High-temperature chemistry of Y, La and lanthanide trifluorides in RF3–R'F3 systems. Part 1. Chemical classification of systems

Highlights: • Chemical classification of RF₃-R'F₃ (R=Y,La–Lu) systems by ΔZ=|Z(R)-Z(R′)| atomic numbers is given • Four groups have been identified in RF₃-R'F₃ systems by ΔZ and phase diagram topology. • Interaction of RF₃ and R'F₃ in systems results in isomorphism and morphotropic phase reactions. A chemical classification of rare earth (R) trifluorides and some studied RF₃-R'F₃ systems with their participation by the difference ΔZ=|Z(R)–Z(R′)| of atomic numbers R and R′ has been developed. The classification has three levels. The first level divides 16 RF₃ (without ScF₃) in structure and phase transformations into four structural subgroups (SSs): A (LaF₃-NdF₃); B (PmF₃-GdF₃); C (TbF₃-HoF₃), and D (ErF₃-LuF₃, YF₃) with PmF₃ belonging to SS B. The second level forms from 10 types of RF₃-R'F₃ systems (TSs), in which chemical interactions of the components increase with increasing ΔZ_max. Jumps of ΔZ_max between some TSs distinguish the third level – four groups of systems (GSs). The jumps in ΔZ_max are associated with changes in the topological features of the phase diagrams. GS-4 systems are not well understood.