Structure breaking and electron localization in liquid cryolite–sodium solutions

Abstract

The continuous nonmetal-metal transition, which occurs on dissolving metals in molten salts, may be shifted to higher metal concentration in solutions involving polyvalent metal ions by the possibility that these metals go into lower oxidation states. We evaluate a microscopic model for these processes in the specific case of solutions of sodium metal in molten cryolite (AlF₃ 3NaF). The structure of the ionic melt is understood and calculable at the microscopic level in terms of a dominant six-fold coordinated trivalent state of the A1 ion (the (AlF₆)^3- complex) with some admixture of a four-fold coordinated trivalent state (the (AlF₄)⁻ complex). The sodium metal is assumed to enter the ionic liquid in the form of monovalent ions and electrons. Our calculations demonstrate how these added components break up the structure of the ionic melt to yield localization by the formation of Al ions in reduced valence states, and provide order-of-magnitude estimates for the free-energy changes involved in these processes. Specifically, we find that with increasing metal concentration the equilibrium between (AlF₆)^3- and (AlF₄)⁻ shifts in favour of the latter, while Al³⁺ ions are released in the melt and bind the available electrons to form Al²⁺ and Al⁺ ions. The latter eventually become the most stable ones and also destabilize the (AlF₄)⁻ complex. This scenario is consistent with available macroscopic observations. We also briefly discuss how the treatment could be extended to examine other events that may arise with increasing metal content, such as the formation of dimers or small metal clusters.