Thermally Activated Electron Exchange in Cu12–xFexSb4S13 (x = 1.3, 1.5) Tetrahedrites: A Mössbauer Study

Abstract

Mixed-valence iron Fe(III/II) tetrahedrites Cu10.7Fe1.3Sb4S13 and Cu10.5Fe1.5Sb4S13 have been prepared, characterized, and studied by Mössbauer spectroscopy. The low-temperature Mössbauer spectra (T ≤ 13 K) of all samples confirm the valence-localized nature of the ground state for the distinct Fe2+ and Fe3+ sites. At elevated temperatures the spectral transformations are characterized by a coexistence of valence-localized states Fe3+/Fe2+ (iron surrounded only by copper ions in the second coordination sphere) and valence-averaged spectral components corresponding to the electron hopping (FeA2+FeB3+) ↔ (FeA3+FeB2+). Our experiments suggest that these tetrahedrites belong to Robin–Day class II mixed-valence systems that display thermally activated charge transfer. Hopping frequencies (Ωhop) have been determined from spectral simulations using the stochastic line shape theory. Analysis of the ⟨Ω⟩(T) temperature dependences reveals the existence of a thermally activated process between (quasi)degenerate iron sites. The activation energies for Cu10.7Fe1.3Sb4S13 (EA ≈ 5.3 meV) and Cu10.5Fe1.5Sb4S13 (EA ≈ 6.6 meV) differ and suggest an important influence of the local crystal surroundings of iron ions on electron hopping. The isomer shift and quadrupole splitting values for the Fe3+ sites reveal a relatively strong and nonmonotonic change with temperature that suggests the occurrence of intrinsic charge delocalization in addition to the intersite electron hopping.