureau, New York (1964); Ref.[9] . See also Herber, R .H . , J. Chem. ed . 180 (in press).

113

u s e d in t h i s w o r k w a s 1 1 9Sif"02 i n c o r p o r a t e d i n t o a c e r a m i c m a t r i x of B 2 0 3 - N a 2 C 0 3 [ 3 ] , D e s p i t e t h e l i n e b r o a d e n i n g due t o a s m a l l e x t e n t of q u a d r u p o l e s p l i t t i n g [12] in t h i s s o u r c e , t h e l a r g e r e c o i l - f r e e f r a c t i o n of e m i s s i o n e v e n t s c o m p e n s a t e s f o r t h e l a c k of r e s o l u t i o n of s u c h a s o u r c e . In a t y p i c a l s p e c t r u m , a t l e a s t 105 c o u n t s p e r c h a n n e l w e r e s c a l e d s o t h a t t h e m e a n s t a t i s t i c a l d e v i a t i o n w a s ~ 0.3%.

R E S U L T S AND DISCUSSION

T h e p a r a m e t e r s e x t r a c t e d f r o m t h e M ö s s b a u e r d a t a a r e s u m m a r i z e d i n T a b l e I . A b s o r b e r s ( 1 - 6 ) a r e e x a m p l e s of c o m p o u n d s i n w h i c h t h e t i n a t o m i s known t o b e 6 - c o o r d i n a t e , bu t w h i c h s h o w n o m e a s u r a b l e r e s o n a n c e ef fec t - at r o o m t e m p e r a t u r e . T h e h e x a - c o o r d i n a t i o n of Sn in t h e 8 o x y q u i n o -l a t e s (1, 2) in t h e 1 ,10 p h e n a n t h r o l i n a t e (3) and in t h e 2.21 b i p y r i d y l c o m p o u n d (4) i s b a s e d on i . r . d a t a a n d m o l e c u l a r w e i g h t s t u d i e s [13] . F r o m t h e s e r e s u l t s i t i s c l e a r t h a t t h e b i d e n t a t e l i g a n d s e f f e c t i n t r a - m o l e c u l a r b o n d i n g t o t h e s a m e m e t a l a t o m . T h e b o n d i n g i n p h t h a l o c y a m i n e t i n d i c h l o r i d e (5) a n d r e l a t e d c o m p o u n d s h a s b e e n i n v e s t i g a t e d b y K r o e n k e a n d K e n n e y [14] who c o n c l u d e d t h a t t h e m e t a l a t o m o c c u p i e s t h e c e n t r e of a n e s s e n t i a l l y s q u a r e p l a n a r a r r a y of f o u r n i t r o g e n a t o m s w i t h t h e t w o h a l o g e n l i g a n d s a b o v e a n d b e l o w t h i s p l a n e . T h e n e a r i d e n t i t y of t h e 531 c m " 1 b a n d i n t h e p h t h a l o c y a m i n e d i f l u o r i d e w i t h t h e 560 c m - 1 b a n d i n SnF62" a n d t h a t of t h e 299 c m - 1 b a n d i n t h e p h t h a l o c y a m i n e d i c h l o r i d e w i t h t h e b a n d s o b s e r v e d in t h e 3 3 0 - 2 7 5 c m " 1 r e g i o n of t h e i . r . s p e c t r a of SnCl4 c o m p l e x e s i s t a k e n a s e v i d e n c e f o r t h e n o n - b r i d g i n g of t h e h a l o g e n a t o m s in (5). A s a b o v e , t h i s n o n - p o l y m e r i c , 6 - c o o r d i n a t e t i n c o m p o u n d h a s R ~ 0. S i m i l a r e v i d e n c e f o r t h e n o n - p o l y m e r i c h e x a - c o o r d i n a t e n a t u r e of ( С Н з ) г З п ( а с а с ) г (6) h a s ' b e e n g i v e n b y T o b i a s e t Eil. [15] .

A b s o r b e r s ( 7 - 1 0 ) a r e e x a m p l e s of a n i n f i n i t e c h a i n of r e p e a t i n g u n i t s . In s u c h c o m p o u n d s R > 0, t h e s p e c i f i c v a l u e be ing d e p e n d e n t on t h e s t r e n g t h of t h e c h e m i c a l b o n d s in t h e p o l y m e r . T h e m o l e c u l a r s y m m e t r y of s t a n n i c f l u o r i d e (7) h a s b e e n d e t e r m i n e d b y X - r a y m e t h o d s [16] a n d c o n s i s t s of a d i s t o r t e d o c t a h e d r o n i n w h i c h t h e t w o f l u o r i n e s a t 1 .88 A a r e b o n d e d t o o n e

о Sn a t o m , a n d t h e f o u r r e m a i n i n g fluorines at 2 .02 A s e r v e a s b r i d g i n g l i g a n d s . F o r c e c o n s t a n t s f o r t h e s e b r i d g i n g S n - F - S n b a n d s c a n n o t be c a l c u l a t e d f r o m e x i s t i n g d a t a b u t t h e M ö s s b a u e r r e s u l t s s u g g e s t a p p r e c i a b l e b o n d i n g b e -t w e e n t h e S n F i m o i e t i e s , c o n s i s t e n t w i t h t h e h i g h m e l t i n g and b o i l i n g p o i n t s of t h i s s o l i d . E v i d e n c e f o r t h e h e x a - c o o r d i n a t i o n of t h e m e t a l a t o m in S n 0 2 ( 8 ) h a s b e e n d i s c u s s e d i n d e t a i l e a r l i e r [12] a n d a p p e a r s w e l l e s -t a b l i s h e d . T h e p o l y m e r i c n a t u r e of b i s ( to luene d i th io l ) t i n (9) h a s b e e n d i s -c u s s e d by P o l l e r [17] who n o t e d t h a t t h e r e d , o r g a n i c - s o l v e n t i n s o l u b l e , h i g h -m e l t i n g c o m p o u n d c a n b e s t b e f o r m u l a t e d a s h a v i n g s u l p h u r - t o - t i n b o n d s f o r m i n g a n e x t e n d e d n e t w o r k s t r u c t u r e . T h e p o l y m e r i c n a t u r e of d i m e t h y l t i n d i f o r m a t e (10) h a s b e e n d i s c u s s e d i n d e t a i l b y O k a w a r a a n d O h a r a . [18] A d e t a i l e d e x a m i n a t i o n of t he i . r . s p e c t r u m of bo th t he so l id and of so lu t ions of t h i s c o m p o u n d , a n d a c o m p a r i s o n of t h e s e d a t a w i t h t h o s e f o r d i a l k y l and t r i a l k y l a c e t a t e s s h o w s t h a t t h e f o r m a t e s a r e p o l y m e r i c not only in t h e s o l i d s t a t e b u t a l s o i n c y c l o h e x a n e a n d n - h e p t a n e s o l u t i o n . In c o n t r a s t , t h e c o r -r e s p o n d i n g a c e t a t e s a r e e x c l u s i v e l y m o n o m e r i c in s o l u t i o n [19] .

114

TABLE I

M Ö S S B A U E R P A R A M E T E R S F O R S O M E P E N T A - A N D H E X A - C O O R D I N A T E D T I N C O M P O U N D S

I S- , Q.S. Absorber m m sec"1 R Commen t s

~ , at 77° К ( w . r . t . S n O j )

1: Hexa-coord ina te t in, non-polymer ic

1. (CsHs)¡>Sn(8-0 quin)2

2. ( C I ^ ^ S n ( 8 - 0 quin)2

3. (CH3)¿SnCl2( l ,10 phenan) 2

4. (СЦ)?ЗпС12(2,2' bipyridyl)

5. Phthaloc. Sn Cl 2

6. ( C H j J ^ n (acac) 2

0.637

0.771

1.32

1.55

0.31

1.18

1.53

1.98

4.03

4.09

0.977

3 . 9 3

2.40

2.83

3.05

2.68

3.15

3.33

7. SnF4

8. S n 0 2

9. Sn ( toluene 3 , 4 di thiol) 2

10. (CHjJüSn (OCHO)2

KzSnFj

CSjSnFj

-0 .47

0

1.40

1.45

-0 .50

- 0 . 4 4

2: Hexa -coord ina t e tin, po lymer ic

1.8

Ó.2

1.52

4.72

1.08

3.26

0.73 Ref. [ 8 . 2 ]

0 .74 Ref. [8]

0 .33 Ref. [ 7 ] , see also Ref. [28]

0 .44

0 .38 Ref. [8]

0 .44 Ref. [8]

TRS 10/10 T a b l e I continued

Absorber I . S .

m m sec-i ( w . r . t . Sn0 2 )

Q . S . . a t 77°К C o m m e n t s

3: Pen ta -coord ina te t in , po lymer ic

11. (CH3)jßnF

12. (CH$)3Sn i m i d a z o l e

13. (CH3>¡Sn (1, 2, 4 t r iazole)

(CH3)3SnN3

1.18

.1.16

1.27

1.24

3.47

2.76

2.96

3 .23

2 .93

2.37

2.33

2.60

0 .12 Ref. [11] , see also Ref . [29]

0 .065

0.095

0.058 Ref. [24]

Mg2Sn

(CH3)3SnOH

(CH3)3Sn(OCHO)

- 1 . 8 2

1.07

1.35

4; Miscel laneous t i n compounds

0

2 .71

3 .52

2.53

2.61

0 .35 Ref. [30] , Anti C a F j s tructure ( b . c . c . )

0 known to b e d i m e r i c in

solution.Ref. [31]

0

(CH3)3Sn(OCOCH3)

(Cf¡H5)3Sn(OCOCH3)

( n Q H9)3Sn (OCOCH3)

Sn (OCOCH3)4

1.34

1.20

1.38

0.18

3.50

3.17

3.67

0

2.61

2 .64

2.66

0

0.078

0

0

(CH3)2Sn(OCOC f i l5)2

( n Q H ^ n (OCOCgH5J2

1.40

1.62

3.96

3 . 4 4

2 .83

2.12

TRS 1 0 / 1 1 T a b l e I cont inued

Absorber I .S .

m m sec i ( w . r . t . S n 0 2 )

Q . S . a t 77°К

R Commen t s

[(CHj^Sn^ (OCOC'CCOJ 1.40 3 .81 2 .71 o'

[ (nC4H,) £ n ] 2 (OCOC — CCO)2 1.25 3.62 2.90 0

[ (CH3) £ n (OCOCH3)]2 О 1.38 3.57 2.58 0.059 Sn-O-Sn and SnOCOSn

bridging groups. Ref. [32]

Sn (grey) 2 .1 0 0 .115 R in terpola ted f r o m

da ta of Ref. [33]

T h e f a c t t h a t h e x a - c o o r d i n a t i o n of t h e t i n a t o m i s no t a n e c e s s a r y c o n -d i t i o n f o r R > 0 i s s u p p o r t e d b y t h e d a t a f o r a b s o r b e r s ( 1 1 - 1 3 ) f o r w h i c h p e n t a - c o o r d i n a t i o n of t h e m e t a l a t o m h a s b e e n i n f e r r e d f r o m a v a r i e t y of p h y s i c a l m e a s u r e m e n t s . T h e s t r u c t u r e of t r i m e t h y l t i n f l u o r i d e (11) h a s b e e n d i s c u s s e d by C l a r k e t a L [ l l ] , A c c o r d i n g t o t h e s e a u t h o r s t h e f l u o r i n e a t o m p o s i t i o n s a r e d i s o r d e r e d with t he h a l o g e n a t o m s occupy ing any p o s i t i o n o n a s p h e r i c a l s u r f a c e a b o u t 2 . 1 5 Â a w a y f r o m o n e t i n a t o m . T h e b r i d g i n g S n - F . . . . S n b o n d i s t h u s not l i n e a r and h e n c e t h e c h e m i c a l bond ing i n v o l v e s c o n s i d e r a b l e c o v a l e n t i n t e r a c t i o n b e t w e e n t h e f l u o r i n e and bo th m e t a l a t o m s . S i m i l a r s t r u c t u r e s invo lv ing p l a n a r o r n e a r l y p l a n a r (СНз)з Sn m o i e t i e s bon-d e d by v a r i o u s b r i d g i n g g r o u p s h a v e b e e n s u g g e s t e d f o r (СНз)з SnOH, [20, 21] (СНз)з S n N 0 3 , [ 2 2 , 23] ( C H 3 ) 3 S n C 1 0 4 [ 2 3 ] a n d ( C H 3 ) 3 S n N 3 [ 2 4 ] .

A f u r t h e r g r o u p of c o m p o u n d s in wh ich t h e t i n a t o m i s p e n t a - c o o r d i n a t e d i n c l u d e s t h o s e i n v o l v i n g b i d e n t a t e n i t r o g e n h e t e r o c y c l i c l i g a n d s s u c h a s t r i -m e t h y l t i n i m i d a z o l e (12) and t r i m e t h y l t i n 1, 2, 4 t r i a z o l e (13). T h e s e c o m -p o u n d s h a v e b e e n p r e p a r e d and s t u d i e d in d e t a i l by v a n d e r K e r k and h i s c o -w o r k e r s [ 2 5 - 2 7 ] w h o h a v e i n f e r r e d a p o l y m e r i c c h a i n s t r u c t u r e i n v o l v i n g ( C H 3 ) 3 S n g r o u p s b r i d g e d b y t h e l i g a n d s f r o m i . r . d a t a , m e l t i n g p o i n t s a n d c h e m i c a l r e a c t i v i t y a n d s o l u b i l i t y .

F r o m t h e s e d a t a i t i s c l e a r t h a t h e x a - c o o r d i n a t i o n - p r e s u m a b l y i n -v o l v i n g s p 3 d 2 h y b r i d i z a t i o n of t h e m e t a l a t o m - i s n e i t h e r a n e c e s s a r y n o r a s u f f i c i e n t cond i t i on f o r o b s e r v i n g a m e a s u r a b l y l a r g e M ö s s b a u e r r e s o n a n c e e f f e c t at r o o m t e m p e r a t u r e (ie : R > 0). T h e d e c i s i v e f a c t o r wh ich g o v e r n s w h e t h e r o r not r e c o i l - f r e e у - r a y a b s o r p t i o n o c c u r s t o any a p p r e c i a b l e ex ten t a t r o o m t e m p e r a t u r e i s t h e m a g n i t u d e of t h e i n t e r m o l e c u l a r b o n d i n g f o r c e s p r e s e n t in t h e s o l i d . On t h e b a s i s of t h e v e r y s i m p l e m o d e l s u g g e s t e d above ( E q s . 5, 7 a n d 8) a n o r d e r - o f - m a g n i t u d e c a l c u l a t i o n of t h e f o r c e c o n s t a n t s c a n b e m a d e f r o m t h e a v a i l a b l e i . r . d a t a .

C l a r k and O ' B r i e n [23] have a s s i g n e d the 1212 cm* 1 band in t he s p e c t r u m of t r i m e t h y l t i n P e r c h l o r a t e t o t h e O - C l - O a s y m m e t r i c s t r e t c h . T h i s i s 3 0 t o 60 c m - 1 s m a l l e r t h a n t h e c o r r e s p o n d i n g a b s o r p t i o n i n i o n i c P e r c h l o -r a t e s . Wi th a r e d u c e d m a s s c a l c u l a t e d on ly f r o m t h e oxygen and t i n a t o m i c w e i g h t s , a n d i g n o r i n g t h e n o n - l i n e a r i t y of t h e S n - O - C l b o n d , t h e c a l c u l a t e d i n t e r m o l e c u l a r f o r c e c o n s t a n t i s 1 .74X 106 dyn c m " 1 . A s i m i l a r c a l c u l a t i o n f r o m t h e i . r . d a t a of T h a y e r a n d W e s t [24] f o r (СНз) 3SnN3 l e a d s t o a f o r c e c o n s t a n t ( ba sed on t h e 2045 c m - i b a n d a s s i g n e d by t h e s e a u t h o r s t o t h e N-N-N a s y m m e t r i c s t r e t c h ) of 1.73 X 10® dyn c m r l .

C lea r ly" , t h e s e i n t e r m o l e c u l a r f o r c e s a r e n o t n e g l i g i b l e , a n d t h e i r m a r k e d e f f e c t on t h e p h o n o n l e v e l s of t h e s e s o l i d s c a n r e a d i l y b e o b s e r v e d i n t h e m a g n i t u d e of t h e r e s o n a n c e e f f e c t in t h e M ö s s b a u e r s p e c t r a of t h e s e c o m p o u n d s . A m o r e q u a n t i t a t i v e r e l a t i o n s h i p b e t w e e n i n t e r m o l e c u l a r b o n d i n g f o r c e s a n d e m u s t b e b a s e d o n a d e t e r m i n a t i o n of d e / d T o v e r a r e a s o n a b l y w i d e t e m p e r a t u r e r a n g e r a t h e r t h a n f o c u s s i n g a t t e n t i o n on t h e R r a t i o a t t w o a r b i t r a r y t e m p e r a t u r e s . S u c h m e a s u r e m e n t s - w h i c h r e -q u i r e p r e c i s i o n c r y o g e n i c t h e r m o s t a t i n g o v e r e x t e n d e d p e r i o d s of t i m e w i t h -ou t t h e i n t r o d u c t i o n of e x t r a n e o u s v i b r a t i o n s i n t o t h e e x p e r i m e n t a l s y s t e m -a r e a t p r e s e n t u n d e r w a y in t h e s e l a b o r a t o r i e s .

It i s a l s o e v i d e n t t h a t s u c h q u a n t i t a t i v e k n o w l e d g e of t h e t e m p e r a t u r e c o e f f i c i e n t s of f 1 f o r m e t a l o r g a n i c c o m p o u n d s c a n b e e x p l o i t e d in t h e s t u d y

118

of p o l y m e r i z a t i o n k i n e t i c s of i n t e r m o l e c u l a r l y b o n d e d c o v a l e n t p o l y m e r s a s w e l l a s i n t h e e l u c i d a t i o n of b o n d i n g i n t h e m o l e c u l a r u n i t s t h e m s e l v e s . C o r r e l a t i o n of s u c h d a t a w i t h s h i f t s o b s e r v e d i n t h e i . г . . s p e c t r a of m o n o -m e r i c a n d p o l y m e r i c m o l e c u l e s , a s w e l l a s w i t h o t h e r m o l e c u l a r s p e c t r o -s c o p i c p a r a m e t e r s s h o u d l e a d t o a b e t t e r i n s i g h t i n t o t h e a r c h i t e c t u r e of m e t a l - o r g a n i c c o m p o u n d s .

A C K N O W L E D G E M E N T S

T h i s w o r k w a s s u p p o r t e d in p a r t by t h e US A t o m i c E n e r g y C o m m i s s i o n , t h e P e t r o l e u m R e s e a r c h F u n d of t h e A m e r i c a n C h e m i c a l S o c i e t y a n d t h e R e s e a r c h C o u n c i l of R u t g e r s U n i v e r s i t y . T h i s s u p p o r t i s h e r e b y g r a t e f u l l y a c k n o w l e d g e d . T h e a u t h o r s a r e a l s o i n d e b t e d t o P r o f e s s o r R. S. T o b i a s a n d M . K e n n e y a n d D r . G . J . M . v a n d e r K e r k , W. C o n s i d i n e , L . M . E p s t e i n , W. T . R e i c h l e , W. G e r r a r d and R. B a r b i e r i f o r t h e i r g e n e r o s i t y in m a k i n g t h e o r g a n o - t i n c o m p o u n d s a v a i l a b l e f o r s t u d y .

R E F E R E N C E S

[1] GOLDANSKII, V . l . , e t a l . , Dokl. Akad. Nauk SSSR 151 (1963) 357. ibid, 156 (1964) 909; ibid, 147 (1962) 127.

[2] CORDEY-HAYES, M . , J. inorg. nucl . Chem. 26 (1964)915. [3] HERBER, R .H. , STÖCKLER, H . A . , Proc. N . Y . A c a d . Sei. 26 (1964)929. [4] HERBER. R .H. , STÖCKLER, H . A . , REICHLE. W . T . , J. chem. Phys. (in press) [5] SHAPIRO, V . G . , SHPINEL, V . S . , Soviet Phys. IETP (Engl. Trans i . ) 19 (1964) 1321 e t an te . [6] BRYUKHANOV, V . A . , DELYAGIN, N . N . , KUZMIN, R.N., Soviet Phys. JETP (Engl.Transi.cf. ) 19(1964)

98. et ante. [7] EPSTEIN, L.M. , 149th Amer. Chem. Soc. Meeting, Detroit, April 1965. [8] GOLDANSKII, V . l . , e t a l . Dokl. Akad. Nauk SSSR156 (1964) 400. [9] WERTHEIM, G. K., The Mössbauer Effect; Principles and Applications, Academic Press, New York (1964).

[10] BRYUKHANOV, V . A . , DELYAGIN, N . N . , SHPINEL, V .S . , Soviet Phys. JETP 47 (1964) 80; Engl. Trans. 20 (1965) 55.

[11] CLARK, H . C . , O'BRIEN, R.J. , TROTTER, J . , Proc. chem. Soc. (1963) 85. [12] HERBER, R.H. , SPUKERMAN, J . J . , J. chem. Phys. (in press) [13] BEATTIE, I .R. , MCQUILLAN, G.P . , RULE, L., WEBSTER, M . , J. chem. Soc. (1963) 1514;

BARBIERI, R., e t a l . , J. Organometal. Chem._1_(1964)427. [14] KROENKE, W., KENNEY, M.E. , Inorg. Chem. _3_(1964) 251 loc. cit . (1964) 696. [15] TOBIAS, R. S. , private communication. [16] HOPPE, R., DAEHNE, W., Naturwissenschaften 49 (1962) 254. [17] POLLER, R.C. , Proc. chem. Soc.- 10 (1963) 312. [18] OKAWARA, R., OHARA, M. , J. Organometal. Chem. J J 1 9 6 4 ) 360. [19] JANNSSEN, M . I . , LUITJEN, J . G . A . , VAN DER KERK, G . J . M . , Reel. Trav. chim. Pays-Bas Belg. 82

(1963) 90. [20] OKAWARA, R., YASUDA, K., J. Organometal. Chem. 1 (1964)356. [21] KASAI, N . , YASUDA, K., OKAWARA, R., J. Organometal. Chem._3_(1965) 172. [22] YASUDA, K., OKAWARA R., J. Organometal. Chem. 3 (1965) 76. [23] CLARK, H . C . , O'BRIEN, R.J . , Inorg. Chem. 2, (1963) 740. [24] THAYER, J . S . , WEST, R., Inorg. Chem._3_(1964) 406. [25] LUITJEN, J . G . A . , JANSSEN, M . J . , VAN DER KERK, G . J . M . , Reel. Trav. c h i m . Pays-Bas Belg. 81

(1962) 202.

119

[26] JANSSEN, M.J . . LUITJEN, J . G . A . , VAN DER KERK, G . J .M. , J. Organometal. Chem. 1_(1964) 286. [27] LUITJEN, J .G .A. , VANDERKERK, G . J .M. , Reel. Trav. chim. Pays-Bas Belg. 82^(19637ll81. [28] HERBER, R.H., Rev. mod. Phys. 3£A (1964) 461. [29] KRIEGSMANN, H. , PISCHTSCHAN, S. , Z. anorg. allg. Chem. 308 (1961) 212. [30] BRYUKHANOV, V .A . , DELYAGIN, N.N. , KUZ'MIN, R.N. , Soviet Phys. JETP, (Engl. Transi.) 19

(1964) 98. —

[31] KRIEGSMANN, H. , PISCHTSCHAN, S. , Z. anorg. allg. Chem. 315 (1962) 283. [32] REICHLE, W . T . , J. Polym. Sei. 49 (1961) 521. [33] BOYLE, A . J . F . , HALL, H.E. , Rep. Prog. Phys. 25 (1962)441.

D I S C U S S I O N

V. I . G O L D A N S K I I p o i n t e d ou t t h a t i s w a s r i s k y t o g i v e r u l e s w i t h o u t t ak ing in to accoun t a d d i t i o n a l c o o r d i n a t i o n , dir - pir bonds and i n t e r m o l e c u l a r r e l a t i o n s . F o r i n s t a n c e , m o s t of t h e c o m p o u n d s in which t i n w a s c o n n e c t e d w i t h f o u r c a r b o n a t o m s d i d n o t s h o w q u a d r u p o l e s p l i t t i n g , b u t i n (С2Нб)з Sn C H 2 C O C H 3 t h e r e w a s a d i s t i n c t q u a d r u p o l e s p l i t t i n g p r o b a b l y due t o S n - O bond ing . T h e dir - p^ b o n d s l e d t o a s t r o n g q u a d r u p o l e s p l i t t i n g (Д) of 1 m m / s [I. S. = 1 . 4 m m / c r e f e r r e d t o a SnC>2 s o u r c e ] in ReSn - С s C-SnR.3. In (СбH5)3 S n C 6 F s , a g a i n wi th f o u r Sn -C b o n d s , t h e s t r o n g a c c e p t o r p r o p e r -t i e s of C 6 F 5 l e d t o a A v a l u é of 1 .1 m m / s . S i m i l a r l y t h e d i f f e r e n c e i n t h e Д v a l u e s of t h e c o m p o u n d s

( С 6 Н 5 ) з S n C l Д 2 .44 m m / s б 1 .3 5 m m / s

a n d ( С б F 5 ) з S n C l Д 1 .55 m m / s 6 1.2 m m / s

w a s due t o s t r o n g a c c e p t o r p r o p e r t i e s of t he C6F5 g r o u p . N . N. G R E E N W O O D a s k e d w h e t h e r t h e a u t h o r w a s th ink ing of Sn IV c o m -

p o u n d s . R . H. H E R B E R r e p l i e d tha t he was th ink ing of Sn° o r SnIV .

120

MÖSSBAUER PARAMETERS FOR METAL-ORGANIC (Fe, Sn) COMPOUNDS

R.H. HERBER THE STATE UNIVERSITY

NEW BRUNSWICK, N.J. , UNITED STATES OF AMERICA

In t he a c c o m p a n y i n g t a b l e s , t h e i s o m e r s h i f t ( I .S . ) and q u a d r u p o l e s p l i t -t i n g (Q.S. ) p a r a m e t e r s r e p o r t e d i n t h e l i t e r a t u r e h a v e b e e n t a b u l a t e d . A l -t h o u g h t h i s c o m p i l a t i o n h a s m a d e u s e of t h e o p e n l i t e r a t u r e a n d s o m e u n -p u b l i s h e d r e s u l t s of l i m i t e d c i r c u l a t i o n , no c l a i m i s m a d e f o r a l l - i n c l u s i v e n e s s , n o r s h o u l d t h e r e s u l t s a n d r e f e r e n c e s w h i c h a r e l i s t e d b e t a k e n a s i m p l y i n g any j u d g e m e n t s c o n c e r n i n g p r i o r i t y of c l a i m s o r a c c u r a c y of d a t a . T h e a u t h o r would w e l c o m e any c o m m e n t s on c o r r e c t i o n s o r a d d i t i o n s which m a y b e a p p r o p r i a t e .

T h e i s o m e r s h i f t s f o r o r g a n o - i r o n c o m p o u n d s have b e e n r e c a l c u l a t e d a s s h i f t s f r o m t h e c e n t r e of t h e s o d i u m n i t r o p r u s s i d e N a 2 [ F e ( C N ) 5 N O ] 2 H 2 0 s p e c t r u m . T h i s c a l c u l a t i o n h a s m a d e u s e of t h e f o l l o w i n g s h i f t s w h i c h a r e t h o u g h t t o b e t h e b e s t c u r r e n t l y a c c e p t e d v a l u e s f o r a r o o m t e m p e r a t u r e n i t r o p r u s s i d e a b s o r b e r a n d a r o o m t e m p e r a t u r e s o u r c e ( i n m m s e c - 1 ) :

S o u r c e Sh i f t f r o m s t d . S o u r c e S h i f t f r o m s t d .

P t + 0 . 6 0 7 K 4 F e ( C N ) 6 • 3 H 2 0 + 0 . 2 1 5

C u + 0 . 4 8 3 3 1 0 S S , V a c r o m . + 0 . 1 8 2

P d + 0 . 4 4 2 3 0 2 S S + 0 . 1 7 5

F e + 0 . 3 4 8 C r + 0 . 0 7 5

+ 0 . 3 5 1 N a 2 [ F e ( C N ) 5 N O ] • г и г е 0

T h e i s o m e r s h i f t s f o r o r g a n o - t i n c o m p o u n d s h a v e b e e n r e c a l c u l a t e d a s s h i f t s f r o m t h e c e n t r e of t h e SnÛ2 s p e c t r u m . T h e c h o i c e of s t a n n i c oxide -d e s p i t e t h e f a c t t h a t t h i s m a t e r i a l w h e n u s e d e i t h e r a s a s o u r c e o r a b s o r b e r g i v e s r i s e t o a w i d e ( o r q u a d r u p o l e s p l i t ) l i n e - i s b a s e d o n t h e w i d e u s e of t h i s m a t e r i a l a s a M ö s s b a u e r s o u r c e . W h e r e r e c a l c u l a t i o n s of i s o m e r s h i f t d a t a w e r e n e c e s s a r y , t h e f o l l o w i n g v a l u e s w e r e u s e d ( in m m s e c - 1 ) :

Sn ( g r e y ) Sn ( w h i t e ) M g 2 S n S n 0 2

+ 2. 10 + 2 . 70 + 1 . 8 2 0

Q u a d r u p o l e s p l i t t i n g s a r e e s s e n t i a l l y t h o s e r e p o r t e d in t h e r e f e r e n c e s c i t e d . S o m e of t h e e a r l y R u s s i a n l i t e r a t u r e r e f e r s t o l i n e p o s i t i o n s of t h e t w o c o m p o n e n t s , a n d t h e r e t h e o b v i o u s c a l c u l a t i o n h a s b e e n c a r r i e d o u t .

121

TABLE I .

ORGANO-IBON COMPOUNDS

T e m p . I . S. Q . S . Absorber , / . к . . j . Ref.

( К) ( m m sec ) (mm sec )

I . Non-ir bonding carbonyls

Fe (CO) s

Fe 2 (CO) 9

Fe3( CO) 12

Fe(CO) 4 I 2

Na 2 Fe(CO) 4

N a 2 F e 3 ( C O ) u

78

78

140

143

158

184

203

227

147 (soin)

201 (soin)

78

143

153

158

298

78

143

147

188

78

78

147

298

298

298

0 . 1 7

0.201

0 .160

0 Л 7

0 .167

0 . 1 3 1

0 .132

0 . 1 4 1

0 .181

0.165

0 .36

0 .415

0 .412

0 .405

0 . 4 3

0 . 2 9 ) 0 . 3 6 I

0 . 3 0 5 1 0 . 3 6 5 ]

0 . 3 0 3 0 .356

0 .85

0 . 3 9 9

0 . 7 9 7

0 . 8 4

0 .110

2 . 5 7

2 . 6 0

2 . 5 3

2 . 5 3

2 . 5 3 7

2 . 5 4 3

2 . 5 4 3

2 . 5 0 5

2 . 5 6

2 . 5 8

0 . 5 4

0 . 4 0

0 .400

0 .392

0 . 3 7 0 }

1.12 I

0 1.10

0 1 . 0 9 3

0 . 3 8 j 0 ) 0 . 4 3 j 1 .05 J

0 .85

0 . 3 8

0 . 7 4

0 .72

0 . 26 ) 1 .32 1 .28 J 0.2 I

[ 7 , 1 4 ]

[10]

[9]

[ 5 ]

[16] [16] [16] [ 1 6 , 9 ]

[ 9 , 1 6 ]

[ 9 , 1 6 ]

[ 7 . 1 4 ]

[ 1 5 ]

[ 9 , 1 6 ]

[16] [8.16]

[ 7 , 1 4 ]

[ 1 5 ]

[ 9 , 1 6 ]

[ 9 , 1 6 ]

[ 7 , 1 4 ]

[11] [9]

[ 1 4 ]

[1] [1]

122

TABLE II ( c o n t . )

Absorber T e m p .

C K )

I . S .

(mm sec"1)

Q . S .

(mm sec"1) Ref.

( C H 3 C H 2 ) 3 P F e ( C O ) 4 78 0 .135 2 . 3 1 [ 1 1 ]

( C 6 H 5 ) 3 P F e ( C O ) 4 78 0 . 1 4 2 . 4 2 [ 7 ]

78 0 .169 2 . 5 4 [ 1 1 ]

t ( C , ¡ H s ) 3 P ] 2 F e ( C O ) 3 78 0 .159 2 . 7 6 [ 1 0 ]

{ [ ( C H 3 ) j N ] 3 P } 2 F e ( C O ) j 78 0 . 1 2 2 . 2 7 [ П

[ ( CH 3) 2N] З PFe( C O ) 4 78 0 .12 2 . 2 2 [ 7 ]

C 3 F 7 F e ( C O ) 4 I 78 0 . 2 3 0 .56 [ 7 ]

H g [ F e ( C O ) 2 N O P ( C 6 H s ) 3 ] 2 78 0 . 2 4 1 . 4 2 [ 7 ]

H g [ F e ( C O ) 3 N O ] 2 78 0 . 2 4 1 .26 [ 7 ]

[ ( C 4 H , ) 2 S n F e ( C O ) 4 ] 2 78 0 . 2 4 0 . 2 [ 7 ]

[ F e ( C O ) 3 S C H 3 ] 2 78 0 . 2 8 0 . 8 9 [ 7 ]

[ F e ( C O ) 3 S C H 2 C H 3 ] 2 1 0 0 - 3 0 0 0 . 1 8 0 . 8 6 [ 1 2 ]

[ F e ( C O ) 3 S C ( C H 3 ) 3 ] 2 78 0 . 2 9 0 . 9 3 [ 7 ]

( C 6 H s ) 3 P F e ( C O ) ( N O ) 2 78 0 .26 0 . 5 0 [ 7 ]

C 2 F 4 S 2 F e 2 ( C O ) 6 78 0 . 3 0 1 .05 [ 7 ]

C 4 F 6 S 2 F e 2 ( C O ) 6 78 0 . 3 0 1 . 3 4 [ 7 ]

F e 3 ( C O ) 9 S e 2 1 0 0 - 3 0 0 0 . 3 2 0 . 5 4 [ 1 2 ]

F e 3 ( C O ) 8 S 2 1 0 0 - 3 0 0 0 . 3 0 0 . 8 4 [ 1 2 ]

II . ir-bonding carbonyls

Trans [ ( C 6 H5)CH=CHCHC0 Fe( CO) 4 78 0 .245 1 .75 [ 1 1 ]

{[CH2=CHCH2 OH] Fe( CO)4}+BF4" 78 0 .325 1 . 0 1 [ U ]

(CH 2 =CH-CH=CH 2 )Fe (CO) 3 78 0 .285 1 , 4 6 [ 1 0 ]

78 0 .22 1 . 2 3 [ 7 ]

[ HÇOCH=C HÇOO] Fe( CO ) 4 78 0 . 2 6 9 1 . 4 1 [11 ]

[ CH3 C H = C H - C H = C H - C 0 2 H] Fe( CO ) 3 78 0 .305 1 . 6 3 [ 1 0 ]

78 0 . 2 9 1 .737 [ 7 ]

C 5 H7 CONH2 Fe( CO ) 3 78 0 . 3 0 1 . 5 4 [ 7 ]

{ [ CH3 CH=CHCH(OH)CH 3 ] Fe(CO)4}+BF4" 78 0 . 3 0 9 1 .16 [ 1 1 ]

G 6 H j F e ( C O ) 3 78 0 . 2 4 1 . 4 3 [ 7 ]

C e H I 0 C O F e ( C O ) 3 78 0 . 2 8 1 . 4 9 [ 7 ]

[ C H 3 C H ( O H ) C H = C H - C H = C H 2 ] F e ( C O ) 3 78 0 . 2 8 1 1. 56 [ 1 0 ]

[CH s CH(OCH 3 )CH=CH-CH=CH 2 ]Fe(CO) 3 78 0. 265 1 . 7 1 [10]

123

TABLE II (cont . )

Absorber T e m p .

C K ) I . S .

(mm sec" 1 ) Q . S .

(mm sec" 1 ) Ref.

C 7 H 6 O F e ( C O ) 3 78 0 . 2 9 1 . 4 9 [ 7 ]

7 - ( O C O C H 3 K G , H i ) F e ( C O ) 3 78 0 .292 2 . 0 1 [ 1 0 ]

C 8 H 8 F e 2 ( C O ) 6 78 0 .26 1 .32 [ 7 . 8]

C , H 8 F e ( C O ) 3 78 0. 31 1 . 2 3 [ 7 . 8 ]

78 0 . 3 4 3 1 .26 [ 1 0 ]

C i H , F e 2 ( C O ) 6 B F 4 78 0 . 3 7 1 . 2 0 [ 7 ]

C e H 9 F e 2 ( C O ) 6 P F 6 78 0 . 3 8 1 . 1 3 [ 7 ]

C 8 H 8 O F e 2 ( C O ) 6 78 0 . 3 3 0 . 8 4 [ 7 ]

C , H , S F e 2 ( C O ) 6 78 0 . 2 4 1 . 1 1 [ 7 ]

C 1 0 H 6 ( C H 2 ) 2 F e ( C O ) 4 78 0 .247 1 . 7 8 [ 1 1 ]

[ ( C 6 H 5 ) C H = C H - C H = C H 2 ] F e ( C O ) 3 78 0 .305 1 . 5 9 [ 1 0 ]

C , 2 H 8 F e 2 ( C O ) 6 78 0 . 6 7 0 . 8 8 [ 7 ]

(5, 6 d ime thy l -1 , 3, 7, 9 t e t r a -d e c a e n e ) F e 2 ( C O ) 6 ( r a c e m i c ) 78 0 . 2 6 7 1 . 6 9 [ 1 0 ]

( m e s o ) 78 0 .275 1 . 5 8 [ 1 0 ]

Al loocimine F e ( C O ) 3 78 0 .269 1 . 7 5 [ 1 0 ]

[ C 3 H 5 F e ( C O ) 3 I ] 2 78 0 .35 1 . 2 8 [ 7 ]

III. Cyclopentadienyls

Ferrocene 4 . 2 0 .76 2 . 3 2 [ 1 3 ]

20 0 .77 2 . 3 8 [ 2 ]

78 0 . 7 6 2 . 3 7 [ 2 ]

78 0 . 7 8 2 . 3 9 [ 3 ]

78 0 . 7 8 2 . 4 0 [ 4 ]

78 0 . 8 4 2 . 3 8 [ 5 ]

295 0 . 7 0 2 . 3 4 [ 6 ]

298 0 . 7 1 4 2 . 3 5 4 [ 1 ]

298 0 . 6 8 2 . 3 6 [ 2 ]

( C 5 H 5 ) F e ( C 5 H 4 ) C O O H 298 0 . 6 9 8 2 . 1 6 4 [ 1 ]

( C 5 H 5 ) F e ( C 5 H 4 ) ( C H 2 ) 3 C O O H 298 0 .700 2 .342 [ 1 ]

( C 5 H 5 ) F e ( C 5 H 4 ) C O C H 3 298 0 .678 2 . 2 3 7 [ 1 ]

( C s H 5 ) F e ( C 5 H 4 ) N 0 2 298 0 . 6 9 3 2 . 2 6 7 [ 1 ]

[ ( C 5 H 5 ) F e ( C s H 4 ) ] 2 C H 2 C H 2 298 0 .700 2 .312 [ 1 ]

( C 5 H 5 ) F e ( C s H 4 ) C ( C H 3 ) 3 298 0 .703 2 .325 [ 1 ]

124

TABLE II (cont . )

Absorber T e m p .

C K ) I . S .

( m m sec" 1 ) Q . S .

(mm sec"1) Ref.

( C 5 H 5 ) F e ( C 5 H 4 ) C H 3 18 0 . 7 7 2 . 3 9 [13 ]

( C 5 H 5 ) F e ( C 5 H 4 ) C H 2 O H 78 0 . 7 7 2 . 3 9 [ 3 ]

( C 5 H 5 ) F e ( C 5 H 4 ) C N 78 0 . 7 8 2 . 3 6 [ 3 ]

( C 5 H 5 ) F e ( C 5 H 4 ) C O C 1 5 H 3 1 - 78 0 . 8 0 2 . 3 0 [ 3 ]

(C 5 H 5 )Fe (C 5 H 4 )COCH 3 78 0 . 8 0 2 . 2 5 [ 3 ]

F e [ C 5 H 4 ( C O C l 2 H 2 5 ) ] 2 78 0 . 3 1 2 . 2 3 [ 3 ]

( C 5 H 5 ) F e ( C 5 H 4 ) ( C H 2 ) 2 C O O H 78 0 . 7 8 2 . 2 2 [ 3 ]

F e [ C 5 H 4 ( C H 2 ) 5 C O O H ] 2 78 0 . 7 9 2 . 2 2 [ 3 ]

Fe [C5 H4 ( COCH3 ) ] 2 78 0 . 7 8 2 . 1 5 [ З ]

( C H 3 C O ) ( C 5 H 3 ) F e ( C 5 H 4 ) ^ЗНз CH2 CH 2

J

78 0 . 7 7 2 . 0 5 [ 3 ]

( C 5 H 4 ) F e ( C 5 H 4 ) GH2 CH2

78 0 . 7 4 2 . 3 0 [ 2 ]

( Ç 5 H 4 ) F e ( C 5 H 4 ) 1 ' CH( CH3)OCH( СН 3 )~ '

78 0 . 7 1 2 . 3 2 [ 2 ]

[ ( C 5 H 5 ) F e ( C 5 H 4 ) ] 2

bifer rocenyl 78 0 . 7 7 2 . 3 4 [ 2 ]

[ ( C 5 H 5 ) F e ( C 5 H 4 ) ] 2 H G difer rocenyl mercury

78 2 . 3 2 [ 2 ]

( C s H s J F e í C s ^ J H g C l 78 2 . 3 4 [ 2 ]

[ ( C 5 H 5 ) F e ( C 0 ) 2 ] 2 C 0 ( C F 2 ) 3 ¿ 0 78 0 . 2 7 1 . 8 5 [ 7 ]

( C 5 H5 )Fe( CO ) 2 COCH 3 78 0 . 2 7 1 . 6 8 [ 7 ]

{ ( C 5 H 5 ) F e ( C O ) 2 P [ N ( C H 3 ) 2 ] 3 } l 78 0 . 2 8 1 . 7 8 [ 7 ]

( C 5 H 5 ) F e ( C O ) 2 C O N ( C 2 H 5 ) 2 78 0 . 2 9 1 . 7 3 [ 7 ]

C 5 H5 Fe (CO ) 2 ( CH 2) 3 COFe( CO ) 2 C2 H5 78 0 . 2 9 1 . 6 6 [ 7 ]

[ C 5 H 5 F e ( C O ) 2 ] 2 C O ( C H 2 ) 4 C O 78 0 . 3 0 1 . 7 0 [ 7 ]

{ C 5 H 5 F e ( C O ) S [ C ( C H 3 ) 3 ] } 2 78 0 . 5 4 1 . 7 0 [ 7 ]

[ C s H 5 F e ( C O ) S ( C H 3 ) ] 2 78 0 . 5 3 1 . 6 6 [ 7 ]

C 5 H 5 F e ( C O ) 2 B r 78 0 . 4 8 1 . 8 7 [ 7 ]

C 5 H 5 F e ( C O ) 2 C l 78 0 . 4 7 1 . 8 8 [ 7 ]

C 5 H 5 F e ( C O ) 2 I 78 0 . 4 6 1 . 8 3 [ 7 ]

[ C 5 H 5 F e ( C O ) 2 ] 2 H g 78 0 . 3 6 1 . 6 0 [ 7 ]

C 5 H5Fe( CO) 2 SCOC6 H5 78 0 . 4 0 1 . 7 9 [ 7 ]

C 5 Hg Fe( CO ) 2 Mn( CO) 5 78 0 . 4 4 1 . 6 8 [ 7 ]

125

TABLE II (cont . )

. . . T e m p . I . S. Q . S, Absorber . - к . - к Ref .

( К) (mm sec ) ( m m sec )

[ C 5 H s F e ( C O ) 2 ] 2 78 0 . 4 6 1 . 8 9 [ 7 ]

[ ( C 5 Hg) Fe( CO) 2 ] 2 CH2( CHj) г CH2 78 0 . 3 1 1 . 6 7 [ 7 ]

[ C 6 H 5 F e ( C O ) 3 ] + PFj 78 0 . 3 1 1 . 8 8 [ 7 ]

(C5H5)2FeBr 20 0 .75 0 . 1 [ 2 ]

78 0 . 7 0 0 . 2 [ 2 ]

( C s H ^ F e B F i 298 0 . 5 4 0 . 7 6 [ 6 ]

( C 6 H 6 ) 2 F e ( p i c r . ) 78 0 ,85 ~ 0 [ 4 ]

IV. Other я-bonding ligands

( C 9 H 7 ) 2 F e 78 0 .85 2 . 5 4 [ 7 ]

[ C 9 H 7 F e ( C O ) 2 ] 2 78 0 . 4 9 1 . 7 2 [ 7 ]

C 7 H 9 F e ( C O ) 2 I 78 0 .45 1 . 5 2 [ 7 ]

[Cg H7 Fe( CO) 3 ] + B F ¿ 78 0 . 3 4 1 . 5 6 [ 7 ]

[C 7 H 9 Fe(CO) 3 ] + BF 4 " 78 0 . 3 3 1 . 5 4 [ 7 ]

V. Misœl laneous

[C4 F6 S2 FeNO] 2 78 0 . 5 2 1 . 2 0 [ 7 ]

{ [ ( C H j ) 2 N ] 3 P } 2 F e ( N O ) 2 78 0 . 2 9 0 . 8 7 [71

Ferredoxin (protein) 77 0 . 7 0 0 . 9 3 [ 1 7 ]

195 0 . 6 5 0 . 7 4 [ 1 7 ]

298 0 . 6 0 0 . 7 0 [ 1 7 ]

Haemln ( C M H 3 2 0 4 N « F e C l ) 4 0 . 6 7 0 . 7 6 [ 1 8 ]

20 0 . 0 7 broad [ 1 8 ]

126

TABLE II (cont. )

ORGANO-TIN COMPOUNDS

Absorber T e m p . C K )

I . S . ( m m sec" 1 )

O . S . ( m m sec" 1 )

Ref.

(CH 3 ) 2 SnL2

( C H 3 ) 2 S n ( O C H 3 ) 2 78 0 . 9 9 0 2 . 3 1 [ 3 4 ]

( CH3 ) 2 Sn( CH 3 COCH2 CH2 COCH3) 78 1 . 1 8 3 . 9 3 [ 3 4 ]

( C H 3 ) 2 S n ( O C O C 5 H 4 N ) 2 78 1 . 2 8 4 . 4 3 [ 3 4 ]

( C H 3 ) 2 S n C l 2 ( l , 10 phenan) 78 1 . 3 2 4 . 0 3 [ 3 4 ]

( C H 3 ) 2 S n ( O C O C 6 H 5 ) 2 78 1 . 4 0 3 . 9 6 [ 3 4 ]

( C H 3 ) 2 S n ( O C H O ) 2 78 1 . 4 5 4 . 7 2 [ 3 4 ]

298 1 .37 4 . 5 2 [ 3 4 ]

( C H 3 ) 2 S n C l 2 78 1 . 5 3 3 . 4 1 [ 3 1 ]

(CH 3 ) 3 SnL

( C H 3 ) 3 S n O H 78 1 . 0 7 2 . 7 1 [ 3 1 ] .

( C H 3 ) 3 S n ( i m i d ) 78 1 . 1 6 2 . 7 6 [ 3 1 ]

( C H ^ S n F 78 1 . 1 8 3 . 4 7 [ 3 1 ]

78 1 . 2 6 3 . 7 7 [ 3 4 ]

298 1 . 2 3 . 6 [ 3 4 ]

( C H 3 ) 3 S n ( B z . imid ) 78 1 . 2 0 2 . 8 9 [ 3 4 ]

( C H 3 ) 3 S n N 3 ' 78 1 . 2 4 3 . 2 3 [ 3 1 ]

( C H 3 ) s S n ( l , 2 , 4 t r iazo le) 78 1 . 2 7 2 . 9 6 [ 3 4 ]

( C H 3 ) 3 S n ( 1 , 2 , 3 t r i azo le ) 78 1 . 3 1 3 . 1 8 [34]

(CH 3 ) 3 SnOCOCH 3 78 1 . 3 4 3 . 4 7 [ 3 1 ]

78 1 .35 3 . 5 2 [ 3 4 ]

( C H 3 ) 3 S n C l 78 1 . 4 0 3 . 0 9 [ 3 1 ]

( C H 3 ) 3 S n [ 0 C 0 C = C C 0 2 ] (CH 3 ) 3 Sn 78 1 . 4 0 3 . 8 1 [ 3 4 ]

(C 2 H5) 2 SnL 2

( C 2 H 5 ) 2 S n C l 2 77 1 . 6 3 . 4 [ 2 3 ]

( C 2 H 5 ) 3 S n L

( C 2 H 5 ) 3 S n N ( N 0 2 ) C H 3 78 1 . 4 9 3 . 5 4 [ 2 8 ]

( C 2 H 5 ) 3 S n [ O C O C ( C H 3 ) = C H 2 ] 77 1 . 5 3 . 0 [ 2 2 ]

127

TABLE II (cont . )

Absorber T e m p .

C K ) I . S .

( m m s e c - 1 ) O.S.

( m m sec" 1 ) Ref .

( C 2 H 5 ) 3 S n ( O C O C H 3 ) n 1 . 6 3 . 2 [ 2 3 ]

( C 3 H7 )2 SnL

( C 3 H 7 ) 2 SnCl2 n 1 . 7 3 . 6 [ 2 3 ]

( C 4 H 9 ) 2 S n L 2

( C 4 H 9 ) 2 S n S 78 0 . 9 1 . 9 [ 2 4 ]

( C 4 H 9 ) 2 S n S 0 3 78 1 . 3 4 . 0 [ 2 4 ]

( n C 4 H 9 ) 2 S n ( O C O C H 3 ) 2 78 1 . 3 4 3 . 5 0 [ 1 9 ]

( nC 4 H 9 ) 2 Sn( O C O C u H 2 3 ) 2 78 1 . 3 4 3 . 3 5 [ 1 9 ]

( nC 4 H9) 2Sn( OCOC y His ) 2 78 1 . 3 5 3 . 4 5 [ 1 9 ]

( n C 4 H 9 ) 2 S n ( O C O C 1 7 H 3 5 ) 2 78 1 . 3 6 3 . 5 6 [ 1 9 ]

( C 4 H 9 ) 2 Sn(Cl ) O C H O . H z O 78 1 . 4 4 3 . 4 7 [ 3 4 ]

( C 4 H 9 ) 2 SnF2 78 1 . 4 5 3 . 9 [ 2 4 ]

( C 4 H 9 ) 2 S n C l 2 11 1 . 5 3 . 4 [ 2 3 ]

( C 4 H 9 ) 2 S n C l 2 78 1 . 6 3 . 2 5 [ 2 4 ]

( C 4 H 9 ) 2 S n ( O C O C 6 H 5 ) 2 78 1 . 6 2 3 . 4 4 [ 3 4 ]

(C4H9)2SnBr2 78 1 . 7 3 . 1 5 [ 2 4 ]

( C 4 H 9 ) 2 S n I 2 78 1 . 8 2 . 9 [ 2 4 ]

( C 4 H 9 ) 2 S n S 0 4 78 1 . 8 4 . 8 [24 ,25]

( C 4 H 9 ) 3 S n L

( C 4 H9) 3 Sn( 0 C 0 C H = C H C 0 2 ) 78 1 . 2 5 3 . 6 2 [ 3 4 ]

( C 4 H 9 ) 3 S n O C O C H 3 78 1 . 3 8 3 . 6 7 [ 3 4 ]

( C 6 H s ) S n L 3

( C 6 H 5 ) S n C l 3 77 2 . 8 4 . 8 [ 2 3 ]

(C 6 H 5 )2SnL 2

( C 6 H s)2Sn( 8 - o x y q u i n ) 2 78 0 . 6 3 7 1 . 5 3 [ 3 4 ]

( C 6 H 5 ) 2 S n [ O C ( C 6 H 5 ) ( C H 2 ) 2 C ( C H 3 ) C O ] 78 0 . 7 1 6 2 . 0 3 [ 3 4 ]

( C 6 H 5 ) 2 S n C l 2 77 1 . 4 • 2 . 8 [ 2 3 ]

78 1 . 3 1 2 . 6 6 [ 3 1 ]

80 1 . 3 2 2 . 9 8 [ 3 0 ]

128


Absorber T e m p .

( • К ) I . S.

( m m sec*1) Q . S .

( m m s e c - 1 ) Ref .

( C s H s ) 3 S n L

( C 6 F s ) 3 S n C l 80 1 . 0 1 1 . 5 5 [ 3 0 ]

( C 6 H 5 ) 3 S n [ O C O C ( C H 3 ) = C H 2 ] 77 1 . 1 5 2 . 1 [ 2 3 ]

( C 6 H 5 ) 3 S n F 78 1 . 1 7 3 . 3 4 [ 3 1 ]

80 1 . 1 9 3 . 9 0 [ 3 0 ]

( C 6 H 5 ) 3 S n ( C 6 f s ) 80 1 . 1 8 1 . 1 0 [ 3 0 ]

C 6 H 5 ) 3 S n O H 78 1 . 1 8 2 . 6 8 [ 3 1 ]

78 1 . 3 5 2 . 7 [ 1 9 ]

( C 6 H 5 ) 3 S n ( B z . I m i d ) 78 1 . 1 9 2 . 5 9 [ 3 4 ]

( C 6 H 5 ) 3 S n B r 80 1 . 2 0 2 . 5 1 [ 3 0 ]

[mCF 3 ( C 6 H4) ] 3 SnBr 78 1 . 2 2 1 . 9 4 [ 3 1 ]

[ p F ( C 6 H s ) ] 3 S n I 78 1 . 2 3 1 . 9 2 [ 3 1 ]

( C 6 H 5 ) 3 S n ( l , 2 , 4 t r i a z o l e ) 78 1 . 2 9 2 . 7 6 [ 3 4 ]

( C 6 H 5 ) 3 Sn( 1 , 2 , 3 Bz . t r i a z o l e ) 78 1 . 3 1 2 . 9 8 [ 3 1 ]

[ £ C l ( C 6 H 4 ) ] 3 S n C l 78 1..37 2 . 4 9 [ 3 1 ]

( C 6 H 5 ) 3 S n C l 77 1 . 4 2 . 4 [ 2 3 ]

78 1 . 3 7 2 . 4 5 [ 3 1 ]

80 1 . 2 5 2 . 5 5 [ 3 0 ]

78 1 . 3 8 2 . 5 [ 1 9 ]

( C 6 H 5 ) 3 S n N 3 . 78 1 . 4 0 3 . 1 9 [ 3 1 ]

( n e o p h y l ) 3 SnL

[ ( C H j ) 2 C ( C 6 H 5 ) C H 2 ] S SnOH 78 1 . 1 3 1 . 0 8 [ 3 1 ]

[ ( C H j J j C C C j H s J C H l j S n N j 78 1 . 3 3 2 . 4 8 [ 3 1 ]

[ ( C H s ) 2 C ( C 6 H s ) C H ] 3 S n F 78 1 . 3 3 2 . 7 9 [ 3 1 ]

[ ( C H j ) a C( C 6 Hs) C H ] 3 SnOCOCHj 78 1 . 3 5 2 . 4 5 [ 3 1 ]

[ ( C H 3 ) 2 C ( C 6 H 5 ) C H ] 3 S n C l 78 1 . 3 9 2 . 6 5 [ 3 1 ]

[ ( C H 3 ) 2 C ( C 6 H 5 ) C H ] 3 S n N 0 3 78 1 . 4 0 3 . 1 8 [ 3 1 ]

[ ( C H 3 ) 3 C ( C 6 H 5 ) C H ] 3 S n I 78 1 . 4 1 2 . 4 0 [ 3 1 ]

[ ( C H 3 ) 2 C ( C 6 H 5 ) C H ] 3 S n B r 78 1 . 4 2 2 . 6 5 [ 3 1 ]

[ ( C H 3 ) 2 C( C 6 H, ) C H ] 3 SnC104 78 1 . 5 7 3 . 8 3 [ 3 1 ]

1 2 9


^ T e m p . I . S . Q . S . Absorber . - i . , - к Ref.

( K) ( m m sec *) ' — *

SnL4

[ H C £ C C 0 2 l 4 S n 78 0 - [ 2 1 ]

S n ( O C O C H 3 ) 4 78 0 . 1 7 6 0 [ 2 8 ]

( C 6 F 5 ) 4 S n 80 1 . 0 2 0 [ 3 0 ]

S n ( S C S N C 2 H 5 ) 4 78 1 . 0 9 0 [ 2 8 ]

( C 6 H s ) 4 S n 78 1 . 2 0 8 0 [ 2 9 , 3 1 ]

80 1 . 2 0 0 [30]

? 1 . 3 5 0 [20]

77 1 . 2 0 [23]

77 1 . 1 0 0 [ 2 6 ]

( C H 3 ) 4 S n 78 1 . 2 2 0 [ 3 3 ]

[ m C F 3 ( C 6 H 5 ) ] 4 S n 78 1 . 2 8 0 [ 3 1 ]

[ p C F 3 ( C 6 H 4 ) ] 4 S n 78 1 . 2 9 0 [ 3 1 ]

[ ( C H s ) 2 C ( C 5 H 5 ) C H ] 4 S n 78 1 . 3 4 0 [ 3 1 ]

( n C 4 H 9 ) 4 S n > 1 . 3 5 0 [ 2 0 ]

78 1 . 3 0 [ 2 4 ]

( C e H u ) 4 S n 78 1 . 5 2 0 [ 3 1 ]

Binuc lear compounds

[ ( C j H 5 ) s S n ] 2 > 1 . 2 5 0 [ 2 0 ]

{ [ P F ( C 6 H 5 ) ] s S n } 2 78 1 . 3 3 0 [ 3 1 ]

[ ( C 2 H s ) 2 S n C l ] 2 78 1 . 3 4 3 . 3 4 [ 1 9 ]

{ [ m C F 3 ( C 6 H 4 ) ] 3 S n } 2 78 1 . 4 0 0 [ 3 1 ]

[ ( C 6 H 5 ) 3 S n ] 2 78 1 . 4 1 0 [ 3 1 ]

[ p - C l ( C 6 H 4 ) 8 S n ] 2 78 1 . 4 4 0 [ 3 1 ]

[ ( C 2 H 5 ) 2 S n C l ] 2 77 1 .45 3 . 1 [ 2 3 ]

Oxides and sulphides

[ £ l ( C 6 H 4 ) ] 2 S n O 78 0 . 8 4 1 . 7 3 [ 1 9 ]

[ £ C l ( C 6 H 4 ) ] 2 S n O 78 0 . 8 8 1 . 7 3 [ Í 9 ]

[ £ B r ( C 6 H 4 ) ] 2 S n O 78 0 . 9 2 . 1 . 8 3 [ 1 9 ]

[ ( C H 3 ) 2 S n O ] x 78 0 . 9 2 4 1 . 9 2 [ 3 1 , 3 4 ]

78 0 . 9 2 1 . 8 2 [ 1 9 ]

130

TABLE II (cont . )

Absorber T e m p .

C K ) I . S .

(mm sec" 1 ) Q . S .

( m m sec~ ' ) Ref

[ ( C 6 H s ) 2 S n q i x 18 0 . 9 6 1 1 . 8 7 [ 3 1 ,

78 0 . 8 8 1 . 7 3 [ 1 9 ]

n ( C 4 H 9 ) 2 S n O 78 1 . 0 2 . 0 6 [ 1 9 ]

78 0 .95 2 . 2 [ 2 4 ]

[ ( C 6 H 5 ) 3 S n ] 2 0 78 1 . 0 8 2 . 1 5 [ 3 1 ]

n ( C 5 H u ) 2 S n O 78 1 . 1 2 . 1 1 [ 1 9 ]

[ ( C , H , ) 2 S n O ] x 77 1 . 1 2 . 4 [ 2 3 ]

78 1 . 1 2 . 1 [ 1 9 ]

[ ( C 6 H 5 ) s S n ] 2 S 78 1 .22 1 . 1 7 [ 3 1 ]

[ ( nC 4 H9) 2Sn( OCOCH3) ] 2 0 78 1 . 3 0 3 . 2 4 [ 1 9 ]

[ ( C H 3 ) 2 S n 0 C 0 C H 3 ] 2 0 78 1 . 3 8 3 . 5 7 [ 3 4 ]

298 1 . 3 3 3 . 5 4 [ 3 4 ]

Hydrides

(CH 3 ) 2 SnH 2 78 1 . 2 3 0 [ 3 3 ]

SnHi 78 1 . 2 7 0 [ 3 3 ]

[ p F ( C 6 H 4 ) ] 3 S n H 78 1 .37 0 [ 3 1 ]

78 1 .26 0 [ 3 3 ]

[ (CH 3 ) 2 CH]SnH 3 78 1 . 4 0 0 [ 3 3 ]

( C H j ) s S n H 78 1 . 2 4 0 [ 3 3 ]

Miscellaneous compounds

( C H 3 ) 2 S n ( 8 - o x y q u i n ) 2 78 0 .771 1 . 9 8 [ 3 4 ]

[ ( n C 4 H 9 ) 2 S n ] x ? 1 .15 - [20 ]

(CH 3 ) 2 SnCl 2 ( 2 , 2 ' b i p y ) 78 1 .55 4 . 0 9 [34 ]

[ ( C 6 H s ) 2 S n ] x ! 1.42 - [20 ]

( C H 3 ) 3 S n C l - p y 78 1*42 3 .35 [ 3 4 ]

( C 5 H a ) 2 S n C l 2 78 1 .59 3 . 4 0 [ 3 1 ]

Sn( T D T ) 2 (b ipy) 78 0 .82 0 .92 [ 2 7 ]

Sn( T D T ) 2 ( o - p h e n ) 2 78 1 . 0 1 1 . 2 6 [27 ]

S n ( T D T ) 2 [ ( C 2 H 5 ) 3 N ] 2 78 0 .45 0 . 8 4 [ 2 7 ]

S n ( T D T ) 2 78 0 .95 1 .62 [ 2 7 ]

131

TABLE II ( con t . )

Absorber T e m p .

C K ) I . S .

( m m sec" 1 ) Q . S .

( m m s e c " 1 ) Ref .

S n ( T D T ) 2 78 1 . 4 0 1 . 5 2 [ 2 7 ]

198 1 . 2 6

Sn(EDT) 2 78 OÍ 96 1 . 0 0 [ 2 7 ]

Sn(EDT) 2 *2py 78 1 . 0 5 1 . 8 4 [ 2 7 ]

S n ( T D T ) 2 ' 2 p y 78 0 . 9 5 3 1 . 6 9 [ 2 7 ]

S n ( C 6 H 5 ) ( T D T ) 78 1 . 3 1 2 . 0 2 [ 2 7 ]

78 1 . 4 1 1 . 9 4 [ 2 7 ]

S n ( C 6 H s ) 2 ( E D T ) 78 1 . 3 7 1 . 6 9 [ 2 7 ]

S n ( C 2 H s ) ( T D T ) 78 1 . 4 1 2 . 6 [ 2 7

S n ( C H 3 ) 2 ( E D T ) 78 1 . 3 2 2 . 3 5 [ 2 7 ]

S n ( E D T ) 2 78 1 . 4 1 1 . 0 6 [ 2 7 ]

TABLE III

ORGANO-(Fe , Sn) COMPOUNDS

Absorber T e m p .

C K ) I . S .

( m m s e c " 1 ) Q . S .

( m m s e c " 1 ) Ref .

[ C s H s F e C C O J ü S n C l ü 78 F e : 0 . 3 6 1 1 . 6 4 [ 3 5 ]

78 Sn: 1 . 9 4 2 . 3 7

[ ^ s H j Fe( CO ) 2 ] SnCl 3 78 Fe : 0 . 4 8 8 1 . 9 4 [ 3 5 ]

78 Sn: 1 . 9 0 1 . 9 0

[C 5 H5 Fe( СО)г ] 2 Sn( C H 3 ) 2 78 F e : 0 . 3 8 0 - 1 , 7 1 5 [ 3 5 ]

78 Sn: 1 . 6 8 0

[ C 5 H 6 F e ( C O ) 2 ] 2 S n ( C 2 H 5 ) 2 78 F e : 0 . 3 7 3 1 . 8 8 [ 3 5 ]

78 Sn: 1 . 7 4 0

[ C s H s F e i C O ) , ] S n ( C 6 H 5 ) 3 78 F e : 0 . 3 8 9 1 . 8 8 [ 3 5 ]

78 Sn: 1 . 4 3 0

' [ ( n - C 4 H 9 ) 2 S n F e ( C O ) 4 ] 2 78 F e : 0 . 2 3 8 0 [ 3 5 ]

78 Sn: 1 . 7 0 1 . 2 6

132


T h e p r e p a r a t i o n of t h i s c o m p i l a t i o n w a s s u p p o r t e d i n p a r t b y t h e US A t o m i c E n e r g y C o m m i s s i o n , t he P e t r o l e u m R e s e a r c h F u n d and t h e R e s e a r c h Counc i l of R u t g e r s , T h e S t a t e U n i v e r s i t y , T h e a u t h o r i s i ndeb ted to M e s s r s , G . I . P a r i s i , H . A . S t ö c k l e r a n d A . H o f f m a n , a n d t o M r s . E . K e e n w h o m a t e r i a l l y h e l p e d i n a s s e m b l i n g t h e s e d a t a .

R E F E R E N C E S

[1] ERICKSON, N. E. , Ph.D. Thesis, University of Washington (1964). [2] WERTHEIM, G. K. , HERBER, R. H. , J. chem. Phys. 38 (1963) 2106. [3] LESIKAR, A. V. , J. chem. Phys. 40 (1964) 2746. [4] EPSTEIN, L.M. , J. chem. Phys. 36 (1962) 2731. [5] WERTHEIM, G. К . , HERBER, R. H. , Proc. Second Int. Conf. on the Mössbauer Effect (COMPTON, D.M.J. ,

SCHOEN, A.H. , Eds.) J. Wiley and Sons, New York (1962). [6] ZAHN, U. , KIENLE, P. . EICHER, H. , ibid. Ref. [5] . [7] HERBER, R. H. , KING, R. В. , WERTHEIM, G.K. , Inorg. Chem. 3 (1964) 101. [8] WERTHEIM, G. K. , HERBER. R. H. , J. Am. chem. Soc. 84 (1962) 2274. [9] FLUCK, E. , KERLER, W. , NEUWIRTH, W.', Angew. Chem. , Int. ed. 2 (1963) 277.

[10] COLLINS, R.L. , PETTIT, R. , J. Am. chem. Soc. 85 (1963) 2332. [11] COLLINS, R.L. , PETTIT, R. , J. chem. Phys. 39 (1963) 3433. [12] KALVIUS, M. , WIEDEMANN, W. , ZAHN, U. , KIENLE, P. , Bull. A.P.S. II,_9_ (1964) 634. [13] COLLINS, R.L. , J. chem. Phys. 42 (1965) 1072. [14] HERBER, R.H. , KINGSTON, W. R. , WERTHEIM, G.K. , Inorg. Chem. 2 (1963) 153. [15] KERLER, W. Î NEUWIRTH, W. , FLUCK, E. , Z. Physik 175 (1963) 200. [16] KERLER, W. , NEUWIRTH, W. , FLUCK, E. , KUHN, P. , ZIMMERMAN, В. , Z. Physik 173 (1963) 321. [17] PILLINGER, W. L. , STONE, J. A. , TID-4500, 26 DP-878. [18] SHULMAN, R. G. , WERTHEIM, G.K. , Rev. mod. Phys. 36 (1964) 459. [19] GOLDANSKII, V . l . , MAKAROV, E. F. , STUKAN, R. A. , TRUKHTANOV, V. A. , KHRAPOV, V. V. ,

Dokl. Akad. Nauk SSSR 151 (1963) 357. [20] GOLDANSKII, V . l . , ROGAEV, V. Ya. , KHRAPOV, V .V . , Dokl. Akad. Nauk SSSR 156 (1964) 909. [21 ] GOLDANSKII, V . l . e t al. , Dokl. Akad. Nauk SSSR 156 (1964) 400. [22] BRYUKHANOV, V.A. , GOLDANSKII, V . l . , DELYAGIN, N. N. , MAKAROV, E. F. , SHPINEL, V. S. ,

Soviet Phys. JETP 42 (1962) 637. [23] BRYUKHANOV, V .A . et al . , Soviet Phys. JETP, (Engl. Trans. ) ¿ 6 (1963) 321.

[24] ALEKSANDROV, A.Yu. e t al. , Soviet Phys. JETP, (Engl. Trans. ) ^ 6 (1963) 879.

[25] ALEKSANDROV, A.Yu. e t al. , Soviet Phys. JETP, (Engl. Trans. )_16 (1963) 1467.

[26] BRYUKHANOV, V.A. , DELYAGIN, N. N. , OPALENKO, A . A . , SHPINEL, V. S. , Soviet Phys. JETP 43

(1962) 432.

[27] EPSTEIN, L.M. , STRAUB, D. K. , Paper No. 6, Div. Phys. C h e m . , Am. chem. Soc. 149th Meeting,

Detroit 1965.

[28] HERBER, R. H. , unpublished data.

[29] HERBER, R. H. , STÖCKLER, H . A . , in Chemical Effects of Nuclear Transformations, И, IAEA, Vienna (1965) 403.

[30] CORDEY-HAYES, M. , J. inorg. nucl. Chem. 26 (1964) 2306. [31] HERBER, R. H. , STÖCKLER, H . A . , REICHLE, W.T. , J. chem. Phys. (in press). [32] HERBER, R.H. , STÖCKLER, H . A . , Trans. N.Y. Acad. Sei. 26 (1964) 929. [33] HERBER, R. H. , PARISI, G. I. , (to be published) cf. Recent Advances in Mössbauer Technology, New

England Nuclear -T.M. C. Symposium, Jan. 1965 (in press). [34] HERBER, R.H. , STÖCKLER. H.A. , unpublished data. [35] HERBER. R.H. , HOFFMAN. A. . unpublished data.

133

APPLICATION OF MÖSSBAUER SPECTROSCOPY TO CHEMISTRY OF IRON AND TO

BIOCHEMICAL PROBLEMS

E. FLUCK UNIVERSITY OF HEIDELBERG, FEDERAL REPUBLIC OF GERMANY

A . A P P L I C A T I O N S T O CHEMISTRY O F IRON

T h e M ö s s b a u e r s p e c t r o s c o p i c i n v e s t i g a t i o n of i r o n c o m p o u n d s p r o v i d e s t w o m a g n i t u d e s w h i c h a r e e s s e n t i a l l y of i n t e r e s t t o t h e c h e m i s t , n a m e l y : (a) t h e i s o m e r i c s h i f t ( I . S . ) ; and (b) t h e q u a d r u p o l e s p l i t t i n g .

1. The isomer shift

F o r i r o n c o m p o u n d s t h a t a r e p r i n c i p a l l y of i o n i c n a t u r e , t h e i s o m e r s h i f t g i v e s d i r e c t i n f o r m a t i o n about t h e o x i d a t i o n s t a t e of i r o n , s i n c e c o m -pounds conta in ing i r o n with the oxidat ion n u m b e r s +6, +3 and + 2 a r e c h a r a c -t e r i z e d by s p e c i f i c r a n g e s depending on the t e m p e r a t u r e t l - 4 ] . F o r example , r e f ë r r e d t o a s o u r c e c o n s i s t i n g of 5 7 Co i n p l a t i n u m and a t a t e m p e r a t u r e of -120°C, t h e s e r a n g e s c o r r e s p o n d to a ve loc i ty of 0 . 9 - 1 . 0 m m / s f o r i r o n ( H ) s a l t s and 0 . 1 - 0 . 2 m m / s f o r i ron(II I ) s a l t s . F e r r a t e s , wi th i ron(VI ) , show a n i s o m e r s h i f t of - 1 . 20 m m / s [5, 6] . A c c o r d i n g to W e r t h e i m and H e r b e r i n R e f . [ 7 ] , t h e i s o m e r s h i f t s c a n b e s t b e r e f e r r e d t o s o d i u m n i t r o s y l p r u s s i a t e , N a 2 [ F e ( C N ) 5 N O ] - 2 H2O, in which c a s e t h e r a n g e s a r e a s fo l lows: i ron(II) s a l t s , 1 . 5 - 1 . 6 m m / ' s ; iron(III) s a l t s , 0 . 7 - 0 . 9 m m / s ; and f e r r a t e (УЦ, '—0.6 m m / s . T h e s h i f t of t h e r e s o n a n c e l i n e s t o w a r d l o w e r o r n e g a t i v e v e l o c i t i e s wi th i n c r e a s i n g o x i d a t i o n n u m b e r i s due t o t h e i n c r e a s i n g s -e l e c t r o n d e n s i t y a t t h e n u c l e u s of t h e i r o n a t o m a s a r e s u l t of d i m i n i s h e d s c r e e n i n g b y t h e 3d e l e c t r o n s [ 8 ] ; s e e a l s o [3] and t h e r e f e r e n c e s g i v e n t h e r e . At p r e s e n t we a r e i n v e s t i g a t i n g v a r i o u s c o m p o u n d s c o n t a i n i n g i r o n wi th o x i d a t i o n n u m b e r s + 4 and +5 , e . g . B a F e Û 3 and N a 3 F e Û 4 .

C o - o r d i n a t i o n c o m p o u n d s of i r o n in wh ich t h e bonds b e t w e e n m e t a l and l i g a n d a r e p r i n c i p a l l y c o v a l e n t o r d i n a r i l y r e v e a l s i m i l a r i s o m e r s h i f t s , q u i t e i n d e p e n d e n t l y of t h e f o r m a l o x i d a t i o n s t a t e of i r o n in t h e c o m p o u n d s . R e f e r r e d to 57Co in p l a t i n u m , t h e s e s h i f t s l i e in the r ange - 0 . 6 to +0.1 m m / s , a r a n g e wh ich a p p l i e s a l s o f o r m e t a l l i c i r o n and i r o n a l l o y s . T h i s i s to be e x p e c t e d on t h e b a s i s of P a u l i n g ' s e l e c t r o n e u t r a l i t y p r i n c i p l e , a c c o r d i n g t o w h i c h t h e c e n t r a l m e t a l a t o m in a l l s t a b l e m e t a l c o m p l e x e s s h o u l d h a v e a c h a r g e s m a l l e r t h a n ± 1 [9, 10] . T h e ox ida t ion n u m b e r of i r o n in co -o r d i n a t i o n c o m p o u n d s c a n b e r e c o g n i z e d f r o m q u a d r u p o l e s p l i t t i n g of t h e r e s o n a n c e l i n e s and t h e i r d e p e n d e n c e on t e m p e r a t u r e ( s e e s e c t i o n A s u b -h e a d i n g 2) .

T h e d e t e r m i n a t i o n of t h e o x i d a t i o n s t a t e of i r o n in h o m o g e n e o u s c o m -p o u n d s of known c o m p o s i t i o n i s o f t en not d i f f i c u l t , but i t b e c o m e s c o n s i d e r -a b l y m o r e c o m p l e x w i t h s u b s t a n c e s of b i o l o g i c a l n a t u r e . Ill t h e l a t t e r i n s t a n c e , h o w e v e r , t h i s k n o w l e d g e i s of e s p e c i a l i m p o r t a n c e b e c a u s e t h e change of ox ida t ion s t a t e i s f r e q u e n t l y r e s p o n s i b l e f o r a p a r t i c u l a r b iological f unc t i on ( s ee s e c t i o n B) . F u r t h e r m o r e , i t h a s p r e v i o u s l y been v e r y d i f f icu l t

134

t o a s c e r t a i n t h e o x i d a t i o n n u m b e r of i r o n i n c o m p l e x m i n e r a l s c o n t a i n i n g t h i s m e t a l . T h e r e s u l t s of M ö s s b a u e r s p e c t r o s c o p i c i n v e s t i g a t i o n s of i r o n s i l i c a t e s h a v e shown t h a t t h i s p r o b l e m c a n now be so lved s u c c e s s f u l l y [11-13] . M o r e o v e r , i t i s p o s s i b l e by t h i s m e t h o d to d e t e r m i n e q u a n t i t a t i v e l y t he r a t i o , i r o n ( I I ) / i r o n ( I I I ) , i n s u c h m a t e r i a l s .

T h e i s o m e r s h i f t i n o r g a n o - i r o n c o m p o u n d s c a n b e c o n s i d e r e d a s a n a d d i t i v e m o l e c u l a r p a r a m e t e r [14] . T h i s a s s u m p t i o n i s s u p p o r t e d b y t h e o b s e r v a t i o n t h a t c i s - t r a n s i s o m e r s i n d i c a t e t h e s a m e i s o m e r s h i f t [15] . In f e r r o c e n e d e r i v a t i v e s , t he i s o m e r s h i f t i s i n d e p e n d e n t of s u b s t i t u t i o n i n t h e r i n g ( s ) [16] .

