Solid State Communications, Vol. 106, No. 9, 621-626. 1998 pp. 0 1998 Elsevier Science Ltd

Printed in Great Britain. All rights reserved 0038-1098/98 $19.00+.00

PII: sl0038-1098(98)00101-x

EPR STUDY OsF La2_xM,Cu04 (M = Sr, Ba)

Shakeel Khan, Arti Singh and R.J. Singh*

Department of Physics, Aligarh Muslim University, Aligarh, 202002 India

(Received 16 September 1997; accepted 29 January 1998 by C.N.R. Rao)

EPR spectra of deoxygenated La i.slSra.l&u04 and La l.94Ba0.06Cu04 have been investigated. In both of them 4 Cu2+ ions are found to combine ferromagnetically to give S =: 2 spectra. At low Sr/Ba content the spins probably start getting polarised in the ab plane. Spin-Hamiltonian para- meters for all the spectra have been determined. Dependence of T, on the content of Sr or Ba has been attempted to be explained. It seems that holes which are generated on the substitution of La3+ by Sr2+ or Ba2+ and are current carriers first get uniformly distributed over the La0 plane but on increasing Sr2+ or Ba2+ content, they may combine together to form O:- peroxide ions and become unable to carry current. On further increasing the Sr2+ or Ba2+ content, the same process is repeated, until the substance becomes metallic or until further amounts of the substituents can not be dissolved in the parent compound. 0 1998 Elsevier Science Ltd. All rights reserved

Keywords: A. superconductors, D. superconductivity.

1. INTRODUCTION

La2Cu04 and its strontium and barium derivatives (along with all other cuprate family high temperature super-

conductors have been reported to be EPR silent [l-5]. But it has been found that CuO, BaCu02, CaCuOz and Y2Cu20s, the constituents of 1 2 3 superconductors [6-lo] and even YBa2Cu307_6 when deoxygenated produce complex spectra which can be analysed in terms of Cu2+-monomers and ferromagnetically coupled Cu2+-dimers and tetramers. The coupling proceeds

through superexchange interaction in the Cu-Cl-Cu and (CUO)~ entities in the Cu02 plane. These entities

are able to give spectra when they get magnetically isolated from the bulk. There have been found more than one variety of these complexes. The objectivIes of

this paper are two-fold.

(i) To extend the study carried out on 1 2 3-superconductor and its constituents to 2 1 4- superconductors because Cu02 plane is common to all these compounds.

(ii) To understand x-dependence of T, in Laz_,M,CuO+ The interesting feature of La2_,M,CuOd system is the dependence of T, on x [ 11, 121, that is,

* Corresponding author.

small x gives it semiconducting property and then T, increases with x and after some value, it starts decreasing. In barium containing compounds, T, increases from

x = 0.04 to 0.08, then starts decreasing and at x = 0.13, T, becomes zero and then again starts increas- ing, goes to another maximum at x = 0.20 and again T, becomes zero at x = 0.25. In strontium containing

compounds, T, starts increasing after x = 0.04 and goes to the maximum at x = 0.15 and then starts decreasing

and at x = 0.34 the substance becomes metallic.

2. EXPERIMENTAL

1. The single crystals of La 1.81Sr0.&u04 and La l.94Ba0.06Cu04 were supplied to us by Dr. K. Schlenga [ 131, after they have performed megnetoresistance

studies on them. Both, these materials have Sr or Ba content which show superconductivity at nearly the highest possible T, for this class of material.

2. La l,96Sro,04Cu04 was prepared by usual solid state reaction route at the calcination temperature of 1000°C. The heating was done for five days with intermittent grindings. The specimen was in powder form.

3. The spectra were recorded on RE-2X, ESR spectrometer (JEOL, JAPAN) working in X-band with 100 kHz field modulation at room temperature.

