STRUCTURAL AND SPECTROSCOPIC STUDY OF THE (MG, NI)(2)(OH)(ASO4) ARSENATES

Compounds of general formula (Mg2-xNix)(OH)(AsO4) (x=0 and 0.45) have been synthesized and characterized. The crystal structures of these co mpounds have been refined using X-ray powder diffraction data. The str ucture of the compounds contains two kinds of oxygen polyhedra coordin ating to the metal atoms, an octahedron and a trigonal bipyramid. In t he magnesium-nickel hydroxyarsenate the NI(II) ions are preferentially placed in the hexacoordinated polyhedra. Diffuse reflectance study of the magnesium-nickel compound shows bands belonging to the two chromo phores of the Ni(II) ions. The ESR spectrum of the (Mg1.55Ni0.45)(OH)( AsO4) phase doped with 1% Co2+ exhibits signals of this cation in both octahedron and trigonal bipyramid coordination polyhedra. A comparati ve study with the homologous cobalt-hydroxyarsenate phases has also be en carried out. (C) 1997 Academic Press.