Bi(F, O)2.45:: An anion-excess fluorite defect structure deriving from rhombohedral LnFO type

During a reinvestigation of the Bi2O3-BiF3 system, a nonstoichiometric oxidefluoride, of composition range BI(F, O)(2.50)-Bi(F, O)(2.43) at 500 degrees C, was characterized by annealing at temperatures higher than 300 degrees C. Its crystal structure was solved by X-ray diffraction on a single crystal of composition Bi(F, O)(2.45) in the R-3m space group with cell parameters: a = 4.1378(9) Angstrom and c = 20.321(3) Angstrom (Shelxl program: wR(2) = 7.2%, R-1 = 3.5%), The structure of BiF1.90O0.55 derives from the well-known rhombohedral LnFO type by formation of 1:0:3 (or 1:0:2) clusters orientated along the [001] axis of the hexagonal associated cell in a partly ordered way. This orientation of the clusters preserves the O/F long range order characteristic of the LnFO type, despite a statistical replacement of almost half the O anions by F ones. As in the tetragonal anion-excess LaF1+2xO1-x and the orthorhombic SmF1+2xO1-x fluorite-related phases, the clustering mainly affects the F site: F anionic vacancies are associated to F-i interstitial anions and to F-r anions relaxed from the normal F site. In the clusters, Bi cations are in 9-fold or 10-fold coordination and the higher limit of anionic insertion (Bi2F4O composition) corresponds to a complete juxtaposition of 1:0:3 clusters, Near this limit, the excess anions tend to form quasi-continuous irregular 3(6) sheets perpendicular to the [001] axis of the hexagonal cell. (C) 1999 Academic Press.