The crystal structure of Ni-5(AsO4)(2)(OH)(4) and its comparison to other M(5)(XO(4))(2)(OH)(4) compounds

The structure of a new nickel arsenate hydroxide, Ni-5(AsO4)(2)(OH)(4), has been determined by single-crystal X-ray diffraction (R(F) = 0.045 for 1469 observed reflections with F-0 > 4 sigma(F-0)). This new compound is isostructural with the mineral reppiaite, Mn-5(VO4)(2)(OH)(4). its unit-cell has C2/m symmetry with the parameters a = 9.291(2), b = 9.008(2), c = 5.149(1) Angstrom, beta = 98.70(3)degrees and Z = 2. The Ni-5(AsO4)(2)(OH)(4) structure is based on a regular cubic close-packing of oxygen atoms and is built of alternating octahedral (NiO6) and tetrahedral (AsO4) layers, stacked in a four-layer repeat along the a direction. Bond valence calculations have been used to locate the OH groups in the structure. The structural similarities and differences with ether M(5)(XO(4))(2)(OH)(4) compounds are described.