Anharmonic lattice dynamics of bcc sodium under high pressures

Abstract

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.