High-temperature chemistry of Y, La and lanthanide trifluorides in RF3-R′F3 systems. Part 3. Phase composition of the studied systems

The phase composition of 34 T-x RF3-R'F-3 systems of trifluorides of rare-earth elements (R REE) is described. The isomorphism of REE ions in three types of RF3 structures has been quantitatively characterized. The growth of the difference of atomic numbers Delta Z = vertical bar Z(R)-Z(R')vertical bar of the REE limits isomorphism by morphotropic transformations (MTs) of solid solutions (ss). The studied systems show phase reactions of MT: peritectic (MT-I) and eutectic (MT-II). MTs are invariant phase reactions of the two ss with the melt. MT express high-temperature chemical interactions of RF3. The parameters MT (T-MT and x) were determined for 11 MT-I and 13 MT-II. As a result of MT, the following phases are formed: 16 t-ss (LaF3 type); 24 beta-ss (beta-YF3 type), and 16 alpha-ss (alpha-YF3 type). The compositions of all detected phases are given in the chemical equations of phase reactions. The prospects of using R1-xR'F-x(3) crystals for photonics and ionics of solids are considered.