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Nanostructured Crystals of Fluorite Phases Sr 1 – x R x F 2 + x ( R Are Rare-Earth Elements) and Their Ordering. 13: Crystal Structure of SrF 2 and Concentration Dependence of the Defect Structure of Nonstoichiometric Phase Sr 1 –   x La x F 2 +   x As Grown ( x = 0.11, 0.20, 0.32, 0.37, 0.47)

Nanostructured Crystals of Fluorite Phases Sr 1 – x R x F 2 + x ( R Are Rare-Earth Elements) and... Abstract The defect structure of as-grown SrF2 and nonstoichiometric phases Sr1 – xLaxF2 + 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 Sr1 – xLaxF2 + 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 Sr1 – xLaxF2 + x phase is proposed, according to which interstitial fluorine ions and impurity cations La3+ are grouped into clusters of the [Sr1 – nLanF26] 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) Å3. The volume of the anion cluster core decreases from 2.52(7) to 2.42(7) Å3, 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) Å3 at х = 0.47. Dynamic thermal displacement of matrix anions in Sr1 – xLaxF2 + 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. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Crystallography Reports Springer Journals

Nanostructured Crystals of Fluorite Phases Sr 1 – x R x F 2 + x ( R Are Rare-Earth Elements) and Their Ordering. 13: Crystal Structure of SrF 2 and Concentration Dependence of the Defect Structure of Nonstoichiometric Phase Sr 1 –   x La x F 2 +   x As Grown ( x = 0.11, 0.20, 0.32, 0.37, 0.47)

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References (37)

Publisher
Springer Journals
Copyright
2019 Pleiades Publishing, Inc.
ISSN
1063-7745
eISSN
1562-689X
DOI
10.1134/s1063774519010279
Publisher site
See Article on Publisher Site

Abstract

Abstract The defect structure of as-grown SrF2 and nonstoichiometric phases Sr1 – xLaxF2 + 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 Sr1 – xLaxF2 + 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 Sr1 – xLaxF2 + x phase is proposed, according to which interstitial fluorine ions and impurity cations La3+ are grouped into clusters of the [Sr1 – nLanF26] 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) Å3. The volume of the anion cluster core decreases from 2.52(7) to 2.42(7) Å3, 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) Å3 at х = 0.47. Dynamic thermal displacement of matrix anions in Sr1 – xLaxF2 + 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.

Journal

Crystallography Reports – Springer Journals

Published: Jan 1, 2019

Keywords: Crystallography and Scattering Methods

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