Exploratory synthesis of new heavy main group chalcogenides

We have recently commenced to investigate mixed valent tin chalcogenides as well as polychalcogenides, namely SrSn₂Se₄, Sr₂SnSe₅, Ba₂SnSe₅, and Ba₂SnTe₅. These materials exhibit (calculated and in part experimentally confirmed) band gaps between 0.2 eV (Ba₂SnTe₅) and 1.2 eV (Ba₂SnSe₅). Molecular units are predominant in these materials, namely SnSe₄^4- tetrahedra, Sn₃Se₁₀^8- and Sn₃Te₁₀^8uilding blocks, respectively, as well as Se₃^2- and Te₅^4- units. Three-dimensionally interconnected covalent networks were found in the new quaternaries Ba₃Cu₂Sn₃Se₁₀, BaCu₂SnSe₄, and BaAg₂SnSe₄, including the Ba²⁺ cations in one-dimensional channels. Despite the small band gaps (e.g., 0.2 eV for BaAg₂SnSe₄), the as-prepared samples exhibit rather small electrical conductivities, but large Seebeck coefficients (above +500 μV/K at 300 K). Our most recently discovered polytellurides Ba₃Cu_14-δTe₁₂ and Ba₂Cu_4-δTe₅ comprising extended Cu atom substructures appear to have thermoelectric properties superior to the tin chalcogenides.