Exploring thallium compounds as thermoelectric materials: Seventeen new thallium chalcogenides

We have begun investigating thallium-containing compounds as candidate thermoelectric materials. Alkali metal chalcogenide systems have produced compounds with promising thermoelectric properties, and the chemistry of the Tl ⁺ cation is similar to that of the alkali metals. However, Tl is less electropositive and heavier than all alkali metals (except Fr). This may lead to compounds with lower electrical resistivity and lower thermal conductivity, both important for thermoelectric performance. Unlike the alkali metals, Tl ⁺ has a lone pair of electrons which can be stereochemically active and can influence coordination geometries. Here we report the results of our initial study of quaternary Tl chalcogenide systems. We present the synthesis and structure of 17 new compounds. Some are isostructural to known alkali metal compounds, and some are new structure types. We discuss the activity of the Tl lone pairs. In addition, we have measured the thermoelectric properties (electrical resistivity, thermal conductivity, and thermopower) and band gaps of some of these materials. All of the measured thermal conductivities are extremely low; however, most of the materials are too resistive for thermoelectric applications, at least as currently prepared.