Complex phase behavior induced by repulsive interactions

For a solid in which the interactions have a hard core plus a simple soft repulsive tail we show, using a perturbation theory, that the possible stable crystalline structures give rise to a rich phase behavior. We find two concomitant critical points each corresponding to phase transitions separating bcc and fcc structures, respectively, and the occurrence of a transition between fcc and bcc phases without change in density. This novel phenomenology may be relevant to the behavior of some metallic systems, colloids, and to water.