Synthesis and Crystal Structures of the First Quaternary Tantalum Thiogermanates,ATaGeS5 (A = K,Rb, Cs)

The new quaternary thiogermanates, ATaGeS₅ (A = K, Rb, Cs) were prepared with the use of halide fluxes and the crystal structures of the compounds were determined by single‐crystal X‐ray diffraction methods. The compounds are isostructural and crystallize in space group Pbar{1} of the triclinic system with two formula units in a cell of dimensions: a = 6.937(1) Å, b = 6.950(2) Å, c = 8.844(3) Å, α = 71.07(2)°, β = 78.56(2)°, γ = 75.75(2)°, V = 387.6(2) Å³ for KTaGeS₅; a = 6.996(3) Å, b = 7.033(3) Å, c = 8.985(4) Å, α = 70.33(3)°, β = 78.12(4)°, γ = 75.63(4)°, V = 399.6(3) Å³ for RbTaGeS₅; a = 7.012(4) Å, b = 7.202(3) Å, c = 9.267(5) Å, α = 68.55(3)°, β = 77.27(4)°, γ = 74.75(4)°, V = 416.2(4) Å³ for CsTaGeS₅. The structures of ATaGeS₅ (A = K, Rb, Cs) are comprised of anionic infinite two‐dimensional {}_infty^2 [TaGeS₅^–] layers separated from one another by alkali metal cations (A⁺). Each layer is made up of tantalum centered sulfur octahedra and pairs of edge‐sharing germanium centered sulfur tetrahedra. The classical charge valence of these compounds should be represented by [A⁺][(Ta⁵⁺)(Ge⁴⁺)(S^2–)₅]. UV/Vis diffuse reflectance measurements indicate that they are semiconductors with optical bandgaps of ca. 2.0 eV.