Crystal Structures of M Bi 2 Br 7 ( M = Rb, Cs) – Filled Variants of AX 7 Sphere Packing

The reinvestigation of the pseudo‐binary systems M Br–BiBr 3 ( M = Rb, Cs) revealed two new phases with composition M Bi 2 Br 7 . Both compounds are hygroscopic and show brilliant yellow color. The crystal structures were solved from X‐ray single crystal diffraction data. The isostructural compounds adopt a new structure type in the triclinic space group P $\bar{1}$ . The lattice parameters are a = 755.68(3) pm, b = 952.56(3) pm, c = 1044.00(4) pm, α = 76.400(2)°, β = 84.590(2)°, γ = 76.652(2)° for RbBi 2 Br 7 and a = 758.71(5) pm, b = 958.23(7) pm, c = 1060.24(7) pm, α = 76.194(3)°, β = 83.844(4)°, γ = 76.338(3)° for CsBi 2 Br 7 . The crystal structures consist of M + cations in anticuboctahedral coordination by bromide ions and bromidobismuthate(III) layers 2 ∞ [Bi 2 Br 7 ] – . The 2D layers comprise pairs of BiBr 6 octahedra sharing a common edge. The Bi 2 Br 10 double octahedra are further connected by common vertices. The bismuth(III) atoms increase their mutual distance in the double octahedra by off‐centering so that the BiBr 6 octahedra are distorted. The CsBi 2 Br 7 type can be interpreted as a common hexagonal close sphere packing of M and Br atoms, in which 1/4 of the octahedral voids are filled by Bi atoms. The structure type was systematically analyzed and compared with alternative types of common packings. The existence of a compound with the suggested composition CsBiBr 4 could not be verified experimentally.