Thermoelectric Properties of New Thallium Tellurides

Abstract

ABSTRACTTernary thallium chalcogenides of the formula Tl4MQ4, whereM= Zr and Hf andQ= S, Se, and Te were synthesized and characterized. Our X-ray diffraction studies on suitable single crystals reveal that the sulphides and selenides are isostructural, with monoclinic space groupP, whereas the corresponding tellurides crystallize in the rhombohedral crystal system (R). The structures of the sulphides and selenides are comprised of zigzag chains of edge-sharingMQ6octahedra, whereas theMTe6octahedra are interconnected via common faces to form linear trimeric units. In all cases, the atoms adopt common oxidation states, namely Tl+,M4+, andQ2–. The electronic structure calculations using the linear muffin tin orbital (LMTO) method predicted band gaps of 1.7 eV, 1.3 eV and 0.3 eV for the sulphides, selenides and tellurides, respectively, implying sulphides and selenides are large band gap materials, and the tellurides narrow gap semiconductors. Their electronic transport properties are also evaluated with respect to the thermoelectric energy conversion.