Synthesis, crystal structures and ionic conductivities of Bi14P4O31 and Bi50V4O85. Two members of the series Bi18-4mM4m,O27+4m (M = P, V) related to the fluorite-type structure

摘要

The two hitherto unknown compounds Bi14P4O31 and Bi50V4O85 were prepared by the direct solid-state reaction of Bi2O3 and (NH4)H2PO4 or V2O5, respectively. Bi14P4O31 crystallizes in a C-centred monoclinic symmetry (C2/c space group) with the unit-cell parameters: a = 19.2745(2) angstrom, b = 11.3698(1) angstrom, c = 52.4082(2) angstrom and beta = 93.63(1)degrees (Z = 16). The syrrimetry of Bi5V4O85 is also monoclinic (I2/m space group) with lattice parameters of a = 11.8123(3) angstrom, b = 11.7425(2) angstrom, c = 16.5396(2) angstrom and beta = 90.14(1)degrees (Z = 2). Both structures correspond to a fluorite-type superstucture where the Bi and P or V atoms are ordered in the framework. An idealized structural model is proposed where the structures result of the stacking of mixed atomic layers of composition [Bi4M4O31] and [Bi18O27] respectively. This new family can be formulated Bi18-4mM4mO27+4m with M = P, V and where the parameter m (0 <= m <= 1) represents the ratio of the number of [Bi(50)V(4)4O(31)] layers to the total number of layers in the sequence. Bi14P4O31 corresponds to m = 1 when Bi50V8O85 corresponds to m = 1/3. In this last case, the structural sequence is simply one [Bi14V4O31] layer to two [Bi18O27] layers. As predicted by the proposed structural building principle, Bi14P4O31 is not a good ionic conductor. The conductivity at 650 degrees C is 4 orders of magnitude lower from those found in Bi46M8O89 W = P, V) (m = 2/3) and Bi5OV4085 (in = 1/3). (c) 2005 Elsevier Inc. All rights reserved.