Crystal Structure Refinements of the Lead(II) Oxoarsenates(V) Pb2As2O7, Pb(H2AsO4)2, Pb5(AsO4)3OH and NaPb4(AsO4)3 from Single-Crystal X-ray Data

Single-crystals of lead(II) oxoarsenates(V) were grown from the melt (Pb2As2O7), from solution (Pb(H2AsO4)(2) and Pb-5(AsO4)(3)OH), and under hydrothermal conditions (NaPb4(AsO4)(3)). Crystal structure refinements from single-crystal X-ray diffraction data revealed isotypism for both Pb2As2O7 and Pb(H2AsO4)(2) with the corresponding barium and phosphate phases. A quantitative comparison of the crystal structures showed a high similarity for the isotypic M2X2O7 structures (M = Pb, Ba; X = As, P), whereas for the M(H2XO4)(2) structures only the pair Pb(H2AsO4)(2) and Pb(H2PO4)(2) is similar, but not Ba(H2AsO4)(2). Pb-5(AsO4)(3)OH adopts the apatite structure type in space group P6(3)/m, with the hydroxyl group disordered around Wyckoff position 2 b (0, 0, 0) in the channels of the structure. NaPb4(AsO4)(3) represents a lacunar apatite with two of the three metal positions occupationally disordered by Pb and Na. In contrast to a previous X-ray powder study of NaPb4(AsO4)(3) that reported an apatite-type structure in space group P6(3)/m, the current single-crystal data clearly revealed a symmetry reduction to space group P3 over bar . Hence, NaPb4(AsO4)(3) is the first lacunar apatite that comprises only tetrahedral anions and adopts the belovite structure type.

Automated summary

Single crystals of lead(II) oxoarsenates(V) were successfully grown and their crystal structures were compared through X-ray diffraction. It was also found that NaPb4(AsO4)(3) is the first lacunar apatite comprising only tetrahedral anions and adopting the belovite structure type.