Rare-Earth-Element (REE) Trifluorides in the RF3–R'F3 (R, R' = REEs) Systems: 1. Classification of the Systems by Chemical Proximity of Components

Abstract

The T–x (temperature–composition) phase state diagrams of 120 systems RF3–R'F3 (pair combinations of 16 trifluorides of rare-earth elements R and R' without ScF3) represent the interaction of components in terms of phase composition depending on the chemical proximity of RF3 and R'F3, which is defined by the difference ΔZ = Z(R) – Z(R') of the atomic numbers of R and R' (except YF3). The lanthanide contraction of R3+ with increasing Z together with temperature causes polymorphism and morphotropy of RF3 and $${{{\text{R}}}_{{{\text{1}}\,\,-\,\,x}}}{\text{R}}_{x}^{'}{{{\text{F}}}_{{\text{3}}}}.$$ The series of RF3 is divided into structure subgroups (SSs), pair combinations of RF3 of which give 10 types of systems. A classification of RF3–R'F3 systems by ΔZ is proposed, which divides 10 types of systems into four groups of systems (GSs): GS-1 contains 25 systems of RF3 of identical SSs with ΔZ = 2–3; GS-2, 43 systems of RF3 of neighboring SSs with ΔZ = 6–7; GS-3, 32 systems of RF3 separated by one SS with ΔZ = 10; and GS-4, 20 systems of RF3 separated by two SSs with ΔZ = 14.