Low-Temperature Structure and Thermoelectric Properties of Pristine Synthetic Tetrahedrite Cu12Sb4S13

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 I (4) over bar 3m, 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 Cu-6 octahedron, the shift of the copper atom toward the triangle S-3 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.