Local features of the crystal structure of superconducting iron chalcogenides Fe(TeSe)$_{1-\delta}$

The local crystal structure of superconducting powders of iron chalcogenides FeTe$_{x}$Se$_{1-x}$ ($x$ = 0.1, 0.22, 0.49, 0.8, 0.9) prepared by dry synthesis (without mineralizer) has been studied by EXAFS spectroscopy above the $K$ Se and $K$ Fe absorption edges in the temperature range of 80–300 K. The dependences of Se–Fe, Fe–Te, and Fe–Fe interatomic bond lengths and degrees of their local disordering (Debye–Waller factors) on the tellurium content and temperature have been obtained. Einstein temperatures characterizing the stiffness of each bond have been determined. The correlation of the Se–Fe bond stiffness with the dependence of the critical temperature of the superconducting transition $T_c$ on the composition of the samples under study have been established, which indicates the specific role of the Se–Fe bond in the superconducting state formation in iron chalcogenides FeTe$_{x}$Se$_{1-x}$.