In a d d i t i o n t o t h e o x i d a t i o n s t a t e of i r o n , t h e i s o m e r s h i f t c a n a l s o p r o -v i d e i n f o r m a t i o n o n t h e b o n d i n g r e l a t i o n s h i p s , e s p e c i a l l y w i t h r e s p e c t t o tf-bond s t r e n g t h i n c o v a l e n t c o - o r d i n a t i o n c o m p o u n d s of i r o n [1, 9 , 1 7 , 18] . In t h e p r u s s i a t e s , i . e . h e x a c y a n o f e r r a t e d e r i v a t i v e s in which a cyano g r o u p i s r e p l a c e d b y s o m e o t h e r l i g a n d , t h e i s o m e r s h i f t d e p e n d s m a i n l y o n t h e 7T-bond s t r e n g t h of t h i s p a r t i c u l a r l i g a n d . Wi th i n c r e a s i n g j r-bond s t r e n g t h , t h e d - e l e c t r o n d e n s i t y of t h e i r o n a n d c o n s e q u e n t l y t h e s c r e e n i n g of s -e l e c t r o n s d e c r e a s e s . T h i s r e s u l t s i n a s h i f t of t h e r e s o n a n c e l i n e t o w a r d p o s i t i v e v e l o c i t i e s w i t h d e c r e a s i n g j r -bond s t r e n g t h i n t h e m a n n e r i n d i c a t e d b y t h e f o l l o w i n g s e r i e s [1, 2 , 1 7 , 1 8 ] :

P r u s s i a t e

N a [ F e ( C N ) _ N O ] . 2 H O

N a 3 [ F e ( C N ) 5 C O ]

N a 5 [ F e ( C N ) 5 S 0 3 l

N a 3 [ F e ( C N ) 5 P ( C 6 H 5 ) 3 ] . 2 H 2 0

N a 4 [ F e ( C N ) 5 N 0 2 ]

N a 3 [ F e ( C N ) 5 N H g ] . 2 H 2 Ö

N a 3 [ F e ( C N ) 5 A s ( C 6 H 5 ) 3 I

In n i t r i t o - a n d a m m i n e p r u s s i a t e , no m o r e тг-bonds c a n b e e s t a b l i s h e d b e -t w e e n t h e c e n t r a l i r o n a t o m a n d t h e n i t r o g e n a t o m s of t h e l i g a n d s , s i n c e t h e n i t r o g e n h a s no o r b i t a l s wh ich a r e a v a i l a b l e f o r t h i s p u r p o s e . A c c o r d i n g l y , t he i s o m e r s h i f t s f o r t h e s e two c o m p o u n d s a r e t h e s a m e . T h e s e s h i f t s should a l s o b e t he m o s t p o s i t i v e o n e s in t h e s e r i e s , a s s u m i n g t h a t t h e c y a n o g r o u p s in a l l p r u s s i a t e s h a v e t h e s a m e jr-bond s t r e n g t h . T h i s s i m p l i f i e d concep t i s b a s e d on t h e a s s u m p t i o n t h a t t h e c j -bonds of t h e s ix th l i gand in a l l p r u s s i a t e s m a k e t h e s a m e c o n t r i b u t i o n t o t h e i s o m e r s h i f t . T h i s a s s u m p t i o n i s c e r t a i n l y no t f u l l y c o r r e c t , bu t a t l e a s t i t a l l o w s u s e f u l q u a l i t a t i v e c o n -c l u s i o n s t o b e m a d e on t h e i r -bond s t r e n g t h of t h e v a r i o u s l i g a n d s .

I n a n u m b e r of c a s e s , t h e i s o m e r s h i f t w a s of a s s i s t a n c e i n d e c i d i n g t h e q u e s t i o n of h y b r i d i z a t i o n i n v o l v e d i n t h e f o r m a t i o n of ст-bond s y s t e m s . In p o t a s s i u m f e r r a t e ( V I ) , K 2 F e Û 4 , i n w h i c h F e h a s t h e o x i d a t i o n n u m b e r + 6, t h e 3d o r b i t a l s of i r o n w o u l d b e o c c u p i e d by o n l y t w o e l e c t r o n s if t h e ion [ F e 0 4 F ~ w e r e c o m p l e t e l y i o n i c i n n a t u r e . T h i s shou ld r e s u l t in a v e r y

j g j r - b o n d s t r e n g t h

p o s i t i v e

135

s l i g h t s c r e e n i n g of t h e i r o n n u c l e u s , l e a d i n g t o a l a r g e n e g a t i v e s h i f t . In f a c t , a n i s o m e r s h i f t ' o f - 1 . 2 0 m m / s ( r e f e r r e d t o 5 7Co i n p l a t i n u m ) i s o b -s e r v e d [1, 2, 18] . T h i s s i g n i f i e s t h a t t h e d - e l e c t r o n d e n s i t y m u s t b e c o n -s i d e r a b l y g r e a t e r , b o t h a c c o r d i n g t o t h e o r i g i n a l d i a g r a m of W a l k e r , W e r t h e i m and J a c c a r i n o , in which the o v e r - a l l s - e l e c t r o n d e n s i t y of the i ron a t o m i s d e s c r i b e d f o r v a r i o u s d - e l e c t r o n c o n f i g u r a t i o n s a n d i s r e l a t e d t o t h e i s o m e r s h i f t , and the c o r r e c t i o n s f o r t h i s d i a g r a m a s p r o p o s e d b y D a n o n [18] . A c c o r d i n g l y , of t h e two e l e c t r o n c o n f i g u r a t i o n s d 3 s and s p 3 w h i c h a r e p o s s i b l e f o r a t e t r a h e d r a l bond a r r a n g e m e n t , t he f o r m e r would a p p e a r t o a p p l y in t h e c a s e of [ F e 0 4 ] z " . T h i s i s s u b s t a n t i a t e d by t h e f a c t t h a t t h e r e s o n a n c e l i n e of t h e s p e c t r u m i s no t s p l i t , a s would b e e x p e c t e d f o r an sp 3

c o n f i g u r a t i o n . It shou ld be no ted t h a t t he o b s e r v e d l i ne s h i f t i s d e p e n d e n t on t e m p e r a -

t u r e , i . e . i t i s c o m p o s e d of the a c t u a l i s o m e r sh i f t i t s e l f and a t e m p e r a t u r e s h i f t . T h e c o n t r i b u t i o n of t h e t e m p e r a t u r e s h i f t t o t h e t o t a l l i n e s h i f t i s u s u a l l y s m a l l c o m p a r e d wi th t h e i s o m e r s h i f t and i s of t he o r d e r of 0 . 5 m m / s a t 100°C. In t h e r a n g e -120° to +80°C, t h e t e m p e r a t u r e sh i f t i s p r a c t i c a l l y l i n e a r i l , 2J . T h e t e m p e r a t u r e s h i f t a s w e l l a s t h e m a g n i t u d e of the D e b y e - W a l l e r f a c t o r a r e a m a n i f e s t a t i o n of t h e p r o p e r t i e s of t h e v i -b r a t i o n a l s p e c t r u m .

2. Quadrupole splitting

T h e i n t e r a c t i o n of t h e e l e c t r i c q u a d r u p o l e m o m e n t of t h e i r o n n u c l e u s in t h e e x c i t e d s t a t e w i th a n e l e c t r i c f i e l d g r a d i e n t i n t h e e l e c t r o n s h e l l of t h e a t o m can be a s o u r c e of m u c h va luab le i n f o r m a t i o n f o r the c h e m i s t . The e l e c t r i c f i e l d in f r e e i ron( I I I ) i o n s h a s a s p h e r i c a l c h a r g e d i s t r i b u t i o n and c o n s e q u e n t l y s h o w s a s i n g l e r e s o n a n c e l i n e . In a c r y s t a l f i e l d , h o w e v e r , t h e d e g e n e r a t e s t a t e of t h e 3d o r b i t a l s no l o n g e r e x i s t s , and q u a d r u p o l e s p l i t t i n g i s o b s e r v e d , a l b e i t to only a r e l a t i v e l y s m a l l ex ten t in m o s t c a s e s .

On t h e o t h e r h a n d , i r on ( I I ) s a l t s s h o w e x t e n s i v e q u a d r u p o l e s p l i t t i n g o w i n g t o a n e x t r a e l e c t r o n in a d d i t i o n t o t h e h a l f - f i l l e d 3d o r b i t a l . In c o -o r d i n a t i o n c o m p o u n d s in w h i c h i r o n h a s t h e o x i d a t i o n n u m b e r + 2 o r 0, no e l e c t r i c f i e l d g r a d i e n t in t h e e l e c t r o n s h e l l a r i s e s w h e n t h e a r r a n g e m e n t of t h e l i g a n d s i s s u f f i c i e n t l y s y m m e t r i c a l . T h i s condi t ion i s s a t i s f i e d when a t l e a s t two t h r e e - f o l d o r h i g h e r a x e s of s y m m e t r y a r e p r e s e n t .

W h e r e a s in c o - o r d i n a t i o n c o m p o u n d s c o n t a i n i n g i r o n wi th o x i d a t i o n n u m b e r +2 o r 0 a l l t he 3d o r b i t a l s a r e f i l l ed , one e l e c t r o n i s l ack ing in sub -s t a n c e s h a v i n g i r o n wi th o x i d a t i o n n u m b e r +3 , wi th t h e r e s u l t t h a t s i m i l a r c o n d i t i o n s a p p l y a s f o r t h e f r e e i ron( I I ) i o n . Such c o m p o u n d s , e . g . h e x a -c y a n o f e r r a t e ( I I I ) i on , r e v e a l q u a d r u p o l e s p l i t t i n g even when t h e o r i e n t a t i o n of t h e l i g a n d s i s o c t a h e d r a l . W i t h i n c r e a s i n g t e m p e r a t u r e t h e o c c u p a t i o n of t h e s p l i t - e l e c t r o n t e r m s in the c r y s t a l f i e ld b e c o m e s m o r e u n i f o r m . T h i s s t a t e s h o u l d a c t u a l l y b e a t t a i n a b l e a t h i g h t e m p e r a t u r e , if i t w e r e not f o r t h e t h e r m a l i n s t a b i l i t y of t h e c o m p o u n d s in m o s t i n s t a n c e s .

E x t e n s i v e q u a d r u p o l e s p l i t t i n g i s o b s e r v e d f o r i ron ( I I ) c o - o r d i n a t i o n c o m p o u n d s when the i r o n a t o m i s s u r r o u n d e d by v a r i o u s l i g a n d s , a s in the p r u s s i a t e s , a l though even c h e m i c a l l y d i f f e r e n t l i gands m a y s o m e t i m e s have a n e q u i v a l e n t e f f e c t on t h e e l e c t r o n s h e l l of t he i r o n a t o m . T h i s i s t r u e f o r

136

h a e m o g l o b i n w h i c h c a r r i e s CO, i . e . w h e n f o u r c o - o r d i n a t i n g p o s i t i o n s of t h e o c t a h e d r a l e n v i r o n m e n t of t h e i r o n a t o m a r e o c c u p i e d b y t h e n i t r o g e n a t o m s of t h e p o r p h y r i n s y s t e m , one p o s i t i o n i s t a k e n up by the globin r e s idue , a n d CO i s a t t a c h e d a t t h e s i x t h p o s i t i o n . T h e M ö s s b a u e r s p e c t r u m of CO-h a e m o g l o b i n r e v e a l s o n l y a s i n g l e r e s o n a n c e l i n e . H o w e v e r , h a e m o g l o b i n w h i c h i s c h a r g e d w i t h o t h e r g a s e s ( e . g . 0 2 ) a l w a y s s h o w s m a r k e d s p l i t t i n g of t h e r e s o n a n c e l i n e [ 1 9 ] .

I n i r o n c o m p l e x e s w h i c h a r e of c h i e f l y c o v a l e n t c h a r a c t e r , t h e t e m p e r a t u r e - d e p e n d e n t q u a d r u p o l e s p l i t t i n g p r o v i d e s i n f o r m a t i o n o n t h e f o r m a l o x i d a t i o n s t a t e . O c t a h e d r a l i ron( I I ) c o m p o u n d s i n d i c a t e no s p l i t t i n g of the r e s o n a n c e l i ne when t h e s i x l i g a n d s a r e i d e n t i c a l o r c h e m i c a l l y e q u i v a -l e n t . If t h e r e s o n a n c e l i n e i s s p l i t , t h i s s p l i t t i n g i s p r a c t i c a l l y i n d e p e n d e n t of t e m p e r a t u r e . O n t h e o t h e r h a n d , t h e s p l i t t i n g of r e s o n a n c e l i n e s i n c o -o r d i n a t i o n c o m p o u n d s c o n t a i n i n g i ron( I I I ) i s s t r o n g l y t e m p e r a t u r e - d e p e n d e n t .

I n m a n y i n s t a n c e s , t h e c o n c l u s i o n s d r a w n f r o m t h e m a g n i t u d e of t h e i s o m e r s h i f t a n d t h e d e g r e e of q u a d r u p o l e s p l i t t i n g h a v e s e r v e d i n s o l v i n g s t r u c t u r a l p r o b l e m s , c f . R e f s . [1, 5, 2 0 - 2 4 ] .

B . A P P L I C A T I O N S T O B I O C H E M I C A L P R O B L E M S

In t h i s s e c t i o n , a t t e n t i o n wi l l b e devo ted t o a s c i e n c e in which M ö s s b a u e r s p e c t r o s c o p i c s t u d i e s , t o g e t h e r w i t h t h e r e s u l t s d e s c r i b e d in t h e p r e v i o u s s e c t i o n , m a y p r o v e t o b e v e r y v a l u a b l e , n a m e l y b i o c h e m i s t r y .

S y s t e m s c o n t a i n i n g i r o n a r e of p a r t i c u l a r i m p o r t a n c e in b i o c h e m i s t r y . F o r a long t i m e , h a e m a n d r e l a t e d s u b s t a n c e s w e r e t he o n l y i r o n - c o n t a i n i n g s y s t e m s k n o w n , b u t l a t e r s t u d i e s b y B e i n e r t 125 ] , S a n d s a n d o t h e r s , l e d t o t h e d i s c o v e r y of p r o t e i n s w h i c h a l s o b e l o n g to t h i s c l a s s . F o r e x a m p l e , a p r o t e i n c o n t a i n i n g i r o n w a s f o u n d i n t h e m i t o c h o n d r i a of b e e f h e a r t a n d a l s o in t h e i r e x t r a c t s wh ich w e r e p r e p a r e d wi th t h e h e l p of s u c c i n i c d e h y d r o -g e n a s e a n d D P N H d e h y d r o g e n a s e . S i m i l a r l y , m a n y o t h e r e n z y m e s w h i c h a r e e s s e n t i a l c a t a l y s t s i n l i v i n g p r o c e s s e s c o n t a i n i r o n . It i s k n o w n , f o r e x a m p l e , t h a t h a e m o g l o b i n i s a n e c e s s a r y a g e n t f o r t r a n s p o r t i n g o x y g e n , b u t t h i s f u n c t i o n m a y a l s o b e p e r f o r m e d in o t h e r i r o n - c o n t a i n i n g e n z y m e s .

S i n c e M ö s s b a u e r s t u d i e s c a n b e c a r r i e d o u t w i t h o u t a f f e c t i n g t h e m a -t e r i a l u n d e r i n v e s t i g a t i o n , i t m a y p r o v e p o s s i b l e t o o b t a i n i n f o r m a t i o n abou t t h e t r a n s i e n t s t a t e of i r o n in s u c h s y s t e m s , e s p e c i a l l y in t he l igh t of t he f a c t t h a t c e r t a i n e n z y m e s a r e i n c o r p o r a t e d in to a l a r g e r a n d c o m p o s i t e s y s t e m . F o r t h i s r e a s o n , i t i s i m p o r t a n t t o b e a b l e t o i n v e s t i g a t e e n z y m e s y s t e m s a s a w h o l e i n m i t o c h o n d r i a .

A l t h o u g h t h e i r o n c o n t e n t i n s u c h s y s t e m s i s n o r m a l l y v e r y s m a l l , v a r i o u s s t u d i e s h a v e b e e n u n d e r t a k e n w i t h s y s t e m s w h i c h c o n t a i n n a t u r a l i r o n o n l y . H o w e v e r , i t i s of a d v a n t a g e t o c o n c e n t r a t e t h e c o n t e n t of 5 7 F e . T h i s h a s b e e n s u c c e s s f u l l y d o n e w i t h f e r r e d o x i n , a n e l e c t r o n - t r a n s p o r t i n g p r o t e i n which c o n t a i n s s e v e n n o n - h a e m i r o n a t o m s and which h a s a m o l e c u l a r w e i g h t of 6000 [ 2 6 ] . I t i s p r o b a b l e t h a t t h e i r o n i n f e r r e d o x i n p l a y s a n i m -p o r t a n t p a r t i n t h e f u n c t i o n of e l e c t r o n t r a n s f e r in t he m o l e c u l e . F e r r e d o x i n i s p r o d u c e d b y t h e b a c t e r i u m C l o s t r i d i u m p a s t e u r i a n u m . L o v e n b e r g , B u c h a n a n a n d R a b i n o w i t z [27] s h o w e d t h a t a l l t h e i r o n a t o m s in f e r r e d o x i n c a n be s u b s t i t u t e d by i ron( I I I ) i ons a f t e r t r e a t m e n t of t h e p r o t e i n wi th sod ium

137

m e r s a l y l , a n d t h a t a c t i v e f e r r e d o x i n c a n b e r e g e n e r a t e d b y a d d i t i o n of m e r c a p t o e t h a n o l . In t h i s w a y , 5 7 F e c o u l d b e c o n c e n t r a t e d t o a n e x t e n t of 80% in f e r r e d o x i n . B y c o n c e n t r a t i n g 5 7 F e b y c u l t i v a t i o n i n B u r k ' s m e d i u m c o n t a i n i n g 5 7 F e C l 2 i n s t e a d of 56FeCl 2 , B e i n e r t w a s a b l e t o o b t a i n a n o t h e r p r o t e i n c o n t a i n i n g i r o n f r o m A z o t o b a c t e r v i n e l a n d i i [28 ] . T h e d e g r e e of c o n c e n t r a t i o n of 5 7 F e i n t h i s c a s e w a s a b o u t 70%. A t s u c h c o n c e n t r a t i o n , i n v e s t i g a t i o n s w o u l d a p p e a r t o b e p o s s i b l e i n s p i t e of t h e l o w i r o n c o n t e n t .

T h e f o l l o w i n g a m o u n t s of m a t e r i a l c o n t a i n 1 g r a m a t o m of i r o n , i . e . 57 g F e , w h e n t h e i r o n i s c o m p l e t e l y e x c h a n g e d b y 5 7 F e 125] :

12 000 g c y t o c h r o m e 1 0 0 0 - 6 000 g f e r r e d o y i n 6 000 g h a e m e r y t h r i n

30 000 g i r o n - c o n t a i n i n g p r o t e i n f r o m A z o t o b a c t e r v i n e l a n d i i 30 000 g i r o n f l a v o p r o t e i n ( d i h y d r o o r o t i c d e h y d r o g e n a s e ) 25 000 g s u c c i n i c d e h y d r o g e n a s e .

M o s t p r o t e i n s c a n b e c o n c e n t r a t e d t o a b o u t 10% a n d a t t h e b e s t t o 20% i n s o l u t i o n .

V a r i o u s i r o n - p o r p h y r i n s ( h a e m c h l o r i d e , b r o m i d e , iod ide and h a e m a t i n ) a s w e l l a s s e v e r a l h a e m p r o t e i n s , n a m e l y : o x y h a e m o g l o b i n , c y t o c h r o m e С a n d c a t a l a s e , h a v e b e e n i n v e s t i g a t e d by K a r g e r [29J; o t h e r a u t h o r s [19, 20, 30] s t u d i e d h a e m o g l o b i n a n d h a e m i n . H a e m o g l o b i n c o n t a i n s f o u r h a e m g r o u p s . In t h e h a e m g r o u p , t h e i r o n i s a t t a c h e d t o f o u r n i t r o g e n a t o m s wh ich b e l o n g t o t h e p o r p h y r i n r i n g , a s m e n t i o n e d b e f o r e . P o s i t i o n & of t h e o c t a h e d r a l e n v i r o n m e n t of t h e i r o n a t o m i s o c c u p i e d b y t h e g l o b i n m o l e c u l e , w h e r e b y i t i s c o m m o n l y a s s u m e d t h a t t h e i r o n a t o m i s a t t a c h e d t o t h e g l o b i n v i a a n i t r o g e n a t o m . P o s i t i o n 6 i s o c c u p i e d b y a l i g a n d w h i c h c a n b e e a s i l y e x -c h a n g e d . F o r e x a m p l e , t h e f o l l o w i n g e x c h a n g e t a k e s p l a c e i n t h e r e d b lood c e l l s :

0 2 - h a e m o g l o b i n + C Q ^ C 0 2 - h a e m o g l o b i n + 0 2 .

G o n s e r , G r a n t a n d K r e g z d e [19, 30] e x a m i n e d t h e r e d b l o o d c e l l s of h u m a n b lood a n d t h o s e of r a t b lood in w h i c h 5 7 F e w a s c o n c e n t r a t e d to abou t 3 . 5 % . In b o t h c a s e s , two d o u b l e t s , a w e a k o n e a n d a s t r o n g o n e , w e r e o b -s e r v e d and t h e s e a t t r i b u t e d t o 0 2 - h a e m o g l o b i n and C 0 2 - h a e m o g l o b i n . Wi th h u m a n b l o o d , a n a d d i t i o n a l w e a k , s i n g l e r e s o n a n c e l i n e a p p e a r e d , w h o s e o r i g i n i s s t i l l u n k n o w n .

On t h e b a s i s of l a t e r w o r k , t h e s e a u t h o r s a s s u m e t h a t i n C 0 2 - h a e m o g l o b i n t h e C 0 2 i s not a t t a c h e d to t h e i r o n but t o a n o t h e r p a r t of t he m o l e c u l e , s i n c e t h e s p e c t r a of b l o o d w h i c h w a s e x p o s e d i n o n e i n s t a n c e t o a n a t m o s p h e r e o f n i t r o g e n a n d i n a n o t h e r i n s t a n c e t o c a r b o n d i o x i d e w e r e p r a c t i c a l l y t h e s a m e . I t h a s n o t b e e n e s t a b l i s h e d w h e t h e r p o s i t i o n 6 of t h e i r o n i n C 0 2 -h a e m o g l o b i n i s v a c a n t o r w h e t h e r i t i s o c c u p i e d b y a m o l e c u l e of w a t e r .

A s s t a t e d , in s e c t i o n A , s u b - h e a d i n g 2, t he s p e c t r u m of C O - h a e m o g l o b i n r e v e a l s o n l y a s i n g l e r e s o n a n c e l ir ie w h i c h h a s a b o u t t h e s a m e i s o m e r s h i f t a s t h a t i n o x y h a e m o g l o b i n . C h a n g e s i n t h e h a e m o g l o b i n m o l e c u l e c a n t h u s b e r e a d i l y d e t e c t e d w i t h t h e h e l p of M ö s s b a u e r s p e c t r a . T h e s u c c e s s a l -r e a d y a t t a i n e d i n t h e a p p l i c a t i o n of M ö s s b a u e r s p e c t r o s c o p y to b i o c h e m i c a l

138

p r o b l e m s s h o w s t h a t t h i s m e t h o d c o n s t i t u t e s a u s e f u l t o o l f o r r e s e a r c h i n t h i s d i r e c t i o n .

R E F E R E N C E S

[1] FLUCK, E . , "The Mössbauer effect and its application in chemistry", Advances in Inorganic Chemistry and Radiochemistry, Vol .6 , Academic Press, New York and London (1964) p.433.

[2] FLUCK, E. , KERLER, W., NEUWIRTH, W., "Der Mössbauer-Effekt und seine Bedeutung für die Chemie". Angew. Chem. 75 (1963) 461; Angew. Chem. Int. e d . , in English 2 (1963) 277; J. Japan. C h e m . , in Japanese, 19(1965) 27.

[3] FLUCK, E . , KERLER, W . , NEUWIRTH, W . , "Application of the Mössbauer effect in coordination chemistry, Proc. 8th Int. Conf. on Coordination Chemistry, Vienna, Sep. 7-11 (1964) p. 1.

[4] GOLDANSKII, V. I . , "The Mössbauer effect and its application in chemistry", G. E.C. a tom. Energy Rev. , 1 (1963) 3 .

[5] KERLER, W. , NEUWIRTH. W., FLUCK, E. . KUHN, P . . ZIMMERMANN, В., "Untersuchung komplexer ' und kovalenter Eisenverbindungen mi t Hilfe des Mössbauer-Effektes von 5 7Fe", Z. Physik, 173 (1963) 321.

[6] WERTHEIM, G . K . , HERBER, R .H . , "Resonant gamma- ray absorption in potassium ferrate" , J. chem. Phys. 36 (1962) 2497.

[7] HERBER, R . H . , "Mössbauer spectroscopy: some recent applications to chemical problems", Mössbauer Symposium, New York, Jan. 26 (1965).

[8] DUNCAN, J . F . , GOLDING, R .M. , "The Mössbauer effect and chemical theory", Proc. 8th Int. Conf. on Coordination Chemistry, Vienna, Sep. 7-11 (1964) p. 11.

[9] FLUCK, E . , KERLER, W. , NEUWIRTH, W . , "Zur Struktur komplexer Eisenverbindungen", Z. anorg. allg. Chem. 333 (1964) 235.

[10] KERLER, W . , NEUWIRTH, W . , FLUCK, E . , "Isomerieveischiebung und Quadrupolaufepaltung beim Mössbauer-Effekt von 51 Fe in Eisenverbindungen", Z . Physik 175 (1963) 200.

[11] DE COSTER, M . , POLLAK, H . , AMELINCKX, S . , "A study of Mössbauer absorption in iron silicates", Phys. Stat. Sol. 3 (1963) 283.

[12] DE COSTER, M . , AMELINCKX, S . , "Evidence from Mössbauer experiments for the association of aliova-lent ions and vacancies in potassium chloride", Phys. Lett. ^ (1962) 245.

[13] POLLAK, H . , DE COSTER, M . , AMELINCKX, S . , "Mössbauer effect in Biotite", Phys. Stat. Sol. 2 (1962) 1653.

[14] HERBER, R. H. , KING, R .B . , WERTHEIM, G . K . , "Systematics of Mössbauer isomer shifts of i ron-organic compounds", Inorg. Chem. ¿ ( 1 9 6 4 ) 101.

[15] HERBER, R. H . , HAYTER, R. G . , "Mössbauer e f fec t in cis-trans isomers", J. Amer. chem. Soc. 86

(1964) 301.

[16] WERTHEIM, G . K . , HERBER, R.H. , "S 7Fe Mössbauer effect in ferrocene derivatives", J. chem. Phys. 38

(1963) 2106.

[17] DANON, J . , "Nuclear isomer shift and covalency in iron complexes", J. chem. Phys. 39 (1963) 236.

[18] DANON, J . , "Investigations on the electronic structure of transition metal complexes using the Mössbauer

effect" , Proc. 8th Int. Conf. on Coordination Chemistry, Vienna, Sept. 7-11 (1964) p. 5.

[19] GONSER, U . , GRANT, R. W., KREGZDE, J . , "Mössbauer effect in haemoglobin with different ligands".

Science 143 (1964) 680.

[20] ABLOV, A. B. , BERSUKER, I. B. , GOLDANSKII, V. I . , "Interpretation of the resonance absorption of

y-quanta by some complexes of Fe, taking into account the covalence of bonds and induction effects",

Dokl. Akad. Nauk S. S.S.R. 152 (1963) 1391.

[21] ABLOV. A . B . , BELOZERSKY, G . N . . GOLDANSKII, V . l . , MAKAROV, E . F . . TRUKHTANOV. V . A . ,

KHRAPOV, V. V . , "Mössbauer spectra of Fe complexes with diacetyl thiosemicarbazone oxime", Dokl.

Akad. Nauk S. S. S. R. 151 (1963) 1352.

[22] ABLOV, A . V . , BERSUKER, I . B . , GOLDANSKII, V . l . , "Mössbauer-spectra of iron complexes with

thiosemicarbazone of d iacetyloxime and their interpretat ion", Proc. 8th Int. Conf. on Coordination

Chemistry, Vienna, Sept. 7-11 (1964) p. 11. Abstracts printed by Springer-Ver lag , Vienna.

[23] HARRIS, A . D . , HERBER, R.H. , JONASSEN, H. В., WERTHEIM, G . K . , "Complexes of iron(II) and seme 5-substituted tetrazoles", J. Amer. chem. Soc. 85 (1963) 2927.

139

[24] WERTHEIM, G . K . , KINGSTON, W. R . , HERBER, R . H . , "Mössbauer-effect In iron(III) ace ty l ace tona te and chemica l consequences of К capture in cobalt(III) a c e t y l a c e t o n a t e " , I . c h e m . Phys. 31 (1962) 687.

[25] BEINERT, H . , pr ivate communica t ions . [26] BLOMSTROM, D . C . , KNIGHT, W. , PHILLIPS, W . D . , WEIHER, I . F . , "The nature of iron in ferredoxin",

Proc. natn. Acad. Sei. j i l (1964) 1085. [27] LOVENBERG, W., BUCHANAN, B.B., RABINOWITZ, I . e . , "Studies on the c h e m i c a l nature of Clostr idial

f e r r edox in , " J. b i o l . C h e m . 238 (1963) 3899. [28] SHETHNA, Y. I . . WILSON, P. W . , HANSEN', R. E . , BEINERT, H . , " Iden t i f i ca t ion by isotopic subst i -

tu t ion of the EPR signal a t g = 1 . 9 4 in a non -ha e m iron prote in from Azo tobak te r " , Proc. na tn . Acad . Se i . 52 (1964) 1263.

[29] KARGER, W . , "Mössbauer-Spektroskopie an Eisenporphyriñen und H ä m - P r o t e i n e n " , Ber. Bunsenge-

se l l schaf t 68 (1964) 793.

[30] GONSER, U . , GRANT, R. W . , KREGZDE, J . , "Determinat ion of the chemica l structure of haemoglobin

using the Mössbauer e f f e c t " , Appl. Phys. Lett. 3 (1963) 189.

Further references:

COSTA, N. L . , DANON, I . , MOREIRA XAVIER, R . , "Measurement of nuclear quadrupole interactions

in iron complexes using the Mössbauer e f f e c t " , Physics Chem. Solids 23 (1962) 1783.

FRAUENFELDER, H . , The Mössbauer Effect , W. A. Benjamin , New York (1962).

HERBER, R . H . , KINGSTON, W. R . , WERTHEIM, G . K . , "Mössbauer e f f ec t in iron pentacarbonyl and

re la ted carbonyls" , Inorg. C h e m . 2 (1963) 153.

ITO, A . , ONO, К . , ISHIKAWA, Y . , "A study of the low temperature transition in magnet i te" , I . phys. Soc. lapan 18 (1963) 1465. KALVIUS, M . , ZAHN, U . , KIENLE, P . , EICHER, H . . "Hyperfeinstruktur des 1 4 , 4 keV-Zustandes von 5 , F e , gebunden in verschiedenen Eisencarbonylen", Z . Naturf . 17A (1962) 494.

N . N . G R E E N W O O D m e n t i o n e d . t h a t a n e w X - r a y i n v e s t i g a t i o n by L . D a h l ( U n i v e r s i t y of W i s c o n s i n , USA) i n d i c a t e d t h e s t r u c t u r e of t r i - i r o n d o d e c a -c a r b o n y l t o b e a s s h o w n in F i g . A . T h i s w a s c o m p a t i b l e wi th t he o b s e r v e d M ö s s b a u e r s p e c t r u m .

D I S C U S S I O N

Fe

Fe

FIG. A. Tri- i ron dodecarbonyl

140

J . A . S T O N E s u g g e s t e d t h a t f e r r i t i n (a n a t u r a l l y o c c u r r i n g c o m p o u n d ) s h o u l d b e i n v e s t i g a t e d w i t h t h e h e l p of M ö s s b a u e r s p e c t r o s c o p y .

R . H . H E R B E R m e n t i o n e d t h a t t h e i n v e s t i g a t i o n of a d e n o s i n e p h o s p h a t e s a t d i f f e r e n t pH v a l u e s of s o l u t i o n s c o n t a i n i n g i r o n y i e l d e d v a l u a b l e r e s u l t s .

V . l . GOLDANSKII m e n t i o n e d t h a t c o m p l e x e s of Fe111 w i t h g u a n i n e , g u a n o s i n e a n d r i b o s e h a d b e e n i n v e s t i g a t e d . F u r t h e r m o r e RNA a n d DNA h a d b e e n i n v e s t i g a t e d . In R N A a n d DNA i r o n w a s u s u a l l y b o n d e d t h r o u g h t h e s u g a r g r o u p s , though s o m e bonds b e t w e e n i r o n and n i t r o g e n e x i s t e d . The i m p o r t a n c e of M ö s s b a u e r s p e c t r o s c o p y f o r b i o c h e m i c a l p r o b l e m s w a s d e m o n s t r a t e d b y t h e f a c t t h a t , f o r i n s t a n c e , t o b a c c o m o s a i c v i r u s l o s t i t s i n f e c t i o n p r o p e r t i e s with t h e l o s s of i r o n . An add i t ion of the o r i g i n a l p r o p e r -t i e s w e r e r e g a i n e d .

R . M . G O L D I N G p o i n t e d ou t t h a t in s e v e r a l i r o n c o m p l e x e s , s u c h a s i r o n d i t h i o c a r b a m a t e s a n d c y t o c h r o m e C, a t l e a s t o n e e x c i t e d s t a t e w a s l o w -l y i n g . S u c h a c o m p l e x w o u l d be e x p e c t e d t o g i v e a n u n u s u a l t e m p e r a t u r e -d e p e n d e n t A E Q v a l u e , in f a c t i t m i g h t be o b s e r v e d t h a t A E Q d e c r e a s e d wi th d e c r e a s i n g t e m p e r a t u r e o v e r a p a r t of t h e t e m p e r a t u r e r a n g e . T h e s e c o n d i n t e r e s t i n g p o i n t a b o u t t h e s e c o m p l e x e s w a s t h a t t h e u n p a i r e d d e l e c t r o n s a f f e c t e d t he e l e c t r o n - s p i n d e n s i t y t h r o u g h o u t t h e m o l e c u l e . He had o b s e r v e d i n i r o n d i t h i o c a r b a m a t e s a s m a l l c h a n g e i n t h e s p i n d e n s i t y of a h y d r o g e n s o r b i t a l ( a r i s i n g f r o m s p i n - p o l a r i z a t i o n ) a s f a r a w a y a s e ight bond d i s t a n c e s f r o m t h e i r o n a t o m . Such e f f e c t s , i . e . l o w - l y i n g e x c i t e d s t a t e s a n d l o n g -r a n g e s p i n p o l a r i z a t i o n , m i g h t a c c o u n t f o r s o m e of t he u n u s u a l p h y s i c a l p r o p e r t i e s of m o l e c u l e s in b i o l o g i c a l s y s t e m s .

141

STRUCTURAL INVESTIGATIONS (continued)

(Sess ion 4)

MÖSSBAUER SPECTRA OF SOME IRON COMPOUNDS

Т . е . GIBB AND N.N GREENWOOD UNIVERSITY OF NEWCASTLE UPON TYNE

UNITED KINGDOM

T h i s p a p e r c o n c e r n s t h e a p p l i c a t i o n of M ö s s b a u e r s p e c t r o s c o p y " t o t h e s o l u t i o n of t h r e e d i f f e r e n t t y p e s of p r o b l e m in t h e c h e m i s t r y of i r o n :

(1) T h e d i r e c t i o n of e l e c t r o n t r a n s f e r i n f e r r o m a g n e t i c i r o n b o r i d e s ; (2) T h e o x i d a t i o n s t a t e s of i r o n i n s i l i c a t e m i n e r a l s ; a n d (3) T h e b o n d i n g in t h e h i g h - s p i n t e t r a h e d r a l c o m p l e x F e C l | \ T h e a p p a r a t u s u s e d w a s b u i l t by M r . J . D. C o o p e r a n d i n c o r p o r a t e d a

L a b e n 5 1 2 - c h a n n e l a n a l y s e r [1] . T h e f i r s t t o p i c w a s i n v e s t i g a t e d in c o l l a b o r a t i o n w i t h D r . R . V. P a r i s h .

A . F E R R O M A G N E T I C I R O N B O R I D E S

C o n s i d e r a b l e i n t e r e s t i s a t t a c h e d to t h e s t r u c t u r e and b o n d i n g in m e t a l b o r i d e s s i n c e t h e y f r e q u e n t l y con ta in both b o r o n - b o r o n and b o r o n - m e t a l b o n d s [2-4] . T h e r e h a s b e e n m u c h d i s c u s s i o n of t h e n a t u r e of t he bonding involved a n d , in p a r t i c u l a r , of t h e d i r e c t i o n of e l e c t r o n t r a n s f e r b e t w e e n t h e m e t a l and b o r o n a t o m s .

T h e s u g g e s t i o n t h a t bond ing in m e t a l b o r i d e s M 2 B and M B invo lves e l e c -t r o n t r a n s f e r f r o m m e t a l t o b o r o n i s due t o P a u l i n g [5, 6] . By u s i n g h i s r e -l a t i o n b e t w e e n bond l e n g t h and bond o r d e r , P a u l i n g s u g g e s t e d t h a t about 0. 3 of an e l e c t r o n m u s t b e t r a n s f e r r e d t o t h e b o r o n a t o m s in F e B ; t h e b o r o n -b o r o n b o n d s a r e h a l f - b o n d s and t h e i r o n - b o r o n b o n d s h a v e an o r d e r b e t w e e n o n e - h a l f and o n e - t h i r d . F o r t h e M 2 B b o r i d e s , i t i s n e c e s s a r y t o p o s t u l a t e e i t h e r t h a t t h e o r d e r of t he m e t a l - b o r o n bond r i s e s [7], o r t h a t the p e r c e n t a g e d c h a r a c t e r of t h i s bond i n c r e a s e s , a s M v a r i e s t h r o u g h t h e s e r i e s Mn, F e , Co, N i . In e, i ther c a s e t h e m e t a l - b o r o n bond i s e x p e c t e d to b e c o m e s t r o n g e r a s t he a t o m i c n u m b e r of t h e m e t a l r i s e s .

I t h a s b e e n s t a t e d t h a t s u c h a t r e n d in b o n d s t r e n g t h s i s i n c o m p a t i b l e w i t h o b s e r v a t i o n s of t h e p a r t i t i o n of p a i r s of m e t a l s b e t w e e n t h e p h a s e s M B and M 2 B in e q u i l i b r i u m [8]. In a l l c a s e s s t u d i e d t h e m e t a l of l o w e r a t o m i c n u m b e r w a s c o n c e n t r a t e d in t h e p h a s e which w a s r i c h e r in b o r o n . T h i s w a s i n t e r p r e t e d a s showing t h a t t h e l i g h t e r e l e m e n t had the g r e a t e r bond s t r e n g t h t o b o r o n . T h i s a r g u m e n t i s n o t v a l i d , h o w e v e r , s i n c e t h e o b s e r v e d p a r t i -t i on r a t i o i s t he r e s u l t of a b a l a n c i n g of m e t a l - b o r o n (as we l l a s b o r o n - b o r o n and m e t a l - m e t a l ) b o n d s t r e n g t h s f o r t h e t w o p h a s e s . T h u s , t h e o b s e r v e d

143

r a t i o m i g h t r e s u l t f r o m a h i g h e r b o n d s t r e n g t h f o r t h e l i g h t e r m e t a l in t h e p h a s e M B o r f o r t h e h e a v i e r m e t a l in t h e M 2 B p h a s e .

A s e v i d e n c e f o r e l e c t r o n t r a n s f e r f r o m b o r o n to m e t a l , K i e s s l i n g [7, 9J n o t e d t h a t t h e m a x i m u m b o r o n c o n t e n t o b t a i n a b l e in an a l l o y d e c r e a s e s a s t h e a t o m i c n u m b e r of t h e m e t a l i n c r e a s e s ( i . e . t h e n u m b e r of h o l e s in t h e d b a n d d e c r e a s e s ) , and i t h a s no t b e e n p o s s i b l e t o ob t a in a b o r i d e of c o p p e r . T h i s a r g u m e n t i s not w i thou t d a n g e r s , h o w e v e r , s i n c e i t i s p r o b a b l e 110, 11] t h a t i n b o r i d e s of t y p e M B 2 t h e s t r u c t u r e a p p r o a c h e s M 2 + (B") 2 . K i e s s l i n g l

[3] h a s a l s o c r i t i c i z e d t h e s u g g e s t i o n of m e t a l - t o - b o r o n e l e c t r o n t r a n s f e r in F e B on t h e g r o u n d s t h a t i t wou ld m a k e b o r o n - b o r o n doub le b o n d i n g m o r e l i k e l y , w h i c h wou ld r e s u l t in s h o r t e r b o r o n - b o r o n d i s t a n c e s t h a n t h o s e o b -s e r v e d a n d wou ld f a v o u r a b o n d a n g l e of a b o u t 180° . T h i s d e p e n d s on how m a n y e l e c t r o n s a r e i n v o l v e d in b o n d i n g to t h e m e t a l a t o m s . P a u l i n g ' s s u g -g e s t i o n of b o r o n - b o r o n h a l f - b o n d s i s c o m p a t i b l e wi th t h e o b s e r v e d bond a n g l e [6].

T h e m o s t d i r e c t e v i d e n c e c o n c e r n i n g t h e d i r e c t i o n of e l e c t r o n t r a n s f e r c o m e s f r o m m e a s u r e m e n t s of s a t u r a t i o n m a g n e t i z a t i o n w h i c h h a v e b e e n m a d e f o r M n B , C o B and N i B [12, 13] and f o r F e 2 B [14] and Co 2 B [15]. T h e r e s u l t s f o r t h e m o n o b o r i d e s h a v e b e e n i n t e r p r e t e d by L u n d q u i s t , M y e r s and W e s t i n [4, 13] in t e r m s of a t h r e e - b a n d m o d e l ; t h e t h r e e b a n d s b e i n g a 3d b a n d of t h e t r a n s i t i o n m e t a l , a 4 s c o n d u c t i o n b a n d , and a band f o r m e d f r o m t h e h y b r i d i z e d 2s and 2p o r b i t a l s of t h e b o r o n a t o m s . T h i s l a t t e r b a n d i s r e s p o n s i b l e f o r t h e b o r o n - b o r o n b o n d i n g a n d i t i s a s s u m e d t h a t i t d o e s n o t c o m b i n e wi th t h e 4 s b a n d . T h e 2sp and 3d b a n d s a r e a s s u m e d to b e l o c a l i z e d on t h e b o r o n a n d m e t a l a t o m s r e s p e c t i v e l y . T h e p o p u l a t i o n of t h e 3d b a n d i s g i v e n ( n e g l e c t i n g c o n t r i b u t i o n s f r o m o r b i t a l m o m e n t s and t h e c o n d u c t i o n band ) by t h e B o h r m a g n e t o n n u m b e r , wh ich i s 1. 92, 1. 12 and 0. 28 ( p e r MB u n i t ) f o r M n B , F e B a n d C o B r e s p e c t i v e l y . A s s u m i n g t h a t one of t h e s p i n s u b - b a n d s i s f u l l , t h i s g i v e s 8. 1, 8. 9 and 9. 7 3d e l e c t r o n s p e r m e t a l a t o m r e s p e c t i v e l y . T h e s e v a l u e s a r e a l l l a r g e r t h a n t h e n u m b e r s of o u t e r e l e c -t r o n s f o r t h e f r e e m e t a l s . T h e 3d b a n d h a s t h e r e f o r e g a i n e d e l e c -t r o n s a t t h e e x p e n s e of t h e b o r o n a t o m s . T h e p o p u l a t i o n of t h e o t h e r t w o b a n d s a r e n o t k n o w n bu t w e r e e s t i m a t e d [4] b y P a u l i n g ' s s u g g e s t e d b o r o n -b o r o n b o n d n u m b e r s . F o r M n B and F e B t h e r e w i l l t h u s b e a p p r o x i m a t e l y o n e e l e c t r o n in t h e 2 s p b a n d a n d t h e r e m a i n d e r i n t h e 4 s b a n d . T h e c o n -f i g u r a t i o n f o r F e B c a n t h e r e f o r e b e w r i t t e n ( 2 s p ) ~ 1 ( 3 d ) 8 - 9 ( 4 s ) ~ 1 1 .

A s i m i l a r p r o c e d u r e m a y b e f o l l o w e d f o r F e 2 B and Co 2 B f o r w h i c h t h e m a g n e t o n n u m b e r s a r e 1. 8 and 0. 8 r e s p e c t i v e l y , g i v i n g p o p u l a t i o n s in t h e 3d b a n d of 8. 2 a n d 9. 2. In t h i s c a s e t h e b o r o n - b o r o n b o n d s a r e v e r y l o n g and t h e popu la t ion of t he 2 s p band wi l l be c o r r e s p o n d i n g l y low. P a u l i n g s u g -g é s t s [6] t h a t t h e b o r o n - b o r o n b o n d s a r e o n e - s i x t h b o n d s , w h i c h would g ive a c o n f i g u r a t i o n f o r è F e 2 B of (2sp )~° - 3 (3d) 8 - 2 (4s )~ 1 .

In s h o r t , t h e s a t u r a t i o n m a g n e t i z a t i o n r e s u l t s s u g g e s t a t r a n s f e r of e l e c t r o n s f r o m b o r o n t o m e t a l . F o r f u r t h e r i n f o r m a t i o n on t h i s p o i n t t h e M ö s s b a u e r s p e c t r a of F e 2 B and F e B h a v e n o w b e e n i n v e s t i g a t e d . In p r i n -c i p l e , t h e M ö s s b a u e r e f f e c t i s w e l l s u i t e d to t h i s p r o b l e m , s i n c e i n f o r m a t i o n m a y b e o b t a i n e d abou t t h e e l e c t r o n d e n s i t y and t h e h y p e r f i n e m a g n e t i c f i e l d a t t h e i r o n n u c l e i , bo th of w h i c h q u a n t i t i e s w i l l d e p e n d on t h e d e t a i l e d e l e c -t r o n i c s t r u c t u r e of t h e i r o n a t o m .

1 4 4

G o o d s p e c t r a f o r t h e p o w d e r e d b o r i d e s w e r e o b t a i n e d w i t h a b o u t 200 000 c o u n t s p e r c h a n n e l a n d t h e a b s o r p t i o n m a x i m a c o u l d b e l o c a t e d t o ± 1 c h a n n e l (0. 045 m m / s ) . F o r F e 2 B a l l s i x l i n e s w e r e w e l l r e s o l v e d a n d t h e p a r a m e t e r s w e r e c a l c u l a t e d b y t h e m e t h o d of l e a s t s q u a r e s . F o r F e B t h e c e n t r a l p a i r of l i n e s w e r e n o t w e l l r e s o l v e d a n d t h e f o u r p a r a m e t e r s w e r e c a l c u l a t e d f r o m t h e p o s i t i o n s of t he f o u r o u t s i d e l i n e s only . T h e v a l u e s o b t a i n e d ( m e a n of t h r e e s e p a r a t e d e t e r m i n a t i o n s ) a r e s h o w n in T a b l e I .

TABLE I

M Ö S S B A U E R P A R A M E T E R S F O R I R O N A N D I R O N B O R I D E S ( m m / s r e l a t i v e t o p u r e i r o n )

Fe Fe2B FeB

C h e m i c a l shift 0 +0. 16 +0 .28

Quadrupole split t ing 0 +0. 03 +0. 08

g 0 (ground state) 3. 924 a 2. 92 1 . 4 0

g j (exc i ted state) 3 . 2 4 5 a 1. 61. 0 . 8 0

Hyperf ine f ie ld (kOe) 3 3 0 a 242 118

Bohr magne ton number '3 2 . 2 1 . 9 " l . l

a ^ These values were obtained f rom Ref. [16] .

For Fe2B and FeB units. Values per Fe atoms are 1 . 7 and 0. 9 respect ive ly .

T h e c o n s i d e r a b l e r e d u c t i o n in h y p e r f i n e m a g n e t i c f i e l d on p a s s i n g f r o m i r o n t o FegB and F e B c l e a r l y i n d i c a t e s a s u b s t a n t i a l r e d u c t i o n in t h e n u m b e r of u n p a i r e d 3d e l e c t r o n s . (The d i p o l a r c o n t r i b u t i o n of t h e s e e l e c t r o n s , which a r e n o t in c u b i c s y m m e t r y , i s e x p e c t e d t o b e v e r y s m a l l . ) A s o n e of t h e s p i n s u b - b a n d s i s f u l l , t h e d e c r e a s e in h y p e r f i n e m a g n e t i c f i e l d m u s t b e c a u s e d by an i n c r e a s e in t h e n u m b e r of 3d e l e c t r o n s . T h i s i m p l i e s t r a n s f e r of e l e c t r o n s f r o m b o r o n t o i r o n , s i n c e t h e o n l y o t h e r s o u r c e of e l e c t r o n s would be t h e 4 s band and any s u b s t a n t i a l change in the popula t ion of t h i s band would g ive a m u c h l a r g e r c h e m i c a l s h i f t t h a n t h a t o b s e r v e d ( s ee be low) . T h i s r e s u l t i s t h u s in a g r e e m e n t wi th t h e i n t e r p r e t a t i o n of t he s a t u r a t i o n m a g n e t i -za t i on r e s u l t .

S m a l l p o s i t i v e c h e m i c a l s h i f t s a r e o b s e r v e d f o r F e g B r e l a t i v e t o i r o n a n d f o r F e B r e l a t i v e t o F e 2 B . T h e c h e m i c a l s h i f t f o r F e 2 B c a n n o t b e i n -t e r p r e t e d d i r e c t l y s i n c e t h e e l e c t r o n i c s t r u c t u r e of m e t a l l i c i r o n i s not we l l u n d e r s t o o d . F o r t h e t w o b o r i d e s , the- o b s e r v e d c h e m i c a l s h i f t m a y b e i n -t e r p r e t e d in t e r m s of t h e i n c r e a s e in popu la t ion of t h e 3d b a n d . T h e r e s u l t s d i s c u s s e d a b o v e s h o w t h a t t h i s p o p u l a t i o n i n c r e a s e s f r o m 8. 2 (Fe2B) t o 8. 9 ( F e B ) e l e c t r o n s p e r b o r i d e u n i t . T h e i n c r e a s e d s h i e l d i n g t h u s p r o d u c e d wou ld b e s u f f i c i e n t t o a c c o u n t f o r t h e o b s e r v e d s m a l l c h e m i c a l s h i f t w i thou t r e q u i r i n g a n y c h a n g e i n p o p u l a t i o n f o r t h e 4 s b a n d . T h e e x t r a e l e c t r o n s g o i n g i n t o t h e 3d b a n d c a n n o t c o m e f r o m t h e 4 s b a n d s i n c e t h i s w o u l d g i v e

145

a m u c h l a r g e r p o s i t i v e c h e m i c a l s h i f t t h a n i s o b s e r v e d . T h e s e e l e c t r o n s m u s t , t h e r e f o r e , c o m e f r o m t h e 2 s p b a n d .

T r a n s f e r of e l e c t r o n s o n t h e s c a l e e n v i s a g e d b y P a u l i n g (a t l e a s t 0. 3 e l e c t r o n t r a n s f e r r e d f o r F e B r e l a t i v e t o Fe2B) w o u l d g i v e t o o l a r g e a c h e -m i c a l s h i f t if t h e y w e r e a l l t a k e n f r o m t h e 4 s b a n d . R e m o v a l of e l e c t r o n s f r o m b o t h t h e 4 s and 3d b a n d s would a c c o u n t f o r t h e o b s e r v e d c h e m i c a l s h i f t ( t he d e c r e a s e i n 4 s e l e c t r o n d e n s i t y a t t h e n u c l e u s b e i n g o f f s e t by r e d u c i n g s h i e l d i n g of 4 s e l e c t r o n s b y 3d e l e c t r o n s ) b u t t h i s i s n o t c o m p a t i b l e w i t h t h e m a g n e t i c p r o p e r t i e s . W e c o n c l u d e , t h e r e f o r e , t h a t b o t h t h e c h a n g e in t h e h y p e r f i n e m a g n e t i c f i e l d and t h e s m a l l c h e m i c a l s h i f t a r e due t o t r a n s f e r of e l e c t r o n s f r o m b o r o n t o i r o n .

T h r e e o t h e r c o m p a r a b l e b i n a r y s y s t e m s of i r o n h a v e b e e n i n v e s t i g a t e d b y t h e M ö s s b a u e r t e c h n i q u e . T h e r e s u l t s r e s e m b l e c l o s e l y t h o s e o b t a i n e d w i t h t h e i r o n b o r i d e s a n d a c o n s i s t e n t i n t e r p r e t a t i o n f o r a l l f o u r s y s t e m s i s t h e r e f o r e p o s s i b l e . S t e a r n s [17] i n v e s t i g a t e d t h e F e + Si s y s t e m u p t o 27 a t o m % s i l i c o n and found t h a t am i n c r e a s e in t h e n u m b e r of s i l i con n e a r e s t n e i g h b o u r s t o a n i r o n a t o m d e c r e a s e d t h e i n t e r n a l m a g n e t i c f i e l d and g a v e a s m a l l p o s i t i v e c h e m i c a l s h i f t . It w a s c o n c l u d e d t h a t t h e s i l i c o n a t o m s c o n t r i b u t e e l e c t r o n s t o t h e i r o n d s h e l l . In a l a t e r p a p e r [18J S t e a r n s d e s c r i b e d a s i m i l a r e f f e c t i n t h e F e + Al s y s t e m . In n e i t h e r s y s t e m w a s a q u a d r u p o l e e f f e c t m e n t i o n e d , b u t s i n c e a l l t h e s p e c t r a r e s u l t e d f r o m t h e s u p e r p o s i t i o n of s e v e r a l h y p e r f i n e f i e l d s , s m a l l s h i f t s w o u l d h a v e b e e n a l -m o s t i m p o s s i b l e t o d e t e c t . T h e f e r r o m a g n e t i c a l l oy Fe^N in w h i c h t h e n i -t r o g e n i s a t t h e b o d y - c e n t r e of a f a c e - c e n t r e d c u b i c l a t t i c e of i r o n a t o m s h a s a l s o b e e n i n v e s t i g a t e d [19] . T w o h y p e r f i n e f i e l d s a r e o b s e r v e d ; o n e , of 345 k O e , i s a t t r i b u t e d t o t h e i r o n a t o m s a t t h e c u b e c o r n e r s , and i s s a i d t o i n d i c a t e a S d ^ s 1 c o n f i g u r a t i o n , wh i l e t h e s e c o n d , of 215 kOë; i s a s s o c i a t e d wi th t h e i r o n a t o m s a t t h e f a c e - c e n t r e , t h e s e h a v i n g a c h i e v e d a p p r o x i m a t e l y a S d ^ s 1 c o n f i g u r a t i o n by a c c e p t i n g e l e c t r o n s f r o m t h e n i t r o g e n .

B . OXIDATION S T A T E S O F IRON IN C R O C I D O L I T E AND AMOSITE

C r o c i d o l i t e i s a h igh ly c o l o u r e d , n a t u r a l l y o c c u r r i n g a m p h i b o l e m i n e r a l [20] c o n t a i n i n g i n f i n i t e s i l i c a t e r i b b o n s of s t o i c h i o m e t r y S i 4 Oiî . T w o p r o b -l e m s a r e p o s e d b y t h i s m i n e r a l . T h e f i r s t i s t h e o r i g i n of i t s b r i g h t b l u e c o l o u r a n d t h e s e c o n d c o n c e r n s t h e n a t u r e of t h e i r o n s p e c i e s f o r m e d o n o x i d a t i o n by m e a n s of o x y g e n o r r e d u c t i o n by m e a n s of h y d r o g e n . T h e d e -t a i l e d s t r u c t u r a l c h a n g e s w h i c h o c c u r d u r i n g t h e s e p r o c e s s e s a r e a l s o u n -k n o w n . It s e e m e d t h a t s i g n i f i c a n t i n f o r m a t i o n on t h e s e p r o b l e m s m i g h t b e o b t a i n e d by M ö s s b a u e r s p e c t r o s c o p y .

B o l i v i a n c r o c i d o l i t e , w h i c h i s t h e on ly o n e f o r w h i c h d e t a i l e d s t r u c t u r a l d a t a a r e a v a i l a b l e [21] c o n t a i n s d o u b l e - s t r a n d e d i n f i n i t e c h a i n s of Si0 4 t e t r a -h e d r a bound t o g e t h e r s o t h a t a l t e r n a t e S i 0 4 u n i t s s h a r e two and t h r e e c o r n e r s . V a c a n t s p a c e s in t h e o x y g e n l a t t i c e a r e f i l l e d b y h y d r o x y l g r o u p s . C h a r g e n e u t r a l i t y i s a c h i e v e d b y m e t a l c a t i o n s c o o r d i n a t e d t o o x y g e n , t h e r e b e i n g f o u r b a s i c a l l y d i f f e r e n t c a t i o n s i t e s M j , M 2 , M3 and M4 wi th a n o n - r a n d o m d i s t r i b u t i o n of Mg 2 + , F e 2 + , F e a + a n d A l a + c a t i o n s on t h e s e s i t e s . M j , M 2

and M 3 a r e e f f e c t i v e l y in o c t a h e d r a l c o o r d i n a t i o n , bu t M4 i s m o r e i r r e g u l a r

146

a n d h a s a c o o r d i n a t i o n n u m b e r of e i g h t . In B o l i v i a n c r o c i d o l i t e M 4 i s o c -c u p i e d c h i e f l y b y N a + , K+ , C a 2 + a n d Mg 2 + , w h e r e a s F e 2 + a n d F e 3 + a p p e a r i n M x , M 2 , a n d M 3 i n c o n j u n c t i o n w i t h s o m e M g 2 + .

T h e s a m p l e s u s e d in t h i s i n v e s t i g a t i o n [22] w e r e f r o m Sou th A f r i c a and w e r e k i n d l y s u p p l i e d b y D r . W. E . A d d i s o n of N o t t i n g h a m U n i v e r s i t y . T h e y c o n t a i n e d a h i g h e r p e r c e n t a g e of i r o n t h a n in t h e B o l i v i a n f o r m a n d t y p i c a l a n a l y s e s [23] g a v e F e 2 + 15. 86% a n d F e 3 + 1 2 . 17%. O x i d a t i o n s t u d i e s [23] h a v e s h o w n t h a t F e 2 + c a n b e o x i d i s e d t o F e 3 + , t h e r e a c t i o n b e i n g s u s t a i n e d b y m i g r a t i o n of p r o t o n s and e l e c t r o n s a l o n g t h e c h a i n s . T h e f o l l o w i n g e q u a t i o n s w e r e p o s t u l a t e d :

4 F e 2 + + 4 0 H - + 0 2 = 4 F e 3+ + 4 0 2 " + 2 H 2 0

4 F e 2 + + 2 0 H " + 0 2 = 4 F e 3 + + 3 0 2 _ + H 2 0

4 F e 2 + + 0 2 = 4 F e 3 + + 202~

T h e k i n e t i c s of t h e o x i d a t i o n [ 2 4 ] i m p l y t h a t r e a c t i o n o c c u r s a t t h e e n d s of t h e c h a i n s . C r o c i d o l i t e h a s a l s o b e e n s h o w n t o h a v e s e m i - c o n d u c t o r p r o -p e r t i e s a l o n g t h e f i b r e a x i s [25] .

R e d u c t i o n of c r o c i d o l i t e b y h y d r o g e n h a s b e e n s t u d i e d [26 ] a t 4 5 0 ° b u t t h e e x p e c t e d r e a c t i o n

4 F e 3 + + 4 0 2 " + 2H 2 = 4 F e 2 + + 4 0 H "

i s n o t f o l l o w e d a b s o l u t e l y ; q u a n t i t a t i v e m e a s u r e m e n t s s h o w e d e l i m i n a t i o n of w a t e r ( and t h u s l a t t i c e oxygen) and t h e f o r m a t i o n of i r o n in a v a l e n c y s t a t e l o w e r t h a n t w o ( p o s t u l a t e d t o b e F e ° by A d d i s o n and S h a r p [26]) .

M ö s s b a u e r s p e c t r a w e r e r u n a t r o o m t e m p e r a t u r e a n d c a l i b r a t e d w i t h t h e h y p e r f i n e s p e c t r u m of m e t a l l i c i r o n a s b e f o r e [1 ], a n d s h i f t v a l u e s a r e q u o t e d in m m / s w i t h r e f e r e n c e t o t h i s s t a n d a r d . T h e m e a s u r e m e n t s w e r e r e p e a t e d s e v e r a l t i m e s t o c h e c k f o r c o n s i s t e n c y of l i n e s h a p e a n d p e a k -i n t e n s i t y r a t i o s . T h e s p e c t r a i l l u s t r a t e d in F i g s . 1 and 2 s h o w e x p e r i m e n t a l p o i n t s f r o m t y p i c a l i n d i v i d u a l r u n s b u t t h e c u r v e s d r a w n t h r o u g h t h e p o i n t s i n c o r p o r a t e a l l d a t a f r o m t h e r e p e a t s p e c t r a a n d a r e t h e r e f o r e n o t n e c e s -s a r i l y t h e b e s t f i t s t o t h e a c t u a l d a t a d i s p l a y e d .

A s b e s t o s m i n e r a l s a r e f i b r o u s , and t h e p o s s i b i l i t y of a n i s o t r o p i c s i n g l e c r y s t a l e f f e c t s w e r e t h e r e f o r e i n v e s t i g a t e d . F i b r e s of n a t u r a l c r o c i d o l i t e , s a m p l e R . S. 13, w e r e l a i d p a r a l l e l t o one a n o t h e r and h e l d b e t w e e n s e l l o t a p e a t an a v e r a g e d e n s i t y of a b o u t 100 m g / c m 2 . S p e c t r a o b t a i n e d w i t h t h e f i b r e a x i s p e r p e n d i c u l a r t o t h e 7 - r a y b e a m and t h e n a t an a n g l e of 50° t o t h e b e a m s h o w e d n o o b s e r v a b l e d i f f e r e n c e . T h i s r e s u l t i s n o t s u r p r i s i n g a s t h e i r o n a t o m s a r e i n s e v e r a l s i t e s of d i f f e r i n g o r i e n t a t i o n , n o n e of w h i c h l i e a l o n g t h e f i b r e a x i s , a n d s o a n y r o t a t i o n e f f e c t w i l l b e g r e a t l y r e d u c e d .

T h e s p e c t r u m of c r o c i d o l i t e R . S. 13 ( F i g . l a ) c o m p r i s e s t w o b r o a d l i n e s of u n e q u a l i n t e n s i t y a n d h a l f - w i d t h . T h e r i g h t - h a n d p e a k i s a t t r i b u t e d t o o n e c o m p o n e n t of a q u a d r u p o l e s p e c t r u m f r o m F e 2 ^ ^ ) . P u b l i s h e d d a t a [27 , 2 8 ] i n d i c a t e t h a t c h e m i c a l s h i f t s i n Fe2+ c o m p o u n d s c o n t a i n i n g o x y g e n f a l l a p p r o x i m á t e l y in t h e r a n g e 1. 1 - 1 . 3 m m / s w i t h r e s p e c t t o m e t a l l i c i r o n .

147

VELOCITY ( m m / s re la t ive to iron)

FIG. 1. Spectra for (a) natural crocidolite; (b) crocidolite after oxidation and reduction at 425° - 475°K; (c) crocidolite after reduction at 560°K.

It i s t h u s s a f e t o c o n c l u d e t h a t t h e o t h e r c o m p o n e n t of t h e q u a d r u p o l e s p e c -t r u m c a n b e d r a w n a s t h e m i r r o r i m a g e about an a x i s a t б = 1. 2 m m / s . T h e s u b t r a c t e d c u r v e t h e n s h o w s t h e c o n t r i b u t i o n of Pe3+(t2g3e|) a s a s i n g l e b r o a d l i n e i n w h i c h any q u a d r u p o l e e f f e c t s a r e u n r e s o l v e d . T h e p e a k i n t e n s i t i e s c o m p a r e f a v o u r a b l y wi th t h e a n a l y t i c a l f i g u r e s of F e 3 + = 12. 2%, F e 2 + = 15. 9%. T h e c h e m i c a l s h i f t s , 6, and q u a d r u p o l e s p l i t t i n g s , Д, a r e :

F e 2+ б = 1. 2 0 ± 0 . 10 m m / s , Д = 2. 4 2 ± 0 . 20 m m / s

F e 3+ б = 0. ,44± 0. 10 m m / s , A = < 0 . 2 m m / s

S ince b o t h Fe 2 + and F e 3 + c a n be s e e n a s d i s c r e t e e n t i t i e s in t he M ö s s b a u e r s p e c t r u m i t f o l l o w s t h a t t h e b l u e c o l o u r of t h e m i n e r a l c a n n o t b e d u e t o a " r a p i d o s c i l l a t i o n of v a l e n c e " of t h e t y p e w h i c h f o r m a n y y e a r s w a s though t

148

VELOCITY ( m m / s re la t ive to iron)

FIG. 2. Spectra for (a) oxidized crocidolite ; (b) amosite

t o l e a d t o t h e i n t e n s e c o l o u r of P r u s s i a n b l u e and r e l a t e d c o m p o u n d s . Such a p r o c e s s w o u l d r e s u l t in an a v e r a g e d s p e c t r u m b e i n g o b s e r v e d . S ince t h e h a l f - l i f e of t h e e x c i t e d 5 7 F e n u c l e u s i s 10-7 s i t f o l l o w s t h a t any c h a r g e -t r a n s f e r p r o c e s s o c c u r s l e s s f r e q u e n t l y t h a n 107 t i m e s p e r s e c o n d f o r any g i v e n i r o n a t o m . A s i m i l a r c o n c l u s i o n h a s r e c e n t l y b e e n r e a c h e d f r o m a m o l e c u l a r o r b i t a l and M ö s s b a u e r s t u d y of P r u s s i a n b lue [29]. In t h i s c o m -pound i t w a s shown t h a t t h e o p t i c a l e l e c t r o n s w e r e 99% l o c a l i z e d on t h e f e r -r o u s s i t e s in t h e g r o u n d s t a t e and t h a t e a r l i e r a t t e m p t s t o e x p l a i n t h e c o l o u r in t e r m s of a r a p i d o s c i l l a t i o n o r r e s o n a n c e b e t w e e n s t r u c t u r e s F e i i ( C N ) 6 F e H i <—» F e i n (CN)gFei1 w e r e i n v a l i d . H o w e v e r , n e i t h e r t h e w o r k on P r u s s i a n b l u e n o r t h e p r e s e n t r e s u l t s r u l e out t h e p o s s i b i l i t y of a c h a r g e -t r a n s f e r o c c u r r i n g b e t w e e n t h e f e r r o u s a n d f e r r i c s t a t e s w h e n an i n c i d e n t pho ton i s a b s o r b e d , a s t h i s p r o c e s s o c c u r s v e r y i n f r e q u e n t l y f o r any g iven i r o n a t o m a t n o r m a l i n t e n s i t i e s of i l l u m i n a t i o n .

T h e s e o b s e r v a t i o n s m a y a l s o b e r e l e v a n t t o t h e m e c h a n i s m of ox ida t i on of c r o c i d o l i t e by m o l e c u l a r oxygen . It h a s b e e n p o s t u l a t e d [23] t h a t t he s u p -

149

ply of f e r r o u s i ó n s t o t h e s u r f a c e , f o r o x i d a t i o n , o c c u r s a s in p o s i t i v e h o l e c o n d u c t i o n ; i t i s e n v i s a g e d t h a t t h i s i n v o l v e s t r a n s f e r of an e l e c t r o n f r o m o n e f e r r o u s ion t o a n e i g h b o u r i n g f e r r i c ion a n d t h a t t h i s p r o c e s s c o n t i n u e s u n t i l t h e r e i s a f e r r o u s ion on t h e s u r f a c e . T h e e l e c t r o n - j u m p p r o c e s s p r e -s u m a b l y o c c u r s r a n d o m l y in t h e a b s e n c e of o x i d a t i o n w h i c h m e r e l y m a k e s t h e p r o c e s s d i r e c t i o n a l b y s u p p l y i n g o x y g e n a t o m s at t h e s u r f a c e t o c a p t u r e e l e c t r o n s f r o m t h e f e r r o u s - f e r r i c p r o c e s s . T h e o b s e r v a t i o n of d i s c r e t e f e r r o u s a n d f e r r i c M ö s s b a u e r r e s o n a n c e s i m p l i e s t h a t t h e e l e c t r o n - j u m p p r o c e s s o c c u r s c o n s i d e r a b l y l e s s f r e q u e n t l y t h a n 10 7 t i m e s p e r s e c o n d a t r o o m t e m p e r a t u r e . T h e s i t u a t i o n i s t h u s n o t c o m p a r a b l e t o t h a t o b t a i n i n g in m a g n e t i t e ( F e 3 0 4 ) w h e r e s u c h e l e c t r o n j u m p s b e t w e e n f e r r o u s and f e r r i c i o n s i n t h e o c t a h e d r a l s i t e s of t h e s p i n e l s t r u c t u r e r e s u l t i n a n a v e r a g e d M ö s s b a u e r s p e c t r u m r a t h e r t h a n d i s t i n c t s e t s of l i n e s f r o m F e 2 + and F e 3 +

i n d i v i d u a l l y [30] .

F i g u r e l b s h o w s t h e M ö s s b a u e r s p e c t r u m of c r o c i d o l i t e R . 5 13 a f t e r o x i d a t i o n and t h e n r e d u c t i o n in t h e t e m p e r a t u r e r a n g e 4 2 5 - 4 7 5 ° K ( e s t i m a t e d F e 3 + = 9. 3%, F e 2 + = 1 5 . 8%, F e ° = 3. 0% [31]), and F i g . 1 с s h o w s a n a l o g o u s da t a a f t e r r e d u c t i o n a t 560°K ( e s t i m a t e d F e 3 + = 6 . 2%, F e z + = 16. 9%, F e ° = 5 . 0 % [31]) . F o r a s t r a i g h t f o r w a r d r e d u c t i o n of F e 3 + t o F e 2 + , o n e w o u l d e x p e c t t h e c o n t r i b u t i o n f r o m F e 3 + t o d i s a p p e a r , a n d t h e s p e c t r u m t o b e c o m e , i n t h e l i m i t , a s y m m e t r i c a l d o u b l e t . A s c a n b e s e e n , t h e t w o c o m p o n e n t s of t h e s p e c t r u m do b e c o m e m o r e n e a r l y e q u a l b u t d e v e l o p an a s y m m e t r i c b r o a d e n i n g t o w a r d s t h e c e n t r e of t h e s p e c t r u m .

T o i n t e r p r e t t h e s e s p e c t r a in d e t a i l , i t i s n e c e s s a r y t o o b t a i n m o r e i n -f o r m a t i o n on t h e i n d i v i d u a l c o n t r i b u t i o n s of F e 3 + a n d F e 2 + in t h e s e m i n e r a l s and on t h e e f f e c t of s i t e s y m m e t r i e s on l i n e - w i d t h s . A c c o r d i n g l y , s a m p l e s of o x i d i s e d c r o c i d o l i t e R . S. 10 ( e s t i m a t e d F e 3 + = 27. 8, Fe 2 + = 1. 3%) and f r e s h a m o s i t e P . R . S. 4 . ( e s t i m a t e d F e 3 + = 0%. F e 2 + = 31 . 9%) w e r e s t u d i e d . T h e M ö s s b a u e r s p e c t r u m of o x i d i s e d c r o c i d o l i t e i s shown in F ig . 2a. Two s h a r p l y d e f i n e d l i n e s a r e o b s e r v e d wi th б = 0.45 ± 0 .10 m m / s and A = 1.02 ± 0.10 m m / s . T h e on ly i n t e r p r e t a t i o n c o n s i s t e n t with t h e s e f i g u r e s i s a q u a d r u p o l e sp l i t t ing due t o l i g a n d e f f e c t s on t h e s y m m e t r i c a l d 5 F e 3 + i on . T h e s i z e of t he sp l i t t i ng i s g r e a t e r t h a n u s u a l l y o b s e r v e d f o r t h i s i o n t h o u g h l a r g e v a l u e s h a v e p r e -v i o u s l y b e e n o b s e r v e d f o r o t h e r i r o n s i l i c a t e s [32]. T h e r e s u l t s i m p l y t h a t t h e r e i s c o n s i d e r a b l e d i s t o r t i o n of t h e o x y g e n l i g a n d s and t h a t t h e M x , M 2

a n d M 3 s i t e s a r e b a s i c a l l y s i m i l a r , l e a d i n g t o s o m e w h a t n a r r o w e r l i n e -w i d t h s . T h e d i f f e r e n c e i n p e a k h e i g h t s m a y b e a s i n g l e c r y s t a l e f f e c t . T h e q u a d r u p o l e s p l i t t i n g i s g r e a t e r t h a n f o r F e 3 + in n a t u r a l c r o c i d o l i t e . As t h e F e 3 + i o n h a s a s p h e r i c a l l y s y m m e t r i c a l e l e c t r o n d i s t r i b u t i o n t h i s i n c r e a s e d q u a d r u p o l e s p l i t t i n g i n d i c a t e s t h a t t h e l a t t i c e h a s d i s t o r t e d t o a c c o m m o d a t e . t h e i n c r e a s e d c h a r g e and p o l a r i z i n g p o w e r of t h e c a t i o n s w h i c h a r e c h i e f l y a l o n g t h e c e n t r a l a r e a of t h e c h a i n . S i n c e t h e M j , M 2 and M 3 s i t e s a r e o b -s e r v e d t o b e s o s i m i l a r , i t m a y b e t h a t t h e i n d i v i d u a l s i l i c a t e c h a i n s h a v e p a c k e d c l o s e r t o g e t h e r c a u s i n g t h e e n v i r o n m e n t r o u n d the Fe 3 + i ons to d i s t o r t in to a f l a t t e n e d o c t a h e d r a l s y m m e t r y .

A m o s i t e i s s i m i l a r t o c r o c i d o l i t e bu t h a s on ly a s m a l l c o n c e n t r a t i o n of a l k a l i c a t i o n s and no F e 3 + c a t i o n s . T h e c o n c e n t r a t i o n of F e 2 + i s 31. 9% and t h i s m e a n s t h a t F e 2 + m u s t occupy the M4 s i t e s a s we l l a s Mi , M 2 and М э . The

150

M ö s s b a u e r s p e c t r u m ( F i g . 2b) s h o w s t w o p a i r s of l i n e s w h i c h a r e s h a r p l y d e f i n e d . T h e c h e m i c a l s h i f t s a n d q u a d r u p o l e s p l i t t i n g s a r e

5 = 1. 1 3 ± 0. 10 m m / s , Д = 2. 8 2 ± 1. 10 m m / s

ô = 1. 0 6 ± 0. 10 m m / s , Д = 1. 63 ± 0. 10 m m / s

T h e s i m i l a r v a l u e s of c h e m i c a l s h i f t i n d i c a t e t h a t b o t h p a i r s of l i n e s d e r i v e f r o m F e 2 + ; t h e d i f f e r i n g v a l u e s of t h e q u a d r u p o l e s p l i t t i n g i m p l y t h a t t h e i o n s a r e in s i t e s y m m e t r i e s w h i c h a r e no t c o m p a r a b l e . T h e l e s s i n t e n s e , i n n e r p a i r s of p e a k s a r e a s s i g n e d t o t h e ( l e s s n u m e r o u s ) M4 s i t e s , a n d t h e o u t e r p e a k s c o r r e s p o n d t o t h e d i s t o r t e d o c t a h e d r a l s i t e s M j , M 2 , M3 wh ich c o n t a i n t h e g r e a t e r n u m b e r of Fe2+ c a t i o n s . T h e M 4 s i t e s a r e 8 - c o o r d i n a t e wi th f o u r n e a r and f o u r d i s t a n t n e i g h b o u r i n g o x y g e n s . T h e m a g n i t u d e of t he q u a d r u p o l e s p l i t t i n g i n h i g h - s p i n F e 2 + c o m p o u n d s d e p e n d s b o t h on t h e c o n -t r i b u t i o n of t h e s i x t h d e l e c t r o n t o t h e f i e l d g r a d i e n t at t h e n u c l e u s and on the a s y m m e t r y of t h e l i g a n d s . A p p r o x i m a t e c a l c u l a t i o n s [33] on t h e m a g n i t u d e and s i g n Of t h e s e two c o n t r i b u t i o n s i n d i c a t e t h a t t h e v a l u e of Д f o r F e 2 + c o m -p o u n d s d e c r e a s e s w i t h d i s t o r t i o n f r o m p e r f e c t o c t a h e d r a l s y m m e t r y c o n s i s -t e n t w i t h t h e above a s s i g n m e n t s .

R e t u r n i n g t o t h e s p e c t r u m of r e d u c e d c r o c i d o l i t e i t a p p e a r s t h a t t h e b r o a d e n i n g of t h e p e a k s t o w a r d s t h e c e n t r e c a n b e e x p l a i n e d in t e r m s of t h e e x t e n s i v e d i s t o r t i o n s in s i t e s y m m e t r y c a u s e d by t h e e l i m i n a t i o n of l a t t i c e o x y g e n on r e d u c t i o n . S i t e s r e d u c e d t o 5 - c o o r d i n a t i o n w i l l h a v e a s m a l l e r e l e c t r i c f i e l d g r a d i e n t a t t h e n u c l e u s and t h u s a s m a l l e r q u a d r u p o l e sp l i t t i ng , and t h e w i d e v a r i a t i o n in i n d i v i d u a l s i t e s y m m e t r i e s i n e v i t a b l y b r o a d e n s t h e r e s o n a n c e l i n e s . A d s o r p t i o n due to t h e r e s i d u a l Fe 3 + m a y a l s o be b r o a d e n e d , and cou ld accoun t f o r t h e s l i gh t o b s e r v e d a s y m m e t r y .

If new s p e c i e s of i r o n s u c h a s F e + o r Fe° w e r e f o r m e d d u r i n g r e d u c t i o n t h e s e would b e l o c a t e d a t o r n e a r a po in t of l a t t i c e b r e a k d o w n and would give on ly b r o a d , w e a k , u n r e s o l v e d l i n e s . T h e e x i s t e n c e of s u c h s p e c i e s c a n n o t t h e r e f o r e e a s i l y b e e s t a b l i s h e d b y M ö s s b a u e r s p e c t r o s c o p y . T h e r e d u c e d m i n e r a l i s e f f e c t i v e l y in an i n t e r m e d i a t e s t a t e of c h e m i c a l d e c o m p o s i t i o n , bu t a g g r e g a t i o n of n e w p h a s e s i s p r e v e n t e d by t h e s i l i c a t e l a t t i c e . T h e r e i s no e v i d e n c e t o c o n f i r m t h a t t h e o n l y o x y g e n a t o m s r e m o v e d a r e t h o s e c o -o r d i n a t e d t o t h e M 2 s i t e s w h e r e A d d i s o n [26] h a s p o s t u l a t e d a c h a n g e f r o m F e 3 + t o F e ° . A m o r e q u a n t i t a t i v e e v a l u a t i o n of t h e r e s u l t s d o e s n o t a p p e a r f e a s i b l e a t t h e p r e s e n t s t a g e .

