PHASE-DIAGRAMS OF SYSTEMS OF PARTICLES INTERACTING VIA REPULSIVE POTENTIALS

We use a recently developed density-functional perturbation theory, wh ich has been applied successfully to predict phase diagrams of systems of attractive particles, to describe the phase diagram of particles i nteracting via repulsive potentials. We consider potentials composed o f a hard-sphere core plus a repulsive term. Specifically, we have inve stigated square shoulder and repulsive Yukawa terms. We show that, whe n the range of the interaction is very short, the shoulder potential l eads to solid-solid coexistence involving two face-centered cubic stru ctures, in analogy to an attractive square-well potential. Comparison with simulation results shows that the theory is quantitatively correc t. If the range of the potentials is sufficiently long, we also find t hat a body-centered cubic structure can be stabilized. By considering the phase behavior at zero temperature, we argue that several triple p oints, involving coexistence of fluid and/or solid phases, may occur. A repulsive Yukawa term also shows a region of body-centered cubic sta bility but, contrary to the square shoulder and attractive Yukawa case s, there is no isostructural solid-solid coexistence. The role of the functional dependence of the interaction potential on particle separat ion at short distances is discussed and shown to be crucial to generat e a solid/solid transition in systems of repulsive particles. Availabl e computer simulation results for this system indicate that the densit y-functional approximation for the hard-sphere system used in this wor k, as well as all other currently available approximations, although q ualitatively correct, may be overestimating the stability of the body- centered cubic phase. (C) 1997 American Institute of Physics.