Displacements in the Cationic Motif of Nonstoichiometric Fluorite Phases Ba₁₋<i>_x</i><i>R_x</i>F₂₊<i>_x</i> as a Result of the Formation of {Ba₈[<i>R</i>₆F_68–69]} Clusters: III. Defect Cluster Structure of the Nonstoichiometric Phase Ba_0.69La_0.31F_2.31 and Its Dependence on Heat Treatment

Abstract

The defect structure of Ba0.69La0.31F2.31 single crystals in as-grown state and after annealing at 1173 K for 336 h was studied by X-ray diffraction analysis. Both crystals belong to the CaF2 structure type (sp. gr. Fm3¯m). They have vacancies in the main anion motif and interstitial fluorine anions in Wyckoff positions 48i and 4b. Relaxation (static displacement of some main anions to Wyckoff position 32f) is observed in the annealed crystal. It was established that annealing leads to a change in the type of displacement of the main anions in Wyckoff positions 8c from dynamic to static. Displacement of La3+ cations to Wyckoff position 32f is observed in both crystals. A model of the defect structure of Ba0.69La0.31F2.31 is proposed, according to which interstitial fluorine anions and La3+ cations are aggregated into [Ba14−nLanF64+n] clusters with the cuboctahedral anionic core formed by interstitial fluorine anions in Wyckoff positions 48i. Ba2+ cations are located in the cluster in the centers of the faces, and the La3+ cations are shifted by 0.24 Å from the vertices of the cluster along the three-fold axis towards the center of the cluster. The study establishes the relationship between the defect structure of crystals and their structurally sensitive properties, and to develop approaches to their management.

Keywords

• nonstoichiometric fluorides
• defect crystal structure
• clusters
• rare earths
• fluorite