Role of iron in synthetic tetrahedrites revisited.

The valence state of iron in Cu 12− x Fe x Sb 4 S 13 tetrahedrites have been revisited by the combination of the crystallographic results, Mössbauer spectroscopy, and magnetization measurements. The crystal structure solution for Cu 11.0 Fe 1.0 Sb 4 S 13 (space group I 4 ¯ 3 m , 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 Fe 3+ 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 Fe 3+ and Fe 2+ centers, whereas the rest of iron atoms exists as valence-localized Fe 3+ and Fe 2+ cations. Electron transfer is frozen out at 77 K, where a combination of two Fe 3+ sites and one high-spin Fe 2+ site is observed. Paramagnetic effective moments extracted from the magnetic susceptibility data point at the Fe 3+ state of iron at x =0.8, while a mixture of Fe 2+ and Fe 3+ is presumed in the samples with higher Fe content. [ABSTRACT FROM AUTHOR]