Role of iron in synthetic tetrahedrites revisited

The valence state of iron in Cu_12-xFe_xSb₄S₁₃ tetrahedrites have been revisited by the combination of the crystallographic results, Mössbauer spectroscopy, and magnetization measurements. The crystal structure solution for Cu_11.0Fe_1.0Sb₄S₁₃ (space group I4¯3m, a=10.3253(12), z=2, R=0.011) proved that iron substitutes for copper only in the Cu1 position. At the iron content of x=0.8, 1.0, and 1.2, the presence of two nonequivalent and non-interacting Fe³⁺ cations was inferred from Mössbauer spectra. At higher levels of substitution (x=1.5 and 2.0), room-temperature Mössbauer spectra indicate the electron hopping between part of Fe³⁺ and Fe²⁺ centers, whereas the rest of iron atoms exists as valence-localized Fe³⁺ and Fe²⁺ cations. Electron transfer is frozen out at 77 K, where a combination of two Fe³⁺ sites and one high-spin Fe²⁺ site is observed. Paramagnetic effective moments extracted from the magnetic susceptibility data point at the Fe³⁺ state of iron at x=0.8, while a mixture of Fe²⁺ and Fe³⁺ is presumed in the samples with higher Fe content.