Nanostructured Crystals of Fluorite Phases Sr_1 – xR_xF_2 + x (R Are Rare-Earth Elements) and Their Ordering. 14: Concentration Dependence of the Defect Structure of Nonstoichiometric Phases Sr_1 – xNd_xF_2 + x As Grown (x = 0.10, 0.25, 0.40, 0.50)

Abstract

The defect structure of as grown Sr_1 – xNd_xF_2 + x (x = 0.10–0.50) crystals grown from a melt by directional solidification under identical conditions has been studied by X-ray diffraction. All crystals belong to the CaF₂-type structure (sp. gr. $Fm\bar {3}m$). The change in the type of structural defect cluster with a change in the NdF₃ content, which occurs in some range of x rather than stepwise, was traced for the first time in the entire homogeneity range of fluorite nonstoichiometric phase Sr_1 – xNd_xF_2 + x. Clusters of two types coexist in crystals with х = 0.10 and 0.25: tetrahedral [Sr_4 – nNd_nF₂₆] and cuboctahedral [Sr_14 – nNd_nF_64 + n]. With a further increase in the NdF₃ content in the Sr_1 – xNd_xF_2 + x structure, a transition from two types of clusters to only one type occurs, and only tetrahedral clusters are formed in crystals with х > 0.40.