KBiMS4 (M=Si, Ge): Synthesis, structure, and electronic structure

Two new bismuth sulfides KBiSiS4 and KBiGeS4 have been synthesized by means of the reactive flux method. They adopt the RbBiSiS4 structure type and crystallize in space group P21/c of the monoclinic system. The structure consists of ∞2[BiMS4-] (M=Si, Ge) layers separated by bicapped trigonal-prismatically coordinated K atoms. The M atom is tetrahedrally coordinated to four S atoms and the Bi atom is coordinated to a distorted monocapped trigonal prism of seven S atoms. The optical band gap of 2.25(2) eV for KBiSiS4 was deduced from the diffuse reflectance spectrum. From a band structure calculation, the optical absorption for KBiSiS4 originates from the ∞2[BiSiS4-] layer. The Si 3p orbitals, Bi 6p orbitals, and S 3p orbitals are highly hybridized near the Fermi level. The orbitals of K have no contributions on both the upper of valence band and the bottom of conduction band. - Graphical abstract: Two new bismuth sulfides KBiSiS4 and KBiGeS4 have been synthesized and characterized. The figure is the view down [0 1 0] of the crystal structure of KBiSiS4.