SYNTHESIS, STRUCTURES, MAGNETIC-PROPERTIES, AND PHASE-TRANSITION OF MANGANESE(II)DIVANADATE - MN2V2O7

The alpha-beta phase transition of Mn2V2O7 has been studied with the u se of single crystals grown from hydrothermal reactions. The structure of the high temperature beta-phase (thortveitite) was previously dete rmined from neutron powder diffraction data. Here we report the struct ure determination of a new phase, alpha-Mn2V2O7, and the structure ref inement of beta-Mn2V2O7 from single-crystal X-ray diffraction data. al pha-Mn2V2O7 crystallizes in the space group P (1) over bar (no. 2) wit h a = 6.868(2) Angstrom, b = 7.976(2) Angstrom, c = 10.927(2) Angstrom , alpha = 87.81(1)degrees, beta = 72.14(1)degrees, gamma = 83.08(1)deg rees, V = 564.5(5) Angstrom(3), Z = 4 at T = 293 K. beta-Mn2V2O7 cryst allizes in the space group C2/m (no. 12) with a = 6.7129(6) Angstrom, b 8.7245(5) Angstrom, c = 4.9693(4) Angstrom, beta = 103.591(8)degrees , V = 282.88(4) Angstrom(3), Z = 2 at T = 323 K. beta-Mn2V2O7 adopts t he thortveitite structure containing edge-sharing MnO6 octahedra and c orner-sharing V2O74- divanadate groups, which have staggered conformat ion and linear V-O-b-V. The structure of alpha-Mn2V2O7, differs from t hat of beta-Mn2V2O7 mainly by the bending of V-O-b-V moieties, as obse rved in the low-temperature forms of other thortveitite like compounds . Differential scanning calorimetry (DSC) indicated that a reversible first-order phase transition occurs at similar to 296 K. The low-tempe rature phase exhibits a paramagnetic-antiferromagnetic transition at T -N = 16.0(5)K and follows Curie-Weiss behavior (mu(eff) = 5.9 mu(B)/Mn atom) at higher temperature. A change of Weiss constants (theta p = - 28 K for alpha-Mn2V2O7; -4 K for beta-Mn2V2O7) relevant to the alpha-b eta phase transition was observed between 290 and 300 K. (C) 1996 Acad emic Press, Inc.