621

622 EPR STUDY OF La2_,M,Cu04 (M = Sr, Ba) Vol. 106, No. 9

3. RESULTS AND DISCUSSION

La,.96Sr0.&~04: It gave only a very broad signal. La ,,sISro,,&u04 (LaSr): The spectrum consists of

two sets of 4 line patterns attributable to two kinds of copper-tetramers generated in the substance on deoxygenation. The position and spacings between the lines of the 4 line set were so much that they can not be attributed to the hyperfine structure of Cu2+ ion as

expected in La,,s,Sr0,,gCu04. So they are assigned to fine structure of a S = 2 system which may arise due to the ferromagnetic coupling of the spins of 4 Cu2+ ions in the (CuO)., entities in the Cu02 plane, when these entities get dissociated from the network by the rupture of 8 oxygen bonds surrounding each (CUO)~ unit. A representative spectrum containing these two sets is shown in Fig. l(a) and the angular variation of the second set in different crystallographic planes is shown in Fig. 2. The Spin-Hamiltonian proposed by Stankowski et al. [14-161 was used for determining parameters for the four lines patterns of S = 2 system. The energy of transitions according to Spin-Hamiltonian are

w2 - wI = gflH + 312~ + 512~

wI -w. = g/3H + 1/2u - 5v

I

30 3000 41 MAGNETIC FIELD(Gauss)

30

Fig. 1. Representative EPR spectra (a) La,,s,Srr,,gCuO~ having two sets of 4 lines pattern; la,6,c,d and 20,b,c.d at an angle of 70” from a-axis in ab plane. (b) La 1.94Ba0.&u04 having two sets of 4 lines pattern; lo.6,c,d and 20,6.c,d at an angle of 30” from b-axis in ab plane. In La 1.94Ba0.06Cu04 ( 1 b), some lines appear on high field side (-) for a small stretch of angles in angular variation study. They may be inter-level transitions [8].

I 1 I 0 30 60 90a 30 60 90

ANGLE@)

Fig. 2. Angular variation plot of second set of 4 line pattern of LaislSrO,,&uO~ (crystallographic axes a, b, c specified).

WO-W_~ =gPH-1/2u+5v

w_, --w_~ =gfiH-3/2u-Y2v

where

u=D(3cos20-l)+3Ecos2$sin28;

v = (a/l0 - b/2)(5 cos* 0 + 3 - 8 sin28)

+ c cos 24 sin2 8 X (7 cos2 8 - 1)

+ (a/2 + 7/2b) cos 46 sin4 0

The Spin-Hamiltonian parameters along with the direc- tion cosines (d.c.) of the g tensor employing aforesaid Spin-Hamiltonian have been given in Table 1.

La I .94Ba0.&u04 (LaBa): This also gave two sets of 4 lines each. A representative spectrum of the two sets is given in Fig. l(b) and the angular variation of the first set

in different crystallographic planes is given in Fig. 3. The Spin-Hamiltonian parameters along with the d.c.‘s of “g” tensor are given in Table 1. In this compound, the signal intensity reduces as time passes after deoxygena- tion and after a few days ( = 10 days), it completely vanishes. This may be due to migration of disordered

oxygen to original position. The possibility of the crystal, catching the oxygen from the atmosphere is ruled out, because no weight gain was recorded. The spectra dis- cussed here are those which were taken on the first day of the quenching of the sample.

There is a big difference in the spectra of La2Cu04 [17] and those of La,,s,Sro.rgCuOl and Lai,94Bao,06Cu04. There is only one set of 4 line pattern in LazCuOd, whereas in each of LaSr and LaBr there are two sets of 4 lines each. In La2Cu04, either only one set is present or if both present, the second one has been masked by the overwhelming presence of Cu-monomer signal. By comparison of intensity and position, the one set of La2Cu04 can be correlated with the first set of LaSr and LaBa. The mean g value = ([gz + gz + gf]/3)“2 for

Tab

le

1. S

pin-

Ham

ilton

ian

para

met

ers

of L

a2C

u04,

L

ar.s

lSra

.r&

uO~

and

La

1,94

Ba0

,MC

u04

and

the

dire

ctio

n co

sine

s of

g t

enso

r

Syst

em

Prin

cipa

l D

irec

tion

cosi

nes

D

E a

b g

valu

es

(Gau

ss)

(Gau

ss)

(Gau

ss)

(Gau

ss)

FGau

ss)

3

Laz

Cu0

4 g,

=

3.50

0 -

- -

%

g,

= 3.

