The crystal structure of namibite, Cu(BiO)(2)VO4(OH), and revision of its symmetry

The crystal structure of namibite, Cu(BiO)(2)VO4(OH) [triclinic, space grou p P (1) over bar, a = 6.210(1), b = 7.398(1), c = 7.471(1) Angstrom, alpha = 90.10(1), beta = 108.73(1), gamma= 107.47(1)degrees, V= 308.22(8) Angstro m(3), Z= 2] was determined using single-crystal X-ray diffraction data. The refinement, based on 2140 unique reflections with F-o > 4 sigma(F-o), gave R1 = 3.61%. The structure determination showed namibite to be triclinic-ps eudomonoclinic; the previously reported C-centered monoclinic cell is a pse udocell. The structure contains two unique Bi atoms, with nine Bi-O bonds b etween 2.17 and 3.39 Angstrom. The Bi coordinations show some stereochemica l influence of the lone pair of electrons on Bi3+. A three-dimensional netw ork is formed by linkages of Bi-O polyhedra to one slightly distorted V5+O4 tetrahedron that decorates chains of corner-sharing CuO6 polyhedra extendi ng parallel to the b axis. The CuO6 polyhedra show the Jahn-Teller-distorte d [4+2]-coordination of Cu2+. Layers of Bi atoms parallel to (100) alternat e with layers of parallel heteropolyhedral [Cu(VO4)O-2(OH)] chains. Accordi ng to bond-valence calculations and geometrical considerations, the 1-1 ato m of the OH group is probably involved in disordered or trifurcated H bondi ng. The structure of namibite represents a slightly distorted ordered varia nt of the monoclinic structure of brendelite, (Bi,Pb)(2)(Fe3+,Fe2+)O-2(OH)( PO4). Further relations to synthetic Cu3Bi4V2O14, to the 7 Angstrom chain-s tructure phosphates and sulfates, and to linarite are outlined.