Landau free energy for a bcc-hcp reconstructive phase transformation - art. no. 224114

We study the bcc-hcp phase transition in Ti and Zr with the use of first-pr inciples calculations. We have determined the complete energy surface from the bcc to hcp structure. The results are used to find an appropriate Landa u free energy density for describing this transformation. The proposed Land au free energy density has two relevant order parameters: shear and shuffle . Through first-principles calculations, we show that the bcc structure is unstable with respect to the shuffle of atoms (TA(1) N-point phonon) rather than the shear. Therefore, we reduce the two order parameter Landau free e nergy to an effective one order parameter (shuffle) potential, which is a r easonable approximation. In general, the effective Landau free energy is a triple-well potential. From the phonon dispersion data and the change in en tropy at the transition temperature we find the free energy coefficients fo r Ti and Zr.