ditellurides of 3d transition metals studied by 57 fe and 125 te mössbauer spectroscopy

Ditellurides of 3d transition metals studiedDitellurides of 3d transition metals studiedby by 5757Fe and Fe and 125125Te Mössbauer spectroscopyTe Mössbauer spectroscopy

Piotr FornalPiotr FornalCracow University of TechnologyCracow University of Technology

Institute of PhysicsInstitute of Physics30-083 Krakow, Poland30-083 Krakow, Poland

Jan StanekJan StanekJagiellonian UniversityJagiellonian University

Marian Smoluchowski Institute of PhysicsMarian Smoluchowski Institute of Physics30-059 Krakow, Poland30-059 Krakow, Poland

“Gütlich, Bill, Trautwein: M össbauer Spectroscopy and Transition Metal Chemistry@�� Springer-Verlag 2009”

“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”

25

Mn3d54s2

26

Fe3d64s2

27

Co3d74s2

28

Ni3d84s2

For MeTe2, Me=Mn, Fe, Ni, the crystal structures evolve from cubic pyrite type (Pa3)

for MnTe2 through orthorhombic marcasite type for FeTe2 (Pnnm) to hexagonal CdI2

(C3m) type for NiTe2. Metal ions are six coordinated by Te atoms which form

compressed, quasi-planar trygonal ant-prism in MnTe2 and NiTe2 or compressed

octahedra in FeTe2 with quasi-linear coordination. Te atoms form Te2 molecules in

MnTe2 and FeTe2, while in NiTe2 each Te is coordinated by 3 Te atoms, forming

double anion layers.

The series of the 3d transition metal

ditellurides is an ideal object for the

study of the interplay between

crystal structure and bonds, here

described by the local electronic

states of anions observed by 125Te

and cation states by 57Fe Mössbauer

spectroscopy, if 57Fe probes are

introduced into the structures

without its altering.

52

Te5s25p4


Coordination of Te and cations in MnTe2 , FeTe2 and NiTe2

MnTe2 FeTe2

NiTe2


Methodological aspects of 125Te Mössbauer spectroscopy

The decay scheme for 125mTe

The most popular source is thermal neutron activated metastable 125mTe in Mg3TeO6, as the matrix [H. Binczycka, S.S. Hafner, J. Stanek, M.

Tromel, Phys. Lett. A, 131, 135 (1988) ]. Its Debye temperature, ΘD = 352(3)K, yields the recoil free fraction fs=0.392(5) at 295 K which enables measurements to be made with this source at room temperature. The activation of 25 mg of Mg3TeO6, enriched in 124Te to 90%, in a thermal neutron beam of 1014 neutrons/cm2·s for 20 days results in the final 125mTe source activity of 150 mCi (5.6 GBq),which allows measurements for many months.


The observed ranges of isomer shift and quadrupole splitting are of the order of the experimental line width

Simplified plot of the ranges of the 125Te isomer shift and quadrupole splitting compared to the natural line width.


The relatively high energy of Mössbauer transition in 125mTe leads to the strong

temperature dependence of the recoil-free fraction between 80 K and 300 K which

facilitates the dynamical studies.

125Te Mössbauer spectra of FeTe2 and NiTe2 at different temperatures.

125Te Mössbauer spectra of MnTe2 at different temperatures. At 78 K (below the

Néel transition) the spectrum exhibits a magnetic splitting.


Results from 125Te Mössbauer spectroscopy

The experimentally determined hyperfine interaction parameters may be

transparently interpreted in terms of 5s and 5p shell population. One hole in

the 5pz -orbital produces a quadrupole splitting between 12 mm/s and 15

mm/s (assuming that the 5px and 5py orbitals remain fully populated ) and one

5s electron increases the isomer shift by 2.4 mm/s which is modified by

contributions of 5p electrons - one 5p electron reduces the isomer shift by 0.4

mm/s, due to enhanced shielding. Consequently, it was possible to determine

the electron configuration of Te and its effective charge in the investigated

series

[J. Stanek, A.M. Khasanov, S.S. Hafner, Phys. Rev. B, 45, 56 (1992)].


Results from 125Te Mössbauer spectroscopy: electronic states of tellurium

1.0 1.2 1.4 1.6IS [m m /s]

0

2

4

6

8QS

[ m

m/s

]

C rTe

MnTe

FeTe

CoTe

NiTe

2

QS-IS relation of 125Te

in 3d transition-metal ditellurides

2.5 3.0 3.5Te - Te distance [ A ]

-2.0

-1.5

-1.0

Te

eff

ecti

ve c

har

ge

o

Mn

Fe

Co

Ni

Effective charges of Te in 3d transition metal ditellurides


Results from 125Te Mössbauer spectroscopy: electronic states of tellurium

The effective charges of Te imply the corresponding

cationic charges:

Mn +2

Co, Fe +3

Ni, +4

Mn+2 (Te-Te) -2

Fe+3 (Te-Te) -3

Ni +4 Te-2 - Te-2


Results from 57Fe Mössbauer spectroscopy

Samples

Samples of Mn1-x57FexTe2, FeTe2 and Ni1-x

57FexTe2 (x=0, 0.03, 0.1) were synthesized

from high purity elements employing evacuated silica tube technique. All specimens

were heated, quenched, reground and annealed at 400ºC for several weeks until

single phase, tested by X-ray diffraction, was attained.

