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°C, was characterized by annealing at temperatures higher than 300°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) ? and c=20.321(3) ? (Shelxl program: wR2=7.2%, R1=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 Fi interstitial anions and to Fr 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 36 sheets perpendicular to the [001] axis of the hexagonal cell.