610

- -

- 27

1 -1

-1

0 5

5

(One

se

t on

ly)

s g,

=

3.16

9 *

La,

RlS

rn !O

CuO

d g_

=

2.63

5 n ~75

n/l527

nn7r

0

. .._.w

".7"&

_"."JJ

a

gv =

2.71

7 -0

.484

0.

875

-0.0

37

42

65

4 1

1 r

(Fir

st

set)

g,

=

3.39

1 0.

013

0.04

9 0.

999

2 k Lai.8+ro.l9Cu04

g, =

2.12

0 0.

912

0.00

0 -0

.411

g,

=

2.15

8 0.

290

0.70

7 0.

645

104

-8

2 2

-1

s (S

econ

d se

t)

gz =

2.

376

0.29

0 -0

.707

0.

645

2 L

a r.

94B

aO.c

&uG

4 g,

=

2.64

5 0.

686

0.72

6 0.

039

II

gY =

2.

784

0.36

8 -0

.393

0.

843

42

84

2 1

4 Y

(F

irst

se

t)

g,

= 2.

846

0.62

8 -0

.564

-0

.537

m

E

L

ar.9

4Bao

.&uG

4 g,

=

2.43

4 0.

830

-0.5

56

-0.0

49

gY =

2.

268

0.21

4 0.

236

0.94

8 12

8 -1

0 -8

1

-1

(Sec

ond

set)

g,

=

2.08

9 0.

516

0.79

7 -0

.315

ml. NT

discussed, that is, in LaSr and LaBa, T, depends on I I I

0 30 60 9O,o 30 60 90 x in an anomalous fashion. Sudden depression of

ANGLE(B) conductivity in LaBa at x = 0.13 has been explained

Fig. 3. Angular variation plot of first set of 4 line pattern by Rameev et al. [18] to be due to some sort of

of La,.94Ba0.&u04 (crystallographic axes a, b, c localization of charge carriers. But what is the mechanism

specified). of localization has not been dealt with. A tentative explanation is given as follows. When strontium or

La2Cu04 is 3.435; for the first set of Lal.s1Sr0.&u04 it is equal to 2.934 and for the first set of La1.94Ba0.06Cu04 it is equal to 2.759. The D and E values are quite different in LazCuOb from those of LaSr and LaBa as can be seen from Table 1. It can be interpreted that in LazCuOd, the 3dq electron of Cu2+ has three dimensional inter-

action whereas in the latter two, it gets confined to two dimensional CuOz plane or there is spin polarisation in ab plane. When small amount of strontium is added to the La2Cu04 as in La,,96Sr0.04Cu04, the Cu-spins which originally had three-dimensional spread might start chan- ging to two-dimensional spread and the process might be so fast that the relaxation time is so much reduced that the signal is broadened beyond detection. When x or Sr 2+ 2- 2-t 2- / 2+ 2- 2+ 2- 2+ 2- z* concentration exceeds certain value ( = 0.05) they prob- cu 3 4J cz cu s 4 3 4 6 b ably get settled in the plane.

As regards occurrence of two kinds of Cu-tetramers it may be said that one of them may arise due to planar

(CUO)~ unit and the other set might have been formed by the attachment of one or two disordered oxygen ion to the planar (CuO)d unit giving it square pyramidal or distorted octahedral shape. The displacement of oxygen

ions from their normal position is quite possible in the process of quenching. On the basis of intensity, the first set of both LaSr and LaBa may be attributed to planar

(CuO)d tetramer and the second to the Cu-tetramers with oxygen attachment. On quenching there is more chance of planar (CuO)J units being present than those with oxygen attachments and hence its intensity should be more than that of other species.

