Crystal structures of the ionic conductors Bi 46M8O 89 ( M=P, V) related to the fluorite-type structure

Abstract

The crystal structures of the two oxides Bi 46 M 8O 89 ( M = P , V) have been solved from single crystals X-ray data at room temperature. Bi 46P 8O 89 crystallizes in the monoclinic symmetry (space group C 2 / 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 ) ° . The symmetry of Bi 46V 8O 89 is also monoclinic but the space group is P 2 1 / 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 ) ° . 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 M O 4 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.