In s u m m a r y : M ö s s b a u e r s p e c t r o s c o p y h a s s h o w n t h a t t h e b l u e c o l o u r of c r o c i d o l i t e i s n o t d u e t o a r a p i d o s c i l l a t i o n of v a l e n c e b e t w e e n Fe2+ a n d F e 3 + . C h a n g e s in q u a d r u p o l e s p l i t t i n g w h i c h a r e o b s e r v e d a f t e r c h e m i c a l t r e a t m e n t a r e i n t e r p r e t e d in t e r m s of c h a n g e s in s i t e s y m m e t r i e s of t h e i r o n a t o m s . N o e v i d e n c e w a s f o u n d f o r o x i d a t i o n s t a t e s of i r o n b e l o w + 2 in r e -d u c e d s p e c i m e n s .

C. B O N D I N G IN H I G H - S P I N , T E T R A H E D R A L F e C l ^ "

T h e M ö s s b a u e r s p e c t r u m of i r o n ( I I ) in a d i s c r e t e t e t r a h e d r a l e n v i r o n -m e n t h a s no t p r e v i o u s l y b e e n r e p o r t e d . P u b l i s h e d a c c o u n t s of the M ö s s b a u e r

151

e f f e c t i n h i g h - s p i n i r o n ( I I ) c o m p o u n d s h a v e d e a l t a l m o s t e x c l u s i v e l y w i t h an o c t a h e d r . a l e n v i r o n m e n t of s i x l i g a n d s b o n d i n g t h r o u g h oxygen [27, 28, 3 4 ] , a l t h o u g h Fe2+ h a d b e e n o b s e r v e d in t e t r a h e d r a l s i t e s i n s p i n e l s a n d o t h e r o x i d e s [35]. T h i s s e c t i o n d e s c r i b e s m e a s u r e m e n t s on a c o m p o u n d c o n t a i n i n g t h e t e t r a h e d r a l i o n F e C l | " ; t h e s e a r e of a d d i t i o n a l i n t e r e s t b e c a u s e of t h e a n o m a l o u s m a g n e t i c m o m e n t of t h i s i o n [36] .

S a m p l e s of v a r i o u s t e t r a h a l o g e n o f e r r a t e s w e r e k i n d l y s u p p l i e d b y P r o f e s s o r R . S. N y h o l m a n d D r . B . T a y l o r of U n i v e r s i t y C o l l e g e , L o n d o n . P o w d e r e d s a m p l e s w e r e p r e s s e d b e t w e e n a l u m i n i u m f o i l in a c o p p e r m o u n t a n d c o o l e d t o v a r i o u s t e m p e r a t u r e s i n a g l a s s c r y o s t a t i n s u l a t e d w i t h f o a m p l a s t i c . S p e c t r a f o r t e t r a e t h y l a m m o n i u m t e t r a c h l o r o f e r r a t e ( I I ) a t 80°K and 190°K a r e s h o w n in F i g . 3. S p e c t r a c o u l d n o t b e o b t a i n e d a t r o o m t e m p e r a -t u r e b e c a u s e of t h e l o w r e c o i l - f r e e f r a c t i o n . N o r cou ld t h e M ö s s b a u e r e f f e c t b e o b s e r v e d in t e t r a e t h y l a m m o n i u m t e t r a b r o m o f e r r a t e ( I I ) o r c a e s i u m t e t r a -c h l o r o f e r r a t e ( I I ) e v e n a t 80°K b e c a u s e of t h e l a r g e r m a s s a b s o r p t i o n c o e f ^ f i c i e n t a s s o c i a t e d w i t h t h e h e a v i e r e l e m e n t s i n t h e s e c o m p o u n d s .

95

95

- 3 - 2 - 1 0 1 2 3 i 5 6

VELOCITY ( m m / s r e l a t i v e to i r o n )

FIG. 3. Spectra for te t raethylammonium tetrachloroferrate(II) at 80" and 190°K

T h e c h e m i c a l s h i f t s , 6, r e l a t i v e t o n a t u r a l i r o n , t h e q u a d r u p o l e s p l i t -t i n g s , Д , a n d t h e 7 - r a y r e s o n a n c e l i n e - w i d t h s a t h a l f h e i g h t , Г, f o r t e t r a -e t h y l a m m o n i u m t e t r a c h l o r o f e r r a t e ( I I ) a r e g i v e n in T a b l e II ( ± 0 . 10 m m / s ) .

T h e s e s p e c t r a s h o w t h a t , a t t h e t e m p e r a t u r e s u s e d , t h e c o m p o u n d g i v e s a s y m m e t r i c a l d o u b l e t t y p i c a l of h i g h - s p i n F e 2 + , e | t | g . T h e r e i s n o i n d i -c a t i o n of a s i x - l i n e s p e c t r u m d u e t o a h y p e r f i n e m a g n e t i c f i e l d r e s u l t i n g

152

TABLE II (cont. )

M Ö S S B A U E R P A R A M E T E R S F O R ( E t 4 N ) 2 F e C Í 4

T e m p . CK)

5 m m / s Д mm/s Г m m / s

80

190

1 . 0 0

0 . 9 3

2 . 4 9

1. 63

0.79-

0 . 7 5

f r o m f e r r o - o r a n t i f e r r o - m a g n e t i c i n t e r a c t i o n s . T h i s i s c o n s i s t e n t w i t h t he known t e m p e r a t u r e d e p e n d e n c e of t he m a g n e t i c s u s c e p t i b i l i t y of t h i s c o m -pound down t o 95°K [36]. T h e q u a d r u p o l e s p l i t t i n g i s c a u s e d by the odd e l e c -t r o n o u t s i d e t h e h a l f - f i l l e d d s h e l l in t h e l o w e r eg(dy) l e v e l s . И t h e s e l e v e l s w e r e i n c o m p l e t e l y d e g e n e r a t e g r o u p s i t c a n b e s h o w n , i n an a n a l o g o u s way t o t h e o c t a h e d r a l c a s e [33], t h a t t h e r e would b e no q u a d r u p o l e s p l i t t i n g . T h e t e m p e r a t u r e d e p e n d e n c e of t h e q u a d r u p o l e s p l i t t i n g r e s u l t s f r o m a s m a l l s p l i t t i n g of t h e dy e n e r g y l e v e l s . A f u l l i n t e r p r e t a t i o n of t h i s d e p e n d e n c e w o u l d f o l l o w t h e a p p r o a c h of I n g a l l s [37] and w o u l d i n c l u d e t h e dy s p l i t t i n g , Д 1 , t h e e f f e c t of c o v a l e n c y on t h e r a d i a l d e p e n d e n c e f u n c t i o n of t h e s i x t h d e l e c t r o n , and t h e s p i n - o r b i t c o u p l i n g e f f e c t . T h i s i s i m p r a c t i c a b l e w i t h t h e d a t a o b t a i n a b l e f r o m t h e p r e s e n t s y s t e m . H o w e v e r , a s i m p l e t r e a t m e n t of t h e t e m p e r a t u r e d e p e n d e n c e , n e g l e c t i n g c o v a l e n c y and s p i n - o r b i t coupl ing, h a s b e e n g iven f o r t h e o c t a h e d r a l c a s e [33]. R e c a l c u l a t i n g t h i s f o r a t e t r a -h e d r a l e n v i r o n m e n t g i v e s t h e f o l l o w i n g r e l a t i o n b e t w e e n t h e q u a d r u p o l e s p l i t t i n g , Д, a n d t h e t e m p e r a t u r e T :

1 . е-Д1ДкТ) A - л 0 ! + e - V ( k T )

w h e r e Д 0 i s t h e l o w - t e m p e r a t u r e l i m i t of Д a s T a p p r o a c h e s a b s o l u t e z e r o , Д ^ э t h e e n e r g y d i f f e r e n c e b e t w e e n t h e two dy l e v e l s , and к i s B o l t z m a n n 1 s c o n s t a n t . At h i g h t e m p e r a t u r e s Д a p p r o a c h e s z e r o , i . e . t h e t w o dy l e v e l s h a v e e q u a l e l e c t r o n p o p u l a t i o n when k T » Aj . T h e two v a l u e s of t he q u a d r u -p o l e s p l i t t i n g in t h e T a b l e l e a d t o a v a l u e of ~ 2 . 7 m m / s f o r До a n d an a p -p r o x i m a t e v a l u e of 1 8 5 ± 5 0 c m " 1 f o r t h e d y e n e r g y d i f f e r e n c e Д1# T h i s i s s m a l l e r t h a n t h e r e s o l u t i o n of n o r m a l u l t r a v i o l e t and v i s i b l e s p e c t r o m e t e r s , b u t m i g h t b e o b s e r v a b l e d i r e c t l y in t h e i n f r a r e d and low t e m p e r a t u r e s . It i s un l i ke ly t h a t t he i m p l i e d m i n u t e d i s t o r t i o n of t he f o u r c h l o r i n e a t o m s f r o m p e r f e c t t e t r a h e d r a l s y m m e t r y about t he i r o n a tom could be d e t e c t e d by X - r a y d i f f r a c t i o n t e c h n i q u e s . T h e r e s u l t s c o n s t i t u t e t h e f i r s t e v i d e n c e f o r s u c h a d i s t o r t i o n i n an i o n of t h i s t y p e .

T h e c h e m i c a l s h i f t s in t h e T a b l e a r e s m a l l e r t h a n t h o s e n o r m a l l y o b -s e r v e d f o r h i g h - s p i n i r o n ( I I ) b o n d e d to o x y g e n [27, 28, 34]; t h i s i n d i c a t e s t h a t r e p l a c e m e n t of s i x b r i d g i n g ox ide l i g a n d s by f o u r (non -b r idg ing ) c h l o r i d e l i g a n d s c a u s e s an o v e r a l l i n c r e a s e in t h e s - e l e c t r o n d e n s i t y a n d t h e i r o n

153

n u c l e u s . T h i s , i n t u r n , i m p l i e s an i n c r e a s e i n c o v a l e n c y , t h a t i s , an i n -c r e a s e d i n v o l v e m e n t of t h e i r o n 4 s and 3d o r b i t a l s in t h e b o n d i n g . T h e p r e -c i s e m a g n i t u d e of t h e c h e m i c a l s h i f t s of F e C l ^ " c o m p a r e d w i t h F e 2 + i n a n o x i d e l a t t i c e c a n n o t b e c a l c u l a t e d q u a n t i t a t i v e l y , bu t q u a l i t a t i v e l y i t c a n . b e s e e n t o d e p e n d on t h e i n t e r p l a y of s e v e r a l f a c t o r s . P r e d o m i n a n t a m o n g s t t h e s e i s t h e i n c r e a s e i n 4 s e l e c t r o n d e n s i t y a t t h e i r o n n u c l e u s a s a r e s u l t of t h e i n c r e a s e d c o v a l e n c y . T h i s i s c o u n t e r a c t e d s l i g h t l y b y t h e i n c r e a s e d s h i e l d i n g of t h e n u c l e u s w h i c h a r i s e s f r o m t h e i n c r e a s e d 3d o c c u p a t i o n . A f u r t h e r a u x i l l i a r y e f f e c t i s t h e e x p a n s i o n of t h e o u t e r o r b i t a l s of t h e i r o n a t o m by t h e n e p h e l a u x e t i c e f f e c t wh ich d i m i n i s h e s t h e 3d s h i e l d i n g t h u s aga in i n c r e a s i n g t h e t o t a l s - e l e c t r o n d e n s i t y a n d r e d u c i n g t h e c h e m i c a l s h i f t .

In s u m m a r y , t h i s s e c t i o n r e p o r t s t h e f i r s t M ö s s b a u e r s p e c t r u m of a d i s c r e t e , t e t r a h e d r a l , h i g h - s p i n i r o n ( I I ) c o m p l e x . T h e m a g n i t u d e of t h e c h e m i c a l s h i f t in F e C l | " i s c o n s i s t e n t w i t h an i n c r e a s e in c o v a l e n c y c o m -p a r e d w i t h F e 2+ in o x i d e l a t t i c e s , a n d t h e e x i s t e n c e of a t e m p e r a t u r e -d e p e n d e n t q u a d r u p o l e s p l i t i s c o n s i s t e n t w i th a d i s t o r t i o n wh ich l i f t s t h e d e -g e n e r a c y of t he l o w e r e g l e v e l s by about 185 c m - i . T h i s i s t h e f i r s t r e p o r t e d e v i d e n c e f o r t h e d i s t o r t i o n e x p e c t e d in a t e t r a h e d r a l d 1 o r h i g h - s p i n d 6 c o m -p l e x . T h e r e i s no i n d i c a t i o n of f e r r o r o r a n t i f e r r o - m a g n e t i c e x c h a n g e down t o 80°K; t h e e n h a n c e d m a g n e t i c m o m e n t a b o v e t h e s p i n - o n l y v a l u e i s t h u s s t i l l n o t s a t i s f a c t o r i l y e x p l a i n e d .

R E F E R E N C E S

[1] COOPER, GIBB, GREENWOOD, PARISH, Trans Faraday Soc. 60 (1964) 2097. [2] ARONSSON, Ark. Kemi 16 (1960) 379. [3] KIESSLING, Acta chem. scand. 4 (1950) 209. [4] LUNDQUIST, Ark. Fys. 23 (1962) 65. [5] PAULING, Proc. R. Soc. A 196 (1949) 343. [6] PAULING, KIESSLING, J. e lectrochem. Soc. 98 (1951) 518. [7] KIESSLING, J. e lectrochem. Soc. 98 (1951) 166. [8] HÄGG, KIESSLING, J. Inst. Metals 81 (1952-3) 57. [9] KIESSLING, Forschr. chem. Forsch. 3 (1953) 41.

[10] JOHNSON, DA A NE, J. chem. Phys. 38 (1963) 425. [11] SILVER, KOSHIDA, J. chem. Phys. 38 (1963) 865. [12] LUNDQUIST, MEYERS, Ark. Fys. 20 (1961) 463. [13] LUNDQUIST, MEYERS, WESTIN, Phil. Mag. 7 (1962) 1187. [14] WEISS, FORRER, Annls Phys. 12 (1929) 279. [15] FRUCHART, Compt. rend. 256 (1963) 3304. [16] WERTHEIM, HERBER, J. chem. Phys. 38 (1963) 2106. [17] STEARNS, Phys. Rev. 129 (1963) 1136. [18] STEARNS, J. appl. Phys. 35 (1964) 1095. [19] SHIRANE, TAKEI, RUBY, Phys. Rev. 126 (1962) 49. [20] WELLS, Structural Inorganic Chemistry, 3rd ed . , Oxford (1962). [21] WITTAKER, Acta crystallogr. 2 (1944) 312. [22] GIBB, GREENWOOD, Trans Faraday Soc. 61 (1965) in press. [23] ADDISON, ADDISON, NEAL, SHARP, J. chem. Soc. (1962) 1468. [24] ADDISON, NEAL. SHARP, J. chem. Soc. (1962) 1472. [25] SHARP, Ph. D. Thesis, Nottingham (1963). [26] SHARP, ADDISON, J. chem. Sot. (1962) 3693. [27] GOLDANSKII, The Mössbauer Effect and the Application in Chemistry, New York, Consultants Bureau

(1964).

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[28] FLUCK, Advances in Inorganic C h e m . and R a d i o c h e m . (Emeleus, H. J . , Sharpe, A.C., Eds.) 6 (1964) 433.

[29] ROBIN, Inorg. C h e m . 1 (1962) 337. [30] BAUM INGER, COHEN, MARINOV, OFER, SEGAL, Phys. Rev. 122 (1961) 1447. [31] ADDISON, p r iva te c o m m u n i c a t i o n . [32] De COSTER, POLLAK, AMELINCKX, Phys. S ta t . Sol. 3 (1963) 283. [33] INGALLS, T e c h . Rpt No. 2 . Ca rneg ie Inst. T e c h . (1962). [34] BRADY, WIGLEY, DUNCAN, Rev. Pure appl . C h e m . 12 (1962) 165. [35] MIZOGUCHI, TANAKA, J. phys. Soc. Japan 18 (1963) 1301. [36] GILL, J. c h e m . Soc. (1961) 3512. [37] INGALLS, Phys. Rev. 133 (1964) A787.

D I S C U S S I O N

J . DANON p o i n t e d out t h a t t h e r e w a s a l s o a s m a l l q u a d r u p o l e s p l i t t i n g in Fe l i l CI4 w h i c h a g a i n p r o b a b l y i n d i c a t e d a s m a l l d i s t o r t i o n f r o m r e g u l a r t e t r a h e d r a l (Danon , J . , R e v . m o d . P h y s . 36 (1964) 459) .

P . H I L L M A N m e n t i o n e d t h a t t h e q u a d r u p o l e s p l i t t i n g w a s of t h e s a m e s i z e and had the s a m e t e m p e r a t u r e d e p e n d e n c e a s t h a t in F e O . No d i s t o r t i o n of t h e c u b i c s t r u c t u r e w a s s e e n i n X - r a y m e a s u r e m e n t s b u t t h e c o m p o u n d w a s n o n - s t o i c h i o m e t r i c .

N . N . G R E E N W O O D did n o t t h i n k t h e d i s t o r t i o n s w e r e l a r g e e n o u g h t o b e s e e n i n X - r a y m e a s u r e m e n t s .

R . M . GOLDING s u g g e s t e d t h a t t h e r e m i g h t be a d y n a m i c J a h n - T e l l e r e f f ec t , in w h i c h c a s e t h e X - r a y d a t a wou ld i n d i c a t e a r e g u l a r s t r u c t u r e bu t t h e M ö s s b a u e r d a t a wou ld n o t .

G. K. W E R T H E I M a s k e d a t w h a t t e m p e r a t u r e t h e m e a s u r e m e n t s s h o w n in T a b l e I w e r e m a d e and w h a t w a s t h e C u r i e t e m p e r a t u r e .

N . N . G R E E N W O O D s a i d t h e y w e r e m a d e a t r o o m t e m p e r a t u r e bu t t h e C u r i e t e m p e r a t u r e w a s n o t k n o w n .

G. K. W E R T H E I M s a i d h e w a s d o u b t f u l abou t t h e i n t e r p r e t a t i o n f o r t h e i r o n b o r i d e s s i n c e an a l t e r n a t i v e i n t e r p r e t a t i o n c o u l d b e t h a t t h e d e c r e a s e of t he h y p e r f i n e f i e l d w a s s i m p l y due t o a d e c r e a s e in t he i n t e r a c t i o n be tween ' t h e i r o n a t o m s a s t h e b o r o n con t en t w a s e n r i c h e d .

N . N . GREENWOOD thought ' the change in d i s t a n c e be tween the i r on a t o m s w a s no t v e r y g r e a t . In i r o n i t s e l f t h e r e w e r e 8 n e a r e s t n e i g h b o u r s at 2.48 Â; in F e 2 B e a c h i r o n h a d 11 n e a r e s t n e i g h b o u r s at 2. 41 Â and in F e B t h e r e w e r e 6 i r o n n e a r e s t n e i g h b o u r s at 2. 6 - 2 . 7 Â and 4 at about 2. 9 Â.

G. K. W E R T H E I M r e f e r r e d t o s e c t i o n В and a s k e d how m a n y e l e c t r o n s h a d t o j u m p t o g ive t h e b l u e c o l o u r .

N . N . G R E E N W O O D a n s w e r e d i t d e p e n d e d on t h e i n t e n s i t y . V. I. GOLDANSKII r e f e r r e d to a s u g g e s t i o n of I. B. B e r s u k e r to s tudy the"

e f f e c t t h a t a L a s e r b e a m i r r a d i a t i o n w o u l d h a v e on t h e M ö s s b a u e r s p e c t r a a s a r e s u l t of t h e f o r m a t i o n of o p t i c a l l y e x c i t e d s t a t e s .

E . F L U C K a s k e d how good w e r e t h e f i g u r e s f o r t h e p e r c e n t a g e s of F e 2 +

and Fe3+. N . N . GREENWOOD s a i d n u m b e r s w e r e f r o m Addison (Re f s . 123, 26, 31])

and t h a t q u a n t i t a t i v e r e s u l t s f r o m M ö s s b a u e r s p e c t r a w e r e good wi th in about ±10%.

155

CHEMICAL ASPECTS OF ISOMER SHIFTS AND QUADRUPOLE SPLITTINGS IN TIN COMPOUNDS

M. CORDEY-HAYES BIRMINGHAM UNIVERSITY

UNITED KINGDOM

T h i s p a p e r b r i e f l y o u t l i n e s t h e e s s e n t i a l f e a t u r e s of t h e i s o m e r s h i f t s a n d q u a d r u p o l e s p l i t t i n g s of t h e 2 3 . 8 - k e V l e v e l of 1 1 9 Sn i n t i n c o m p o u n d s . A s u m m a r y Of t h e d a t a f o r t h e i n o r g a n i c t i n c o m p o u n d s i s g i v e n i n T a b l e I . T h e r e s u l t s w e r e t a k e n f r o m t h e p u b l i c a t i o n s of B o y l e e t a l . [1], B r y u k h a n o v e t a l . [2] a n d C o r d e y - H a y e s 13]. A s h o r t d i s c u s s i o n of t h e r e s u l t s w i l l p r o -v i d e a f r a m e w o r k f o r t h e m o r e r e c e n t m e a s u r e m e n t s o n o r g a n i c t i n c o m p o u n d s .

I S O M E R S H I F T S

T h e i s o m e r s h i f t s a r e a l l w r i t t e n r e l a t i v e t o a g r e y t i n a b s o r b e r a n d t h e p o s i t i v e s i g n i n d i c a t e s a s h i f t t o h i g h e r e n e r g i e s . T h e r e s u l t s f a l l i n t o t w o d i s t i n c t g r o u p s : t h e c o m p o u n d s w i t h a p o s i t i v e s h i f t a r e t h e s t a n n o u s c o m p o u n d s a n d t h o s e i n t h e n e g a t i v e g r o u p a r e a l l s t a n n i c c o m p o u n d s . A n e u t r a l t i n a t o m h a s f o u r t e e n e l e c t r o n s o u t s i d e t h e k r y p t o n c o r e , t h e i r c o n -f i g u r a t i o n b e i n g 4 d 1 0 5 s 2 5 p 2 . C o m p l e t e l y i o n i c s t a n n i c c o m p o u n d s h a v e t h e c o n f i g u r a t i o n 4 d 1 0 a n d t h e i o n i c s t a n n o u s c o m p o u n d s h a v e t h e c o n f i g u r a t i o n 4 d 1 0 5 s 2 . T h e c o v a l e n t c o m p o u n d s h a v e an i n t e r m e d i a t e n u m b e r of 5 s e l e c -t r o n s d e p e n d i n g o n t h e n a t u r e of t h e h y b r i d i z a t i o n a n d o n t h e p a r t i a l i o n i c c h a r a c t e r of t h e b o n d s i n v o l v e d . F o r e x a m p l e , g r e y t i n h a s a d i a m o n d s t r u c -t u r e a n d i s c o m p l e t e l y q u a d r i c o v a l e n t w i t h t h e h y b r i d i z e d c o n f i g u r a t i o n 5 s 5 p 3 . T h e r e f o r e , g r e y t i n h a s i n e f f e c t o n e 5 s e l e c t r o n a s s o c i a t e d w i t h e a c h a t o m . B e c a u s e t h e i s o m e r s h i f t i s s e n s i t i v e t o c h a n g e s in t h e s - e l e c t r o n d e n s i t y i t i s c l a s s i f i e d a c c o r d i n g t o t h e o x i d a t i o n s t a t e . M o r e o v e r , t h e s h i f t f r o m g r e y t i n t o S n F 4 c o r r e s p o n d s t o a d e c r e a s e in t h e 5 s - e l e c t r o n d e n s i t y a n d r e p r e s e n t s a n i n c r e a s e i n t h e i o n i c c h a r a c t e r of t h e c h e m i c a l b o n d s . T h e i s o m e r s h i f t r e s u l t s f o r t h e q u a d r i v a l e n t c o m p o u n d s c l e a r l y s h o w i n -c r e a s i n g i o n i c c h a r a c t e r w i t h i n c r e a s i n g e l e c t r o n e g a t i v i t y d i f f e r e n c e (F ig . 1). F r o m t h i s c o r r e l a t i o n s e v e r a l e s t i m a t e s of t h e p a r t i a l i o n i c c h a r a c t e r of t h e S n - X b o n d s h a v e b e e n m a d e . P r o b a b l y t h e b e s t v a l u e s a r e t h o s e o b -t a i n e d [4] b y c a l i b r a t i n g t h e i s o m e r s h i f t s c a l e w i t h v a l u e s of i on i c c h a r a c t e r d e t e r m i n e d b y i n d e p e n d e n t m e t h o d s .

F o r t h e d i v a l e n t c o m p o u n d s t h e r e i s n o c l e a r c o r r e l a t i o n b e t w e e n t h e s h i f t d a t a a n d a s i m p l e e l e c t r o n e g a t i v i t y s c a l e . T h e r a n g e of e l e c t r o n e g a -t i v i t i e s t h a t h a v e t o b e a s s o c i a t e d w i t h t h e d i f f e r e n t s t a t e s of h y b r i d i z a t i o n m a y o b s c u r e a n y c o r r e l a t i o n . A l s o , c o r r e c t i o n s f o r i m p o r t a n t s c r e e n i n g e f f e c t s h a v e t o b e m a d e . F o r e x a m p l e , a 5 s - v a l e n c y e l e c t r o n v i a t h e p e n e -t r a t i o n of t h e c l o s e d s h e l l s p a r t i a l l y s h i e l d s t h e i n n e r s e l e c t r o n s f r o m t h e n u c l e a r c h a r g e ; s o t h a t , on t h e r e m o v a l of t h e v a l e n c y e l e c -t r o n t h e b i n d i n g e n e r g y of t h e i n n e r e l e c t r o n s i s i n c r e a s e d and t h e s e s h e l l s

1 5 6

TABLE II (cont. )

I S O M E R S H I F T A N D Q U A D R U P O L E S P L I T T I N G A T 8 0 ° K

T h e i s o m e r s h i f t s a r e r e l a t i v e t o t h e g r e y t i n a b s o r b e r . A l l v a l u e s a r e i n m m s e c - i w i t h a n a c c u r a c y of 0 . 1 m m s e c - 1

Compound borner

shift Quadrupole

, , , Electron configuration splitting 6

4d1 05s2

SnCl j + 2 . 4

SnBr2 + 2 . 1

SnS0 4 + 1 . 8 1 . 0

SnC204 +1.7 1 . 4

SnCSlj. 2H.O +1 .6 1 . 0

SnF2 + 1 . 6 1 . 7

SnS + 1 . 4 1 . 0

S n3 ( p °4 ) s + 1 . 1 1 . 6

SnO +0.7 1 . 5

Sn (white) +0 .6

Sn (grey) 0 . 0 4d1°5s5p s

Snl 4 - 0 . 3

SnS2 - 0 . 8 .

SnBr4 - 1 . 0

SnCl4 - 1 . 3

(NH4)2SnCl6 - 1 . 9

SnCl4. 5 ^ 0 - 1 . 9

Sn(S04)2 - 2 . 0

Sn02 - 2 . 2

SnF4 - 2 . 5 1 - 8 4d 1 0

m o v e t o w a r d s t h e n u c l e u s , t h u s i n c r e a s i n g t h e i r c o n t r i b u t i o n t o фЩ,0). If 6E 5 s i s t h e t h e o r e t i c a l s h i f t f o r t h e r e m o v a l of o n e 5s e l e c t r o n t h e n t h e o b -s e r v e d s h i f t 6EOBS = (1 - <Z)ÔE5s , w h e r e a i s t h e s c r e e n i n g c o r r e c t i o n . B y u s i n g t h e t h e o r y of C r a w f o r d a n d S c h a w l o w [5] and H u m b a c h [6], d e v e l o p e d f o r t h e i s o t o p e s h i f t of o p t i c a l s p e c t r a , B o y l e e t a l . [1 ] a n d C o r d e y - H a y e s [3] h a v e t a k e n t h e s c r e e n i n g c o r r e c t i o n a t o b e 0. 2 . A c o r r e c t i o n of 0 . 1 w a s a l s o m a d e f o r t h e s h i e l d i n g of t h e 5 s e l e c t r o n s b y t h e 5p e l e c t r o n s . M o r e r e c e n t l y G o l d a n s k i i a n d M a k a r o v [7] h a v e g i v e n t h e r e s u l t of c a l c u l a t i o n s m a d e t a k i n g i n t o a c c o u n t n o t o n l y t h e v a r i a t i o n s i n t h e i n n e r s s h e l l s b u t

157

t 2

ELECTRONEGATIVITY DIFFERENCE

FIG. 1. The isomer shifts (relative to grey tin) and the corresponding ionic character of the Sn-X bonds for the quadrivalent inorganic compounds

a l s o t h e c o m p r e s s i o n of t h e v a l e n c e s h e l l d u e t o i o n i z a t i o n . T h e i r r e s u l t i n d i c a t e s t h a t a > 1 ; t h u s t h e ne t e f f e c t of l o s i n g a 5s e l e c t r o n i s to i n c r e a s e фЩ,0) t h e t o t a l s - e l e c t r o n d e n s i t y a t t h e n u c l e u s . A l t h o u g h t h i s r e s u l t r e -q u i r e s t h e c h a n g e in s i g n of 6 R / R i t d o e s n o t a l t e r t h e i s o m e r s h i f t i o n i c c h a r a c t e r e s t i m a t e s f o r t h e q u a d r i v a l e n t c o m p o u n d s s i n c e t h e y w e r e b a s e d on ly on a p r o p o r t i o n a l i t y c o n c e p t . H o w e v e r , a m o n g t h e p r o b l e m s t h a t could b e e x p l a i n e d b y t h e r e s u l t i s t h e s c a t t e r in t h e s h i f t s f o r t h e d i v a l e n t c o m -p o u n d s . G o l d a n s k i i h a s i n d i c a t e d t h a t m o r e d e t a i l e d c a l c u l a t i o n s a r e in p r o -g r e s s . An e x p e r i m e n t a l t e s t and t h e o r e t i c a l c a l c u l a t i o n of t h e m a g n i t u d e of t h e s c r e e n i n g c o r r e c t i o n i s a l s o in p r o g r e s s in B i r m i n g h a m . V e r y r e c e n t l y S u m b a y e v a n d M e z e n t s e v [8] h a v e p u b l i s h e d d a t a on t h e c h e m i c a l s h i f t s of t h e К s e r i e s X - r a y l i n e s i n t i n m e t a l a n d t i n o x i d e . T h e i r e x p e r i m e n t a l d a t a e n a b l e a f u r t h e r c o m p a r i s o n to be m a d e wi th t he t h e o r e t i c a l ca l cu l a t i ons . Bu t u n t i l s u c h c o m p a r i s o n s b e t w e e n t h e o r y and e x p e r i m e n t h a v e b e e n m a d e t h e i s o m e r s h i f t s f o r t h e d i v a l e n t c o m p o u n d s r e m a i n d i f f i c u l t t o a n a l y s e .

Q U A D R U P O L E S P L I T T I N G

E x t r a n u c l e a r f i e l d s c a n s p l i t t h e e m i s s i o n o r a b s o r p t i o n l ine into a n u m -b e r of c o m p o n e n t s . In d i a m a g n e t i c t i n t h e r e i s no m a g n e t i c i n t e r a c t i o n bu t t h e e l e c t r i c q u a d r u p o l e m o m e n t p r o d u c e d by t h e a s y m m e t r i c n u c l e a r c h a r g e

158

d i s t r i b u t i o n c a n i n t e r a c t w i th e n v i r o n m e n t a l e l e c t r i c f i e l d g r a d i e n t s g iv ing r i s e t o a s p l i t t i n g of t h e e n e r g y l e v e l s . F o r ax ia l ly s y m m e t r i c e l e c t r i c f ie ld g r a d i e n t s t h e q u a d r u p o l e h y p e r f i n e s p l i t t i n g of a n u c l e a r l e v e l [9] i s :

A E ™ i = № T ) [ 3 m i 2 " I ( I + 1 ) ]

w h e r e e q I s t h e e l e c t r i c f i e l d g r a d i e n t at t h e n u c l e a r s i t e and eQ i s t h e n u c -l e a r q u a d r u p o l e m o m e n t . I i s t h e t o t a l sp in and m ¡ r e f e r s t o t he o r i e n t a t i o n s u b s t a t e s , t h e r e b e i n g d e g e n e r a c y in ± m j . F o r t h e g r o u n d s t a t e of 1 1 9 Sn ( s p i n I = i ) t h e r e i s n o s p l i t t i n g b u t a n o n - z e r o v a l u e of Д Е o c c u r s f o r t h e e x c i t e d s t a t e (1-3/2), in w h i c h c a s e t h e s p l i t t i n g of t h e t w o s u b s t a t e s i s e 2 q Q / 2 .

T h e q u a d r u p o l e s p l i t t i n g d a t a f o r t h e i n o r g a n i c t in c o m p o u n d s a r e given i n c o l u m n 2 of T a b l e I . T h e d a t a c a n b e i n t e r p r e t e d b y u s i n g t h e s i m p l e r e l a t i o n s h i p s b e t w e e n e l e c t r i c f i e l d g r a d i e n t s a n d a t o m i c w a v e f u n c t i o n s f i r s t p r o p o s e d b y T o w n e s a n d D a i l e y 110]. U n d i s t u r b e d c l o s e d s h e l l s and s u b s h e l l s a r e s p h e r i c a l l y s y m m e t r i c and t h u s do no t c o n t r i b u t e t o t h e q u a -d r u p o l e i n t e r a c t i o n . T h e c o n t r i b u t i o n f r o m s e l e c t r o n s ou t s ide c l o s e d s h e l l s i s s i m i l a r l y z e r o . It i s only t h e s p h e r i c a l l y a s y m m e t r i c c h a r g e d i s t r i bu t i on of v a l e n c e p e l e c t r o n s w h i c h p r o d u c e l a r g e f i e l d g r a d i e n t s a t t h e n u c l e u s . F u r t h e r m o r e , w h e n a l l t h r e e p o r b i t a l s a r e e q u a l l y p o p u l a t e d t h e i r s u p e r -p o s i t i o n f o r m s a s p h e r i c a l l y s y m m e t r i c c h a r g e ^d i s t r ibu t ion . T h u s , q u a d r u -p o l e s p l i t t i n g in c o v a l e n t c o m p o u n d s i s a s s o c i a t e d w i t h u n b a l a n c e d 5p-e l e c t r o n d e n s i t y . T h e f i e l d g r a d i e n t f o r o n e u n b a l a n c e d 5p e l e c t r o n in t i n = - 4 / 5 е(1/гзУ= - 3 . 0 X 1 0 1 6 e . s . u . In ion ic c r y s t a l s t he long r a n g e f ie ld g r a d i e n t s a r e t o o s m a l l t o p r o d u c e m e a s u r a b l e s p l i t t i n g s . A s i n g l e e l e c -t r o n i c c h a r g e on a n e i g h b o u r i n g ion a t a t y p i c a l d i s t a n c e of 2Â f r o m the r e -s o n a n t n u c l e u s p r o d u c e s a f i e l d g r a d i e n t at t h e n u c l e u s of -10 1 4 (1 - y) e . s . u . , w h e r e y i s t h e S t e r n h e i m e r a n t i s h i e l d i n g f a c t o r wh ich i s e s t i m a t e d 111] t o be a p p r o x . - 1 0 f o r Sn4+.

T h u s , i n g e n e r a l , s p l i t t i n g h a s b e e n o b s e r v e d on ly in t h e c o v a l e n t c o m -p o u n d s . In t h e d i c o v a l e n t c o m p o u n d s t h e s p l i t t i n g i s c o n s i s t e n t wi th e i t h e r of t h e g e n e r a l l y p r o p o s e d t in(II ) s t r u c t u r e s , n a m e l y t h e n o r m a l s 2 p 2 v a l e n c e s t a t e o r a h y b r i d i z e d s t r u c t u r e c o n t a i n i n g an u n s h a r e d p a i r o r v a c a n t p o r -b i t a l . In s e v e r a l c a s e s t h e m a g n i t u d e of t h e s p l i t t i n g h a s b e e n c o r r e l a t e d w i t h t h e known c r y s t a l s t r u c t u r e . F o r e x a m p l e , in SnS and Sn(OH2)Cl2. H 2 0 t h e t i n a t o m c o n t a i n s t h r e e s p 3 b o n d i n g o r b i t a l s and a n o n - b o n d i n g p a i r t h a t i s s t e r e o c h e m i c a l l y i m p o r t a n t . F o r c o m p l e t e l y e q u i v a l e n t sp 3 b o n d s t h e r e i s an equa l popu la t ion of t h e t h r e e p o r b i t a l s but t h e e l e c t r o n s in the u n s h a r e d p a i r p r o v i d e a p - e l e c t r o n i m b a l a n c e and sp l i t t i ng i s o b s e r v e d .

No s p l i t t i n g w a s o b s e r v e d in t h e q u a d r i c o v a l e n t i n o r g a n i c c o m p o u n d s , t h u s s u p p o r t i n g a s p 3 h y b r i d i z e d s t r u c t u r e c o n t a i n i n g f o u r e q u i v a l e n t bonds . T h e o n l y e s s e n t i a l l y i o n i c c o m p o u n d in w h i c h s p l i t t i n g h a s b e e n o b s e r v e d i s SnF4. In c o n t r a s t t o t h e o t h e r h a l i d e s w h i c h a r e t e t r a h e d r a l m o l e c u l a r s o l i d s , S n F 4 h a s an o c t a h e d r a l s t r u c t u r e c o n t a i n i n g f o u r b r i d g i n g f l u o r i n e a t o m s w h i l s t t w o f u r t h e r f l u o r i n e a t o m s a r e bound d i r e c t l y t o e a c h t in a tom t o c o m p l e t e t h e o c t a h e d r a l e n v i r o n m e n t [12]. T h e o b s e r v e d q u a d r u p o l e s p l i t t i n g in t h i s c o m p o u n d i s a s s o c i a t e d wi th t h e i n e q u i v a l e n c e of t he fluorine b o n d s and i s f u r t h e r s u p p o r t f o r t h e p r o p o s e d b r i d g i n g s t r u c t u r e .

159

ORGANIC TIN C O M P O U N D S

T h e M ö s s b a u e r s p e c t r a of m a n y o r g a n i c t i n h a l i d e s a n d r e l a t e d c o m -p o u n d s h a v e b e e n s t u d i e d [13 -15 ] . T h e m a g n i t u d e s of t h e i s o m e r s h i f t s and q u a d r u p o l e s p l i t t i n g s a t 80°K f o r s o m e of t he c o m p o u n d s a r e l i s t e d i n T a b l e l l . A g a i n t h e s h i f t s a r e w r i t t e n r e l a t i v e t o a g r e y t i n a b s o r b e r . When c r y s t a l s t r u c t u r e s a r e no t a v a i l a b l e i t i s not a l w a y s p o s s i b l e to m a k e an un iequ ivoca l a n a l y s i s of t h e M ö s s b a u e r d a t a ; f o r t h i s r e a s o n o n l y s e l e c t e d r e s u l t s of p e r s o n s i l i n t e r e s t a r e d i s c u s s e d .

TABLE II

ISOMER SHIFT (RELATIVE T O GREY TIN) AND QUADRUPOLE SPLITTING AT 80°K

A l l v a l u e s a r e i n m m s e c " 1

Compound Isomer shift Quadrupole splitt ing

(CH3)4Sn - 0 . 56

(C,H¡)4Sn -0 . 80

( C j F ^ S n - 0 . 98

( C 6 ^ ) 3 S n C l - 0 . 75 2. 55

( C j H & S n C l j - 0 . 68 2. 98

(CH3)3SnI - 0 . 63 3 . 1 9

(CH3)3SnBr - 0 . 65 3 . 4 0

(CH3)3SnCl - 0 . 7 0 3 . 5 5

(CH3)3SnF - 0 . 85 4 . 0 3

(C4H<,)2SnS - 1 . 2 1 . 9

(C 4R,) 2SnS0 3 - 0 . 6 4 . 0

(C4H9)2SnSQ4 - 0 . 3 4 . 7

E l e c t r o n d i f f r a c t i o n m e a s u r e m e n t s on s e v e r a l of t h e h a l i d e s have shown t h a t t h e m o l e c u l e s a r e t e t r a h e d r a l in t h e v a p o u r s t a t e . H e r e a s a f i r s t a p -p r o x i m a t i o n i t i s a s s u m e d t h a t t h e s o l i d s a r e t e t r a h e d r a l m o l e c u l a r c r y s t a l s . C e r t a i n l y f o r t h e s y m m e t r i c a l R 4 Sn c o m p o u n d s t h e r e s o n a n c e s a r e s i n g l e l i n e s w i t h i s o m e r s h i f t s c o n s i s t e n t w i t h r e g u l a r t e t r a h e d r a l s t r u c t u r e s . By c o r r e l a t i n g t h e r e s u l t s w i t h t h e i n o r g a n i c c o m p o u n d d a t a i t i s found t h a t t he

160

c a r b o n - t i n b o n d s a r e p r e d o m i n a n t l y c o v a l e n t bu t t h e s m a l l d i f f e r e n c e be tween t h e s h i f t s f o r t h e v a r i o u s o r g a n i c c o m p o u n d s m a k e i t p o s s i b l e to a r r a n g e t h e g r o u p s in t h e f o l l o w i n g o r d e r of d e c r e a s i n g e l e c t r o n e g a t i v i t y :

ВД>(С6Н5)> (CHg)

T h e i n t r o d u c t i o n of an i n o r g a n i c s u b s t i t u e n t f o r one of t h e o r g a n i c r a d i -c a l s p r o d u c e s a l a r g e q u a d r u p o l e s p l i t t i n g , e . g . t h e s p l i t t i n g in (C 6 H 5 ) 3 SnCl i s 2. 6 m m s e c - 1 . T h i s s p l i t t i n g c a n s t i l l b e i n t e r p r e t e d in t e r m s of an e s -s e n t i a l l y t e t r a h e d r a l s t r u c t u r e b u t n o w w i t h o n e of t h e f o u r . b o n d s h a v i n g a d i f f e r e n t p a r t i a l i o n i c c h a r a c t e r . A h a l i d e w h i c h i s e l e c t r o n e g a t i v e w i t h r e s p e c t t o t i n r e m o v e s c h a r g e бе f r o m t h e t i n a t o m . T h e e l e c t r i c f i e l d g r a d i e n t a s s o c i a t e d w i t h t h e c h a r g e on t h e e l e c t r o n e g a t i v e s u b s t i t u e n t i s

бе - p f (1 - y ) , w h e r e 7 i s t h e S t e r n h e i m e r a n t i s h i e l d i n g f a c t o r a n d r t h e S n -

h a l o g e n b o n d l e n g t h . In a d d i t i o n t o p r o d u c i n g a s m a l l i o n i c f i e l d g r a d i e n t , t h e t r a n s f e r of c h a r g e t o t h e s u b s t i t u e n t a l s o p r o d u c e s a n i n e q u i v a l e n c e in t h e s p 3 h y b r i d o r b i t a l s and c o n s e q u e n t l y an i m b a l a n c e of v a l e n c e p - e l e c t r o n d e n s i t y o n t h e t i n a t o m . T h e f i e l d g r a d i e n t a t t h e t i n n u c l e u s a s a r e s u l t of t h i s i m b a l a n c e i s p r o p o r t i o n a l t o бе and of m a g n i t u d e С б е < Т / R 3 w h e r e

1 /R^> i s a n a v e r a g e o v e r t h e r a d i a l w a v e f u n c t i o n f o r a 5p e l e c t r o n a n d С i s a c o n s t a n t . H e n c e , i f o t h e r e f f e c t s a r e u n i m p o r t a n t , t h e t o t a l f i e l d g r a d i e n t 6 e [ C < ^ l / R 3 > - (1 - 7 ) / r 3 ] d e c r e a s e s t o z e r o a s бе d e c r e a s e s . T h e q u a d r u p o l e s p l i t t i n g d a t a f o r t h e p h e n y l and fluorophenyl t i n h a l i d e s e x t r a -p o l a t e t o n e a r z e r o w h e n p l o t t e d a g a i n s t e l e c t r o n e g a t i v i t y d i f f e r e n c e ( F i g . 2a , b ) . T h u s t h e d a t a f o r t h e s e h a l i d e s a r e c o n s i s t e n t w i t h a m o l e c u l a r c r y s t a l i n w h i c h t h e t e t r a h e d r a l s t r u c t u r e i s r e t a i n e d b u t w i t h t h e h a l i d e b o n d h a v i n g a g r e a t e r i o n i c c h a r a c t e r . In t h e s e c o m p o u n d s t h e r e l a t i v e d e -g r e e s of i o n i c i t y of t h e f l u o r i d e , o x i d e , c h l o r i d e , b r o m i d e and i o d i d e s u b -s t i t u e n t s [ 1 4 , 1 5 ] a r e 1. 0 : 0 . 85 : 0. 65 : 0 . 65 : 0. 50 (±0. 05) .

FIG. 2. Magni tude of the quadrupole spli t t ing in (a) ( Q H 5 ) 3 S n X , (b) (C 6 F s ) 3 SnX and (c) (CH 3 ) s SnX c o m -pounds (where X includes F, О , CI , Br and I) against e lec t ronega t iv i ty d i f fe rence

161

T h e d a t a f o r t h e m e t h y l t i n h a l i d e s , ( F i g . 2c) a r e no t c o n s i s t e n t wi th t h i s s i m p l e m o d e l , t h u s s u g g e s t i n g t h a t e f f e c t s o t h e r t h a n t h e p a r t i a l i on i c c h a -r a c t e r of t h e t i n - h a l o g e n b o n d s a r e r e q u i r e d t o i n t e r p r e t t h e r e s u l t s . O t h e r s o u r c e s of f i e l d g r a d i e n t a r e p o s s i b l e t h r o u g h :

(1) O r b i t a l r e h y b r i d i z a t i o n a t t h e t i n a t o m (2) I n e q u i v a l e n t b o n d s ( a s in SnF 4 ) (3) D a t i v e d^-p^ p a r t i a l d o u b l e b o n d s . A p p r e c i a b l e d^-p^ b o n d i n g wi th t h e s a t u r a t e d c a r b o n a t o m s of t he m e t h y l

g r o u p i s u n l i k e l y . A l s o b e c a u s e of t h e 1 / R 3 d e p e n d e n c e , d e l e c t r o n s a r e n o t e x p e c t e d t o c o n t r i b u t e d i r e c t l y t o t h e e l e c t r i c f i e l d g r a d i e n t a t t h e t i n n u c l e u s . H o w e v e r , if s u b s t a n t i a l c o n t r a c t i o n of t h e d o r b i t a l s o c c u r s in t h e f o r m a t i o n of d h y b r i d s ( C r a i g e t a l . [16]) t h e n t h e i r c o n t r i b u t i o n m a y b e s i g n i f i c a n t .

F r o m a c r y s t a l s t r u c t u r a l a n a l y s i s of t r i m e t h y l t in f l u o r i d e , C l a r k e t a l . [17] h a v e e s t a b l i s h e d t h a t in t h i s c o m p o u n d t h e r e g u l a r t e t r a h e d r a l s t r u c t u r e i s d e s t r o y e d a n d t i n i s e s s e n t i a l l y 5 - c o o r d i n a t e . T h e y g i v e e v i d e n c e t h a t t h e s t r u c t u r e c o n s i s t s of n e a r l y p l a n a r ( C H ^ S n g r o u p s l i n k e d b y b r i d g i n g fluorine a t o m s and a r r a n g e d a l o n g c h a i n s . F u r t h e r m o r e t h e F - S n . . . F bonds a r e n o t l i n e a r . T h i s s t r u c t u r e i s p r o b a b l y s o m e f o r m of q u a s i - i o n i c s p 3 d h y b r i d , b u t s i n c e t h e s t e r e o c h e m i s t r y i s n o t s i m p l e and a l s o s i n c e t h e r o l e of d - h y b r i d i z a t i o n i s d i f f i c u l t t o e v a l u a t e , t h e a n a l y s i s of t h e M ö s s b a u e r d a t a i s n o t y e t c o m p l e t e . H o w e v e r , t h e r e s u l t s do i m p l y t h a t m e t h y l t i n c h l o r i d e , b r o m i d e a n d i o d i d e m a y a l s o c o n t a i n p e n t a - c o o r d i n a t e t i n . Bu t i t i s t o b e n o t e d t h a t t h e c o m p o u n d s a r e k n o w n t o h a v e a t e t r a h e d r a l s t r u c -t u r e in t h e v a p o u r s t a t e [18]. F u r t h e r e x p e r i m e n t s in p r o g r e s s i n c l u d e t h e m e a s u r e m e n t of t h e s i g n , a n i s o t r o p y a n d t e m p e r a t u r e d e p e n d e n c e of t h e e l e c t r i c f i e l d g r a d i e n t .

T h e i s o m e r s h i f t a l s o p r o v i d e s i n f o r m a t i o n on t h e m e c h a n i s m of t h e h a l i d e s u b s t i t u t i o n . H o w e v e r , s e c o n d a r y c h a n g e s in t h e s h i e l d i n g a c t i o n of t h e i n n e r e l e c t r o n s c o m p e n s a t e t h e p r i m a r y r e m o v a l of e l e c t r o n s w i t h t h e r e s u l t t h a t t h e i s o m e r s h i f t s a r e l e s s s e n s i t i v e t o i n o r g a n i c s u b s t i t u t i o n .


It i s a p l e a s u r e t o t h a n k D r . R . D. P e a c o c k f o r h e l p f u l d i s c u s s i o n s and M r . M . Vuc l i c f o r t h e p r e p a r a t i o n of t h e c o m p o u n d s u s e d in t he B i r m i n g h a m m e a s u r e m e n t s .

R E F E R E N C E S

[1] BOYLE, A . J . . BUNBURY, D. P. . EDWARDS, C . , Proc. phys. Soc. 79 (1962) 416. [2] BRYUKHANOV, V . A . , DELYAGIN, N. N . . OPALAKO, A . A . , SHPINEL, V. S . , Zh. éksp. teor. Fiz. 43

(1962) 432. [3] CORDEY-HAYES, M . . J. inorg. nucl. Chem. 26 (1964) 915. [4] GOLDANSKII, V. I . , Dokl. Akad. Nauk SSSR 147 (1962) 127; G. E. C. a tom. Energy Rev. 4 (1963) 3. [5] CRAWFORD, M. F . , SCHAWLOW, A . L . , Phys. Rev. 76 (1949) 1310. [6] HUMBACH, W. , Z. Phys. 141 (1955) 59.

162

и GOLDANSKII, V . l . , MAKAROV, E. F . , Phys. Lett. 2(1965) 111. [8] SUMBAYEV, O . I . , , MEZENTSEV, A . F . , Zh. éksp. teor. Fiz. 48 (1965)445. [9] DAS, T. P . , HAHN, E. L. , Solid State Physics. Suppl. 1, Academic Press.

[10] TOWNES, C. H . , DA ILE Y, B. P . , J. chem. Phys. 17 (1949) 782. [11] BORSA, F . , BARNES, R. G . . Phys. Rev. Lett. 12 (1964) 281. [12] DAHNE, W. , 7th Int. Conf. on Coordination Chemistry, Stokholm (1962). L13J ALEKSANDROV, A . , DELYAGIN, N. N . , MITROFANOV, KTPT. POLAK, L. S . . SHPINEL. V. S . , Zh.

éksp. teor. Fiz. 43 (1962) 1242. [14] BRYUKHANOV, V . A . , GOLDANSKII. V . l . , DELYAGIN, N. N. , KORYTKO, L . A . , MAKAROV, E .F . ,

SUZADALEV, I.P., SHPINEL, V. S . , Zh. éksp. teor. Fiz. 43 (1962) 448. [15] CORDEY-HAYES. M . , J. inorg. nucl. Chem. 26 (1964) 2306. [16] CRAIG, D. P . , MACCOLL, A . , NYHOLM. R. S . . ORGEL, L. E., SUTTON, L.E. , J. chem. Soc. (1964)

332. [17] CLARK. H . C . . O'BRIEN. R.J . , TROTTER, J . , J. chem. Soc. (1963) 83. [18] SKINNER, H . A . , SUTTON, L. E., Trans. Faraday Soc. 40 (1944) 164.

D I S C U S S I O N

R . H . H E R B E R p o i n t e d o u t t h a t m o l e c u l a r w e i g h t d e t e r m i n a t i o n s on f r o z e n s o l u t i o n s w o u l d h e l p in d e c i d i n g if t h e m e t h y l t i n h a l i d e s w e r e 5 - c o o r d i n a t e .

N . N . G R E E N W O O D p o i n t e d out t h a t sp 3 d h y b r i d i z a t i o n w a s no t t h e on ly p o s s i b l e d e s c r i p t i o n f o r 5 - c o o r d i n a t e t i n b u t a g r e e d t h a t i t w a s t h e m o s t l i k e l y o n e .

MÖSSBAUER SPECTRA OF SOME ORGANO-TIN COMPOUNDS

T . C . GIBB AND N .N . GREENWOOD UNIVERSITY OF NEWCASTLE UPON TYNE, UNITED KINGDOM

A s e r i e s of t r i p h e n y l s t a n n y l c o m p o u n d s h a s b e e n s t u d i e d . D e s p i t e t he a b s e n c e of a r e g u l a r t e t r a h e d r a l e n v i r o n m e n t f o r t h e t i n a t o m in m o s t of t h e c o m p o u n d s , no o b s e r v a b l e q u a d r u p o l e e f f e c t w a s d e t e c t e d . T h i s r e s u l t , a n d t h e o b s e r v e d c h e m i c a l s h i f t s , a r e d i s c u s s e d i n t e r m s of s t r u c t u r e and b o n d i n g i n t h e s e c o m p o u n d s .

T h e e x p e r i m e n t a l d a t a a r e l i s t e d in T a b l e I a n d c o m p a r e d w i t h o t h e r p u b l i s h e d d a t a on r e l a t e d c o m p o u n d s . L i n e - w i d t h s a r e quoted f o r an a b s o r b e r t h i c k n e s s of 60 m g . c m - 2 e x c e p t f o r ЭпОг a n d ß-Sn w h i c h w e r e 20 a n d 40 m g . c m - 2 r e s p e c t i v e l y . T h e s p e c i m e n of h e x a p h e n y l d i - t i n w a s k i n d l y s u p p l i e d b y D r . R . H . P r i n c e ( C a m b r i d g e U n i v e r s i t y ) a n d t h e s a m p l e s of ( P h 3 S n ) 4 S n , ( P h 3 S n ) 4 G e , (Ph3Sn>4 P b and P h 8 S n 3 w e r e supp l i ed by D r . J . G . A. L u i j t e n ( O r g a n i s c h C h e m . I n s t . T . N. O. U t r ech t ) . T y p i c a l s p e c t r a a r e shown in F i g . 1 .

163

TABLE II (cont. )

M Ö S S B A U E R P A R A M E T E R S F O R S O M E O R G A N O - T I N C O M P O U N D S ( m m / s r e f e r r e d t o S n 0 2 )

Compound •к Chemical shift, 6

Line-width, Г

Quadrupole split, Д

Ref.

Sn0 2 300 0 .00 1 . 7 4 1 0 . 1 2 -

B-Sn 300 2 .50 1 .28 ± 0 . 1 2 -

Ph^Sn 80 1 . 1 5 ± 0 . 1 0 1 . 5 6 1 0 . 2 0 0

(PbjSn)2 80 1 . 3 0 ± 0 . 1 0 1 .56 ± 0 . 2 0 0

(Ph3Sn)4 Sn 80 1 . 3 3 ± 0 . 1 0 ' 1 . 68 ± 0 . 2 0 0

(Ph3Sn)4 Ge 80 1 .13 i 0 .10 1 . 5 3 ± 0 . 1 0 0

(Ph3Sn)4 Pb 80 1 . 3 9 ± 0 . 1 0 1 .56 ± 0 . 2 0 0

PhsSn. Ph2Sn. SnPh3 80 1 . 0 6 ± 0 , 1 0 1 . 3 9 ± 0 .20 0

Bu2nSnO 80 1 .15 ± 0.10 1 .56 ± 0 .20 2 . 08 ± 0 . 2 0

Ph4Sn 80 1 .208 1 .31 0 [ 1 ]

(Ph3Sn)2 80 1.413 1 .31 0 [ 1 ]

Bu2nSnO 80 1 . 0 3 ± 0 . 03 1 .95 ± 0 . 0 3 [ 1 ]

80 0 . 9 5 ± 0 . 1 0 2 . 2 ± 0 . 2 [ 2 ]

Bu2nSnH2 80 1 .45± 0.07 0 [ 3 ]

Ph3SnH 80 1 . 4 5 1 0 . 0 5 0 [ 3 ]

Ph3SnLi 80 1 .40 è 0. 07 0 [ 3 ]

Me3SnCh: CH2 80 1 .30 è 0. 05 0 [ 3 ]

The c h e m i c a l s h i f t s in the s e r i e s ( P h 3 S n ) 4 G e , (Ph3Sn)4 Sn and (Ph 3 Snk Pb a r e c o n s i s t e n t w i t h t h e i n c r e a s i n g e l e c t r o p o s i t i v e c h a r a c t e r of t h e c e n t r a l m e t a l a t o m . T h e s h i f t f o r РЬз Sn. P h 2 S n . S n P h 3 i s u n u s u a l l y low but r e p r o -d u c i b l e v a l u e s w e r e o b t a i n e d on s e v e r a l r u n s . In b o t h ( P h 3 S n ) 4 S n and P h 3 S n . P h 2 S n . S n P h 2 t h e t i n a t o m s a r e in two c h e m i c a l l y d i f f e r e n t e n v i r o n -m e n t s but on ly one l i ne w a s o b s e r v e d , i m p l y i n g t h a t t he c h e m i c a l s h i f t s a r e t o o s i m i l a r t o i n t r o d u c e a n y m e a s u r a b l e a s y m m e t r y i n t h e l i n e s h a p e .

T a b l e I i n d i c a t e s t h a t t he l i n e - w i d t h s of the s i n g l e - l i n e s p e c t r a a r e c o m -p a r a b l e t o t h e c o m p o n e n t l i n e - w i d t h s of B u 2 n S n O , and t o t he l i n e - w i d t h of P h 4 S n w h i c h i s t e t r a h e d r a l a n d s h o u l d h a v e z e r o f i e l d g r a d i e n t a t t h e t i n n u c l e u s . It c a n t h e r e f o r e be conc luded tha t in the t r i p h e n y l s t a n n y l compounds a n y q u a d r u p o l e s p l i t t i n g i s l e s s t h a n the e x p e r i m e n t a l r e s o l u t i o n of 0. 2-0 . 3 m m / s . S i m i l a r r e s u l t s h a d p r e v i o u s l y b e e n r e p o r t e d f o r B u 2 n S n H 2 , P h 3 S n H , P h 3 S n L . i a n d M e 3 S n C H : C H 2 ( s e e T a b l e I) .

By c o n t r a s t , the fo l lowing c o m p o u n d s have been found to have quadrupole s p l i t t i n g s of 1 - 4 m m / s [1, 2, 4, 5] : R 3 S n X , R 2 S n X 2 , R S n X 3 , ( R 3 S n ) 2 0 ,

164

VELOCITY ( m m / s relat ive to S n 0 2 )

FIG. 1. Mössbauer s p e c t r a of s o m e o r g a n o - t i n c o m p o u n d s

( R 2 S n O ) x , R 3 S n O H , ( R 3 S n ) 2 S , R 3 S n N 3 , R 3 S n C 1 0 4 , R 3 S n O C O M e , R 3 S n N 0 3 , R2SnSC>4 a n d R 2 S n S 0 3 , w h e r e R i s a n a l k y l o r a r y l g r o u p a n d X i s a h a l o g e n . I t i s s i g n i f i c a n t t h a t if o n e o r m o r e of t h e a t o m s d i r e c t l y b o n d e d t o t h e t i n a t o m p o s s e s s e s a n o n - b o n d i n g p a i r of e l e c t r o n s (e . g . F , Cl , B r , I, O, S, N) t h e n a q u a d r u p o l e s p l i t t i n g i s o b s e r v e d , w h e r e a s if none of the d i r e c t l y bonded a t o m s h a s a l o n e p a i r ( e . g . H, L i , C, Ge , Sn, P b ) t h e n no q u a d r u p o l e s p l i t -t i n g i s o b s e r v e d . T h e o n l y a p p a r e n t e x c e p t i o n i s R 3 S n C H 2 . CO. C H 3 but t h i s p r o b a b l y h a s a 5 - c o o r d i n a t e s t r u c t u r e w i t h a d i r e c t b o n d b e t w e e n t i n a n d o x y g e n [ 4 ] .

F o u r e x p l a n a t i o n s of t h e d i f f e r e n c e b e t w e e n t h e s e , two c l a s s e s of o r g a n o -t i n c o m p o u n d c a n be c o n c e i v e d :

(i) It c o u l d b e a s s u m e d t h a t t h e r e i s a f i n i t e f i e l d g r a d i e n t in a l l c o m -p o u n d s bu t , in t h o s e c o m p o u n d s w i t h z e r o q u a d r u p o l e s p l i t t i n g , t h i s i s t i m e -a v e r a g e d t o z e r o o v e r t h e l i f e - t i m e of t h e e x c i t e d r s t a t e . T h i s e x p l a n a t i o n i s r e j e c t e d , f i r s t l y , b e c a u s e t h e r e a p p e a r s t o be no m e c h a n i s m f o r s u c h a v e r -

165

a g i n g and, s e c o n d l y , b e c a u s e i t i s no t c l e a r why t h i s s h o u l d b e i n f l u e n c e d by t h e p r e s e n c e o r a b s e n c e of n o n - b o n d i n g p e l e c t r o n s on t h e a t o m s a t t a c h e d t o t h e t i n a t o m .

(ii) It cou ld be a s s u m e d t h a t t h e o b s e r v e d e f f e c t s a r e due s i m p l y to v a -r i a t i o n s in a s y m m e t r y in the t in 5p s h e l l a s a r e s u l t of bonding. T h i s i n t e r -p r e t a t i o n i m p l i e s t h a t t h e r e i s n e g l i g i b l e d i f f e r e n c e b e t w e e n bonds to c a r b o n on t h e o n e h a n d , a n d t o h y d r o g e n , l i t h i u m , g e r m a n i u m , t i n o r l e a d on t h e o t h e r h a n d . T h i s e x p l a n a t i o n i s r e j e c t e d s i n c e i t d o e s no t i n d i c a t e w h y i n s o m e c o m p o u n d s a p p a r e n t e q u i v a l e n c e i s o b s e r v e d i n t h e f o u r b o n d s t o t i n w h e r e a s i n o t h e r c o m p o u n d s t h e f o u r b o n d s b e h a v e n o n - e q u i v a l e n t l y a s f a r a s t h e f i e l d g r a d i e n t i s c o n c e r n e d . T h u s it m i g h t be expec ted tha t t h e r e would b e s o m e c o r r e l a t i o n w i t h e l e c t r o n e g a t i v i t y d i f f e r e n c e s but t h i s i s no t s o a s s h o w n by T a b l e II . F o r e x a m p l e (Ph 3 Sn)4 P b s h o w s no q u a d r u p o l e s p l i t t i n g y e t t h e d i f f e r e n c e in e l e c t r o n e g a t i v i t y b e t w e e n c a r b o n and l e a d i s g r e a t e r t h a n b e t w e e n c a r b o n and a n y of t he o t h e r e l e m e n t s l i s t e d ; l i k e w i s e P h 3 S n - S n P h 3

TABLE II

E L E C T R O N E G A T I V I T Y DIFFERENCES*1 B E T W E E N CARBON AND OTHER E L E M E N T S

No quadrupole spli t t ing Quadrupole spli t t ing observed

Element Дх Element Дх

H •0.40 F +1 .60

LI •1.53 Cl +0 .33

G e •0.48 Br + 0 . 2 4

Sn •1.80 I - 0 . 2 9

Pb •1.95 О +1 .00

S - 0 . 0 6

N +0 .57

a Allred-Rochow values used, C = 2 . 5 0 ; Pauling values and Mull iken values lead to s imi lar conclusions.

s h o w s a s i n g l e l i n e y e t t h e e l e c t r o n e g a t i v i t y d i f f e r e n c e b e t w e e n c a r b o n a n d t i n i s g r e a t e r t h a n b e t w e e n c a r b o n and any of t h e o t h e r e l e m e n t s excep t l ead ; by c o n t r a s t b r o m i n e , i o d i n e and s u l p h u r h a v e e l e c t r o n e g a t i v i t i e s v e r y c l o s e t o t h a t of c a r b o n y e t o r g a n o - t i n c o m p o u n d s i n w h i c h one of t h e s e e l e m e n t s i s b o n d e d t o t i n a l l s h o w w e l l - d e f i n e d q u a d r u p o l e s p l i t t i n g s .

( i i i) A t h i r d i n t e r p r e t a t i o n of t h e r e s u l t s c o n c e r n s t h e p o s s i b i l i t y of a d -m i x i n g s o m e 5cJ. c h a r a c t e r i n t o t h e b o n d i n g w a v e - f u n c t i o n s of t h e t i n ' a t o m . G e n e r a l e q u a t i o n s f o r t h e e v a l u a t i o n of t h e f i e l d g r a d i e n t i n t e r m s of t h e a t o m i c w a v e - f u n c t i o n s h a v e b e e n g i v e n by T o w n e s and D a i l e y [ 6 ] . F o r a n y

166

g iven w a v e - f u n c t i o n ф d e s c r i b i n g a bond d i r e c t e d in t he z d i r e c t i o n , t he f i e ld g r a d i e n t a t t he n u c l e u s i s g iven by

Л Vzz = e J ф* [ ( 3 c o s 2 в - l ) / r ]фdT

w h e r e 0 i s the a z i m u t h a l ang le , r i s the d i s t a n c e f r o m n u c l e u s , and the w a v e -f u n c t i o n c o n t r i b u t i o n i s i n t e g r a t e d o v e r a l l s p a c e . T h e f u n c t i o n ф n e a r t h e t i n n u c l e u s c a n b e e x p a n d e d i n t e r m s of t h e a t o m i c w a v e - f u n c t i o n s of t i n i t s e l f , s i n c e t h e f a c t o r 1 / r 3 d i m i n i s h e s t h e c o n t r i b u t i o n of t he o t h e r bonded a t o m s a t t h i s p o i n t .

T h e d o m i n a n t t e r m s i n t h e e x p a n s i o n i n t e g r a l s f o r t h e f i e l d g r a d i e n t a r e t h o s e c o n t a i n i n g . t h e (5p, 5p) and (5s, 5d) p a i r s of o r b i t a l s ; t h e s e t e r m s w i l l b e l a r g e r t h a n t h o s e d e r i v i n g f r o m t h e (5p, 5f) a n d (5p, 6p) i n t e g r a l s . H e n c e , p r o v i d i n g tha t bo th t he 5s and 5d t i n o r b i t a l s h a v e a r e a s o n a b l e e l e c -t r o n p o p u l a t i o n , t h e r e w i l l be a n a p p r e c i a b l e c o n t r i b u t i o n t o t h e f i e l d g r a -d i e n t . If i t i s now a s s u m e d t h a t t he a s y m m e t r y w h i c h i s i n t r o d u c e d in to t h e s p 3 h y b r i d s w h e n R 4 S n g o e s t o R 3 S n X h a s o n l y a s l i g h t e f f e c t on t h e f i e l d g r a d i e n t a t t h e n u c l e u s , t h e n t h e p r e s e n c e of a q u a d r u p o l e s p l i t i n c e r t a i n o r g a n o - t i n c o m p o u n d s c a n be e x p l a i n e d p r o v i d e d t h a t a m e c h a n i s m c a n b e f o u n d f o r i n v o l v i n g t h e t i n 5d o r b i t a l s i n t h e s e i n s t a n c e s . I t h a s a l r e a d y b e e n p o i n t e d ou t t h a t q u a d r u p o l e s p l i t t i n g i s o n l y o b s e r v e d w h e n o n e of t h e a t t a c h e d a t o m s h a s a n o n - b o n d i n g p a i r of e l e c t r o n s in а р я o r b i t a l . T h e r e i s t h u s t h e p o s s i b i l i t y in t h e s e i n s t a n c e s of p^d^ i n t e r a c t i o n wi th t h e o t h e r -w i s e v a c a n t d o r b i t a l s on t h e t i n a t o m , t h u s p r o v i d i n g a m e c h a n i s m f o r i n -v o l v i n g t h e s e o r b i t a l s i n b o n d i n g .

(iv) T h e f o u r t h p o s s i b l e i n t e r p r e t a t i o n i s t o p o s t u l a t e t h a t s , p a n d d m i x i n g o c c u r s t o s u c h a n e x t e n t t h a t 5 - c o o r d i n a t e c o m p o u n d s a r e o b t a i n e d i n t h o s e c o m p o u n d s f o r w h i c h a q u a d r u p o l e s p l i t t i n g i s o b s e r v e d . T h u s , of t h e c o m p o u n d s m e n t i o n e d e a r l i e r , i t h a s a l r e a d y b e e n e s t a b l i s h e d t h a t M e 3 S n F [ 7 ] , M e 3 S n O H [8] a n d M e 3 S n O C O M e [9] a r e 5 - c o o r d i n a t e i n t h e s o l i d s t a t e . It m a y w e l l b e t h a t t h e o t h e r s a r e t o o , p a r t i c u l a r l y s i n c e a g r o w i n g n u m b e r of s u c h c o m p o u n d s a r e b e i n g r e p o r t e d i n t h e l i t e r a t u r e .

T h e c r u c i a l e x p e r i m e n t s t o be done on t i n c o m p o u n d s a r e t h e r e f o r e a s f o l l o w s :

(a) X - r a y s t r u c t u r e d e t e r m i n a t i o n s t o e s t a b l i s h w h e t h e r , in f ac t , t h o s e c o m p o u n d s w i t h a p p r o x i m a t e l y z e r o q u a d r u p o l e s p l i t t i n g a r e 4 - c o o r d i n a t e a n d t h o s e w i t h a p p r e c i a b l e s p l i t t i n g a r e 5 - c o o r d i n a t e .

(b) M o l e c u l a r w e i g h t m e a s u r e m e n t s in n o n - p o l a r s o l v e n t s , c o u p l e d wi th i n f r a r e d s t u d i e s of l iquid and on f r o z e n so lu t i ons , and M ö s s b a u e r s t u d i e s on f r o z e n s o l u t i o n s . T h u s , i t h a s b e e n r e p o r t e d L4J t h a t t h e c h e m i c a l s h i f t a n d q u a d r u p o l e s p l i t t i n g i n c r y s t a l l i n e E t 2 S n C l 2 a r e i d e n t i c a l w i t h t h o s e o b t a i n e d f r o m a f r o z e n s o l u t i o n in d i c h l o r o e t h a n e , but i t w a s not s t a t e d w h e t h e r the s o l u t e w a s p r e s e n t a s a 4 - c o o r d i n a t e m o n o m e r o r a 5 - c o o r d i n a t e d i m e r .

(c) C h l o r i n e n u c l e a r q u a d r u p o l e r e s o n a n c e e x p e r i m e n t s t o d e t e c t t h e p r e s e n c e o r a b s e n c e of b r i d g i n g c h l o r i n e a t o m s .

E x p e r i m e n t s a l o n g t h e s e l i n e s a r e b e i n g u n d e r t a k e n .

167

R E F E R E N C E S [ 1 ] HERBER, R . H . , private communicat ion. [ 2 ] ALEKSANDROV. A. Y u . , DELYAGIN, N . N . , MITROFANOV, K . P . , POLAK, L . S . , SHPINEL, V . S . ,

Soviet Phys. JETP 16 (1963) 879. [ 3 ] ALEKSANDROV, A . Y u . , OKHLOKYSTIN, O .Yu . , POLAK, L .S . , SHPINEL, V . S . , Dokl. Akad. Nauk

SSSR 157 (1964) 934. [ 4 ] GOLDANSKII, V . l . , The Mössbauer Effect and its Application in Chemistry, New York, Consultants

Bureau (1964) . [ 5 ] BRYUKHANOV, V . A . , GOLDANSKII, V . l . , DELYAGIN, N . N . , KORYTKO, L . A . , MAKAROV, E . F . ,

SUZDALEV, I . P . , SHPINEL, V . S . , Soviet Phys. JETP 16 (1963) 321. [ 6 ] TOWNES, C . H . , DAILEY, B .P . , J. chem. Phys. Д ( 1 9 4 9 ) 782. [ 7 ] CLARK, H . C . , O'BRIEN, R .J . , TROTTER, J . , Proc. chem. Soc. (1963) 85. [ 8 ] KASAI, N . . YASUDA, K . , OKAWARA, R. , J. organometal . Chem. 3 (1965) 172. [ 9 ] JANSSEN, M . I . , LUIJTEN, J . G . A . , VAN DERKERK, G . J . M . , Reel Trav. chim. Pays-Bas Belg. 82

(1963) 90.

D I S C U S S I O N

V . l . GOLDANSKII s u g g e s t e d t h a t d i f f e r e n c e s in bond l e n g t h s would be i m p o r t a n t i n t h e ( P h 3 S n ) 4 P b , ( P h 3 S n ) 4 G e , ( P h 3 S n ) 4 Sn s e r i e s .

И З У Ч Е Н И Е О Л О В О О Р Г А Н И Ч Е С К И Х ПРОИЗВОДНЫХ Б А Р Е Н О В М Е Т О Д О М М Е С С Б А У Э Р О В С К О Й

С П Е К Т Р О С К О П И И

A . Ю . А Л Е К С А Н Д Р О В , В . И . Б Р Е Г А Д З Е , B . И . Г О Л Ь Д А Н С К И Й , Л . И . 3 А Х А Р К И Н ,

О . Ю . 0 Х Л О Б Ы С Т И Н и В . В . Х Р А П О В

И Н С Т И Т У Т Х И М И Ч Е С К О Й ФИЗИКИ АН С С С Р , И Н С Т И Т У Т Э Л Е М Е Н Т О О Р Г А Н И Ч Е С К И Х

С О Е Д И Н Е Н И Й АН С С С Р , МОСКВА С С С Р

ABSTRACT

USE OF MÖSSBAUER SPECTROSCOPY TO STUDY "BARENES" DERIVED FROM STANNO-ORGAN1C COM-POUNDS. The Mössbauer spectra of some stanno-organic compounds with the carborane group are described. The very strong acceptor nature of this group leads to quite unusual properties of Mössbauer spectra, unobserved in other examples of -Sn-C- bonds.

В последние г о д ы большое внимание у д е л я е т с я изучению свойств ново-г о к л а с с а б о р о р г а н и ч е с к и х с о е д и н е н и й - б а р е н о в ( к а р б о р а н о в - 1 0 [ 1 - 5 ] ) . Н е о б ы ч н о е в а л е н т н о е с о с т о я н и е у г л е р о д а в б а р е н а х , с и л ь н ы е э л е к т р о н о -а к ц е п т о р н ы е с в о й с т в а бареновой с и с т е м ы в сочетании с ее исключительной

168

с т а б и л ь н о с т ь ю д е л а ю т особенно и н т е р е с н ы м и с с л е д о в а н и е м е т а л л о о р г а н и -ч е с к и х п р о и з в о д н ы х б а р е н о в , с о д е р ж а щ и х прямую сг-связь м е т а л л - б а р е н о -вый у г л е р о д . Т а к и е соединения , о б р а з о в а н н ы е с у ч а с т и е м т я ж е л ы х м е т а л -л о в ( о л о в а , р т у т и и д р . ) , по р я д у х и м и ч е с к и х с в о й с т в с у щ е с т в е н н о о т л и -ч а ю т с я о т о б ы ч н ы х м е т а л л о о р г а н и ч е с к и х с о е д и н е н и й [6] .

П о с к о л ь к у с е й ч а с у ж е н а к о п л е н з н а ч и т е л ь н ы й о п ы т в и с п о л ь з о в а н и и м е с с б а у э р о в с к о й с п е к т р о с к о п и и для и с с л е д о в а н и я х и м и ч е с к о г о с т р о е н и я и реакционной способности оловоорганических соединений [7] , представлялось и н т е р е с н ы м и з у ч и т ь с п е к т р ы р е з о н а н с н о г о п о г л о щ е н и я у - к в а н т о в р я д а баренильных производных о л о в а и, т е м с а м ы м , сравнить влияние б а р е н и л ь -ной г р у п п ы н а э л е к т р о н н о е о к р у ж е н и е я д р а о л о в а с в л и я н и е м б о л ь ш о г о ч и с л а о б ы ч н ы х о р г а н и ч е с к и х з а м е с т и т е л е й .

