SPECIFIC FEATURES OF DEFECT STRUCTURE OF Na_0.39Y_0.61F_2.22 CRYSTALS. Zhurova, E.A., Maximov, B.A., Hull S., Keen, D.A., Wilson, C.C., Sobolev B.P., Simonov, V.I. Inst. of Crystallography RAS, Moscow, Russia Rutherford Appleton Laboratory, England

The specific features of structure and electron density distribution have been studied in the temperature range 10-296 K in the Na_0.39Y_0.61F_2.22 fluorite crystal.

The structure refinements of the crystal were realized to R~ 0.5 % for the X-ray experiment and to R ~ 3-7 % for the neutron ones at different temperatures. It was found, that there are three different F atom sites, the main one (8c) F, relaxed (32f) FR with coordinates (0.29, 0.29, 0.29) and supplementary (48i) F' with coordinates (0.5, 0.13, 0.13). Cationic sites are splitted about 0.1 A in the directions of the coordinate axes. If we take into account the multiplicity of the (32f) site, the occupancy of the FR atom per one formula unit is close to Na content, and we can suppose that there are two sublattices in the Na_0.39Y_0.61F_2.22 crystal. One of them contains Y and F atoms, the other one contains Na and FR, and one of the sublattices is displaced relative to the other by a distance of ~ 0.1 A.

The temperature dependence of the lattice parameter indicates a possible change in the crystal structure model at ~110 K. However, such a specific feature was not revealed in the course of structure refinement.

On the deformation electron density maps there are peaks ~ 0.1 e/A3 in height, which are slightly displaced from Na(Y)-F line. They can be due to partial covalent character of the Y-F chemical bond.