DEFECT STRUCTURE OF NA0.39Y0.61F2.22 CRYSTALS

The structure of an Na0.39Y0.61F2.22 crystal has been studied by X-ray and neutron diffraction methods in the temperature range 10-296 K. It has been established that, in this crystal, the fluorine atoms occupy three different positions: (1) the basic (8c) F, (2) the additional p osition (48i) F', and (3) the relaxed position (32f) F-R with the coor dinates (0.29, 0.29, 0.29). The latter can be considered as the result of the displacement of the fluorine atoms along the threefold symmetr y axes from their basic (8c) position. It is established that the cati on position is split for a distance of about 0.1 Angstrom along the co ordinate axes. chemical bonding between cations and fluorine atoms is partly covalent.