С э т о й целью мы изучили м е с с б а у э р о в с к и е с п е к т р ы р а з л и ч н ы х б а р е н -о р г а н и ч е с к и х с о е д и н е н и й ч е т ы р е х в а л е н т н о г о о л о в а . И з м е р е н и е с п е к т р о в р е з о н а н с н о г о поглощения проводилось на у с т а н о в к а х с переменной и п о с т о -янной с к о р о с т ь ю с п р и м е н е н и е м в к а ч е с т в е д е т е к т о р о в излучения сцинтил-л я ц и о н н о г о и р е з о н а н с н о г о с ч е т ч и к о в [8] . В с е и с с л е д о в а н н ы е о б р а з ц ы о х л а ж д а л и с ь до т е м п е р а т у р ы 7 7 ° К ; и с т о ч н и к у - к в а н т о в S n 0 2 н а х о д и л с я при комнатной т е м п е р а т у р е . Р е з у л ь т а т ы и з м е р е н и й представлены в т а б л и -це , в которой п р и в е д е н ы з н а ч е н и я к о н с т а н т к в а д р у п о л ь н о г о р а с щ е п л е н и я Д и величины и з о м е р н ы х с д в и г о в 6. Как правило , квадрупольное расщепление в соединениях ч е т ы р е х к о в а л е н т н о г о олова проявляется только в том случае , если з а м е с т и т е л и з н а ч и т е л ь н о р а з н я т с я друг от д р у г а по с в о е й э л е к т р о о т -р и ц а т е л ь н о с т и и по м е н ь ш е й м е р е о д н а и з о б р а з о в а н н ы х о л о в о м с в я з е й з а м е т н о о т л и ч а е т с я о т т р е х о с т а л ь н ы х по с т е п е н и и о н н о с т и [ 7 , 9 ] . З а н е м н о г и м и и с к л ю ч е н и я м и [10, 11] , о б у с л о в л е н н ы м и , п о - в и д и м о м у , о б р а з о -в а н и е м пятой — координационной — с в я з и о л о в а с к и с л о р о д о м , либо индук-т и в н ы м э ф ф е к т о м , с п е к т р ы с о е д и н е н и й , в к о т о р ы х а т о м о л о в а с в я з а н с ч е т ы р ь м я у г л е р о д н ы м и а т о м а м и , п р е д с т а в л я ю т с о б о й с и н г л е т н у ю линию. М е ж д у т е м , как э т о видно из п р и в е д е н н ы х в т а б л и ц е д а н н ы х , в с е б а р е н и л ь -ные соединения с ч е т ы р ь м я S n - C с в я з я м и дают квадрупольное расщепление м е с с б а у э р о в с к и х линий в области Д = 0 , 7 - 1,7 м м / с е к . Отсюда с н е с о м н е н -н о с т ь ю с л е д у е т в ы в о д , ч т о б а р е н и л ь н а я с и с т е м а в о т л и ч и е от б о л ь ш о г о ч и с л а о б ы ч н ы х о р г а н и ч е с к и х з а м е с т и т е л е й о б л а д а е т с и л ь н ы м э л е к т р о н о -а к ц е п т о р н ы м д е й с т в и е м по о т н о ш е н и ю к а т о м у о л о в а . Х а р а к т е р н о , ч т о к в а д р у п о л ь н о е р а с щ е п л е н и е в с п е к т р а х т р и ф е н и л б а р е н и л о л о в о х л о р и д а (N2 5) и д и ф е н и л б а р е н и л о л о в о д и х л о р и д а (N2 6) з н а ч и т е л ь н о м е н ь ш е , ч е м в с п е к т р а х " т р и ф е н и л о л о в о х л о р и д а (А = 2 ,4 м м / с е к . ) . Э т о о з н а ч а е т , ч т о в д в у х н а з в а н н ы х б а р е н и л ь н ы х п р о и з в о д н ы х о л о в а и о н н о с т и с в я з е й S n - C l и S n - б а р е н и л б л и з к и , а п о т о м у с и м м е т р и я э л е к т р и ч е с к о г о п о л я н а я д р е з н а ч и т е л ь н о выше , чем в а н а л о г и ч н ы х фенильных производных , в с л е д с т в и е с и л ь н о г о э л е к т р о н о а к ц е п т о р н о г о в л и я н и я б а р е н и л ь н ы х з а м е с т и т е л е й .

И з с р а в н е н и я с п е к т р о в ф е н и л б а р е н и л т р и ф е н и л о л о в а (N21 ) и п е н т а ф т о р ф е -н и л т р и ф е н и л о л о в а [12] с л е д у е т , ч т о ф е н и л б а р е н и л ь н а я г р у п п а о к а з ы в а е т на э л е к т р о н н о е о к р у ж е н и е я д р а о л о в а т а к о й же э ф ф е к т , как и п е н т а ф т о р ф е -н и л ь н а я . Э т о т в ы в о д можно с о п о с т а в и т ь с ч а с т о т а м и Я К Р C l 3 5 в п е н т а -ф т о р х л о р б е н з о л е (39 ,42 м г г ц ) , т р и ф т о р х л о р м е т а н е ( 3 8 , 0 8 9 м г г ц ) [13] и т р и н и т р о х л о р м е т а н е ( 4 2 , 9 4 8 м г г ц ) [14] , ч т о п о з в о л я е т о р и е н т и р о в о ч н о о ц е н и т ь э л е к т р о н о а к ц е п т о р н у ю с п о с о б н о с т ь ф е н и л б а р е н и л ь н о й г р у п п ы .

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И м е н н о ф е н и л б а р е н и л ь н а я г р у п п а как а к ц е п т о р с и л ь н е е т р и ф т о р м е т и л ь н о й г р у п п ы , п р и м е р н о р а в н а п е н т а ф т о р ф е н и л ь н о й и з н а ч и т е л ь н о у с т у п а е т т р и -н и т р о м е т и л ь н о й .

И н т е р е с н о о т м е т и т ь , с у щ е с т в е н н у ю р а з н и ц у в с п е к т р а х фенилбаренил-т р и ф е н и л о л о в а (N91) и б а р е н и л т р и ф е н и л о л о в а (№8). С а м ф а к т влияния з а -м е с т и т е л я , н а х о д я щ е г о с я в ß- положении к а т о м у о л о в а , в м е с с б а у э р о в с к о й спектроскопии оловоорганических соединений наблюдается впервые* . Одна-ко , у в е л и ч е н и е х и м и ч е с к о г о (изомерного) с д в и г а и квадрупольного р а с щ е п -л е н и я при з а м е н е в т о р о г о Н(С ) - а т о м а на фенил не м о ж е т б ы т ь о б ъ я с н е н о с точки з р е н и я о б ы ч н ы х п р е д с т а в л е н и й об индукционном влиянии . Т р у д н о д о п у с т и т ь , ч т о у в е л и ч е н и е а к ц е п т о р н о й с п о с о б н о с т и ф е н и л б а р е н и л ь н о г о о с т а т к а по сравнению с б а р е н и л ь н ы м происходит и з - з а э л е к т р о н о а к ц е п т о р -н о г о влияния фенильной г р у п п ы , к о т о р о е не м о ж е т б ы т ь с т о л ь з н а ч и т е л ь -н ы м . В о з м о ж н о , причина э т о г о отличия з а к л ю ч а е т с я в к а к о м - т о с п е ц и ф и -ч е с к о м в з а и м о д е й с т в и и э л е к т р о н н ы х с и с т е м б а р е н о в о г о и фенильного я д е р .

В т е х с л у ч а я х , к о г д а с б а р е н о в ы м я д р о м с в я з а н ы д в а а т о м а о л о в а (№10) , п р о и с х о д и т з а к о н о м е р н о е у м е н ь ш е н и е е г о в л и я н и я н а к а ж д ы й и з а т о м о в Sn (Д п а д а е т до 0 ,7 м м / с е к ) ; в к о л ь ч а т о й с и с т е м е , о б р а з о в а н н о й д в у м я а т о м а м и Sn и д в у м я б а р е н и л е н о в ы м и г р у п п а м и (N94) э т о в л и я н и е вновь с т а н о в и т с я з н а ч и т е л ь н ы м . До сих пор не были изучены м е с с б а у э р о в -с к и е с п е к т р ы о л о в о о р г а н и ч е с к и х соединений, о б р а з о в а н н ы х д в у х в а л е н т н ы м о л о в о м . О л о в о д и а л к и л ы R 2 S n , содержащие ф о р м а л ь н о д в у х в а л е н т н о е олово , в д е й с т в и т е л ь н о с т и п о л и м е р н ы [ ( R 2 S n ) x ] и с о д е р ж а т ч е т ы р е х в а л е н т -н о е о л о в о [16] . Х а р а к т е р н о , ч т о с в е ж е п р и г о т о в л е н н о е д и ф е н и л о л о в о в отличие от е г о а л и ф а т и ч е с к и х а н а л о г о в мономерно [18] , что , п о - в и д и м о м у , с в я з а н о с р а с с р е д о т о ч е н и е м д в у х S - э л е к т р о н о в д в у х в а л е н т н о г о о л о в а по ф е н и л ь н ы м к о л ь ц а м . У ч и т ы в а я с и л ь н ы й э л е к т р о н о а к ц е п т о р н ы й х а р а к т е р ф е н и л б а р е н о в ы х г р у п п , м о ж н о было о ж и д а т ь , ч т о и д и ф е н и л б а р е н и л о л о в о о к а ж е т с я не м е н е е с т а б и л ь н ы м ,

М е с с б а у э р о в с к и е с п е к т р ы соединений двух- и ч е т ы р е х в а л е н т н о г о олова р е з к о р а з л и ч а ю т с я [7] . Химический с д в и г линий органических и н е о р г а н и -ч е с к и х п р о и з в о д н ы х ч е т ы р е х в а л е н т н о г о о л о в а о т н о с и т е л ь н о tf-Sn ( с е р о г о олова с целиком к о в а л е н т н ы м и ч е т ы р ь м я гибридными Sp 3 - с в я з я м и ) о т р и ц а -т е л е н , т о г д а к а к п р о и з в о д н ы е д в у х к о в а л е н т н о г о о л о в а о б л а д а ю т п о л о ж и -т е л ь н ы м и о т н о с и т е л ь н о a - S n х и м и ч е с к и м и с д в и г а м и , т . е . б г 2 ,1 м м / с е к о т н о с и т е л ь н о S n 0 2 . Е с т е с т в е н н о было о ж и д а т ь , что аналогичное р а з л и ч и е проявится и в о л о в о о р г а н и ч е с к и х соединениях . Д е й с т в и т е л ь н о , по величине х и м и ч е с к о г о с д в и г а д и ф е н и л б а р е н и л о л о в о , п о л у ч е н н о е д е й с т в и е м ф е н и л -барениллития на б е з в о д н о е двухлористое олово в э ф и р е , м о ж е т быть н а д е ж -но о т н е с е н о к п р о и з в о д н ы м д в у х в а л е н т н о г о о л о в а ( с м . р и с . , к р и в а я а ) .

При стоянии э ф и р н о г о р а с т в о р а дифенилбаренилолова постепенно начал о б р а з о в ы в а т ь с я о с а д о к , и с п е к т р с у с п е н з и и принял вид , и з о б р а ж е н н ы й на кривой б . Форма с п е к т р а р е з к о отлична от лоренцевской . Указанное и з м е -нение с п е к т р а м о г л о б ы т ь о б у с л о в л е н о ч а с т и ч н о й п о л и м е р и з а ц и е й д в у х в а -л е н т н о г о с о е д и н е н и я . Для проверки э т о г о предположения ч е р е з 7 с у т о к после

* Е с л и не с ч и т а т ь т р и а л к и л а ц е т о н а т о в о л о в а , г д е , однако , возникновение к в а д р у п о л ь -ного р а с щ е п л е н и я о б у с л о в л е н о не просто в в е д е н и е м кислорода в ß - п о л о ж е н и е к а т о м у олова , но о б р а з о в а н и е м координационной с в я з и э т о г о кислорода с о л о в о м [15] .

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Р и с . М е с с б а у э р о в с к и е с п е к т р ы олова в н е к о т о р ы х производных б а р е н о в .

П А Р А М Е Т Р Ы М Е С С Б А У Э Р О В С К И Х С П Е К Т Р О В ОЛОВА В Р Я Д Е П Р О И З В О Д Н Ы Х Б А Р Е Н О В

№ п / п

Соединение Д, м м / с е к 6, м м / с е к № п / п

Д, м м / с е к 6, м м / с е к

1 . C e H 5 C - C - S n ( C 6 H 5 ) 3

в 1 0 н 1 0

1 , 2 0 ± 0 , 0 6 1,30 ± 0 , 0 7

2 . ( C 3 H 7 ) 3 S n - C - C H V ' / В10Н10

1,65 ± 0 , 0 8 1,45 ± 0 , 0 7

3 . (С 3 Н 7 )3 S n - C ^ C - S n ( C 3 H , ) 3

В 10 Н 10

1,50 ± 0 , 0 5 1,45 ± 0 , 0 5

B i o H i o ' ¡ Л

( С 4 Н 9 ) 2 S n C ^ I ^ S n ( C 4 H 9 )2

в 1 0 н 1 0

4 .

B i o H i o ' ¡ Л

( С 4 Н 9 ) 2 S n C ^ I ^ S n ( C 4 H 9 )2

в 1 0 н 1 0

1,60 ± 0 , 0 4 1,20 ± 0 , 0 3

5 . ( С 6 Н 6 С - С ) 3 — SnCl v v В 10 Н 10

0,4 ± 0 , 0 4 1,20 ± 0 , 0 5

6 . ( C 6 H 5 C - C ) 2 S n C l 2 \ - / В ю Н 1 0

0,90 ± 0 , 0 5 1,25 ± 0 , 0 6

7 . ( С 6 Н 5 С - C ) 2 S n B r 2 \ с /

В ю н 1 0

0 ,80 ± 0 , 0 4 0 ,90 ± 0 , 0 5

8 . H C - C - S n ( C 6 H 5 ) 3 0 ,95 ± 0 , 0 5 1,05 ± 0 , 0 5 V ' / В 10Н10

9 . ( C 6 H 5 C 7 C ) 2 S n ( C 4 H 9 ) 2 V - / В10Н10

1,70 ± 0 , 0 8 1,20 ± 0 , 0 6

10. (С6 Н5 )3 Sn - С - С - Sn(C 6 Н5 )3 \ t > / В ю Н ю

0,70 ± 0 , 0 4 0 ,95 ± 0 , 0 5

11 . C g H s C - C - S n f C s H T b

В10Н10

1,50 ± 0 , 0 7 1,35 ± 0 , 0 5

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н а ч а л а опыта были о т д е л ь н о с н я т ы с п е к т р ы р а с т в о р а и о с а д к а (кри-в ы е в и г ) . О к а з а л о с ь , ч т о о с а д о к с о д е р ж и т ч е т ы р е х в а л е н т н о е о л о в о (6 = 1,35 м м / с е к ; А = 1,30 м м / с е к ) , т о г д а к а к в р а с т в о р е п о - п р е ж н е м у н а -х о д и т с я д в у х в а л е н т н о е о л о в о (6 = 2 ,95 м м / с е к ; Д = 1 , 9 0 м м / с е к ) . С п е к т р р а с т в о р а , о т д е л е н н о г о от о с а д к а , о с т а в а л с я н е и з м е н н ы м в т е ч е н и е д в у х н е д е л ь .

Т а к и м о б р а з о м , дифенилбаренилолово д е й с т в и т е л ь н о о к а з а л о с ь в е с ь м а у с т о й ч и в ы м с о е д и н е н и е м , полимеризация к о т о р о г о п р о т е к а е т о т н о с и т е л ь н о м е д л е н н о . Д а н н ы е о химических с в о й с т в а х дифенилбаренилолова и кинети-ке е г о полимеризации будут опубликованы о т д е л ь н о .

Л И Т Е Р А Т У Р А

[1] ЗАХАРКИН Л .И . , CTAHKO В . И . , Б Р А Т Ц Е В B . C . , ЧАПОВСКИЙ Ю . А . и О Х Л О Б Ы С -ТИН О . Ю . И з в . АН С С С Р , с е р . х и м . 1 9 6 3 , 2 2 3 9 .

[2] З А Х А Р К И Н Л . И . , С Т А Н К О В . И . , Б Р А Т Ц Е В В . А . , Ч А П О В С К И Й Ю . А . . К Л И М О В А А . И . , О Х Л О Б Ы С Т И Н О . Ю. и П О Н О М А Р Е Н К О A . A . Д о к л . АН С С С Р . 1 5 5 . 6 ( 1 9 6 4 ) .

[3] ЗАХАРКИН Л . И . , Б Р Е Г А Д З Е В . И О Х Л О Б Ы С Т И Н О . Ю . И з в . АН С С С Р , с е р . х и м . 1964, 1539 .

[4] HEYING T . L . , AGEH J . W . , C L A R K С. L . , MANGOLD D . , GOLDSTEIN H . , HILLMAN M . , P O L A K R . , SZYMANSKY J . , I n o r g . C h e m . 2^ 1089 (1963).

[5] F E I N M . , BOBINSY J . , W A Y E S N . . S C H W A R T Z N . . C O H E N M . , I n o r g . C h e m . 2, 1111 (1963) .

[6] З А Х А Р К И Н Л . И . , Б Р Е Г А Д З Е В . И . , О Х Л О Б Ы С Т И Н О . Ю . / в п е ч а т и / . 17] Г О Л Ь Д А Н С К И Й В . И . , " Э ф ф е к т М е с с б а у э р а и е г о п р и м е н е н и е в х и м и и " , И з д а т . АН

С С С Р , 1 9 6 3 . [8] М И Т Р О Ф А Н О В К . П . , Ш П И Н Е Л Ь В . С . , И Л Л А Р И О Н О В А H . A . , П Т Э , № 3 ( 1 9 6 3 ) . [9] А Л Е К С А Н Д Р О В А . Ю . , О Х Л О Б Ы С Т И Н О . Ю . , ПОЛАК Л . С . , Ш П И Н Е Л Ь B . C . ДАН

С С С Р . (101 А Л Е К С А Н Д Р О В А . Ю. , М И Т Р О Ф А Н О В К . П . , О Х Л О Б Ы С Т И Н О . Ю . , П О Л А К Л . С . ,

ШПИНЕЛЬ B . C . , ДАН С С С Р . [11] Г О Л Ь Д А Н С К И Й В . И . , Х Р А П О В В . В . , М А К А Р О В Е . Ф . , 3 6 , 4 6 1 ( 1 9 6 4 ) . [12] C O R D E Y - H A Y E S M . , J . i n o r g . n u c l . C h e m . 26 , 2 3 0 7 ( 1 9 6 4 ) . [13] Ф Е Д И Н Э . И . , С Е М И Н Г . К . , Ж . с т р у к т у р . х и м и и , 4 G 4 ( 1 9 6 0 ) . [14] С Е М И Н Г . К . , Ф А Й Н З И Л Ь Б Е Р Г А . Я . , Ж . с т р у к т у р , х и м и и , 5 , ( 1 9 6 5 в п е ч а т и ) . [15] GOLDANSKII V . l . , T h e M ö s s b a u e r E f f e c t and i t s A p p l i c a t i o n s in C h e m i s t r y , C o n s u l t a n t s

B u r e a u , New Y o r k , 1964. [16] Г О Л Ь Д А Н С К И Й В . И . , Р О Ч Е В В . Я . , Х Р А П О В B . B . , ДАН С С С Р , 1 5 6 , 9 0 9 ( 1 9 6 4 ) .

D I S C U S S I O N

H. F R A U E N F E L D E R ( C h a i r m a n ) s a i d t h a t a s a n " i r o n m a n " h e w a s i m p r e s s e d b y t h e n u m b e r of t i n c o m p o u n d s s t u d i e d , bu t w a n t e d t o a s k how m u c h h a d M ö s s b a u e r e f f e c t a d d e d to t h e c h e m i s t r y of t i n .

A g e n e r a l d i s c u s s i o n f o l l o w e d , t h e m a i n p o i n t s of w h i c h w e r e : R . H . H E R B E R s u g g e s t e d a n a n a l o g y w i t h t h e e a r l y d a y s of n u c l e a r

m o d e l s ; i n b o t h c a s e s a g r e a t d e a l p f e x p e r i m e n t a l d a t a w e r e c o l l e c t e d and t h e g e n e r a l p a t t e r n s o b s e r v e d f o r t h e d e c a y s c h e m e s e n a b l e d d e v e l o p m e n t of n u c l e a r m o d e l s t o be m a d e l a t e r .

V . l . G O L D A N S K I I a g r e e d i n p r i n c i p l e a n d s a i d t h a t n o w a t r a n s i t i o n f r o m q u a n t i t y t o q u a l i t y c o u l d be o b s e r v e d . T h e n u m e r o u s s p e c t r o s c o p i c d a t a h a d a l r e a d y g i v e n s o m e n e w and i m p o r t a n t r e s u l t s on t h e s t r u c t u r e of

1 7 2

t i n h a l i d e s , t i n - o r g a n i c o x i d e s a n d o t h e r t i n - o r g a n i c c o m p o u n d s , o n t h e n a t u r e of t h e S n - C bond in c a r b o r a n e s and on the b e h a v i o u r of Sn c o m p o u n d s in s o m e s o l v e n t s and in s o m e m a g n e t i c m a t e r i a l s .

ISOMER SHIFTS IN RARE EARTH S

W. HENNING, S. HÜFNER, P. KIENLE, D. Q U I T M A N N A N D E. STEICHELE TECHNISCHE HOCHSCHULE,

DARMSTADT, FEDERAL REPUBLIC OF GERMANY

I s o m e r s h i f t s of r e c o i l l e s s e m i t t e d y - r a y s f r o m r a r e - e a r t h n u c l e i h a v e b e e n i n v e s t i g a t e d r e c e n t l y [1-12] . T h e y h a v e b e e n m e a s u r e d b e t w e e n d iva len t , t r i v a l e n t , m e t a l l i c and i n t e r m e t a l l i c r a r e - e a r t h c o m p o u n d s . T h e p r i m a r y i n t e r e s t w a s c o n c e n t r a t e d on e x p l o i t i n g t h e n o v e l n u c l e a r i n f o r m a t i o n abou t the change of t he n u c l e a r c h a r g e d i s t r i b u t i o n r e s u l t i n g f r o m the y - t r a n s i t i o n .

T h i s p a p e r d o e s not d e a l wi th t h e s e r e s u l t s but d i s c u s s e s p r o b l e m s c o n -c e r n i n g :

(a) T h e v a l i d i t y of t h e a c c e p t e d b a s i c c o n c e p t s of t h e i s o m e r s h i f t ; (b) T h e c a l i b r a t i o n of e l e c t r o n d e n s i t i e s ; (c) T h e a p p l i c a t i o n of i s o m e r - s h i f t r e s u l t s t o s t u d y b o n d i n g i n r a r e -

e a r t h c o m p o u n d s .

(a) Concepts of the isomer shift

F o r t h e i n t e r p r e t a t i o n of t h e i s o m e r s h i f t of n u c l e a r y - r a y s o n e c a n f a v o u r a b l y m a k e u s e of t h e a d v a n c e d t h e o r i e s [13] p r o p o s e d f o r t h e i s o t o p e sh i f t of o p t i c a l l i n e s , which m e a s u r e t h e c o m p l e m e n t a r y e f f e c t in t he a t o m i c s h e l l s . F o l l o w i n g t h e s e t h e o r i e s t h e i s o m e r s h i f t i s g i v e n i n f i r s t o r d e r p e r t u r b a t i o n t h e o r y b y :

6E = EX - E 2 = (2тг/3) Z e2 [ I - | ^ ( 0 ) | 2 ] 6 < г 2 > (1)

wi th

6 < r 2 > = < r 2 > e - < r 2 > g (2)

and

x r 2 \ s / p ( r ) r 2 d T ' / p ( r ) d T

E i 2 a r e t h e - y - t r a n s i t i o n e n e r g i e s i n e l e c t r o n c o n f i g u r a t i o n s h a v i n g t o t a l e l e c t r o n d e n s i t i e s | ^ ( 0 ) | 2 "2 a t t h e n u c l e u s w h i c h h a v e m e a n s q u a r e c h a r g e r a d i i < . r 2 > e ^ i n t h e e x c i t e d s t a t e (e ) a n d t h e g r o u n d s t a t e (g) , d e f i n e d b y r e l a t i o n (3); p ( r ) d e n o t e s t h e n u c l e a r c h a r g e d e n s i t y . R e l a t i o n (1) i s e x a c t

173

t o t h e f i r s t o r d e r , w h e n t h e e l e c t r o n d e n s i t y s t a y s c o n s t a n t i n t h e r e g i o n of t h e n u c l e u s . T h e d e c r e a s e of t h e e l e c t r o n d e n s i t y t o w a r d s t h e e d g e of t h e n u c l e u s i s t a k e n i n t o a c c o u n t by a s m a l l c o r r e c t i o n [ 1 3 ] .

T h e e f f e c t of a p o l a r i z a t i o n of t h e n u c l e u s b y t h e e l e c t r o n s i s n o t c o n t a i n e d in r e l a t i o n (1) . S u c h e f f e c t s h a v e b e e n s u g g e s t e d l o n g a g o by B r e i t e t a l . [14] a n d h a v e b e e n r e c o n s i d e r e d r e c e n t l y b y R e i n e r a n d W i l e t s [ 1 5 ] , w h o e s t i m a t e d t h e q u a d r u p o l e p o l a r i z a b i l i t y of d e f o r m e d n u c l e i . T h o u g h t h i s e f f e c t d o e s n o t s e e m t o b e n e g l i g i b l e i t i s s m a l l a n d w a s n o t i n v e s t i -g a t e d e x p e r i m e n t a l l y u n t i l r e c e n t l y [ 7 ] . Y e t q u i t e r e c e n t l y A t z m o n y a n d O f e r [7] r e p o r t e d a n o m a l o u s i s o m e r s h i f t s of t h r e e - y - t r a n s i t i o n s wh ich s e e m t o i n d i c a t e e f f e c t s in t h e i s o m e r s h i f t u n t i l n o w not c o n s i d e r e d . T h e r e s u l t s on i s o m e r s h i f t s of t h e 21. 7 -keV -y - r ays of 1 5 1 E u , t h e 9 7 - k e V and t h e 103-keV • y - t r a n s i t i o n s i n 1 5 3 E u m e a s u r e d i n E u m e t a l a n d EuSC>4 r e l a t i v e t o EU2O3 s e e m t o s h o w t h a t t h e r a t i o of t h e t w o s h i f t s a r e u n e q u a l i n a l l t h r e e t r a n -s i t i o n s , i n d i c a t i n g a d r a s t i c b r e a k d o w n of r e l a t i o n (1) . W e d i d n o t n o t i c e s u c h a n e f f e c t in a s i m i l a r i n v e s t i g a t i o n r e p o r t e d r e c e n t l y , but t h i s m a y h a v e b e e n b e c a u s e o u r e x p e r i m e n t w a s not s u f f i c i e n t l y s e n s i t i v e t o d e t e c t s u c h a n e f f e c t . P r e s e n t e d h e r e a r e p r e l i m i n a r y r e s u l t s of a r e i n v e s t i g a t i o n of t h e e f f e c t r e p o r t e d by A t z m o n y and O f e r [ 7 ] . T h e s e r e s u l t s c o m p a r e t h e i s o m e r s h i f t s of t h e 2 1 . 7 - k e V t r a n s i t i o n i n i s i E u and t h e 97 . 5 - k e V Y - r a y s of ^ E u . T o do t h i s f a v o u r a b l y , w e p l o t t e d i n F i g . 1 t h e i s o m e r s h i f t of t h e 2 1 . 7 - k e V l i n e i n s o m e c o m p o u n d s r e l a t i v e t o Е и 2 О з v e r s u s t h e s h i f t of t h e 97 . 5 - k e V l i n e i n t h e s a m e c o m p o u n d s . A l l e x i s t e n t d a t a a r e i n c l u d e d . If r e l a t i o n (1) h o l d s f o r t h e i s o m e r s h i f t , a l l p o i n t s shou ld l i e on a s t r a i g h t l i ne t h r o u g h t h e o r i g i n d e t e r m i n e d by t h e E u 2 0 3 r e f e r e n c e p o i n t s . T h i s c o n d i t i o n s e e m s t o b e f u l -f i l l e d f o r a l l m e a s u r e m e n t s w i t h i n t h e l i m i t s of e r r o r s , a l t h o u g h t h e r e m i g h t

FIG. 1. Isomer shift of the 97. 5-keV y-rays of 153Eu versus the isomer shift of the 21. 7-keV y-rays of " 'Eu

2.0

ДЕ (97keV) [cm/s]

174

be an i n d i c a t i o n t h a t EUSO4 b e h a v e s s o m e w h a t i r r e g u l a r l y . One s h o u l d no t e t h a t t h e i s o m e r s h i f t of t h e 21 . 7 - k e V y - r a y s i n EUSO4 h a s b e e n m e a s u r e d a t r o o m t e m p e r a t u r e , w h e r e a s a l l t h e s h i f t s of t h e 9 7 - k e V l i n e h a v e b e e n d e t e r m i n e d a t 4 . 2 ° K . P r e v i o u s e x p e r i m e n t s s h o w e d t h a t t h e i s o m e r s h i f t of t h e 21 . 7 - k e V l i n e i n EUCI2» E u S a n d E u m e t a l d id not c h a n g e n o t i c e a b l y a t l o w t e m p e r a t u r e s c o m p a r e d w i t h t h e r e s u l t s g a i n e d a t r o o m t e m p e r a t u r e . T h i s m i g h t no t h o l d f o r E u S 0 4 . T h e p r e s e n t r e s u l t s d o not s e e m t o s u p p o r t t h e a n o m a l o u s i s o m e r s h i f t of t h e 21 . 7 - k e V and t h e 9 7 - k e V t r a n s i t i o n s , a l -t h o u g h i t i s s t i l l p r e m a t u r e t o d e r i v e a f i n a l c o n c l u s i o n i n v i e w of t h e u n -c e r t a i n t i e s of t h e p r e s e n t r e s u l t s . W e a r e in t h e p r o c e s s of i m p r o v i n g t h e e x p e r i m e n t a l d a t a , i n c l u d i n g m e a s u r e m e n t s of t h e i s o m e r s h i f t of t h e 1 0 3 - k e V t r a n s i t i o n i n 153EU.

(b) The calibration of electron densities

In a p r e v i o u s p a p e r w e p r o p o s e d a c a l i b r a t i o n s c h e m e f o r t h e i s o m e r s h i f t s i n E u a n d s u g g e s t e d i t s e x t e n s i o n t o i s o m e r s h i f t s f o r 7 - r a y s of o t h e r r a r e - e a r t h n u c l e i m e a s u r e d b e t w e e n t h e m e t a l s and t h e c o r r e s p o n d i n g i o n i c c o m p o u n d s [ 6 ] . T h e c h a n g e of t h e e l e c t r o n d e n s i t i e s a t t h e n u c l e u s b e t w e e n t h e e l e c t r o n c o n f i g u r a t i o n s 4 f 6 5 s 2 5 p 6 ( E u 3 + ) a n d , 4 f 5 s 2 5 p 6 (Eu2+) w a s d e -d u c e d f r o m i s o t o p e s h i f t d a t a of S m a t o m s a n d h y p e r f i n e s t r u c t u r e r e s u l t s on 1 5 1EU . T h e r e s u l t of t h i s c a l i b r a t i o n w a s :

1 0 ( 0 ) 1 « . - k ( 0 ) | 2f , = 1 . 9 X 1 0 * « c m " 3

T h e d e n s i t y of t h e c o n d u c t i o n e l e c t r o n s a t t h e n u c l e u s f o r E u m e t a l w a s a l s o d e d u c e d b y t h i s c a l i b r a t i o n a n d t h e i s o m e r s h i f t of t h e 21 . 7 - k e V y - r a y s i n E u m e t a l w i t h r e s p e c t t o d i v a l e n t E u . T h e r e s u l t w a s :

I ^ ( ° ) I C E = 0 . 9 X 1 0 « c m " 3

It w a s t h e n p r o p o s e d t o d e t e r m i n e t h e c o n d u c t i o n e l e c t r o n d e n s i t y at t h e n u c -l e u s i n a l l r a r e - e a r t h m e t a l s b y u s i n g t h e r e s u l t f o r E u m e t a l m u l t i p l i e d b y t h e r a t i o of t h e c o n d u c t i o n e l e c t r o n d e n s i t i e s d e t e r m i n e d f r o m a t o m i c v o l u m e s of t h e r a r e - e a r t h m e t a l s r e l a t i v e to E u . P o s i t r o n ann ih i l a t i on r a t e s [16] in r a r e - e a r t h m e t a l s i n d i c a t e t h a t t h e c o n d u c t i o n e l e c t r o n d e n s i t i e s a r e i n v e r s e l y p r o p o r t i o n a l t o t h e d i f f e r e n c e s of t h e a t o m i c and t h e ion ic v o l u m e s . With t h e i n t e n t i o n t o t e s t t h i s c a l i b r a t i o n s c h e m e , s t u d i e s w e r e m a d e on t h e i s o m e r s h i f t of t h e 2 2 - k e V 7 - r a y s of 1 4 9 S m i n S m m e t a l a n d d i v a l e n t S m C l 2 r e l a t i v e t o t r i v a l e n t Э т г О з . T h e r a t i o of t h e s e s h i f t s w o u l d b e p r e d i c t e d b y o u r c a i i D r a n o n t o :

E ( m e t a l ) - E ( S m 3 f ) K E ( S m 2 + ) - E ( S m 3 + )

U n f o r t u n a t e l y 6 < r 2 > = 3X10" 4 f m 2 i s s o s m a l l f o r t h e 2 2 - k e V t r a n s i t i o n in 1 4 9 S m t h a t t h e s h i f t s o b s e r v e d a m o u n t t o only about 3% of t h e e x p e r i m e n t a l l i n e - w i d t h . T h e r a t i o of t h e s h i f t s m e a s u r e d i s

175

T h e e r r o r s q u o t e d i n c l u d e o n l y t h e s t a t i s t i c a l e r r o r s of t h e m e a s u r e m e n t . T h e r e s u l t d o e s n o t c o n t r a d i c t t h e p r e d i c t i o n of t h e c a l i b r a t i o n s c h e m e .

(c) The application of isomer shift results to study bonding in rare-earth compounds

A s u m m a r y of i s o m e r s h i f t s o b s e r v e d f o r t h e 21 . 7 - k e V 7 - r a y s of 1 5 1 E u i s g i v e n i n T a b l e I . T h e s e s h i f t s a r e m e a s u r e d r e l a t i v e t o E u F 3 . T h e r e -s u l t s exh ib i t s o m e i n t e r e s t i n g f e a t u r e s . T h e l a r g e n e g a t i v e s h i f t s in d i v a l e n t c o m p o u n d s ( 4 f 7 5 s 2 5 p 6 ) a r e a t t r i b u t e d t o a s m a l l e r e l e c t r o n d e n s i t y a t t h e

TABLE I

I S O M E R S H I F T S F O R T H E 2 1 . 7 - k e V 7 - L I N E O F 151EU T h e o b s e r v e d D o p p l e r s h i f t s v a r e g i v e n r e l a t i v e t o E u F 3

Compound v (cm/s) Compound v (cm/s)

Eu2 S3 + 0.14 [3] EuS - 1.17 [3]

EU 2Te з + 0.10 [3] EuTe - 1.25 [3 ]

EuIG a + 0. 050 [5] EuSe - 1.22 [3]

Eu(C2H5 SO4 )a + 0. 025 [5] EU(C2H5)2 - 1.32 [5]

Eu2 o3 0. 005 EUC12 - 1.34 [3]

Eu metal - 0. 78 [1] EUC03 - 1.37 [6]

Eu m e t a l b - 0. 82 [3] Èu(OH)2 - 1.49 [5]

EuOb - 1. 09 [5] EUSO4 - 1. 5

EuO - 1. 18 [6] EUS04 - 1.43 [8]

EuSb - 1. 12 [5]

a Europium iron garnet b Ref. [1] c Ref. [4]

n u c l e u s t h a n i n t r i v a l e n t E u ( 4 f 6 5 s 2 5 p 6 ) . I t i s m a i n l y p r o d u c e d b y t h e i n -

c r e a s e d s h i e l d i n g of t h e s e l e c t r o n s in t h e 4f7 c o m p a r e d w i t h t h e 4 f 6 e l e c t r o n

c o n f i g u r a t i o n . T h e i s o m e r s h i f t s b e t w e e n t h e t r i v a l e n t c o m p o u n d s a r e e x -

t r e m e l y s m a l l b u t s t i l l m e a s u r a b l e . T h e l a r g e s t s h i f t , w h i c h i s e x p e r i -

m e n t a l l y c e r t a i n ( (0 . 0 5 0 ± 0 . 0 0 7 ) c m / s b e t w e e n Е и г 0 3 a n d E u I G ) i s on ly a b o u t 3% of t h e s h i f t b e t w e e n E u 3 + a n d d i v a l e n t E U S O 4 . T h i s r e v e a l s o n l y a v e r y s m a l l d i f f e r e n c e in t h e i n f l u e n c e of t h e l i g a n d b o n d s on t h e e l e c t r o n c o n f i g u -r a t i o n of t r i v a l e n t E u . It s e e m s s u r p r i s i n g t h a t e v e n t h e 6 s c h a r a c t e r of t h e c h e m i c a l b o n d d o e s no t c h a n g e n o t i c e a b l y . T h e r e s u l t s a r e d r a s t i c a l l y d i f f e r e n t i n d i v a l e n t E u , w h i c h d i f f e r s f o r m a l l y o n l y b y o n e a d d i t i o n a l 4f e l e c t r o n f r o m E u 3 + . T h e i s o m e r s h i f t s i n v a r i o u s c o m p o u n d s c h a n g e u p t o 0 . 4 c m / s w h i c h i s 27% of t h e l a r g e s t o b s e r v e d s h i f t b e t w e e n E u 2 + a n d

176

Eu3+. It i s r e m a r k a b l e t h a t t h e c o m p o u n d s w i t h s t r o n g l y e l e c t r o n e g a t i v e l i g a n d s ( e . g . C I " , SOf") h a v e t h e s m a l l e s t e l e c t r o n d e n s i t i e s a t t h e E u n u c l e u s . T h i s d i f f e r e n c e i n b e h a v i o u r of t h e i s o m e r s h i f t i n c o m p a r i s o n w i t h t r i v a l e n t c o m p o u n d s i s d i f f i c u l t t o u n d e r s t a n d . It m a y be c a u s e d p a r t i a l l y by t h e f a c t t h a t t h e a d d i t i o n a l 4f e l e c t r o n i n c r e a s e s t h e r a d i a l e x -t e n s i o n of t h e 4f and t h e o t h e r o u t e r s h e l l s , w h i c h m a k e s t h e m m o r e s u i t e d f o r c o v a l e n t b o n d i n g . Only s u c h o r b i t a l s t h a t i n c r e a s e t h e e l e c t r o n d e n s i t y a t t he n u c l e u s can p a r t i c i p a t e in t he cova l en t bond ing . T h i s e x c l u d e s a t once t h e 5 s 2 e l e c t r o n s . So i t i s s u g g e s t e d t h a t t h e d i f f e r e n t e l e c t r o n d e n s i t y i n t h e d i v a l e n t E u c o m p o u n d i s c a u s e d by a d i f f e r e n t d e g r e e of c o v a l e n t b o n d i n g by t h e 4f and 5p o r b i t a l s .

T h e a u t h o r s w o u l d l i k e t o t h a n k P r o f e s s o r B r i x f o r m a n y h e l p f u l d i s -c u s s i o n s a n d P r o f e s s o r H e l l w e g e f o r h i s f r i e n d l y s u p p o r t of t h i s w o r k .

R E F E R E N C E S

[1] BARRET, P. H. , SHIRLEY, D.A. , Phys. Rev. ¿31 (1963) 123. [2] SHIRLEY, D.A. , FRANKEL, R. В. . WICKMANN. H.H. , Rev. mod. Phys. 36 (1964) 392. [3] KIENLE, P. , Rev. mod. Phys. 36 (1964) 372. [4] NO WIK, I. , OFER, S. , Rev. mod. Phys. 36 (1964) 392.

[53 BAUMINGER, E. R. , GRODZINS, L. , FREEMAN, A.J. , Rev. mod. Phys. 36 (1964) 392; Bull. Amer, phys. Soc. 9 (1964) 451.

[6] BRIX, P. , HUFNER, S. , KIENLE, P. , QUITMANN, D. , Phys. Lett. W (1964) 140. [7] ATZMONY, U. , OFER, S. , Phys. Lett. 14 (1965) 284. [8] STEICHELE, E. , HUFNER, S. , KIENLE, P. , Phys. Lett. 14 (1965) 321. [9] OFER, S., SEGAL. E.,'N0WIK, I., BAUMINGER, E. R. , GRODZINS, L., FREEMAN, A.J., SCHIEBER, M. ,

Phys. Rev. 137 (1965) 627. [10] OFER, S. e t a l . , to be published in Phys. Rev. (1965). [11] FINK, J. , KIENLE, P. , Preprint (1965). [12] QUITMANN, D. , HUFNER, S. , KIENLE, P. , Phys. Verh. 5 (1965) 101. [13] FRADKIN, E. E. , Soviet Phys. JETP _15 (1960) 550. [14] BREIT, G. , ARFKEN, G.B. , GLENDENIN, W.W. , Phys. Rev. 78 (1950) 390. [15] REINER, A.S. , WILETS, L. , Nucl. Phys. 36 (Í962) 457. [16] RODDA, J.L. , STEWART, M.G. , Phys. Rev. 131 (1963) 131.

D I S C U S S I O N

P . H I L L M A N a s k e d w h e t h e r t he a u t h o r s 1 d a t a w e r e in c o n t r a d i c t i o n wi th t h o s e of O f e r [Ref . 7]

P . K I E N L E r e f e r r e d t o t h e f i g u r e in h i s p a p e r a n d s a i d t h a t t h e d a t a w a s i n a g r e e m e n t but d i f f e r e n t c o n c l u s i o n s w e r e m a d e .

J . DANON s u g g e s t e d t h a t t h e d i f f e r e n c e in s p r e a d of t he i s o m e r s h i f t s f o r t h e d i v a l e n t and t r i v a l e n t c o m p o u n d s m i g h t b e due t o d i f f e r e n c e s in t h e c o n t r i b u t i o n s f r o m 6s e l e c t r o n s ; t he 6s s h e l l would be r e l a t i v e l y m o r e f i l l e d e v e n in t h e m o r e i o n i c t r i v a l e n t r a r e - e a r t h c o m p o u n d s .

V. I . GOLDANSKII a s k e d what w e r e t h e a b s o l u t e v a l u e s of SR. P . K I E N L E g a v e v a l u e s

f o r 1 5 1 E u 6<+ 2 > = + 0. 030 ± 0. 01 ( F e r m i ) 2

f o r 1 5 3 E u 6<+ 2 > = - 0. 17 ± 0. 05 f o r t he 9 7 - k e V t r a n s i t i o n = - 0 . 18 i 0. 05 f o r t he 103 -keV t r a n s i t i o n .

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G. К . W E R T H E I M p o i n t e d out t h a t d i v a l e n t E u w a s s t a b l e only b e c a u s e one h a d a h a l f - f i l l e d s h e l l a n d s u g g e s t e d t h a t t h i s i s of i m p o r t a n c e i n t h e i n t e r p r e t a t i o n of t h e d a t a .

P . K I Ê N L E r e p l i e d t h a t t h e r a d i c a l e x p a n s i o n of 4 f 7 w a s l a r g e r t h a n 4 f 6 , a n d a d d e d t h a t e l e c t r o s t a t i c e f f e c t s a r i s i n g f r o m t h e h a l f - f i l l e d s h e l l m i g h t b e i m p o r t a n t .

H. F R A U E N F E L D E R ( C h a i r m a n ) conc luded tha t t h e r e w a s a good f u t u r e f o r the M ö s s b a u e r e f f e c t in the r a r e - e a r t h c h e m i s t r y , e s p e c i a l l y if the s m a l l e f f e c t s could be e n l a r g e d by u s i n g s c a t t e r i n g g e o m e t r y .

P . K I E N L E a g r e e d , bu t one l i m i t a t i o n i n c h e m i s t r y w a s f o r t h e o d d -n e ü t r o n i s o t o p e s i n w h i c h 6 R / R w a s s m a l l .

J . DANON a s k e d if t h e r e w a s any da ta f r o m i so tope s h i f t s on the n u c l e a r p o l a r i z a t i o n e f f e c t s .

P . K I E N L E s t a t e d t h e r e s e e m e d t o b e no e x p e r i m e n t a l l y e s t a b l i s h e d e x i s t e n c e f o r s u c h e f f e c t s .

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STRUCTURAL INVESTIGATIONS (continued)

(Sess ion 5)

MÖSSBAUER E F F E C T STUDIES WITH ACTINIDES*

J . A. STONE E. I . Du PONT DE NEMOURS,& C o . , AIKEN, S. C.


A. INTRODUCTION

^ M ö s s b a u e r r e s o n a n c e s t u d i e s in t he a c t i n i d e e l e m e n t s o f f e r a new t e c h -n i q u e f o r m e a s u r i n g s o l i d - s t a t e p r o p e r t i e s t o a r e g i o n of t he p e r i o d i c c h a r t w h e r e s u c h i n f o r m a t i o n i s r e l a t i v e l y s p a r s e . It i s w e l l known t h a t t h e a c t i -n i d e s , t h e e l e m e n t s wi th a t o m i c n u m b e r s f r o m 90 to 103, f o r m a t r a n s i t i o n s e r i e s d u e t o f i l l i n g of t h e 5f e l e c t r o n s h e l l , a n a l o g o u s t o t h e r a r e - e a r t h s e r i e s i n w h i c h t h e 4f s h e l l i s f i l l e d . L i k e t h e r a r e e a r t h s , t h e a c t i n i d e m e t a l s a n d c o m p o u n d s a r e e x p e c t e d t o e x h i b i t a v a r i e t y of i n t e r e s t i n g m a g n e t i c p r o p e r t i e s , bu t , u n l i k e t h e r a r e e a r t h s , t h e r e h a v e b e e n f e w s t u d i e s of t h e m a g n e t i c b e h a v i o u r of a c t i n i d e s , and t h e s e p r o p e r t i e s a r e l a r g e l y u n k n o w n ^ T h e c h e m i c a l p r o p e r t i e s of t h e a c t i n i d e s h a v e b e e n s tud i ed s o m e w h a t m o r e e x t e n s i v e l y , a n d , i n c o n t r a s t t o t h e r a r e e a r t h s , f o r m a m u l t i p l i c i t y of s t a b l e v a l e n c e s t a t e s , e s p e c i a l l y in t h e l i g h t e r m e m b e r s of t he s e r i e s i s j u s t t h e s e p r o p e r t i e s , m a g n e t i c and c h e m i c a l , f o r which t h e M ö s s b a u e r e f f e c t i s a v a l u a b l e p r o b e ; , s e n s i t i v e t o t he m a g n e t i c and e l e c t r i c e n v i r o n m e n t of a n a t o m j T h e r a r e - e a r t h s e r i e s h a s b e e n a p a r -t i c u l a r l y f r u i t f u l r e g i o n in t e r m s of t he n u m b e r of e l e m e n t s wh ich h a v e b e e n s h o w n to e x h i b i t t h e M ö s s b a u e r e f f e c t , and f o r t h i s r e a s o n t h e e x p l o i t a t i o n of t h e M ö s s b a u e r e f f e c t t o y i e l d n e w s o l i d - s t a t e a n d c h e m i c a l i n f o r m a t i o n on t h e r a r e e a r t h s i s a h igh ly a c t i v e f i e ld of r e s e a r c h t o d a y . T h e r e i s e v e r y r e a s o n t o b e l i e v e t h a t t h e a c t i n i d e s c a n be s i m i l a r l y s t u d i e d b y the M ö s s b a u e r e f f e c t .

T h e s p e c i a l n a t u r e of t he a c t i n i d e e l e m e n t s l e a d s to c o m p l i c a t i n g f a c t o r s w h i c h do not o c c u r in M ö s s b a u e r e x p e r i m e n t s wi th o t h e r e l e m e n t s . O r d i -n a r i l y o n e r e q u i r e s a s t a b l e i s o t o p e f r o m w h i c h t o f a b r i c a t e a n a b s o r b e r ; h o w e v e r , a l l of t h e a c t i n i d e i s o t o p e s a r e r a d i o a c t i v e . T h i s n e e d no t b e a s e r i o u s d i s a d v a n t a g e , b e c a u s e a n i s o t o p e w h o s e h a l f - l i f e i s l o n g c o m p a r e d wi th t h e t i m e of t he e x p e r i m e n t m a y be c o n s i d e r e d " e f f e c t i v e l y s t a b l e " f o r u s e a s a n a b s o r b e r . B e c a u s e m a n y of t h e a c t i n i d e i s o t o p e s h a v e h a l f - l i v e s

*

The information contained in this article was developed during the course of work under contract AT(07-2)-l with the US Atomic Energy Commission.

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r a n g i n g f r o m d e c a d e s t o m i l l i o n s of y e a r s , t h e " e f f e c t i v e s t a b i l i t y " c r i -t e r i o n can be m e t in a n u m b e r of i s o t o p e s t h a t a r e of i n t e r e s t f o r M ö s s b a u e r e x p e r i m e n t s . T h e r a d i o a c t i v i t y of t h e a c t i n i d e s i s a n u i s a n c e f o r m o s t of t h e p h y s i c a l m e t h o d s u s e d t o d e t e r m i n e b u l k p r o p e r t i e s ; i n M ö s s b a u e r r e -s o n a n c e s t u d i e s , h o w e v e r , t h i s r a d i o a c t i v i t y c a n be put t o u s e a s an i n t e g r a l p a r t of t h e m e a s u r e m e n t t e c h n i q u e , a s t h e s o u r c e of r e c o i l - f r e e r a d i a t i o n . T h e a b s o r b e r r a d i o a c t i v i t y wi l l s t i l l p r o v i d e unwan ted b a c k g r o u n d r a d i a t i o n , bu t in m o s t e x p e r i m e n t s i t s h o u l d be p o s s i b l e t o m i n i m i z e t h i s c o n t r i b u t i o n and to m a k e s u i t a b l e c o r r e c t i o n s f o r i t .

A f u r t h e r c o m p l i c a t i o n i s t h a t t h e p r e d o m i n a n t m o d e of d e c a y i n t h e a c t i n i d e s e r i e s i s b y t h e e m i s s i o n of a l p h a p a r t i c l e s . T h e r e c o i l e n e r g y of the r e s i d u a l n u c l e u s a f t e r a lpha e m i s s i o n i s of t he o r d e r of 100 keV, which i s s u f f i c i e n t t o r e m o v e the r e c o i l i n g a t o m f r o m i t s l a t t i c e s i t e and to d i s r u p t l o c a l c r y s t a l l i n e b i n d i n g f o r c e s . One m i g h t e x p e c t t h e p r o b a b i l i t y f o r r e -c o i l l e s s y - r a y e m i s s i o n a f t e r s u c h an e v e n t t o be v e r y s m a l l o r n e g l i g i b l e . I t h a s b e e n s h o w n e x p e r i m e n t a l l y , h o w e v e r , t h a t t h e M ö s s b a u e r e f f e c t c a n b e o b s e r v e d a f t e r a l p h a d e c a y [2]. T h u s , t h e r e i s no a p r i o r i r e a s o n t o e x -c lude a - e m i t t i n g s o u r c e s f r o m c o n s i d e r a t i o n a s c a n d i d a t e s f o r u s e in M ö s s b a u e r a b s o r p t i o n s t u d i e s .

T h o r i u m and u r a n i u m a r e t h e on ly a c t i n i d e e l e m e n t s t h a t e x i s t in n a t u r e i n s i g n i f i c a n t q u a n t i t i e s ; a l l of t h e e l e m e n t s h e a v i e r t h a n u r a n i u m a r e m a n -m a d e . T h u s t h e q u e s t i o n of a v a i l a b i l i t y of a c t i n i d e i s o t o p e s i n s u f f i c i e n t q u a n t i t i e s f o r s o u r c e s and a b s o r b e r s i s of i m m e d i a t e p r a c t i c a l i m p o r t a n c e i n d i s c u s s i n g M ö s s b a u e r e f f e c t e x p e r i m e n t s w i t h a c t i n i d e s . F o r t u n a t e l y , m a n y of t h e h e a v y e l e m e n t i s o t o p e s a r e b y - p r o d u c t s f r o m t h e o p e r a t i o n of n u c l e a r r e a c t o r s ; s i z e a b l e q u a n t i t i e s of s o m e of t h e l o n g e r - l i v e d i s o t o p e s h a v e b e e n b u i l t up in t h i s w a y . R e s e a r c h q u a n t i t i e s of t h e h e a v i e s t i s o t o p e s h a v e b e e n p r o d u c e d by s p e c i a l r e a c t o r l o a d i n g s and h i g h - f l u x n e u t r o n i r r a d i -a t i o n f o r a n e x t e n d e d p e r i o d of t i m e [3] . S o m e of t h e a c t i n i d e i s o t o p e s a r e a v a i l a b l e in c h e m i c a l l y s e p a r a t e d and i s o t o p i c a l l y e n r i c h e d f o r m s ; i n p r i n -c i p l e i t i s a l w a y s p o s s i b l e t o p e r f o r m t h e s e o p e r a t i o n s on any s a m p l e of m a -t e r i a l c o n t a i n i n g a n i s o t o p e of i n t e r e s t . T h a t t h e w o r l d ' s s u p p l y of a c t i n i d e e l e m e n t s w i l l c o n t i n u e t o i n c r e a s e i n t h e f o r e s e e a b l e f u t u r e s e e m s t o b e a j u s t i f i a b l e a s s e r t i o n .

N e a r l y e v e r y a c t i n i d e i s o t o p e i s a p o t e n t i a l c a n d i d a t e f o r M ö s s b a u e r e f f e c t e x p e r i m e n t s by the u s u a l s t a n d a r d of having a l o w - e n e r g y g a m m a t r a n -s i t i o n t o t h e g r o u n d s t a t e . I f , in a d d i t i o n , o n e a p p l i e s t h e c r i t e r i a of e f f e c t i v e s t a b i l i t y a n d r e a s o n a b l e a v a i l a b i l i t y d i s c u s s e d a b o v e , a r e a l i s t i c l i s t of a c t i n i d e M ö s s b a u e r e f f e c t c a n d i d a t e s i s o b t a i n e d . T h e s e ; y - t r a n s i t i o n s , t o -g e t h e r wi th s o m e of t h e i r p r o p e r t i e s and o t h e r r e l e v a n t i n f o r m a t i o n , a r e p r e -s e n t e d in T a b l e s I and II, w h e r e i t i s c o n v e n i e n t t o s e p a r a t e t h o s e t r a n s i t i o n s i n o d d - m a s s i s o t o p e s f r o m t h o s e i n e v e n - e v e n i s o t o p e s . A m o n g t h e y -t r a n s i t i o n s i n o d d - m a s s i s o t o p e s , g i v e n i n T a b l e I , a r e t h e s o - c a l l e d a n o m a l o u s E l t r a n s i t i o n s b e t w e e n d i f f e r e n t i n t r i n s i c s t a t e s of d e f o r m e d n u c l e i [6 ] . T h e s e t r a n s i t i o n s h a v e e s p e c i a l l y f a v o u r a b l e l i f e t i m e s a n d c o n v e r s i o n c o e f f i c i e n t s , a n d a p p e a r t o h a v e t h e g r e a t e s t p r o m i s e a s a c t i n i d e M ö s s b a u e r c a n d i d a t e s . F o r t h e e v e n - e v e n i s o t o p e s , t h e g r o u n d s t a t e s a r e a l w a y s s p i n 0, and the f i r s t e x c i t e d s t a t e i s i n v a r i a b l y t h e s p i n 2 m e m b e r of t he g r o u n d -s t a t e r o t a t i o n a l b a n d . G e n e r a l l y , t h e E2 t r a n s i t i o n b e t w e e n t h e s e two l e v e l s

180

i s t h e o n l y one of p o s s i b l e i n t e r e s t f o r M ö s s b a u e r e x p e r i m e n t s in t h e e v e n -e v e n a c t i n i d e i s o t o p e s . B e c a u s e of t h e g r e a t s i m i l a r i t y a m o n g t h e m a n y t r a n s i t i o n s of t h i s t y p e e n c o u n t e r e d in t h e a c t i n i d e s , t h e y h a v e b e e n g r o u p e d t o g e t h e r i n T a b l e I I . No s u i t a b l e g a m m a t r a n s i t i o n s a r e known i n t h e o d d -odd i s o t o p e s .

A s i n o t h e r c o m p i l a t i o n s of t h i s t y p e , t h e p o t e n t i a l u s e f u l n e s s of t h e v a r i o u s t r a n s i t i o n s l i s t e d in T a b l e s I a n d II r a n g e s f r o m m a r g i n e d t o v e r y good . P e r h a p s t h e m o s t p r o m i s i n g c a n d i d a t e in t h e a c t i n i d e s i s t h e 5 9 . 6 - k e V t r a n s i t i o n i n 2 3 7 Np. A t t h e t i m e of w r i t i n g , 237Np i s t h e o n l y a c t i n i d e i s o t o p e i n w h i c h t h e M ö s s b a u e r e f f e c t h a s b e e n o b s e r v e d [2], but i t i s a l s o t h e on ly one i n w h i c h r e p o r t e d e x p e r i m e n t s h a v e b e e n a t t e m p t e d . T h e p u r p o s e of t h e r e m a i n d e r of t h i s p a p e r i s t o r e v i e w d e v e l o p m e n t s w i t h 23TNp M ö s s b a u e r e f f e c t e x p e r i m e n t s . In t h e f o l l o w i n g s e c t i o n s an a c c o u n t of s o m e of t h e e x -p e r i m e n t a l d e t a i l s of t h i s w o r k i s g i v e n , a n d t h e c u r r e n t e x p e r i m e n t a l a n d t h e o r e t i c a l s i t u a t i o n w i t h 2 3 7Np i s p r e s e n t e d , wh i l e t h e f i n a l s e c t i o n s u r v e y s t h e g e n e r a l a r e a s w h e r e t h i s new r e s o n a n c e i s e x p e c t e d to be u s e f u l .

B . E X P E R I M E N T A L D E T A I L S

T h e f o l l o w i n g i s a d e s c r i p t i o n of a p p a r a t u s a n d p r o c e d u r e s t h a t h a v e b e e n u s e d a t t h e S a v a n n a h R i v e r L a b o r a t o r y f o r w o r k w i t h t h e M ö s s b a u e r e f f e c t i n 2 3 7 N p .

1. Velocity spectrometer

T h e SRL v e l o c i t y s p e c t r o m e t e r f o r 2 3 ' N p e x p e r i m e n t s i s a l o u d s p e a k e r -t y p e d e v i c e d r i v e n w i t h a t r i a n g u l a r v e l o c i t y w a v e f o r m i n o r d e r t o s w e e p r e p e a t e d l y t h r o u g h a r a n g e of v e l o c i t i e s w i t h a p p r o x i m a t e l y e q u a l t i m e a t e a c h v e l o c i t y . T h e i n s t r u m e n t i s o p e r a t e d i n t h e p u l s e - h e i g h t m o d e , w i t h p u l s e s f r o m a s i n g l e - c h a n n e l a n a l y s e r b e i n g m o d u l a t e d w i t h t h e v e l o c i t y w a v e f o r m . P u l s e - h e i g h t s o r t i n g i s p e r f o r m e d w i t h a n R I D L 4 0 0 - c h a n n e l a n a l y s e r t o g i v e d a t a i n t h e f o r m of c o u n t i n g r a t e a s a f u n c t i o n of v e l o c i t y . R e a d - o u t i s on p u n c h e d c a r d s s u i t a b l e f o r c o m p u t e r a n a l y s i s . T h e s p e c t r o -m e t e r c a n a c h i e v e a v e l o c i t y r a n g e of ±3 c m / s e c w i t h a m p l i t u d e l i n e a r i t y of 0 .5% o v e r t h e c e n t r a l t h r e e - q u a r t e r s of t h e s p e c t r u m [7] .

T h e m e c h a n i c a l p o r t i o n of the s p e c t r o m e t e r c o n s i s t s of t w o l o u d s p e a k e r m a g n e t s ; one d r i v e s t h e s o u r c e s i t u a t e d a t t h e end of a long , s t a i n l e s s - s t e e l t u b e e x t e n d i n g i n t o a c r y o s t a t , a n d t h e o t h e r s e r v e s a s a v e l o c i t y - s e n s i n g d e v i c e . T h e d e s i g n of t h i s t r a n s d u c e r i s q u i t e s i m i l a r t o t h a t p u b l i s h e d b y L y n c h a n d B a u m g a r d n e r [8] , w i t h t h e e x c e p t i o n t h a t t h e m a g n e t g a p s a r e t u r n e d t o f a c e e a c h o t h e r . A c r o s s - s e c t i o n of t h e t r a n s d u c e r i s s h o w n i n F i g . 1 .

S e v e r a l t y p e s of 7 - r a y d e t e c t o r s h a v e b e e n u s e d , i nc lud ing s o d i u m iod ide s c i n t i l l a t i o n c r y s t a l s a n d p r o p o r t i o n a l c o u n t e r s f i l l e d w i t h e i t h e r x e n o n o r k r y p t o n . F o r 7 - r a y s l o w e r i n e n e r g y t h a n a b o u t 75 k e V , t h e p r o p o r t i o n a l c o u n t e r s h a v e s i g n i f i c a n t l y b e t t e r r e s o l u t i o n t h a n t h e s c i n t i l l a t i o n c o u n t e r . C o u n t e r s f i l l e d w i t h a 90% X e - 10% CI14 m i x t u r e a n d h a v i n g a b e r y l l i u m s i d e - w i n d o w w e r e u s e d in m o s t of t h e e x p e r i m e n t s t o b e d e s c r i b e d w i t h t h e 5 9 . 6 - k e V 7 - r a y of 2 3 7 Np. It w a s conven ien t to s e t the window of the s i n g l e - c h a n n e l

181

TABLE IX

Possibilities for Mössbauer Gamma Transitions Odd-Mass Actinide IsotopesГ1,51

ill

Ground State Excited State Gamma Ray Parent Absorber Isotope

Availability Г 3 1 Half-Life Spin

Energy (keV)

Half-Life (nsec) Spin Multipolar!ty

Conversion Coefficient 6 1 Isotope

Decay Mode Half-Life

3 2 7 Ac Decigrams 22y 3/2+ 27.5 29.9

4 El Ml

L: 2.8 231pa a 3.4xl04y

aa3Th 7340y 5/2+ 29.I 12.4 71.4 97.3

(7/2) 7/2+

3/2 or 5/2 9/2+

Ml(?) Ml + É2

E2

I22® Pa |f33U

EC a

1.5d 1.62xl05y

z 3 1Pa Orajns 3.43xl04y 3/2- 58.5 76.6 84.2 41

7/2-5/2-5/2+

E2 Ml El 2.8

F 3 1Th |f31U

P" EC

25.64h 4.3d

гззра

j

27d 3/2- 57 86.3 37

7/2-5 / 2 +

E2 El 1.9

Í Í 3 3 T h

L237Kp P~ a

22.4m 2.2xlOey

/аззц Kilogram 1.62xl05y 5/2+ 40 0.37 7 / 2 + E2 + Ml 2 3 3 p a P" 27d 237Np Kilogram 2.2xlOey 5/2+ 33.2

59.6 63 7/2+ 5 / 2 -

Ml + E2 El 1.0.

p 3 7 u 2 3 7 P U

I ? * 1 Am

P" EC a

6.75d 45.6d 458y

239рц Kilogram & greater

244l3y 1/2+ 57 5 / 2 + E2 P 3 8Hp l ? 3 s A m

ß-EC

2.35d 12h

3 4 3 Am Hundreds of grams

7951У or 7650y

5/2- 8 3 . 9 2 5 / 2 + El 0.20 P 4 3 P u L247Bk

P" a

4.98h 104y

2 4 5 Cm Hundreds of grams

1.2xl04y (?)

7/2+ 36 2 4 5 Am P" 2h

248Bk Grams 3l4d 7/2+ 42 9/2+ Ml 2S3Es a 20d

TABLE IX

Mössbauer Effect Possibilities with 2+ -» 0+ E2 Gamma Transitions In Even-Even Actinide Isotopes^, 51

G r o u n d S t a t e E x c i t e d S t a t e A b s o r b e r A v a i l a b i l i t y E n e r g y H a l f - L i f e EC, ß + P a r e n t 0 - P a r e n t a P a r e n t

I s o t o p e Г 3 1 H a l f - L i f e ( k e V ) ( n s e c ) I s o t o p e H a l f - l i f e I s o t o p e H a l f - L i f e I s o t o p e H a l f - U f e

2 2 e B a Gram 1622У o r 1 5 9 0 y

66 0 . 6 3 2 2 e A c 2 9 h " • f t 8 x l 0 4 y

2 3 ° T h M i l l i g r a m s t o g r a m s

8 i l 0 4 y 5 3 2 a 0 P a 17« 234ц 2 . 4 8 x l 0 5 y

2 3 a T h U n l i m i t e d 1 . 3 9 x l 0 l o y 50 O . 3 I 5 2 3 8 ц 2 . 3 9 x l O T y

232ц 7 * y 1 7 . 5 О .254 2 3 2 N p 13m 2 9 2 P a 1 . 3 1 d 2 3 e P u 2 . 8 5 y

234ц M i l l i g r a m 2 . 4 8 x l O s y W 0 . 2 6 6 2 3 4 N p 4 . 4 0 d 2 3 4 P a 2 3 4 m p a

6 . 6 6 h 1 . 1 7 5 m

z ? e P u 8 6 Л у

23 е и M i l l i g r a m 2 . 3 9 * 1 0 7 y 0 . 2 3 2 2 3 e H p -22h 2 4 ° P u 6 5 8 0 y

238ц Unlimited H.56x10"у W О . 2 2 5 2 4 2 P u З . 7 9 х 1 0 3 у

2 3 e P ü K i l o g r a m 8 6 . 4 y H4 O.I83 a s s N p 2 . 1 0 d , 2 4 a C m 1 6 3 d

2 4 ° P u M i l l i g r a m & g r e a t e r ( - 1 0 0 * )

6 5 8 0 y • 3 0 . 1 7 3 2 4 0 A m 5 1 b 2 4 0 H p 7 . 3 m 2 4 4 C m 1 7 . 9 y o r

1 9 . 2 y

2 * 2 P u Grams t o h u n d r e d s o f g r a m s ( - 1 0 0 ^ )

3 . 7 9 * 1 0 s y 4 4 . 6 2 4 2 A m I 6 h

2 4 2 C m H u n d r e d s o f 1 6 3 d 4 2 . 2 2 4 2 A m 1 6 h Z 4 e 0 f 3 7 . 5 h g r a m s

340d 2 4 4 C m Grams t o h u n d r e d s o f g r a m s

1 7 . 9 У o r 1 9 . 2 y

4 5 0 . 0 9 7 2 4 ® C f 340d

2 4 e C m Grams t o h u n d r e d s o f g rams

* 0 0 0 y ( ? ) 4 2 . 9 2 = ° C f 1 0 . 9 y

2 4 8 C m Mg. t o h u n d r e d s o f mg.

4 . 7 x l O s y 4 2 2 = a C f 2 . 6 y

2 5 ° C f M i c r o g r a m s t o t e n s o f m i l l i g r a m s

1 0 . 9 y 4 1 3 . 1 3 h и * й » 3 . 2 4 h

FIG. 1. SRL velocity transducer and liquid helium Dewar

a n a l y s e r a c r o s s t he xenon e s c a p e p e a k of 59 .6 -keV t r a n s i t i o n ; the 5 9 . 6 - k e V p h o t o -p e a k , of l e s s e r i n t e n s i t y , w a s no t s u i t a b l e b e c a u s e i t w a s not r e s o l v e d f r o m t h e e s c a p e p e a k of t h e К X - r a y s . T h i s m e t h o d w a s s a t i s f a c t o r y , but s u b s e q u e n t w o r k h a s shown tha t a b e t t e r coun t ing a r r a n g e m e n t i s p o s s i b l e . When the f i l l g a s i s a 90% K r - 1 0 % C H 4 m i x t u r e , t h e 5 9 . 6 - k e V p h o t o p e a k and i t s e s c a p e p e a k a r e a d j a c e n t i n t h e g a m m a - r a y s p e c t r u m , a n d bo th m a y be e n c o m p a s s e d by t h e s i n g l e - c h a n n e l a n a l y s e r w i n d o w . T h e r e a r e no i m p o r t a n t i n t e r f e r i n g r a d i -a t i o n s o t h e r t h a n t h e u s u a l C o m p t o n b a c k g r o u n d . B e c a u s e of t h e l o w e r a t o m i c w e i g h t of k r y p t o n , t h i s c o u n t e r h a s s o m e w h a t l e s s e f f i c i e n c y t h a n

184

t h e x e n o n c o u n t e r s . H o w e v e r , i t w a s f o u n d t h a t r e a s o n a b l e c o u n t i n g r a t e s cou ld b e o b t a i n e d wi thou t n o t i c e a b l e l o s s of r e s o l u t i o n by p a s s i n g t h e r a d i -a t i o n i n t o t h e c o u n t e r l o n g i t u d i n a l l y t h r o u g h a 0 . 0 1 0 - i n . a l u m i n i u m end w i n d o w .

2. Cryogenics [7]

T h e v e l o c i t y s p e c t r o m e t e r w a s d e s i g n e d f o r o p e r a t i o n wi th bo th s o u r c e and a b s o r b e r at t h e s a m e c r y o g e n i c t e m p e r a t u r e . E x p e r i m e n t s h a v e b e e n p e r f o r m e d a t 77°K a n d a t 4 . 2 °K . T h e t r a n s d u c e r , d r i v e r o d , a n d s o u r c e -a b s o r b e r c h a m b e r a r e c o m p l e t e l y e n c a s e d to f o r m a c l o s e d c r y o s t a t s y s t e m t h a t may b e e v a c u a t e d and f i l l e d wi th e x c h a n g e g a s . G a m m a r a y s f r o m jthe d r i v e n s o u r c e p a s s t h r o u g h t h e a b s o r b e r a n d ou t of t h e c r y o s t a t v i a a b e r y l l i u m window. F o r l i q u i d n i t r o g e n e x p e r i m e n t s , a s i m p l e g l a s s D e w a r wi th b e r y l l i u m windows in t h e b o t t o m w a s u s e d . L i q u i d h e l i u m e x p e r i m e n t s w e r e p e r f o r m e d in t h e m e t a l D e w a r shown in F i g . 1.

T h i s D e w a r h a s a c a p a c i t y of f i v e l i t r e s of l i q u i d h e l i u m and c a n m a i n -t a i n a s a m p l e t e m p e r a t u r e of 4.2°K f o r about f o u r d a y s wi thout r e f i l l i n g . B e -c a u s e t h e t o p of t h e D e w a r i s b l o c k e d b y t h e t r a n s d u c e r a s s e m b l y , a f i x e d l i q u i d - h e l i u m t r a n s f e r t u b e i s b u i l t i n t o t h e s y s t e m . A v a c u u m - j a c k e t e d c o u p l i n g c o n n e c t s t h e f i x e d a n d t h e m o v a b l e p o r t i o n s of t h e t r a n s f e r t u b e . T h e l i q u i d h e l i u m l e v e l i n t h e D e w a r i s m o n i t o r e d w i t h t h r e e s m a l l c a r b o n r e s i s t o r s p l a c e d a t d i f f e r e n t h e i g h t s a l o n g t h e d r i v e - r o d h o u s i n g t u b e ; t h e l iqu id l e v e l i s d e t e c t e d by the change in r e s i s t a n c e above and below the l e v e l . P r e c o o l i n g b e f o r e t r a n s f e r of l i q u i d h e l i u m i s a c h i e v e d by b l o w i n g co ld h e l i u m g a s i n to t h e D e w a r . In t h i s w a y , t h e D e w a r c a n b e c o o l e d t o a b o u t 90°K, a f t e r w h i c h a r e a s o n a b l y e f f i c i e n t t r a n s f e r of l i q u i d h e l i u m c a n be p e r f o r m e d .

3. Preparation of sources and absorbers

N e p t u n i u m - 2 3 7 i s t h e p r o d u c t of t h r e e m o d e s of r a d i o a c t i v e d e c a y : e l e c t r o n c a p t u r e of 2 3 7 P u , b e t a d e c a y of 2 3 7 U, a n d a l p h a d e c a y of 2 4 1 A m . S o u r c e s of t h e l a s t t w o i s o t o p e s h a v e b e e n u s e d i n M ö s s b a u e r a b s o r p t i o n e x p e r i m e n t s . A p a r t i a l d e c a y s c h e m e i s g i v e n i n F i g . 2 . V a r i o u s m e t h o d s of p r o d u c i n g t h e s e i s o t o p e s a r e d i s c u s s e d b e l o w , f o l l o w e d b y a d e s c r i p t i o n of t h e c h e m i c a l p r o c e d u r e s u s e d to m a k e s o u r c e s and a b s o r b e r s f r o m t h i s m a t e r i a l .

B o t h 2 3 7Np and 2 4 1 A m a r e c r e a t e d d u r i n g p r o l o n g e d n e u t r o n i r r a d i a t i o n of u r a n i u m a n d a r e t h u s b y - p r o d u c t s of n u c l e a r r e a c t o r o p e r a t i o n . G r a m q u a n t i t i e s , and g r e a t e r , of c h e m i c a l l y s e p a r a t e d n e p t u n i u m and a m e r i c i u m a r e a v a i l a b l e ; m a t e r i a l f o r t he M ö s s b a u e r a b s o r p t i o n e x p e r i m e n t s was t a k e n f r o m s t o c k s o l u t i o n s of t h e s e e l e m e n t s . T h e i s o t o p i c a b u n d a n c e of 2 3 7 Np i s u s u a l l y 100%, w h e r e a s 2 4 1 Am m a y b e m i x e d wi th v a r y i n g a m o u n t s of 2 4 3 A m , d e p e n d i n g u p o n t h e d u r a t i o n of n e u t r o n i r r a d i a t i o n . H o w e v e r , m a -t e r i a l r i c h in 2 4 1 Am i s a v a i l a b l e , and the s p e c i f i c a c t i v i t y of 2 4 1 Am i s g r e a t e r t h a n t h a t of 2 4 3 Am by a f a c t o r of 17, s o t h e p r e s e n c e of 2 4 3 A m p r e s e n t s no p a r t i c u l a r p r o b l e m in 2 3 7 Np e x p e r i m e n t s .

T h e 7 - d a y ß - e m i t t e r 2 3 7 U i s m o s t e a s i l y f o r m e d b y t h e r m a l n e u t r o n i r r a d i a t i o n of 2 3 6 U, w h i c h i s a v a i l a b l e wi th e n r i c h m e n t s of 95% and b e t t e r .

185

"'Pu ( 12.9 y)

FIG. 2. Partial decay scheme to levels of '237Np

F o r e x a m p l e , 1 m g of 236u3 Og i r r a d i a t e d i n a t h e r m a l n e u t r o n f l u x of 2X 101 4 n / c m 2 s f o r one week gave a y ie ld of about 40 m C i of 237u. One migh t c o n s i d e r f o r t h e p r o d u c t i o n of 237y a n a l t e r n a t e m e t h o d w h i c h h a s t h e a d -v a n t a g e of b e i n g s e l f - r e g e n e r a t i n g . T h e r e a r e s e v e r a l m i l l i c u r i e s of 2 3 7 U i n s e c u l a r e q u i l i b r i u m w i t h g r a m q u a n t i t i e s o f . 2 4 1 P u , d u e t o a v e r y w e a k a - d e c a y b r a n c h [4] . P r e s u m a b l y o n e m i g h t p e r i o d i c a l l y m i l k t h e 2 3 7 U a c -t i v i t y f r o m t h e m a s s of 2 4 i p U j w h e n e v e r a f r e s h s o u r c e w a s r e q u i r e d .

A l t h o u g h 237pu h a s no t b e e n u s e d a s a s o u r c e in M ö s s b a u e r a b s o r p t i o n e x p e r i m e n t s , if i t w e r e a v a i l a b l e i t wou ld be i n t e r e s t i n g t o s t u d y a s a t h i r d r o u t e t o 237]\jpt a n d i t s h a l f - l i f e of 45 d a y s i s q u i t e f a v o u r a b l e . S p a l l a t i o n r e a c t i o n s s u c h a s 2 3 5 U (a , n ) , 2 3 7 N p 2 n ) , a n d 2 3 7 N p ( p , n ) m a y b e u s e d t o p r o d u c e 2 3 7 P u in a c y c l o t r o n , bu t t h e c r o s s - s e c t i o n s a r e s m a l l [3], s o t h a t h igh b e a m - c u r r e n t s and long b o m b a r d m e n t t i m e s would b e r e q u i r e d t o p r o -d u c e t h e a m o u n t of a c t i v i t y n e e d e d .

M o r e t h a n ha l f t h e a c t i v i t y o b t a i n e d f r o m t h e r m a l n e u t r o n i r r a d i a t i o n of e n r i c h e d 236u j s d u e t o f i s s i o n p r o d u c t s of 2 3 5 U, a f e w p e r c e n t of w h i c h i s u s u a l l y p r e s e n t . T h e f o l l o w i n g c h e m i c a l p r o c e d u r e w a s u s e d f o r p u r i f i -c a t i o n of t h e 237u a c t i v i t y . T h e t a r g e t m a t e r i a l w a s d i s s o l v e d w i t h 6 M H C l a n d H 2 O 2 . A f t e r h e a t i n g t o d e c o m p o s e e x c e s s H 2 0 2 , t h e s o l u t i o n w a s e v a p o r a t e d n e a r l y t o d r y n e s s and t h e n t a k e n up wi th 12M H C l . T h e a c t i v i t y w a s p l a c e d on a n a n i o n e x c h a n g e c o l u m n a n d w a s h e d w i t h 1 2 M H C 1 ; u n d e r t h e s e c o n d i t i o n s u r a n i u m i s a d s o r b e d on t h e c o l u m n w h i l e f i s s i o n p r o d u c t s and o t h e r i m p u r i t i e s a r e w a s h e d t h r o u g h . F i n a l l y , t h e u r a n i u m w a s e l u t e d wi th 1M HC1 to y i e l d r a d i o c h e m i c a l l y p u r e 237xj; a s d e t e r m i n e d by the y - r a y s p e c t r u m .

