X-RAY STUDY OF ANION-EXCESSIVE FLUORITE-TYPE PHASE CA0.80DY0.20F2.20

Specific features of single crystals of the anion-excessive fluorite-t ype phase of the composition Ca0.80Dy0.20F2.20 [sp. gr. Fm3m, Z = 4, a = 5.5115(3) Angstrom] have been studied using X-ray diffraction data (lambda AgKalpha radiation). The structure is refined in the anharmoni c approximation of the atomic thermal vibrations up to R = 0.0070. The number of crystallographically nonequivalent structure factors is 106 , the number of the parameters refined is 19, and max sin theta/lambda - 1.28 Angstrom(-1). It is established that interstitial fluoride ato ms in the structure statistically occupy the positions 48i (1/2, u, u) with u = 0.119(3) and 32f (v, v, v) with v = 0.415(2) located on two- and threefold symmetry axes, respectively. Some fluoride atoms are di splaced from the basic 8c position along the threefold axis to the pos ition 32f (w, w, w) with w = 0.323(2).