Crystal structure and fluoride ion conducting behavior of nonstoichiometric rare-earth oxyfluorides

Abstract

Nonstoichiometric rare-earth oxyfluorides (RO_1−xF_1+2x) were synthesized and the relationship between the crystalline phase and fluoride-ion-conducting properties were investigated. In the case of stoichiometric ROF (x = 0.0) except for YbOF, a trigonal structure (R3m) was formed, in which the RO₄F₄ unit was distorted as the ionic radius of R³⁺ decreased. The introduction of F⁻ ions into the lanthanum oxyfluoride (LaOF) structure, which initially had the rectangular RO₄F₄ unit, resulted in a transformation to the tetragonal P4/nmm phase. However, in the case of nonstoichiometric RO_1−xF_1+2x with an increased distortion in the RO₄F₄ unit compared to the stoichiometric ROF (x = 0.0), lower symmetric orthorhombic Pmmn and Abm2 phases were obtained. The electrical conductivity of the RO_1−xF_1+2x solids is significantly influenced by their structural symmetry, and the LaO_1−xF_1+2x solid having the most symmetric RO₄F₄ units in the structure exhibited the highest ion conductivity.