186

S o ü r c e s i n c o r p o r a t i n g 231U into the hos t l a t t i c e s of T h 0 2 and N p 0 2 have been p r e p a r e d . Addi t ion of NH¿OH to a so lu t ion of 236.237uo§+ and Th4 + w ü l c o p r e c i p i t a t e a m m o n i u m u r a n a t e and t h o r i u m h y d r o x i d e ; i gn i t i on in a i r f o l l o w e d by r e d u c t i o n in a h y d r o g e n a t m o s p h e r e f o r t h r e e h o u r s a t 1200°C p r o d u c e s a U 0 2 - T h 0 2 s o l i d s o l u t i o n [9] f r o m w h i c h a s o u r c e m a y b e f a b r i c a t e d . Di lu te so l id s o l u t i o n s of U 0 2 in NpC^ a r e p r e p a r e d in the s a m e m a n n e r . S o u r c e s of 2 4 1 Am a s A m 0 2 in t h e h o s t l a t t i c e s T h 0 2 , N p 0 2 , and A m 0 2 a r e m a d e by the s a m e p r o c e d u r e , e x c e p t t ha t t he h y d r o g e n r e d u c t i o n s t e p i s o m i t t e d . A d i f f e r e n t type of s o u r c e w a s p r e p a r e d by e l e c t r o p l a t i n g 236,237UO2 on the s u r f a c e of a s t a i n l e s s - s t e e l d i s c and t h e n d i f f u s i n g the a c -t iv i ty in to the d i s c at 900°C u n d e r a h y d r o g e n a t m o s p h e r e .

A b s o r b e r s of NpC>2 and NpAl2 h a v e b e e n u s e d . N e p t u n i u m d i o x i d e i s p r e p a r e d by s i m p l e hyd rox ide p r e c i p i t a t i o n and ign i t ion . The i n t e r - m e t a l l i c c o m p o u n d NpAl2 h a s b e e n p r e p a r e d b y h e a t i n g s t o i c h i o m e t r i c q u a n t i t i e s of t he m e t a l s t o g e t h e r at 700°C in an i n e r t a t m o s p h e r e . A h o m o g e n e o u s c o m -pound (as d e t e r m i n e d by X - r a y p o w d e r p a t t e r n s [10]) i s obta ined by r e p e a t e d c r u s h i n g and r e h e a t i n g of t h i s m i x t u r e . Th in a b s o r b e r s (and a l l the s o u r c e s ) w e r e e n c a p s u l a t e d by a n c h o r i n g the m a t e r i a l t o a f i l t e r p a p e r b a s e wi th p o l y e s t e r t a p e . T h i c k e r a b s o r b e r s , . con ta in ing a s m u c h a s 250 m g / c m 2 of m a t e r i a l , w e r e s e a l e d in c l e a r p l a s t i c h o l d e r s .

C . MÖSSBAUER E F F E C T IN 237Np

It i s c o n v e n i e n t f o r p u r p o s e s of d i s c u s s i o n t o d iv ide t h e e x p e r i m e n t a l w o r k wi th 2 3 7 Np in to t h r e e p h a s e s :

(a) E x p e r i m e n t s des igned to d e m o n s t r a t e the ex i s t ence of the M ö s s b a u e r e f f e c t w i t h t h e 5 9 . 6 - k e V t r a n s i t i o n of 2 3 7 Np. T h i s p h a s e of t h e s t u d y h a s b e e n s u c c e s s f u l l y c o m p l e t e d and i s d e s c r i b e d i n t h i s s e c t i o n .

(b) E x p e r i m e n t s in s e a r c h of a n a r r o w s i n g l e l i n e in e i t h e r a s o u r c e o r a n a b s o r b e r . T h e c u r r e n t r e s u l t s of t h i s w o r k , s t i l l i n p r o g r e s s , a r e g i v e n in t h i s s e c t i o n .

(c) E x p e r i m e n t s which apply the 237Np r e s o n a n c e to yield s p e c i f i c i n f o r -m a t i o n on n u c l e a r , ' s o l i d - s t a t e , and c h e m i c a l p r o b l e m s . H e r e , of c o u r s e , l i e s t he r e a l v a l u e of t h i s new r e s o n a n c e , and the p u r p o s e of p h a s e s (a) and (b) i s to m a k e p o s s i b l e w o r k in p h a s e (c). Some of t h e s e f u t u r e p o s s i b i l i t i e s a r é d i s c u s s e d in a s e p a r a t e s e c t i o n .

1. Experiments with both source and absorber in NpO¡ host lattices

In l o o k i n g f o r t h e M ö s s b a u e r e f f e c t w i t h t h e 5 9 . 6 - k e V 7 - r a y of 2 3 7 Np, t h e p r o b l e m w a s to c h o o s e s u i t a b l e c o n d i t i o n s u n d e r w h i c h t h e e f f e c t , if i t e x i s t e d , cou ld b e o b s e r v e d . In t h e a b s e n c e of a n y d a t a t o t h e c o n t r a r y , i t was a s s u m e d tha t the r e c o i l - f r e e f r a c t i o n might be s m a l l and that t h e r e might be m a n y h y p e r f i n e l i n e s of unknown spac ing , so e x p e r i m e n t a l condi t ions w e r e c h o s e n a c c o r d i n g l y . Low t e m p e r a t u r e s w e r e c o n s i d e r e d a n e c e s s i t y to o b -t a in a r e a s o n a b l e r e c o i l - f r e e f r a c t i o n , and the in i t i a l e x p e r i m e n t s w e r e p e r -f o r m e d at 4.2°K. If t he s p a c i n g s of t he h y p e r f i n e e m i s s i o n l i n e s of a s o u r c e a r e i d e n t i c a l w i t h t h o s e of t h e a b s o r p t i o n l i n e s of a n a b s o r b e r , t h e n t h e r e

187

w i l l b e a s t r o n g r e s o n a n c e i n t h e v e l o c i t y s p e c t r u m a t t h e v e l o c i t y c o r r e s -p o n d i n g t o c o m p l e t e o v e r l a p b e t w e e n t h e s o u r c e a n d a b s o r b e r l i n e s . T h e r e wi l l be s m a l l e r r e s o n a n c e s on e i t h e r s i d e of t h i s , a t v e l o c i t i e s c o r r e s p o n d i n g t o p a r t i a l o v e r l a p . I d e n t i c a l h y p e r f i n e s p l i t t i n g of s o u r c e a n d a b s o r b e r i s m o s t p r o b a b l e if t h e r a d i o a c t i v e s o u r c e a t o m s a r e e m b e d d e d i n t h e s a m e c h e m i c a l c o m p o u n d , o r h o s t l a t t i c e , a s t h e a b s o r b e r . T h e s e c o n s i d e r a t i o n s l e d to t he c h o i c e of NpO^ a s t h e a b s o r b e r and a s t he h o s t l a t t i c e f o r s o u r c e s . In a d d i t i o n t o b e i n g t h e m o s t r e a d i l y a v a i l a b l e s o l i d c o m p o u n d of n e p t u n i u m , NpC>2 h a s a c u b i c c r y s t a l s t r u c t u r e , of t h e C a F 2 t y p e [11], a n d t h u s s h o u l d h a v e no e l e c t r i c f i e l d g r a d i e n t a t t h e n u c l e u s a s a r e s u l t of t h e l a t t i c e . If i n d e e d t h e h y p e r f i n e s p l i t t i n g due to the q u a d r u p o l e i n t e r a c t i o n i s e l i m i n a t e d , t h e p r o b l e m i s g r e a t l y s i m p l i f i e d . H o w e v e r , a t low t e m p e r a t u r e s NpO z m a y h a v e m a g n e t i c h y p e r f i n e s p l i t t i n g , s i n c e h e a t c a p a c i t y d a t a i n d i c a t e a m a g n e t i c t r a n s i t i o n a t a b o u t 25°K [12] .

- 2 0 2 Velocity (mm/sec)

-10 -5 0 . »5 »10 Velocity (mm/sec)

FIG. 3. Mössbauer absorption spectra of the 59.6-keV gamma-ray of 23,Np (Absorber: Np02 , 250 mg/cm 2 . Source: ^ A m or 237 U in Np02 )

FIG. 4. Mössbauer ab-sorption spectrum of 237 NpOg at 4.2°K (Source: 237 U in Np02)

T h e d a t a of S t o n e and P i l l i n g e r [2] f o r M ö s s b a u e r a b s o r p t i o n of t h e 5 9 . 6 - k e V y - r a y of 237Np w i t h N p O a h o s t l a t t i c e s f o r b o t h s o u r c e s a n d a b -s o r b e r s a r e g i v e n in F i g s . 3 and 4 . T h e s a m e p o l y - c r y s t a l l i n e NpO z

a b s o r b e r , 250 m g / c m 2 t h i ck , w a s u s e d in e a c h e x p e r i m e n t . With a. 237U s o u r c e a t 4 .2°K a b r o a d r e s o n a n c e w i t h s u g g e s t i o n s of s t r u c t u r e w a s o b t a i n e d ( F i g s . 3(c) and 4) . Wi th a n 2 4 1 A m s o u r c e a t 4.2°K, a s i m i l a r l y b r o a d v e l o c i t y s p e c t r u m w a s o b s e r v e d , but w i th only a 1% c h a n g e in count r a t e at z e r o v e -l o c i t y . At 77°K the t o t a l r e s o n a n c e e f f e c t s w e r e s m a l l e r than a t 4.2°K. How-e v e r , t h e y w e r e e a s i e r t o o b s e r v e t h a n i n t h e c o r r e s p o n d i n g e x p e r i m e n t s a t 4 .2°K, b e c a u s e t h e r e s o n a n c e s w e r e s h a r p e r a n d g a v e l a r g e r c h a n g e s in

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c o u n t r a t e . T h e i n c r e a s e d w i d t h of t h e r e s o n a n c e a t 4 . 2 ° K i s a s s u m e d t o b e r e l a t e d t o t h e m a g n e t i c t r a n s i t i o n in N p 0 2 . T h e v e l o c i t y s p e c t r a a t 77°K f o r t h e 2 « A m s o u r c e ( F i g . 3(a)) and f o r t h e 2 3 7 U s o u r c e ( F i g . 3(b)) a r e s t i l l m u c h b r o a d e r t h a n n a t u r a l l i n e - w i d t h ( 2 Г а 0 . 0 7 m m / s ) . H o w e v e r , i t s hou ld b e n o t e d t h a t s o m e l i n e b r o a d e n i n g i s t o b e e x p e c t e d f r o m r e s o n a n t s e l f -a b s o r p t i o n i n t h e s o u r c e s and f r o m t h i c k n e s s e f f e c t s i n t h e a b s o r b e r . T h e v e l o c i t y s p e c t r a s h o w n a r e u n c o r r e c t e d f o r b a c k g r o u n d , w h i c h r a n g e d f r o m 5 0 - 7 0 % of t h e t o t a l c o u n t r a t e . V e l o c i t y s p e c t r a w e r e a l s o t a k e n a t 298°K, b u t t h e r e s o n a n c e e f f e c t w a s t o o s m a l l t o m e a s u r e a t t h i s t e m p e r a t u r e .

2. Related theoretical work

T h e o b s e r v a t i o n of a r e s o n a n c e e f f e c t f o l l o w i n g t h e » - d e c a y of 2 4 1 Am i s p a r t i c u l a r l y i n t e r e s t i n g b e c a u s e t h e M ö s s b a u e r e f f e c t c a n a c t a s a s e n s i t i v e p r o b e f o r t h e l o c a l e n v i r o n m e n t of t h e r e c o i l i n g n u c l e u s a t t h e t i m e of y - r a y e m i s s i o n . S t o n e a n d P i l l i n g e r [2] p o i n t e d o u t t h a t t h e o b s e r v a t i o n of r e -s o n a n c e a b s o r p t i o n f o l l o w i n g a l p h a d e c a y o f f e r s e v i d e n c e t h a t t h e a — r e c o i l s t o p p i n g t i m e and t h e d i e l e c t r i c r e l a x a t i o n t i m e of NpC>2 a r e s h o r t c o m p a r e d wi th t h e l i f e t i m e of t h e 5 9 . 6 - k e V l e v e l of 2 3 7 N p . A r e c e n t c o m p u t a t i o n of t h e a - r e c o i l s t o p p i n g t i m e l e n d s s u p p o r t t o t h i s i d e a [13] . In a s o m e w h a t d i f -f e r e n t a p p r o a c h i t h a s b e e n s h o w n t h a t t h e e x p e r i m e n t a l r e s u l t s c a n a l s o be r e a s o n a b l y we l l e x p l a i n e d b y c o n s i d e r i n g t h e t h e r m a l a f t e r - e f f e c t s a s s o c i a t e d wi th t h e » - p r o c e s s [14] .

K a p l a n , u s i n g r e c e n t l y d e v e l o p e d s t o p p i n g t h e o r y [15] t o c a l c u l a t e t h e p a t h l e n g t h of h e a v y n u c l e a r r e c o i l s i n a s t o p p i n g m e d i u m , h a s o b t a i n e d a n e s t i m a t e of t h e r e c o i l s t o p p i n g t i m e [13] . A t h e o r e t i c a l r a n g e - e n e r g y c u r v e w a s c o m p u t e d f o r 2 3 7 N p s t o p p i n g in NpC>2, g i v i n g t h e r a n g e of a 2 3 1 Np n u c l e u s w i t h 9 3 - k e V r e c o i l e n e r g y a s 2 . 3 6 X 10-6 c m . K a p l a n s h o w s t h a t t h e s t o p p i n g t i m e m a y be d e r i v e d f r o m t h e r a n g e - e n e r g y c u r v e by n u m e r i c a l m e t h o d s . T h e t i m e r e q u i r e d f o r t h e r e c o i l v e l o c i t y t o d r o p to a v a l u e c o r -r e s p o n d i n g t o c h e m i c a l b i n d i n g e n e r g i e s i s e s t i m a t e d t o b e 3 . 9 X 1 0 _ 1 3 , s . T h i s r e s u l t , w h i c h i s m u c h s h o r t e r t h a n t h e l i f e t i m e of t h e n u c l e a r e x c i t e d s t a t e , 6 . 3 X 1 0 " 8 s , i n d i c a t e s t h a t t h e » - r e c o i l s t o p p i n g t i m e i s n o t a n i m p o r t a n t f a c t o r in c o n s i d e r i n g s u b s e q u e n t r e c o i l l e s s 7 - r a y e m i s s i o n . H o w -e v e r , e l e c t r o n i c r e l a x a t i o n p r o c e s s e s m i g h t s t i l l h a v e c h a r a c t e r i s t i c t i m e s a p p r o a c h i n g t h e n u c l e a r l i f e t i m e .

In a r e c e n t p a p e r , M u l l e n [14] t a k e s t h e v i e w p o i n t t h a t t h e d e c r e a s e d r e c o i l - f r e e f r a c t i o n a f t e r » - d e c a y ( a s c o m p a r e d w i t h t ha t a f t e r | 3 -decay) c a n b e e x p l a i n e d b y p o s t u l a t i n g a n e f f e c t i v e t e m p e r a t u r e i n t h e v i c i n i t y of t h e r e c o i l a t o m c o n s i d e r a b l y h i g h e r t h a n t h e a m b i e n t t e m p e r a t u r e . I f , a t t h e t i m e of a - d e c a y , a p o r t i o n of t h e k i n e t i c e n e r g y of t h e e v e n t i s t r a n s f e r r e d t o t h e l a t t i c e i n t h e f o r m of h e a t , e x t r e m e l y h i g h l o c a l t e m p e r a t u r e s a r e p r o d u c e d , and t h e l o c a l h e a t d i f f u s e s o u t w a r d w i t h t i m e . T h e o b s e r v e d r e -d u c t i o n i n r e c o i l - f r e e f r a c t i o n w i t h a - d e c a y a t 77°K c o r r e s p o n d s t o a n e f -f e c t i v e t e m p e r a t u r e of 215°K; f r o m t h i s M u l l e n d e r i v e s a l o w e r l i m i t of t h e m e a n t h e r m a l c o n d u c t i v i t y of NpC>2 f r o m c l a s s i c a l d i f f u s i o n t h e o r y a s 1.6 X 10"5 c a l c m - 1 deg" 1 s e c " 1 . O r d e r - o f - m a g n i t u d e a g r e e m e n t wi th s o m e w h a t s p a r s e t h e r m a l c o n d u c t i v i t y d a t a f o r a c t i n i d e o x i d e s s u g g e s t s t h a t t h e t h e o r e t i c a l a s s u m p t i o n s i n h e r e n t i n t h e c a l c u l a t i o n m a y b e a r c o n s i d e r a b l e v a l i d i t y .

1 8 9

3. Experiments with dissimilar host lattices [16]

I n c o n s i d e r i n g s u i t a b l e h o s t l a t t i c e s f o r s o u r c e s w h i c h m i g h t g i v e a s i n g l e n a r r o w e m i s s i o n l i n e , ThC>2 h a s b e e n a p r i m e c a n d i d a t e . T h o r i u m d i o x i d e h a s a c u b i c c r y s t a l s t r u c t u r e i s o m o r p h o u s w i t h U O 2 , N p Q 2 , a n d А т О г [IV], s o t h a t s o u r c e a t o m s s h o u l d f i t e a s i l y i n t o t h e s t r u c t u r e a t l a t t i c e s i t e s , a n d t h e c u b i c s y m m e t r y s h o u l d g i v e a v a n i s h i n g q u a d r u p o l e i n t e r -a c t i o n . T h e e l e c t r o n i c c o n f i g u r a t i o n of T h 4 + i s t h a t of r a d o n , w i t h n o e l e c t r o n s b e y o n d c l o s e d s h e l l s , a n d T h O s i s t h e r e f o r e d i a m a g n e t i c [ 1 8 ] . W i t h no a p p a r e n t s o u r c e s of m a g n e t i c o r q u a d r u p o l e s p l i t t i n g , TI1O2 s e e m e d i d e a l a s a h o s t l a t t i c e f o r t h e a c t i n i d e s y s t e m s u n d e r c o n s i d e r a t i o n . B y u s i n g t h e s a m e t h i c k NpC>2 a b s o r b e r w h i c h h a d p r e v i o u s l y b e e n s h o w n t o g i v e a r e s o n a n c e e f f e c t , a n u m b e r of e x p e r i m e n t s h a v e b e e n p e r f o r m e d w i t h b o t h 2 3 7 U a n d 2 4 1 A m s o u r c e s , a t 4 .2 °K a n d 7 7 ° K . A f e w e x p e r i m e n t s w è r e

r u n w i t h t h e 2 4 i A m s o u r c e a t 4 .2°K a n d t h e a b s o r b e r a t 77°K. I n a l l of t h e

e x p e r i m e n t s n o r e s o n a n c e a b s o r p t i o n h a s b e e n o b s e r v e d . T h e r e a s o n s f o r

t h e s e n e g a t i v e r e s u l t s w i t h TI1O2 h o s t l a t t i c e s a r e no t u n d e r s t o o d a t t h i s t i m e .

F u r t h e r s t u d y of t h e ThC>2 h o s t l a t t i c e i s p l a n n e d .

A s o u r c e of p u r e 2 4 1 А т О г w a s o b s e r v e d t o g i v e a r e s o n a n c e w h e n r u n a t 77°K w i t h t h e NpC>2 a b s o r b e r ( 2 5 0 m g / c m 2 ) . T h e v e l o c i t y s p e c t r u m i s v e r y s i m i l a r t o t h a t i n F i g . 3 (a ) e x c e p t t h a t t h e c o u n t - r a t e c h a n g e i s o n l y a b o u t 1%, a n d t h e r e i s a c h e m i c a l s h i f t of + 0 . 7 m m / s f o r 2 4 1 A m i n А т О г w i t h r e s p e c t t o 2 4 1 A m i n N p Û 2 . W i t h t h e 2 4 1 A m 0 2 s o u r c e a t 4 . 2 ° a n d t h e

a b s o r b e r a t 7 7 ° K , n o e f f e c t w a s o b s e v r e d .

S o m e p r e l i m i n a r y e x p e r i m e n t s h a v e b e e n p e r f o r m e d w i t h 2 3 7 U i n a

s t a i n l e s s - s t e e l h o s t l a t t i c e . T h i s m a t e r i a l w a s c h o s e n b e c a u s e i t i s a c u b i c ,

n o n m a g n e t i c m e t a l , bu t t h e l o c a l e n v i r o n m e n t a n d c h e m i c a l s t a t e of u r a n i u m

i n t h e l a t t i c e i s u n k n o w n . T h e r e s u l t s of e x p e r i m e n t s a t 4 . 2 ° K w i t h NpC>2

a b s o r b e r s w e r e n e g a t i v e .

B i n a r y i n t e r m e t a l l i c c o m p o u n d s w i t h t h e c o m p o s i t i o n AB2 a n d w i t h t h e

c u b i c , L a v e s - p h a s e c r y s t a l s t r u c t u r e h a v e b e e n u s e d s u c c e s s f u l l y a s s i n g l e -

l i n e a b s o r b e r s in r a r e - e a r t h M ö s s b a u e r e f f e c t s t u d i e s [19, 20] . An a n a l o g o u s

a p p r o a c h i s c u r r e n t l y u n d e r w a y i n t h e a c t i n i d e s t u d i e s . A n a b s o r b e r of t h e

L a v e s - p h a s e c o m p o u n d N p A l 2 ( ~ 2 0 0 m g / c m 2 ) h a s b e e n i n v e s t i g a t e d a t 4 . 2 °

a n d 77TC w i t h a s o u r c e of 2 4 1 A m i n NpC>2, bu t a n y r e s o n a n c e a b s o r p t i o n w a s

t o o s m a l l t o m e a s u r e . E x p e r i m e n t s w i t h 2 3 7 U s o u r c e s a r e in p r o g r e s s . T h e

m a g n e t i c p r o p e r t i e s of N p A l a a r e n o t k n o w n , b u t t h e r e i s a s t r o n g p o s s i -

b i l i t y t h a t i t m a y b e i n a m a g n e t i c s t a t e a t l o w t e m p e r a t u r e s . T h e c o r r e s -

p o n d i n g r a r e - e a r t h c o m p o u n d s , R A 1 2 , h a v e C u r i e p o i n t s i n t h e r a n g e f r o m

4 - 1 7 6 ° K [21 ] .

4. Discussion

H a v i n g d e m o n s t r a t e d t h a t t h e M ö s s b a u e r e f f e c t c a n b e o b t a i n e d w i t h t h e 5 9 . 6 - k e V y - r a y of 2 3 7 N p , o n e m u s t t h e n s e a r c h f o r m a t e r i a l s w h i c h c a n a c t a s s i n g l e - l i n e s o u r c e s and a b s o r b e r s . T h e n e e d f o r s u c h m a t e r i a l s i s c l e a r , f o r t h e y c a n b e u s e d a s p r o b e s t o s t u d y c o m p l e x h y p e r f i n e s t r u c t u r e s i n o t h e r s u b s t a n c e s , i n a t r a c t a b l e m a n n e r . T h e e x p e r i m e n t a l a p p r o a c h b e i n g u s e d i n t h e s e a r c h f o r s i n g l e - l i n e m a t e r i a l s f o r 2 3 7 Np w o r k i s o n e of s u c c e s s i v e

190

a p p r o x i m a t i o n s ; t h a t i s , m a n y c o m b i n a t i o n s of s o u r c e s a n d a b s o r b e r s a r e t o b e t r i e d , and if a p a r t i c u l a r c o m b i n a t i o n g i v e s a n a r r o w e r v e l o c i t y s p e c -t r u m t h a n t h e b e s t p r e v i o u s one , t h e s o u r c e o r a b s o r b e r r e s p o n s i b l e f o r t h e i m p r o v e m e n t w i l l b e u s e d i n s u c c e e d i n g e x p e r i m e n t s u n t i l a b e t t e r c o m b i -n a t i o n i s f o u n d . A t p r e s e n t w e k n o w t h a t t h e c o m b i n a t i o n of 2 3 7 U i n И р О г wi th an NpC>2 a b s o r b e r - a t 77°K g i v e s a v e l o c i t y s p e c t r u m wi th the a p p e a r a n c e of a b r o a d e n e d s i n g l e l i n e . H o w e v e r , we d o n o t a s y e t k n o w w h e t h e r t h i s i s due to c o m p l e t e o v e r l a p of i d e n t i c a l c o m p l e x s o u r c e and a b s o r b e r pa t t e rns , o r t o t h e o v e r l a p of b r o a d s i n g l e l i n e s . T o p r o c e e d f u r t h e r , i t h a s b e e n a s s u m e d t h a t t h e l a t t e r i s t h e c a s e ; t h u s , u n k n o w n s o u r c e s a r e t o b e r u n a g a i n s t t h e NpC>2 a b s o r b e r a t 77°K, a n d u n k n o w n a b s o r b e r s a r e t o b e r u n a g a i n s t t h e s o u r c e of 2 3 7U in N p 0 2 a t 77°K. T h i s p r o c e d u r e w i l l be r e -e v a l u a t e d f r o m t i m e t o t i m e t o e n s u r e t h a t i t i s l e a d i n g i n t h e d i r e c t i o n of t h e d e s i r e d r e s u l t , a t r u l y n a r r o w s i n g l e - l i n e s o u r c e a n d a b s o r b e r .

In a d d i t i o n t o t h e s i n g l e - l i n e p o s s i b i l i t i e s a l r e a d y m e n t i o n e d , 2 3 7U i n ThC>2 a s a s o u r c e and NpAl 2 a s an a b s o r b e r , s e v e r a l o t h e r m a t e r i a l s a p p e a r p r o m i s i n g . If t h e L a v e s - p h a s e 2 3 7UA12 [22] c a n b e p r e p a r e d i t w i l l b e s t u d i e d a s a s o u r c e . A l s o 2 3 7 U s o u r c e s a s v e r y d i l u t e s o l i d s o l u t i o n s of u r a n i u m in a l u m i n i u m m a y b e u s e d ; t h e m a x i m u m s o l i d s o l u b i l i t y i s o n l y 0 .06 w t . % U [23], bu t t h e a t o m i c a b s o r p t i o n c r o s s - s e c t i o n of a l u m i n i u m f o r 5 9 . 6 - k e V r a d i a t i o n i s s m a l l e n o u g h t h a t t h i c k n e s s e s of Al up to 1 c m c a n be t o l e r a t e d . In e i t h e r of t h e s e s o u r c e s the p o s s i b i l i t y e x i s t s , if u l t r a p u r e 236U (>99 .9% e n r i c h m e n t ) c a n be o b t a i n e d , of f a b r i c a t i n g s o u r c e s w h i c h cou ld b e r e g e n e r a t e d m a n y t i m e s by t h e r m a l n e u t r o n i r r a d i a t i o n s , wi thout the n e -c e s s i t y of p e r f o r m i n g c h e m i c a l s e p a r a t i o n s e a c h t i m e . O t h e r p r o m i s i n g c a n d i d a t e s f o r s i n g l e - l i n e a b s o r b e r s a r e t h e i n t e r m e t a l l i c c o m p o u n d s NpAl 3

a n d NpBei3, w h i c h a r e k n o w n t o h a v e d i f f e r e n t d i s t i n c t i v e c u b i c s t r u c t u r e s [10, 24] .

D . F U T U R E P O S S I B I L I T I E S W I T H 2 3 7 N p

A s s u m i n g t h a t good s i n g l e - l i n e s o u r c e s and a b s o r b e r s c a n be found , one m a y o u t l i n e t h e m o r e o b v i o u s a p p l i c a t i o n s of t h e M ö s s b a u e r e f f e c t in 2 3 7Np. T h e f o l l o w i n g i s a d i s c u s s i o n of t h e d i r e c t i o n s t h i s r e s e a r c h c a n r e a s o n a b l y be e x p e c t e d t o t a k e ; it i s c o n v e n i e n t to c l a s s i f y t h e s e g e n e r a l a r e a s a s g iving n u c l e a r , c h e m i c a l , and s o l i d - s t a t e i n f o r m a t i o n .

1. Nuclear physics

B e c a u s e t r a n s i t i o n s b e t w e e n n u c l e a r h y p e r f i n e l e v e l s a r e d i r e c t l y o b -s e r v e d wi th t h e M ö s s b a u e r e f f e c t , and b e c a u s e t h e h y p e r f i n e s p l i t t i n g i s r e -l a t e d to t h e n u c l e a r m o m e n t s , i t h a s b e e n p o s s i b l e in m a n y c a s e s to e x t r a c t v a l u e s f o r t h e n u c l e a r m o m e n t s f r o m M ö s s b a u e r - e f f e c t d a t a [25] . F u r t h e r -m o r e , i t i s i m p o r t a n t t h a t t h e n u c l e a r p r o p e r t i e s of a M ö s s b a u e r i s o t o p e b e w e l l u n d e r s t o o d b e f o r e a t t e m p t i n g to d r a w c h e m i c a l i n f e r e n c e s f r o m r e -s o n a n c e a b s o r p t i o n d a t a [26 , 27 ] . F o r t h e c a s e of 2 3 7 Np, t h e n u c l e a r m o -m e n t s of t h e g r o u n d s t a t e a n d t h e 5 9 . 6 - k e V s t a t e a r e e i t h e r u n k n o w n o r u n -c e r t a i n . T h e g r o u n d - s t a t e m a g n e t i c m o m e n t h a s b e e n d e r i v e d f r o i i i p a r a -m a g n e t i c r e s o n a n c e m e a s u r e m e n t s ; a v a l u e of ± { 6 . 0 ± 2 . 5 ) n . m . w a s ob ta ined

191

in R b ( N p 0 2 ) ( N 0 3 ) 3 [28], w h e r e a s t he r e s u l t f r o m N p F 6 w a s ±2,70 n . m . [29] . T h e m a g n e t i c m o m e n t of t h e 5 9 . 6 - k e V s t a t e h a s b e e n r e p o r t e d t o b e +(2 .0 ± 0 . 5 ) n . m . f r o m t h e m e t h o d of p e r t u r b e d a n g u l a r c o r r e l a t i o n of y - r a y s [30]. T h e q u a d r u p o l e m o m e n t of n e i t h e r s t a t e i s k n o w n , a l t h o u g h v a l u e s of s e v e r a l b a r n s a r e e x p e c t e d in t h i s r e g i o n of h i g h l y d e f o r m e d n u c l e i .

F r o m a r e s o l v e d h y p e r f i n e s p e c t r u m due only t o the i n t e r a c t i o n b e t w e e n t h e n u c l e a r m o m e n t s a n d t h e e f f e c t i v e m a g n e t i c f i e l d a t t h e n u c l e u s , o n e c a n ob t a in two q u a n t i t i e s : /ujHgff and /u0

Heff < w h e r e / U j and ц0 a r e t h e m a g -n e t i c m o m e n t s of t h e e x c i t e d a n d g r o u n d s t a t e s , a n d Heff i s t h e e f f e c t i v e f i e l d . In g e n e r a l , Heff wi l l not be known, s o t h a t t h e h y p e r f i n e p a t t e r n wi l l y i e l d on ly t h e r a t i o p = ß i / ß 0 . F o r an e x c i t e d s t a t e wi th s p i n I x = 5 / 2 a n d t h e g r o u n d s t a t e w i t h I 0 = 5 / 2 , s u c h a s we h a v e w i t h 2 3 7 N p , e a c h s t a t e h a s s i x m a g n e t i c s u b l e v e l s w i th q u a n t u m n u m b e r s m and e n e r g i e s p r o p o r t i o n a l t o ß m ; f o r d ipo l e r a d i a t i o n t h e r e a r e 16 a l l o w e d y - r a y t r a n s i t i o n s b e t w e e n the l e v e l s . T h e p a r a m e t e r p i s d e t e r m i n e d by the r e l a t i v e p o s i t i o n s and i n t e n -s i t i e s of t he 16 l i n e s . T h e r e l a t i v e e n e r g i e s of the l i n e s f o r t h i s s p i n s y s t e m h a v e b e e n c o m p u t e d a s f u n c t i o n s of p f o r p u r e m a g n e t i c s p l i t t i n g a n d a r e g i v e n in T a b l e III a l o n g wi th t h e r e l a t i v e i n t e n s i t i e s . It i s c o n v e n i e n t t o e x -p r e s s t h e r e l a t i v e e n e r g i e s of t h e l i n e s on a n o r m a l i z e d s c a l e w h e r e t h e

TABLE III

Relative Energy Expressions for = 5/2 to I 0 = 5/2 Transitions with Magnetic Splitting

Relative ni! -> mo Intensity Numerators

+ 5/2 + 5/2 25 -5P + 5 + 5/2 + 3/2 10 -5P + 3 + 3/2 + 5/2 10 -3P + 5 + 3/2 + 3/2 9 -3P + 3 + 3/2 + 1/2 16 -3p + 1 + 1/2 + 3/2 16 -p + 3 + 1/2 + 1/2 1 -p + 1 + 1/2 - 1/2 18 -p - 1 - 1/2 + 1/2 18 p + 1 - 1/2 - 1/2 l p - 1 - 1/2 - 3/2 16 p - 3 - 3/2 - 1/2 16 3P - 1 - 3/2 - 3/2 9 3P - 3 - 3/2 - 5/2 10 3P - 5 - 5/2 - 3/2 10 5p - 3 - 5/2 - 5/2 25 5p - 5

Denominators : P < 0 5 - 5p

0 < P < +1 5P - 3 P > +1 5 - 3P

192

l o w e s t - e n e r g y l i ne a l w a y s i s a t - 1 , and t h e h i g h e s t - e n e r g y l ine a l w a y s i s at +1. T h u s , t h e r e l a t i v e e n e r g y e x p r e s s i o n s in T a b l e III h a v e b e e n c o n -s t r u c t e d by a d j u s t i n g the z e r o of e n e r g y t o half the e n e r g y be tween the o u t e r -m o s t l i n e s , and d i v i d i n g t h e a d j u s t e d e n e r g y of e a c h l i n e b y t h e a d j u s t e d e n e r g y of t h e h i g h e s t - e n e r g y l i n e . T h e d e n o m i n a t o r of t h e e x p r e s s i o n c h a n g e s a t c e r t a i n v a l u e s of p c o r r e s p o n d i n g t o t h e p o i n t s w h e r e l e v e l c r o s s i n g s g i v e a d i f f e r e n t p a i r of o u t e r m o s t l i n e s .

An i n t e r e s t i n g f e a t u r e of t h e m a g n e t i c a l l y s p l i t 5 / 2 —» 5 / 2 s p i n s y s t e m i s t h a t t h e h y p e r f i n e p a t t e r n a t a n y v a l u e of p i s i n d i s t i n g u i s h a b l e f r o m t h e p a t t e r n a t 1 / p . T h i s i s c l e a r l y i l l u s t r a t e d in a l o g a r i t h m i c p lo t of r e l a t i v e e n e r g y v e r s u s p, w h i c h s h o w s c o m p l e t e r e f l e c t i o n s y m m e t r y t h r o u g h t h e p = 1 l i n e . In F i g . 5 t h e r e l a t i v e e n e r g y of e a c h l i ne i s p l o t t e d a g a i n s t log | p | s e p a r a t e l y f o r p o s i t i v e p and n e g a t i v e p. It wi l l be no ted tha t t h e o r d e r i n g of the l i n e s by i n t e n s i t y d i f f e r s f o r the two s i g n s of p and thus g ives a m e a n s of d e t e r m i n i n g t h e s i g n . T h e r e f o r e , : if t h e m a g n e t i c h y p e r f i n e s t r u c t u r e of 2 3 7Np c a n be o b s e r v e d it wi l l g ive a r a t i o of t he m a g n e t i c m o m e n t s of t h e ground s t a t e and the 5 9 . 6 - k e V s t a t e but wil l not t e l l which s t a t e has the l a r g e r m a g n e t i c m o m e n t ; i t w i l l t e l l w h e t h e r t h e two m a g n e t i c m o m e n t s h a v e t h e s a m e o r o p p o s i t e s i g n s . An i n d e p e n d e n t d e t e r m i n a t i o n of no , p e r h a p s by ENDOR t e c h n i q u e s , would be r e q u i r e d t o c o m p l e t e t h e p i c t u r e and g i v e ц-у a n d Heff .

A s i m i l a r t r e a t m e n t m a y b e c a r r i e d out f o r a p u r e q u a d r u p o l e i n t e r -a c t i o n in an a x i a l l y s y m m e t r i c f i e ld g r a d i e n t . H e r e the h y p e r f i n e s p e c t r u m y i e l d s t h e r a t i o of t h e q u a d r u p o l e m o m e n t s , к = Q i / Q o - F o r t h e 5 / 2 - > 5 / 2 s p i n s y s t e m , e a c h s t a t e i s s p l i t i n to t h r e e s u b l e v e l s c h a r a c t e r i z e d b y ±m and whose e n e r g i e s a r e p r o p o r t i o n a l to Qfrn2 - 35/12) ; t h e r e a r e 7 a l lowed d ipo le 7 - r a y t r a n s i t i o n s . T h e n o r m a l i z e d r e l a t i v e - e n e r g y e x p r e s s i o n s , a s f u n c t i o n s of к f o r t he 7 l i n e s , w e r e c o n s t r u c t e d j u s t a s i n the c a s e of m a g -ne t i c sp l i t t i ng , and the r e l a t i v e i n t e n s i t i e s a r e given in Tab l e IV. As b e f o r e , t h e s e r e l a t i v e e n e r g i e s a r e p l o t t e d a g a i n s t l o g | к | in F i g . 6, w h i c h s h o w s t h a t t h e s i g n of к c an be d e t e r m i n e d f r o m the o r d e r of t he l i n e s . F o r n e g a t i v e к one cannot d i s t i n g u i s h b e t w e e n к and l / к ; h o w e v e r , if к i s p o s i -t ive i t s m a g n i t u d e m a y b e u n a m b i g u o u s l y d e t e r m i n e d .

2. Structural chemistry

It h a s b e c o m e c u s t o m a r y t o a t t e m p t t o c o r r e l a t e w i t h c h e m i c a l p r o -p e r t i e s the i s o m e r sh i f t and quad rupo l e sp l i t t i ng da ta f r o m M ö s s b a u e r e f f ec t e x p e r i m e n t s . T h i s p r o c e d u r e h a s b e e n p a r t i c u l a r l y f r u i t f u l f o r i r o n and t in compounds us ing the 5TFe and 119Sn M ö s s b a u e r r e s o n a n c e s [31]. R a r e - e a r t h c h e m i s t r y i s s o m e w h a t l e s s v a r i e d , s i n c e t h e +3 v a l e n c e s t a t e d o m i n a t e s the s e r i e s in ion ic c o m p o u n d s , and t h e r e i s v e r y l i t t l e t e n d e n c y f o r the r a r e e a r t h s t o e n t e r in to c o v a l e n t b o n d s ; t he M ö s s b a u e r e f f e c t h a s b e e n c o r r e s -p o n d i n g l y l e s s u s e f u l f o r p u r e l y c h e m i c a l s t u d i e s w i th t h e r a r e e a r t h s . In c o n t r a s t t o t h i s b e h a v i o u r , t he l i g h t e r a c t i n i d e s , inc lud ing n e p t u n i u m , h a v e s e v e r a l s t a b l e v a l e n c e s t a t e s and f o r m both ion i c and c o v a l e n t c o m p o u n d s . It s e e m s r e a s o n a b l e t o c o n c l u d e t h a t t h e M ö s s b a u e r e f f e c t i s a p o t e n t i a l l y u s e f u l t o o l in e l u c i d a t i n g t h e c h e m i c a l p r o p e r t i e s of n e p t u n i u m a n d o t h e r a c t i n i d e s .

193

FIG. 5. Hyperfine line positions from magnetic splitting for 5/2 -»5/2 transitions (p= Mi/Mo)

T h e c h e m i s t r y of n e p t u n i u m h a s not b e e n v e r y t h o r o u g h l y s t u d i e d a s c o m p a r e d w i t h i t s t e c h n o l o g i c a l l y i n a p o r t a n t n e i g h b o u r s , u r a n i u m a n d p l u t o n i u m . H o w e v e r , t h e b a s i c c h e m i s t r y of t h e e l e m e n t i s w e l l known [1], a n d i t s p r o p e r t i e s a r e found t o b e i n t e r m e d i a t e b e t w e e n t h o s e of U a n d P u . N e p t u n i u m h a s t h e s t a b l e v a l e n c e s 0, +3, +4, +5, a n d +6, w i t h t h e +5 a n d +6 s t a t e s t e n d i n g t o e x i s t a s N p O | a n d NpO§*. F o r e a c h v a l e n c e s t a t e s e -v e r a l e x a m p l e s of s o l i d c o m p o u n d s a r e k n o w n , w i t h t h e i r n u m b e r p e r h a p s b e i n g l i m i t e d b y t h e p a u c i t y of i n v e s t i g a t i o n s . S e v e r a l r e v i e w s of t h e c h e m i s t r y a n d c o m p o u n d s of n e p t u n i u m a r e a v a i l a b l e [1, 32, 33 ] .

W h e n a s u i t a b l e s i n g l e - l i n e s o u r c e i s f o u n d f o r 237Np M ö s s b a u e r r e -s o n a n c e , i t s hou ld be u s e d to e x a m i n e t h e v e l o c i t y s p e c t r a of Np c o m p o u n d s

194

TABLE II

Relative Energy Expressions for Ij. = 5/2 to I 0 = 5/2 Transitions with Quadrupole Splitting

Relative . Numerators mi mo Intensity к < 0 0 < к < 1/2 1/2 < к < 2 Р > 2

± 5/2 ± 5/2 50 3 3K kK - 3 К - 1 3* - 4 ± 5/2 ± 3/2 20 -1 - 3 К Чк + 1 К + 1 Зк ± 3/2 ± 5/2 20 3 + к - 3 -1 - к -4 - к

± 3/2 ± 3/2 18 -1 + к •i- 1 1 - к - к

± 3/2 ± 1/2 32 -3 + к + 3 2 - к 2 - к

± 1/2 ± 3/2 32 -1 + Зк 1 - 2к 1 - 2к - Зк

± 1/2 ± 1/2 38 -3 + Зк 3 - 3 К . 2 - 2к 2 - 3 «

Denominators : 3/c - 3' 3 к. + 1 Зк

f o r w h i c h t h e bond ing i s r e a s o n a b l y w e l l u n d e r s t o o d . In t h i s w a y one would b u i l d u p a b o d y of d a t a on q u a n t i t i e s m e a s u r e d w i t h t h e M ö s s b a u e r e f f e c t , w h i c h w o u l d b e c o r r e l a t e d w i t h t h e c h e m i c a l a n d s t r u c t u r a l p r o p e r t i e s of Np c o m p o u n d s and w h i c h w o u l d t h e n b e u s e f u l i n s t u d y i n g c o m p o u n d s w i t h u n k n o w n p r o p e r t i e s . A f e w e x a m p l e s a r e of p a r t i c u l a r i n t e r e s t . T h e 5 Í 1

c o n f i g u r a t i o n of NpV I c o m p o u n d s h a s b e e n s t u d i e d e x t e n s i v e l y b e c a u s e i t i s r e a d i l y s u s c e p t i b l e t o t h e o r e t i c a l a n a l y s i s . O t h e r t h a n the nep tuny l c o m -p o u n d s , t h e on ly known NpVI c o m p o u n d i s N p F g , f o r w h i c h c o n s i d e r a b l e s p e c t r o s c o p i c and s t r u c t u r a l d a t a e x i s t [34, 35] . S o m e new NpV I c o m p l e x e s w i t h o c t a h e d r a l c o o r d i n a t i o n m a y s o o n be a v a i l a b l e f o r s t u d y [36] . A l s o i n t e r e s t i n g i s t h e r e c e n t l y d i s c o v e r e d t r i o x i d e , NpÛ3 [37] . S ince t he 5f o r -b i t a l s a r e s p a c i a l l y m o r e e x t e n s i v e t h a n t h e i r 4f c o u n t e r p a r t s , t h e y a r e c a -p a b l e of t a k i n g p a r t , in v a r y i n g d e g r e e s , in c o v a l e n t bonding ; t he M ö s s b a u e r e f f e c t shou ld be u s e f u l i n s t u d y i n g t h i s type of bond ing . F i n a l l y , i t i s hoped t h a t t h e a v a i l a b i l i t y of a new p h y s i c a l t oo l s o s p e c i f i c a l l y h e l p f u l in s t u d y i n g n e p t u n i u m c o m p o u n d s w i l l e n c o u r a g e c h e m i s t s t o look f o r new and d i f f e r e n t t y p e s of m a t e r i a l s c o n t a i n i n g n e p t u n i u m .

3. Solid-state physics

S i n c e t h e m a g n e t i c p r o p e r t i e s of n e p t u n i u m and i t s c o m p o u n d s a r e l a r g e l y u n k n o w n , t h e M ö s s b a u e r e f f e c t m i g h t b e c o m e t h e f i r s t t o o l t o f i n d s o m e of t h e i r l o w - t e m p e r a t u r e p h a s e t r a n s i t i o n s . T h e on ly w e l l - k n o w n c a s e i s t h e p a r a m a g n e t i c - a n t i f e r r o m a g n e t i c t r a n s i t i o n i n NpO z a t 25°K [12] ; i t wou ld be u s e f u l t o s t u d y t h e v e l o c i t y s p e c t r u m of N p O s a s a f u n c t i o n of t e m p e r a t u r e n e a r t h e t r a n s i t i o n . A l though a n y of t h e n e p t u n i u m c o m p o u n d s m i g h t h a v e a n o r d e r e d m a g n e t i c s t a t e , t h e m e t a l a n d i t s a l l o y s h a v e t h e g r e a t e s t i n t e r e s t f o r m a g n e t i c s t u d i e s . T h e a c t i n i d e m e t a l s , l i k e t h e r a r e -e a r t h m e t a l s , f o r m s o l i d s o l u t i o n s wi th e a c h o t h e r , bu t h a v e v e r y l i t t l e t e n d e n c y t o do s o w i t h o t h e r m e t a l s ; i n s t e a d t h e y f o r m i n t e r m e t a l l i c c o m -p o u n d s wi th w e l l - d e f i n e d s t o i c h i o m e t r y . A f ew i n t e r m e t a l l i c c o m p o u n d s of

195

к Negotive

-0.5 О +0.5 Relative Velocity, A v

FIG. 6. Hyperfine line positions from quadrupole splitting for 5/2-* 5/2 transitions (K = Q1/Q0)

n e p t u n i u m h a v e b e e n r e p o r t e d [10 , 24] , bu t u n d o u b t e d l y m a n y m o r e c o u l d b e m a d e . F o r e x a m p l e , U F e 2 a n d P u F e 2 a r e k n o w n [22 , 38] , b u t N p F e 2 h a s n o t b e e n r e p o r t e d . T h e l a t t e r c o m p o u n d w o u l d b e p a r t i c u l a r l y i n t e r e s t i n g t o s t u d y b e c a u s e M ö s s b a u e r a b s o r p t i o n e x p e r i m e n t s c o u l d b e r u n w i t h b o t h N p a n d F e , a n d t h i s i n f o r m a t i o n w o u l d c o m p l e m e n t e x i s t i n g 5 7 F e d a t a i n U F e 2 [ 3 9 ] .

It m a y b e p o s s i b l e t o s t u d y t h e s o l i d - s t a t e a n d c h e m i c a l p r o p e r t i e s n o t o n l y of a b s o r b e r s , b u t of s o u r c e s a s w e l l , t o t h e e x t e n t t h a t t h e s e p r o p e r t i e s a r e n o t m a s k e d b y t h e e f f e c t s of r a d i o a c t i v e d e c a y a n d r a d i a t i o n d a m a g e . I n d e e d , t h e l a s t two p h e n o m e n a c a n b e s t u d i e d in t h e i r own r i g h t , but i t shou ld a l s o b e p o s s i b l e , f o r e x a m p l e , t o d e t e r m i n e w h e t h e r o r no t t h e s o u r c e n u c l e i

196

a r e i n a m a g n e t i c e n v i r o n m e n t . In t h i s w a y one m i g h t g a i n new i n f o r m a t i o n on m a t e r i a l s s u c h a s U H 3 a n d U 0 2 w h i c h a r e known t o h a v e m a g n e t i c p h a s e t r a n s i t i o n s [40, 41] . O b s e r v a t i o n s of t h e M ö s s b a u e r e f f e c t in 237Np f r o m t h e e l e c t r o n - c a p t u r e d e c a y of 2 3 7 Pu shou ld a l s o b e m a d e in a c o m p a r a t i v e s t u d y of t h e t h r e e m o d e s of d e c a y to 2 3 7Np. Such s t u d i e s m i g h t r e v e a l t he c h a r g e s t a t e s of t h e n e p t u n i u m a t o m f o r m e d i m m e d i a t e l y a f t e r d e c a y .

L a t t i c e d y n a m i c s s t u d i e s m a y m a k e u s e of t h e f a c t t h a t 2 3 7Np i s t he h i g h e s t m a s s i s o t o p e y e t f o u n d w h i c h g i v e s t h e M ö s s b a u e r e f f e c t . T h e o r e t i c a l s t u d i e s on h e a v y i m p u r i t i e s i n l i g h t h o s t l a t t i c e s i n d i c a t e t h a t t h e p h o n o n s p e c t r u m i s s e n s i t i v e t o t h e r a t i o of t h e a t o m i c m a s s e s [42] . W i t h 2 3 7 N p t h e p o s s i b i l i t y e x i s t s of v e r y h i g h m a s s r a t i o s , a n d t h e i r i n f l u e n c e on t h e p h o n o n s p e c t r u m s h o u l d b e m a n i f e s t e d i n t h e b e h a v i o u r of t h e r e c o i l - f r e e f r a c t i o n .

T h e M ö s s b a u e r e f f e c t f o l l o w i n g a - d e c a y w i l l g i v e i n f o r m a t i o n o n t h e e n v i r o n m e n t of t h e r e c o i l n u c l e u s a t t h e t i m e of - y - e m i s s i o n . T h i s m a y be u s e f u l i n s t u d y i n g t h e a t o m i c r e l a x a t i o n p r o c e s s e s w h i c h o c c u r a f t e r t h e r e c o i l h a s s t o p p e d ; t h e s e m i g h t b e e x p e c t e d t o be d i f f e r e n t in i n s u l a t o r s and i n c o n d u c t o r s . One p o s s i b l e t e c h n i q u e would b e t o m e a s u r e t h e r e c o i l - f r e e f r a c t i o n a t d i f f e r e n t t i m e i n c r e m e n t s a f t e r t h e e m i s s i o n of t h e a - p a r t i c l e , i n a n a - 7 c o i n c i d e n c e e x p e r i m e n t . A l p h a - e m i t t i n g n u c l e i e m b e d d e d i n a s o l i d c r e a t e i n t e n s i v e r a d i a t i o n d a m a g e i n t h e l a t t i c e ; c u m u l a t i v e r a d i a t i o n d a m a g e c a n b e s t u d i e d by m e a s u r i n g t h e r e c o i l - f r e e f r a c t i o n of t h e s o u r c e f r o m t i m e t o t i m e o v e r a n e x t e n d e d p e r i o d . S o m e r a d i a t i o n d a m a g e i s b e -l i e v e d t o b e s e l f - a n n e a l i n g a t a l l e x c e p t t h e l o w e s t t e m p e r a t u r e [43] ; t h i s c o u l d b e e a s i l y s t u d i e d w i t h M ö s s b a u e r a b s o r p t i o n e x p e r i m e n t s a n d m i g h t g ive t he p e c u l i a r e f f e c t of d e c r e a s i n g the r e c o i l - f r e e f r a c t i o n a s the t e m p e r a -t u r e i s l o w e r e d .

E . SUMMARY AND CONCLUSION

T h e a c t i n i d e e l e m e n t s o f f e r m a n y p o s s i b l e i s o t o p e s which m i g h t exh ib i t t h e M b ' s s b a u e r e f f e c t . Of t h e s e on ly 2 3 7 Np h a s b e e n s t u d i e d , a n d t h e r e s o -n a n c e h a s b e e n found a f t e r bo th t he ß - d e c a y of 237U and the » - d e c a y of 2 4 1 Am. T h e e x p e r i m e n t a l a r r a n g e m e n t and c h e m i c a l p r o c e d u r e s f o r t h i s w o r k h a v e b e e n d e s c r i b e d , and the r e s u l t s of 2 3 7Np s t u d i e s , both e x p e r i m e n t a l and t h e o -r e t i c a l , h a v e b e e n r e v i e w e d . C u r r e n t w o r k w i t h 2 3 7 N p i s p o i n t e d t o w a r d s f i n d i n g s o u r c e s and a b s o r b e r s wh ich h a v e s i n g l e n a r r o w l i n e s . If t h i s s e a r c h i s s u c c e s s f u l , t h e w a y w i l l b e o p e n e d f o r a v a r i e t y of e x p e r i m e n t s w h i c h s h o u l d g ive new i n f o r m a t i o n on s u c h d i v e r s e t o p i c s a s t h e n u c l e a r m o m e n t s of 2 3 7Np, s t r u c t u r a l and bonding d a t a on Np c o m p o u n d s , m a g n e t i c p r o p e r t i e s , l a t t i c e d y n a m i c s , and r a d i a t i o n d a m a g e . T h e p r o s p e c t s l ook v e r y good f o r d e v e l o p i n g t h e M ö s s b a u e r e f f e c t i n t o a t o o l w h i c h c a n b e u s e d t o s t u d y t h e e n t i r e a c t i n i d e r e g i o n .

197

R E F E R E N C E S

[1] KATZ, J . J . . SEABORG, G. T . , The Chemistry of the Actinide Elements, John Wiley & Sons, Inc., New York (1957).

[2] STONE, J. A., PILUNGER, W. L., "Recoilless gamma-ray emission after alpha decay", Phys. Rev. Lett. , 13 (1964) 200.

[3] HYDE, E.K., PERLMAN, I . , SEABORG, G. T . , The Nuclear Properties of the Heavy Elements, Vol. I, Systematics of Nuclear Structure and Radioactivity, Prentice-Hall, Inc., Englewood Cliffs, New Jersey (1964) 303.

[4] STROMINGER, D., HOLLANDER, J. M. , SEABORG, G. T . , "Table of isotopes", Rev. mod. Phys. 30 (1958) 585.

[5] SCHEER, J . , Energie-Niveaus der schweren Kerne, A =213 bis A = 257; Landolt-Börnstein, New Series, Vol. 1/1, Springer-Verlag, Berlin (1961) 3-1.

[6] ASARO, F., STEVENS, F. S., HOLLANDER, J. M., PERLMAN, I., "Anomalous electric dipole conversion coefficients in odd-mass isotopes of the heavy elements", Phys. Rev. 117 (1960) 492.

[7] PILLINGER, W.L., unpublished. [8] LYNCH, F. J. , BAUMGARDNER, J. B. . "Transducer for Mössbauer experiments", Proc. 2nd Int. Conf.

on the Mössbauer Effect, Saclay, France, Sept. 1961; John Wiley & Sons, Inc. , New York (1962) 54. [9] TRZEBIATOWSKI, W. , SELWOOD, P. W. , "Magnetic susceptibilities of urania-thoria solid solutions",

J. Amer. chem. Soc. 72 (1950) 4504. [10] RUNNALLS, O.J. C. , "Neptunium-aluminium intermetallic compounds", J. Metals 5, AIME Trans.

197 (1953) 1460. [11] ZACHARIASEN, W.H., "The crystal structure of NpOj and NpO", The Transuranium Elements, Research

Papers, Natl. Nuclear Energy Ser. , Div. IV, _14B (1949) 1489. [12] WESTRUM, E. F. ,Jr. , HATCHER, J. B. , OSBORNE, D.W., "The entropy and low temperature heat

capacity of neptunium dioxide", J. chem. Phys. 21 (1953) 419. [13] KAPLAN, M., "Influence of recoil stopping time on the Mössbauer effect following alpha decay"

(unpublished). [14] MULLEN, J. G., "The Mó'ssbauer effect following alpha decay: its relation to heat conduction in solids",

Phys. Lett. 15 (1965) 15. L15] KAPLAN, M., FINK, R. D., "Recoil properties of M2Sm from nuclear reactions induced by heavy ions.

I. Samarium compound systems", Phys. Rev. 134 (1964) B30. [16] PILLINGER, W. L., STONE, J. A. , unpublished results. [17] ZACHARIASEN, W. H. , "Crystal radii of the heavy elements", Phys. Rev. 73 (1948) 1104. [18] OSBORNE, D.W., WESTRUM, E. F. ,Jr . , "The heat capacity of thorium dioxide from 10 to 305* K. The

heat capacity anomalies in uranium dioxide and neptunium dioxide", J. chem. Phys. 21 (1953) 1884. [19] MEYER-SCHÜTZMEISTER, L., HANNA, S.S., HEBERLE, J. , DIAZ, J . , RENO, R. W., "Internal magnetic

field in gadolinium", Rev. mod. Phys. 36 (1964) 392. [20] COHEN, R. L., WERNICK, J. H. . "Nuclear hyperfine structure in 166Er", Phys. Rev. 134 (1964) B503. [21] WILLIAMS, H.J. , WERNICK, J. H., NESBITT, E. A., SHERWOOD, R. C. , "Magnetic properties of rare

earth aluminium compounds with MgCu2 structure", J. phys. Soc. Japan 17, Supp. B-I (1962) 91. [22] GORDON, P., KAUFMANN, A. R., "The alloy systems uranium-aluminium and uranium-iron", J. Metals,

AIME Trans. 188 (1950) 182. [23] ROY, P. R., "Determination of a-aluminum solid solubility limits in the aluminum-uranium and

aluminum-plutonium systems", J. nucl. Mater.. (1964) 59. [24] RUNNALLS, O.J. C. , "The intermetallic phase NpBeu", Acta Cryst. 7 (1954) 222. [25] LINDGREN, I. . "Table of nuclear spins and moments", Perturbed Angular Correlations (E.-KARLSSON,

E.MATHIAS and K. SIEGBAHN, Eds. ), North-Holland Publishing Co. , Amsterdam (1964) 379. [26] PERLOW, G.J., "Ratio of the quadrupole moment of the first excited state of 12®Xe to that of the ground

state of ш Хе" , Phys. Rev. 135 (1964) B1102. [27] GOLDANSKII, V. I . , MAKAROV, E. F., "On the sign of change of the charge radius of the " 'Sn nucleus

in the transition to the 23.8 keV level", Phys. Lett. 14 (1965) 111. [28] BLEANEY. В., LLEWELLYN, P.M., PRYCE, M. H. L., HALL, G. R.. "Paramagnetic resonance in neptunyl

rubidium nitrate", Phil. Mag. 45 (1954) 992. [29] HUTCHISON, C. A. . J r . , WEINSTOCK, В., "Paramagnetic resonance absorption in neptunium hexa -

fluoride", J. chem. Phys. 32 (1960) 56.

1 9 8

[30] KROHN, V. E., NOVEY, Т. В., RABOY, S . , "Gyro magnetic ratio of 6 X 10"8 sec 237 Np by angular -correlation techniques", Phys. Rev. 98. (1955) 1187.

[31] GOLDANSKII, V. I . , "The Mössbauer effect and its applications in chemistry", G. E. C. atom Energy Rev. 1 (4) (1963) 3.

[32] PAGES, M., "Neptunium" Nouveau Traité de Chimie Minérale. XV Uranium et Transuraniens (P. PASCAL, Ed. ) Masson et Cie, Editeures, Paris (1962) 237.

[33] BURNEY, G. A. , DUKES, E,K. , GROH, H.J . , "Analytical chemistry of neptunium". Prog, in Nucl. Energy, Series IX, Analyt. Chem. 5 (to be published).

[34] MALM, J. G. . WEINSTOCK, В., CLAASSEN, H. H. -, "Infrared spectra of NpF6 and PuF6 ", J. chem. Phys. 23 (1955) 2192.

[35] EISENSTEIN, I. C. , PRYCE, M. H. L., "Theory of the magnetic and spectroscopic properties of neptunium hexafluoride", Proc. R. Soc. (London) A255 (1960) 181.

[36] KARRAKER, D. G., private communication. [37] BAGNALL, K.W., LA IDLER, J. B., "Neptunium and plutonium trioxide hydrates", J. chem. Soc. (1964)

2693. [38] RUNNALLS, O.I. C . , "The crystal structure of some intermetallic compounds of plutonium". Can. J.

Chem. 34 (1956) 133. [39] KOMURA, S., KUNITOMI, N., TSENG, P.-K., SHIKAZONO, N. . TAKEKOSHI, H., "Mössbauer effect

of the intermetallic compound UFe2", I. phys. Soc. lapan 16 (1961) 1479. [40] GRUEN, D. M. , "Magnetic properties of uranium hydride". J. chem. Phys. 23 (1955) 1708. [41] JONES, W. M , GORDON, J . , LONG, E. A., "The heat capacities of uranium, uranium trioxide, and

uranium dioxide from 15° К to 300* K", J. chem. Phys. 20 (1952) 695. [42] MARADUDIN, A. A., FLINN, P. A., RADCLIFFE, J. M., "The Mössbauer effect for an impurity nucleus,

L Theoretical results for Bravais crystals", Annls. Phys. (N. Y. ) 26 (1964) 81. [43] DIENES, G.J., VINEYARD, G. H., Radiation Effects in Solids, Interscience Publishers, Inc., New York

(1957) 129.

D I S C U S S I O N

J . A . S T O N E s t a t e d t h a t s u b s e q u e n t t o t h e w r i t i n g of h i s p a p e r , r e s o n a n c e e f f e c t s w e r e o b s e r v e d w i t h a NpAl 2 a b s o r b e r ; t h e i s o m e r s h i f t of t he b r o a d , a p p a r e n t l y s i n g l e l i n e w a s +0.54 c m / s wi th r e s p e c t t o NpÛ2.

A r e s o n a n c e e f f e c t w a s a l s o o b s e r v e d w i t h a 2 3 7U s o u r c e i n T h 0 2 , bu t t h e c h a n g e in c o u n t - r a t e w a s v e r y s m a l l , and the r e s o n a n c e w a s qui te b r o a d .

H. F R A U E N F E L D E R s u g g e s t e d tha t new a-y a n g u l a r c o r r e l a t i o n s tud ies m i g h t g ive t h e e x c i t e d s t a t e m a g n e t i c m o m e n t of 2 3 7Np u n a m b i g u o u s l y .

P . H I L L M A N p o i n t e d out t h a t f o r l a t t i c e d y n a m i c s s t u d i e s wi th a heavy i m p u r i t y in a l i gh t l a t t i c e , a n i s o t o p e wi th a f a i r l y s h o r t - l i v e d e x c i t e d s t a t e w a s d e s i r a b l e .

R . H . H E R B E R ( C h a i r m a n ) , J . A . S T O N E , P . K I E N L E a n d G . K . W E R T H E I M d i s c u s s e d t h e q u e s t i o n of t h e s h i f t e d c e n t r e of g r a v i t y f o r t h e r e s o n a n c e at 4.2°K wi th 2 3 7U in N p 0 2 s o u r c e v e r s u s N p 0 2 a b s o r b e r . P o s s i b l e e x p l a n a t i o n s i n c l u d e d a n a s y m m e t r i c p a t t e r n of q u a d r u p o l e s p l i t t i n g , e f f e c t s d u e t o a v e r y t h i c k s o u r c e , a n d d i f f e r e n t t e m p e r a t u r e s of s o u r c e a n d a b s o r b e r d u e t o r a d i o a f c t i v e h e a t i n g .

V . l . GOLDANSKII d i s c u s s e d t h e h e a t - s p i k e m o d e l of r a d i a t i o n d a m a g e i n t h e a - d e c a y p r o c e s s , po in t ing out t h a t t he l o c a l r i s e in t e m p e r a t u r e a long the r e c o i l p a t h w a s i n v e r s e l y p r o p o r t i o n a l t o t he m e a n l i f e t i m e of the exc i ted s t a t e .

T h u s , h e d o u b t e d t h a t a r e s o n a n c e e f f e c t co u l d b e s e e n if t h i s l i f e t i m e w e r e a s s h o r t a s 10" 1 0 s . A p r a c t i c a l e x a m p l e of t he v e r y high l o c a F t e m p e r a t u r e s

199

w a s t h e " n u c l e a r s p o t - w e l d i n g " of p o l y m e r s by t h e p r o d u c t s of t h e 1 0 B (n, a) 7 L i n u c l e a r r e a c t i o n .

J . DANON s t a t e d t h a t c r y s t a l l i n e s o l i d s of 2 1 0 P o b e c a m e a m o r p h o u s b e c a u s e of t h e i r own h i g h a l p h a r a d i a t i o n f i e l d .

P . K I E N L E d i s a g r e e d wi th the h e a t - s p i k e m o d e l , quo t ing w o r k on t h e i r r a d i a t i o n of u r a n i u m w i t h n e u t r o n s and a - p a r t i c l e s a t 4 .2°K. A n n e a l i n g c u r v e s s h o w e d t h a t t h e d a m a g e v e r y q u i c k l y a n n e a l e d e v e n a t t h i s low t e m p e r a t u r e .

R . H . H E R B E R ( C h a i r m a n ) though t t h a t N p 0 2 w a s a f o r t u i t o u s l y good c h o i c e f o r an a b s o r b e r , b e c a u s e t h e e n e r g y - l o s s m e c h a n i s m of t h e r e c o i l a t o m w a s s o m e w h a t l ike n e u t r o n m o d e r a t i o n . T h e l a s t th ing the r e c o i l a t om did w a s t o k ick out a Np a t o m and t ake i t s p l a c e in a l a t t i c e s i t e . M e t a l l i c Np would not h a v e b e e n a s good .

J . DANON be l i eved t h e m o s t i n t e r e s t i n g f e a t u r e of t h i s work f o r c h e m i s t s w a s t h e p o s s i b i l i t y of s tudy ing 5f bonding .

R . M . GOLDING s a i d one had t o b e c a r e f u l in c a l c u l a t i n g w a v e f u n c t i o n s f o r t h e s e e l e m e n t s . S p i n - o r b i t c o u p l i n g w a s p r o b a b l y l a r g e .

P . K I E N L E a g r e e d and s a i d the t h e o r e t i c a l w o r k of J u d d and e x p e r i -m e n t a l op t i ca l s p e c t r a showed tha t the s i t u a t i o n was v e r y c o m p l i c a t e d . T h e a c t i n i d e s w e r e m o r e l i k e t h e 3d t r a n s i t i o n s e r i e s t h a n t h e r a r e e a r t h s i n t h a t t h e c r y s t a l f i e ld w a s t h e s a m e o r d e r of m a g n i t u d e a s t h e L - S coupl ing ;

P . K I E N L E e x p r e s s e d s u r p r i s e tha t T h 0 2 hos t l a t t i c e s f o r s o u r c e s had not g iven s ing le , n a r r o w l i n e s . Single c r y s t a l s of T h 0 2 have been u s e d s u c -c e s s f u l l y a s m a t r i c e s f o r r a r e e a r t h s in e p r w o r k . As f o r R Al 2 compounds , w o r k w i t h E r A l 2 gave a l i n e w h i c h w a s 6 t i m e s b r o a d e r t h a n n a t u r a l l i n e -width, and even though the l i f e t i m e of the t r a n s i t i o n was f a i r l y s h o r t (2 n a n o -s e c o n d s ) , s o m e q u a d r u p o l e sp l i t t i ng was o b s e r v e d . Cohen had u s e d t e r n a r y L a v e s - p h a s e c o m p o u n d s c o n t a i n i n g y t t r i u m t o g i v e a l o w C u r i e - p o i n t .

G . K . W E R T H E I M s t a t e d t h a t w i th i n t e r m e t a l l i c s i t w a s e s s e n t i a l t o h a v e good s t o i c h i o m e t r y a n d good X - r a y d i f f r a c t i o n a n a l y s i s .

ISOMER SHIFT OF THE 8 1 - k e V GAMMA LINE OF i33Cs

W. HENNING, S. HÜFNER, P. KIENLE, D. QUITMAN AND E. STEICHELE TECHNISCHE HOCHSCHULE, DARMSTADT. FEDERAL REPUBLIC OF GERMANY

ГтЬе M ö s s b a u e r e f f e c t of the 81 -keV y - r a y s of i 3 3 C s h a s b e e n o b s e r v e d r e c e n t l y [1, 3] . T h i s t r a n s i t i o n , w h i c h p r o c e e d s f r o m t h e 1= 5 / 2 + e x c i t e d s t a t e t o the 1 = 7 / 2 + g round s t a t e , i s a h igh ly h i n d e r e d Ml r a d i a t i o n (T¿ =6 .0 ns)3, whic-h i s c o m p a t i b l e w i th a ds / 2 and a g 7 / 2 s i n g l e - p a r t i c l e a s s i g n m e n t f o r t h e l a s t odd p r o t o n . A l s o , t h i s i n t e r p r e t a t i o n d o e s not c o n t r a d i c t the m a g -n e t i c m o m e n t s d e t e r m i n e d f o r bo th s t a t e s -И*. J

T o s tudy t h e c h a n g e of m e a n s q u a r e c h a r g e r a d i i f o r s o m e p r o t o n t r a n -s i t i o n s , we began to i n v e s t i g a t e the M ö s s b a u e r e f fec t of the 81-keV t r a n s i t i o n

200

i n i33Cs . W e w i s h t o r e p o r t h e r e t h e f i r s t r e s u l t s on i s o m e r s h i f t s of t h i s t r a n s i t i o n , w h i c h a r e s o m e w h a t u n e x p e c t e d .

B a r i u m - 1 3 3 , w h i c h d e c a y s b y e l e c t r o n c a p t u r e t o e x c i t e d s t a t e s of 1 3 3 Cs, w a s u s e d a s t h e s o u r c e f o r t h e 8 1 - k e V r a d i a t i o n . A c o n v e n t i o n a l t r a n s -m i s s i o n - t y p e r e s o n a n c e a b s o r p t i o n e x p e r i m e n t w a s c a r r i e d o u t . T h e y - r a y s t r a n s m i t t e d t h r o u g h r e s o n a n c e a b s o r b e r s w e r e d e t e c t e d w i t h a l f - i n . - d i a m . 2 - m m - t h i c k N a l ( T l ) c o u n t e r . T h e s o u r c e w a s m o v e d i n r e s p e c t t o t h e a b -s o r b e r w i t h a f e e d - b a c k r e g u l a t e d l oud s p e a k e r d r i v e . T h e s i n u s o i d a l v e l o -c i t y f o r m w a s c a l i b r a t e d w i t h t h e h y p e r f i n e s p e c t r a of t h e 14. 4 - k e V y-t r a n s i t i o n i n m e t a l l i c i r o n . In a l l e x p e r i m e n t s s o u r c e a n d a b s o r b e r w e r e c o o l e d t o 4 . 2°K. I n a s e a r c h f o r a s o u r c e m a t e r i a l g i v i n g a h i g h f r a c t i o n of r e c o i l l e s s e m i s s i o n a n d a n u n s p l i t l i n e , s o u r c e s of BaSC>4, ВаС1г a n d B a s u b s t i t u t e d i n C a F 2 w e r e e x a m i n e d . T h e m a x i m u m r e s o n a n c e a b -s o r p t i o n e f f e c t s m e a s u r e d w i t h t h e s e s o u r c e s a n d a 4 4 0 - m g / c m 2 - t h i c k C s C l a b s o r b e r i s g i v e n i n T a b l e I . F o r a l l f u r t h e r e x p e r i m e n t s t h e s o u r c e w i t h B a i n C a F 2 , w h i c h g i v e s a h i g h r e c o i l l e s s e m i s s i o n , w a s u s e d . T h e e m i s s i o n l i n e of t h i s s o u r c e w a s s t u d i e d w i t h a C s C l a b s o r b e r . F o r C s C l one e x p e c t s a n a b s o r p t i o n l i n e w i t h n a t u r a l l i n e - w i d t h a s n o q u a d r u p o l e s p l i t t i n g i s e x -p e c t e d b e c a u s e of t h e c u b i c s t r u c t u r e of С s C l . O n e c a n a l s o a c c o u n t f o r l i n e b r o a d e n i n g c a u s e d b y t h e f i n i t e a b s o r b e r t h i c k n e s s b e c a u s e t h e D e b y e t e m p e r a t u r e of C s C l i s known . F i g u r e l a s h o w s t h e r e s u l t of a m e a s u r e m e n t w i t h a 4 4 0 - m g / c m 2 - t h i c k C s C l a b s o r b e r . T h e t r a n s m i s s i o n a s a f u n c t i o n of t h e D o p p l e r s h i f t s h o w s a s i n g l e L o r e n t z i a n - s h a p e d l i n e w i t h a f u l l w i d t h a t h a l f m a x i m u m of 1 m m / s . A f t e r p r o p e r c o r r e c t i o n f o r t h e a b s o r b e r t h i c k n e s s Г е х р = 1. 2 X 1 0 " ' e V c o m p a r e d w i t h r n a t = 1. 5 X 1 0 " 7 e V i s d e r i v e d .

TABLE I

T H E R E S O N A N C E A B S O R P T I O N O F T H E 8 1 - k e V y - L I N E W I T H T H E 1 3 3 B A A C T I V I T Y I N V A R I O U S S O U R C E M A T E R I A L S A N D

A C s C l A B S O R B E R ( 4 4 0 m g / c m 2 )

Source Absorption

m

ш В а in BaS04 1.2

BaCl2 0.3

CaF2 1 .3

T h i s e x p e r i m e n t s h o w s t h a t t h e r e i s n o q u a d r u p o l e s p l i t t i n g i n t h e s o u r c e a n d n o d e t e c t a b l e e f f e c t s of t h e p r e c e d i n g e l e c t r o n c a p t u r e d e c a y o n t h e e m i s s i o n l i n e .

F u r t h e r e x p e r i m e n t s w e r e m a d e w i t h C S 2 S O 4 , C S C I O 3 , С в г С г г О ? , C s B i 2 a n d C s m e t a l a b s o r b e r s t o i n v e s t i g a t e q u a d r u p o l e s p l i t t i n g (CS2SO4 and C s C 1 0 3 f o r w h i c h t h e q u a d r u p o l e c o u p l i n g of t h e g r o u n d s t a t e i s known [4]) a n d i s o m e r s h i f t s ( e s p e c i a l l y C s B i 2 a n d C s m e t a l ) . T r a n s m i s s i o n s p e c t r a f o r CS2SO4 and C s B i 2 a r e s h o w n in F i g s , l b a n d l c . No e v i d e n c e w a s f o u n d

2 0 1

v in c m / s - 1 4 -1.2 -0.8 - O í О 0.4 0.8 1.2 1.6

- I i -1.2 -0.8 -0.4 0 0 4 0.6 12 1.6

V in c m / S

FIG. 1. Relative transmission N(v)/N(«>) of the 81-keV y-rays of 133Cs as a function of Doppler velocity, v, for a source df 133Ba in CaF2 and various resonance absorbers at 4. 2°K

f o r q u a d r u p o l e s p l i t t i n g in a l l a b s o r b e r s i n v e s t i g a t e d . S m a l l i s o m e r s h i f t s i n Cs2 S 0 4 , CSCIO3, C s 2 C r 2 0 7 a n d C s B i 2 r e l a t i v e t o C s C l a r e i n d i c a t e d by t h e m e a s u r e m e n t s . T h e s e r e s u l t s a r e s u m m a r i z e d in T a b l e II. Unfo r tu -n a t e l y no r e s o n a n c e e f f e c t w a s f o u n d i n C s m e t a l b e c a u s e of t h e low D e b y e t e m p e r a t u r e of t h i s s u b s t a n c e . T h i s c o m p l i c a t e s t h e i n t e r p r e t a t i o n of the o b s e r v e d i s o m e r s h i f t s . On t h e o t h e r hand , one n o t e s t h a t t h e 7 - l i n e in t h e i n t e r m e t a l l i c c o m p o u n d CsBÍ2 , which i s s u p e r c o n d u c t i n g a t 4. 2°K, i s sh i f t ed t o w a r d s h i g h e r e n e r g i e s w i t h r e s p e c t t o t h e l i n e s in a l l i o n i c c o m p o u n d s . T h e m e a s u r e d e l e c t r i c d i p o l e m o m e n t of t h e C s C l m o l e c u l e i n d i c a t e s 75% i o n i c c h a r a c t e r of t h e b o n d . C s B Í 2 i s an i n t e r m e t a l l i c c o m p o u n d of t h e s t r o n g l y e l e c t r o p o s i t i v e C s w i t h t h e e l e c t r o n e g a t i v e B i . A c c o r d i n g t o P a u l i n g [ 5 ] , a t r a n s f e r of e l e c t r o n s f r o m t h e e l e c t r o n e g a t i v e Bi to t h e e l e c -t r o p o s i t i v e C s s t a b i l i z e s t h i s compound . T h i s e f f ec t i s e s p e c i a l l y p ronounced i n i n t e r m e t a l l i c c o m p o u n d s of a l k a l i m e t a l s . T h e r e f o r e i t s e e m s p r o p e r t o a s s u m e t h a t t h e e l e c t r o n d e n s i t y at the C s n u c l e u s i s h i g h e r in C s B i 2 t han in C s C l . T h e p o s i t i v e i s o m e r s h i f t i n C s B i 2 c o m p a r e d w i t h C s C l i m p l i e s

202

TABLE II

R E S U L T S F O R T H E 8 1 - k e V y - L I N E O F i 3 3 C s I N V A R I O U S A B S O R B E R S

Absorber ÓE

( m m / s) &E

(10-9 eV) a r exp

( m m / s ) Гехр

2 r n a t

CsBi2 +0.095 ± 0 . 0 2 2 +26 ±6 0 . 4 1 . 1 1 .9

CsCl 0 .00 i 0 ,008 0 . 0 4 2 . 1 1 . 3 1 .0 1.7

Cs 2 S0 4 - 0 . 0 3 0 ± 0 . 0 0 6 - 8 . 1 ± 1 . 6 1 . 6 1 . 1 1 .9

CsC103 - 0 . 0 3 5 ± 0 . 0 0 6 - 9 . 5 ± 1 .6 1 . 5 1 .0 1.7

C s 2 C r 2 0 7 - 0 . 039 ± 0. 008 - 1 0 , 5 ± 2 . 1 1 .3 1 . 0 1 .7

6E= isomer shifts, a = max imum resonance absorption, Г е Х р = exper imental l ine-width, r e X p / 2 r n a t = ratio of the experimental to natural l ine-width .

t ha t the n u c l e a r c h a r g e r a d i u s i s l a r g e r in t he 8 1 - k e V s t a t e than in the ground s t a t e . It i s d i f f i c u l t t o d e t e r m i n e 6<^r2 ( the c h a n g e of t h e m e a n s q u a r e c h a r g e r a d i u s f o r t he 8 1 - k e V t r a n s i t i o n ) q u a n t i t a t i v e l y , b e c a u s e of t he l a c k of a n e x a c t k n o w l e d g e of t h e d i f f e r e n c e of t h e e l e c t r o n d e n s i t i e s i n t h e i n -v e s t i g a t e d a b s o r b e r s . It i s p r o b a b l y a good g u e s s to a s s u m e tha t the i s o m e r s h i f t b e t w e e n C s B i 2 and C s 2 C r 2 0 7 , w h i c h s h o w s t h e l a r g e s t n e g a t i v e s h i f t , i s a t t r i b u t e d t o a b o u t o n e 6 s e l e c t r o n . I t s d e n s i t y c a n be d e d u c e d [б] f r o m t h e k n o w n h y p e r f i n e c o u p l i n g c o n s t a n t a6S a n d t h e g f a c t o r of t h e g r o u n d s t a t e , and y i e l d s (A(0)26S= 0. 545 X l 0 26 c m 3 a f t e r a p p l y i n g a r e l a t i v i s t i c c o r -r e c t i o n f a c t o r . W i t h t h e i s o m e r s h i f t 6E g i v e n by t h e r e l a t i o n :

6E = (27r/3)Ze 2 x | ^ ( 0 ) | 26 s X 6 < r 2 >

6 < r 2 y ^ +0. 004 f m 2 i s d e r i v e d . T h i s r e s u l t d i s a g r e e s wi th t h e p r e d i c t i o n f o r a p r o t o n t r a n s i t i o n b e t w e e n s i n g l e p a r t i c l e d 5 / 2 and g 7 / 2 s t a t e s in a f in i t e s q u a r e we l l p o t e n t i a l , c a l c u l a t e d by E i s i n g e r and J a c c a r i n o [7J to b e Ó ( r 2 ) = - 0 . 14 f m 2 . T h e s m a l l 6<^r 2 y s u g g e s t s t h a t t h e n u c l é o n s i n b o t h s t a t e s o c c u p y t h e s a m e o r b i t a l s a n d d i f f e r o n l y by t h e i r c o u p l i n g s c h e m e .

