Modeling and Rietveld-refinement of the crystal structure of Bi4Ta2O11 based on that of Bi7Ta3O18

A P (1) over bar model for the crystal structure of Bi4Ta2O11 has been prop osed based on units from the P1 crystal structure of Bi7Ta3O38. The model w as found to be plausible in terms of bond valence sums. When refined via th e 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 k 0) planes (perpendicular to [111](fluorite)) Regular TaO6 octahedra form co rner-connected columns. Bismuth atoms are in high coordination environments similar to those found in fluorite-type. Bi7Ta3O18 and Bi4Ta2O11 are discu ssed in terms of their structural relationships to one another, to adjacent phases in the Bi2O3-Ta2O5 system, and to structures proposed by earlier au thors based on an archetypal fluorite-type substructure. (C) 1999 Academic Press.