Nanostructured Crystals of Fluorite Phases Sr_1 – xR_xF_2 + x (R Are Rare-Earth Elements) and Their Ordering. 13: Crystal Structure of SrF₂ and Concentration Dependence of the Defect Structure of Nonstoichiometric Phase Sr_1 – xLa_xF_2 + x As Grown (x = 0.11, 0.20, 0.32, 0.37, 0.47)

The defect structure of as-grown SrF₂ and nonstoichiometric phases Sr_1 – xLa_xF_2 + x (x = 0.11, 0.20, 0.32, 0.37, 0.47) single crystals, grown from a melt under identical conditions, has been studied by X-ray diffraction analysis. All crystals belong to the CaF₂-type structure, sp. gr. \(Fm\bar {3}m\). Deficit of fluorine anions is found in the 8c site in SrF₂. Interstitial anions are not visualized in SrF₂ in difference electron-density maps. The Sr_1 – xLa_xF_2 + x phases exhibit the presence of vacancies in the main anion motif and interstitial fluorine ions of three types: in two sites 32f (w, w, w) with different coordinates w and in one site 4b. A model of the defect structure of Sr_1 – xLa_xF_2 + x phase is proposed, according to which interstitial fluorine ions and impurity cations La³⁺ are grouped into clusters of the [Sr_1 – nLa_nF₂₆] tetrahedral configuration. Calculations based on structural data revealed that the average number of La³⁺ ions per cluster linearly increases from 2.6 to 3.13 with an increase in the LaF₃ concentration. The average crystal volume corresponding to one cluster decreases from 1170.6(3) to 336.1(5) ų. The volume of the anion cluster core decreases from 2.52(7) to 2.42(7) ų, passing through a minimum in the composition with x = 0.32, which is similar to that of congruently melting phase, and then increases to 2.44(9) ų at х = 0.47. Dynamic thermal displacement of matrix anions in Sr_1 – xLa_xF_2 + x is observed in the [111] direction towards the cubic void center in the anion sublattice. Therefore, according to the mechanism of electrical conductivity, anion jumps are most likely in this direction.