@) --

’ I v z-

An important question is, why the spins in the (CuO),, IdI

units become ferromagnetically coupled when the Fig. 4. (a) Extended network of CuO;! plane in the high T,

original compound is 3D antiferromagnetic insulator. superconductors. An isolated copper tetramer enclosed

The reason may be breaking of all neighbouring by solid thick lines. (b) Square planar copper tetramer.

oxygen bonds of (CUO)~ entities and its magnetic (c) Copper tetramer attached with one oxygen ion giving

isolation from bulk. The ferromagnetically coupled square pyramidal structure. (d) Copper tetramer attached with two oxygen ions giving distorted octahedral

Cu-tetramers have been observed by us [6-lo] in many structure.

EPR STUDY OF La2_XM,Cu04 (M = Sr, Ba) Vol. 106, No. 9

instances where CuOz plane exists. However it will be very interesting to show theoretically that the ground state of isolated (CUO)~ unit has 4 spins aligned in the same direction. An isolated (CuO), entity in the extended CuO:! plane is shown in Fig. 4(a). A planar (CUO)~ entity, (CUO)~ coordinated to one oxygen ion in square pyramidal configuration and (CuO)d coordinated to two oxygen ions in a distorted octahedral configuration have been shown in Fig. 4(b), 4(c) and 4(d) respectively.

Now, the second objective of this paper will be

624

Vol. 106, No. 9 EPR STUDY OF La2_xMXCu04 (M = Sr, Ba) 625

barium is incorporated in La2Cu04, it gets distributed in the La0 plane replacing La and creating corresponding number of holes. It is conjectured that the influence of each Sr or Ba ion extends over certain area (area of influence) covering few unit cells in the La0 planar sub lattice and so long there is vacant area, the new Sr or Ba ions will not enter the areas of influence of already present ions. New ions will enter new areas and the addition of more ions will proportionately increase the number of holes or charge carriers resulting into increased conductivity. After a certain concentration, the new ions will start encroaching in the already occupied area of influence and producing two holes in that area. The two holes may combine in a manner that they are no longer able to carry charge. The seats of the holes are oxygen ions in the plane converting O*- to O’- ion. The two nearby O’- ions may combine to form O:- molecule, thus localizing both the holes and thus suppressing superconductivity. Coordination of peroxide Oz- ion to metal centers or metal complexes has been observed in many cases [I93 and its stretching vibrational frequency has been found to be approximately 800 cm-‘. Raman spectra of La2Cu04, LaSr and LaBa have been investigated by Sugai er al. [ 19,201 and it has been observed that in going from parent compound to LaSr and LaBa, the spectrum undergoes remarkable change. The most notable change has been seen in the b,, spectral line of La,.$r&u04 (polarisatbn of incident light parallel to (1 1 0) and of the scattered light (1 i 0) direction) where a new line which did not exist in present compound appears whose frequency is 800 cm-‘. Origin of this frequency has not been interpreted by Sugai et al. because the authors while doing this had another problem in mind, i.e. to show whether the system behaves as Fermi liquid or marginal Fermi liquid. It will be shown a little later that the O:- vibration created in LaSr system will have polariza- tion directed along (1 i 0) or (1 1 0) direction. A challenging and useful experiment will be to record this vibration in enriched LaBa with 0 ” replacing 0 16. In that case due to enhanced “reduced mass” and :same force constant, the reduction in the frequency of the order of 24 cm-’ should be observed. The sample La,,s,Sr0,,&u04 chosen by Sugai et al. had enough concentration of Sr so that each area of influence contains two Sr ions or two holes so that O:- concentration becomes sufficient to show up in the Raman spectra. When all the areas of influence have been occupied by two holes, introduction of additional Sr or Ba ions will create unpaired holes in each area which will cause conductivity and again after exceeding certain concen- tration, the third unpaired hole may get paired off thus again reducing conductivity. Here only rise and fall of electrical conductivity has been explained and

not superconductivity. The superconductivity mechanism may arise from some other kind of pairing, which may take place while the holes are still unpaired and certainly not the pairing discussed here which results into the formation of peroxide O,‘- radical.

