The crystal structures of the binary mixed valence compound (Bi3BiO7)-Bi-(III)-O-(V) and isotypic Bi3SbO7 as determined by high resolution X-ray and neutron powder diffraction

The crystal structures of the mixed valence compound (Bi3BiO7)-Bi-(IIII)-O-(v) (1) and isotypic (Bi3SbO7)-Sb-(IIII)-O-(v) (2) were determined ab initio from powder diffraction data. At room temperature, both structures crystallize in space group P (1) over bar (Z=2) with a=6.7253(2), b=6.9950(2), c=7.7961(2)Angstrom, alpha = 72.566(2), beta = 88.842(2), gamma = 76.925(2)degrees, and V= 340.39(2) Angstrom(3) for (1) and a = 6.6044(3), h = 7.0146(3), c = 7.6048(3) Angstrom, alpha = 73.388(2), beta = 89.225(2), gamma = 76.190(2)degrees, and V= 327.28(2) Angstrom(3) 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. (C) 2002 Elsevier Science.