Crystal structures of the ionic conductors Bi46M8O89 (M = P, V) related to the fluorite-type structure
Résumé
The crystal structures of the two oxides Bi46M8O89 (M=P, V) have been solved from single crystals X-ray data at room temperature. Bi46P8O89 crystallizes in the monoclinic symmetry (space group C2/m) with the cell parameters a = 19.6073(4) A, b = 11.4181(4) A, c = 21.1119(4) A and β=112.14(3)°. The symmetry of Bi46V8O89 is also monoclinic but the space group is P21/c with the unit-cell parameters: a = 20.0100(4) A, b = 11.6445(4) A, c = 20.4136(4) A and β=107.27(3)°. Both structures derive from an oxygen deficient fluorite-type structure where the Bi and M cations (M=P, V) are ordered in the framework. The structures are characterised by isolated MO4 tetrahedra (M=P, V) which contradicts the previous results. The difference between the two structures is only due to a different order of the M atoms (M=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.