<i>Ab initio</i> calculation of lattice dynamics and thermodynamic properties of beryllium

Abstract

We investigate the phase transition, elastic constants, phonon dispersion curves, and thermal properties of beryllium (Be) at high pressures and high temperatures using density functional theory. By comparing the Gibbs free energy, in the quasiharmonic approximation (QHA), of hexagonal-closed-packed (hcp) with those of the face-centered cubic (fcc) and body-centered-cubic (bcc) we find that the hcp Be is stable up to 390 GPa, and then transforms to the bcc Be. The calculated phonon dispersion curves are in excellent agreement with experiments. Under compression, the phonon dispersion curves of hcp Be do not show any anomaly or instability. At low pressure the phonon dispersion of bcc Be display imaginary along Γ-N in the T1 branches. Within the quasiharmonic approximation, we predict the thermal equation of state and other properties including the thermal expansion coefficient, Hugoniot curves, heat capacity, Grüneisen parameter, and Debye temperature.