Nanostructured Crystals of Fluorite Phases Sr1 – xRxF2 + x (R Are Rare-Earth Elements) and Their Ordering. 15. Concentration Dependence of the Defect Structure of As Grown Nonstoichiometric Phases Sr1 – xRxF2 + x (R = Sm, Gd)

Abstract

The defect structure of as grown single crystals Sr_1 – xSm_xF_2 + x (x = 0.14, 0.26) and Sr_1 – xGd_xF_2 + x (x = 0.12, 0.19, 0.30) grown under identical conditions was studied by X-ray diffraction. All crystals belong to the CaF₂ structure type, sp. gr. \(Fm\bar {3}m\). All phases contain interstitial fluoride ions in the 48i site and vacancies in the main anionic motif. In Sr_0.70Gd_0.30F_2.30 interstitial fluoride anions were found also in the 4b site; in Sr_0.88Gd_0.12F_2.12, in the 32f site. Relaxation displacements of a part of F_(8c) anions to the 32f site are observed in Sr_0.86Sm_0.14F_2.14. Displacements of cations to the 24e site occur in all the phases under study; in the phases with R = Gd, there are also displacements to the 32f site. A model of the defect structure of the phases Sr_1 ‒ xR_xF_2 + x (R = Sm, Gd) is proposed. According to this model, interstitial fluoride anions and R³⁺ cations are grouped into octacubic clusters [Sr_14 – nR_nF_64 + n]. The average number of R³⁺ per cluster decreases from 6 to 4.5 with increasing х, the volume of the cluster core increases from 61.2(1) to 65.9(2) ų, and the volume per cluster decreases from 2419.9(5) to 783.5(2) ų.