Crystal Structures of MBi2Br7 (M = Rb, Cs) - Filled Variants of AX7 Sphere Packing

The reinvestigation of the pseudo-binary systems MBrBiBr3 (M = Rb, Cs) revealed two new phases with composition MBi2Br7. 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 (1) over bar. The lattice parameters are a = 755.68(3) pm, b = 952.56(3) pm, c = 1044.00(4) pm, alpha = 76.400(2)degrees, beta = 84.590(2)degrees, gamma = 76.652(2)degrees for RbBi2Br7 and a = 758.71(5) pm, b = 958.23(7) pm, c = 1060.24(7) pm, alpha = 76.194(3)degrees, beta = 83.844(4)degrees, gamma = 76.338(3)degrees for CsBi2Br7. The crystal structures consist of M+ cations in anticuboctahedral coordination by bromide ions and bromido-bismuthate(III) layers (2)(infinity)[Bi2Br7](-). The 2D layers comprise pairs of BiBr6 octahedra sharing a common edge. The Bi2Br10 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 BiBr6 octahedra are distorted. The CsBi2Br7 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 CsBiBr4 could not be verified experimentally.