Thermoelectric Properties of New Thallium Tellurides

ABSTRACTTernary thallium chalcogenides of the formula Tl4MQ4, where M = Zr and Hf and Q = 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 group P, 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-sharing MQ6 octahedra, whereas the MTe6 octahedra are interconnected via common faces to form linear trimeric units. In all cases, the atoms adopt common oxidation states, namely Tl+, M4+, and Q2–. 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.