Exploratory synthesis of new heavy main group chalcogenides

We have recently commenced to investigate mixed valent tin chalcogenides as well as polychalcogenides, namely SrSn/sub 2/Se/sub 4/, Sr/sub 2/SnSe/sub 5/, Ba/sub 2/SnSe/sub 5/, and Ba/sub 2/SnTe/sub 5/. These materials exhibit (calculated and in part experimentally confirmed) band gaps between 0.2 eV (Ba/sub 2/SnTe/sub 5/) and 1.2 eV (Ba/sub 2/SnSe/sub 5/). Molecular units are predominant in these materials, namely SnSe/sub 4//sup 4-/ tetrahedra, Sn/sub 3/Se/sub 10//sup 8-/ and Sn/sub 3/Te/sub 10//sup 8u/ilding blocks, respectively, as well as Se/sub 3//sup 2-/ and Te/sub 5//sup 4-/ units. Three-dimensionally interconnected covalent networks were found in the new quaternaries Ba/sub 3/Cu/sub 2/Sn/sub 3/Se/sub 10/, BaCu/sub 2/SnSe/sub 4/, and BaAg/sub 2/SnSe/sub 4/, including the Ba/sup 2+/ cations in one-dimensional channels. Despite the small band gaps (e.g., 0.2 eV for BaAg/sub 2/SnSe/sub 4/), the as-prepared samples exhibit rather small electrical conductivities, but large Seebeck coefficients (above +500 /spl mu/V/K at 300 K). Our most recently discovered polytellurides Ba/sub 3/Cu/sub 14-/spl delta//Te/sub 12/ and Ba/sub 2/Cu/sub 4-/spl delta//Te/sub 5/ comprising extended Cu atom substructures appear to have thermoelectric properties superior to the tin chalcogenides.