Synthesis, crystal structures and ionic conductivites of Bi14P4O31 and Bi50V4O85. Two members of the series Bi18-4mM4mO27+4m (M = P, V) related to the fluorite-type structure
Résumé
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) A, b = 11.3698(1) A, c = 52.4082(2) A and β=93.63(1)° (Z=16). The symmetry of Bi50V4O85 is also monoclinic (I2/m space group) with lattice parameters of a = 11.8123(3) A, b = 11.7425(2) A, c = 16.5396(2) A and β=90.14(1)° (Z=2). Both structures correspond to a fluorite-type superstructure 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 [Bi14M4O31] and [Bi18O27] respectively. This new family can be formulated Bi18−4mM4mO27+4m with M=P, V and where the parameter m (0less-than-or-equals, slantmless-than-or-equals, slant1) represents the ratio of the number of [Bi14M4O31] 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 °C is 4 orders of magnitude lower from those found in Bi46M8O89 (M=P, V) (m=2/3) and Bi50V4O85 (m=1/3).