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

The new quaternary thiogermanates, ATaGCS(5) (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 P (l) over bar of the triclinic system with two formula units in a cell of dimensions: a = 6.937(1) angstrom, b = 6.950(2) angstrom, c = 8.844(3) angstrom, alpha = 71.07(2)degrees, beta = 78.56(2)degrees, gamma = 75.75(2)degrees, V= 387.6(2) angstrom(3) for KTaGeS5; a = 6.996(3) angstrom, b = 7.033(3) angstrom, c = 8.985(4) angstrom, alpha = 70.33(3)degrees, beta = 78.12(4)degrees, gamma = 75.63(4)degrees, V = 399.6(3) angstrom(3) for RbTaGeS5; a = 7.012(4) angstrom, b = 7.202(3) angstrom, c = 9.267(5) angstrom, alpha = 68.55(3)degrees, beta = 77.27(4)degrees, gamma = 74.75(4)degrees, V = 416.2(4) angstrom(3) for CsTaGeS5. The structures of ATaGeS(5) (A = K, Rb, Cs) are comprised of anionic infinite two-dimensional (2)(infinity)[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+)(S2-)(5)]. UV/Vis diffuse reflectance measurements indicate that they are semiconductors with optical bandgaps of ca. 2.0 eV.