LAUSR

The defect structure of as grown Sr1 – xNdxF2 + 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 Sr1 – xNdxF2 + x. Clusters of two types coexist in crystals with х = 0.10 and 0.25: tetrahedral [Sr4 – nNdnF26] and cuboctahedral [Sr14 – nNdnF64 + n]. With a further increase in the NdF3 content in the Sr1 – xNdxF2 + 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.