X-RAY STUDY OF NONSTOICHIOMETRIC FLUORITE-TYPE CA0.65ND0.35F2.35 SINGLE-CRYSTALS

Specific features of single crystals of the nonstoichiometric fluorite -type phase of the compositions Ca0.65Nd0.35F2.35 [Sp. gr. Fm3m, Z = 4 , a = 5.6095(3) Angstrom] have been studied using X-ray diffraction da ta (lambda AgKalpha radiation). The structure is refined in the anharm onic approximation of atomic thermal vibrations up to R = 0.0087 (the number of crystallographically nonequivalent structure factors 98, the number of the parameters refined 14, sin theta/lambda less than or eq ual to 1.24 Angstrom(-1)). The main structural features that distingui sh this phase from the defect-free CaF2 phase are determined. It is es tablished that interstitial atoms in the structure statistically occup y the crystallographic positions gc. Fluorine atoms liberated from the position sc and fluorine atoms compensating the substitution of Ca2by Nd3+ cations are located on threefold axes in the centers of cubic voids of the fluorite structure in the position F '' 32f (v, v, v) wit h v = 0.415(2). Moreover, in the vicinity of the basic fluorine positi on, one can reliably localize F''' atoms in the position 32f (w, w, w) with w = 0.320(2). The corresponding peaks should be interpreted as r elaxation of some anions from the basic position 8c along the threefol d axis.