Bi(F, O)2.45: AN ANION-EXCESS FLUORITE DEFECT STRUCTURE DERIVING FROM RHOMBOHEDRAL LnFO TYPE

摘要

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 >300 C. Its crystal structure was solved by XRD on a single crystal of composition Bi(F, O)2.45, in the R-3m space group with cell parameters: a=4.1378(9) angstroms and c=20.321(3) angstroms (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. 23 refs.