FLUID-SOLID EQUILIBRIUM OF A CHARGED HARD-SPHERE MODEL

The fluid-solid equilibrium of a system made of charged hard spheres w ith positive and negative ions of the same size is considered. At high temperatures freezing occurs in a substitutionally disordered close p acked structure, the face centered cubic solid (fcc). At low temperatu res freezing occurs in the ordered cesium chloride structure (CsCl). A s the latter solid coexists with the Ecc structure at high densities, two triple points exist on the phase diagram. By using computer simula tion we determine the precise location of both triple points. In the f irst of them, vapor, liquid, and solid (CsCl) are in equilibrium at T = T/[q(2)/(sigma k epsilon)] = 0.0225, where q is the charge of the i ons, sigma their diameters, il the Boltzmann constant, and epsilon the dielectric constant. In the other triple point, occurring al T = 0.2 4, the three coexisting phases are the fluid, a CsCl solid, and the fe e solid. The vapor-liquid-solid triple point temperature is found to b e about one-third of the critical temperature, in good agreement with the experimental ratio for a number of molten salts. An implementation of the cell theory for the solid phases of charged hard spheres is pr esented. It is shown that this simple theory provides a reasonable des cription of the proper ties of solid charged hard spheres. When the th eory is combined with an accurate theory for the fluid phase a very sa tisfactory description of the phase diagram of charged hard spheres is obtained. The cell theory predictions are better than those recently reported using a density functional scheme.