The thermodynamic stability of Ln(III) (Ln: La, Ce, Pr, Tb, and Er) and U(III) complexes in LiCl-KCl system at 923 K was studied by electrochemical techniques. The Gibbs free energy change of the Ln 3+/Ln 0 and U 3+/U 0 redox reactions, ΔG3/0′, was investigated under various mole fraction of KCl, χ K = 0-0.65. In general, ΔG3/0′ increased linearly with the decrease of χ K due to the distortion of octahedral symmetry of LnCl63- and UCl63-, but a few exceptions were found for lighter Ln elements, e.g., La and Ce, at smaller χ K of 0-0.2. The discrepancy suggests an increase of the number of Cl - ions around La 3+ and Ce 3+.
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