In LaSr, the transition temperature T, increases continuously from x = 0.15 and then starts decreasing. In LaBa, T, increases continuously from x = 0.05 to 0.08 and then starts decreasing, vanishes at x = 0.13 and again starts increasing upto x = 0.20 and then decreasing and again falls to zero at x = 0.25. With this dependence of T, on x or the chemical composition of the compound giving highest T,, the area of influence of one hole substitution can be estimated in each case. A two dimensional rectangular sublattice of LaSr and LaBa is shown in Fig. 5(a) and 5(b) respectively. The figures show that if a hole enters the square ticked in the figure, its area of influence extends upto all squares marked by crosses. In LaSr, one hole may cover area of 13 squares enclosed by dashed lines [Fig. 5(a)] and in LaBa an area of 25 squares also enclosed by dashed lines [Fig. 5(b)]. It shows that out of 13 La atoms in LaSr, one has been replaced by Sr or other is 7.7% probability of Sr and 92.3% probability of La which is in agreement with the composition La1.s5Sr0.15Cu04. In LaBa, out of 25 La, one is replaced by Ba and thus making com- position La, ,92Bao.osCu04; these are the chemical compositions which show highest T, in the respective compounds.

As shown in the Fig. 5(a) the two ions which can form peroxide O:- ions lie along (1 1 0) or (1 i 0) direction and the polarization direction should also be along the stretching direction of OS- molecule as has been observed in the Raman spectrum [ 19, 201 of LaSr compound.

The separation of these two 02- ions in the undisturbed lattice of LaSr is calculated to be 2.67 A whereas reported values of Oi- separation is 1.49 A [2 11. It has been found invariably that when an ion of ionic radius different from the original one is substituted in the lattice, the lattice gets locally deformed. In the present case, the ionic radius of La3+ is 1.04 A and that of the substituent S?+ = 1.20 A and Ba*’ = 1.38 A. Therefore squares containing Sr*+ or Ba*+ may be deformed or puckered so that two O’- ions are pushed near each other. Moreover the original 0 *- ions now become 0 ‘- ion after the advent of the hole thus decreasing repulsion between them and thus there is much likelihood of two O’- ions coming closer than the original separation.

Another question that needs discussion is why the area of influence of Ba*+ = 25 squares [Fig. 5(b)] is much more than that of Sr*+ = 13 squares [Fig. 5(a)]. This may be explained as due to larger ionic radius of Ba*+ = 1.38 A in comparison to that of Srzf = 1.20 A.

626 EPR STUDY OF La2_1MXCu04 (M = Sr, Ba) Vol. 106, No. 9

Fig. 5. La-O extended network in LaI.ssSro.i5Cu04 system; square with a hole is ticked. The other 12 squares (crossed) show the area of influence. (b) La-O extended network in La,,92Bao,osCu04 system; square with a hole is ticked. The other 24 square (crossed) show the area of influence.

In our discussion, we have seen that the holes are generated in La0 planes but for superconductivity the holes should flow in Cu02 planes. But by now it has been established [22, 231 that La0 planes are reservoirs of holes but they flow through CuOz planes to cause superconductivity.

Acknowledgements-Our sincere thanks are due to Dr. K. Schlenga (Physikalisches Instut iii, der Universitk Erlangan-Nurberg, Germany) who kindly supplied single crystals and relevant literature on the subject.

1. 2.

3.

4. 5.

6.

7.

8.

9.

10.

11. 12. 13.

14.

15.

16.

17.

18. 19.

20.

21.

22.

23.

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