STABILITY OF CHLOROCOMPLEXES IN TRIVALENT METAL ALKALI-HALIDE MIXTURES

Raman scattering experiments on yttrium-alkali chloride and lanthanum-alkali chloride mixtures have demonstrated that Y3+ and La3+ ions in these melts are in a state of rather stable octahedral-type coordination by chlorine first neighbours. In this paper the earlier ionic model of Akdeniz and Tosi for solutions of polyvalent cation chlorides in molten alkali chlorides, allowing for the possible formation of long-lived tetrahedral chlorocomplexes in chemical equilibrium with free species, is extended to octahedral Y-alkali halide and La-alkali halide mixtures. Complex stability is evaluated as a function of the alkali chloride solvent and of solute concentration, c, the relevant free energies being determined from charged-hard-sphere interactions, with a statistical treatment of the melt based on the mean spherical approximation. The binding free energy of the complexes is calculated from ionic-model treatments of species in vacuo. The results are compared with the Raman scattering experiments.