Defect Structures of La1 – ySryF3 – y, La1 – yBayF3 – y, and Nd1 – yCayF3 – y (y = 0.05, 0.10) Nonstoichiometric Tysonite Phases

Abstract

Characteristic features of defect structures of La_{1 – y}Sr_yF_{3 – y}, La_{1 – y}Ba_yF_{3 – y}, and Nd_{1 – y}Ca_yF_{3 – y} (y = 0.05, 0.10) nonstoichiometric phases of different compositions are determined from X-ray diffraction data. Interest in subtle details of their structure is determined by the possibility of ion transport over fluorine vacancies and by a strong compositional dependence of the ionic conductivity. The La_0.95Sr_0.05F_2.95, La_0.95Ba_0.05F_2.95, and Nd_0.95Ca_0.05F_2.95 phases, as well as the La_0.9Ba_0.1F_2.9 phase, crystallize as β-LaF₃ (sp. gr. P3̅c1, Z = 6). The La_0.9Sr_0.1F_2.9 and Nd_0.9Ca_0.1F_2.9 phases lose their superstructure and are described by a cell whose volume is three times smaller (sp. gr. P6₃/mmc, Z = 2). Defects of crystal structure R_{1 – y}M_yF_{3 – y} are not exhausted by vacancies in fluorine positions. All crystals with a “large” cell are twinned according to the merohedral twin law. The majority of atomic positions in models with a “small” cell are split by group symmetry elements and are occupied statistically.