Syntheses, Structures, and Properties of Six Novel Alkali Metal Tin Sulfides: K₂Sn₂S₈, α-Rb₂Sn₂S₈, β-Rb₂Sn₂S₈, K₂Sn₂S₅, Cs₂Sn₂S₆, and Cs₂SnS₁₄

Six alkali metal tin polysulfides and one monosulfide, K₂Sn₂S₈ (I), α-Rb₂Sn₂S₈ (II), β-Rb₂Sn₂S₈ (III), K₂Sn₂S₅ (IV), Cs₂Sn₂S₆ (V), and Cs₂SnS₁₄ (VI), respectively, were synthesized by a molten salt technique. I and IV were made by heating mixtures of Sn/K₂S/S (molar ratio 1/2/8) at 275 and 320 °C, respectively, for 4–6 days. II and III were made by heating mixtures of Sn/Rb₂S/S (molar ratio 1/2/12 for II and 1/1/8 for III at 330 and 450 °C, respectively, for 4–6 days. V and VI were made by heating mixtures of Sn/Cs₂S/S (molar ratio 1/3/8 for V and 1/2/8–12 for VI at 275 °C for 4–6 days. The crystals form in a K₂S_X, Rb₂S_x, and Cs₂S_x flux, respectively. Orange crystals of I crystallize in the monoclinic space group P2₁/n with a = 9.580(8) Å, b = 10.004(5) Å, c = 14.131(7) Å, β = 107.82(6)°, and Z = 4. Orange crystals of II and III have the same anionic frameworks as I. II crystallizes also in the monoclinic space group P2₁/n with a = 9.788(3) Å, b = 9.978(3) Å, c = 14.360(2) Å, and β = 106.70(2)°, and Z = 4. III crystallizes in the orthorhombic space group Pbcn with a = 9.987(5) Å, b = 19.635(3) Å, c = 13.747(3) Å, and Z = 8. The yellow-orange IV crystallizes in the monoclinic space group C2/c with a = 11.804(3) Å, b = 7.808(1) Å, c = 11.539(1) Å, β = 108.35(1)°, and Z = 4. The yellow V crystallizes in the triclinic space group P1̄ with a = 7.289(4) Å, b = 7.597(3) Å, c = 6.796(3) Å, α = 114.80(3)°, β = 108.56(4)°, γ = 97.54(4)°, and Z = 1. Red crystals of VI are monoclinic, space group P2₁/n, with a = 6.964(6) Å, b = 18.66(1) Å, c = 14.80(1) Å, β = 99.39(1)°, and Z = 4. The structures of these six compounds have been determined by single-crystal X-ray diffraction analysis. IR and Raman spectra for these compounds are reported. I–III have novel two-dimensional structures. Each [Sn₂S₈]_n²ⁿ⁻ layer is composed of [Sn₂S₄]_n parallel chains, which contain octahedral SnS₆ and tetrahedral SnS₄, cross-linked by S₄²⁻ ligands. Charge-compensating potassium or rubidium cations are found between the layers. IV has the Tl₂Sn₂S₅ structure type and has a three-dimensional structure, with [SnS₃]_n²ⁿ⁻ chains formed by distorted SnS₅ trigonal bipyramids sharing two of their common edges with one another. Those chains are then cross-linked by sharing the remaining vertices of the trigonal bipyramids to generate parallel tunnels in which potassium cations are located. The structure of V is closely related to IV. It also comprises [SnS₃]_n²ⁿ⁻ chains which in a different fashion are cross-linked by S₂²⁻ to form an extended two-dimensional structure. VI contains a molecular [SnS₁₄]²⁻ complex anion with octahedral Sn⁴⁺ ligated by two S₄²⁻ and one S₆²⁻ chelating ligands. The UV/vis optical properties of I–V are reported. The optical band gaps are 2.15 eV for I–III, 2.36 eV for IV, and 2.44 eV for V.