Thermoelectric Properties of New Thallium Tellurides
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Thermoelectric Properties of New Thallium Tellurides Cheriyedath Raj Sankar, Savitree Bangarigadu-Sanasy, and Holger Kleinke Department of Chemistry, University of Waterloo, Waterloo, ON, Canada N2L 3G1. ABSTRACT Ternary 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 P1¯ , whereas the corresponding tellurides crystallize in the rhombohedral crystal system (R3¯ ). 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. INTRODUCTION Thermoelectric materials convert heat directly into electricity, and the thermoelectric energy conversion is regarded as a promising route for energy conversion, especially from waste heat. Many of the advanced thermoelectric materials are either chalcogenides or pnictides of heavy elements, which are narrow band gap semiconductors [1-6]. Often, the efficiency of a thermoelectric material is expressed in terms of the dimensionless figure-of-merit given by, ZT = TS2
–1 –1
(1)
Numerous reports on the optimization of existing thermoelectric materials, such as Bi2Te3 and PbTe, were published, in order to produce higher ZT values [6-8]. Doping, nanostructuring, and forming multilayers often offer significant improvements of ZT. Several ternary thallium chalcogenides were reported to be good thermoelectrics [9-13]. Thallium, being a heavy element, appears to cause a low lattice contribution to the thermal conductivity, which helps improving the ZT of the material. The structure and bonding characteristics of some Tl4M'Q4 (M' = Ti, Si, Ge and Sn, Q = S and Se) representatives can be found in the literature [14,15]. Tl4TiQ4 and Tl4SnQ4 (Q = S and Se) crystallize in the monoclinic crystal system with space group P21/c, where as Tl4SiS4 and Tl4GeS4 are reported to be monoclinic, Cc, and Tl4SiSe4 as crystallizing in C2/c. The Tl4M'Q4 chalcogenides all share isolated tetrahedra of [M'Q4]4– as common structural feature, which are separated by Tl atoms. In all these cases, weak Tl–Tl contacts in the range between 3.40 Å and 4.00 Å are observed. Several analogous A4M'Q4 chalcogenides, where A is a monovalent ion of
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the alkali metal elements, exist without comparable A–A contacts [14-20]. All of these compounds also contain isolated tetrahedral ani
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