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

A chemical classification of rare earth (R) trifluorides and some studied RF3-R'F-3 systems with their participation by the difference Delta Z = vertical bar Z(R)-Z(R')vertical bar 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'F-3 systems (TSs), in which chemical interactions of the components increase with increasing Delta Z(max). Jumps of Delta Z(max) between some TSs distinguish the third level - four groups of systems (GSs). The jumps in Delta Z(max) are associated with changes in the topological features of the phase diagrams. GS-4 systems are not well understood.