Mn1-x57FexTe2

Below 85 K, in the antiferromagnetic state, the spectra showed mixed magnetic-

quadrupole interaction with H=9.3 T at 20 K, the electric field gradient being axial

with the main axis parallel to H and positive Vzz, as calculated in the point charge

approximation


Results from 57Fe Mössbauer spectroscopy : Mn1-x

57FexTe2

-1 0 0 1 0v e lo c ity [m m /s]

293 K

102 K

85 K

75 K

20 K

The 57Fe Mössbauer spectra of Mn1-x57FexTe2.

The magnetic spectra were fitted using a

Hamiltonian-solving program for mixed

magnetic-quadrupole interactions.


Results from 57Fe Mössbauer spectroscopy : FeTe2

-3 0 3v e lo c ity [m m /s]

4.2 K 0 T

4.2 K4.5 T

The well known Mössbauer spectrum

of FeTe2 is in form of weakly

temperature dependent quadrupole

doublet. The spectrum recorded in

external magnetic field, fitted using

the Gabriel-Ruby procedure, shows

that electric field gradient is fairly

axial (η<0.2) and Vzz is negative, as

calculated in the point charge

approximation [J. Stanek, P. Fornal.

Nukleonika, 49 (2004) 63-65]. The fitted

hyperfine field is reduced in

comparison to the applied one. The 57Fe Mössbauer spectra of

FeTe2 at 4.2 K at zero field (top)

and in external magnetic field of

4.5 T (bottom)


Results from 57Fe Mössbauer spectroscopy : Ni1-x57FexTe2

-2 0 2v e lo c ity [m m /s]

4.2 K

293 K 90

293 K 45

o

o

The asymmetry of the quadrupole doublet depends on the orientation of the sample against the gamma beam which suggests the occurrence of texture, confirmed by the electron microscopy study. If one assumes that the main axis of the field gradient is perpendicular to the sample plane, the more intense high energy line shows that Vzz is positive, as calculated in the point charge approximation[J. Stanek, S.S. Hafner, P.Fornal, Hyperfine Interaction C, 5, (2001) 355-358 ].

The 57Fe Mössbauer spectra of Ni1-x57FexTe2 at

4.2 K (top) and at room temperature: middle: sample perpendicular to the gamma beam, bottom: sample at 45º to the gamma beam.

The scanning electron microscopy picture of the powder sample of

Ni1-x57FexTe2.


Summary57Fe Mössbauer parameters

1. The increase of the 125Te isomer shift is correlated with the decrease of the 57Fe isomer shift.

2. The quadrupole splitting of 57Fe is weakly temperature dependent.

3. The sign of Vzz acting on 57Fe is the same as that obtained from calculation within the point charge approximation lattice contribution.

4. The value of the hyperfine magnetic ion 57Fe in MnTe2 is close to the transferred magnetic field on 125Te.

5. The measured magnetic field at FeTe2 was less by 2% than the applied one.

Sample T [K] [mm/s] [mm/s] H [T]

Mn0.9Fe0.1Te2 293 0.78 (+)1.28

247 0.82 (+)1.29

197 0.85 (+)1.31

102 0.91 (+)1.34

85 0.91 + 1.34 2.76

75 0.92 + 1.37 4.60

20 0.94 + 1.45 9.32

Mn0.97Fe0.03Te2 293 0.78 (+)1.28

FeTe2 293 0.47 (-)0.52

4.2* 0.46 - 0.52

Ni0.9Fe0.1Te2 293 0.42 (+)0.33

150* 0.44 (+)0.41

4.2* 0.40 (+)0.44

Ni0.97Fe0.03Te2 293 0.42 (+)0.31(*) source and absorber at the same temperature


Discussion (1)1. The starting point for the discussion is FeTe2, where Fe is in the diamagnetic

low spin FeII state with nominal 5d6 configuration. This spin state is

confirmed by Mössbauer spectroscopy (cf. lattice origin of efg, reduced vs.

applied value of the magnetic field. However, the charge of about 3 electrons

is transferred from Fe to Te2 molecule and the charge of Fe should be +3. The

only possibility is to write the electronic configuration as 3(d6)x with x= 5/6.

2. Applying the same arguments for Mn1-x57FexTe2 and Ni1-x

57FexTe2 one may write

the Fe configuration as 3(d6)1 and 3(d6)4/6 which lead to the +2 effective charge

of Fe and +4 in Mn1-x57FexTe2 and Ni1-x

57FexTe2, respectively.

3. These simple minded arguments point out that the terms “electronic state”

and “charge state” or “valence state” should not be used equivalently. In the

studied samples iron is in FeII low spin electronic state, with “charge state”

varying from +2 in Mn1-x57FexTe2 through +3 in FeTe2 up somehow

controversial +4 state in Ni1-x57FexTe2.


Discussion (2)

Increase of the charge transfer to Te

Increase in the number of 3d electrons (Mn, Fe, Ni)

Weakening of the Te-Te bonds in Te2 molecule

Increase of the Te-Te distances

Structure evolution from pyrite-type through marcasite-type to CdI2-type

Assuming that the local states of Fe reasonably reproduce the properties of the

substituted Mn and Ni cations the simple ionic model well reproduce the

bonding-structure interplay in the investigated series, according to the following

scenario:

ditellurides of 3d transition metals studied by 57 fe and 125 te mössbauer spectroscopy

Documents

te isomer shift

te mssbauer spectra

te mssbauer spectra

electron configuration

fe mssbauer spectroscopy

te atoms form te2 molecules

quadrupole splitting

fete2 pnnm