CRYSTAL-STRUCTURES AND CHARACTERIZATION OF A NEW MANGANESE(III) ARSENATE, MNASO4.1.2H2O, AND MANGANESE(II) PYROARSENATE, MN2AS2O7

The synthesis and crystal structure of a new manganese(III) arsenate, MnAsO4.1.2H2O, are reported. This compound crystallizes in a monoclinic space group, C2/c (a = 7.030 (1) angstrom, b = 7.824 (1) angstrom, c = 7.469 (1) angstrom, beta = 112.14 (1)-degrees, Z = 4) and is isostructural with MnPO4.H2O. The structure has been refined from laboratory X-ray powder diffraction data by the Rietveld method (R(I) = 3.3%, R(WP) = 11.0%) and even the H position has been refined without constraints, showing that the water molecule is hydrogen-bonded to the arsenate groups. Whether the additional 0.2 water molecules are also present in the structure or in a second amorphous phase is unclear. A strong Jahn-Teller effect was found, in agreement with the diffuse-reflectance spectrum, and the TGA-DTA curves and the IR spectrum of the manganese(III) arsenate hydrate are discussed. The thermal decomposition product of MnAsO4.1.2H2O, manganese(II) pyroarsenate, Mn2As2O7, crystallizes with a thortveitite-type structure (a = 6.7493 (8) angstrom, b = 8.7589 (8) angstrom, c = 4.7991 (5) angstrom, beta = 102.83 (1)-degrees, Z = 2, space group C2/m) and has been refined (R(I) = 2.9%, R(WP) = 6.7%) with a split-site model to take account of the disorder of the bridging oxygen due to bending of the pyroarsenate groups.