Bidimensional Cationic Ordering and Thermal Dependence in betaPbxV2O5 Bronzes

The monoclinic lead vanadium oxide bronzes -PbxV2O5 were investigated for their cationic ordering within the host lattice. Superstructure reflections appear on X-ray powder patterns beyond x0.30. For the formal x=0.30 lead ratio, the crystal structure was refined from X-ray single crystal data in the (a, 2b, c) unit cell, a=15.478(10) ?, 2b=7.288(5) ?, c=10.123(6) ?, =109.29(1)°, 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 our Xray 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.