No abstract

We have examined the low-temperature crystal structure and thermoelectric properties of unsubstituted synthetic tetrahedrite, Cu12Sb4S13, a parent compound for modern state-of-the-art thermoelectric materials for midtemperature heat-to-power conversion. The crystal structure, space group I4̅3m, was probed by X-ray powder diffraction with synchrotron radiation at different temperatures within the range of 10–293 K. It displays subtle changes at the temperature of the metal-to-semiconductor transition (MST) near 90 K, at which a concerted displacement of two independent atoms occurs without symmetry reduction. The displacement of the sulfur atom toward the face of the Cu6 octahedron, the shift of the copper atom toward the triangle S3 plane composed of two independent sulfur atoms, and a sharp elongation of the CuSb separation upon the MST largely affect the transport properties of Cu12Sb4S13. It displays sharp increase in electrical resistivity and a maximum in thermopower below the MST. On the contrary, the lattice part of thermal conductivity increases smoothly in the entire temperature range. Low thermal conductivity of Cu12Sb4S13 is associated with the quazi-localized out-of-plane rattling of three-coordinated copper atoms, which softens with decreasing temperature responding to subtle structural changes upon the MST.

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 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 noninteracting 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.

No abstract

No abstract