Polymorphism, Isomorphism, and Morphotropy in Trifluorides of Rare-Earth Elements (R) and RF₃–R'F₃ Systems

Abstract

The relationship between the polymorphism, isomorphism, and morphotropy has been traced for the homologous series of 16 trifluorides of rare-earth elements (REEs) (without ScF₃) and R_1 – xR$_{x}^{'}$F₃ phases (R is an REE) based on the T–x diagrams of RF₃–R'F₃ systems. The polymorphism is determined by the REE atomic numbers Z and Т values, which change the cation/anion radius ratio r₊/r_–. Four structural RF₃ subgroups are selected according to the polymorphism and types of LaF₃, β-YF₃, and α-YF₃ (α-UO₃) structures: A (R = La–Nd), B (R = Pm–Gd), C (R = Tb–Ho), and D (R = Er–Lu, Y). Combinations of RF₃ form 10 types of RF₃–R’F₃ systems. Isomorphism (both perfect and limited) manifests itself in the homogeneity range of R_1 – xR$_{x}^{'}$F₃ phases. It affects the R_1 – xR$_{x}^{'}$F₃ structure via the r₊/r_– ratio by preparing (jointly with Т) their morphotropic transformations (MTs). The morphotropy of R_1 – xR$_{x}^{'}$F₃ is regulated by the parameters Т and х via the ratio r₊/r_– and is implemented by means of phase reactions of melt with R_1 – xR$_{x}^{'}$F₃ and R_1 – yR$_{y}^{'}$F₃ of different structures at peritectic (MT-I) and (or) eutectic (MT-II) temperatures. Morphotropic transformations of R_1 – xR$_{x}^{'}$F₃ structures occur at the boundary of GdF₃–TbF₃ (Z = 64.43–64.51; МT-I; 1186 ± 10°С) and HoF₃–ErF₃ (Z = 67.67–67.36; МT-II; 1120 ± 10°С). A definition of true morphotropy for systems in T–x coordinates is given.