First-principles study of the pressure-induced bcc-hcp transition in Ba

The pressure-induced bcc-hcp phase transition in barium has been studied with use of first-principles total-energy calculations. The volume dependence of the complete energy surface from the bcc to the hcp structure has been determined as a function of the atomic displacements corresponding in the bcc phase to the ${\mathrm{T}}_1$ N-point phonon mode and a long-wavelength shear. The results predict that the frequency of this ${\mathrm{T}}_1$ N-point phonon mode decreases with increasing pressure, however, before the mode goes soft, the hcp phase becomes the lowest-energy structure and the transition is first order.