A very simple lattice model is presented for mixtures of alkali metals and alkali halides, like Kx(KCl)1-x. The valence electrons are assumed to be excluded from the Wigner-Seitz spheres associated with the anions, while they are uniformly distributed over the cation Wigner-Seitz spheres. The calculated excess volume of mixing turns out to be positive over most of the concentration range, in qualitative agreement with experiment, provided the surface energy of the electrons, associated with the boundary of the anion Wigner-Seitz spheres, is taken into account. The results of this model differ considerably from earlier predictions based on linear electron screening theory.

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Journal J. Phys.: Condens. Matter
Dijkstra, Marjolein, Hansen, J. -P, & Meroni, A. (1994). A simple lattice model for the mixing properties of molten Kx(KCl)1-x solutions. J. Phys.: Condens. Matter, 6, 2129–2136.