STRUCTURES OF ORTHORHOMBIC AND MONOCLINIC NI3(ASO4)2

The crystal structures of two hydrothermally grown polymorphs of nickel orthoarsenate omicron- and m-Ni3(AsO4)2 have been determined by single-crystal X-ray diffraction. Crystal data: omicron-Ni3(AsO4)2, M(r) = 454.01, orthorhombic, Cmca, Z = 4, F(000) = 859.95, lambda-(mo K-alpha) = 0.71069 angstrom, T = 295 (1) K, a = 5.943 (2), b = 11.263 (4), c = 8.164 (3) angstrom, V = 546.5 (3) angstrom3, D(x) = 5.517 g cm-3, mu-(mo K-alpha) = 228.69 cm-1, R = 0.035, wR = 0.030 for 439 independent reflections; m-Ni3(AsO4)2, monoclinic, P2(1)/c, Z = 4, F(000) = 859.95, lambda-(Mo K-alpha) = 0.71069 angstrom, T = 295 (1) K, a = 5.764 (1), b = 9.559 (2), c = 10.194 (2) angstrom, beta = 92.95-degrees, V = 560.9 (1) angstrom3, D(x) = 5.374 g cm-3, mu-(Mo K-alpha) = 222.76 cm-1, R = 0.040, wR = 0.031 for 1640 independent reflections. The crystal chemistry of nickel orthoarsenate is intermediate between those of the corresponding phosphate and vanadate, and its dimorphism is closely related to the olivine-spinel phase relations.