The crystal structures of the binary mixed valence compound Bi₃^(III)Bi^(V)O₇ and isotypic Bi₃SbO₇ as determined by high resolution X-ray and neutron powder diffraction

The crystal structures of the mixed valence compound Bi₃^(IIII)Bi^(V)O₇ (1) and isotypic Bi₃^(III)Sb^(V)O₇ (2) were determined ab initio from powder diffraction data. At room temperature, both structures crystallize in space group P1̄ (Z = 2) with a = 6.7253(2), b = 6.9950(2), c = 7.7961(2) Å, α = 72.566(2), β = 88.842(2), γ = 76.925(2)°, and V = 340.39(2) ų for (1) and a = 6.6044(3), b = 7.0146(3), c = 7.6048(3) Å, α = 73.388(2), β = 89.225(2), γ = 76.190(2)°, and V = 327.28(2) ų for (2). Combined Rietveld refinements based on both synchrotron and neutron powder patterns were performed (T = 295 K, R-Bragg = 5.4% for (1), R-Bragg = 6.4% for (2)). The crystal structures contain five crystallographically different anion polyhedra in two different coordination types and can be related to the CaF₂-type structure with the cations forming a distorted face centered cubic lattice and the oxygen atoms filling positions close to seven of the eight tetrahedral cavities.