STRUCTURAL-PROPERTIES OF TRANSITION-METAL PYROARSENATES CO2AS2O7,MN2AS2O7,NI2AS2O7

Single phase powder samples of the transition metal pyroarsenates, M2As2O7 (M = Co, Mn, Ni), have been prepared and analyzed by X-ray diffraction, chemical analysis, high resolution powder neutron diffraction, and differential scanning calorimetry measurements. Co2As2O7 and Ni2As2O7 were found to undergo high temperature structural phase transitions between α- and β-forms at 452 and 691 K, respectively. The high temperature β-modifications of Co2As2O7 and Ni2As2O7 are isomorphous with Sc2Si2O7 (thortveitite) of space group C2 m, with cell constants a = 6.6316(1) Å, b = 8.5541(1) Å, c = 4.7633(1) Å, β = 103.56(1)°, and a = 6.5391(1) Å, b = 8.5007(1) Å, c = 4.7437(1) Å, β = 103.19(1)°, respectively. Mn2As2O7 does not exhibit a high temperature phase transition and adopts the thortveitite structure at room temperature; a = 6.7442(2) Å, b = 8.7545(3) Å, c = 4.8025(2) Å, β = 102.76(1)°. The structures of all three thortveitite phases have been refined, and positional and thermal parameters are given. The α-M2As2O7 (M = Co, Ni) are triclinic (P1 or P1) but the structures could not be refined satisfactorily from powder neutron data. © 1990.