X-ray and neutron diffraction study of the defect crystal structure of the as-grown nonstoichiometric phase Y0.715Ca0.285F2.715

This work begins a series of papers aimed at studying the defect structure of nonstoichiometric phases R1−yCayF3−y with a tysonite-type (LaF3) structure. In the single-crystal structure of Y0.715Ca0.285F2.715 with a tysonite-type small unit cell (sp. gr. P63/mmc, a = 3.9095(2) Å, c = 6.9829(2) Å; Z = 2; Rw = 2.16%), the displacements of Y3+ cations and F2− anions from 63 symmetry axes were observed for the first time. The X-ray diffraction pattern shows weak satellites insufficient for structural calculations. The LaF3 structure type is stabilized up and down on the temperature scale due to anion vacancies and the symmetrizing effect of Ca2+ cations lying on 63 symmetry axes. At 120°C the fluoride-ion conductivity in the nonstoichiometric phase Y0.715Ca0.285F2.715 is five orders of magnitude higher than that in the stoichiometric phase β-YF3. The transition to a superionic state is caused by a deviation from stoichiometry and is not associated with reconstructive phase transformation