Defect fluorite structures containing Bi2O3: the system Bi2O3-Nb2O5

A previously described solid solution in the system Bi2O3-Nb2O5 based on the δ-Bi2O3 structure has been reinvestigated by electron diffraction and high resolution electron microscopy. Four distinct phases have been found, all of which are based upon defect fluorite structures, and the structural principles of the phase with the greatest compositional range have been elucidated. This involves an adaption of elements of the pyrochlore structure with an ordering of niobium-oxygen octahedra into groups lying on the {111} planes of an enlarged cubic unit cell, separated by a matrix of δ-Bi2O3 structure.