Crystal structures of the ionic conductors Bi46M8O89 (M=P, V) related to the fluorite-type structure

The crystal structures of the two oxides Bi46M8O89 (M=PM=P, V) have been solved from single crystals X-ray data at room temperature. Bi46P8O89 crystallizes in the monoclinic symmetry (space group C2/mC2/m) with the cell parameters a=19.6073(4)Å, b=11.4181(4)Å, c=21.1119(4)Å and β=112.14(3)°β=112.14(3)°. The symmetry of Bi46V8O89 is also monoclinic but the space group is P21/cP21/c with the unit-cell parameters: a=20.0100(4)Å, b=11.6445(4)Å, c=20.4136(4)Å and β=107.27(3)°β=107.27(3)°. Both structures derive from an oxygen deficient fluorite-type structure where the Bi and MM cations (M=PM=P, V) are ordered in the framework. The structures are characterised by isolated MMO4 tetrahedra (M=PM=P, V) which contradicts the previous results. The difference between the two structures is only due to a different order of the MM atoms (M=PM=P, V) in the fluorite-type superstructure. It will be shown that some oxygen sites are partially occupied in both structures which can explain the ion conduction properties of these phases. A structural building principle will be proposed that can explain the large domain of solid solution related to the fluorite-type observed in both systems.

View of the [Bi18O27] and [Bi14V4O31] layers in Bi46V8O89. The oxygen atoms have been omitted.Figure optionsDownload as PowerPoint slide