Nanostructured Crystals of Fluorite Phases Sr-1 (-) (x)R(x)F2+x (R Are Rare-Earth Elements) and Their Ordering. 15. Concentration Dependence of the Defect Structure of As Grown Nonstoichiometric Phases Sr1-xR(x)F2+x (R = Sm, Gd)

摘要

The defect structure of as grown single crystals Sr1 - xSmxF2 + x (x = 0.14, 0.26) and Sr1 - xGdxF2 + 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 (3) over barm. All phases contain interstitial fluoride ions in the 48i site and vacancies in the main anionic motif. In Sr0.70Gd0.30F2.30 interstitial fluoride anions were found also in the 4b site; in Sr0.88Gd0.12F2.12, in the 32f site. Relaxation displacements of a part of F-(8c) anions to the 32f site are observed in Sr0.86Sm0.14F2.14. 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 Sr1-xRxF2 + x (R = Sm, Gd) is proposed. According to this model, interstitial fluoride anions and R3+ cations are grouped into octacubic clusters [Sr14-nRnF64+n]. The average number of R3+ per cluster decreases from 6 to 4.5 with increasing., the volume of the cluster core increases from 61.2(1) to 65.9(2) angstrom(3), and the volume per cluster decreases from 2419.9(5) to 783.5(2) angstrom(3).