Mossbauer spectroscopy evidence of intrinsic non-stoichiometry in iron telluride single crystals

The FeTe parent compound for iron-superconductor chalcogenides was studied applying Mossbauer spectroscopy accompanied by ab initio calculations of electric field gradients at the iron nuclei. Room-temperature (RT) Mossbauer spectra of single crystals have shown asymmetric doublet structure commonly ascribed to contributions of overstoichiometric iron or impurity phases. Low-temperature Mossbauer spectra of the magnetically ordered compound could be well described by four hyperfine-split sextets, although no other foreign phases different from Fe1.05Te were detected by XRD and microanalysis within the sensitivity limits of the equipment. Density functional ab initio calculations have shown that over-stoichiometric iron atoms significantly affect electron charge and spin density up to the second coordination sphere of the iron sub-lattice, and, as a result, four non-equivalent groups of iron atoms are formed by their local environment. The resulting four-group model consistently des