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, Cu 12 Sb 4 S 13, a parent compound for modern state-of-the-art thermoelectric materials for midtemperature heat-to-power conversion. The crystal structure, space group I 4̅3 m, 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 Cu 12 Sb 4 S 13 . 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 Cu 12 Sb 4 S 13 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.