Nanostructured Crystals of Fluorite Phases Sr1–xRxF2+x (R Are Rare-Earth Elements) and Their Ordering. 16: Defect Structure of the Nonstoichiometric Phases Sr1–xRxF2+x (R = Pr, Tb–Yb) As Grown

Abstract

The defect structure of as grown Sr0.72Pr0.28F2.28, Sr0.87Tb0.13F2.13, Sr0.88Dy0.12F2.12, Sr0.89Ho0.11F2.11, and Sr0.9R0.1F2.1 single crystals (R = Y, Er, Tm, Yb) prepared under identical conditions has been studied by X-ray diffraction analysis. All crystals belong to the CaF2 structure type (sp. gr. $$Fm\bar {3}m$$ ) and have vacancies in the main anion motif and interstitial fluorine anions at site 32f in the crystal with R = Pr and site 48i for the other R. Interstitial fluorine anions are found near site 4b in the crystals with R = Tb and Dy (sites 32f and 24e, respectively) and at site 4b in the crystal with R = Er. Relaxation (static displacement of some main anions to site 32f) is observed in all crystals (except for R = Tb). Displacements of some Sr2+ cations to sites 32f and 24e is simultaneously observed in crystals with R = Tb, Dy, Tm, and Yb. In crystals with R = Ho, Y, and Er, some Sr2+ cations are displaced only to site 32f; no cation displacement is observed for R = Pr. A model of the defect structure of the phases is proposed based on the structure of crystals studied, according to which interstitial fluorine anions and R3+ cations are aggregated into [Sr4–nRnF26] clusters of tetrahedral configuration for the phase with R = Pr or [Sr14–nRnF64+n] clusters of octacubic configuration for other R.