Defect Crystal Structure of \(\alpha {\text{-N}}{{{\text{a}}}_{{0.5-x}}}{{R}_{{0.5 + x}}}{{{\text{F}}}_{{2 + 2x}}}\) (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

Abstract

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. \(Fm\bar {3}m\)). 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  \(\alpha {\text{-N}}{{{\text{a}}}_{{0.5-x}}}{{R}_{{0.5 + x}}}{{{\text{F}}}_{{2 + 2x}}}\) 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 \(\alpha {\text{-N}}{{{\text{a}}}_{{0.5-x}}}{{R}_{{0.5 + x}}}{{{\text{F}}}_{{2 + 2x}}}\) 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.