Defect Crystal Structure of (R = Dy–Lu, Y) According to X-ray and Electron Diffraction Data. II: Defect Structure of the α-Na0.4R0.6F2.2 (R = Ho–Lu, Y) Nanostructured Crystals.

The α-Na_0.4R_0.6F_2.2 crystals (R = Ho–Lu, Y) have been studied by X-ray diffraction analysis at 293 and 85 K. A unified cluster model of nanostructured crystals with a fluorite-type structure, based on the polymorphism of KR₃F₁₀ (R = Er, Yb), was used to model their defect structure. The α-Na_0.4R_0.6F_2.2 matrix component contains Na⁺ and R³⁺ ions in a ratio of 1 : 1. Part of the matrix anions are shifted from the 8c to 32f site (sp. gr. ). Excess R³⁺ cations form, jointly with Na⁺, octa-cubic clusters with cores in the form of cuboctahedra {F₁₂}, consisting of interstitial anions at the 48i site. The α-Na_0.4R_0.6F_2.2 cluster component is formed by octa-cubic clusters of type i. The electron diffraction study showed that the clusters are shaped as plates about 5 nm thick with superstructural ordering. Their structural model based on the K_0.265Gd_0.735F_2.47 structure was proposed. Experimental confirmation of the affiliation of to nanostructured crystals was obtained for the first time by electron diffraction. When temperature decreases from 293 to 85 K, the type of the cluster component of the defect α-Na_0.4R_0.6F_2.2 structure with R = Ho–Lu, Y does not change. At 293 K, the boundary of the change in the defect structure type in the series is located between R = Dy (Z = 66) and Ho (Z = 67). With a decrease in temperature from 293 to 85 K the position of the boundary does not change. [ABSTRACT FROM AUTHOR]