Bidimensional cationic ordering and thermal dependence in β-PbxV2O5 bronzes

The monoclinic lead vanadium oxide bronzes β-PbxV2O5 were investigated for their cationic ordering within the host lattice. Superstructure reflections appear on X-ray powder patterns beyond x ∼ 0.30. For the formal x = 0.30 lead radio, the crystal structure was refined from X-ray single crystal data in the (a, 2b, c) unit cell, a = 15.478(10) angstrom, 2b = 7.288(5) angstrom, c = 10.123(6) angstrom, β = 109.29(1)degree, S. G. P21/m and final R = 0.047, Rw = 0.054. It shows evidence of an intratunnel ordering involving a Pb2+ zigzag sequence running parallel to the b axis while ordered and disordered tunnels alternate along the a axis. Pb2+ display different interaction with the V-O framework according to the kind of tunnels it occupies. Electron diffraction patterns confirmed the X-ray results. A supplementary 2a periodicity was sometimes observed in small crystals as a subcondition of the previous ordering. Weissenberg photographs and electronic diffraction patterns allowed the proposition of a bidimensional cationic ordering model in the fourfold (2a, 2b, c) unit cell. In that case, the intertunnel ordering involves strong local Pb2+ electrostatic effects despite the shielding of the tunnel walls. The thermal analysis (DTA and high temperature electronic microscopy) evidenced the irreversible disappearing of the superstructure on heating while a second rearrangement is observed on cooling. 28 refs., 12 figs., 5 tabs