Complex phase behavior induced by repulsive interactions

For a solid in which the interactions have a hard core plus a simple soft r epulsive tail we show, using a perturbation theory, that the possible stabl e crystalline structures give rise to a rich phase behavior. We find two co ncomitant critical points each corresponding to phase transitions separatin g bce and fee structures. respectively. and the occurrence of a transition between fee and bce phases without change in density. This novel phenomenol ogy may be relevant to the behavior of some metallic systems, colloids, and to water.