Atomic distribution, electron transfer, and charge compensation in artificial iron-bearing colusites Cu26-xFexTa2-γSn6S32

Abstract

The crystal and electronic structures of synthetic Cu26−xFexTa2Sn6S32 (0 ≤ x ≤ 4.0) colusites were studied by single-crystal X-ray diffraction, 57Fe and 119Sn Mössbauer spectroscopy, and X-ray photoelectron spectroscopy (XPS). It has been shown that iron-bearing colusites contain a maximum of 3.5 ÷ 4.0 iron atoms per formula unit, with iron substituting copper only on the Cu(1) site of the cubic lattice. Presence of localized mixed-valence states Fe3+/Fe2+ for colusites with x ≥ 2 has been confirmed by 57Fe Mössbauer spectroscopy at low temperatures (T < 40 K). Increasing temperature leads to the valence-averaged spectral component corresponding to the electron hopping Fe2+ + Cu2+ ↔ Fe3+ + Cu+. XPS data have shown that Ta5+, Sn4+ and S2- do not change their valence states in any samples, whereas Cu2+/Cu+ and Fe3+/Fe2+ ratios increase with increasing the iron content (x). The Cu 2p core XPS spectra analyzed by the charge transfer (CT) approach revealed mixing of the d9 (Cu2+) and d10L (Cu+) configurations. This was attributed to the 3d-Cu and 3d-Fe hybridization mediated by the S-3p orbitals, which can initiate the intersite electron hopping between iron and copper atoms. The mechanisms of charge compensation at different levels of heterovalent iron doping are discussed.