New quaternary bismuth sulfides: Syntheses, structures, and band structures of AMBiS₄ (A = Rb, Cs; M = Si, Ge)

Four new compounds, RbSiBiS₄, RbGeBiS₄, CsSiBiS₄, and CsGeBiS₄, have been synthesized by means of the reactive flux method. The isostructural compounds RbSiBiS₄, RbGeBiS₄, and CsGeBiS₄ crystallize in space group P2₁/c of the monoclinic system with four formula units in cells of dimensions at 153 K of a = 6.4714(4) Å, b = 6.7999(4) Å, c = 17.9058(11) Å, and β = 108.856(1)° for RbSiBiS₄, a = 6.5864(4) Å, b = 6.8559(4) Å, c = 17.9810(12) Å, and β = 109.075(1)° for RbGeBiS₄, and a = 6.5474(4) Å, b = 6.9282(4) Å, c = 18.8875(11) Å, and β = 110.173(1)° for CsGeBiS₄. CsSiBiS₄ crystallizes in a different structure type in space group P2₁/c of the monoclinic system with four formula units in a cell of dimensions at 153 K of a = 9.3351(7) Å, b = 6.9313(5) Å, c = 12.8115(10) Å, and β = 109.096(1)°. The two structure types are closely related and consist of ²∞[MBiS₄ ^-] (M = Si, Ge) layers separated by bicapped trigonal-prismatically coordinated alkali-metal atoms. In each, the M atom is coordinated to a tetrahedron of four S atoms and the Bi atom is coordinated to seven S atoms comprising five close S atoms at the corners of a square pyramid with Bi near the center of the basal plane and the sixth and seventh S atoms further away to complete a distorted monocapped trigonal prism. The optical band gaps of 2.23 eV for RbGeBiS₄ and 2.28 eV for CsGeBiS₄ were deduced from their diffuse reflectance spectra. From a band structure calculation, the optical absorption for RbGeBiS₄ originates from the ²∞[GeBiS₄ ^-] layer. The Ge 4p orbitals, Bi 6p orbitals, and S 3p orbitals are highly hybridized.