Displacements in the cationic motif of nonstoichiometric fluorite phases Ba_{1 − x} R _x F_{2 + x} as a result of the formation of {Ba₈[R ₆F_68–69]} clusters: I. The Ba_0.78Tm_0.22F_2.22 crystals

Abstract

The displacements of Ba²⁺ cations in the cationic motif of Ba_0.78Tm_0.22F_2.22 crystals, which are representatives of nonstoichiometric fluorite phases Ba_{1 − x} R _x F_{2 + x} , are proved for the first time with the use of precision investigations of the fine atomic structure. It is shown that the cation displacements, like the previously revealed displacements in the anionic motif, reflect the formation of {Ba₈[R ₆F_68–69]} superclusters of structural defects with nanometer linear sizes. The Ba ²⁺ cations are displaced from the fluorite crystallographic positions 4a (space group Fm $\bar 3$ m) to the positions 32f. The static nature of the cation displacements is confirmed by the fact that these displacements are retained at a temperature of 110 K. The correctness of the interpretation of the correlation between the cation displacements and the formation of superclusters of structural defects is supported by the coincidence of the intercationic distances determined in the disordered phase Ba_0.78Tm_0.22F_2.22 with those found in the previously studied ordered phases Ba₄ R ₃F₁₇ (R = Y, Yb). The model with splitting of cationic positions is appropriate for testing in structural investigations of crystals of the fluorite phases M _{1 − x} R _x F_{2 + x} at room temperature.