T h e a b s o r p t i o n l i n e s of a l l i o n i c c o m p o u n d s i n v e s t i g a t e d s h o w s m a l l n e g a t i v e i s o m e r s h i f t s r e l a t i v e to C s C l . T h i s i n d i c a t e s a s m a l l e r e l e c t r o n d e n s i t y a t t h e n u c l e u s t h a n in C s C l , r e f l e c t i n g an i n c r e a s i n g ion ic c h a r a c t e r of t he bond b e t w e e n Cs and the [ S 0 4 ] 2 " , [СЮ 3 ]" and [Cr2C>7j2- an ions . The d i f f e r e n c e of t h e i s o m e r s h i f t b e t w e e n C s 2 C r 2 O i a n d C s C l i s a b o u t 29% of t h e s h i f t b e t w e e n С з 2 С г 2 0 7 a n d C s 2 B i , wh ich we a t t r i b u t e d t o t he c o n t r i b u -t i o n of abou t one 6 s e l e c t r o n .

T h i s r e s u l t would p r e d i c t 71% ion ic c h a r a c t e r f o r t he bond in C s C l , which a g r e e s s u r p r i s i n g l y w e l l w i t h a n i o n i c c h a r a c t e r of 75% d e d u c e d f r o m t h e e l e c t r i c d i p o l e m o m e n t of C s C l . I t c o n f i r m s t h e c a l i b r a t i o n s c h e m e f o r i s o m e r s h i f t s i n C s w h i c h w a s p r o p o s e d a b o v e . F u r t h e r m o r e i t s u g g e s t s t h a t C s 2 S 0 4 , CSCIO3 a n d C s 2 C r 2 0 7 a r e h i g h l y i o n i c c o m p o u n d s .

203

In t h i s c o n t e x t o n e s h o u l d a l s o m e n t i o n t h a t t h e s o u r c e s h o w s a s m a l l p o s i t i v e i s o m e r s h i f t 6 E = (+1. 8 ± 2 . 9 ) X 1 0 ' 9 eV r e l a t i v e to C s C l in a c c o r d a n c e w i t h a s l i g h t l y s m a l l e r i o n i c c h a r a c t e r of C s F a s c o m p a r e d w i t h C s C l . It would be i n t e r e s t i n g to m e a s u r e t he c h e m i c a l s h i f t s in n u c l e a r m a g n e t i c r e -s o n a n c e f o r v a r i o u s C s . c o m p o u n d s t o o b t a i n t h e i s o m e r s h i f t c a l i b r a t i o n s c h e m e .


T h e a u t h o r s w o u l d l i k e t o t h a n k P r o f e s s o r H e l l w e g e f o r h i s f r i e n d l y s u p p o r t of t h i s w o r k . M i s s F o r g a t s c h ' s d e v o t e d e n d e a v o u r t o s o l v e o u r c h e m i c a l p r o b l e m s i s g r a t e f u l l y a p p r e c i a t e d .

R E F E R E N C E S

[1] MARSHALL, J . H . , PERLOW, G . J . , RUBY, S.L. , Bull. Amer. phys. Soc., Series II, 10 (1965) 64. 10(1965) 64.

[2] STEICHELE, E.. HENNING, W., QUITMANN, D., HÜFNER, S., KIENLE, P., Phys. Verh. 5(1965) 102. [3] Nuclear Data Sheets, NCR 61-2-90. [4] EMSHWILLER, M., HAHN, E.L.. KAPLAN, D. Phys. Rev. 118 (1960) 414. [5] PAULING, L., The Nature of Chemical Bond, Cornell University Press (1960). [6] KOPFERMANN, H. , Kernmomente, Akademische Verlagsgesellschaft mbH., Frankfurt am Main(1956). [7] EISINGER, J . , IACCARINO, V.J., Rev. mod. Phys. 30 (1958) 528.

D I S C U S S I O N

J . D A N O N a s k e d w h e t h e r t h e a u t h o r s h a d r u n C s l . . P . K I E N L E r e p ü e d t h a t t h e y w e r e d o i n g i t n o w a n d t h a t t h e r e w e r e

n m r d a t a on a l l t h e c a e s i u m h a l i d e s . J . D A N O N r e m a r k e d t h a t M ö s s b a u e r m e a s u r e m e n t s s h o w e d t h a t a l l

t h e c a e s i u m h a l i d e s s e e m e d t o h a v e a b o u t t h e s a m e i o n i c i t y . R . H. H E R B E R ( C h a i r m a n ) i n q u i r e d abou t t he s t a t u s of t he i s o m e r sh i f t

c a l i b r a t i o n f o r i od ine , and a s k e d if a C s l e x p e r i m e n t cou ld be done f r o m the i o d i n e s i d e .

V . l . G O L D A N S K I I a n s w e r e d t h a t t h e i o d i n e c a l i b r a t i o n n e e d e d t o b e r e d o n e a n d t h a t n 2 r d a t a w e r e n e e d e d f o r a l l t h e c a e s i u m h a l i d e s .

P . K I E N L E s a i d t h a t t h e r e w e r e n 2 r d a t a f o r C s C 1 0 3 , but t h i s m a t e r i a l w a s s o d i f f i c u l t to w o r k wi th tha t they did not t r y i t .

V . l . G O L D A N S K I I s t a t e d t h a t t h e e f f e c t of djr-ртг b o n d i n g s h o u l d b e c o n s i d e r e d .

P . K I E N L E a g r e e d a n d a d d e d t h a t p h o l e s m i g h t a l s o be s t u d i e d by t h e M ö s s b a u e r t e c h n i q u e .

N . N . G R E E N W O O D s a i d t h a t s i n c e t h e o b s e r v e d s h i f t s w e r e s o s m a l l , one h a d to q u e s t i o n t h e s t a t i s t i c s of t h e d a t a a n d t h e c u r v e - f i t t i n g m e t h o d s , and a l s o e n q u i r e a b o u t t h e r e p r o d u c i b i l i t y of t h e s h i f t s .

204

P . KIENLE a n s w e r e d tha t the s m a l l s i ze of the s h i f t s m a d e the e x p e r i -m e n t s v e r y d i f f i cu l t , but tha t m a n y e x p e r i m e n t s w e r e p e r f o r m e d , and the r e p r o d u c i b i l i t y w a s good . T h e e r r o r s quo ted i n c l u d e d not on ly s t a t i s t i c s , bu t a l s o e s t i m a t e s of the s y s t e m a t i c e r r o r s , and w e r e v e r y c o n s e r v a t i v e .

R .M. GOLDING asked if t h e r e w e r e d i f f e r e n c e s in the n m r ch emi ca l sh i f t s f o r the c a e s i u m h a l i d e s .

P . K I E N L E r e p l i e d t h a t t h e r e w e r e d i f f e r e n c e s , but t h e y w e r e s m a l l .

THE MÖSSBAUER EFFECT IN THE NOBLE GASES

A REVIEW

P. HILLMAN THE WEIZMANN INSTITUTE OF SCIENCE

REHOVOTH, ISRAEL

A. INTRODUCTION

^Since t h e M ö s s b a u e r e f f e c t i s z e r o f o r a f r e e g a s , i t i s n e c e s s a r y t o l o c a l i z e a M ö s s b a u e r a t o m by p h y s i c a l o r c h e m i c a l m e a n s . T h i s c a n b e done f o r a nob le g a s by f r e e z i n g , by " p h y s i c a l " t r app ing such a s by ad-s o r p t i o n o r i n c l a t h r a t e compounds , o r , s ince the r e c e n t syn thes i s of noble-g a s compounds , by c h e m i c a l b inding. Apar t f r o m n u c l e a r i n fo rma t ion , with which we a r e n o t - h e r e - p r i m a r i l y c o n c e r n e d , t he kind of i n fo rma t ion that h a s b e e n o b t a i n e d f r o m M ö s s b a u e r e x p e r i m e n t s on t h e n o b l e g a s e s c o n c e r n s m a i n l y t he n a t u r e of t he b ind ing . T h i s a l l o w s ' u s t o divide t h i s r e p o r t in to two s e c t i o n s . The f i r s t c o n c e r n s s i t u a t i o n s of ma in ly c h e m i c a l , and t h e r e -f o r e n e a r l y h a r m o n i c , b ind ing , and p r e s e n t s i n f o r m a t i o n m o s t l y abou t t h e e x i s t e n c e a n d n a t u r e of n o b l e - g a s c o m p o u n d s , J < r - T h e s e c o n d d i s c u s s e s t h e m o r e u n u s u a l s i t u a t i o n of " p h y s i c a l " and, in g e n e r a l , n o n - h a r m o n i c b ind ing , w h e r e the i n f o r m a t i o n p r i m a r i l y c o n c e r n s the n a t u r e and g e o m e t r y of the f o r c e s involved .

T h e i s o t o p e s wi th which m o s t of the w o r k r e p o r t e d in t h e two s e c t i o n s w a s done w e r e 8 3 K r and 1 2 9 Xe; 1 3 1Xe was a l s o u s e d j The r e l e v a n t da ta on t h e s e n u c l e i a r e shown in T a b l e I.

B. CHEMICAL BINDING

The compounds of xenon a r e e a s i e r to p r e p a r e and s t a b l e r than those of k r y p t o n , s o a l l t h e p u b l i s h e d w o r k h a s in f á c t d e a l t w i t h t h e m . Indeed , a t t e m p t s at s e e i n g the M ö s s b a u e r e f f e c t in t h e c o m p o u n d Krlfc h a v e f a i l e d u n e x p e c t e d l y , f o r a s y e t unknown r e a s o n s [2]. Al l t h e p u b l i s h e d xenon

205

TABLE I

DATA ON ISOTOPES USED

Isotope Natural

abundance (%)

Energy of level

(keV)

Ground-state spin

Excited-state spin

Life t ime of leve l

(ris) Patent

Decay of

parent

Life t ime of parent

83 K r 1 1 . 5 9 . 3 9 /2 7 /2 147 8 3 K r m У 1 .9 h

8 3 B r , e" 2. 3 h

s3Rb EC 83 d

"S Xe 2 6 . 4 40. 0 1/2 3 / 2 0 . 7 i 2 9 X e m У 8 . 0 d

129J В" 1. 6X10 7 y

' " C s EC 30 .7 h

ш x e 2 1 . 2 80 .2 3 /2 1/2 0 . 3 131Хе m У 12 d

l S J j ß " 8 . 1 d

c o m p o u n d w o r k h a s b e e n b y G . J . P e r l o w a n d c o l l e a g u e s , m o s t l y w i t h t h e 4 0 - k e V l e v e l of 1 2 9 X e , u s i n g t h e 1 2 9 I p a r e n t ( s e e T a b l e I). In a l l t h e i r w o r k b o t h s o u r c e a n d a b s o r b e r w e r e a t l i q u i d h e l i u m t e m p e r a t u r e t o m a x i -m i z e t h e e f f e c t f o r t h e r a t h e r h i g h l e v e l (40 k e V ) . T h e l a t e s t v e r s i o n of t h e i r a p p a r a t u s i s d e s c r i b e d i n R e f . [3 ] .

It i s i m p o r t a n t t o r e m e m b e r t h a t a l l c o n c l u s i o n s d e r i v e d about t h e xenon c o m p o u n d s c r e a t e d in t h e 1 2 9I s o u r c e s c o n c e r n t h e f i r s t n a n o s e c o n d a f t e r t he c r e a t i o n of 1 2 9 X e , a n d g i v e n o i n f o r m a t i o n a b o u t l o n g e r p e r i o d s , s i n c e t h e r e l e v a n t g a m m a h a s t h e n l e f t t h e e n v i r o n m e n t . C o n c l u s i o n s about a b s o r b e r s , of c o u r s e , c o n c e r n s t a b l e s i t u a t i o n s . L e t u s t h e r e f o r e f i r s t c o n s i d e r t h e l a t t e r .

T h e s t a b l e c o m p o u n d s w h i c h h a v e b e e n s t u d i e d b y P e r l o w e t a l . , a p a r t f r o m t h e h y d r o q u i n o n e - X e - c l a t h r a t e s , w h i c h w i l l b e d i s c u s s e d b e l o w , a r e s o d i u m p e r x e n a t e , x e n o n t e t r a f l u o r i d e , x e n o n d i f l u o r i d e , a n d t h e o x i d e Х е О з [ 3 - 5 ] .

VELOCITY ( m m / t e c )

FIG. 1. Representative spectra taken with NaI29I source [4]

F i g u r e 1 s h o w s s o m e of t h e i r r e s u l t s , w i t h â s o u r c e of N a l 2 9 I . A l l l i n e - w i d t h s e x c e p t t h o s e of t h e d i f l u o r i d e a r e n e a r l y e q u a l , w i th in t h e l i m i t s of e r r o r , a n d c o n s i s t e n t w i t h t h e e l e c t r o n i c a l l y m e a s u r e d l i f e t i m e of t h e l e v e l c o n c e r n e d , i n d i c a t i n g t h a t t h e s o u r c e , t h e c l a t h r a t e a n d t h e p e r x e n a t e

207

h a v e v e r y s m a l l e l e c t r i c f i e l d g r a d i e n t s (EFG.). T h e l i n e b r o a d e n i n g in t h e d i f l u o r i d e i s not u n d e r s t o o d ; if t h e s a m e m e c h a n i s m i s p r e s e n t in t h e k r y p t o n d i f l u o r i d e i t m a y e x p l a i n R u b y ' s f a i l u r e t o s e e a n e f f e c t , s i n c e t h e l i n e i n k r y p t o n i s i n t r i n s i c a l l y m u c h n a r r o w e r a n d m i g h t b e c o m p l e t e l y s m e a r e d o u t .

S o d i u m p e r x e n a t e h a s a n u n c e r t a i n f o r m u l a , n o m i n a l l y Na4XeC>6. п Н г О w i t h n = 2, 5 o r 8. X - r a y e v i d e n c e s h o w s an a p p r o x i m a t e l y r e g u l a r o c t a h e -d r o n of o x y g e n a t o m s s u r r o u n d i n g t h e x e n o n , w i t h a x e n o n - o x y g e n d i s t a n c e of 1.9 A. Such a c u b i c e n v i r o n m e n t p r o v i d e s (by s y m m e t r y ) no c r y s t a l l i n e and t h e r e f o r e no a t o m i c E F G a s s e e n in t he u n s p l i t M ö s s b a u e r l i ne . E v e n in t h e p r e s e n c e of a s m a l l c r y s t a l l i n e E F G (too s m a l l t o b e d i r e c t l y s e e n ) , t h e a t o m i c E F G would v a n i s h , at l e a s t in the l i m i t of ion ic b ind ing , if t he xenon h a s v a l e n c e s i x o r e i g h t a s e x p e c t e d h e r e , s i n c e t h e a t o m w o u l d i n e i t h e r c a s e b e i n a s i n g l e t s t a t e .

X e n o n d i f l u o r i d e i s a l i n e a r m o l e c u l e w i t h 2.OA s e p a r a t i o n s , a n d t h e t e t r a f l u o r i d e i s a 2 .7 Â s q u a r e w i t h t h e x e n o n a t t h e c e n t r e . T h e o b s e r v e d q u a d r u p o l e sp l i t t i ng i s a l w a y s the p r o d u c t of t he E F G and the n u c l e a r q u a d r u -p o l e m o m e n t Q; t h e l a t t e r w a s m e a s u r e d b y P e r l o w [5] b y c o m p a r i n g t h e s p l i t t i n g o b s e r v e d i n 1 2 9 X e E i w i t h t h a t m e a s u r e d i n t h e s a m e c o m p o u n d of

1 3 l X e , w h o s e q u a d r u p o l e m o m e n t w a s known f r o m o p t i c a l and a t o m i c b e a m m e t h o d s . T h e E F G in t h i s c o m p o u n d i s t h e n f o u n d t o b e c o n s i s t e n t w i t h a s i m p l e m o d e l in w h i c h t h e bond ing of t h e x e n o n i s a t t r i b u t e d t o t h e p x and py o r b i t a l s i n t h e p l a n e of t h e m o l e c u l e . T h e s e e l e c t r o n s a r e s t r e t c h e d o u t -w a r d , a n d b e c a u s e of t h e c u b i c d e p e n d e n c e of t h e E F G on r a d i u s , m o s t of t h e E F G at t he n u c l e u s c o m e s f r o m the r e m a i n i n g doubly occup ied p z o r b i t a l . T h e c a l c u l a t e d E F G p r o d u c e d b y t h e s e t w o e l e c t r o n s a l o n e i s a l i t t l e t o o l a r g e , s i m p l y i n d i c a t i n g a n o n - z e r o c o n t r i b u t i o n f r o m t h e px a n d p y o r b i t a l s . A s i m i l a r m o d e l f o r t h e d i f l u o r i d e a s c r i b e s t he bonding t o t he two p z e l e c t r o n s (z b e i n g a l o n g t h e m o l e c u l e ) w h i c h a r e d r a w n o u t w a r d s l e a v i n g an E F G p r o -d u c e d b y t h e t w o h o l e s e x a c t l y e q u a l in m a g n i t u d e (but of o p p o s i t e s i g n ) t o t h e E F G p r o d u c e d in t h e t e t r a f l u o r i d e b y t h e t w o p z o r b i t a l s , in a g r e e m e n t w i t h e x p e r i m e n t ( s e e F i g . 1).

T h e o x i d e Х е О з a l s o e x h i b i t s a q u a d r u p o l e s p l i t t i n g , r a t h e r s m a l l e r t h a n t h e f l u o r i d e s , c o r r e s p o n d i n g t o 0 . 4 of a p z h o l e .

C o n s i d e r i n g n o w t h e b e h a v i o u r of t h e 1 2 9I a n d 1 3 1 I s o u r c e s , w e n o t e d a b o v e t h a t N a l p r o d u c e s a n u n s p l i t l i n e . If t h e ß" d e c a y of t h e i o d i n e d o e s no t k n o c k t h e a t o m ou t of t h e l a t t i c e s i t e , t h e Xe w i l l b e in a c u b i c s i t e , s o t h a t t h e E F G i s z e r o . T h e a t o m i c c o n t r i b u t i o n to the E F G should in any c a s e b e z e r o , s i n c e t h e ß~ d e c a y of I" s h o u l d u s u a l l y l e a d t o Xe° ; ß~ d e c a y i s known g e n e r a l l y t o l e a v e t h e e l e c t r o n n u m b e r i n s t a n t a n e o u s l y u n c h a n g e d [6].

A n o t h e r s o u r c e w h i c h w a s f o u n d t o g i v e a n u n s p l i t l i n e w a s I2 . In t h i s c a s e t h e l a t t i c e i s n o t c u b i c a n d t h e d e c a y of I o m i g h t b e e x p e c t e d t o l e a d t o X e + . T h e r e s u l t s h o w s t h a t t h e c r y s t a l l i n e c o n t r i b u t i o n t o t h e E F G i s a p p a r e n t l y n e g l i g i b l e and t h a t , p o s s i b l y , t h e xenon w i th in a n a n o s e c o n d a f t e r t h e d e c a y s t e a l s a n e l e c t r o n f r o m i t s n e i g h b o u r i n g i o d i n e , b e c o m i n g Xe° , w h i c h of c o u r s e h a s no E F G .

In a s u c c e s s f u l a t t e m p t t o c r e a t e x e n o n c o m p o u n d s i n t h e ß~ d e c a y of t h e i o d i n e , P e r l o w e t a l . [3 , 4] . c h o s e a n u m b e r of c o m p o u n d s i n w h i c h t h e i o d i n e h a d a h i g h f o r m a l v a l e n c e .

208

T h e f a c t t h a t t h e s p e c t r u m f o r N a l O ß i s s t r o n g l y s p l i t i n d i c a t e s t h a t s o m e c o m p o u n d i s i n d e e d f o r m e d , s i n c e t h e c r y s t a l l i n e c o n t r i b u t i o n t o t h e s p l i t t i n g i s a p p a r e n t l y g e n e r a l l y s m a l l . T h e M ö s s b a u e r e f f i c i e n c y i s a l s o h i g h e r t h a n t h a t s e e n f o r n e u t r a l x e n o n . T h a t t h e c o m p o u n d f o r m e d i s ХеОз i s d e d u c e d f r o m t h e e q u a l i t y of t h e o b s e r v e d s p l i t t i n g (w i th in 2%) a n d t h e i s o m e r i c s h i f t w i t h t h o s e o b s e r v e d f o r c h e m i c a l Х е О з a s a n a b s o r b e r , and t h e s i m i l a r i t y of t h e d e d u c e d f i e l d g r a d i e n t w i t h t h a t o b s e r v e d by q u a d r u p o l e r e s o n a n c e in KIOs . B y u s i n g a s c a l e f a c t o r t o m u l t i p l y t h e i s o m e r i c s h i f t s o b s e r v e d in t h e M ö s s b a u e r e f f e c t in i o d i n e d e d u c e d b y e q u a t i n g I C I 4 a n d X e C l 4 , t h e i s o m e r i c s h i f t s of Х е О з a n d К Ю з a r e a l s o e x t r e m e l y c l o s e .

T h e p r o d u c t i o n of a c o m p o u n d i n t h e d e c a y of K I O 4 , N a 3 H 2 l 0 6 a n d И а г Н з Ю б i s t e s t i f i e d b y t h e o b s e r v a t i o n of an e x c e p t i o n a l l y h i g h M ö s s b a u e r e f f i c i e n c y , i n d i c a t i n g s t r o n g c h e m i c a l b o n d i n g . In a l l t h r e e c a s e s , t h e l i n e i s u n s p l i t , a s in N a é X e O e a n d in t h e M ö s s b a u e r e f f e c t in i od ine in KIO4. T h i s

i s c o n s i s t e n t w i t h t h e t e t r a h e d r a l a n d o c t a h e d r a l e n v i r o n m e n t s in t h e K I O 4

a n d t h e o t h e r t w o c o m p o u n d s r e s p e c t i v e l y . S t r o n g e v i d e n c e t h a t t h e c o m -

p o u n d f o r m e d a t l e a s t i n t h e K I O 4 d e c a y i s X e Û 4 c o m e s f r o m t h e a l m o s t

e x a c t e q u a l i t y ( w i t h t h e s c a l e f a c t o r m e n t i o n e d a b o v e ) of t h e i s o m e r i c s h i f t

w i t h t h a t f o r i o d i n e in K I O 4 .

T h e d e c a y of KICI4 . H 2 O in a l i g n e d c r y s t a l l i n e f o r m l e a d s t o a s p l i t l i n e

w i t h u n e q u a l c o m p o n e n t s . T h e i n t e n s i t y r a t i o of t h e c o m p o n e n t s i s j u s t t h a t

e x p e c t e d if t h e p l a n a r m o l e c u l e Х е С Ц i s f o r m e d w i t h no c h a n g e i n t h e m o l e -c u l a r a x i s . T h e i s o m e r i c s h i f t a l s o a g r e e s p e r f e c t l y w i t h t h a t of i o d i n e in KICI4 . H 2 0 . A s u b s t a n t i a l p r o d u c t i o n of X e C l 2 i s e x c l u d e d b e c a u s e t he l i n e a r X e C l 2 m o l e c u l e s w o u l d b e p r o d u c e d a l m o s t i s o t r o p i c a l l y a n d g i v e e q u a l p e a k s . An a t t e m p t t o p r o d u c e Х е С 1 г i t s e l f b y t h e d e c a y of K I C I 2 . H 2 O w a s p r o b a b l y s u c c e s s f u l , bu t t h e p i c t u r e w a s c l o u d e d b y t h e a p p a r e n t a p p e a r a n c e of a c o n s i d e r a b l e q u a n t i t y of X e C l 4 .

C . P H Y S I C A L B I N D I N G

V e r y l i t t l e M ö s s b a u e r w o r k h a s b e e n d o n e on f r o z e n k r y p t o n , a n d n o n e a p p a r e n t l y on x e n o n . It i s r a t h e r d i f f i c u l t t o p r o d u c e a g o o d u n i f o r m f r o z e n a b s o r b e r , b u t it i s p o s s i b l e t o w o r k w i t h f r o z e n s o u r c e s , and a f u r t h e r s t u d y of t h i s i d e a l V a n d e r W a a l s c r y s t a l s y s t e m s e e m s c a l l e d f o r . T h e n a r r o w t e m p e r a t u r e r a n g e i s of c o u r s e a d r a w b a c k . H a z o n y a n d H i l l m a n [8] o b -s e r v e d a M ö s s b a u e r e f f i c i e n c y of t h e o r d e r of 50% f o r a f r o z e n k r y p t o n s o u r c e a t a b o u t 8 7 ° K w i t h a n u n s p l i t ( t h o u g h p r o b a b l y b r o a d e n e d ) l i n e , a s e x p e c t e d .

R u b y [2] a t t e m p t e d t o p r o d u c e a k r y p t o n a b s o r b e r b y a d s o r b i n g t h e g a s on t o c h a r c o a l . A l t h o u g h a d e q u a t e q u a n t i t i e s of g a s w e r e a d s o r b e d , no e f f e c t w a s o b s e r v e d . T h i s m i g h t p e r h a p s i m p l y s o m e k i n d of s u r f a c e f r e e d o m f o r t h e a d s o r b e d g a s , b u t t h i s i a h i g h l y s p e c u l a t i v e w i t h o u t f u r t h e r w o r k .

T h e g r e a t b u l k of t h e w o r k o n p h y s i c a l l y b o u n d s y s t e m s h a s b e e n d o n e o n t h e h y d r o q u i n o n e c l a t h r a t e . T h i s s t r u c t u r e i s a t h r e e - d i m e n s i o n a l n e t -w o r k of n e a r l y s p h e r i c a l c a v i t i e s f o r m e d b y t w o i n t e r p e n e t r a t i n g l a t t i c e s of h y d r o q u i n o n e . F o r e i g n a t o m s o r m o l e c u l e s of a p p r o p r i a t e s i z e c a n b e t r a p p e d in t h e c a v i t i e s d u r i n g t h e c r y s t a l l i z a t - i o n ' p r o c e s s in an a t o m i c r a t i o

209

g i v e n b y З С 6 Н 4 (ОН)г X M, w h e r e M i s t h e g u e s t a t o m o r m o l e c u l e . T h e i n t e r e s t in t h i s s y s t e m l i e s no t i n t h e s t r u c t u r e of t h e c l a t h r a t e , bu t i n t h e n a t u r e of t h e b i n d i n g of t h e g u e s t a t o m a n d i t s i n t e r a c t i o n w i t h t h e c r y s t a l d y n a m i c s . T h e r m o d y n a m i c e v i d e n c e [9] i s t h a t t he b ind ing i s , a s e x p e c t e d , a n h a r m o n i c , a n d t h i t f t h e g u e s t a t o m s a c t m o r e o r l e s s i n d e p e n d e n t l y of t he c r y s t a l d y n a m i c s . A f a i r f i t t o t h e t e m p e r a t u r e d e p e n d e n c e of t h e s p e c i f i c h e a t of t h e g u e s t " g a s " , a s s u m e d e q u a l to t h e d i f f e r e n c e b e t w e e n the s p e c i f i c h e a t s of t he f i l l e d and e m p t y c l a t h r a t e s , i s o b t a i n a b l e wi th a L e n n a r d - J o n e s -D e v o n s h i r e t y p e of p o t e n t i a l . H o w e v e r , t h e r e s u l t s a r e not v e r y s e n s i t i v e to t h e s h a p e of t he p o t e n t i a l o r t h e d e g r e e of coup l ing to t he c r y s t a l .

M o s t of t h e M ö s s b a u e r w o r k h a s b e e n done on t h e 8 3 K r c l a t h r a t e a s a ' s o u r c e o r a b s o r b e r [8 , 10] . T h e x e n o n c l a t h r a t e w a s f r e q u e n t l y u s e d b y P e r l o w e t a l . a s a c o n v e n i e n t u n s p l i t n a r r o w l i n e a b s o r b e r , bu t no s p e c i a l s t u d y of the s y s t e m w a s m a d e . Al though the n a t u r a l l i n e - w i d t h in 5 3 Kr i s two h u n d r e d t i m e s l e s s t h a n t h a t f o r t h e x e n o n i s o t o p e s , no c l e a r s p l i t t i n g w a s e v e r o b s e r v e d . T h e l i n e w a s h o w e v e r b r o a d e n e d by a m i n i m u m of a f a c t o r of t h r e e , i n c r e a s i n g wi th r a d i a t i o n d a m a g e and d e c r e a s i n g t e m p e r a t u r e , but i n d e p e n d e n t of t h e p e r c e n t a g e of t h e h o l e s f i l l e d . T h e l i n e b r o a d e n i n g p r e -s u m a b l y o r i g i n a t e s i n s o m e w a y f r o m t h e c r y s t a l l i n e E F G , a l t h o u g h i t i s no t c l e a r why i t s e f f e c t on t h e n u c l e u s s h o u l d no t a v e r a g e t o z e r o o v e r t h e r a p i d w a n d e r i n g s of t h e a t o m a r o u n d t h e n e a r l y s p h e r i c a l c a v i t y . It i s p o s -s i b l e t h a t t h e d e p a r t u r e f r o m s p h e r i c i t y i s s u f f i c i e n t t o g i v e a n o n - z e r o a v e r a g e .

< m СП 0.4 (Л

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2 0.3

0.2

0.1

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FIG. 2. Temperature dependence of the Mössbauer fraction in the krypton clathrate [8]

T o s t u d y t h e s h a p e of t h e b i n d i n g of t h e k r y p t o n , H a z o n y a n d H i l l m a n s t u d i e d in d e t a i l t h e t e m p e r a t u r e d e p e n d e n c e of t h e M ö s s b a u e r f r a c t i o n . T h e i r r e s u l t s a r e s h o w n i n F i g . 2. T h e o b s e r v e d d e p e n d e n c e c o n t r a s t s s h a r p l y wi th t h a t u s u a l l y o b s e r v e d f o r a c h e m i c a l l y bound M ö s s b a u e r i so tope , and c l e a r l y i n d i c a t e s t he a n h a r m o n i c i t y of t h e b ind ing . S ince the M ö s s b a u e r f r a c t i o n d e p e n d s e s s e n t i a l l y on t h e a v a i l a b l e v o l u m e f o r t h e a t o m to m o v e in, t h e h i g h - t e m p e r a t u r e f l a t n e s s of t h e M ö s s b a u e r f r a c t i o n i m m e d i a t e l y

210

s u g g e s t s a s t e e p - w a l l e d we l l , w h e r e a s t h e r a p i d r i s e in t he f r a c t i o n a r o u n d 100°K c a l l s f o r a s h a r p d e c r e a s e in v o l u m e . Q u a n t i t a t i v e l y i t i s p o s s i b l e t o f i t t h e h i g h - t e m p e r a t u r e r e g i o n w i t h a s p h e r i c a l , s q u a r e w e l l of r e a s o n -a b l e d i m e n s i o n s ( - 0 . 9 Â d i a m . ) , a n d t h e h i g h v a l u e n e a r 80°K w i t h a h a r -m o n i c w e l l w i t h 0D = 140°K. Ii d o e s n o t , h o w e v e r , s e e m p o s s i b l e t o f i n d a s i n g l e s t a t i c w e l l wh ich wi l l r e p r o d u c e bo th t h e s e r e g i o n s - a n d in p a r t i c u l a r t h e s h a r p t r a n s i t i o n b e t w e e n t h e m . I n d e e d D i s a t n i k [11] h a s s h o w n t h a t in g e n e r a l no s t a t i c m o d e l c a n m a t c h t h e s t e e p n e s s of t he c u r v e and ye t be con-s i s t e n t wi th t h e o b s e r v e d s p e c i f i c h e a t . T h e m o s t obv ious c o n c l u s i o n i s t ha t t h e r e i s in f a c t a n i n t e r a c t i o n of t h e g u e s t a t o m w i t h t h e l a t t i c e d y n a m i c s , a l t h o u g h why i t s h o u l d a p p e a r s o s h a r p l y i n t h i s t e m p e r a t u r e r e g i o n i s no t c l e a r . A l t e r n a t i v e e x p l a n a t i o n s m i g h t b e t h a t t h e r e i s s o m e k i n d of h i g h -o r d e r p h a s e t r a n s i t i o n in t h e c r y s t a l w h i c h a f f e c t s t h e e f f e c t i v e w e l l s h a p e (none h a s b e e n o b s e r v e d in t h e s p e c i f i c h e a t m e a s u r e m e n t s ) ; o r p o s s i b l y t h a t t h e t h e r m a l v i b r a t i o n s of t h e w a l l s of t h e c a v i t i e s h a v e a s u b s t a n t i a l r o l e i n d e t e r m i n i n g t h e i r s h a p e . T h e l a t t e r m a y n o t b e u n r e a s o n a b l e ; a l i ke ly s h a p e f o r t h e p o t e n t i a l would b e s o m e w h a t l ike a wine b o t t l e ( L e n n a r d -J o n e s - D e v o n s h i r e p o t e n t i a l ) a n d if t h e v i b r a t i o n a m p l i t u d e of t h e w a l l s i s c o m p a r a b l e w i t h t h e w i d t h of t h e c e n t r a l b u m p , t h e e f f e c t i v e v o l u m e of t h e b u m p c o u l d d e c r e a s e r a p i d l y w i t h i n c r e a s i n g t e m p e r a t u r e .

A n i n t e r e s t i n g p o i n t i s t h e h i g h D e b y e t e m p e r a t u r e (140°K) n e e d e d t o f i t t he l o w - t e m p e r a t u r e M ö s s b a u e r f r a c t i o n . T h i s i s not c o n s i s t e n t wi th t he s m a l l m a s s e s of t h e h y d r o q u i n o n e c o m p o n e n t s a n d t y p i c a l o r g a n i c D e b y e t e m p e r a t u r e s . It i s c l e a r t h a t g r o u p s of t h e h y d r o q u i n o n e a t o m s a r e a c t i n g c o o p e r a t i v e l y , g i v i n g a l a r g e r e f f e c t i v e m a s s . T o p u t i t a n o t h e r w a y , t h e a c o u s t i c a l m o d e of t h e c r y s t a l i s p a r t i c i p a t i n g i n t h e p r o c e s s , b u t no t t h e o p t i c a l m o d e . T h i s c o n c e p t of i n c r e a s e d e f f e c t i v e m a s s , o r t h e d i s t i n c t i o n b e t w e e n t h e o p t i c a l a n d a c o u s t i c m o d e s of c r y s t a l s , s e e m s v i t a l t o t h e a n a -l y s i s of m a n y M ö s s b a u e r s y s t e m s .

T h e M ö s s b a u e r l i n e o b s e r v e d [8] i n t h e ß~ d e c a y of 8 3 B r in A g B r i s v e r y m u c h b r o a d e n e d . T h i s i s p e r h a p s s u r p r i s i n g in v i ew of t he cubic s t r u c -t u r e of t h e c r y s t a l , bu t p e r h a p s t h e B r r e c o i l s i n to a n i n t e r s t i t i a l p o s i t i o n ; one w o u l d no t h a v e e x p e c t e d a n a t o m i c c o n t r i b u t i o n t o t h e E F G in t h i s l ong l i f e t i m e l e v e l , g iving the k r y p t o n a t o m p l e n t y of t i m e to be n e u t r a l i z e d . Note tha t in t h e c l a t h r a t e , H a z o n y and H i l l m a n p r e s e n t s t r o n g e v i d e n c e tha t r a d i -a t ion d a m a g e f r o m t h e (n, y) f o r m a t i o n of t h e 8 3 K r i s not r e s p o n s i b l e f o r t h e l ine b r o a d e n i n g .

F u r t h e r s tudy of t he c l a t h r a t e s and o t h e r " p h y s i c a l " b inding s y s t e m s wi l l c l e a r l y be m o s t r e w a r d i n g .

R E F E R E N C E S

[1] HYMAN, H., Noble-Gas Compounds, University of Chicago (1963). [2] RUBY, S., private communication. [3] PERLOW, G.J., PERLOW, M.R., Argonne Report SM-57/83 (1965). [4] PERLOW, G.J. , PERLOW, M. R., Rev. mod. Phys. 36 (1964) 353. [5] PERLOW, G.J. , Phys. Rev. 135 (1964) B1102. [6] CARLSON, T .A. , Phys. Rev. 131 (1963) 676.

211

[7] PERLOW, G.I . , PERLOW, M.R., J. chem. Phys. 41 (1964) 1157. [8] HAZONY, Y., HILLMAN, P., submitted to Phys. ~Rev. [9] GREY, N.R., PARSONAGE, N.G., STAVELEY, L.A.K., Mol. Phys. 4 (1961) 153.

[10] HAZONI, Y„ HILLMAN, P., PASTERNAK, M, RUBY, a , Phys. Lett. 2.(1962)337. [11] DISATNIK, Y., private communication.

D I S C U S S I O N

R . H. H E R B E R ( C h a i r m a n ) , J . DANON and G. K. W E R T H E I M a l l m a d e s u g g e s t i o n s f o r e x p l a i n i n g t h e t e m p e r a t u r e b e h a v i o u r of t h e r e c o i l - f r e e f r a c t i o n f o r S 3 K r in a c l a t h r a t e .

R . H. H E R B E R ( C h a i r m a n ) p r o p o s e d a m o d e l wi th two a t o m s in the c l a t h -r a t e , w i t h d i f f e r e n t Debye t e m p e r a t u r e s .

J . DANON s u g g e s t e d t h a t m o v e m e n t of t h e K r a t o m i n s i d e t h e c l a t h r a t e m i g h t c h a n g e a t a f i x e d t r a n s i t i o n t e m p e r a t u r e .

G . K. W E R T H E I M s u g g e s t e d a p o t e n t i a l w e l l w i t h " h o l e s " i n it f o r t h e a t o m t o f a l l i n t o , s o t h a t a t l ow t e m p e r a t u r e s t h e a t o m w a s b o u n d , bu t a t h i g h e r t e m p e r a t u r e s i t c o u l d m o v e a r o u n d .

P . H I L L M A N s t a t e d t h a t t h e c l a t h r a t e w a s a t i gh t s t r u c t u r e , a n d m u c h d i s -t o r t i o n wou ld b e r e q u i r e d t o a c c o m o d a t e two K r a t o m s i n s i d e i t .

It w a s k n o w n t h a t h e l i u m c o u l d l e a k ou t of s u c h a c l a t h r a t e a n d s i n c e t h e He a t o m w a s s m a l l e r t h e r a t e of l e a k i n g w a s p r o b a b l y e x p o n e n t i a l . The f r e e m o v e m e n t of t h e k r y p t o n w a s on ly 1 Â . F u r t h e r m o r e , t h e s a m e t e m -p e r a t u r e d e p e n d e n c e w a s o b s e r v e d f o r a l l c o n c e n t r a t i o n s of K r . It w a s not p o s s i b l e t o g i v e a p o t e n t i a l w e l l w h i c h w o u l d a c c o u n t f o r t h e o b s e r v e d b e -h a v i o u r . It w a s n o t a c o o p e r a t i v e p h e n o m e n o n .

H . F R A U E N F E L D E R a s k e d if t h e c o m p r e s s i b i l i t y of t h e c l a t h r a t e w a s k n o w n .

P . H I L L M A N s a i d i t w a s v e r y f r a g i l e - t h e g a s c a m e out q u i t e e a s i l y . J . S P I J K E R M A N s a i d s o m e c l a t h r a t e s w e r e p r e p a r e d by p u s h i n g t h e

g a s i n t o t h e c a g e a t s e v e r a l a t m o s p h e r e s p r e s s u r e , bu t no t k i l o b a r s . R . M. G O L D I N G a s k e d how c l o s e t h e s t r u c t u r e w a s t o i d e a l . P . H I L L M A N r e p l i e d tha t it w a s p r e t t y c l o s e to i d e a l a s shown by X - r a y

a n a l y s i s , and t h e h igh r e p r o d u c i b i l i t y of t h e r e s u l t s . R a d i a t i o n d a m a g e r e d u c e d t h e r e c o i l - f r e e f r a c t i o n , bu t not v e r y m u c h .

T h e r e f o r e , d e f e c t s d id no t s e e m t o b e t oo i m p o r t a n t in t h i s w o r k . J . S P I J K E R M A N e x p l a i n e d t h a t s o m e w o r k a t NBS h a d shown tha t K r in

c a r b o n s i e v e s g a v e no e f f e c t . H e r e t h e c a v i t y w a s m u c h b i g g e r , about 5 A on a s i d e . A l s o j K r F 2 h a d b e e n t r i e d , w i t h n o e f f e c t o b s e r v e d .

V . l . GOLDANSKII a n d P . K I E N L E a s k e d if t he t e m p e r a t u r e d e p e n d e n c e of x e n o n in c l a t h r a t e s h a d b e e n w o r k e d out .

P . H I L L M A N s a i d t h a t it h a d n o t , a n d t h a t 1 3 1 X e h a d no t b e e n r u n in c l a t h r a t e a t a l l .

212

UNCONVENTIONAL MÖSSBAUER STUDIES*

M. А Т А С , H. FRAUENFELDER AND M . GARRELL UNIVERSITY OF ILLINOIS, URBANA, ILL.



L i p k i n ' s h i s t o r y of t h e M ö s s b a u e r e f f e c t J X Í c a n b e con t inued by the a d -d i t i o n of two n e w p e r i o d s : (1) t h e c h e m i s t r y a g e ( 1 9 6 1 - 1 9 6 3 ) , in w h i c h c h e m i s t s d i s c o v e r t he M ö s s b a u e r e f f e c t ; (2) t h e s e c o n d p h y s i c s a g e (1964 to d a t e ) , in w h i c h p h y s i c i s t s t r y t o r e c o v e r l o s t g r o u n d . S i n c e t h e c h e m i s t r y a g e , m o s t p u b l i c a t i o n s no l o n g e r j u s t a n n o u n c e t h e d i s c o v e r y of a n e w M ö s s b a u e r i s o t o p e , bu t c o n t a i n s y s t e m a t i c s t u d i e s of v a r i o u s s o l i d s , c o m -p o u n d s a n d a l l o y s . Now in t h e s e c o n d p h y s i c s a g e , n e w t e c h n i q u e s a p p e a r a n d t h e r a n g e of p o s s i b l e e x p e r i m e n t s i n c r e a s e s .

In t h e p r e s e n t r e p o r t , we c o n c e n t r a t e o n t h e e f f e c t s t h a t c h a r a c t e r i z e t h e s e c o n d p h y s i c s a g e , bu t a l s o m a k e r e m a r k s o n c h e m i c a l a p p l i c a t i o n s . T h e m a t e r i a l c o v e r e d w i l l m a k e i t c l e a r t h a t a l l t h e n u c l i d e s d i s c u s s e d be low o f f e r exc i t i ng p o s s i b i l i t i e s f o r u s e in c h e m i s t r y , m e t a l l u r g y and s o l i d -

i ^ s t a t e p h y s i c s . Л J&&«~discUss t h e fo l lowing f o u r a r e a s : (1) T h e u s e of t h e s c a t t e r i n g g e o m e t r y of s t u d y y - r a y s w i t h e n e r g i e s

h i g h e r t h a n a b o u t 90 k e V . (2) T h e d i r e c t e x c i t a t i o n of M ö s s b a u e r t r a n s i t i o n s b y n e u t r o n c a p t u r e .

C o u l o m b e x c i t a t i o n , o r c h a r g e d p a r t i c l e r e a c t i o n s . (3) T h e u s e of n u c l i d e s w h e r e t h e m o t h e r s u b s t a n c e i s s h o r t - l i v e d o r

w h e r e t h e g r o u n d s t a t e i s r a d i o a c t i v e . (4) T h e d i s c o v e r y of v e r y n a r r o w r e s o n a n c e s . R a t h e r t h a n d i s c u s s a l l t h e n u c l i d e s t h a t b e a r on o n e o r m o r e of t h e s e

t o p i c s , we s e l e c t t y p i c a l e x a m p l e s t h a t p r o m i s e to b e c o m e p a r t i c u l a r l y i n t e r e s t i n g . T h e s e n u c l i d e s a r e s h o w n in T a b l e I .

B . P O T A S S I U M - 4 0

A l l M ö s s b a u e r e x p e r i m e n t s d o n e i n t h e p a s t h a v e i n v o l v e d t h e u s e of r a d i o a c t i v e p a r e n t n u c l i d e s to p o p u l a t e t h e e x c i t e d s t a t e to be s t u d i e d . S o m e s t a t e s e x i s t t h a t c a n n o t b e r e a c h e d by s u c h a m e t h o d bu t wh ich would o t h e r -w i s e b e u s a b l e f o r M ö s s b a u e r e x p e r i m e n t s . T h e f i r s t e x c i t e d s t a t e of 4 0 K i s a n e x a m p l e ; i t h a s a n e x c i t a t i o n e n e r g y of 2 9 . 4 k e V a n d a h a l f - l i f e of 3 . 9 n s , but it i s no t f ed by a r a d i o a c t i v e d e c a y 1 . H o w e v e r , i t c a n b e m a d e

b y m a n y n u c l e a r r e a c t i o n s s u c h a s 3 9 K ( n , y) 4 0 K*, 3 9 K ( d , p ) 4 0K* 4 0 K(p, p ' p K * , 4 0 A(p , n ) 4 9K*. R e c e n t l y , t h e M ö s s b a u e r e f f e c t of t h e 2 9 . 4 - k e V t r a n s i t i o n h a s indeed b e e n o b s e r v e d by H a f e m e i s t e r and S h e r a [2] u s i n g n e u t r o n c a p t u r e a n d b y R u b y a n d H o l l a n d [3] u s i n g t h e (d, p) r e a c t i o n . A t t h e p r e s e n t

* Supported by the US Office of Naval Research under contract 1834(05). 1 Incidentally, the ground state of 40K is radioactive and decays with a lifetime of 1 . 2 8 x 1 0 ' y.

Hence 40K is also an example of class (3) mentioned in the introduction.

213

TABLE II

M Ö S S B A U E R N U C L I D E S D I S C U S S E D I N P R E S E N T R E P O R T

Nuclide Transition E y (keV) T ^ s )

Feature of interest* Maximum observed e f fec t

ЩЦ 2 9 . 4 3. 9(-9) Excitation by neutron capture, (d, p) reaction

22% at 4°K

" N i 6 7 . 4 5 . 2 ( -9) Coulom b exci tat ion 3. 7% at 78°K

129 J 2 6 . 8 18.5(-9) Radioactive ground s ta te short-lived parent

21% at 78° К

ш T a 6 . 2 6 .8 ( -6 ) Very narrow resonance 5% a t 300°K

U6 0 s 137 5 .1(-10) Scattering expt. 34%

188 OS 155 6 .2( -10) Scattering expt. with "high-energy" y - r ay

6 .5% at 26°K

t i m e , n e u t r o n c a p t u r e y i e l d s b e t t e r r e s u l t s a n d we b r i e f l y d i s c u s s s o m e of H a f e m e i s t e r a n d S h e r a ' s w o r k .

A t h e r m a l n e u t r o n b e a m wi th i n t e n s i t y of a b o u t 3 X 1 0 6 n / s w a s a l l o w e d t o h i t l o w - t e m p e r a t u r e t a r g e t s of n a t u r a l p o t a s s i u m i n t h e f o r m s K, K C l a n d K F . N e u t r o n c a p t u r e p r o d u c e s t h e 7 . 8 - M e V s t a t e in 4 0 K . A f t e r g a m m a d e c a y , a b o u t 30% of t h e n e u t r o n c a p t u r e s l e a d t o t h e 2 9 . 4 - k e V l e v e l . T h e r e s o n a n c e of t h e 2 9 . 4 y - r a y s w a s o b s e r v e d w i t h l o w - t e m p e r a t u r e K C l a b -s o r b e r s , e n r i c h e d to 30 . 3% in 4 0 к . A t y p i c a l s p e c t r u m i s s h o w n in F i g . 1 .

VELOCITY (cm/sec)

FIG ... bauerspectrumofthe29.4-keVy-raysfrom4 0K. Source produced by neutron capture; source and absorber (KCl,i - 4°K. (From Hafemeister and Shera, Ref. [2])

T h e r e c o i l l e s s f r a c t i o n s of t h e s o u r c e s and t h e D e b y e t e m p e r a t u r e s d e r i v e d f r o m t h e e x p e r i n e n t s a r e s h o w n in T a b l e I I , t o g e t h e r w i t h t h e D e b y e t e m p e r a t u r e s d e r i v e d f r o m s p e c i f i c h e a t d a t a .

T h e e m i s s i o n of e n e r g e t i c y - r a y s p r e c e d i n g t h e f o r m a t i o n of the 29 .4 -keV s t a t e l e a v e s t h e 4 0 K n u c l e u s w i t h a d i s t r i b u t i o n of r e c o i l e n e r g i e s u p t o a m a x i m u m of a b o u t 800 e V . S u c h e n e r g i e s c a n e a s i l y d i s p l a c e a t o m s f r o m

214

TABLE II

M Ö S S B A U E R E F F E C T O F T H E 2 9 . 4 - k e V S T A T E IN 4 0 K , P R O D U C E D B Y N E U T R O N C A P T U R E

Source T e m p . f K ) f ©f(°K) ©DÍ°K)

KF 4 0 .23 i 0 .03 145 320

78 0 .14± 0 .02 190 320

KCl 4 0 . 1 8 * 0 . 0 4 120 225

78 0. I l l 0 .02 175 218

К 4 0.036± 0 . 0 1 60 98

f = recoilless fract ion of the source © f = the Debye temperature derived from f © p = the Debye temperature derived from specific hea t data

t h e i r l a t t i c e s i t e s . Two a r g u m e n t s ind ica te , h o w e v e r , tha t r ad i a t i on d a m a g e i s not too s e r i o u s : the o b s e r v e d f v a l u e s a r e about half t he expec ted v a l u e s a n d t h e o b s e r v e d l i n e - w i d t h c o r r e s p o n d s t o a h a l f - l i f e of 4 . 3 ± 0 . 9 n s , a s c o m p a r e d t o t h e e l e c t r o n i c a l l y m e a s u r e d v a l u e of 3 . 9 ± 0 . 35 n s . S e r i o u s r a d i a t i o n d a m a g e would r e d u c e t h e f v a l u e s m u c h m o r e and would r e s u l t in b r o a d e r l i n e s .

The p r e l i m i n a r y r e s u l t s of the two g roups indica te tha t n u c l e a r r eac t ions , p a r t i c u l a r l y n e u t r o n c a p t u r e , can be u s e d to exc i t e l e v e l s tha t a r e not popu-l a t e d in r a d i o a c t i v e d e c a y . Such e x p e r i m e n t s m a y a l s o y i e ld c o n s i d e r a b l e i n f o r m a t i o n abou t r a d i a t i o n d a m a g e ( s ee a l s o R e f s . 4 and 5) .

C . N I C K E L - 6 1

T h e M ö s s b a u e r e f f e c t of t h e 6 7 . 4 - k e V t r a n s i t i o n in 61Ni w a s o b s e r v e d

3+ [6] in 1960 by u s i n g the d e c a y 6 1Co 9 9 m i n> 6 1Ni. Recent ly , Seyboth et a l . [7]

h a v e o b s e r v e d r e c o i l l e s s 7 - r a y e m i s s i o n in t h i s t r a n s i t i o n a f t e r i t w a s i n -duced by Cou lomb e x c i t a t i o n . An e n r i c h e d n i c k e l t a r g e t (92% 61Ni) of t h i ck -n e s s 2 . 8 m g / c m 2 and he ld a t 78°K was b o m b a r d e d with 2 5 - M e V 1 6 0 4 + i o n s . F i g u r e 2 s h o w s one of t h e M ö s s b a u e r s p e c t r a o b t a i n e d in t h i s e x p e r i m e n t . T h e t o t a l e m i s s i o n l i n e had the s a m e a r e a a s the one ob ta ined in the e a r l i e r e x p e r i m e n t [6 ] , but t h e m a x i m u m a b s o r p t i o n w a s found to b e abou t 25% s m a l l e r , i n d i c a t i n g an i n c r e a s e d l i ne -wid th . Rad i a t i on d a m a g e e f f e c t s a r e o b v i o u s l y not v e r y s e r i o u s . T h e e x p e r i m e n t e s t a b l i s h e s t h e f e a s i b i l i t y of M ö s s b a u e r e x p e r i m e n t s w i th C o u l o m b e x c i t a t i o n 7 - r a y s .

215

т i — i — i 1 1 1 1 — i — r

1.01 -

ш ce

.97 - -

9 6 Ь 1 i I I 1 I i I I L ' -10.0 -8 .0 -6.0 -4 .0 -2.0 0 2.0 4.0 6.0 8.0 10.0

V E L O C I T Y (mm/sec)

F I G . 2 . M ö s s b a u e r s p e c t r u m o f t h e 6 7 . 4 - k e V g a m m a t r a n s i t i o n f r o m 6 1 N i , p r o d u c e d by C o u l o m b e x c i t a t i o n .

( F r o m S e y b o t h , O b e n s h a i n a n d C z j z e k , Re f . [ 7 ] )

D . I O D I N E - 1 2 9

Iod ine c o m p o u n d s a r e i m p o r t a n t in i n o r g a n i c and o r g a n i c c h e m i s t r y and v e r y u s e f u l i n m e d i c i n e . T h e a l k a l i - h a l i d e s a r e a m o n g t h e m o s t - s t u d i e d c o m p o u n d s i n s o l i d - s t a t e p h y s i c s . I od ine i s u s e d in p h o t o g r a p h y . B e c a u s e i o d i n e h a s on ly one e l e c t r o n m i s s i n g f r o m a c l o s e d - s h e l l c o n f i g u r a t i o n it can f o r m m a n y d i f f e r e n t f o r m a l v a l e n c e s t a t e s . H o w e v e r , m a n y c o m p o u n d s a r e n o t f u l l y e x p l o r e d and a d d i t i o n a l i n f o r m a t i o n i s v e r y h e l p f u l i n u n d e r s t a n d i n g t h e c h e m i s t r y of i o d i n e . I t i s t h e r e f o r e f o r t u n a t e t h a t i t h a s two i s o t o p e s w i t h t r a n s i t i o n s t h a t s h o w t h e M ö s s b a u e r e f f e c t . S o m e p r o p e r t i e s of t h e s e two i s o t o p e s , 1271 a n d !29x/ a r e s u m m a r i z e d i n T a b l e I I I .

TABLE III

P R O P E R T I E S O F A N D

Property " 7 I 1291

Ground state Stable, 100% abundance Radioactive, T^ = 1 . 6 x l 0 7 y

Mössbauer transition

Energy 59 keV 26. 8 keV

Half-l ife 2 .68 ns 18 .5 ns

Transition 7 + 5 +

2 2

216

B o t h i s o t o p e s g i v e c e r t a i n d i f f i c u l t i e s . 1 2 9 I h a s a r a d i o a c t i v e g r o u n d s t a t e and s p e c i a l c a r e m u s t h e n c e b e t a k e n in w o r k wi th t h e a b s o r b e r s . T h e p a r e n t n u c l e u s t h a t a l l o w s t h e b e s t w o r k , 1 2 9 T e , h a s a g r o u n d - s t a t e h a l f -l i f e of o n l y 70 m i n ; a l l e x p e r i m e n t s m u s t b e d o n e q u i c k l y . l 2 7 I g i v e s o n l y v e r y s m a l l e f f e c t s and i t i s d i f f i c u l t to p r e p a r e v e r y i n t e n s e s o u r c e s . How-e v e r , b e a u t i f u l w o r k h a s b e e n d o n e wi th b o t h i s o t o p e s and t h e r e s u l t s f r o m t h e v a r i o u s p u b l i c a t i o n s c o m p l e m e n t e a c h o t h e r v e r y w e l l . H e r e , we m a i n l y d i s c u s s 1 2 9 I .

T h e M ö s s b a u e r e f f e c t i n 1 2 9 I w a s f i r s t f o u n d b y J h a e t a l . [8] b u t t h e w o r k b y H a f e m e i s t e r e t a l . [9] p r o v i d e s a l l t h e i m p o r t a n t d a t a . T h e s e a u t h o r s f i r s t s h o w e d t h a t e x c i t i n g i n f o r m a t i o n o n n u c l e a r a n d s o l i d - s t a t e p h y s i c s c a n b e o b t a i n e d w i t h 1 2 9 I d e s p i t e t h e r a d i o a c t i v e g r o u n d s t a t e a n d d e s p i t e t h e s h o r t - l i v e d p a r e n t n u c l e u s i 2 9 T e . T h e i n f o r m a t i o n i m p o r t a n t f o r c h e m i s t r y c o m e s f r o m t h e o b s e r v a t i o n of t h e i s o m e r s h i f t and the q u a d r u -po l e s p l i t t i n g .

F i g u r e 3 s h o w s t h e i s o m e r s h i f t s of 1Z9I c o m p o u n d s , a l l m e a s u r e d r e l a -t i v e t o Z n 1 2 9 T e s o u r c e s . A l l b u t o n e of t h e 1 2 9 I v a l u e s a r e t a k e n f r o m t h e p a p e r b y H a f e m e i s t e r e t a l . [ 9 ] ; t h e v a l u e f o r a m m o n i u m t r i h y d r o g e n p a r a p e r i o d a t e , ( N H 4 ) 2 H 3 I 0 6 , i s t a k e n f r o m e x p e r i m e n t s by A t a c e t a l . L10J . A t a c e t a l . a l s o h a v e s t u d i e d m o l e c u l a r i o d i n e ; F i g . 4 s h o w s t h e M ö s s b a u e r s p e c t r u m a t 78°K o b t a i n e d wi th a Z n 1 2 9 T e s o u r c e . T h e i n t e r p r e t a t i o n of t h e m o l e c u l a r i o d i n e s p e c t r u m i s n o t ye t c l e a r , b u t i t a p p e a r s t o h a v e a l a r g e p o s i t i v e i s o m e r s h i f t .

кю3

NH4IO3 Bot lO- i

P b T e l z

ZnTe Csl

-Kl

- К Ю 4

• ( N H 4 ) 2 H 3 I 0 6

F I G . 3 . I s o m e r s h i f t in 1 2 9 I c o m p o u n d s . ( D a t a , I l l i n o i s g r o u p , Refs . [ 9 ] a n d [ 1 0 ] ) ( S o u r c e m o v e s t o w a r d

a b s o r b e r for p o s i t i v e v e l o c i t i e s )

T h e i s o m e r s h i f t s f o r 1 2 71, o b t a i n e d b y P e r l o w and R u b y [11, 1 2 ] , a n d 1 2 9 I , g i v e n i n F i g . 3 , a r e i n r e a s o n a b l e a g r e e m e n t . A s s h o w n in T a b l e I I I ,

217

- .865 cm/aec О с m/яес + .865 cm/sec

10 20 30 40 50 60 70 80 90 100 МО 120 130 140 150 160 170

CHANNEL NUMBERS

FIG. 4. Mössbauer spectrum of the 26. 8-keV transition in 1Z9I, measured with Zn129Te source and 1291г

absorber, both at 78°K. (Illinois group, Ref. [10])

t h e t r a n s i t i o n s i n t h e t w o i s o t o p e s a r e i n v e r t e d . If t h e i n v e r s i o n d o e s n o t a f f e c t o t h e r p r o p e r t i e s of t h e s t a t e s and if t he a c c e p t e d t h e o r y of t h e i s o m e r s h i f t i s c o r r e c t , t h e i s o m e r s h i f t s f o r t h e t w o i s o t o p e s s h o u l d b e o p p o s i t e i n s i g n and a b o u t e q u a l in m a g n i t u d e f o r c o r r e s p o n d i n g c o m p o u n d s . P e r l o w a n d Ruby [ 1 1 , 1 2 ] f i nd f o r t h e r a t i o of i s o m e r s h i f t s 6

6 ( 1 2 7 I ) / 6 ( 1 2 9 I ) = - 0 . 7

T h e e x p e c t e d c h a n g e in s i gn i s found and t h e m a g n i t u d e s a r e t he s a m e wi th in 30%. C o m p l e t e a g r e e m e n t canno t b e e x p e c t e d s i n c e t h e q u a d r u p o l e m o m e n t s of t h e c o r r e s p o n d i n g s t a t e s a r e not i d e n t i c a l .

T h e c o m p o u n d s s h o w n i n F i g . 3 c a n b e d i v i d e d i n t o t h r e e c l a s s e s w i t h t h e e x c e p t i o n of t h e p a r a p e r i o d a t e , a c c o r d i n g to t h e v a l e n c e s t a t e of i o d i n e . T h e t h r e e c l a s s e s , t o g e t h e r w i t h t y p i c a l e x a m p l e s , a r e g i v e n i n T a b l e I V .

TABLE IV

IODINE COMPOINDS, VALENCE STATES AND ISOMER SHIFTS [9]

Valence state Isomer shift relative to " of iodine Z,nTe, in cm/s

Iodides KI 1 - 0.051 ± 0.0025

Iodates КЮ3 5 + +0. 156 ±0.02

Periodate KIO 7+ - 0 . 2 3 4 ±0.006 4

218

F i g u r e 3 and T a b l e IV both s h o w tha t t he i s o m e r s h i f t c h a r a c t e r i z e s t he t h r e e c l a s s e s ; w i t h i n e a c h c l a s s , d i f f e r e n c e s a r e s m a l l . F r o m t h e i s o m e r s h i f t , t h e 5 s - e l e c t r o n d e n s i t y a t t h e n u c l e u s , c a n b e c a l c u l a t e d . D e t a i l s of t h e c a l c u l a t i o n s a r e c o n t a i n e d i n R e f . l 9 J . T h e 5 s - e l e c t r o n d e n s i t y t h e n g i v e s i n f o r m a t i o n a b o u t t h e n a t u r e of t h e c h e m i c a l b o n d . In p a r t i c u l a r , t h e 5 s -e l e c t r o n d e n s i t y d e p e n d s s e n s i t i v e l y o n t h e r e l a t i v e p a r t i c i p a t i o n of t h e 5p and 5s e l e c t r o n s in t h e b o n d s b e t w e e n t h e i od ine and t h e s u r r o u n d i n g a t o m s . The d i s c u s s i o n [9] g i v e s t h e fo l lowing q u a l i t a t i v e exp lana t ion of t he o b s e r v e d i s o m e r s h i f t s : i n t h e a l k a l i i o d i d e s , t h e I " i o n a l m o s t h a s t h e x e n o n c o n -f i g u r a t i o n 5s 2 5 p 6 . I n t h e ( I 0 3 ) ~ c o m p o u n d s , e a c h I a t o m i s s u r r o u n d e d b y s i x О a t o m s in a n i r r e g u l a r o c t a h e d r o n . T h r e e of t h e o x y g e n a t o m s a r e in t h e ( Ю з ) " g r o u p a n d t h e o t h e r t h r e e a r e c o n s i d e r a b l y f a r t h e r a w a y . S i n c e the r e s u l t i n g I - O bond a n g l e s a r e v e r y c l o s e to 90° (the bonds con ta in c h i e f l y p e l e c t r o n s ) t h e s p h y b r i d i z a t i o n i s s m a l l [ 1 3 , 1 4 ] . T h e r e m o v a l of t h e 5p e l e c t r o n s f r o m a t o m i c o r b i t s i n c r e a s e s t h e d e n s i t y of t h e s e l e c t r o n s a t t h e n u c l e u s a n d r a i s e s t h e e n e r g y of t h e g a m m a t r a n s i t i o n s w i t h r e s p e c t t o t h e one in I " . T h e i n c r e a s e in e n e r g y c o r r e s p o n d s t e t h e m o r e p o s i t i v e i s o m e r s h i f t s h o w n i n T a b l e I V . I n t h e ( Ю 4 ) " i o n , t h e i o d i n e a t o m i s s u r r o u n d e d b y a t e t r a h e d r o n of o x y g e n a t o m s . T h e a n g l e b e t w e e n t h e I - O b o n d s i s 109° a n d a c o n s i d e r a b l e s p h y b r i d i z a t i o n t a k e s p l a c e . T h e c o r r e s p o n d i n g r e m o v a l of 5s e l e c t r o n s f r o m t h e a t o m i c o r b i t s r e s u l t s i n a d e c r e a s e of t h e e l e c t r o n d e n s i t y a t t h e n u c l e u s and a c c o r d i n g to a d e c r e a s e of t h e y - r a y e n e r g y . T h e a r g u m e n t s c a n b e m a d e m o r e q u a n t i t a t i v e [9] . T h e i s o m e r s h i f t i n p a r a p e r i o d a t e , ( N H ^ H a l O e , i s v e r y l a r g e a n d n e g a t i v e , 6 = - 0 . 3 1 + 0 . 0 1 c m / s L10j . Such a s h i f t can be u n d e r s t o o d by a s s u m i n g sp? d 2

h y b r i d i z a t i o n .

T h e q u a d r u p o l e s p l i t t i n g c a n a l s o be u s e d to i n f e r i n f o r m a t i o n abou t t he c h e m i c a l b o n d . W h e r e a s t h e i s o m e r s h i f t i s m o s t s e n s i t i v e to t h e s - e l e c t r o n w a v e - f u n c t i o n a t t h e n u c l e u s , t h e q u a d r u p o l e s p l i t t i n g i s p r o p o r t i o n a l t o t h e e l e c t r i c f i e l d g r a d i e n t p r o d u c e d b y p e l e c t r o n s [14] . T h e w e l l - r e s o l v e d s p e c t r u m f o r t h e m o l e c u l a r i dod ine shown in F i g . 4 can be f i t a p p r o x i m a t e l y by a s s u m i n g a q u a d r u p o l e i n t e r a c t i o n of 1509 M c / s a n d a n a s y m m e t r y p a -r a m e t e r of r] = 0 . 2 . H o w e v e r , t h e p e a k h e i g h t s do n o t c o r r e s p o n d t o t h e t h e o r e t i c a l v a l u e s , i n d i c a t i n g t h a t t h e I 2 a b s o r b e r c o u l d b e p a r t i a l l y o r i e n t e d .

E . T A N T A L U M - 1 8 1

D u r i n g t h e p a s t f e w y e a r s t h e s e a r c h f o r a n i d e a l M ö s s b a u e r i s o t o p e , w i th a Q v a l u e ( i . e . Гу/Еу) a s h igh a s t h a t of t h e 9 3 - k e V t r a n s i t i o n in 6 7 Zn and f v a l u e s a s h igh a s t h o s e of 57Fe, h a s n e v e r been g iven u p . T h e 6 . 2 - k e V y - r a y of 1 8 1 Ta s e e m e d a good c a n d i d a t e ; i t h a s a h a l f - l i f e of 6. 8 ц s , a l i n e -wid th of 6 . 7 X 1 0 " 1 1 eV, a Q v a l u e of a b o u t l 6 l 4 , and i t s r e c o i l l e s s f r a c t i o n i s n e a r u n i t y e v e n a t r o o m t e m p e r a t u r e . A l l e a r l y a t t e m p t s t o s e e a M ö s s -b a u e r e f f e c t w e r e u n s u c c e s s f u l , h o w e v e r , T h e m a i n o b s t a c l e s a r e t h e T a L X - r a y s a t 8 . 1 and 9 . 4 keV, t h e l a r g e q u a d r u p o l e m o m e n t of T a , t he l a r g e S t e r n h e i m e r a n t i - s h i e l d i n g f a c t o r , a n d t h e d i f f i c u l t i e s of f r e e i n g T a f r o m i n t e r s t i t i a l i m p u r i t i e s t h a t c a n c a u s e f i e l d g r a d i e n t s .

219

T h e f i r s t s u g g e s t i o n of a p o s i t i v e M ö s s b a u e r e f f e c t w a s found by Cohen e t a l . [15] . C o n c l u s i v e e v i d e n c e , h o w e v e r , h a s o n l y v e r y r e c e n t l y b e e n o b t a i n e d b y S t e y e r t e t a l . [16] . O n e of t h e i r M ö s s b a u e r s p e c t r a i s s h o w n in F i g . 5 . T h e i r b e s t r e s o n a n c e , ob ta ined with t u n g s t e n s i n g l e - c r y s t a l s o u r c e and a t a n t a l u m a b s o r b e r (5 m g / c m ' ) , s h o w s a r a w e f f e c t of about 5%, a width of 0 . 2 6 m m / s , and an i s o m e r s h i f t of + 0 . 9 m m / s [17] . T h e e x p e c t e d width 2 r i s 0 . 0 0 7 c m / s . T h e a r e a u n d e r t h e a b s o r p t i o n c u r v e i n d i c a t e s t h a t t h e r e c o i l l e s s f r a c t i o n i s v e r y l a r g e ; e v i d e n c e f o r q u a d r u p o l e s p l i t t i n g i s p r e s e n t .

T h e s e e x p e r i m e n t s o f f e r exc i t i ng new v i s t a s f o r i n v e s t i g a t i o n s in n u c l e a r and s o l i d - s t a t e p h y s i c s , in c h e m i s t r y and in m e t a l l u r g y .

FIG. 5. Mössbauer spectra of the 6. 2-keV gamma transition from ш Т а , measured with Та absorber, using Та and W sources. (From Steyert, Taylor and Storms, Ref. [16])

F . OSMIUM-186 AND OSMIUM-188

A s a n e x a m p l e of t h e a p p l i c a t i o n of t h e s c a t t e r i n g t e c h n i q u e , we s k e t c h s o m e r e s u l t s [18] o b t a i n e d w i t h 1 8 6 O s and 1 8 8 O s . T h e s c a t t e r i n g t e c h n i q u e i s d i s c u s s e d in m o r e d e t a i l i n a s e p a r a t e r e p o r t .

S o u r c e s w e r e p r e p a r e d b y i r r a d i a t i n g n a t u r a l r h e n i u m w i t h n e u t r o n s . S c a t t e r e r s of o s m i u m m e t a l a n d O s S 2 w e r e u s e d . F i g u r e s 6 a n d 7 s h o w e x a m p l e s of M ö s s b a u e r s c a t t e r i n g s p e c t r a . T h e r e s o n a n c e l i n e s o b t a i n e d wi th t h e OsS2 s c a t t e r e r a r e c o n s i d e r a b l y b r o a d e n e d by u n r e s o l v e d quad rupo le s p l i t t i n g . F r o m t h e b r o a d e n i n g , t h e r a t i o of t h e q u a d r u p o l e m o m e n t s of t h e two i s o t o p e s c a n be d e t e r m i n e d .

2 2 0

FIG. 6. The scattered count rate for the 137-keV transition of 186Os as a function of the effective scatterer velocity which is given by the rod velocity divided by 1. 3. The background is normalized to 1. (Illinois group, Ref. [18])

VELOCITY (cm/sec)

FIG. 7. The scattered count rate for the 155-keV transition of 188Os as a function of the effective scatterer velocity. (Illinois group, Ref. [18])

R E F E R E N C E S

[1] FRAUENFELDER, H. , The Mössbauer Effect, W. A. Benjamin, New York (1962) 13. [2] HAFEMEISTER, D.W. , SHERA, E.B., Phys. Rev. Lett. U(1965) 593. [3] RUBY, S .L. , HOLLAND, R. E., Phys. Rev. Lett. 14(1965) 591. [4] STONE, I . A . , PILLING ER, W.L. , Phys. Rev. Lett. 13 (1964) 200. [5] MULLEN, J .G . , Phys. Lett. 25 (1965) 15.

2 2 1

[6] OBENSHAIN, F. E. , WEGENER, H. , Phys. Rev. 121 (1961) 1344. [7] SEYBOTH, D . . OBENSHAIN, F. E. , CZJZEK, G. V. , to be published. [8] JHA, S . , SEGNAN, R. , LANG, G. , Phys. Rev, 128 (1962) 1160, [9] HAFEMEISTER, D.W. , DePASQUALI, G. , DeWAARD, H. , Phys. Rev. 135 (1964) B1089.

[10] АТАС, M. , DEBRUNNER, P. . DePASQUALI, G. , GARRELL, M. . GRÄNICHER, H., to be published. [11] PERLOW, G . I . , RUBY, S. L., Phys. Lett. 13 (1964) 198. [12] PERLOW, G.L., RUBY, S. L., Bull. Amer. phys. Soc. 10 (1965) 367-HH4. [13] PAULING, L., The Nature of the Chemical Bond, Cornell University Press, Ithaca (1960). [14] DAS, T . P . , HAHN, E. L . , Nuclear Quadrupole Resonance Spectroscopy, Academic Press, Inc . ,

New York (1958). {15] COHEN, S.G. , MARINOV, A . , BUDNICK, J . I . , Phys. Lett. 12 (1964) 38. [16] STEYERT, W. A . , TAYLOR, R.D. , STORMS, E.K. , Phys. Rev. Lett. 14 (1965) 739. [17] TAYLOR, R. D . , private communication. [18] MORRISON. R.J. , A TAC, M . , DEBRUNNER, P . , FRAUENFELDER, H . . Phys. Lett. 12 (1964) 35.

D I S C U S S I O N

V . l . GOLDANSKII a s k e d w h a t kind of t a r g e t t h e O a k R i d g e g r o u p u s e d i n t h e i r C o u l o m b e x c i t a t i o n w o r k on 6 1 N i . I t s h o u l d h a v e b e e n t h e m e t a l t a r g e t , b e c a u s e o t h e r w i s e t h e r e would h a v e b e e n d i f f i c u l t i e s due t o t h e l o c a l h e a t i n g of t h e t a r g e t by 1 6 0 i o n s .

H . F R A U E N F E L D E R r e p l i e d t h a t i t w a s m e t a l l i c and added t h a t a s t u d y of t h e e f f e c t in i n s u l a t o r s a n d in m e t a l s would be i n t e r e s t i n g .

R . H . H E R B E R w o n d e r e d , i n t he l i g h t of t h e r e c e n t p h y s i c s a d v a n c e s , w h e t h e r D r . F r a u e n f e l d e r would c a r e to r e v i s e h i s op in ion ( e x p r e s s e d in an a f t e r - d i n n e r s p e e c h ) t h a t c h e m i s t s wou ld s o o n c o m e t o d o m i n a t e t h e f i e l d of M ö s s b a u e r e f f e c t r e s e a r c h b e c a u s e n u c l e a r p h y s i c i s t s f o u n d t h e y c o u l d g e t a n s w e r s t o t h e i r p r o b l e m s b e t t e r b y o t h e r m e t h o d s .

H . F R A U E N F E L D E R d e c l i n e d to c h a n g e h i s v i e w p o i n t , and s t a t e d t h a t t h e q u a n t i t y of n u c l e a r d a t a o b t a i n a b l e w a s s t i l l v e r y s m a l l c o m p a r e d w i t h c h e m i c a l i n f o r m a t i o n . H o w e v e r , if o n e c o u l d f i n d a l i n e 100 t i m e s n a r r o w e r t h a n t h e s h a r p e s t l i n e known t o d a y , t h e p h y s i c i s t s m i g h t h a v e a c h a n c e . F o r e x a m p l e a R u s s i a n g r o u p had b e e n look ing f o r t h e v e r y n a r r o w Ag l i n e .

P . K I E N L E t h o u g h t o n e s h o u l d b e c a r e f u l a b o u t s t a t e m e n t s on n u c l e a r p h y s i c s . T h e r e w a s s t i l l t h e u n e x p l o r e d f i e l d of t h e d i f f e r e n t o r i e n t a t i o n s of n u c l e a r s t a t e s a f t e r ß - d e c a y .

H . F R A U E N F E L D E R s t a t e d t h a t t h e ß-y e x p e r i m e n t , s u c h a s wi th 1291, m i g h t g i v e i n f o r m a t i o n a f a c t o r of 5 o r 10 b e t t e r t h a n b e f o r e . T h i s s t i l l w o u l d n o t g i v e n e w i n f o r m a t i o n o n w e a k i n t e r a c t i o n s .

P . K I E N L E s u g g e s t e d t h a t s u c h w o r k wou ld g ive n e w n u c l e a r s t r u c t u r e i n f o r m a t i o n a n d s h o u l d t h e r e f o r e b e d o n e . - H . F R A U E N F E L D E R a g r e e d .

M . C O R D E Y - H A Y E S a s k e d if a n y n e w - y - r a y s of v e r y low e n e r g y , s a y a r o u n d 1 k e V , w e r e l i k e l y t o be f o u n d .

