Ternary chlorides of the trivalent late lanthanides

A comprehensive review on phase diagrams, crystal structures and thermodynamic properties of ternary chlorides formed in the systems ACl/LnCl₃ (A=Na, K, Rb, Cs) is presented. It continues an earlier review with the same contents on the lanthanides from La to Gd 1]. In both papers the author's own studies, published since 1985, together with original papers from other scientists are treated. With the three larger cations compounds of the composition A₃LnCl₆, A₂LnCl₅, ALn₂Cl₇ and beginning with holmium Cs₃Ln₂Cl₉ are formed. With sodium the compounds Na₃Ln₅Cl₁₈ (Ln=La to Sm) and NaLnCl₄ (Ln=Eu to Lu) also exist. The stability of a ternary chloride in a system ACl/LnCl₃ is given by the 'free enthalpy of synreaction', the formation of a compound from its neighbour compounds in its system. This must be negative. A surprising result is that the highest – melting compounds in the systems, A₃LnCl₆, are formed from ACl and A₂LnCl₅ with a loss of lattice energy, U. They exist as high-temperature compounds due to a sufficiently high gain in entropy at temperatures where the entropy term TΔS compensates the endothermic ΔH.