Polymorphic phase transition for inverse-power-potential crystals keeping the first-order anharmonic correction to the free energy

An improved phase diagram for the polymorphic fcc-bcc phase transition of inverse-power-potential crystals is obtained through an exact analytic calculation of the crystalline free energy including the first-order [O(${\mathit{T}}^2$), where T is the temperature] anharmonic term in the internal energy. The anharmonic correction reduces the region of bcc stability relative to the harmonic result, in qualitative agreement with recent Monte Carlo results for the 1/${\mathit{r}}^6$ potential.