Bi(F, O)2.45: An Anion-Excess Fluorite Defect Structure Deriving from Rhombohedral LnFO Type
✍ Scribed by J.P. Laval; J.C. Champarnaud-Mesjard; A. Britel; A. Mikou
- Publisher
- Elsevier Science
- Year
- 1999
- Tongue
- English
- Weight
- 558 KB
- Volume
- 146
- Category
- Article
- ISSN
- 0022-4596
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✦ Synopsis
During a reinvestigation of the Bi 2 O 3 +BiF 3 system, a nonstoichiometric oxide6uoride, of composition range Bi(F, O) 2.50 +Bi(F, O) 2.43 at 5003C, was characterized by annealing at temperatures higher than 3003C.
Its crystal structure was solved by X-ray di4raction on a single crystal of composition Bi(F, O) 2.45 , in the R-3m space group with cell parameters: a ؍ 4.1378(9) A s and c ؍ 20.321(3) A s (Shelxl program: wR 2 ,%2.7؍ R 1 .)%5.3؍
The structure of BiF 1.90 O 0.55 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 LaF 1؉2x O 1؊x and the orthorhombic SmF 1؉2x O 1؊x 6uorite-related phases, the clustering mainly a4ects 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 (Bi 2 F 4 O 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.