Modeling and Rietveld-Refinement of the Crystal Structure of Bi4Ta2O11Based on That of Bi7Ta3O18

AP model for the crystal structure of Bi4Ta2O11has been proposed based on units from theP1 crystal structure of Bi7Ta3O38. The model was found to be plausible in terms of bond valence sums. When refined via the Rietveld method using X-ray (synchrotron) and neutron powder diffraction data, the final structure had greatly improved bond valence sums. The metal atom array approximates fluorite-type with regular "step" defects on (h k0) planes (perpendicular to [111]fluorite). Regular TaO6octahedra form corner-connected columns. Bismuth atoms are in high coordination environments similar to those found in fluorite-type. Bi7Ta3O18and Bi4Ta2O11are discussed in terms of their structural relationships to one another, to adjacent phases in the Bi2O3–Ta2O5system, and to structures proposed by earlier authors based on an archetypal fluorite-type substructure.