Non-stoichiometric compounds, with the compositional range of Bi(F,O)2.50 to (F,O)2.43 at 500C, were characterized by annealing at temperatures above 300C. The crystal structure was determined by the X-ray diffractometry of a single crystal with the composition: Bi(F,O)2.45. It belonged to the space group, R-3m, with the cell parameters: a = 0.41378 and c = 2.0321nm. The structure of BiF1.90O0.55 was obtained from the well-known rhombohedral LnFO-type structure via the formation of 1:0:3 (or 1:0:2) clusters which were oriented along the [001] axis of the associated hexagonal cell in a partially ordered way. This orientation of the clusters preserved the O/F long-range order which was characteristic of LnFO, in spite of a statistical replacement of almost half of the O anions by F. As in the case of the tetragonal anion-excess LaF1+2xO1-x and the orthorhombic SmF1+2xO1-x fluorite-related phases, the clustering mainly affected the F site. That is, F anionic vacancies were associated with Fi interstitial anions and with Fr anions which were relaxed from the normal F site. The Bi cations had a 9-fold or 10-fold coordination in clusters, and a higher limit of anionic insertion (Bi2F4O composition) corresponded to a complete juxtaposition of 1:0:3 clusters.

An Anion-Excess Fluorite Defect Structure Derived from Rhombohedral Type. J.P.Laval, J.C.Champarnaud-Mesjard, A.Britel, A.Mikou: Journal of Solid State Chemistry, 1999, 146[1], 51-9