Freezing of Ionic Melts into Normal and Superionic Phases

Abstract

The freezing mechanisms of molten alkali halides typified by RbCl and NaCl on the one hand, and molten fluorite-type materials BaCl₂ and SrCl₂ on the other, are compared and contrasted. Experimentally, the volume change across the solid-liquid transition is large for the alkali halides (∼20%) and almost an order of magnitude smaller for the fluorites, where the freezing transition is to a superionic phase.

For RbCl and NaCl therefore, the free energy gain associated with the large volume change is one major feature in freezing, the other being the strong charge ordering. This is made quantitative and structural predictions for molten RbCl are in excellent agreement with neutron data. The agreement for NaCl is of poorer quality, but still semiquantitative.

In the case of freezing into the superionic phase, it turns out instead that the major features are the strong ordering of the divalent cation component and the difference in the partial molar volumes of anion and cation. The pronounced cation ordering is reminiscent of a tendency of strong repulsive Coulomb interactions inducing a classical Wigner transition, which in this case corresponds to the cation sub-lattice freezing. The microscopic theory presented then indicates that the disordered chlorine ions behave as a lattice liquid, the periodic external potential being created by the largely rigid cation lattice.