The crystal structure of namibite, Cu(BiO) 2 VO 4 (OH), and revision of its symmetry

The crystal structure of namibite, Cu(BiO) 2 VO 4 (OH) [triclinic, space group P1̄, 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) Å 3 , Z = 2] was determined using single-crystal X-ray diffraction data. The refinement, based on 2140 unique reflections with F o > 4σ(F o ), 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 Bi 3+ . A threedimensional network is formed by linkages of Bi-O polyhedra to one slightly distorted V 5+ O 4 tetrahedron that decorates chains of corner-sharing CuO 6 polyhedra extending parallel to the b axis. The CuO 6 polyhedra show the Jahn-Teller-distorted [4+2]-coordination of Cu 2+ . Layers of Bi atoms parallel to (100) alternate with layers of parallel heteropolyhedral [Cu(VO 4 )O 2 (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 (Fe 3+ ,Fe 2+ )O 2 (OH)(PO 4 ). Further relations to synthetic Cu 3 Bi 4 V 2 O 14 , to the 7 Å chain-structure phosphates and sulfates, and to linarite are outlined.