Anharmonic lattice dynamics of bcc sodium under high pressures

Phonon-phonon coupling in body centered cubic (bcc) sodium is studied from first-principles to investigate its anharmonic lattice dynamics under high pressures. The renormalization of the phonon frequencies of the transverse acoustic (TA) modes along the Γ-N direction is found to be distinctly different compared with other phonon modes. We find that these TA modes firstly stiffen with increasing external pressure below 30 GPa; however, at higher pressures, these TA modes exhibit a nonmonotonic behavior and soften unexpectedly. When further taking into account the phonon coupling effects, these TA modes stiffen with increasing temperature. The competing effects of the pressure-induced softening and the temperature-induced stiffening of these TA modes may be related to the anomalous melting behavior of sodium under high pressures.