New Layered Compounds through Polysulfide Flux Synthesis;A₂Sn₄S₉(A=K, Rb, Cs) Present a New Form of the [Sn₄S₉]^2-Network

Crystals of Cs₂Sn₄S₉, Rb₂Sn₄S₉, and K₂Sn₄S₉were synthesized by reacting Sn with Cs₂S_x, Rb₂S_x, or K₂S_xfluxes at 500°°C. They can also be prepared as microcrystalline products through direct combination reactions at temperatures ranging from 720 to 750°C. The new ternary sulfide Cs₂Sn₄S₉crystallizes in the orthorhombicPnmaspace group, the new ternary sulfide Rb₂Sn₄S₉crystallizes in the orthorhombic P2₁2₁2₁space group, and the new ternary sulfide K₂Sn₄S₉appears to be isostructural with Rb₂Sn₄S₉, as determined through X-ray powder diffraction. The structures of Cs₂Sn₄S₉, Rb₂Sn₄S₉, and K₂Sn₄S₉consist of nearly identical [Sn₄S₉]^2-layers which are stacked in two different arrangements. Alkali metal Cs⁺, Rb⁺, and K⁺cations are located between the layers. Raman and far-IR spectroscopic data confirm the presence of the [Sn₄S₉]^2-layer in all three compounds. Single crystal optical spectra indicate that Cs₂Sn₄S₉, Rb₂Sn₄S₉, and K₂Sn₄S₉are wide band-gap semiconductors with bandgaps of 2.66 eV, 2.65 eV, and 2.66 eV, respectively.