Structural Variety of Defect Perovskite Variants M3E2X9 (M = Rb, Tl, E = Bi, Sb, X = Br, I)

Abstract The compounds Rb3Sb2Br9, Rb3Sb2I9, Rb3Bi2Br9, Rb3Bi2I9, and Tl3Bi2Br9 were synthesized and their crystal structures determined from single crystal X‐ray diffraction data. The compounds Rb3Sb2Br9, Rb3Sb2I9, and Rb3Bi2I9 crystallize in the Tl3Bi2I9 type of structure (space group P21/n, no. 14). Rb3Bi2Br9 and Tl3Bi2Br9 crystallize in a new but closely related type of structure (space group P21/a, no. 14). Both structure types feature characteristic double layers comprising corner‐sharing EX6 octahedra. The space groups are set in a way that the stacking direction of the layers is the [001] direction. The group‐subgroup relations to cubic perovskite ABO3 are discussed. Differences between M3E2X9 types are attributed to distortions of the underlying MX3 close packing. Depending on the atomic size ratio, the distortions are quantified by an order parameter.