Investigation into the Existence of Sr3Sn2S7 via Exploratory Synthesis

During our investigation of A3B2Q7 compounds (A = Sr–Ba; B = Si–Sn; Q = O–Te), we solved the mystery of the Sr3Sn2S7 crystal structure. Although Sr3Sn2S7 shares the 3:2:7 stoichiometry with other compounds in the family, it crystallizes in a distinct, new centrosymmetric structure type. Sr3Sn2S7 adopts the monoclinic C2/m space group, with lattice parameters a = 23.207(2) Å, b = 7.9768(8) Å, c = 17.216(3) Å, β = 132.1738(17)°, and Z = 8, rather than the predicted Pbam space group. Unlike previously reported A3B2Q7 compounds, Sr3Sn2S7 is composed of rare SnS5 trigonal bipyramids as well as SnS4 tetrahedra, forming SnS4–SnS5 chains within the crystal structure. Density functional theory (DFT) calculations revealed that the new material is a semiconductor with a calculated band gap of 1.10 eV, while its red color is indicative of a larger gap of 1.8–2.0 eV.