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Ditellurides of 3d transition metals studied by 57 Fe and 125 Te Mössbauer spectroscopy

Ditellurides of 3d transition metals studied by 57 Fe and 125 Te Mössbauer spectroscopy. Piotr Fornal Cracow University of Technology Institute of Physics 30-083 Krakow, Poland Jan Stanek Jagiellonian University Marian Smoluchowski Institute of Physics 30-059 Krakow, Poland.

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Ditellurides of 3d transition metals studied by 57 Fe and 125 Te Mössbauer spectroscopy

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  1. Ditellurides of 3d transition metals studiedby 57Fe and 125Te Mössbauer spectroscopy Piotr Fornal Cracow University of Technology Institute of Physics 30-083 Krakow, Poland Jan Stanek Jagiellonian University Marian Smoluchowski Institute of Physics 30-059 Krakow, Poland “Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”

  2. The series of the3d transition metalditellurides 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. 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.

  3. Coordination of Te and cations in MnTe2, FeTe2 and NiTe2 MnTe2 FeTe2 NiTe2

  4. Methodological aspects of 125Te Mössbauer spectroscopy 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 decay scheme for 125mTe

  5. 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.

  6. 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 MnTe2 at different temperatures. At 78 K (below the Néel transition) the spectrum exhibits a magnetic splitting. 125Te Mössbauer spectra of FeTe2 and NiTe2 at different temperatures.

  7. 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 5pyorbitals 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)].

  8. Results from 125Te Mössbauer spectroscopy: electronic states of tellurium QS-IS relation of 125Te in 3d transition-metal ditellurides Effective charges of Te in 3d transition metal ditellurides

  9. Results from 125Te Mössbauer spectroscopy: electronic states of tellurium Mn+2 (Te-Te) -2 The effective charges of Te imply the corresponding cationic charges: Mn +2 Co, Fe +3 Ni, +4 Fe+3 (Te-Te) -3 Ni +4 Te-2 - Te-2

  10. Results from 57Fe Mössbauer spectroscopy Samples Samples of Mn1-x57FexTe2, FeTe2 and Ni1-x57FexTe2 (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

  11. Results from 57Fe Mössbauer spectroscopy : Mn1-x57FexTe2 The 57Fe Mössbauer spectra of Mn1-x57FexTe2. The magnetic spectra were fitted using a Hamiltonian-solving program for mixed magnetic-quadrupole interactions.

  12. Results from 57Fe Mössbauer spectroscopy : FeTe2 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)

  13. Results from 57Fe Mössbauer spectroscopy : Ni1-x57FexTe2 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 pictureof the powder sample of Ni1-x57FexTe2.

  14. Summary 57Fe 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 isweakly temperature dependent. 3. The sign of Vzz acting on 57Fe is thesame 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 atFeTe2 was less by 2% than the applied one. (*) source and absorber at the same temperature

  15. Discussion (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. • Applying the same arguments for Mn1-x57FexTe2 and Ni1-x57FexTe2 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-x57FexTe2, respectively. • 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.

  16. Discussion (2) 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: Increase in the number of 3d electrons (Mn, Fe, Ni) Increase of the charge transfer to Te Increase of the Te-Te distances Weakening of the Te-Te bonds in Te2 molecule Structure evolution from pyrite-type through marcasite-type to CdI2-type

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