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

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 R1 – 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 R1 – xR $$_{x}^{'}$$ F3 phases. It affects the R1 – xR $$_{x}^{'}$$ F3 structure via the r+/r– ratio by preparing (jointly with Т) their morphotropic transformations (MTs). The morphotropy of R1 – 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 R1 – xR $$_{x}^{'}$$ F3 and R1 – yR $$_{y}^{'}$$ F3 of different structures at peritectic (MT-I) and (or) eutectic (MT-II) temperatures. Morphotropic transformations of R1 – 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.