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.
Highlights
One of the ways of searching for new crystalline materials is the complication of a chemical composition of crystals with saving of their structure type
Using the method of structural analysis of the M1−x Rx F2+x phases, it was found that interstitial fluorine anions appear in difference Fourier syntheses in eight Wyckoff positions: four 32f (w, w, w), two 48i (u, u, 0.5), 48g (q, 0.25, 0.25), 24e (p, 0.5, 0.5) and 4b (0.5, 0.5, 0.5)
The structure of nonstoichiometric fluorite phase Ba0.69 La0.31 F2.31 in as-grown condition and after a long isothermal annealing at 1173 K was studied by X-ray diffraction
Summary
One of the ways of searching for new crystalline materials is the complication of a chemical composition of crystals with saving of their structure type. With the transition to an isostructural solid solution, some physical characteristics vary within a wide range. This approach allows one to efficiently control the physicochemical properties and expands the range of promising crystalline materials for various areas of solidstate physics and chemistry. R = rare earth elements) with the defect fluorite structure are the most commonly used and promising materials in technological applications among known fluoride nonstoichiometric phases. They are formed in 80 MF2 –RF3 systems [1]. The M1–x Rx F2+x phases have different properties compared to the initial MF2 , due to their structural features
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