Structural relationships among bismuth-rich phases in the Bi2O3-Nb2O5, Bi2O3-Ta2O5, Bi2O3-MoO3 and Bi2O3-WO3 systems

Abstract

The crystal structures of bismuth-rich phases in the Bi2O3-M2O5 (M = Nb, Ta) and Bi2O3-M2O6 (M = W, Mo) systems are investigated, and the most probable model for the crystal structure of each phase is used to propose a crystal-chemical scheme unifying key features of these structures in terms of their relationships to a fluorite-type δ-Bi2O3-related parent structure. This scheme is based on the incorporation of pyrochlore-type structural motifs (e.g., strings of M atoms along 〈110〉F directions) into the underlying fluorite-type metal atom array. The relative orientations of these M atom strings and their interactions are driven by the competing requirements of overall charge-balance (a function of mol% M2O5 or M2O6) and local coordination environment (a function of the element M) within the flexible fluorite-type oxygen atom array. These interactions interrelate the oxygen atom array, the metal atom array, and the composition. As the composition moves further from pure Bi2O3, they force the abandonment of pyrochlore-type structural motifs, and hence any meaningful relationship to a fluorite-type δ-Bi2O3-related parent structure is lost. The descriptive and predictive utilities of this crystal-chemical scheme are discussed.