Synthesis, crystal structures and ionic conductivities of Bi14P4O31 and Bi50V4O85. Two members of the series Bi18−4mM4mO27+4m (M=P, V) related to the fluorite-type structure

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)Å, b=11.3698(1)Å, c=52.4082(2)Å and β=93.63(1)°β=93.63(1)° (Z=16Z=16). The symmetry of Bi50V4O85 is also monoclinic (I2/m   space group) with lattice parameters of a=11.8123(3)Å, b=11.7425(2)Å, c=16.5396(2)Å and β=90.14(1)°β=90.14(1)° (Z=2Z=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   (0⩽m⩽10⩽m⩽1) represents the ratio of the number of [Bi14M4O31] layers to the total number of layers in the sequence. Bi14P4O31 corresponds to m=1m=1 when Bi50V8O85 corresponds to m=1/3m=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=2/3m=2/3) and Bi50V4O85 (m=1/3m=1/3).

Structure of Bi14P4O31Figure optionsDownload as PowerPoint slide