Stability of Chlorocomplexes in Molten Salt Mixtures as a Screening-controlled Chemical Equilibrium

We develop a simple ionic model for solutions of polyvalent cation chlorides in molten alkali chlorides, which allows for the possible formation of long-lived tetrahedral chlorocomplexes in chemical equilibrium with free species. 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 parameters adapted to simulate a calcium chloride solute. For a given binding free energy of complexes, their stability is controlled by the ionic screening length of the solution. In particular, we find (i) instability of calcium chlorocomplexes in lithium chloride solvent, marginal stability at low c in sodium chloride solvent and increasing stability through the series of solvents from potassium to cesium chloride; and (ii) rapid dissociation of complexes in each of these latter solvents with increasing solute concentration as c goes through or exceeds the “stoichiometric” composition c = 1/5. The dissociation process is driven by a positive-feedback screening mechanism, the decrease of the screening length with increasing c being primarily determined by free calcium ions. Dissociation of complexes may therefore be viewed as a classical insulator-to-conductor transition, which occurs in a narrow range of composition in correspondence to a “critical value” of the screening length. A stability criterion for complexes is derived and contact is made with the available evidence from thermodynamic and Raman spectroscopy data.