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

The crystal structures of the two oxides Bi46M8O89 (image, V) have been solved from single crystals X-ray data at room temperature. Bi46P8O89 crystallizes in the monoclinic symmetry (space group image) with the cell parameters image, image, image and image. The symmetry of Bi46V8O89 is also monoclinic but the space group is image with the unit-cell parameters: image, image, image and image. Both structures derive from an oxygen deficient fluorite-type structure where the Bi and image cations (image, V) are ordered in the framework. The structures are characterised by isolated imageO4 tetrahedra (image, V) which contradicts the previous results. The difference between the two structures is only due to a different order of the image atoms (image, 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.