Local features of the crystal structure of superconducting iron chalcogenides Fe[.sub.1 - δ]

The local crystal structure of superconducting powders of iron chalcogenides Fe[Te.sub.x][Se.sub.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.sub.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 Fe[Te.sub.x][Se.sub.1-x].