Density-functional theory for fluid-solid and solid-solid phase transitions.

We develop a theory to describe solid-solid phase transitions. The density functional formalism of classical statistical mechanics is used to find an exact expression for the difference in the grand thermodynamic potentials of the two coexisting phases. The expression involves both the symmetry conserving and the symmetry broken parts of the direct pair correlation function. The theory is used to calculate phase diagram of systems of soft spheres interacting via inverse power potentials u(r) = epsilon (sigma/r)(n),where parameter n measures softness of the potential. We find that for 1/n < 0.154 systems freeze into the face centered cubic (fcc) structure while for 1/n >= 0.154 the body-centred-cubic (bcc) structure is preferred. The bcc structure transforms into the fcc structure upon increasing the density. The calculated phase diagram is in good agreement with the one found from molecular simulations.