Synthesis and Crystal Structures of Rb 4 Al 2 S 5 and Cs 4 In 2 S 5

Two new alkali metal thiotrielates with the general composition M 4 T 2 S 5 ( M = Rb, Cs; T = Al, In) were obtained via the azide route and their crystal structures were determined from single‐crystal X‐ray diffraction data. Highly air sensitive Rb 4 Al 2 S 5 crystallizes in pale red rods in the space group Pna 2 1 (no. 33) with lattice constants a = 11.4450(5) Å, b = 11.4605(6) Å, c = 9.5727(4) Å, and a cell volume of V = 1255.6(1) Å 3 with Z = 4. To the best of our knowledge, Rb 4 Al 2 S 5 represents the first rubidium thioaluminate and a new structure type in the field of ternary alkali metal chalkogenometallates. Cs 4 In 2 S 5 crystallizes isotypic to Rb 4 In 2 S 5 in the space group P $\bar{1}$ (no. 2) with lattice constants a = 9.6871(3) Å, b = 11.1549(4) Å, c = 7.7879(3) Å, α = 110.731(3)°, β = 95.760(3)°, γ = 102.766(2)°, and a cell volume of V = 752.75(4) Å 3 with Z = 2. The crystal structures of both solids consist of edge‐linked [ T 2 S 6 ] 6– double tetrahedra, which build up one‐dimensional, polymeric 1 ∞ [ T 2 S 5 4– ] strands by trans‐corner sharing. The anionic chains are separated by rubidium and cesium cations, respectively. Aluminum as well as indium has a slightly distorted tetrahedral surrounding by sulfur, and the four independent rubidium atoms in Rb 4 Al 2 S 5 feature distorted trigonal prismatic and distorted octahedral coordination spheres, respectively. In Cs 4 In 2 S 5 the four different cesium positions show coordination numbers of six and seven. Cs 4 In 2 S 5 was also characterized by Raman spectroscopy. The frequencies of In–S stretching modes are found in the range from 342 cm –1 to 292 cm –1 .