Ternary chlorides of the trivalent early lanthanides - Phase diagrams, crystal structures and thermodynamic properties

A comprehensive review on phase diagrams, crystal structures and thermodynamics of ternary chlorides formed in systems ACl/LnCl(3) (A=Cs, Rb, K, Na; Ln=La-Gd) is presented. The review summarizes the author's own studies, published since 1985, and original papers of other scientists. With the larger alkali metal ions compounds such as A(3)LnCl(6), A(2)LnCl(5) and ALn(2)Cl(7) were obtained. With sodium additional compounds NaLnCl(4) and Na(3)Ln(5)Cl(18) were obtained. The crystal structures are discussed with the concept of ionic radii, which determine the coordination numbers of Ln(3+) and A(+) cations against Cl- anions. The formation enthalpies of the compounds from ACl and LnCl(3) were determined by solution calorimetry. Gibbs' free energies and entropies for these reactions were obtained by e.m.f. measurements vs. temperature. The stability of a ternary chloride in a system ACl-LnCl(3) is given by the 'free enthalpy of symproportionation', that is, the formation of a compound from its neighbour compounds in the system. This DeltaG(syn)(o) must be negative. A surprising result is, that the highest-melting compounds in the systems, A(3)LnCl(6), are formed from ACl(+)A(2)LnCl(5) by a loss in lattice energy. They exist as high-temperature compounds due to sufficiently high gain in entropy at temperatures where TDeltaS>DeltaH.