Defect Structures of La1 – ySr y F3 – y, La1 – yBa y F3 – y, and Nd1 – yCa y F3 – y (y = 0.05, 0.10) Nonstoichiometric Tysonite Phases

Abstract

Characteristic features of defect structures of La1 – ySr y F3 – y, La1 – yBa y F3 – y, and Nd1 – yCa y F3 – 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 La0.95Sr0.05F2.95, La0.95Ba0.05F2.95, and Nd0.95Ca0.05F2.95 phases, as well as the La0.9Ba0.1F2.9 phase, crystallize as β-LaF3 (sp. gr. P3c1, Z = 6). The La0.9Sr0.1F2.9 and Nd0.9Ca0.1F2.9 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 R1 – yM y F3 – 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.