STRUCTURE OF A NONSTOICHIOMETRIC BA0.78ND0.22F2.22 CRYSTAL

Ba0.78Nd0.22F2.22 single crystals are grown from melt by the Stockbarg er technique. The structure of the fluorite-type cubic phase was studi ed by the X-ray diffraction method. The unit cell parameters ate a = 6 .098 and a = 6.075 Angstrom at T = 296 and T = 170 K, respectively; sp . gr. Fm3m. The single-crystal diffraction data indicate the statistic al filling of the main anionic position (gel (1/4 1/4 1/4) by fluorine atoms: it is occupied with the probability 0.869. The difference elec tron-density maps and the structure refinement indicate that two addit ional crystallographic positions are filled with fluorine atoms: (48i) on the twofold axis occupied with the probability 0.037 and (4b) at t he center of an empty cube occupied with the probability 0.033. The be havior of the structure parameters of the crystal in the transition fr om room temperature to 170 K is also studied.