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: , , and β=93.63(1)° (Z=16). The symmetry of Bi₅₀V₄O₈₅ is also monoclinic (I2/m space group) with lattice parameters of , , 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 Bi_18−4mM_4mO_27+4m with M=P, V and where the parameter m (0?m?1) 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).