Solidification of the charged hard-sphere fluid

Monte Carlo simulations of a hard-sphere model with positive and negative charges are carried out to determine the range of temperatures and densities in which the fluid and solid phases are thermodynamically stable. This is a simple model of an ionic liquid. The limiting densities of fluid and solid phases for a given temperature are determined from the discontinuity of the compressibility factor against density dependence at a constant volume. The dependence of density on the potential energy and the heat capacity confirms these predictions. The temperature-density curves showing the limits of fluid and solid phases increase with the increasing density at low temperatures and are nearly temperature independent at high temperatures. The energy drop between the fluid and solid phases depends on temperature. This energy is positive at low temperatures and negative at high temperatures. This unusual behaviour observed at low temperatures appears to be due to the formation of a tetragonal primitive quasi-structure before solid phase. This quasi-structure is stabilised by the electrostatic interactions. The solid phase also forms a tetragonal primitive lattice but with a different c/a ratio, which is a result of the high-density hard core interactions. The translational order parameter of quasi- and crystal structures amounts to 0.565 and 0.576, respectively. Our results are in general in agreement with earlier studies, but some differences were found.