Abstract

The crystal structure of namibite, Cu(BiO)2VO4(OH) [triclinic, space group

\(\mathit{P}\overline{1}\)
, a = 6.210(1), b = 7.398(1), c = 7.471(1) Å, α = 90.10(1), β = 108.73(1), γ = 107.47(1)°, V = 308.22(8) Å3, Z = 2] was determined using single-crystal X-ray diffraction data. The refinement, based on 2140 unique reflections with Fo > 4σ(Fo), gave R1 = 3.61%. The structure determination showed namibite to be triclinic-pseudomonoclinic; the previously reported C-centered monoclinic cell is a pseudocell. The structure contains two unique Bi atoms, with nine Bi-O bonds between 2.17 and 3.39 Å. The Bi coordinations show some stereochemical influence of the lone pair of electrons on Bi3+. A three-dimensional network is formed by linkages of Bi-O polyhedra to one slightly distorted V5+O4 tetrahedron that decorates chains of corner-sharing CuO6 polyhedra extending parallel to the b axis. The CuO6 polyhedra show the Jahn-Teller-distorted [4+2]-coordination of Cu2+. Layers of Bi atoms parallel to (100) alternate with layers of parallel heteropolyhedral [Cu(VO4)O2(OH)] chains. According to bond-valence calculations and geometrical considerations, the H atom of the OH group is probably involved in disordered or trifurcated H bonding. The structure of namibite represents a slightly distorted ordered variant of the monoclinic structure of brendelite, (Bi,Pb)2(Fe3+,Fe2+)O2(OH)(PO4). Further relations to synthetic Cu3Bi4V2O14, to the 7 Å chain-structure phosphates and sulfates, and to linarite are outlined.

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