Mössbauer spectroscopy evidence of intrinsic non‐stoichiometry in iron telluride single crystals

Abstract

The FeTe parent compound for iron-superconductor chalcogenides was studied applying Mössbauer spectroscopy accompanied by ab initio calculations of electric field gradients at the iron nuclei. Room-temperature (RT) Mössbauer spectra of single crystals have shown asymmetric doublet structure commonly ascribed to contributions of overstoichiometric iron or impurity phases. Low-temperature Mössbauer spectra of the magnetically ordered compound could be well described by four hyperfine-split sextets, although no other foreign phases different from Fe 1.05 Te 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 describes the angular dependence of the single crystals Mössbauer spectra as well as intensity asymmetry of the doublet absorption lines in powdered samples at RT. We suppose that our approach could be extended to the entire class of Fe 1+y Se 1−x Te x compounds, which contain excess iron atoms.

Evidence of the Plaquette Structure of Fe1+xTe Iron Telluride: Mössbauer Spectroscopy Study

“…Moreover, it is well known that iron telluride is always synthesized with iron off‐stoichiometry – a feature, which attracts very much attention last time (see recent work and references therein). In our previous study, the off‐stoichiometric iron telluride Fe 1.05 Te was studied making use of a combination of Mössbauer spectroscopy experiments and ab initio calculations. An essential output of our study in ref.…”

“…An essential output of our study in ref. is a four‐center model for the local environment of iron ions in the iron‐surplus lattice of Fe 1+ x Te. Spatially, the constituents of the four‐center model are located around an interstitial excess iron ion (see Figure 4 of ref.…”

“…Spatially, the constituents of the four‐center model are located around an interstitial excess iron ion (see Figure 4 of ref. ) and form something like a plaquette (see Figure 5 of ref. ).…”

“…) and form something like a plaquette (see Figure 5 of ref. ). It consists of interstitial excess iron ion (Fe2) and two coordination rings (nearest and next‐nearest neighbors) of intralayer iron ions (Fe1/1 and Fe1/2, respectively) around it.…”

“…Depending on the concentration of the excess iron, these plaquettes could be either separated by a rest of unperturbed Fe1 ions at low concentration of Fe2 ions (≈5% and lower), or adjoin one another, or eventually overlap when the concentration increases further. In the case of large concentration of excess Fe2 ions, their strong magnetism and the plaquette structure around could be a reason for suppression of spin fluctuations, which mediate the superconductivity in iron‐based superconductors, according to ref. .…”