Defect structure of Na{sub 0.39}Y{sub 0.61}F{sub 2.22} crystals

Abstract

The structure of an Na{sub 0.38}Y{sub 0.61}F{sub 2.22} 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 position (48i) F, and (3) the relaxed position (32f) F{sup R} with the coordinates (0.29, 0.29, 0.29). The latter can be considered as the result of the displacement of the fluorine atoms along the threefold symmetry axes from their basic (8c) position. It is established that the cation position is split for a distance of about 0.1 A along the coordinate axes. Chemical bonding between cations and fluorine atoms is partly covalent.