Fluid-solid equilibrium of a charged hard-spheres (RPM) model
C. Vega, F. Bresme, and J.L.F. Abascal
Physical Review E 54, 2746-2760 (1996)
ABSTRACT
The fluid-solid equilibrium of a system made of charged hard spheres with
positive and negative ions of the same size (RPM) is considered.
At high temperatures freezing occurs in a substitutionally disordered close
packed structure, the face centered cubic solid (fcc).
At low temperatures freezing occurs in the ordered cesium chloride structure
(CsCl).
As the latter solid coexists with the fcc structure at high densities, two
triple points exist on the phase diagram.
By using computer simulation we determine the precise location of both triple
points.
In the first of them, vapor, liquid, and solid (CsCl) are in equilibrium at
T*= kT/[q^2/(sigma epsilon)] = 0.0225 where q is the charge of the
ions, sigma their diameters, k the Boltzmann constant, and
epsilon the dielectric constant.
In the other triple point, occurring at T* =0.24 the three coexisting
phases are the fluid, a CsCl solid and the fcc solid.
The vapor-liquid-solid triple point temperature is found to be 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 presented.
It is shown that this simple theory provides a reasonable description of the
properties of solid charged hard spheres.
When the theory is combined with an accurate theory for the fluid phase a very
satisfactory 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.
