A new structure type in the hexagonal perovskite family; structure determination of the modulated misfit compound Sr_9/8TiS₃

Sr_9/8TiS₃, strontium titanium sulfide, a new phase in the hexagonal perovskite-like Sr_xTiS₃ system, has been prepared and its structure solved from single-crystal X-ray data within the (3 + 1)-dimensional [(3 + 1)D] formalism. Sr_9/8TiS₃ crystallizes with trigonal symmetry [R3m(00γ)0s superspace group], with the following lattice parameters: a_s = 11.482 (3), c_s = 2.9843 (8) Å, q = 0.56247 (7)c* and V_s = 340.7 (3) ų. The structure was considered as commensurate [R3c three-dimensional (3D) space group], but refined within the (3 + 1)D formalism to a residual factor R = 2.79% for 64 parameters and 1084 independent reflections. Original crenel functions were used for the sulfur and strontium description. The structure is different from that of the hexagonal perovskite-like oxide counterparts. The main difference is related to the presence of a new type of polyhedron in the [MS₃] transition metal chains, intermediate between the octahedra classically found in such chains and the trigonal prismatic sites encountered in the oxides.