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

Abstract

The phase composition of 34 ​T-x RF3–R’F3 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 ΔZ ​= ​|Z(R)–Z(R′)| 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 (TMT 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 β-ss (β-YF3 type), and 16 α-ss (α-YF3 type). The compositions of all detected phases are given in the chemical equations of phase reactions. The prospects of using R1-xR’xF3 crystals for photonics and ionics of solids are considered.