STRUCTURE OF SR2BI2O5 FROM X-RAY AND NEUTRON POWDER DIFFRACTION DATA

Bismuth strontium oxide, Sr2Bi2O5, M(r) = 673.20, orthorhombic, Cmcm, T = 295 K, a = 3.8268 (2), b = 14.3142 (6), c = 6.1724 (4) angstrom, V = 338.11 (2) angstrom3, Z = 2, D(x) = 6.612 (1) Mg m-3 . Rietveld refinement using neutron powder diffraction data [lambda = 2.57171 (7) angstrom, F(000) = 120.26 fm, 55 contributing reflections] resulted in R(wp) = 4.31 %. Around Bi lie four 0 atoms at short distances, forming a tetrahedron, approximately. However, the occupation of two of these positions is mutually exclusive, i.e. the effective coordination is threefold. The resulting umbrella-like BiO3 groups are, on average, linked in pairs. The SrO6 Prisms share edges along [001] giving sheets perpendicular to [010]. Bond distances and angles are consistent with related materials containing stereochemically active lone pairs.