"* H . F R A U E N F E L D E R s t a t e d t h a t n u c l e a r s y s t e m a t i c s w e r e w e l l enough u n d e r s t o o d t h a t o n e c o u l d s a y i t w a s i m p r o b a b l e t h a t m a n y m o r e w o u l d b e f o u n d .

M . C O R D E Y - H A Y E S c o m m e n t e d t h a t a l t h o u g h К Ю 4 w a s k n o w n t o b e f e r r o - e l e c t r i c , i t h a d a p p a r e n t l y e s c a p e d t h e a t t e n t i o n of w o r k e r s i n t h e f i e l d t h a t S n 0 2 w a s a l s o .

222

KINETIC INVESTIGATIONS

(Sess ion 6, P a r t 1)

THE INFLUENCE OF THE DIPOLAR APROTIC SOLVENTS ON THE QUADRUPOLE SPLITTING OF

MÖSSBAUER SPECTRAL LINES OF DIBUTYLTIN DICHLORIDE*

V . l . GOLDANSKII. O . Y u . OKHLOBYSTIN, V . Y a . ROCHEV AND V . V . KHRAPOV ACADEMY OF SCIENCES OF THE USSR

MOSCOW

ABSTRACT

Mössbauer spectroscopy is used by the authors to study the effect of dipoiai aprotic solvents on dibutyitin chloride. These solvents, which strongly solvate cations (or the cationoid portion of the molecule) but poorly solvate anions, cause a more or less pronounced distortion of the field symmetry of the metal nucleus. The different solvating powers of these solvents can be estimated by measuring the quadrupole splitting values of the23.8-keV y-line of 115Sn in dibutyitin chloride in a number of solvating solvents. If the solvating power of diethyl ether is taken as 1 the solvating power of some other solvents was found to be: 6 for diethoxyethane; 7 for tetrahydrofuran; 10 for dimethoxyethane; 15.5 for hexamethyl-triamidophosphate; 16 for dimethyl-formamide; and 18 for dimethyl sulphoxide.

D I S C U S S I O N

V. I. GOLDANSKII added to the main talk some considerations about the poss ibi l i t ies of "Mössbauer labell ing" as a new isotope method in different branches of chemis t ry , i . e . fo r the study of f e r rocen ium + FeCl4 s t ructure , as it was done by H . A . Stukan et a l . , f o r the study of absorp t ion on i n -homogeneous s u r f a c e s e t c .

N . N . GREENWOOD pointed out the impor tance of d i f fe rent physical methods for the descr ipt ion of solvent p roper t i es and the i r role in chemical reac t ions .

E . FLUCK discussed the possibility to get the data on solvent propert ies f r o m NMR spec t ra . с

H. FRAUENFELDER pointed out the significance of detailed comparison of d i f fe ren t in format ion which could be obtained by d i f ferent methods, and the p r inc ipa l poss ib i l i t i es of such methods .

R .H. HERBER suggested that Mössbauer spect ra could give much more in fo rma t ion ,on the equi l ibr ium s ta te of d i f ferent compounds than on the kinet ics of t he i r t r a n s f o r m a t i o n s .

* Since this paper will be published in the Journal of Orgahometailic Chemistry 3 (1965), only the abstract is printed here.

2 2 3

P . HILLMAN, J . SPIJKERMAN and V . l . GOLDANSKII discussed the possible use of "Mössbauer labelling" in exper iments with Kr in clathrates, and decided that in this case the re was no need in this new development of the method because the evidence of only one type of s i te f o r Kr nuclei in c la th ra tes was already obtained. The "Mössbauer labelling" could be of use for the study of the isotope exchange on inhomogeneous surfaces .

R.M. GOLDING, P . KIENLE (Chairman) and V.l. GOLDANSKII discussed the p r o p e r t i e s of F e C l 4 and the Mössbauer s p e c t r a of corresponding com-pounds .

2 2 4

RECOIL STUDIES

(Sess ion 6, Par t 2)

LIMIT ON THE LIFETIME OF NON-EQUILIBRIUM F e 3 +

IONS IN SOME IONIC SOLIDS FROM THE MÖSSBAUER E F F E C T *

G . K . WERTHEIM AND H J . GUGGENHEIM BELL TELEPHONE LABORATORIES, MURRAY HILL, N.J.


ABSTRACT

The lifetime of higher valence states produced by Auger effect can be obtained from the Mössbauer effect in some favourable isotopes. It is shown that valence states higher than Fe*+ which are produced after the electron capture decay of divalent 51Co in ZnF, have lifetimes less than 10"® s. A similar limit is obtained for the lifetime of higher charge states of iron produced by divalent cobalt diffused into NaF with the aid of charge compensating sodium vacancies. Trivalent iron has lifetimes greater than 3 x 10"® s in NaF if charge compensation is accomplished by oxygen. The lifetime in hydrated ferrous ammonium sulphate is 5 X 10"8 s. The short lifetimes, even of Fes+ in some divalent lattices, indicate that the lifetimes of higher valences will generally be too short to be observed with the present technique. It also suggests that valence states which do not occur in stable compounds will generally have short lifetimes and be difficult to detect.

DISCUSSION

J . . DANON s u g g e s t e d t h a t i n t h e e x p e r i m e n t s on i r o n in N a F m e n t i o n e d i t w o u l d b e i n t e r e s t i n g t o t r y t o s t u d y t h e e f f e c t of r e d u c t i o n of t h e i r o n t o t h e d i v a l e n t s t a t e b y u s i n g a r e d u c i n g a t m o s p h e r e .

G . K . W E R T H E I M r e p l i e d t h a t in c o n n e c t i o n wi th w o r k on i r o n in c o r u n -d u m h e h a d t r i e d t o u s e r e d u c t i o n t o p r o d u c e d i v a l e n t i r o n but h a d o b t a i n e d o n l y m e t a l l i c i r o n .

J . DANON a s k e d how t h e h y p e r f i n e s p l i t t i n g o b t a i n e d in W e r t h e i m ' s e x -p e r i m e n t s c o r r e s p o n d e d to t h a t of i r o n o x i d e s .

G . K . W E R T H E I M r e p l i e d t h a t i t w a s f o u n d t o b e t h e s a m e a s t h a t of FegOe a n d c o r r e s p o n d e d t o 550 k O e .

P . H I L L M A N s u g g e s t e d t h a t s t u d i e s of t h e t i m e - d e p e n d e n c e of t h e o b -s e r v e d p h e n o m e n a w o u l d p r o v i d e u s e f u l i n f o r m a t i o n . C h a n g e s i n t h e s p e c t r u m w o u l d b e o b s e r v e d .

G . K . W E R T H E I M o p p o s e d the s u g g e s t i o n on t h e g r o u n d s t h a t t h i s would l e a d t o l i n e b r o a d e n i n g . I t wou ld on ly b e of u s e i n t h e c a s e of f e r r o u s a m -m o n i u m s u l p h a t e .

* Since this paper has been published in the Journal of Chemical Physics', 42, 11 (1965) 3873, only the abstract is printed here.

2 2 5

J . DANON, G . K . W E R T H E I M a n d N . S A I T O d i s c u s s e d t h e i n f l u e n c e of t h e r e m o v a l of w a t e r f r o m t h e o r i g i n a l h y d r a t e d f e r r o u s a m m o n i u m s u l -pha t e and the p r o p e r t i e s of the r e m a i n i n g n o n - s t o i c h i o m e t r i c a n h y d r o u s s a l t .

N . S A I T O s u g g e s t e d i n v e s t i g a t i o n s of o t h e r f l u o r i d e s . He m e n t i o n e d t h e i n f l u e n c e of c a t i o n s o n t h e p r o p e r t i e s of t h e a n i o n s b o u n d to t h e m a n d g a v e a n e x a m p l e f r o m r e c o i l c h e m i s t r y .

R . H . H E R B E R a s k e d w h e t h e r t h e o b s e r v a t i o n ( a t l i q u i d n i t r o g e n t e m p e r a t u r e ) of t h e s i x - l i n e h y p e r f i n e s t r u c t u r e s p e c t r u m of t h e 3+ s t a t e of i r o n , v e r y s i m i l a r t o t h e s p e c t r u m o b t a i n e d w i t h f e r r i c o x i d e , w a s s u g g e s t i v e of a c o - o p e r a t i v e p h e n o m e n a .

G. K . W E R T H E I M r e p l i e d t h a t t h i s n e e d no t be s o . It could a l s o be due t o a d i lu t e ion wi th a l ong e l e c t r o n - s p i n r e l a x a t i o n t i m e . H y p e r f i n e sp l i t t i ng n e e d n o t b e d u e t o a m a g n e t i c a l l y o r d e r e d s t a t e . Wi th d i l u t e i r o n i n c o r u n d u m a t l ow t e m p e r a t u r e t h e i n t e r n a l f i e l d of 550 kG w a s due to a s p i n of 5 / 2 i n t h e d s h e l l , w i t h t h e d - s h e l l e x c h a n g e p o l a r i z i n g t h e s e l e c t r o n s and t h e s e l e c t r o n s h a v i n g a F e r m i c o n t a c t i n t e r a c t i o n w h i c h w a s t h e s a m e w h e t h e r t he i r o n w a s s i m p l y p r e s e n t t h e r e in a p a r a m a g n e t i c s t a t e with long r e l a x a t i o n t i m e o r w h e t h e r i t w a s locked in to an o r d e r e d s y s t e m .

J . D A N O N a s k e d a b o u t t h e p o s s i b i l i t y of o b s e r v i n g a s i j n i l a r p h e n o -m e n o n in o t h e r m e t a l s i n s e m i c o n d u c t o r s .

G . K . W E R T H E I M t o l d of e x p e r i m e n t s on 5 7Co in n a n d p t y p e , Si, Ge a n d G a A s in w h i c h no d i f f e r e n c e in i s o m e r s h i f t s w a s o b s e r v e d .

MÖSSBAUER E F F E C T STUDY OF THE ISOMERIC DE-EXCITATION IN l^Sn™*

R.H. HERBER AND H . A . STÖCKLER SCHOOL OF CHEMISTRY, RUTGERS. THE STATE UNIVERSITY

NEW BRUNSWICK, N.J. , UNITED STATES OF AMERICA

f ~ ABSTRACT

As with the k-capture, y-emission decay of 57Co, the chemical consequences of the isomeric decay of H9Snm can be investigated by the techniques of Mössbauer spectroscopy. The de-excitation of the 245-d 11/2-state occurs by the emission of 65.3-keV (M4) and 23.8-keV (M1-E2) y-radiation in cascade. Three

major chemical effects of nuclear transformations that can be investigated by Mössbauer techniques are: (a) consequences of the '"SnCn.yJu'Snm process andeffectsof the concomitant y-radiation during pile ex-posure; (b) recoil effects accompanying the M4 decay; and (c) effects of internal conversion of the M4 decay. These effects have been studied in stannic oxide metallic (grey) tin, and tetraphenyl tin, each labelled with the nuclide u s S n n ' .

In 1 1 9 Sn m 0 2 , neither the y- radiation exposure during pile irradiation, nor the emission (and internal conversion) of the 65.3-keV radiation leads to observable changes in the resonance line shape or position. This observation contrasts with the analogous situation in the decay (k-capture and y-emission) of 5 ,Co O. In

* Since this paper has been published in Chemical Effects of Nuclear Transformations II, IAEA, Vienna (1965) 403, only the abstract is printed here.

226

both cases two oxidation states of the metal oxide are known [Fell, FelH and Snll, Sn l v respectively] but only in "Co О has the presence of both states, as a result of the precursor nuclear event, been observed. Similar negative results obtain in metallic (grey) tin, in consonance with the observation on 5,Co diffused into metallic iron and used as a Mössbauer source.

As with 57Co-labelled cobalticinium tetraphenyl borate, however, the chemical consequences of nuclear de-excitation can be observed in metal-organic compounds labelled with I19Sn'". When 119Snm ©»is used as a source at 78° К with a 17 mg cm"2 SnC^ ceramic absorber, the major resonance is that associated with the single non-quadrupole split resonance line observed with an unlabelled Sn®4 absorber. In addition, there Is a contribution to the resonance pattern by a broad line (or quadrupole split resonance) absorption that reflects the filling of К or L shell vacancies by valence electrons. Direct bond rupture due to recoil is unable to produce this effect since (in the non-relativistic approximation) the recoil energy must be less than ~ 0 . 3 keV (~7 kcal/mole"i) , which is well below the chemical bond energies involved. /

DISCUSSION

P . K I E N L E ( C h a i r m a n ) and H. F R A U E N F E L D E R po in t ed out t he s i g n i -f i c a n c e of t he d i r e c t c o m p a r i s o n of f ' v a l u e s f o r t he s o u r c e and the a b s o r b e r in t h e s a m e c h e m i c a l s t a t e s .

G . K . W E R T H E I M a s k e d abou t t he c o n t r a d i c t i o n b e t w e e n s o m e d a t a o b -t a i n e d f o r Sn02, MgSn and S n P h 4 .

R . H . H E R B E R a n s w e r e d t h a t s u c h c o n t r a d i c t i o n e x i s t e d i n d e e d , b u t w a s n o t e x p l a i n e d y e t .

RECOILLESS GAMMA-EMISSION A F T E R NEUTRON CAPTURE REACTIONS

] . FINK AND P. KIENLE TECHNISCHE HOCHSCHULE, MÜNCHEN

AND TECHNISCHE HOCHSCHULE. DARMSTADT FEDERAL REPUBLIC OF GERMANY

R e c o i l l e s y - e m i s s i o n h a s b e e n p r e d o m i n a n t l y s t u d i e d w h e n i t o c c u r s a f t e r r a d i o a c t i v e d e c a y . S tone a n d P i l l i n g e r [ 1 ] h a v e r e c e n t l y s h o w n t h a t r e c o i l l e s s y - r a y s a r e a l s o e m i t t e d a f t e r a - d e c a y . We w i s h t o r e p o r t t h e f i r s t r e s u l t s of an e x p e r i m e n t t o s t u d y r e c o i l l e s s y - e m i s s i o n by (n, y) r e -a c t i o n s t o p o p u l a t e t h e e x c i t e d n u c l e a r s t a t e . E x p e r i m e n t a l l y , i t s e e m e d m o s t c o n v e n i e n t t o i n v e s t i g a t e f i r s t t h e 8 9 . 0 - k e V a n d t h e 7 9 . 5 - k e V t r a n -s i t i o n s f r o m t h e g r o u n d s t a t e r o t a t i o n a l b a n d s of 1 5 6 Gd a n d i58Gd. T h e y a r e e m i t t e d Elfter n e u t r o n c a p t u r e in i55Gd a n d 157 Gd, r e s p e c t i v e l y . B o t h r e a c t i o n s h a v e f a v o u r a b l y h i g h c r o s s - s e c t i o n s f o r t h e r m a l n e u t r o n s [ 2 ] : a [ i55Gd (n, y) 1 5 6 G d ] = 7 X 104 b , a [157Gd (n, y) i58Gd] = 16 X 104 b . T h e y y i e l d p e r h u n d r e d c a p t u r e r e a c t i o n s [ 2 ] i s 14 pho tons of 8 9 . 0 keV in 15?Gd and 10 p h o t o n s of 7 9 . 5 k e V in i s^Gd. T h e r e a r e a l s o no o t h e r i n t e n s e y - t r a n s i t i o n s in t h i s e n e r g y r e g i o n f r o m t h e c a p t u r e y - r a y d e c a y s [ 2 ] . T h e e n e r g i e s of t h e c o m p o u n d s t a t e s [ 2] f o r m e d b y c a p t u r e of s l o w n e u t r o n s

2 2 7

a r e 8 . 528 MeV in i56Gd and 7 . 913 MeV in i5«Gd. T h e h i g h e r - e n e r g y y - r a y s e m i t t e d in t h e i r d e c a y to t h e 89 . 0 - k e V and 79 . 5 - k e V s t a t e s t r a n s f e r r e c o i l e n e r g i e s u p to a b o u t 220 e V to t h e n u c l e i , w h i c h a r e h igh enough t o r u p t u r e c h e m i c a l b o n d s and c a u s e l a t t i c e d i s p l a c e m e n t s .

T h e s e e f f e c t s of n u c l e a r r e c o i l m a y l e a d t o a c h a n g e of t h e f r a c t i o n of r e c o i l l e s s y - r a y s e m i t t e d . V a r i o u s e l e c t r o n i c s t a t e s of t h e d i s p l a c e d i ons m a y p r o d u c e d i f f e r e n t h y p e r f i n e s p l i t t i n g s of t h e y - r a y s d e t e c t a b l e by t h e M ö s s b a u e r m e t h o d , when t h e n a t u r a l l i n e - w i d t h i s s m a l l e r than the sp l i t t ings . Ye t f o r Gd one e x p e c t s o n l y s m a l l s p l i t t i n g s of t h e y - l i n e я r e l a t i v e to t h e i r w id th , a t l e a s t w h e n one d e a l s wi th t r i v a l e n t Gd i o n s in t h e i r 4f 7 - 8 S m u l t i -p l e t c o n f i g u r a t i o n , b e c a u s e b o t h t h e m a g n e t i c d i p o l e i n t e r a c t i o n a n d t h e e l e c t r i c q u a d r u p o l e i n t e r a c t i o n wi th the 1=2 r o t ä t i o n a l s t a t e s a r e s m a l l . T h i s v e r y p a r t i c u l a r s i t u a t i o n f o r Gd i ons m a k e s t he p r e s e n t e x p e r i m e n t r e l a t i ve ly i n s e n s i t i v e f o r s t u d y i n g t h e d e t a i l s of r e c o i l e f f e c t s , w h e r e a s r e c o i l l e s s e m i s s i o n i s e a s i e r t o o b s e r v e a n d t h e r e f o r e u s e f u l f o r s t u d y i n g h y p e r f i n e i n t e r a c t i o n s in t h e a b s o r b e r , e s p e c i a l l y i s o m e r s h i f t s . T h e s e c o n d i t i o n s a r e d r a s t i c a l l y c h a n g e d f o r o t h e r r a r e - e a r t h i o n s , w i t h l a r g e h y p e r f i n e f i e l d s w h i c h a r e r a t h e r s e n s i t i v e t o t h e d e t a i l s of t h e c r y s t a l f i e l d .

T h e e x p e r i m e n t a l a r r a n g e m e n t u s e d to s t u d y t h e M ö s s b a u e r e f f e c t of n e u t r o n c a p t u r e y - r a y s i s shown in F i g . 1. T h e n e u t r o n b e a m f r o m the c o r e of a r e a c t o r i s c o l l i m a t e d t o a d i a m e t e r of 1. 5 c m . A f i l t e r of B i s i n g l e c r y s t a l s r e m o v e s t h e y - r a y s and e p i t h e r m a l n e u t r o n s f r o m the b e a m . T h e f i l t e r e d b e a m c o n t a i n e d 4 X 10® n / s w i t h i n a n a r e a of 3 c m 2 a t t h e t a r g e t p o s i t i o n . T h e t a r g e t s a n d r e s o n a n c e a b s o r b e r s w e r e c o o l e d t o 4 . 2 ° K b y l i q u i d h e l i u m . T a r g e t s e n r i c h e d in- 1 5 5 Gd o r 1 5 7 G d , o r if m o r e c o n v e n i e n t .

FIG. 1. Experimental arrangement to study Mössbauer effect with y-rays from (n,y) reactions. R reactor, С neutron collimator, F Bi single crystal filter, К liquid He cryostat, L loudspeaker drive, T target, A absorber, D detector

a b s o r b e r s e n r i c h e d i n i 5 6 G d o r 1 5 8 G d , h a v e b e e n u s e d t o i n v e s t i g a t e t h e r e s o n a n c e a b s o r p t i o n of t h e 89 . 0 - k e V o r 79 . 5 - k e V y - t r a n s i t i o n s s e p a r a t e l y . T h e t a r g e t s w e r e s l a n t e d r e l a t i v e t o t h e b e a m d i r e c t i o n ( n o r m a l l y 45°). A N a l c o u n t e r d e t e c t e d t h e y - r a y s e m i t t e d p e r p e n d i c u l a r t o t h e b e a m a f t e r p a s s i n g t h r o u g h t h e r e s o n a n c e a b s o r b e r . T h e t a r g e t s a b s o r b e d abou t 90% of t h e n e u t r o n s . T h e c o u n t - r a t e s w e r e a b o u t 3 0 0 0 / s in m o s t e x p e r i m e n t s .

228

T h e b a c k g r o u n d f r o m e n r i c h e d t a r g e t s a m o u n t s to abou t 60% of t h e d i s c r i m i -n a t e d p u l s e s . T h e t r a n s m i s s i o n of t h e y - r a y s w a s m e a s u r e d c o n v e n t i o n a l l y a s a f u n c t i o n of t h e D o p p l e r s h i f t t r a n s f e r r e d t o t h e y - q u a n t a b y m o v i n g t h e t a r g e t i n r e s p e c t t o t h e a b s o r b e r w i t h a f e e d - b a c k r e g u l a t e d l o u d - s p e a k e r d r i v e of t h e c o n s t a n t a c c e l e r a t i o n t y p e . T h e v e l o c i t y of t h e s y s t e m w a s c a l i -b r a t e d w i t h known h y p e r f i n e s p e c t r a .

R e s u l t s o n r e c o i l e f f e c t s o n t h e e m i s s i o n s p e c t r u m f r o m Gd2C»3 a n d m e t a l l i c Gd t a r g e t s a s w e l l a s s o m e r e m a r k s on t h e i s o m e r s h i f t s of y - r a y s f r o m r o t a t i o n a l s t a t e s b e t w e e n Gd m e t a l and t r i v a l e n t Gd i o n s a r e r e p o r t e d .

F i g u r e 2 s h o w s t r a n s m i s s i o n s p e c t r a a s a f u n c t i o n of t h e D o p p l e r s h i f t f o r v a r i o u s t a r g e t a n d a b s o r b e r c o m b i n a t i o n s . T h e r e s u l t s of a " m i r r o r " e x p e r i m e n t w i t h t h e 7 9 . 5 - k e V y - r a y s of i58Gd i n w h i c h t h e t a r g e t a n d t h e a b s o r b e r s u b s t a n c e s w e r e i n t e r c h a n g e d a r e g i v e n in F i g s . 2(a) and 2(b) . In 2 (a ) t h e t a r g e t w a s Gd m e t a l a n d t h e a b s o r b e r i58Gd¿0 3 ( 9 7 . 5 % i58Gd) c o n -t a i n i n g 13 . 5 m g / c m 2 i58Gd. In 2(b) t h e t a r g e t w a s i 5 7 G d 2 0 3 ( 9 3 . 7% ^ G d ) a n d t h e a b s o r b e r G d m e t a l of n a t u r a l i s o t o p i c a b u n d a n c e c o n t a i n i n g 3 7 . 0 m g / c m 2 1 5 8 G d . T h e e f f e c t i v e a b s o r b e r t h i c k n e s s f o r r e c o i l l e s s r e s o -n a n c e a b s o r p t i o n t = N 0 O f (N = n u c l e i / c m 2 , cr0 = m a x i m u m r e s o n a n c e ' a b -s o r p t i o n c r o s s - s e c t i o n , f = r e c o i l l e s s f r a c t i o n ) w a s e s t i m a t e d a s t = 3 . 9 a n d t = 3 . 2 f o r (a) and (b) r e s p e c t i v e l y , a s s u m i n g an u n s p l i t a b s o r p t i o n l i n e . T h e o b s e r v e d b r o a d e n i n g of t h e l i n e s s u g g e s t s r e a l v a l u e s f o r t , w h i c h a r e a b o u t a f a c t o r of 3 s m a l l e r . Wi th t h e s e r e l a t i v e l y t h i n a b s o r b e r s , t h e t r a n s -m i s s i o n s p e c t r a s h o u l d b e t h e s a m e i n t h e m i r r o r e x p e r i m e n t ( a f t e r p r o p e r c o r r e c t i o n f o r t h e b a c k g r o u n d i n t h e c o u n t i n g c h a n n e l , a n d f o r t h e r a t i o R of r e s o n a n c e a b s o r b i n g n u c l e i p e r c m 2 (R = 2 . 7 5 ) i n (a) and (b ) )when e f f e c t s of n u c l e a r r e c o i l do n o t i n f l u e n c e r e c o i l l e s s y - e m i s s i o n i n b o t h t a r g e t s . T h e e x p e r i m e n t s h o w s t h a t t h e t r a n s m i s s i o n l i n e w i t h t h e G d 2 0 3 t a r g e t i s a b o u t 10% b r o a d e r t h a n w i t h t h e G d m e t a l t a r g e t . T h i s i n d i c a t e s t h a t t h e r e c o i l e f f e c t s p r o d u c e o n l y a s m a l l d i f f e r e n c e i n t h e h y p e r f i n e s t r u c t u r e b e t w e e n a m e t a l l i c a n d a n o x i d e t a r g e t . T h i s m a y r e f l e c t t h e p a r t i c u l a r b e h a v i o u r of t h e h y p e r f i n e s t r u c t u r e f o r an S s t a t e i o n . F r o m the a r e a u n d e r t h e t r a n s m i s s i o n l i n e s o n e d e d u c e s t h e f o l l o w i n g r e l a t i o n b e t w e e n t h e r e l e -v a n t r e c o i l - f r e e f r a c t i o n s of t h e t a r g e t s (t) a n d a b s o r b e r s (a) u s e d i n t h e " m i r r o r " e x p e r i m e n t .

f (t) X f (a) = 0 . 6 f (a) X f ( t ) oxide metal oxide * metal

T h i s s o m e w h a t s u r p r i s i n g r e s u l t i n d i c a t e s t h a t t h e r e c o i l l e s s p r o b a b i l i t y f o r a t o m s a f t e r a p r e c e d i n g (n, y) r e a c t i o n r e l a t i v e t o t h o s e in u n p e r t u r b e d l a t t i c e s i s f o r t h e o x i d e o n l y 0 . 6 t i m e s t h a t f o r t h e m e t a l . F u r t h e r e x p e r i -m e n t s , i n w h i c h a b s o l u t e f v a l u e s a r e d e t e r m i n e d , m a y h e l p u s to u n d e r s t a n d . t h i s e f f e c t , f o r w h i c h w e do n o t h a v e a s i m p l e e x p l a n a t i o n .

F i g u r e 2 ( c ) s h o w s t h e t r a n s m i s s i o n s p e c t r u m of t h e 7 9 . 5 - k e V y - l i n e w i t h a 1 5 7 G d 2 0 3 t a r g e t a n d a 'Gd2C>3 a b s o r b e r . T h e r e i s i n d i c a t i o n of a p a r t l y r e s o l v e d h y p e r f i n e p a t t e r n a t t r i b u t e d t o q u a d r u p o l e s p l i t t i n g of t h e 1=2 e x c i t e d s t a t e i n t h e s o u r c e and t h e a b s o r b e r . A d e t a i l e d a n a l y s i s s h o u l d a l l o w t h e d e t e r m i n a t i o n of t h e e l e c t r i c f i e l d g r a d i e n t s and e l e c t r o n i c s h i e l d i n g f a c t o r s i n a w a y s i m i l a r t o t h e w o r k of B a r n e s e t a l . I 3 J on T m 2 0 3 .

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FIG. 2. Relative transmission of capture y-rays from 156Gd and 158Gd through resonance absorbers as function of the Doppler shift, v. (a) 79. 5 keV from 158Gd, target Gd metal, absorber 158Gd203

(b) 79. 5 keV from 158Gd, target 157Gd203, absorber Gd metal (c) 79. 5 keV from 158Gd, target l s 1Gd203 , absorber Gd203

(d) 89. 0 keV from 156Gd, target155 Gd203 , absorber Gd metal

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F i g u r e 2(d) s h o w s t h e s p e c t r u m of t h e 8 9 . 0 - k e V - y - r a y s of i56Gd w i t h a 1 5 5 G d 2 0 3 t a r g e t a n d a G d m e t a l a b s o r b e r . T h i s s p e c t r u m a n d t h e a n a -l o g o u s s p e c t r a of t h e 79 . 5 - k e V y - r a y s of i 5 8 G d ( F i g s . 2(a) and 2(b)) s h o u l d s h o w e v i d e n c e of i s o m e r s h i f t s of y - t r a n s i t i o n s f r o m a 2+ r o t a t i o n a l s t a t e t o

2 3 0

i t s g r o u n d s t a t e b e t w e e n t r i v a l e n t Gd i o n s and t h e m e t a l . T h e d a t a i n d i c a t e b a r e l y m e a s u r a b l e s h i f t s f o r b o t h t r a n s i t i o n s (6E = E ( m e t a l ) - E (Gd 3 + )):

8 9 . 0 k e V 1 5 6 G d : 6E = + ( 9 . 2 ± 3 . 9 ) X 1 0 ' " e V

7 9 . 5 k e V 1 5 8 G d : 6E = + ( 4 . 4 ± 2 . 0 ) X 10" 8 e V

T h e i s o m e r s h i f t i s u s u a l l y e x p r e s s e d b y t h e r e l a t i o n :

6E = (27г/3)Z е2ХЬф ( 0 f X 6 < r 2 >

w h e r e б ф{О)2 i s t h e d i f f e r e n c e of t h e e l e c t r o n d e n s i t y a t t h e n u c l e u s i n Gd m e t a l and G d 3 + , wh ich i s a t t r i b u t e d to t he c o n d u c t i o n e l e c t r o n d e n s i t y a lone . B r i x e t a l . [ 4 ] d e r i v e d r e c e n t l y a c a l i b r a t i o n s c h e m e w h i c h a l s o e n a b l e s one to e s t i m a t e the conduc t ion e l e c t r o n dens i t y in Gd m e t a l a s 2 .0X10" 2 6 cm" 3 . T h e change of t h e m e a n s q u a r e c h a r g e r a d i u s b e t w e e n the exc i t ed s t a t e ( h e r e t h e ç o t a t i o n a l s t a t e ) a n d t h e g r o u n d s t a t e i s б ^ r 2 ) > = <^r2^ e - "C1-2/^ • T h e r e s u l t s f o r 6<^r 2 ^ a r e :

8 9 . 0 k e V , 1 5 6 G d : 6 < r 2 > = +(2 . 4 ± 1 .0 ) X 1 0 " 3 f m 2

79 . 5 k e V , 1 5 8 G d : 6 < r 2 > = +(1 . 2 ± 0 . 5) X 1 0 ~ 3 f m 2

T h e s e s m a l l 6 " ( r 2 ) v a l u e s f o r r o t a t i o n a l t r a n s i t i o n s i n Gd n u c l e i a r e v e r y p u z z l i n g i n v i e w of t h e p r e d i c t i o n of t h e r o t a t i o n a l v i b r a t i o n a l m o d e l [ 5 ] f o r e v e n n u c l e i . T h i s m o d e l d e s c r i b e s v e r y w e l l t h e o b s e r v e d e n l a r g e m e n t of t h e m o m e n t of i n e r t i a , J , a s a f u n c t i o n of t he a n g u l a r m o m e n t u m I wi th in a r o t a t i o n a l band. In t e r m s of t h i s m o d e l , 6 \ r 2 ) > i s g iven by t h e r e l a t i o n [6] :

6 < r 2 > . ¿ x i ( I + l ) X ( ^ ) 2 X ß 02 X R ;

e = - h 2 / J i s d e t e r m i n e d f r o m t h e e n e r g y of t h e 1=2 s t a t e ; Eg = e n e r g y of t h e ß v i b r a t i o n ; ß0 = d e f o r m a t i o n p a r a m e t e r d e r i v e d f r o m the quad rupo le m o m e n t a n d R 0 = n u c l e a r r a d i u s . W i t h t h i s r e l a t i o n a n d t h e p a r a m e t e r s d e r i v e d f r o m e x p e r i m e n t a l d a t a t h e f o l l o w i n g 6<^r2)> a r e p r e d i c t e d :

8 9 . 0 k e V , 1 5 6 G d : Ô < r 2 > = 3 6 X 1 0 - 3 f m 2

7 9 . 5 k e V , 1 5 8 G d : 6 < r 2 > = 1 9 X 1 0 " 3 f m 2

T h e e x p e r i m e n t a l v a l u e s a r e r o u g h l y a f a c t o r of t en s m a l l e r than the t h e o r e -t i c a l p r e d i c t i o n s 1 . W e b e l i e v e t h a t t h i s l a r g e d i s c r e p a n c y c a n n o t b e d u e t o t h e u n c e r t a i n t y i n б | ^ ( 0 ) | 2 u s e d f o r t h e e v a l u a t i o n of 6 < r 2 > f r o m i s o m e r -s h i f t d a t a . A d e c r e a s e of 6 \ф( 0) | 2 b y a f a c t o r of t e n i s i n c o n t r a d i c t i o n

' The authors have the indication of similar results for the 80.6-keV transition in 166Er and the 84- keV transition in 1,0Yb.

231

w i t h a l l p l a u s i b l e a s s u m p t i o n s f o r c o n d u c t i o n e l e c t r o n d e n s i t i e s and w o u l d a l s o y i e l d u n r e a s o n a b l y h i g h v a l u e s f o r 6<^r2)> f o r o d d p r o t o n t r a n s i t i o n i n 153EU ( 6 < r 2 > = - 1 . 7 f m 2 !) [ 7 ] . '

T h e a u t h o r s w o u l d l i k e t o t h a n k P r o f e s s o r H . M a i e r - L e i b n i t z f o r h i s f r i e n d l y s u p p o r t of t h i s w o r k . D r . W . W i e d e m a n n ' s h e l p i n c r y o g e n i c p r o b l e m s i s g r a t e f u l l y a c k n o w l e d g e d . O n e of t h e a u t h o r s ( P . K . ) i s p a r t i -c u l a r l y g r a t e f u l f o r m a n y d i s c u s s i o n s wi th P r o f e s s o r G r e i n e r , who p a t i e n t l y t a u g h t h i m a l l t h e i n t r i g u i n g p r o p e r t i e s of t h e r o t a t i o n a l v i b r a t i o n a l t h e o r y f o r n u c l e i .

R E F E R E N C E S

[1] STONE, J .A. , PILLINGER, W.L., Phys. Rev. Lett.^13 (1964) 200. [2] Nuclear Data Sheets. [3] BARNES, R.G., KANKELEIT, E., MÖSSBAUER,R.L., POINDEXTER, J .M., Phys. Rev. Lett. 11 (1963)

253; Phys. Rev. 136 (1964) A 175. [4] BRIX, P. , HÜFNER, S., KIENLE, P. , QUITMANN, D. , Phys. Lett. 13 (1964) 140; Z. f. Physik, to

be published. [5] FAESSLER, A.. G REINER, W., Z. Phys. 168 (1962) 425; ibid. 170 (1962) 105; ibid. 177 (1964) 190. [6] GREINER, W., private communication. [7] STEICHELE, E., HÜFNER, S., KIENLE, P., Phys. Lett. 14 (1965) 321.

D I S C U S S I O N

R . H . H E R B E R a s k e d w h e t h e r t h e p o s s i b l e e f f e c t s p r o d u c e d b y t h e e l e c t r o n l o s s e s due to t h e r e c o i l and by e l e c t r o n c o n v e r s i o n w e r e c o m p a r e d .

P . K I E N L E ( C h a i r m a n ) a n s w e r e d t h a t t h e r e c o i l e f f e c t s w e r e of m u c h m o r e i m p o r t a n c e i n h i s c a s e .

R . H . H E R B E R a d d e d t h a t l o w - e n e r g y t r a n s i t i o n s a s w e l l a s t h e r m a l s p i k e s c o u l d p l a y a n i m p o r t a n t r o l e . J . W i l l a r d h a s p o i n t e d o u t t h e i m -p o r t a n c e of i n t e r n a l c o n v e r s i o n e v e n t s l a t e i n t h e s l o w i n g - d o w n h i s t o r y of r e c o i l a t o m s ;

G . K . W E R T H E I M p o i n t e d o u t t h e d e s i r a b i l i t y of m e a s u r e m e n t s of c o -i n c i d e n c e s b e t w e e n M ö s s b a u e r and c a p t u r e y - r a y s .

P . K I E N L E ( C h a i r m a n ) a g r e e d , and a d d e d t h a t t h e H F S of s p e c t r a w a s t o o w e a k in t h e c a s e s t u d i e d . E f f e c t s of r e c o i l s h o u l d b e t a k e n i n t o a c c o u n t a l s o i n d i s c u s s i n g t h e c o n s e q u e n c e s of e l e c t r o n s h e l l s h a k e u p d u e t o t h e К c a p t u r e . H e s h o w e d s o m e d a t a of C a r l s o n o n t h e r e s u l t s of i r r a d i a t i o n of x e n o n .

232

RECOILLESS GAMMA-EMISSION FROM EXCITED ATOMIC STATES IN EUROPIUM

E. STEICHELE, W. HENNING, S. HÜFNER AND P. KIENLE TECHNISCHE HOCHSCHULE, DARMSTADT

FEDERAL REPUBLIC OF GERMANY

High ly i o n i z e d a t o m s a r e p r o d u c e d a f t e r e l e c t r o n c a p t u r e (EC) d e c a y of a n u c l e u s a s a r e s u l t of n o n - r a d i a t i v e t r a n s i t i o n s f i l l i ng i n n e r s h e l l vacanfeies and b e c a u s e of t he n o n - a d i a b a t i c c h a n g e o f t h e n u c l e a r c h a r g e [1]. The r e s u l t i n g c h a r g e s p e c t r a h a v e b e e n e x t e n s i v e l y s t u d i e d i n f r e e a t o m s a n d m o l e c u l e s [2 ] , but on ly v e r y s c a n t y knowledge e x i s t s about t he b a c k - r e a c t i o n p r o c e s s e s of t h e a n o m a l o u s a t o m i c c o n f i g u r a t i o n s to c h e m i c a l l y s t a b l e s t a t e s in s o l i d s . T h e e x p e c t e d v e r y s h o r t r e a c t i o n t i m e s m a k e i t v e r y d i f f i c u l t t o i d e n t i f y t he i n t e r m e d i a t e c o n f i g u r a t i o n s . A l s o we do not know of any da ta on the f o r m a t i o n of e x c i t e d a t o m i c s t a t e s a s a r e s u l t of n u c l e a r d e c a y .

R e c e n t l y v a r i o u s a u t h o r s [3] h a v e s t u d i e d t h e r e c o i l l e s s y - e m i s s i o n s p e c t r a , p r o d u c e d a f t e r t h e E C d e c a y of 57Co and t h e i s o m e r i c t r a n s i t i o n of 119 S n m , w i t h t h e i n t e n t i o n of i d e n t i f y i n g s h o r t - l i v e d a t o m i c c h a r g e s t a t e s f r o m t h e h y p e r f i n e s t r u c t u r e of t h e y - r a y s . T h e h a l f - l i v e s of t h e n u c l e a r s t a t e s ( 1 0 - 7 s and 1 . 8 X 1 0 - B s) c o n f i n e d t h e s e e x p e r i m e n t s to t he o b s e r v a t i o n of r e l a t i v e l y l o n g - l i v i n g c h a r g e s t a t e s . We w i s h to r e p o r t t he i n v e s t i g a t i o n of t h e r e c o i l l e s s e m i s s i o n s p e c t r u m f r o m t h e v e r y s h o r t - l i v e d 9 7 - k e V s t a t e of ^ » E u j T i / s = 2 X 10" 1 0 s ) a f t e r E C d e c a y of 1 5 3 Gd. E v i d e n c e of r e c o i l l e s s y - e m i s s i o n f r o m e x c i t e d s t a t e s of t r i v a l e n t E u i s f o u n d .

T h e M ö s s b a u e r e f f e c t of t h e 9 7 - k e V t r a n s i t i o n i n 1 5 3Eu w a s s t u d i e d r e c e n t l y b y A t z m o n y a n d O f e r 1.4] a n d b y S t e i c h e l e e t a l . 15] . L a r g e i s o m e r s h i f t s b e t w e e n t r i v a l e n t and d i v a l e n t E u c o m p o u n d s w e r e f o u n d - ( 6 E = - 0 . 5 X 1 0 " 5 eV) - w h i c h e x c e e d t h e e x p e r i m e n t a l l i n e - w i d t h of t h e s h o r t - l i v i n g s t a t e . T h e s e f e a t u r e s m a k e t h e 9 7 - k e V t r a n s i t i o n p a r t i c u l a r l y s u i t e d f o r i d e n t i f y i n g v e r y s h o r t - l i v i n g a t o m i c s t a t e s of E u p r o d u c e d a f t e r t h e E C d e c a y of 1 5 3 Gd b y t h e i s o m e r s h i f t of t h e e m i s s i o n s p e c t r u m . T h e d i f f e r e n c e of t h e e l e c t r o n d e n s i t y a t t h e n u c l e u s , e x h i b i t e d by t h e i s o m e r s h i f t b e t w e e n t r i v a l e n t a n d d i v a l e n t E u w i t h e l e c t r o n c o n f i g u r a t i o n s ( a p a r t f r o m c l o s e d i n n e r s h e l l s ) 4 f 6 5s 2 5p 6 and 4f 7 5s 2 5p 6 r e s p e c t i v e l y , i s p r o d u c e d by t h e d i f f e r e n t s h i e l d i n g of c l o s e d - s h e l l s e l e c t r o n s b y t h e 4 f B a n d 4 f 7 c o n -f i g u r a t i o n s . T h e e l e c t r o n d e n s i t y d i f f e r e n c e :

k ( 0 ) | * f , - k ( 0 ) | 24 f e - 1 .9 X 1 0 2 6 c m " 3

w a s d e t e r m i n e d by B r i x et a l . [6] . When the i s o m e r sh i f t i s e x p r e s s e d by the r e l a t i o n :

6E = (27г/3) Z e 5 k(o)i24f7 - i*(o)i

4f 6 < r 2 > (1)

233

a c h a n g e of t h e m e a n s q u a r e n u c l e a r - c h a r g e r a d i u s 6 < r 2 > = 0 .17 f m 2 i s d e -r i v e d , a s c a l c u l a t e d b y [5] f o r t h e 9 7 - k e V t r a n s i t i o n l .

I n t h e e x p e r i m e n t s r e p o r t e d [4, 5 ] , b o t h g r o u p s u s e d G d 2 0 3 a s s o u r c e m a t e r i a l . A m o r e d e t a i l e d a n a l y s i s of t h e t r a n s m i s s i o n s p e c t r a t h r o u g h EU2O3 a b s o r b e r s i n d i c a t e s c l e a r l y a s y m m e t r i c l i n e s ( s e e F i g . 1(a) in R e f . [4] a n d F i g . 1(a) i n R e f . [5] ) s h o w i n g e n h a n c e d a b s o r p t i o n w h e n t h e e m i s s i o n l i n e i s s h i f t e d to h i g h e r e n e r g i e s . T h i s o b s e r v a t i o n l e d u s to r e - i n v e s t i g a t e t h e e m i s s i o n s p e c t r u m . C a r e w a s t a k e n t o c i r c u m v e n t a l l e x p e r i m e n t a l d e f i c i e n c i e s w h i c h m i g h t s i m u l a t e s u c h a n e f f e c t . T h e Gd2C>3 s o u r c e f r o m o u r e a r l i e r w o r k w a s a l s o u s e d f o r t h e p r e s e n t e x p e r i m e n t . T h e r e s o n a n c e a b s o r b e r w a s 2 0 0 - m g / c m 2 - t h i c k Еи^Оз of n a t u r a l i s o t o p i c a b u n d a n c e . T h i s s u b s t a n c e w a s a n a l y s e d f o r s m a l l c o n t e n t s of d i v a l e n t E u w h i c h w o u l d s h o w a b s o r p t i o n a t h i g h e r e n e r g i e s c o m p a r e d w i t h E u 3 + [4, 5] . F o r t h a t p u r p o s e , t h e r e c o i l l e s s r e s o n a n c e a b s o r p t i o n f o r t h e 2 1 . 7 - k e V y - l i n e of 1 5 1 E u , w h i c h s h o w s c l e a r l y s e p a r a t e d l i n e s f o r t r i v a l e n t and d i v a l e n t E u [ 6 ] , w a s i n v e s t i -g a t e d . T h i s e x p e r i m e n t p r o v e d t h a t t h e a b s o r b e r s u b s t a n c e c o n t a i n e d l e s s t h a n 0 . 1 % d i v a l e n t E u . T h e d r i v e s y s t e m to p r o d u c e t h e D o p p l e r s h i f t of t h e y - r a y s f r o m t h e s o u r c e w a s i m p r o v e d by c o m p a r i s o n wi th t h e s e t - u p u s e d in t h e p r e v i o u s e x p e r i m e n t . T h e s i n u s o i d a l v e l o c i t y s p e c t r u m w a s a c c u r a t e t o w i t h i n 0 .3%.

T r a n s m i s s i o n m e a s u r e m e n t s of t h e 9 7 - k e V y - r a y s t h r o u g h the r e s o n a n c e a b s o r b e r , a s a f u n c t i o n of t h e D o p p l e r s h i f t of t h e s o u r c e i n r e s p e c t t o t h e a b s o r b e r , a r e s h o w n i n F i g . l . F i g u r e 1(a) d i s p l a y s t h e t r a n s m i s s i o n s p e c t r u m f o r s o u r c e and a b s o r b e r a t r o o m t e m p e r a t u r e . T h e t r a n s m i s s i o n a s a f u n c t i o n of t h e e n e r g y i s c o n s t a n t w i t h i n 0 . 0 5 % , w h i c h s h o w s t h a t t h e e x p e r i m e n t a l p r o c e d u r e a p p l i e d d o e s n o t i n t r o d u c e a v e l o c i t y - d e p e n d e n t c h a n g e of t h e t r a n s m i s s i o n . F i g u r e 1(b) s h o w s t h e t r a n s m i s s i o n s p e c t r u m w i t h s o u r c e a n d a b s o r b e r c o o l e d t o 4 . 2 ° K . T h i s s p e c t r u m i s c l e a r l y a s y m -m e t r i c u p t o t h e h i g h e s t D o p p l e r s h i f t , w i t h e n h a n c e d a b s o r p t i o n f o r h i g h e r e n e r g i e s .

Q u a d r u p o l e s p l i t t i n g i n t h e s o u r c e and t h e a b s o r b e r m a y p r o d u c e a s y m -m e t r i c t r a n s m i s s i o n s p e c t r a (w i th t h i c k a b s o r b e r s ) . Bu t t h i s e x p l a n a t i o n i s r u l e d o u t b y t h e o b s e r v a t i o n t h a t t h e 1 0 3 - k e V l i n e i n 1 5 3 E u , w h i c h i s s e v e n t i m e s n a r r o w e r t h a n t h e 9 7 - k e V l i n e , d o e s n o t e x h i b i t s u c h a s p l i t t i n g ( s e e R e f . [5] a n d t h e a d d i t i o n a l n a r r o w l i n e i n F i g . 1(b)) a l t h o u g h t h e q u a d r u p o l e i n t e r a c t i o n s h o u l d b e a b o u t t h e s a m e . T h e a b s o r p t i o n l i ne i s t h e r e f o r e s y m -m e t r i c . T h e a s y m m e t r y i n t h e t r a n s m i s s i o n s p e c t r u m m u s t b e d u e t o t h e s o u r c e , w h i c h e m i t s m o r e l o w - e n e r g y y - q u a n t a .

A n a t t e m p t w a s m a d e t o f i t t h e t r a n s m i s s i o n s p e c t r u m w i t h t w o L o r e n t z i a n c u r v e s . T h e r e s u l t i s i n c l u d e d i n F i g . 1(b) . M o s t of t h e i n t e n s i t y (91%) of t h e e m i s s i o n s p e c t r u m i s c o n t a i n e d in a l i n e w i t h n o i s o m e r s h i f t i n r e s p e c t t o t h e a b s o r b e r . I t i s a t t r i b u t e d t o t r i v a l e n t E u w i t h t h e c o n f i g u r a t i o n 4 f 6 5 s 2 5 p 6 . A n a d d i t i o n a l w e a k l i n e ( d e n o t e d b y x) i s f o u n d (9% i n t e n s i t y ) , s h i f t e d b y v = - 1 . 5 c m / s o r 6 E = - 4 . 9 X 10~6 e V i n r e s p e c t t o t h e s t r o n g e r l i n e . I t s l i n e - w i d t h i s v e r y i n a c c u r a t e l y d e t e r m i n e d b u t i t i s n o t b r o a d e r t h a n t h e m a i n l i n e . W i t h ó <(r2^> = - 0 . 1 7 f m 2 o n e c o n c l u d e s t h a t

• | * (0 ) |® - |<&(0)|® = + 1 . 5 X 1 0 2 6 c m " 3

1 The consequences of anomalous isomer shifts reported in Ref. [4] are not considered in this discussion

234

О 1 2 Э < 5 6

v in cm / s

FIG. 1. Relative transmission N(v)/N(») of the 97-keV and 103-keV y-radiation of 15sEu as a function of the Doppler shift v from a Gd203 source through à EU2O3 absorber. (a) At room temperature, (b) At 4. 2°K. The solid line approximates the total transmission spectrum including the narrow contribution of the 103-keV line. The dashed curve is a fit of the spectrum from 4f65s !5p6 con-figuration. (c) Transmission spectrum from anomalous states

E m i s s i o n f r o m c o n f i g u r a t i o n s wi th s t i l l h i g h e r e l e c t r o n d e n s i t i e s at t h e n u c l e u s s e e m s t o b e i n d i c a t e d i n F i g . 1(b). No e v i d e n c e of d i v a l e n t E u c o n -f i g u r a t i o n s , f o r m e d a f t e r t h e E C d e c a y , i s s e e n . T h e e l e c t r o n d e n s i t y d i f -f e r e n c e f o u n d f o r t h e c o n f i g u r a t i o n x in r e s p e c t t o 4 f 6 5 s 2 5 p s i s e x p e c t e d f o r a s t a t e in w h i c h a 4f e l e c t r o n i s p r o m o t e d in to a 5d o r 5p o r b i t a l o r i s e n t i r e l y r e m o v e d f r o m the ion, so s u g g e s t i n g c o n f i g u r a t i o n s of the type 4f 55s 25p 65d (a), 4 Í 5 5s 2 5p 0 6p (b) o r 4 f ö 5 s 2 5 p 6 ( E u 4 + ) f o r the ion x . T h i s fo l l ows f r o m the change of t he e l e c t r o n d e n s i t y a t t h e n u c l e u s a s e v i d e n c e d by i s o t o p e s h i f t m e a s u r e -m e n t s i n S m f o r t h e t r a n s i t i o n 4 f 6 5 s 2 5 p 6 t o 4 f 5 5 s 2 5 p 6 5 d . ( F o r a d i s c u s s i o n s e e R e f . [6]).

D i e k e and C r o s s w h i t e [7] found f r o m o p t i c a l s p e c t r a t h a t t h e c o n f i g u r a t i o n s (a) a n d (b) p r o d u c e m a n y l o w - l y i n g e x c i t e d s t a t e s i n E u 3 + . It i s t h e r e f o r e t e m p t i n g to a s s u m e t h a t t he a n o m a l o u s 7 - e m i s s i o n i s o b s e r v e d when t r i v a l e n t Eu ions a r e f o r m e d in e x c i t e d s t a t e s of the c o n f i g u r a t i o n s (a) and (b). B e f o r e o n e a c c e p t s t h i s f i n a l l y o n e h a s to r u l e out o t h e r c o n f i g u r a t i o n s , e s p e c i a l l y t h e 4 f 5 5 s 2 5 p 6 f r o m Eu 4 + , w h i c h w o u l d a l s o a c c o u n t f o r t h e i s o m e r s h i f t o b -s e r v e d . T h e f o l l o w i n g a r g u m e n t s a r e b a s e d on t h e o b s e r v a t i o n t h a t t h e a n o m a l o u s e m i s s i o n l i n e h a s r e l a t i v e l y low i n t e n s i t y c o m p a r e d wi th t h e e m i s s i o n s p e c t r u m f r o m the 4 f 6 5s 25p6 con f igu ra t i on , but about the s a m e width. T h i s m e a n s tha t the r e s p o n s i b l e e l e c t r o n c o n f i g u r a t i o n i s f o r m e d with a s m a l l p r o b a b i l i t y a n d t h a t i t s h a l f - l i f e i s l ong c o m p a r e d w i t h t h e h a l f - l i f e of t h e

235

n u c l e a r s t a t e (2 X 10"10 s ) . T h e s e r e q u i r e m e n t s a r e p robab ly not fu l f i l l ed f o r the Eu 4 + 4 f 5 5s 2 5p 6 c o n f i g u r a t i o n , b e c a u s e the r e s u l t s of c h a r g e s p e c t r a sugges t [2] t h a t E u 4 + s t a t e s a r e f o r m e d v i a t h e d i r e c t p r o c e s s a n d b y s u c c e s s i v e e l e c t r o n c a p t u r e of t he h i g h e r c h a r g e s t a t e s wi th m u c h h i g h e r p r o b a b i l i t y than c o n f i g u r a t i o n s of Eu 3 + . B u t t h e y - e m i s s i o n s p e c t r u m s h o w s that m o s t of t h e i o n s h a v e E u 3 + c o n f i g u r a t i o n s a f t e r s e v e r a l 10~ l u s a f t e r E C d e c a y . T h u s t h e p r o b a b l y m o s t s t a b l e 4 f 5 5s 2 5p 6 c o n f i g u r a t i o n of Eu 4 + should a l so t r a n s f o r m t o E u 3 + i n a t i m e s h o r t e r t h a n 2 X 10~10 s . T h e y - e m i s s i o n l i n e wou ld t h e n b e b r o a d e n e d . On t h e o t h e r h a n d i t s e e m s q u i t e r e a s o n a b l e t h a t t h e r e i s a s m a l l p r o b a b i l i t y t h a t c e r t a i n s u i t a b l e e x c i t e d s t a t e s of Eu 3 + ( e s p e c i a l l y t hose w i t h 4 f 5 5 s 2 5 p e 6 p ) a r e p r o d u c e d , w h i c h h a v e a l o n g h a l f - l i f e c o m p a r e d w i t h o t h e r s t h a t d e c a y v e r y f a s t t o t h e s t a t e s w i t h a 4 f 6 5 s 2 5 p 6 c o n f i g u r a t i o n , a l -though d i r e c t e v i d e n c e i s no t a v a i l a b l e . It s e e m s p o s s i b l e tha t e x c i t e d s t a t e s of t h e c o n f i g u r a t i o n 4 f 5 5 s 2 5 p 6 6 s a r e r e s p o n s i b l e f o r the y - e m i s s i o n ind ica t ing s t i l l l a r g e r i s o m e r s h i f t s . I m p r o v e d e x p e r i m e n t s m a y p o s s i b l y c l a r i f y t h i s p o i n t .

T h e a u t h o r s w i s h t o t h a n k P r o f e s s o r H e l l w e g e f o r h i s f r i e n d l y s u p p o r t of t h i s w o r k and P r o f e s s o r B r i x f o r m a n y h e l p f u l d i s c u s s i o n s .

R E F E R E N C E S

[1] For a review and for relevant literature see: WEXLER, S., Actions Chimiques et Biologiques des Radia-tions, VIII (M. HAISSINSKY, Ed. ) Masson et Cie., Paris (1964).

[2] SNELL, A. H., PLEASANTON, F., CARLSON, T.A. , Chemical Effects of Nuclear Transformations I. IAEA, Vienna (1961) 147; WEXLER. S., ibid P. 145; CARLSON, T. A. . WHITE, R. M. , Chemical Effects of Nuclear Transformations 1. IAEA, Vienna (1965) 23.

[3] WERTHEIM, G. К., Phys. Rev. 124 (1961) 764; WERTHEIM, G. К., KINGSTON, W.R., HERBER, R.H., J. chem. Phys. 37 (1962) 687; BEARDEN, A.J. , MATTERN, P.L., HART, T.R., Rev. mod. Phys. 36 (1964) 470; HERBER, R. H., STÖCKLER, H.A., Chemical Effects of Nuclear Transformations II, IAEA,

'Vienna (1965) 463; NESMEYANOV, A.N. et al . , ibid p. 419. WERTHEIM, G.K., GUGGENHEIM, H.J.. J. chem. Phys. to be published ; INGALLS, R., De PASQUALI, G., Phys. Lett. 15 (1965) 262.

[4] ATZMONY, U., OFER, S., Phys. Lett. 14 (1965) 284. [5] STEICHELE, E., HÜFNER, S., KIENLE, P., Phys. Lett. 14 (1965) 321. [6] BRIX, P., HÜFNER, S., KIENLE, P., QUITMANN, D., Phys. Lett. 13 (1964)140; Z. Phys. t o b e

published. [7] DIEKE, G.H. , CROSSWHITE, H. M. . Appl. Optics 2 (1963) 675.

D I S C U S S I O N

J . D A N O N a s k e d a b o u t t h e d i f f e r e n c e s i n e n e r g y of v a r i o u s l e v e l s . P . K I E N L E ( C h a i r m a n ) p r o v i d e d s o m e f i g u r e s .

236

MAGNETISM

(Sess ion 7, Par t 1)

SOME RECENT CONTRIBUTIONS TO THE STUDY OF THE MAGNETISM OF METALS, ALLOYS

AND INTERMETALLIC COMPOUNDS

G. K. WERTHEIM BELL TELEPHONE LABORATORIES, MURRAY HILL, N . I .


I N T R O D U C T I O N

M a g n e t i s m i s w i thou t doubt t h e d i s c i p l i n e of s o l i d - s t a t e p h y s i c s to which t h e M ö s s b a u e r e f f e c t h a s m a d e i t s m o s t s i g n i f i c a n t c o n t r i b u t i o n s . The c l a s s i -c a l m e a s u r e m e n t i n t h i s d i s c i p l i n e i s t h a t of the m a g n e t i z a t i o n a s a f u n c t i o n of m a g n e t i c f i e l d and t e m p e r a t u r e . T h e l i m i t a t i o n s a r e a p p a r e n t . In a c o m -p o u n d of m a g n e t i c a t o m s o n l y t h e n e t m o m e n t p e r m o l e c u l e i s a c c e s s i b l e ; t h e d e t a i l s of t h e b e h a v i o u r of t h e s u b l a t t i c e s a r e n o t . M o r e o v e r ; ' r e c e n t e x p e r i e n c e h a s s h o w n t h a t v e r y h igh m a g n e t i c f i e l d s 100 kOe) m a y be r e -q u i r e d t o r e a c h s a t u r a t i o n . A m a j o r b r e a k - t h r o u g h c a m e w i t h t h e a d v e n t of p o l a r i z e d n e u t r o n s c a t t e r i n g w h i c h g a v e i n f o r m a t i o n n o t o n l y on t h e m o m e n t s of t h e s u b l a t t i c e i o n s b u t a l s o on t h e i r s p i n d i r e c t i o n s . C o m p l i -c a t e d c a n t e d , s p i r a l and s c r e w s t r u c t u r e s w e r e shown to e x i s t in n o m i n a l l y s i m p l e s u b s t a n c e s .

T h e i o n i c m o m e n t , t h e f u n d a m e n t a l p r o p e r t y of a t r a n s i t i o n m e t a l ion, m a n i f e s t s i t s e l f in o t h e r m e a s u r a b l e s , ' f o r e x a m p l e in t he n u c l e a r m a g n e t i c h y p e r f i n e s t r u c t u r e ( h f s ) s p l i t t i n g . T h e c o n n e c t i o n i s w e l l u n d e r s t o o d f o r a m e t a l l i k e i r o n . T h e p o l a r i z e d e l e c t r o n s i n t h e i n c o m p l e t e i n n e r s h e l l , i . e . t h e 3d s h e l l w h e r e t h e i o n i c m o m e n t r e s i d e s , e x c h a n g e - p o l a r i z e t h e i n n e r s - s h e l l e l e c t r o n s a s w e l l a s t h e c o n d u c t i o n e l e c t r o n s . T h e r e s u l t i n g u n b a l a n c e d s - e l e c t r o n s p i n d e n s i t y a t t he n u c l e u s r e s u l t s in a F e r m i - c o n t a c t i n t e r a c t i o n w h i c h g i v e s r i s e t o t h e h f s s p l i t t i n g . T h i s s p l i t t i n g , i n t u r n , m a y b e c h a r a c t e r i z e d b y an e f f e c t i v e m a g n e t i c f i e l d , H, t h e h y p e r f i n e f i e l d . T h e r e a r e no o r b i t a l c o n t r i b u t i o n s t o t he h f s f i e l d b e c a u s e t he o r b i t a l a n g u l a r m o m e n t u m i s q u e n c h e d .

Whi le i t m a y b e t e m p t i n g t o a s s u m e t h a t the i on i c m o m e n t sind h y p e r f i n e f i e l d wi l l be p r o p o r t i o n a l u n d e r t h e s e c i r c u m s t a n c e s , i t i s c l e a r t h a t s u c h a r e l a t i o n s h i p c a n a t b e s t h a v e o n l y a p p r o x i m a t e v a l i d i t y . T h e ch ie f r e a s o n i s t h a t t h e m e c h a n i s m r e l a t i n g t h e s e t w o q u a n t i t i e s d e p e n d s on t h e r a d i a l p a r t of t h e e l e c t r o n i c wave f u n c t i o n s which wi l l not be the s a m e in a l l e n v i r o n -m e n t s . T h i s p o i n t i s i l l u s t r a t e d b y t h e c a s e of F e 3 + in o x i d e s w h e r e v a l u e s of H r a n g i n g f r o m 520 t o 620 k O e h a v e b e e n f o u n d f o r t h e S = 5 / 2 i o n w i t h 5 . 92 B o h r m a g n e t o n s . F o r r a r e - e a r t h i s o t o p e s t h e o r b i t a l c o n t r i b u t i o n s

237

t o b o t h t h e i o n i c m o m e n t a n d t h e h y p e r f i n e f i e l d a r e g e n e r a l l y d o m i n a n t , e x c e p t f o r t h e S - s t a t e i o n s G d 3 + a n d E u 2 + . F o r r a r e - e a r t h i o n s t h e e f f e c t of bond ing on Heff i s s m a l l s i n c e the 4f e l e c t r o n s a r e not involved in bonding and a r e b e t t e r s h i e l d e d f r o m the e n v i r o n m e n t t h a n t h e 3d e l e c t r o n s i n i r o n .

T h e r e a r e two t e c h n i q u e s p a r t i c u l a r l y w e l l - s u i t e d f o r the s t u d y of h f s in m a g n e t i c m a t e r i a l - n u c l e a r m a g n e t i c r e s o n a n c e ( n m r ) a n d M ö s s b a u e r e f f e c t . T h e d i s c o v e r y of d o m a i n - w a l l e n h a n c e d n m r and m o r e r e c e n t l y t h e u s e of a p u l s e - e c h o t e c h n i q u e h a v e m a d e p o s s i b l e t h e s t u d y of f e r r o m a g n e t i c m e t a l s a n d a l l o y s b y r a d i o f r e q u e n c y r e s o n a n c e . T h e i n t e r p r e t a t i o n of n m r d a t a h a s u n f o r t u n a t e l y e n c o u n t e r e d c o n s i d e r a b l e d i f f i c u l t y b e c a u s e of p o o r l y - k n o w n e n h a n c e m e n t f a c t o r s . T h e M ö s s b a u e r e f f e c t d o e s no t s u f f e r f r o m t h i s d i f f i c u l t y . It i s g e n e r a l l y l e s s s e n s i t i v e t o l i n e b r o a d e n i n g , bu t g i v e s r e s u l t s w h i c h a r e an o r d e r of m a g n i t u d e l e s s p r e c i s e . I t a l s o a l l o w s s t u d y of t h e n u c l e a r h f s i n i s o t o p e s l i k e 1 6 6 E r w h o s e g r o u n d - s t a t e s p i n i s z e r o .

A . P U R E M E T A L S A N D R A N D O M A L L O Y S

T h e d i s c o v e r y of 5 7 F e a s an e m i n e n t l y s u i t a b l e M ö s s b a u e r i s o t o p e [ 1 ] i n 1959 i n i t i a t e d a s e r i e s of s t u d i e s on f e r r o m a g n e t i c a l l o y s and c o m p o u n d s w h i c h i s s t i l l i n p r o g r e s s . T h e f i r s t m e a s u r e m e n t , t h a t of t h e h f s i n i r o n m e t a l i t s e l f [ 2 ] , w a s s o o n f o l l o w e d b y t h e d e t e r m i n a t i o n of t h e t e m p e r a -t u r e [ 3 ] a n d l a t e r t h e p r e s s u r e [ 4 ] d e p e n d e n c e of t h e h f s i n t e r a c t i o n . T h e s i g n of t h e e f f e c t i v e f i e l d w a s found to b e n e g a t i v e [ 5 ] g iv ing s t r o n g s u p p o r t t o t h e c o r e p o l a r i z a t i o n m e c h a n i s m a s t h e o r i g i n of t h e f i e l d . -

T h e r e f o l l o w e d a s e r i e s of i n v e s t i g a t i o n s of t he h f s of 5 7 F e i n t he a l l o y s y s t e m s F e C o , F e N i [ 6 ] , C u N i (Fe ) [ 7 ] , F e P d [ 8 ] , F e P t [ 9 J a n d F e R h [ 1 0 ] . T h e d o m i n a n t c o n c l u s i o n t h a t e m e r g e d f r o m t h e s e s t u d i e s w a s t h a t t h e e f f e c t i v e f i e l d and h e n c e t h e m o m e n t of t h e i r o n a t o m s in t h e s e m e t a l l i c s o l i d s o l u t i o n s i s s i n g u l a r l y s t a b l e . T h i s w a s p a r t i c u l a r l y s t r i k i n g i n F e P d and F e P t w h e r e H w a s f o u n d to be a l m o s t i n d e p e n d e n t of c o m p o s i t i o n . T h e s y s t e m a t i c b e h a v i o u r f r o m F e t h r o u g h F e C o , F e N i i n t o C u N i ( F e ) w a s a l s o n o t e d [ 1 1 ] . A l l o y s of i r o n w i t h e l e m e n t s of l o w e r Z , i . e . F e V , F e C r and F e M n s h o w e d p r o n o u n c e d l i n e b r o a d e n i n g [ 1 2 ] w h i c h w a s s u b s e q u e n t l y r e -s o l v e d i n d i l u t e a l l o y s a n d w i l l b e d i s c u s s e d i n s e c t i o n B .

In t h e r a r e - e a r t h r e g i o n the h f s of a n u m b e r of p u r e m e t a l s w a s obta ined , i n c l u d i n g E u , Gd, Dy, E r and T m . A l l o y s h a v e n o t b e e n s t u d i e d . T h e e m p h a s i s h a s g e n e r a l l y b e e n on t h e m e a s u r e m e n t of n u c l e a r m o m e n t s .

B . D I L U T E A L L O Y S

1. E f f e c t of impurities on the magnetic properties of iron

M e a s u r e m e n t of t h e m a g n e t i z a t i o n of i r o n c o n t a i n i n g s m a l l a m o u n t s of n o n - m a g n e t i c i m p u r i t i e s s u c h a s s i l i c o n a r e c o n s i s t e n t w i t h t h e a s s u m p t i o n t h a t t h e m a g n e t i c m o m e n t a s s o c i a t e d w i t h t h e h o s t i r o n a t o m s r e m a i n s u n -c h a n g e d w h e r e a s t h e i m p u r i t y c a r r i e s , a t m o s t , a m o m e n t v e r y m u c h s m a l l e r t h a n t h a t of i r o n , i . e . t h e y e x h i b i t w h a t h a s b e e n t e r m e d " s i m p l e d i l u t i o n " .

238

M ö s s b a u e r e x p e r i m e n t s , on t h e o t h e r hand , i n d i c a t e t h a t t h e e f f e c t i v e f i e l d of i r o n a t o m s n e a r s i l i c o n a t o m s i s r e d u c e d b y a s i g n i f i c a n t a m o u n t [ 1 3 ] , s a y 8% b e l o w t h e p u r e i r o n v a l u e .

A s i m p l e c a l c u l a t i o n ( s e e A p p e n d i x ) s h o w s t h a t if h y p e r f i n e f i e l d a n d m a g n e t i c m o m e n t r e m a i n p r o p o r t i o n a l [ 1 4 ]

Ms D = — — . ^ Fe

D i s t h e d i l u t i o n p a r a m e t e r [ 15] w h i c h i s a p p r o x . — + 1 w h e r e ц

/J pg tí с

i s t h e a v e r a g e m a g n e t i c m o m e n t p e r a l l o y a t o m a n d с i s t h e a t o m f r a c t i o n

1 э н of i m p u r i t y ; a i s g iven b y — and t h e s u b s c r i p t s r e f e r s to t h e s o l u t e H Fe "С

i m p u r i t y . F o r n o n - m a g n e t i c i m p u r i t i e s jus ~ 0 and D s h o u l d e q u a l a . F o r

a l l o y s l i k e F e S i w h i c h f o l l o w s i m p l e d i l u t i o n , D ~ 0 . On t h e o t h e r h a n d a , a s d e t e r m i n e d f r o m M ö s s b a u e r r e s u l t s , i s of t h e o r d e r of u n i t y ( T a b l e I ) .

To r e c o n c i l e t h e s e two r e s u l t s , one m u s t abandon the s t r i c t p r o p o r t i o n a -l i t y b e t w e e n h f s f i e l d and a t o m i c m a g n e t i c moment. T w o m e c h a n i s m s h a v e b e e n a d v a n c e d to t h i s p u r p o s e . E i t h e r t h e e f f e c t a r i s e s f r o m a d i s t u r b a n c e of t he c o n d u c t i o n e l e c t r o n p o l a r i z a t i o n by the i m p u r i t y [16 ] o r the d - e l e c t r o n w a v e - f u n c t i o n s a r e a l t e r e d , t h e r e b y chang ing the c o r e p o l a r i z a t i o n . A c l e a r -cu t cho i ce c a n n o t b e m a d e on t h e e v i d e n c e p r e s e n t e d , so f a r .

An u n e x p e c t e d f i n d i n g w a s t h a t 3 d - g r o u p e l e m e n t s wi th Z l e s s t h a n t h a t of i r o n , i . e . T i , V, C r , and Mn, have e f f e c t s on t he h f s f i e l d of ne ighbour ing i r o n a t o m s w h i c h do n o t d i f f e r s i g n i f i c a n t l y f r o m t h o s e of t h e n o n - m a g n e t i c i m p u r i t i e s s u c h a s Si , G e , Sn, A l o r G a [ 1 7 ] . A p p a r e n t l y t h e m a g n e t i c i n t e r a c t i o n b e t w e e n h o s t a n d i m p u r i t y i s n o t i m p o r t a n t i n d e t e r m i n i n g t h e e f f e c t s on t h e h f s i n t e r a c t i o n on n e i g h b o u r i n g i r o n a t o m s ; t he e f f e c t i s b e t t e r t h o u g h t of a s a r i s i n g f r o m t h e r e m o v a l of a n i r o n a t o m t h a n f r o m t h e s u b -s t i t u t i o n of a p a r t i c u l a r i m p u r i t y . ( M a g n e t i z a t i o n m e a s u r e m e n t s a r e n o t h e l p f u l i n t h e c a s e of m a g n e t i c i m p u r i t y i o n s s i n c e one d o e s no t know w h a t m o m e n t to a s s o c i a t e wi th t h e m ) . A r e c e n t f i n d i n g t h a t b e a r s on t h i s p r o b l e m i s t h a t t h e h f s c o u p l i n g of Mn d i s s o l v e d in i r o n d o e s no t h a v e t h e s a m e t e m p e r a t u r e d e p e n d e n c e a s t h e i r o n magnfe t i z a t i on . R a t h e r , i t b e h a v e s l ike a p a r a m a g n e t i c s p i n i n a f i e l d p r o p o r t i o n a l t o t h e i r o n m a g n e t i z a t i o n [ 1 8 ] , i . e . t h e m o m e n t of t h e m a n g a n e s e a t o m i s l o c a l i z e d .

D i l u t e a l l o y s of i r o n w i t h t h e e l e m e n t C o , N i , R h , P r o r P t a r e d i -s t i n c t l y d i f f e r e n t f r o m t h o s e j u s t d i s c u s s e d . In e v e r y c a s e , t h e add i t i on of s m a l l a m o u n t s of t h e s e e l e m e n t s l e a d s t o an i n c r e a s e i n t he a v e r a g e h y p e r -f i n e f i e l d e x p e r i e n c e d b y t h e i r o n a t o m s . T h e n e a r - n e i g h b o u r s t r u c t u r e i s n o t r e s o l v e d , bu t l i n e b r o a d e n i n g i s e v i d e n t . T h i s l e a d s t o t h e c o n c l u s i o n t h a t the e f f e c t s of t h e s e i m p u r i t y a t o m s i s s p r e a d o v e r m a n y n e i g h b o u r s i t e s . T h e e f f e c t .of t h e i m p u r i t i e s i s b e s t e x p r e s s e d i n t e r m s of o , t h e f r a c t i o n a l c h a n g e i n H p e r a t o m f r a c t i o n of i m p u r i t y ( T a b l e I ) .

In e v e r y c a s e t he d i l u t i o n p a r a m e t e r , D, h a s t h e s a m e s i g n but i s s i g n i -f i c a n t l y l a r g e r t h a n a . T h e d i f f e r e n c e s h o u l d b e t h e m a g n e t i c m o m e n t of

239

TABLE II

R E S U L T S F O R VARIOUS M E T A L I M P U R I T I E S

Impurity D<a> a M - > F e( e )

Al -0.2 -0.92 ( b ) 0 ? +0.72

Si. -0.03 -vO +0.63

Ga - -0.89 ( b ) - -

Sn -0.05 -0.64 ( b ) -V.0 +0.59

Ti -0.48 -0.83 ( b ) "°-32i4* +0.67

V -0.21 -0.69 ( b ) -0.18 i 8 +0.66

Cr -0.03 - 0 . 6 8 ^ -0.32i8 +0.97

Mn +0.05 -0.50 ( b ) 0.0Х +0.55

Co

N1

1.50

1.2

0.5I ( b )

0 . 3 6 ^

+0.95 2 3

+0.4l?

+0.04

+0.43

Ru 0.8 -0.24 R( b ) +0.4l 2 3 +0.63

Rh

Pd

1.2

0.8

• 0.48

0.684(d)

+0.23 i 4

+0.05 1 0

+0.49

+ 0 . 0 7

OB 0.6 - 0 . 9 6 5

o . o 2 + 1 . 5 6

Ir 1.2 0.07 c( d ) o . o 2 + 1 . 1

Pt 1 . 8 0.703(d) 0.01 + 1 . 1

(a) From Ref. [18] (b) Derived from Mössbauer data of Ref. [17] (c) Ref. [13] (d) Wertheim, G . K . , Wemick, J . H . , unpublished. (e) Collins, M . F . , Low, G . G . , to be published. * Subscripts denote probable errors.

t h e i m p u r i t y a t o m p r o v i d e d t h e h f s f i e l d and m a g n e t i c m o m e n t r e m a i n p r o -p o r t i o n a l f o r a l l a t o m s i n t h e a l l o y . In t h o s e c a s e s w h e r e t h e m o m e n t of t he i m p u r i t y a t o m i s known f r o m n e u t r o n s c a t t e r i n g e x p e r i m e n t s , a r e s i d u a l d i f f e r e n c e r e m a i n s which i s c o m p a r a b l e to t ha t e n c o u n t e r e d in n o n - m a g n e t i c i m p u r i t y a t o m s (Tab le I) .

240

T h e e f f e c t s of t h e e l e m e n t s R u , O s a n d I r h a v e a l s o b e e n i n v e s t i g a t e d . O s m i u m y i e l d s w e l l - r e s o l v e d s t r u c t u r e w i t h a n e a r - n e i g h b o u r c o e f f i c i e n t а г = - 0 . 1 1 4 a n d a n e x t - n e a r n e i g h b o u r c o e f f i c i e n t ~ 0 . 0 . I r i d i u m d o e s n o t p r o d u c e r e s o l v e d s t r u c t u r e , bu t t h e o u t e r l i n e s a r e s i g n i f i c a n t l y d i s t o r t e d . T h e r e s u l t s a r e s u m m a r i z e d i n T a b l e I .

2. Iron as a dilute impurity in other metals

(a) I s o m e r s h i f t

In o n e of t h e f i r s t d i s c u s s i o n s of t h i s e f f e c t i t w a s s h o w n t h a t t h e i r o n i s o m e r s h i f t a s s u m e s v a l u e s in m e t a l l i c s o l i d s o l u t i o n s which depend on t h e h o s t m e t a l [ 1 9 ] . S u b s e q u e n t m e a s u r e m e n t s h a v e s h o w n t h a t t h e r a n g e of v a l u e s e n c o u n t e r e d in m e t a l l i c e n v i r o n m e n t s i s about half t h a t found in ion ic c o m p o u n d s . M o r e i n t e r e s t i n g , h o w e v e r , i s t h e f i n d i n g t h a t t he i s o m e r s h i f t i n t h e d - g r o u p e l e m e n t s e x h i b i t s s y s t e m a t i c b e h a v i o u r w h i c h s u g g e s t s a r e l a t i o n t o t h e f i l l i n g of t h e d a n d s b a n d s ( F i g . 1) .

D e t a i l e d i n t e r p r e t a t i o n h a s not ye t b e e n g iven, p e r h a p s b e c a u s e of d i f f i -c u l t i e s w h i c h w e r e a p p a r e n t e v e n i n t h e s t u d y of 5 7 F e i n C u - N i a l l o y s [ 7 ] . T h e s e s h o w e d t h a t t h e i n c r e a s e i n t h e i s o m e r s h i f t in go ing f r o m Cuo.6Nio.4 t o p u r e Cu c o r r e s p o n d s t o a d e c r e a s e i n t h e s - e l e c t r o n d e n s i t y a t t h e i r o n a t o m , s u g g e s t i n g t h a t t h e m a j o r e f f e c t on t h e i s o m e r s h i f t i s b e c a u s e of t h e l o c a l i z a t i o n of d e l e c t r o n s on t h e i r o n a t o m s .

No c o r r e l a t i o n b e t w e e n i s o m e r s h i f t and l a t t i c e c o n s t a n t o r l a t t i c e t y p e h a s b e e n found in t h e s e m e t a l l i c h o s t s .

