Syntheses, structures, optical properties, and theoretical calculations of Cs₂Bi₂ZnS₅, Cs₂Bi ₂CdS₅, and Cs₂Bi₂MnS₅

Three new compounds, Cs₂Bi₂ZnS₅, Cs ₂Bi₂CdS₅, and Cs₂Bi ₂MnS₅, have been synthesized from the respective elements and a reactive flux Cs₂S₃ at 973K. The compounds are isostructural and crystallize in a new structure type in space group Pnma of the orthorhombic system with four formula units in cells of dimensions at 153K of a=15.763(3), b=4.0965(9), c=18.197(4)Å, V=1175.0(4)ų for Cs₂Bi₂ZnS₅; a=15.817(2), b=4.1782(6), c=18.473(3) Å, V=1220.8(3) ų for Cs₂Bi ₂CdS₅; and a=15.830(2), b=4.1515(5), c=18.372(2)Å, V=1207.4(2)ų for Cs₂Bi₂MnS₅. The structure is composed of two-dimensional _∞ ²[Bi₂MS₅^2-] (M=Zn, Cd, Mn) layers that stack perpendicular to the [100] axis and are separated by Cs⁺ cations. The layers consist of edge-sharing _∞ ¹[Bi₂S₆^6-] and _∞¹[MS₃^4-] chains built from BiS₆ octahedral and MS₄ tetrahedral units. Two crystallographically unique Cs atoms are coordinated to S atoms in octahedral and monocapped trigonal prismatic environments. The structure of Cs ₂Bi₂MS₅, is related to that of Na ₂ZrCu₂S₄ and those of the AMM′Q ₃ materials (A=alkali metal, M=rare-earth or Group 4 element, M′= Group 11 or 12 element, Q=chalcogen). First-principles theoretical calculations indicate that Cs₂Bi₂ZnS₅ and Cs₂Bi₂CdS₅ are semiconductors with indirect band gaps of 1.85 and 1.75eV, respectively. The experimental band gap for Cs₂Bi₂CdS₅ is ≃1.7eV, as derived from its optical absorption spectrum.