(b) L o c a l i z a t i o n of t h e m o m e n t of F e in d - g r o u p m e t a l s

T h e M ö s s b a u e r e f f e c t h a s a l s o c o n t r i b u t e d s i g n i f i c a n t l y t o o u r u n d e r -s t a n d i n g of a n o t h e r g r o u p of s u b s t a n c e s , t h e a l l o y s of p a l l a d i u m o r p l a t i n u m w i t h s m a l l a m o u n t s of i r o n o r c o b a l t . M a g n e t i z a t i o n m e a s u r e m e n t s [ 2 0 ] in t h e s e a l l o y s show v e r y l a r g e m o m e n t s p e r i r o n a tom; in f ac t , the m o m e n t s a r e m u c h l a r g e r t h a n t h o s e w h i c h c a n e x i s t i n t h e d s h e l l of i r o n . S ince t h e 4d b a n d of p u r e p a l l a d i u m i s j u s t f i l l e d a n d c a n n o t n o r m a l l y c o n t r i b u t e t o t he m o m e n t , we a r e l ed to a s s u m e tha t the p r e s e n c e of i r o n i n d u c e s a m o m e n t i n t h e n e i g h b o u r i n g p a l l a d i u m a t o m s . T h i s a s s u m p t i o n s e e m s r e a s o n a b l e s i n c e t he F e r m i l e v e l in p a l l a d i u m i s n e a r t h e u p p e r edge of t he d band . The m a g n e t i c i n t e r a c t i o n i n t h e s e d i l u t e a l l o y s i s s o s t r o n g t h a t e v e n 0 . 1% of i r o n p r o d u c e s f e r r o m a g n e t i c b e h a v i o u r i n p a l l a d i u m . T h i s i n d i c a t e s t h a t t h e p a l l a d i u m a t o m s a r e i n v o l v e d i n t h e m a g n e t i c c o u p l i n g s i n c e t h e i r o n a t o m s a r e t o o w e l l s e p a r a t e d t o b e e x c h a n g e - c o u p l e d t h e m s e l v e s .

T h e M ö s s b a u e r e f f e c t h y p e r f i n e f i e l d a t t he i r o n n u c l e u s in t h e s e a l l o y s i s s i m i l a r to t h a t in i r o n and i s a l m o s t e n t i r e l y i n d e p e n d e n t of c o m p o s i t i o n I 8 ] . T h e e f f e c t i v e f i e l d in t h e v e r y d i l u t e a l l o y s i s - 2 9 5 kOe a t 0°K. T h i s s u g g e s t s t h a t ~ 2 B o h r m a g n e t o n s a r e a s s o c i a t e d d i r e c t l y with the i r o n a t o m and t h a t t h e r e m a i n i n g m o m e n t i s s h a r e d by t h e n e i g h b o u r i n g p a l l a d i u m a t o m s . T h e i s o m e r s h i f t of i r o n in p a l l a d i u m i s a l s o v e r y c l o s e t o t h a t of i r o n i n i t s n a t u r a l h o s t l a t t i c e . T h i s e v i d e n c e s u p p o r t s t h e c o n c l u s i o n t h a t t he a t o m i c c o n f i g u r a t i o n of the i r o n a t o m s i s s i m i l a r to t h a t in m e t a l l i c i r o n .

241

FIG. 1. The " F e isomer shift for iron in dilute solid solution in d-group elements. Source and absorber are at room temperature. Compiled from various sources and expressed relative to a metallic chromium host lattice as the zero of isomer shift

The Mössbauer effect has also been used to ver ify that the large moment pe r i ron atom obtained f r o m magnetization measurements is associated with a t ightly in te rac t ing c lus ter , i . e . an i ron atom and i t s neighbours, r a the r than being sp read un i formly through the d band of the pal ladium host [ 21] . Th i s was d e m o n s t r a t e d by m e a s u r i n g the hyper f ine s t r u c t u r e of ал a l loy in a s t r o n g magne t i c f ie ld at low t e m p e r a t u r e . The al loy chosen was so dilute that there was no spontaneous magnetization at low tempera ture . When

242

t h i s i s t h e c a s e , t h e i r o n a t o m t o g e t h e r wi th i t s n e i g h b o u r s a c t s a s a p a r a -m a g n e t i c i m p u r i t y , w h i c h i n a n a p p l i e d f i e l d o r i e n t s i t s e l f a c c o r d i n g t o a B r i l l o u i n f u n c t i o n . T h e d e t a i l e d b e h a v i o u r d e p e n d s b o t h on t h e s p i n J a n d д Н / к Т , w h e r e ß i s t h e m a g n e t i c m o m e n t a s s o c i a t e d wi th a p a r a m a g n e t i c c l u s t e r . T h e d a t a i n d i c a t e a m o m e n t of 1 2 . 6 ¿ í B / a t o m . S ince t h i s i s l a r g e r t h a n t h e m a x i m u m m o m e n t a s s o c i a t e d wi th t he i r o n a t o m ' s own d e l e c t r o n s , t h e n e c e s s a r y c o n c l u s i o n i s t h a t t h e ' m a g n e t i c m o m e n t s of t he i r o n a t o m and i t s n e i g h b o u r i n g p a l l a d i u m a t o m s a c t a s a u n i t . T h e s p i n a s s o c i a t e d w i t h t h i s c o m p l e x w a s f o u n d t o b e 1 3 / 2 .

R e s u l t s i n p l a t i n u m a r e s i m i l a r , w i t h 6 . 2 / a t o m a n d w i t h J l y i n g b e t w e e n 5 / 2 a n d 4 .

T h e m o m e n t s a s s o c i a t e d wi th i r o n a t o m s a s d i l u t e s o l u t e s in t h e o t h e r d - g r o u p e l e m e n t s h a v e b e e n s y s t e m a t i c a l l y m e a s u r e d and g ive i n f o r m a t i o n

r e l e v a n t t o t h e d e n s i t y of s t a t e s in t h e d b a n d . T h e c o r r e s p o n d i n g M ö s s b a u e r e x p e r i m e n t s a r e m o r e d i f f i cu l t t h a n t h o s e in p a l l a d i u m s i n c e i r o n i n t h e s e m a t e r i a l s d o e s no t e x h i b i t a n o m a l o u s l y l a r g e m o m e n t s . H o w e v e r , s u c h e x p e r i m e n t s have b e e n c a r r i e d out bo th in B i t t e r m a g n e t s and in s u p e r -c o n d u c t i n g s o l e n o i d s .

It h a s b e e n found [ 2 2 ] t h a t t h e r e i s no l o c a l i z e d s t a t e f o r i r o n d i s s o l v e d i n v a n a d i u m , n i o b i u m o r t a n t a l u m . F o r t u n g s t e n , m o l y b d e n u m , go ld a n d s i l v e r , t h e h f s f i e l d w a s f o u n d to b e s m a l l e r t h a n i n i r o n m e t a l i n d i c a t i n g a r e d u c e d m o m e n t i n t h e d s h e l l . T h e t o t a l a t o m i c m o m e n t a s s o c i a t e d w i t h e a c h i m p u r i t y i r o n a t o m in t h e s e e l e m e n t s did not h a v e the a n o m a l o u s l y high v a l u e s found in p a l l a d i u m and p l a t i n u m .

C . I N T E R M E T A L L I C C O M P O U N D S

I n t e r m e t a l l i c c o m p o u n d s a r e of p a r t i c u l a r i n t e r e s t i n t h e s t u d y of t h e m a g n e t i s m of m e t a l s b e c a u s e t h e y p r o v i d e a m e a n s of c h a n g i n g the e n v i r o n -m e n t of a n a t o m w i t h o u t i n t r o d u c i n g the s t a t i s t i c a l c o m p l i c a t i o n s of a l l o y s . T h e s e c o m p o u n d s a r e o r d e r e d s t r u c t u r e s which f o r m at d e f i n i t e a t o m r a t i o s . T h e y h a v e a t t i m e s b e e n u s e d i n M ö s s b a u e r e x p e r i m e n t s f o r s o m e s p e c i f i c p r o p e r t y , e . g . M n 2 S n a n d Mn 4 Sn [ 2 3 ] t o p r o v i d e a m a g n e t i c a l l y o r d e r e d e n v i r o n m e n t f o r 1 1 9 Sn, o r T m F e 2 to p r o v i d e a c u b i c m a g n e t i c e n v i r o n m e n t f o r a r a r e - e a r t h i o n [ 2 4 ] , bu t t h e i r m a g n e t i c p r o p e r t i e s a r e of i n t e r e s t in t h e i r own r i g h t .

T h e M ö s s b a u e r e f f e c t i s a p a r t i c u l a r l y u s e f u l t o o l , e s p e c i a l l y f o r t h e s t u d y of i n t e r m e t a l l i c c o m p o u n d s of t he r a r e - e a r t h s and t r a n s i t i o n m e t a l s . T h e h f s f i e l d p r o v i d e s a m e a s u r e of t h e s u b l a t t i c e m a g n e t i z a t i o n w i t h o u t t he w e l l - k n o w n c o m p l i c a t i o n s wh ich a r i s e f r o m the n e e d to u s e l a r g e m a g n e t i c f i e l d s t o r e a c h s a t u r a t i o n i n c o n v e n t i o n a l m a g n e t i z a t i o n m e a s u r e m e n t s . It m a k e s i t p o s s i b l e t o m e a s u r e C u r i e and N é e l t e m p e r a t u r e s w i t h o u t e x t r a -p o l a t i o n t o z e r o f i e l d . W i t h c a n t e d s p i n s t r u c t u r e s i t g i v e s a f i e l d p r o p o r -t i o n a l t o t h e t o t a l i o n i c m o m e n t r a t h e r t h a n to i t s f e r r i m a g n e t i c c o m p o n e n t .

A m a j o r f r a c t i o n of t h e s e i n v e s t i g a t i o n s h a s b e e n c o n c e r n e d w i t h t h e f e r r i m a g n e t i c M g C u 2 s t r u c t u r e c u b i c L a v e s p h a s e c o m p o u n d s [ 2 5 ] in which t h e M g s i t e i s o c c u p i e d b y a r a r e - e a r t h i o n a n d t h e Cu s i t e by a l u m i n i u m o r a t r a n s i t i o n m e t a l . T h e f i r s t i n v e s t i g a t i o n s w e r e c o n c e r n e d wi th the i r o n

243

in R F e 2 c o m p o u n d s [ 2 6 ] . T h e s e s h o w e d g e n e r a l l y t h a t t h e h f s f i e l d a t t h e i r o n s i t e d o e s not h a v e a s i g n i f i c a n t c o r r e l a t i o n wi th t h e m o m e n t of t he r a r e -e a r t h i o n . T h a t t h e f i e l d i s l a r g e l y , if n o t e n t i r e l y , d u e t o t h e e x c h a n g e i n t e r a c t i o n w i t h i n F e s u b l a t t i c e i s b e s t s h o w n b y t h e f a c t t h a t t h e f i e l d h a s t h e s a m e m a g n i t u d e e v e n i n Z r F e 2 i n w h i c h t h e Z r c a r r i e s no m o m e n t . It h a s a l s o b e e n s h o w n t h a t t h e l i n e b r o a d e n i n g , w h i c h w a s n o t e d i n t h e e a r l y e x p e r i m e n t s , w a s d u e t o u n r e s o l v e d s t r u c t u r e a r i s i n g f r o m t h e 8% a n i s o -t r o p y of t h e h f s i n t e r a c t i o n i n t h e n o n - c u b i c i r o n s i t e [ 2 7 ] .

E x p e r i m e n t s on r a r e - e a r t h i o n s in t h e s e c o m p o u n d s have been of g r e a t e r i n t e r e s t . In T m F e 2 i t w a s shown t h a t t h e T m s u b l a t t i c e m a g n e t i z a t i o n m a y b e t h o u g h t of a s a r i s i n g d o m i n a n t l y f r o m a m o l e c u l a r f i e l d p r o d u c e d by t h e i r o n s u b l a t t i c e r a t h e r t h a n b y T m - T m e x c h a n g e [ 2 4 ] . T h e m a g n e t i c p r o -p e r t i e s of t h e c o m p o u n d c o u l d b e a c c o u n t e d f o r i n t e r m s of i o n i c m o m e n t s p r o p o r t i o n a l to t h e F e and T m h f s i n t e r a c t i o n s . S u b s e q u e n t m e a s u r e m e n t s of t h e r a r e - e a r t h h f s i n c o m p o u n d s w i t h F e , Co , N i , a n d A l , a s w e l l a s i n t h e r a r e - e a r t h m e t a l , h a v e s h o w n t h a t t h e r a r e - e a r t h h y p e r f i n e f i e l d s a s a g e n e r a l r u l e o b e y t h e i n e q u a l i t y [ 2 8 ] .

H Fe > H _ > H Со Ni •H Al H, metal

T h i s i s p r e s u m a b l y t h e r e s u l t of c o n d u c t i o n e l e c t r o n p o l a r i z a t i o n b y t h e d - g r o u p t r a n s i t i o n m e t a l i o n s , e s p e c i a l l y s i n c e t h e i n c r e m e n t a l f i e l d i s a p p r o x i m a t e l y p r o p o r t i o n a l t o t h e i r m o m e n t .

A l t h o u g h m o s t of t h e M ö s s b a u e r - e f f e c t s t u d i e s of i n t e r m e t a l l i c c o m -p o u n d s h a v e b e e n c o n c e r n e d w i t h m a g n e t i c a l l y o r d e r e d m a t e r i a l s , a b r i e f e x a m p l e w i l l s u f f i c e t o s h o w t h a t i n t e r e s t i n g r e s u l t s c a n a l s o b e o b t a i n e d i n p a r a m a g n e t i c c o m p o u n d s . A r e c e n t i n v e s t i g a t i o n of t h e B 2 0 s t r u c t u r e m a t e r i a l F e S i y i e l d e d t h e t e m p e r a t u r e - d e p e n d e n t q u a d r u p o l e s p l i t t i n g shown in F i g . 2. Both the m a g n i t u d e of t he s p l i t t i n g and i t s t e m p e r a t u r e dependence a r e u n u s u a l . In i o n i c s a l t s , b e h a v i o u r of t h i s t y p e i s k n o w n to a r i s e f r o m t h e s p i n - o r b i t and c r y s t a l - f i e l d s p l i t t i n g , but in a m e t a l t he a b s e n c e of s u c h l e v e l s g e n e r a l l y r e s u l t s i n s m a l l a n d o n l y w e a k l y t e m p e r a t u r e - d e p e n d e n t q u a d r u p o l e i n t e r a c t i o n s .

200 400 600 800 TEMPERATURE С Ю

FIG. 2. The quadrupole splitting of 57Fe in the intermetallic compound FeSi

T h e u n u s u a l n a t u r e of t h i s m a t e r i a l h a s a l s o b e e n f o u n d i n i t s p a r a -m a g n e t i c s u s c e p t i b i l i t y w h i c h i s v e r y l a r g e a t r o o m t e m p e r a t u r e a n d e x -

244

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h i b i t s a n e x p o n e n t i a l d e c r e a s e w i t h 1 / T , b e c o m i n g v e r y s m a l l b e l o w 77°K ( F i g . 3). T h i s b e h a v i o u r s u g g e s t s t ha t the F e r m i l e v e l i s j u s t above a r eg i o n of h igh d e n s i t y of s t a t e s i n t h e d b a n d . T h e t e m p e r a t u r e d e p e n d e n c e of both s u s c e p t i b i l i t y and q u a d r u p o l e i n t e r a c t i o n t h e n a r i s e s f r o m t h e r m a l exc i t a t i on of e l e c t r o n s out of t h e h igh d e n s i t y of s t a t e s r e g i o n [ 2 9 ] .

245

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T h e c o n t r i b u t i o n s of t h e M ö s s b a u e r e f f e c t t o t h e u n d e r s t a n d i n g of t h e m a g n e t i s m of m e t a l s , a l l o y s a n d i n t e r m e t a l l i c c o m p o u n d s h a v e b e e n i l l u -s t r a t e d b y s o m e e x a m p l e s d r a w n f r o m r e c e n t r e s e a r c h .

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T h e i r o n a l l o y s u n d e r d i s c u s s i o n a r e a l l i n t h e b o d y - c e n t r e d c u b i c r a n g e . T h e i m p u r i t y e l e m e n t e n t e r s t h e l a t t i c e s u b s t i t u t i o n a l l y . In t h e b o d y - c e n t r e d c u b i c s t r u c t u r e e a c h a t o m h a s e i g h t n e a r - n e i g h b o u r s a t »У~3/2а ( a i s t h e l a t t i c e c o n s t a n t ) , s i x n e x t - n e a r - n e i g h b o u r s a t a , t w e l v e t h i r d - n e a r - n e i g h b o u r s a W 2 a e t c . T h e h y p e r f i n e f i e l d a t an i r o n a t o m m a y b e e x p r e s s e d i n t e r m s of t h e o c c u p a n c y of t h e s e n e i g h b o u r s h e l l s p r o v i d e d l i n e a r s u p e r p o s i t i o n i s v a l i d . T h e h y p e r f i n e f i e l d i s t h e n w r i t t e n a s a s u m of t h e e f f e c t s of a l l t h e n e i g h b o u r i n g s h e l l s of t h e a t o m

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w h e r e щ i s t h e n u m b e r of i m p u r i t y a t o m s i n t h e i 0 1 s h e l l a n d a i i s t h e c o -e f f i c i e n t c h a r a c t e r i z i n g t h e e f f e c t i v e n e s s of a n i m p u r i t y a t o m i n t h a t s h e l l . T h e p r o b a b i l i t y of t h e r e b e i n g s u c h a c o n f i g u r a t i o n i s

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t h e i t h s h e l l . T h e a v e r a g e h y p e r f i n e f i e l d i s n o w o b t a i n e d b y

H = ^ H (n l f ng, . . .) f(n-j, n 2 , . . .)

w h e r e t h e s u m m a t i o n i n c l u d e s a l l p o s s i b l e c o m b i n a t i o n s of s h e l l o c c u p a n c i e s . T h e s u m m a t i o n y i e l d s t h e f o l l o w i n g s i m p l e r e s u l t

TO

1 + c I a i N i 1=1

= Hq ( 1 + c a )

w h e r e

« - î - A - è ' I S 1 = 1 U

H = H,

246

T h e a v e r a g e h y p e r f i n e f i e l d i s t h u s a l i n e a r f u n c t i o n of c o n c e n t r a t i o n wi th

c o e f f i c i e n t a , w h e r e a i s t h e s u m of t h e p r o d u c t of t h e s h e l l c o e f f i c i e n t s

w i t h t h e c o - o r d i n a t i o n n u m b e r , N¡ , of t h a t s h e l l , i . e . t h e t o t a l n u m b e r of

a t o m s i n t h a t s h e l l . T h i s e q u a t i o n c a n b e c o m p a r e d t o t h a t u s e d i n R e f . [ 1 7 ] s h o w i n g t h a t

a = 8 a + 6 b + к

o r cw

• I a i N i к

1=3

i . e . , the p a r a m e t e r к s i m p l y r e p r e s e n t s t he s u m of t he e f f e c t s a r i s i n g beyond t h e s e c o n d s h e l l .

No te , h o w e v e r , t h a t t h e a v e r a g e h y p e r f i n e f i e l d c a n n o t be c o m p a r e d to t h e a v e r a g e m a g n e t i z a t i o n . T h e y d i f f e r in t h a t H i s c o n c e r n e d on ly wi th t he i r o n a t o m s , w h e r e a s t h e m a g n e t i z a t i o n i s an a v e r a g e o v e r a ü t h e a t o m s in t h e a l l o y , h o s t - l a t t i c e a t o m s a s w e l l a s i m p u r i t y a t o m s .

T h e a v e r a g e m o m e n t i s w r i t t e n

ÏÏ = M - p e U " 0 ) + UgC

w h e r e juFe i s t h e a v e r a g e m o m e n t of t h e i r o n a t o m s in t h e a l l o y a n d fis t h e m o m e n t of t h e d i l u t e i m p u r i t y , a s s u m e d i n d e p e n d e n t of c o m p o s i t i o n . T h e n f o r с « 1

w h i c h d e f i n e s t h e d i l u t i o n p a r a m e t e r of A r r o t t and N o a k e s . If h y p e r f i n e f i e l d a n d m o m e n t a r e p r o p o r t i o n a l

< Ч е 1 Ô.H U F e à o H n ïïc

= a

and

D = a + U s

^ F e

T h e l a s t e q u a t i o n p r o v i d e s a t e s t of w h e t h e r h y p e r f i n e f i e l d and m o m e n t a r e in f a c t p r o p o r t i o n a l .

We s h a l l e x a m i n e t w o r e l a t e d c a s e s w h e r e t h e s o l u t e m o m e n t i s known to b e s m a l l o r c a n s a f e l y be a s s u m e d to b e s m a l l . In F e S n the t i n h y p e r f i n e f i e l d , m e a s u r e d f o r a d i l u t e s o l u t i o n of t i n i n i r o n , w a s found to be 7 8 . 5 kOe. T h i s c o r r e s p o n d s t o a v e r y s m a l l m o m e n t on t h e t i n a t o m . We s h a l l t h e r e -f o r e a s s u m e t h a t ß s c a n b e t a k e n t o b e z e r o . N o t e , h o w e v e r , t h a t w h e r e a s D i s c l o s e t o z e r o , a i s a s i z e a b l e n e g a t i v e n u m b e r . In t h e i r o n - s i l i c o n

247

s y s t e m , w e c a n a s s u m e , o n t h e b a s i s o f a n a l o g y w i t h t h e t i n s y s t e m , t h a t t h e m o m e n t of t h e S i a t o m s i s s m a l l . B y u s i n g t h e M ö s s b a u e r r e s u l t s t o o b t a i n a , a s i m i l a r d i s c r e p a n c y r e s u l t s .

R E F E R E N C E S

[1] POUND, R.V. , REBKA, G . A . , J r . , Phys. Rev. Lett. 3 (1959) 554. [2] HANNA, S.S. et a l . , Phys. Rev. Lett. 4 (1960) 177. [3] NAGLE, D.E. et a l . , Phys. Rev. Lett. 5 (1960) 364. [4] NICOL, M., JURA, G. , Science Ml (1963) 1035. See also LISTER, J .D. , BENEDEK, G.D., J. appl.

Phys. 34 (1963) 688 for nmr measurements of the pressure dependence. [5] HANNA, S.S. et a l . , Phys. Rev. Lett. 4 (1960) 513. [6] JOHNSON, C.E. et a l . , Phys. Rev. Lett. _6_( 1961) 450. [7] WERTHEIM, G.K. , WERNICK, J .H. , Phys.' Rev. 123 (1961) 155. [8] NAGLE, D.E. et a l . , Phys. Rev. 125 (1962) 490. [9] BEMSKI, G. , unpublished.

[10] SHIRANE, G. , et a l . , Phys. Rev. _131 (1963) 183. [11] BOYLE, A.J .F. , HALL, H.E., Rpt. Progr. Phys. 25 (1962) 441. [12] JOHNSON, C.E. et a l . , Proc. phys. Soc. (London) 81 (1963) 1079. [13] STEARNS, M.B., Phys. Rev. 129 (1963) 1136. [14] CRANSHAW, Т .Е . , JOHNSON, C.E. , RIDOUT, M.S. (Proc. of Magnetism Conf., Nottingham (1964)

to be published) discuss contributions to the hfs interaction arising from the loss of cubic symmetry. These are generally smaller than the effects considered here.

[15] ARROTT, A., NOAKES, J.E. (Iron and its Dilute Solid Solutions, Interscience, New York(1963) p.81) give results of recent measurements of the dilution parameter.

[16] STEARNS, M.B., WILSON, S.S. , Phys. Rev. Lett. 13 (1964) 313. [17] WERTHEIM, G . К . , JACCARINO, V. , WERNICK, J .H . , BUCHANAN, D.N.E. , Phys. Rev. Lett. 12

(1964) 24. [18] JACCARINO, V. , WALKER, L.R., WERTHEIM, G . K . , Phys. Rev. Le t t . ^3 (1964) 752. [19] WALKER, L.R., WERTHEIM, G .K. , JACCARINO, V. , Phys. Rev. Lett. _6_(1961) 98. [20] BOZORTH, R.M. et a l . , Phys. Rev. 122 (1961) 1157; CLOGSTON, A.M. et a l . , ibid 125 (1962)

541. [21] CRAIG, P.P. et a l . , Phys. Rev. Lett. 9 (1962) 12. [22] KITCHENS, T .A . , STEYERT, W.A., TAYLOR, R.D., US Atom. Energy Comm. LAMS-3053 "(1964). [23] MEYER-SCHÜTZMEISTER, L. , PRESTON, R.S., HANNA, S.S. , Phys. Rev. 122 (1961) 1717. [24] COHEN, R.L., Phys. Rev. ¿34 (1964) A94. [25] WERNICK, J .H . , GELLER, S . . Trans. AIME 218 (1960) 866. [26] WALLACE, W.E., EPSTEIN, L.M., J. chem. Phys. 35 (1961) 2238; KOCHER, C.W. , BROWN, P.J . ,

J. appl. Phys. Suppl. 33 (1962) 1091; WERTHEIM, G.K. , WERNICK. J .H . , Phys. Rev. 125 (1962) 1937.

[27] WERTHEIM, G.K. , JACCARINO, V . , WERNICK, J .H . , Phys. Rev. 135 (1964) A151. [28] OFER, S. , RAKAVY, M., SEGAL, E., KHURGIN, В., to be published; NOWIK, I . , to be published. [29] WERTHEIM, G.K. , IACCARINO, V. , WERNICK, J .H . , to be published.

D I S C U S S I O N

P . K I E N L E a s k e d W e r t h e i m t o e x p l a i n f u r t h e r t h e r e l a t i o n b e t w e e n t h e t e m p e r a t u r e d e p e n d e n c e o f q u a d r u p o l e s p l i t t i n g a n d t h e b a n d s t r u c t u r e .

G . K . W E R T H E I M g a v e a d e t a i l e d s t a t e m e n t c o n c e r n i n g t h e t e m p e r a t u r e d e p e n d e n c e of q u a d r u p o l e i n t e r a c t i o n s w h i c h g a v e r i s e t o t h e o b s e r v e d v a r i -a t i o n o f t h e s p l i t t i n g i n M ö s s b a u e r s p e c t r a o f F e S i .

2 4 8

J . DANON a s k e d a b o u t S t e r n h e i m e r s h i e l d i n g f a c t o r s f o r i n t e r m e t a l l i c c o m p o u n d s .

G . K . W E R T H E I M m e n t i o n e d " o v e r s h i e l d i n g " ( s e e B u l l . A m e r . P h y s . S o c . 9 (1964) 383) .

P . K I E N L E a s k e d f o r f u r t h e r c l a r i f i c a t i o n of b a n d s t r u c t u r e a n d d - e l e c t r o n p o p u l a t i o n .

G. K . W E R T H E I M s t a t e d t h a t q u a d r u p o l e c o u p l i n g did no t a r i s e f r o m a s i n g l e m e c h a n i s m . T h e r e w e r e c o n t r i b u t i o n s f r o m t h e l a t t i c e a s w e l l a s f r o m t h e t h e r m a l l y e x c i t e d e l e c t r o n s a n d t h e i r s h i e l d i n g .

R . M . G O L D I N G a s k e d a b o u t Z r 2 F e . He a s k e d if t h e t w o t e r m s i n t h e H a m i l t o n i a n f o r t h e h y p e r f i n e f i e l d c o u l d b e s e p a r a t e d .

G . K . W E R T H E I M r e p l i e d t h i s cou ld no t be done a t t he p r e s e n t s t a t e of k n o w l e d g e on t h e b a s i s of t h e e x p e r i m e n t a l d a t a . T h e r e w a s a l s o a f u r t h e r d i s c u s s i o n c o n c e r n i n g d i p o l a r c o n t r i b u t i o n s by i m p u r i t y a t o m s in a m e t a l l i c l a t t i c e .

P . K I E N L E d i s c u s s e d i n t e r n a l f i e l d s in r a r e - e a r t h a l u m i n i u m c o m -p o u n d s . In ЕгА1г a n d D y A l 2 f o r e x a m p l e , t h e f i e l d s ( e x t r a p o l a t e d t o 0°K) w e r e e s s e n t i a l l y f r e e i o n f i e l d s . On t h e o t h e r h a n d HoAl^ s h o w e d no f i e l d e v e n a t 3°K.

P . K I E N L E and G. K. W E R T H E I M d i s c u s s e d f u r t h e r the l a t t i c e o r d e r i n g in r a r e - e a r t h a l u m i n i u m c o m p o u n d s .

G . K . W E R T H E I M p r e s e n t e d s o m e d i s c u s s i o n of m a g n e t i z a t i o n c u r v e s f o r i r o n and f o r i m p u r i t y a t o m s in i r o n . S p e c i f i c a l l y d a t a o n M n in F e s h o w e d t h a t t h e 5 5 Mn h f s c o u p l i n g i s n o t p r o p o r t i o n a l t o t h e m a g n e t i z a t i o n of t h e i r o n h o s t . T h e d e t a i l e d b e h a v i o u r i n d i c a t e s t h a t t he Mn m o m e n t i s l o c a l i z e d ( s e e P h y s . R e v . L e t t . 13_ (1964) 7 5 2 . )

С У П Е Р О Б М Е Н Н О Е И Н Д У Ц И Р О В А Н И Е М А Г Н И Т Н Ы Х П О Л Е Й НА Я Д Р А Х Н Е М А Г Н И Т Н Ы Х А Т О М О В *

В . И . Г О Л Ь Д А Н С К И Й , В . А . Т Р У Х Т А Н О В , М.Н.ДЕВИШЕВА, В . Ф . Б Е Л О В

ИНСТИТУТ ХИМИЧЕСКОЙ ФИЗИКИ АН С С С Р , МОСКВА С С С Р

ABSTRACT**

SUPEREXCHANGE INDUCING OF MAGNETIC FIELDS ON NUCLEI OF DIAMAGNETIC ATOMS. The Mössbauer spectrum of Sn in the garnet Y 3 - x Cax Snx Fe 5 . x 012 (x = 0.25) was obtained at 77°K. The spectra show hyperfine structures both in the ground and the 23.8-keV states of 1I9Sn. The interaction of (diaroagnetic) Sn ions with magnetic Fe ions seems to proceed by indirect (supej) exchange through the О ions. This inter-action induces quite large magnetic fields on u ' S n nuclei: H ~200 kOe at -196°C. With increasing tempera-ture, the magnetic field on 1,9Sn nuclei decreases and, simultaneously, the field on 5 ,Fe nuclei also decreases.

* Since this paper has been published in Physics Letters, 15 (1965) 317, only the abstract is primed here.

* * This abstract has been published in Chemical Abstracts, J 3 (1965) 1394.

249

DISCUSSION

N . N . G R E E N W O O D s t a t e d t h e r e s e e m e d to be two k i n d s of i n t e r a c t i o n s o b s e r v e d ; t h a t i s t h e l a r g e f i e l d o b s e r v e d i n t h e n m r d a t a f o r F e F 2 and t h a t f o r F e S n c o m p o u n d s . H e a s k e d w h a t w a s t h e d i f f e r e n c e i n t h e t w o e f f e c t s w h i c h w e r e o b s e r v e d .

V . l . GOLDANSKII r e p l i e d t h e d i f f e r e n c e w a s qu i t e i m p o r t a n t : i t w a s t he d i r e c t i n d u c t i o n of t h e m a g n e t i c f i e l d i n F e F 2 a n d t h e i n d i r e c t ( s u p e r -e x c h a n g e ) i n d u c t i o n i n S n - O - F e i n g a r n e t s . He t h e n p o i n t e d ou t t h a t t h e r e w e r e l i m i t a t i o n s on t h e r a n g e of Sn :Fe c o n c e n t r a t i o n s which w e r e s t i l l f e r r o -m a g n e t i c and w h i c h c o u l d t h u s b e s t u d i e d . F u r t h e r , i t wou ld b e of i n t e r e s t t o r e p l a c e t h e o x y g e n a t o m s wi th o t h e r l i g a n d s t o s e e wha t t h e r e q u i r e m e n t s w e r e f o r " p a s s i n g a long m a g n e t i c i n f o r m a t i o n " f r o m F e to Sn. T h e s e s t u d i e s m i g h t e l u c i d a t e s o m e b i o c h e m i c a l s t u d i e s w h i c h h a d b e e n c a r r i e d o u t b y B l u m e n f e l d i n t h e I n s t i t u t e of C h e m i c a l P h y s i c s of t h e USSR A c a d e m y of S c i e n c e s .

M. C O R D E Y - H A Y E S s t a t e d t h a t m a g n e t i c d a t a on t i n m o m e n t a a p p e a r e d to f a l l in to two g r o u p s d e p e n d i n g on w h e t h e r f i e l d s w e r e i n t e r n a l o r e x t e r n a l and a s k e d if t h e r e w e r e any e x p l a n a t i o n f o r t h i s .

V . l . GOLDANSKII did no t know, but i t a p p e a r e d to h i m t h a t m o r e p r e -c i s e r e s u l t s w e r e o b t a i n e d wi th t h e l a r g e r i n t e r n a l f i e l d s t h a n wi th t h e c o m -p a r a t i v e l y w e a k e x t e r n a l m a g n e t i c f i e l d s .

P . H I L L M A N ( C h a i r m a n ) s a i d i n t h e d e c o m p o s i t i o n of F e O one o b t a i n e d b o t h F e 3 0 4 a n d F e . P a r t i a l l y d e c o m p o s e d s a m p l e s a t r o o m t e m p é r a t u r e s h o w e d h . f . f i e l d s of F e 3 0 4 and F e bu t no m a g n e t i c h . f . s of F e O . H o w e v e r , w h e n t h e p a r t i a l l y d e c o m p o s e d s a m p l e s w e r e c o o l e d t o a f e w d e g r e e s above t h e b u l k C u r i e t e m p e r a t u r e of F e O a s t r o n g m a g n e t i c o r d e r i n g e f f e c t w a s o b s e r v e d i n t h e M ö s s b a u e r s p e c t r u m of t h e r e s i d u a l F e O . T h e i n t i m a t e m a g n e t i c e n v i r o n m e n t a p p a r e n t l y i m p o s e d a m a g n e t i c o r d e r i n g on the o t h e r -w i s e p a r a m a g n e t i c p a r t i c l e s of F e O .

N . N . G R E E N W O O D w o n d e r e d , s i n c e F e O i s n o t s t o i c h i o m e t r i c a s u s u a l l y p r e p a r e d , w h e t h e r t h i s d id no t l e a d t o a p r o b l e m .

P . H I L L M A N ( C h a i r m a n ) s a i d t h e i n i t i a l m a t e r i a l w a s a p p r o x i m a t e l y F e 0.92 О- H o w e v e r , t h e p a r a m a g n e t i c p e a k o b s e r v e d a t r o o m t e m p e r a t u r e f o r t h e p a r t i a l l y d e c o m p o s e d m a t e r i a l s h o w e d n o q u a d r u p o l e s p l i t t i n g a n d

. h e n c e i t w a s conc luded t h a t the F e O p h a s e r e m a i n i n g w a s e s s e n t i a l l y s t o i c h i o - . m e t r i c . In any e v e n t , t h i s w a s no t too r e l e v a n t t o t he i m p o s e d m a g n e t i z a t i o n .

250

ANALYTICAL MEASUREMENTS AND STANDARDS

(Sess ion 7, Part 2)

MÖSSBAUER RESONANCE ABSORPTION STUDY ON PHOTOLYSIS AND RADIOLYSIS OF FERRIC OXALATE*

N. SAITO, T . TO MINA GA AND F. AMBE THE UNIVERSITY OF TOKYO, HANGO, TOKYO

AND H. SANO

OCHANOMIZU UNIVERSITY, OTSUKA TOKYO, JAPAN

T h e t e c h n i q u e of M ö s s b a u e r r e s o n a n c e a b s o r p t i o n of 7 - r a y s h a s r e c e n t l y b e e n a p p l i e d t o t h e s t u d y of t h e v a l e n c e s t a t e , s t r u c t u r e a n d n a t u r e of c h e m i -c a l b o n d s of v a r i o u s c o m p o u n d s . B e c a u s e of t h e f a v o u r a b l e p r o p e r t i e s of 5 7 F e f o r t h e o b s e r v a t i o n of t h e r e s o n a n c e a b s o r p t i o n s p e c t r a a n u m b e r of i r o n c o m p o u n d s h a v e b e e n i n v e s t i g a t e d . H o w e v e r , t h e M ö s s b a u e r r e s o n a n c e a b s o r p t i o n t e c h n i q u e h a s n o t s o f a r b e e n u s e d t o i n v e s t i g a t e t h e c h e m i c a l c h a n g e i n d u c e d b y i o n i z i n g r a d i a t i o n o r l i g h t i n i r o n c o m p o u n d s * . T h e r e -f o r e , t h e p r e s e n t a u t h o r s h a v e a p p l i e d t h i s t e c h n i q u e t o t h e s t u d y of p h o t o -l y s i s a n d r a d i o l y s i s of f e r r i c o x a l a t e .

^ F e r r i c o x a l a t e , Г е ^ С г О ^ з - пНгО,^ w a s i r r a d i a t e d w i t h 6 0 C o 7 - r a y s a t a m b i e n t t e m p e r a t u r e i n t h e p r e s e n c e of a i r . T h e t o t a l d o s e w a s 2 X 1 0 a R . In c a s e of p h o t o l y s i s t h e s a m e - s a l t w a s e x p o s e d t o t h e l i g h t f r o m a m e r c u r y l a m p . T h e r e s o n a n c e a b s o r p t i o n of 1 4 . 4 k e V 7 - r a y s i n i r r a d i a t e d a n d n o n -i r r a d i a t e d i r o n o x a l a t e s ( a s a b s o r b e r s ) w a s m e a s u r e d w i t h a s o u r c e of 5"?Co d i f f u s e d i n t o c o p p e r f o i l . T h e s o u r c e w a s m o v e d l i n e a r l y a t a c o n s t a n t v e l o c i t y b y m e a n s of a m e c h a n i c a l d r i v e . N o r e m a r k a b l e c h a n g e w a s o b -s e r v e d b e t w e e n t h e s p e c t r a o b t a i n e d w i t h t h e s a m e a b s o r b e r a t 2 2 ° C a n d l i q u i d n i t r o g e n t e m p e r a t u r e ^ T h e i r M ö s s b a u e r s p e c t r a a r e s h o w n i n F i g . 1.

A s s e e n i n F i g . l a , t h e s p e c t r u m of n o n - i r r a d i a t e d f e r r i c o x a l a t e c o n s i s t s of t w o l i n e s a t 0 . 2 7 a n d 0 . 7 0 m m / s ( v e r s u s s t a i n l e s s s t e e l ) . A f t e r i r r a d i -a t i o n w i t h 7 - r a y s , t h e h e i g h t of b o t h l i n e s i s d e c r e a s e d and n e w l i n e s a p p e a r a t a b o u t 0 . 5 a n d 2 . 3 m m / s ( F i g . l b ) . S i n c e t h e p o s i t i o n s of t h e s e n e w l i n e s n e a r l y c o r r e s p o n d t o t h o s e i n t h e s p e c t r u m of n o n - i r r a d i a t e d f e r r o u s o x a l a t e ( F i g . l c ) , i t m a y b e c o n c l u d e d t h a t t h e e l e c t r o n c o n f i g u r a t i o n a r o u n d t h e i r o n

* Presented also at the 8th Symposium on Radiochemistry, Chemical Society of Japan, Osaka, Nov. 1964.

1 The effect of y-rays on the shape of Mössbauer spectra was reported only on some tin compounds: Ref. Aleksandrov, A.Y. e t a l . , Soviet Phys. JETP 16 (1963)1467.

2 n = nearly 5 (determined by gravimetric determination of iron and microelementary analysis of ferric oxalate ).

2 5 1

SOURCE VELOCITY ( m m / s )

FIG. 1. Mössbauer resonance absorption spectra at 22°C. (a) Non-irradiated ferric oxalate, (b) Irradiated ferric oxalate, (c) Non-iriadiated ferrous oxalate

a tom in f e r r i c o x a l a t e i r r a d i a t e d wi th 7 - r a y s b e c o m e s v e r y s i m i l a r to tha t in f e r r o u s o x a l a t e ( t h e r e f o r e , F e 3 + i s r e d u c e d to F e 2 + a s t h e r e s u l t of r a d i o -l y s i s ) . T h e M ö s s b a u e r s p e c t r u m ob ta ined wi th f e r r i c oxa l a t e tha t h a s u n d e r -gone p h o t o l y s i s i s a l m o s t i d e n t i c a l in i t s s h a p e wi th t h a t of t h e y - r a y i r r a d i -a t ed s a m p l e 3 .

The above c o n c l u s i o n m a y b e c o n f i r m e d by i n v e s t i g a t i o n of t h e i r i n f r a r e d s p e c t r a s h o w n i n F i g . 2 . T h e i n f r a r e d s p e c t r a of i r r a d i a t e d and n o n - i r r a d i a t e d i r o n o x a l a t e s w e r e m e a s u r e d a s K B r d i s c by u s ing a Hi tachi E P I - 5 1 0 s p e c t r o -

WAVE NUMBER ( c m " 1 )

FIG. 2. Infrared spectra. (a) Non-irradiated ferric oxalate, (b) Irradiated ferric oxalate, (c) Non-irradiated ferrous oxalate

3 It is well known that ferric oxalate is decomposed by light to yield ferrous oxalate.

252

p h o t o m e t e r . B y e x a m i n i n g t h e r e g i o n s of 7 5 0 - 8 5 0 c m " 1 ( O - C - O b e n d i n g ) and 1 2 5 0 - 1 3 5 0 c m _ 1 ( C - O s t r e t c h i n g ) i t m a y b e c o n c l u d e d t h a t t h e s p e c t r u m of i r r a d i a t e d f e r r i c o x a l a t e ( F i g . 2b) i s o b t a i n a b l e b y s u p e r i m p o s i t i o n of t h e s p e c t r u m of n o n - i r r a d i a t e d f e r r o u s o x a l a t e ( F i g . 2c) on tha t of n o n - i r r a d i a t e d f e r r i c o x a l a t e ( F i g . 2a) in a d e q u a t e p r o p o r t i o n s .

F u r t h e r s t u d y i s in p r o g r e s s to i n v e s t i g a t e t he p o s s i b i l i t y of s e m i q u a n t i -t a t i v e d e t e r m i n a t i o n o f " F e l i + f o r m e d i n f e r r i c o x a l a t e i n t h e c o u r s e of y - r a d i o l y s i s b y t h e M ö s s b a u e r t e c h n i q u e .

D I S C U S S I O N

P . K I E N L E s a i d t h a t W e r t h e i m ' s e x p e r i m e n t s w i t h i m p u r i t y a t o m s s u g g e s t e d t o h im e x p e r i m e n t s wi th a - i r r a d i a t e d s a m p l e s (at l o w t e m p e r a t u r e ) . C o n c u r r e n t e l e c t r i c c o n d u c t i v i t y e x p e r i m e n t s s u g g e s t e d a b o u t 1% r a d i a t i o n d a m a g e ( t h i s i s v e r y h igh) b u t t h e M ö s s b a u e r s p e c t r a s h o w e d n o a n o m a l o u s e f f e c t s .

G . K . W E R T H E I M b e l i e v e d t h e s e r e s u l t s w e r e not t o o s u r p r i s i n g s i n c e a n n e a l i n g a f t e r a - i r r a d i a t i o n u n d o u b t e d l y r e d u c e d t h e l a t t i c e d e f e c t c o n c e n -t r a t i o n s v e r y m a r k e d l y t o l e v e l s w h e r e M ö s s b a u e r m e a s u r e m e n t s did no t s h o w a n y t h i n g .

N . S A I T O s t a t e d t h e i r m e a s u r e m e n t s s h o w e d t h a t t h e r e a p p e a r e d t o b e a r e d u c t i o n of Fe 1 1 1 t o Fe 1 1 on i r r a d i a t i o n . M o r e a c c u r a t e m e a s u r e m e n t s of G v a l u e s a n d c o m p a r i s o n of t h e s e w i t h M ö s s b a u e r r e s u l t s s h o u l d g ive s o m e n e w d a t a on r a d i a t i o n d a m a g e e f f e c t s .

253

A STANDARD REFERENCE MATERIAL FOR MÖSSBAUER SPECTROMETRY OF

IRON AND ITS COMPOUNDS

J.J. SPUKERMAN, F.C. RUEGG AND J. R. DeVOE

NATIONAL BUREAU OF STANDARDS

WASHINGTON, D. C . , UNITED STATES OF AMERICA

It h a s b e c o m e a p p a r e n t i n t h e l a s t y e a r t h a t t h e d i s c o v e r y of t h e M ö s s b a u e r e f f e c t h a s r e s u l t e d i n a n e w s p e c t r o m e t r i c m e t h o d f o r c h e m i c a l s t r u c t u r e a n a l y s i s w h i c h c o m p l i m e n t s t h e o t h e r s p e c t r o m e t r i e s s u c h a s e l e c t r o n s p i n and n u c l e a r m a g n e t i c r e s o n a n c e . S i n c e M ö s s b a u e r s p e c t r o m e t r y i n v o l v e s t h e m e a s u r e m e n t of t h e d i f f e r e n c e i n c e r t a i n l o w - l y i n g e n e r g y l e v e l s of t h e n u c l e u s i n t h e s o u r c e of M ö s s b a u e r r a d i a t i o n ( r e l a t i v e t o t h e d i f f e r e n c e i n t h e a b s o r b e r ) i t i s n e c e s s a r y t o e s t a b l i s h a r e f e r e n c e l e v e l f r o m w h i c h a l l o t h e r d i f f e r e n c e s i n e n e r g y l e v e l s a r e m e a s u r e d . O t h e r w i s e , t h e r e p o r t i n g of r e l a t i v e d i f f e r e n c e s i n e n e r g y l e v e l s f r o m a v a r i e t y of d i f f e r e n t t y p e s of s o u r c e s o r a b s o r b e r s w i l l n e c e s s i t a t e c o n s i d e r a b l e u s e of n o r m a l i z a t i o n f a c t o r s t o e s t a b l i s h a n a r t i f i c i a l r e f e r e n c e l e v e l .

T h e d i f f e r e n c e s i n e n e r g y b e t w e e n t h e s o u r c e s ' M ö s s b a u e r r a d i a t i o n a n d t h e e n e r g y r e q u i r e d b y t h e a b s o r b e r f o r r e s o n a n t a b s o r p t i o n , w h i c h r e s u l t s f r o m the d i f f e r e n c e i n c h e m i c a l e n v i r o n m e n t in t he s o u r c e r e l a t i v e to t h e a b s o r b e r , i s t h e s o - c a l l e d c h e m i c a l ( i s o m e r ) s h i f t . We co u l d c o n s i d e r t he s t a n d a r d r e f e r e n c e m a t e r i a l to be in the f o r m of a s o u r c e o r an a b s o r b e r . T h e r e a r e m a n y r e a s o n s o t h e r t h a n c o n v e n i e n c e f o r s e l e c t i n g an a b s o r b e r a s t h e r e f e r e n c e m a t e r i a l . T h e s t a n d a r d m u s t b e a r e p r o d u c i b l e c h e m i c a l s y s t e m , s t a b l e a t s t a n d a r d t e m p e r a t u r e and p r e s s u r e , a n d i t m u s t h a v e a n a r r o w r e s o n a n c e l i n e - w i d t h and l a r g e r e s o n a n c e e f f e c t at r o o m t e m p e r a t u r e , p l u s a s m a l l c h e m i c a l s h i f t t e m p e r a t u r e c o e f f i c i e n t [1] .

T o a s s u r e tha t t h e c h e m i c a l s y s t e m be a s highly r e p r o d u c i b l e a s p o s s i b l e , i t w a s d e c i d e d t o u s e s i n g l e c r y s t a l s of t h e c o m p o u n d . T h e c h e m i c a l c o m p o u n d c h o s e n i [2] f o r t h e i r o n s t a n d a r d i s d i s o d i u m n i t r o s o p e n t a c y a n o f e r r a t e d i h y d r a t e , N a ¿ F e ( C N ) 5 N O ] • 2НгО, c o m m o n l y c a l l e d s o d i u m n i t r o p r u s s i d e . A s t e r e o g r a p h i c p r o j e c t i o n of s o d i u m n i t r o p r u s s i d e i s s h o w n in F i g . 1. P e r -t i nen t d a t a on t he c h e m i c a l , p h y s i c a l and c r y s t a l p r o p e r t i e s a p p e a r in T a b l e I .

P l a t e l e t s 1 c m X 1 c m X 0 . 0 7 7 5 c m w e r e c u t a n d p o l i s h e d s o t h a t t h e m o l e c u l a r a x i s of s y m m e t r y i s a l o n g t h e p l a n e of t h e p l a t e l e t ( s e e F i g . 2 ) . W h e n t h e c r y s t a l i s m o u n t e d s o tha t t h e M ö s s b a u e r r a d i a t i o n p a s s e s t h r o u g h p e r p e n d i c u l a r (±2°) t o t h e b e p l a n e of t he p l a t e l e t , t h e a b s o r p t i o n s p e c t r u m s h o w n in F i g . 3 i s o b t a i n e d . T h e a b s o r p t i o n s p e c t r a o b t a i n e d w i t h c r y s t a l s c u t p a r a l l e l t o t h e o t h e r two p l a n e s a r e s h o w n in F i g s . 4 and 5. T w o p e a k s a r e o b t a i n e d , s e p a r a t e d b y 0 . 1 7 1 2 ± 0 . 0 0 0 4 c m / s , w h i c h r e s u l t f r o m t h e e l e c t r i c q u a d r u p o l e s p l i t t i n g of t he i r o n n u c l e u s . T h e poin t h a l f - w a y ' b e t w e e n

1 This choice resulted from a discussion between G.K. Wertheim and R.H. Herber, and was accepted as a reference standard for iron at the Inorganic Chemistry Session of the Gordon Conference in the summer of 1964.

254

loo

Ь.11.84 Â C-15.UÂ

FIG. 1. Stereographic projection of sodium nitroprusside

t h e s e two p e a k s i s a s s i g n e d a d i f f e r e n t i a l c h e m i c a l s h i f t v a l u e of z e r o . T h e p r o c e d u r e w i l l b e t o p l a c e t h e s t a n d a r d r e f e r e n c e a b s o r b e r i n t h e s p e c t r o -m e t e r a n d c a l i b r a t e t h e s p e c t r o m e t e r i n a c c o r d a n c e w i t h t h e p o i n t of z e r o d i f f e r e n t i a l c h e m i c a l s h i f t . T h e c h e m i c a l s y s t e m t o b e m e a s u r e d i s t h e n p l a c e d in t h e s p e c t r o m e t e r and t h e d i f f e r e n t i a l c h e m i c a l s h i f t i s m e a s u r e d d i r e c t l y .

T h e s e s i n g l e c r y s t a l p l a t e l e t s w i l l b e c a l i b r a t e d b y a n o p t i c a l i n t e r -f e r o m e t r i c M ö s s b a u e r s p e c t r o m e t e r to an a c c u r a c y of 1 p a r t in 100 000 and a p r e c i s i o n of 1 p a r t i n 10 000 ( s t a n d a r d d e v i a t i o n of a s i n g l e d e t e r m i n a t i o n ) . A M ö s s b a u e r s p e c t r u m w i t h a 5 7 C o - p a l l a d i u m s o u r c e w i t h t h e n a r r o w e s t l i n e - w i d t h o b t a i n a b l e w i l l b e s e n t w i t h e a c h p l a t e l e t . T h i s w i l l g i v e t h e e x p e r i m e n t e r t h e p o s s i b i l i t y of c o m p a r i n g q u a l i t a t i v e l y t he s p e c t r u m on h i s s p e c t r o m e t e r w i t h t h a t of t h e N a t i o n a l B u r e a u of S t a n d a r d s s i n c e t h e l i n e -w i d t h of b o t h s o u r c e s a r e u n l i k e l y t o b e i d e n t i c a l .

255

TABLE II

CHEMICAL, PHYSICAL AND CRYSTAL P R O P E R T I E S OF Na2[Fe(CN)5NO] • 2 H 2 0

Molecular weight 297. 95

Specific gravity 1.72 g/cm 3

Colour Red

Solubility in H 20 40 gД 00 ml at 16*C

Crystallographic data [3]

12 Orthorhombic, space group D2jj - P n n m

Cell constants

a = 6.17 ± 0. 03 К

b = 11. 84 i 0. 06 Â

с = 15.43 è 0. 08 Â

Habit [110] primary rhombic prism

[010] brachy-pinakoid

[011] primary brachy-domal prism

Crystallizes in needles along the b-axis

FIG. 2. Single crystal of sodium nitroprusside, showing habit and orientation of platelet

This is the f i r s t of a s e r i e s of standards that will be made available from the National Bureau of S tandards , USA. Standards fo r tin and iodine a r e being planned. The cost of these standards has not been established. Notices will be sent out when these standards become available.

2 5 6

N a 2 F e ( C N ) 5 N O * 2 H j O SINGLE C R Y S T A L 100 P L A N E ( b e )

FIG. 3. Mössbauer spectra of standard reference material for Fe

N a 2 F e ( C N ) s N 0 * 2 H 2 0 S INGLE CRYSTAL 001 P L A N E ( s c )

FIG. 4. Mössbauer spectra of single crystal of sodium nitroprusside cut parallel to the ac plane [001]

2 5 7

Na 2 Fe(CN> 5 N0»2H 2 0 SINGLE CRYSTAL 010 P L A N E (ab)

FIG. 5. Mössbauer spectra of single crystal of sodium nitroprusside cut parallel to the ab plane [010]

R E F E R E N C E S

[1] HERBER, R.H., "Mössbauer spectroscopy - some recent applications to chemical problems", Mössbauer Symposium, N.E.N.C. - T.M.C. New York, Jan. 1965.

[2] DANON, J.. J. chem. Phys. 41 (1964)3378. [3] MANOHARAN, P.T. , HAMILTON, W.C., "The crystal structure of sodium nitroprusside", Inorg. Chem.

2 (1963) 1043.

D I S C U S S I O N

P . H I L L M A N ( C h a i r m a n ) a s k e d w h a t wou ld b e t h e c o s t of t h e N . B . S . s t a n d a r d i z e d c r y s t a l s .

J . S P I J K E R M A N r e p l i e d i t w a s no t p o s s i b l e t o q u o t e a f i r m p r i c e a t t h i s t i m e . H o w e v e r t h e c o s t of c a l i b r a t e d s a m p l e s w o u l d b e n o m i n a l a n d p r o b a b l y l e s s t h a n a b o u t $ 5 0 . 0 0 .

R . H . H E R B E R s a i d h e would l i k e t o po in t ou t o n c e a g a i n t h a t t h e f i r s t s u g g e s t i o n s c o n c e r n i n g the s u i t a b i l i t y of s o d i u m n i t r o p r u s s i d e a s a s t a n d a r d c a m e f r o m s o m e i n f o r m a l d i s c u s s i o n s b e t w e e n G . K. W e r t h e i m and h i m s e l f , m a k i n g u s e of t h e d a t a of E . F l u c k , W . K e r l e r , W . N e u w i r t h a n d o t h e r s . T h i s d i s c u s s i o n t h e n l ed t h e way to f u r t h e r c o n s i d e r a t i o n s at t h e I n o r g a n i c C h e m i s t r y G o r d o n C o n f e r e n c e i n 1964.

258

J . D A N O N s a i d t h e M ö s s b a u e r s p e c t r u m of s i n g l e c r y s t a l s of s o d i u m n i t r o p r u s s i d e h a d b e e n i n v e s t i g a t e d by h i m a n d h i s c o - w o r k e r s a n d by L . I a n a r e l l a , a n d p r e l i m i n a r y r e s u l t s h a d b e e n p r e s e n t e d a t t h e G o r d o n C o n f e r e n c e ( I n o r g a n i c C h e m i s t r y ) i n 1964. He t h e n c o m m e n t e d on t h e d i s -c r e p a n c y b e t w e e n t h e m o r p h o l o g i c a l and X - r a y i d e n t i f i c a t i o n of t h e c r y s t a l a x e s in s o d i u m n i t r o p r u s s i d e . . It w a s f e l t tha t t h i s d i s c r e p a n c y shou ld be r e s o l v e d a t t h e e a r l i e s t p o s s i b l e t i m e b y d i s c u s s i o n w i t h X - r a y c r y s t a l l o g r a p h e r s .

J . S P I J K E R M A N d i s c u s s e d f u r t h e r a s p e c t s of t h e N . B . S . s t a n d a r d i -z a t i o n p r o g r a m m e a s i t p e r t a i n e d t o M ö s s b a u e r s t a n d a r d s .

M. C O R D E Y - H A Y E S po in t ed out tha t S n 0 2 had a f e r r o - e l e c t r i c t r a n s i t i o n a t 28°C and t h u s w a s no t s u i t a b l e a s a s t a n d a r d f o r t i n M ö s s b a u e r s p e c t r o s -c o p y . M o r e o v e r t h e p r e s e n c e of q u a d r u p o l e s p l i t t i n g i n t h i s s o l i d w i t h i t s p o s s i b l e c o n c o m i t t a n t a s y m m e t r y w a s u n d e s i r a b l e s i n c e t h e s p l i t t i n g w a s not r e a d i l y r e s o l v e d .

R . M . GOLDING, J . S P I J K E R M A N and J. DANON b r i e f l y d i s c u s s e d f u r t h e r the r e q u i r e m e n t s f o r a s t a n d a r d f o r M ö s s b a u e r s p e c t r o s c o p y of t in compounds, but no s u i t a b l e compound was s u g g e s t e d .

259

RECOMMENDATIONS OF THE P A N E L

A. E V A L U A T I O N O F MÖSSBAUER S P E C T R O S C O P Y

1. Developments of the Mössbauer method:

New M ö s s b a u e r t r a n s i t i o n s in v a r i o u s n u c l i d e s a r e being d i s c o v e r e d so t h a t M ö s s b a u e r s p e c t r o s c o p y c a n b e a p p l i e d t o a d d i t i o n a l e l e m e n t s . N e w m e t h o d s a r e c o n t i n u o u s l y b e i n g d e v e l o p e d (e . g . s c a t t e r i n g m e a s u r e m e n t s and c o u l o m b e x c i t a t i o n ) w h i c h i n c r e a s e t h e s c o p e of p r o b l e m s to w h i c h t h e M ö s s b a u e r t e c h n i q u e i s a p p l i c a b l e . T h e o b s e r v a t i o n of t h e e f f e c t i n t r a n -s i t i o n s w i t h v e r y n a r r o w l i n e - w i d t h m a y o p e n u p c o m p l e t e l y n e w a r e a s of i n v e s t i g a t i o n .

F r e q u e n t l y t h e u s e f u l n e s s of t h e m e t h o d i s i n c r e a s e d b y c o m b i n a t i o n w i t h o t h e r m e t h o d s s u c h a s p e r t u r b e d a n g u l a r c o r r e l a t i o n m e a s u r e m e n t s .

2. Examples of applications in fundamental investigations

2 . 1 . P u r e a n d a p p l i e d c h e m i s t r y

(a) S t u d y of c h e m i c a l b o n d i n g a n d m o l e c u l a r s t r u c t u r e . T r a n s i t i o n m e t a l c o m p o u n d s ( e . g . F e , Co , R u , I r , P b , Hg) N o n - t r a n s i t i o n m e t a l c o m p o u n d s ( e . g . Sn, Т е , К) R a r e - e a r t h c o m p o u n d s ( b o t h l a n t h a n i d e s a n d a c t i n i d e s ) N o n - m e t a l l i c e l e m e n t s ( e . g . I, K r , Xe ) M e t a l - o r g a n i c c o m p o u n d s ( e . g . t h o s e of F e , Sn , Au, P t e t c . ) B i o l o g i c a l l y s i g n i f i c a n t c o m p o u n d s .

(b) Study of hot atom- p r o c e s s e s and c h e m i c a l e f f e c t s of n u c l e a r t r a n s f o r m a -t i o n s ; h i g h - e n e r g y c h a r g e d p a r t i c l e i n t e r a c t i o n s wi th m a t t e r . T h e M ö s s b a u e r s p e c t r o s c o p y p r o v i d e s u p p e r l i m i t s of t he t i m e s c a l e of e v e n t s t h e d u r a t i o n of w h i c h c a n f r e q u e n t l y no t b e e s t i m a t e d by o t h e r m e t h o d s . It p r o v i d e s a n a d d i t i o n a l w a y to s t u d y t h e i n t e r a c t i o n of a c -c e l e r a t e d c h a r g e d p a r t i c l e s w i t h m a t t e r .

(c) S tudy of c h e m i c a l k i n e t i c s . C h e m i c a l m e t a t h e s i s r e a c t i o n s P o l y m e r i z a t i o n r e a c t i o n s of o r g a n o - m e t a l l i c c o m p o u n d s , e . g . t i n po ly -m e r i s a t i o n c a t a l y s t s , a g e i n g and d y n a m i c s of p o l y m e r s , c r o s s - l i n k i n g u n d e r i r r a d i a t i o n C h e m i c a l r e a c t i o n s a t c r y o g e n i c t e m p e r a t u r e s .

(d) S tudy of c h e m i c a l p r o c e s s e s on s u r f a c e s and i n t e r f a c e s ; so lven t e f f e c t s and s o l u t e - s o l v e n t i n t e r a c t i o n s .

(e) Study of c h e m i c a l p r o c e s s e s in r a d i a t i o n f i e l d s , (f ) Study of c h e m i c a l p r o c e s s e s a t h igh p r e s s u r e s , (g) U s e of M ö s s b a u e r a t o m s in t r a c e r t e c h n i q u e s .

261

2 . 2 . S o l i d - s t a t e p h y s i c s

(a ) C o r r e l a t i o n of m e a s u r e m e n t s of q u a d r u p o l e a n d m a g n e t i c h y p e r f i n e c o u p l i n g s and i s o m e r s h i f t s with t h e o r y p r o v i d e s c r i t i c a l t e s t s of m o d e l s of s t r u c t u r e s of m o l e c u l e s and c r y s t a l s . Q u a d r u p o l e i n t e r a c t i o n a n d the i s o m e r s h i f t p r o v i d e i n f o r m a t i o n on h y p e r f i n e i n t e r a c t i o n s of a n e l e c t r o s t a t i c n a t u r e .

T h i s i n f o r m a t i o n c a n b e c o n n e c t e d t o e l e c t r o n i c w a v e - f u n c t i o n s of d i a m a g n e t i c m o l e c u l e s .

(b) M a g n e t i c h y p e r f i n e i n t e r c a t i o n s in s o l i d s m a y b e s t u d i e d by t h e M ö s s b a u e r e f f e c t w h e r e t h e N M R t e c h n i q u e i s n o t a p p l i c a b l e b e c a u s e of l i n e -b r o a d e n i n g .

(c) M a g n e t i c p r o p e r t i e s of m a t e r i a l s , e . g . s u b - l a t t i c e m a g n e t i z a t i o n , C u r i e t e m p e r a t u r e s , s u p e r - p a r a m a g n e t i s m and conduc t ion e l e c t r o n p o l a r i s a t i o n .

(d) Study of r e l a x a t i o n e f f e c t s . (e) S tudy of l a t t i c e v i b r a t i o n s , i n p a r t i c u l a r f o r i m p u r i t y a t o m s and d e f e c t

s t r u c t u r e s . (f ) S t u d i e s of d i f f u s i o n in s o l i d s and l i q u i d s , e . g . s t u d i e s of s m a l l p a r t i c l e s

and s t r u c t u r e of w a t e r . (g) S t u d i e s of p h y s i c a l t r a p p i n g of a t o m s , e . g . a d s o r p t i o n on s u r f a c e s and

t r a p p i n g i n c l a t h r a t e s . (h) S tudy of d i l u t e i m p u r i t i e s and of d e f e c t s t r u c t u r e s . ( i ) S t u d i e s of c r y s t a l s t r u c t u r e and p h a s e t r a n s i t i o n s . ( j ) P r o p e r t i e s of s o l i d s u n d e r v e r y h igh p r e s s u r e s .

2 . 3 . N u c l e a r p h y s i c s

(a) Un ique a p p l i c a t i o n s t o t h e s t u d y of t h e c h a n g e of n u c l e a r c h a r g e r a d i i i n 7 - t r a n s i t i o n .

(b ) M e a s u r e m e n t of t h e p r e c i s e r a t i o of q u a d r u p o l e m o m e n t s of n u c l e a r s t a t e s .

( c ) P r e c i s e d e t e r m i n a t i o n of m a g n e t i c m o m e n t s of e x c i t e d n u c l e a r s t a t e s . (d) S tudy of t h e p o l a r i s a t i o n of 7 - r a y s and i t s a p p l i c a t i o n t o s t u d y t h e p o p u -

l a t i o n of n u c l e a r p o l a r i s a t i o n s t a t e s a f t e r r a d i o a c t i v e d e c a y o r n u c l e a r r e a c t i o n s .

(e) M e a s u r e m e n t s of l i f e t i m e s .

3. Applied research and technology

(a) R e s e a r c h i n c a t a l y s i s . E x a m p l e s : i r o n o x i d e c a t a l y s t s a n d u s e of i r o n a s a p r o b e t o s t u d y c a t a l y s t s .

(b) A n a l y s i s of o r e and m i n e r a l s a m p l e s , and p r o s p e c t i n g f o r m e t a l s . P o r t -a b l e a p p a r a t u s h a s b e e n d e v e l o p e d f o r t h e l a t t e r p u r p o s e , e . g . 0 .03% of t i n i n o r e s c a n b e d e t e c t e d i n a f e w m i n u t e s . G e n e r a l l y a p p l i c a b l e f o r F e , Sn a n d r a r e e a r t h s .

( c ) P o l y m e r c h e m i s t r y : s t u d y of s t a b i l i z i n g sind a g e i n g p h e n o m e n a : (d) C e m e n t s , c e r a m i c s a n d g l a s s s t u d i e s : p r o d u c t s of h i g h t e m p e r a t u r e

r e a c t i o n s .

262

(e) A n a l y s i s , e . g . n o n - d e s t r u c t i v e a n a l y s i s , r a r e - e a r t h a n a l y s i s . ( f ) S u r f a c e r e a c t i o n s i n s o i l , e . g . r e a c t i o n s of i r o n p h o s p h a t e s . (g) S t u d i e s of a l l o y s a n d m a g n e t i c m a t e r i a l s . (h) S t u d i e s of s m a l l m o t i o n s . ( i ) S p a c e c h e m i s t r y , e . g . i n o r g a n i c a n a l y s i s .

B . R E C O M M E N D A T I O N S T O T H E A G E N C Y

T h e M ö s s b a u e r e f f e c t p r o v i d e s a v e r y u s e f u l t o o l i n m a n y i m p o r t a n t i n v e s t i g a t i o n s . It c u t s a c r o s s t he d i s c i p l i n e s of n u c l e a r p h y s i c s , s o l i d - s t a t e p h y s i c s a n d c h e m i s t r y , a n d o p e n s u p w a y s of i n v e s t i g a t i o n of m a n y o t h e r d i s c i p l i n e s . I t i s a c o m p a r a t i v e l y i n e x p e n s i v e r e s e a r c h f i e l d , r e l y i n g upon t o o l s wh ich f r e q u e n t l y a r e a l r e a d y in e x i s t e n c e e v e n in deve lop ing c o u n t r i e s . S o m e i n t e r e s t i n g r e s e a r c h p r o g r a m m e s c o m b i n e M ö s s b a u e r s t u d i e s w i t h t h e d i r e c t u s e of r e a c t o r s . M ö s s b a u e r s p e c t r o s c o p y e n a b l e s s c i e n t i s t s t o w o r k i n t h e n u c l e a r f i e l d w i t h o u t h a v i n g to p u r c h a s e e x p e n s i v e e q u i p m e n t . It m a k e s p o s s i b l e the e d u c a t i o n and t r a i n i n g of s c i e n t i f i c p e r s o n n e l in v a r i o u s b r a n c h e s of S c i e n c e r e l a t e d t o n u c l e a r p h y s i c s a n d n u c l e a r c h e m i s t r y . I t i s , t h e r e f o r e , c o n s i d e r e d an i m p o r t a n t f i e l d f o r t h e A g e n c y .

T h e P a n e l c o n s i d e r s t h a t t h e A g e n c y cou l d r e n d e r a u s e f u l c o n t r i b u t i o n

by : (1) P r o v i d i n g f e l l o w s h i p s , r e s e a r c h c o n t r a c t s , and t e c h n i c a l a s s i s t a n c e

in t h e f o r m of e x p e r t s and e q u i p m e n t . (2) A s s i s t i n g in t h e d i s t r i b u t i o n of i n f o r m a t i o n , e . g . by p r o v i d i n g b i b l i o -

g r a p h i e s and i n f o r m a t i o n on i s o t o p e a v a i l a b i l i t y . (3) A c t i n g a s a b r o k è r i n t h e p r o v i s i o n of M ö s s b a u e r s t a n d a r d s a n d

t h o s e M ö s s b a u e r i s o t o p e s w h i c h a r e d i f f i c u l t t o obtain.

T h e P a n e l f u r t h e r r e q u e s t s t h e A g e n è y to a r r a n g e i n 1967 a S y m p o s i u m on " i n t e r a c t i o n of n u c l e a r p h y s i c s w i t h s o l i d - s t a t e p h y s i c s a n d c h e t n i s t r y " , i n c l u d i n g t h e f o l l o w i n g t o p i c s :

(a) M ö s s b a u e r e f f e c t (b) P e r t u r b e d a n g u l a r c o r r e l á t i o n s (c) P o s i t r o n a n n i h i l a t i o n (d) N u c l e a r o r i e n t a t i o n I t f u r t h e r r e c o m m e n d s t h a t a n o t h e r P a n e l b é c o n v e n e d , p r e f e r a b l y i n

O c t o b e r 1966 , o n t h e same t o p i c a s t h e p r e s e n t P a n e l , b u t , p e r h a p s , i n -c l u d i n g n u c l e a r a p p l i c a t i o n s . O n e m e m b e r of t h è P a n e l m e n t i o n e d t h a t t h e R o y a l S o c i e t y of New Z e a l a n d in c o - o p e r a t i o n with t he Roya l Soc ie ty (London) h a d t h e i n t e n t i o n of s u p p o r t i n g s u c h a P a n e l t o b e h e l d i n New Z e a l a n d . It w a s s u g g e s t e d t h a t t h e A g e n c y m i g h t w i s h t o с о - s p o n s o r t h i s P a n e l .

263

APPLICATIONS OF THE MÖSSBAUER E F F E C T IN

CHEMISTRY AND SOLID-STATE PHYSICS

HELD IN VIENNA, 26-30 APRIL 1965

CHAIRMAN RAPPORTEUR

Sess ion 1 J . DANON G . K . WERTHEIM

S e s s i o n 2 N. N. GREENWOOD H. FRAUENFELDER

S e s s i o n 3 V . l . GOLDANSKII E . FLUCK

S e s s i o n 4 H. F R A U E N F E L D E R M. CORDEY-HAYES

S e s s i o n 5 R . H . HERBER J . A . S T O N E

S e s s i o n 6 ( P a r t s 1 and 2) P . KIENLE V . l . GOLDANSKII

Sess ion 7 ( P a r t s 1 and 2) P . HILLMAN R. H. HERBER

SECRETARIAT OF THE P A N E L

Scien t i f ic S e c r e t a r i e s : G. B. COOK Divis ion of R e s e a r c h and L a b o r a t o r i e s , IAEA

O. SUSCHNY Divis ion of R e s e a r c h and L a b o r a t o r i e s , IAEA

265

LIST OF PARTICIPANTS

Name I n s t i t u t i o n Nominating State or Organization

Cook, G.B. Division of Research and Laboratories International Atomic Energy Agency Vienna, Austria

IAEA

Cordey-Hayes, M. Physics Dept. , Birmingham University Birmingham 15

United Kingdom

Danon , I . Centro Brasileiro de Pesquisas Fisicas Av. Wenceslow Bras. 71 Rio de Janeiro

Brazil

Fluck, E. Anorganisch-chemisches Institut Heidelberg Universität

Federal Republic of Germany

Frauenfelder, H. Dept. of Physics and Materials Research Laboratory United States of University of Illinois America Urbana, 111.

Goldanskii, V.l . Academy of Sciences of the USSR Moscow

Union of Soviet Socialist Republics

Golding, R.M. Chemistry Division Dept. of Scientific and Industrial Research Private Bag, Petone

New Zealand

Greenwood, N.N. Dept. of Inorganic Chemistry University of Newcastle-upon-Tyne

United Kingdom

Herber, R.H. School of Chemistry Rutgers, The State University New Brunswick, N.J.

United States of America

Hillmati, P. Weitzmann Institute of Science Rehovoth

Israel

Kienle, P. Institut für Technische Kernphysik Technische Hochschule 61 Darmstadt

Federal Republic of Germany

Saito, N. Dept. of Chemistry The University of Tokyo Hongo, Tokyo, Japan

MEA

Spijkerman, J .J . National Bureau of Standards Washington DC


2 6 6

LIST OF P A R T I C I P A N T S

I n s t i t u t i o n

Experimental Physics Division Savannah River Laboratory E.I. du Pont de Nemours and Co. Aiken. S.C.

Division of Research and Laboratories International Atomic Energy Agency Vienna, Austria

Bell Telephone Laboratories Murray Hill, N.J.

Dept. of Chemistry McGill University Montreal 2, Canada

N o m i n a t i n g S t a t e

o r O r g a n i z a t i o n


IAEA


IAEA

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I S R A E L H e i l i g e r and C o . 3 N a t h a n S t r a u s s S t r ee t J e r u s a l e m

I T A L Y A g e n z i a E d i t o r i a l e I n t e r n a z i o n a l e O r g a n i z z a z i o n i U n i v e r s a l i ( A . E . I . O . U . ) Via Merav ig l i 16 Milan

J A P A N Maruzen Company L t d . 6 , Tor i Nichome N i h o n b a s h i ( P . O . Box 605) T o k y o C e n t r a l

M E X I C O L i b r a r í a I n t e r n a c i o n a l Av. Sonora 206 Mexico 11, D . F .

N E T H E R L A N D S N.V. Mar t inus Ni jhoff L a n g e Voorhout 9 The Hague

N E W Z E A L A N D Whitcombe & T o m b s , L td . G . P . O . Box 1894 Wel l ing ton , C . l

NORWAY J o h a n Grundt T a n u m

Kar l J o h a n s ga te 43

O s l o

P A K I S T A N K a r a c h i E d u c a t i o n S o c i e t y

Haroon C h a m b e r s

South N a p i e r R o a d

( P . O . Bo* No . 4866)

K a r a c h i 2

P O L A N D

O s r o d e k R o z p o w s z e c h n i a n a

Wydavraiciw N a u k o w y c h

P o l s k a A k a d e m i e N a u k

P a ï a c Kul tury i N a u k i

Warsaw

R O M A N I A Car t imex Rue A. Br iand 14-18 B u c a r e s t

SOUTH A F R I C A

Van S c h a i k ' s B o o k s t o r e ( P t y ) L t d .

L i b r i Bu i ld ing Church S t r e e t

( P . O . Box 724)

Pretoria

S P A I N

L i b r a r l a B o s c h

R o n d a de la U n i v e r s i d a d 11

B a r c e l o n a

S W E D E N C . E . F r i t z e s Kung l . H o v b o k h a n d e l

' F r e d s g a t a n 2 S tockholm 16

S W I T Z E R L A N D

L i b r a i r i e P a y o t

Rue Grenus б 1211 G e n e v a 11

T U R K E Y L ib ra i r i e H a c h e t t e 469 , I s t i k l â l C a d d e s i B e y o g l u , I s t a n b u l

UKRAINIAN S O V I E T SOCIALIST R E P U B L I C

See u n d e r USSR

UNION O F S O V I E T SOCIALIST R E P U B L I C S

Mezhduna rodnaya Kniga S m o l e n s k a y a ' S e n n a y a 32-34 Moscow G-200

U N I T E D KINGDOM O F G R E A T BRITAIN AND N O R T H E R N I R E L A N D

Her M a j e s t y ' s S ta t ionery O f f i c e P . O . Box 569 L o n d o n , S . E . I

UNITED S T A T E S O F AMERICA N a t i o n a l Agency for I n t e r n a t i o n a l P u b l i c a t i o n s , I n c . 317 E a s t 34th S t ree t New York , N . Y . 10016

V E N E Z U E L A Sr. B rau l i o G a b r i e l C h a c a r e s Gobernador a Caod i l i t o 37 San t a R o s a l i a (Apar t ado P o s t a l 8092) C a r a c a s D . F .

Y U G O S L A V I A J u ^ o s l o v e n s k a K n j i g a T e r a z i j e 27 B e l g r a d e

IAEA p u b l i c a t i o n s can a l s o be p u r c h a s e d r e t a i l a t the Uni t ed N a t i o n s B o o k s h o p a t Un i t ed N a t i o n s H e a d q u a r t e r s , New York, a t the n e w s - s t a n d a t the A g e n c y ' s H e a d -q u a r t e r s , V i e n n a , and at mos t c o n f e r e n c e s , s y m p o s i a and s e m i n a r s o r g a n i z e d by the A g e n c y .

In order to f a c i l i t a t e the d i s t r i b u t i o n of i t s p u b l i c a t i o n s , the Agency i s p r e p a r e d to a c c e p t p a y m e n t in U N E S C O c o u p o n s or in l o c a l c u r r e n c i e s .

O r d e r s and i n q u i r i e s from c o u n t r i e s where s a l e s a g e n t s h a v e no t y e t b e e n a p p o i n t e d may be s e n t to :

D i s t r i bu t i on and S a l e s Group , I n t e r n a t i o n a l Atomic Energy A g e n c y , Kä rn tne r R ing 11, A-1010, Vienna I, Aus t r i a

INTERNATIONAL ATOMIC ENERGY AGENCY VIENNA, 1966

PRICE: USA and Canada: US $6.00 Austria and elsewhere: S 126,-(£1.16.0; F.Fr.24,-; DM21, )

1 of the mössbauer effect in chemistry 1 and solid-state

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