A first-principles study of the anharmonic lattice dynamics of sodium under pressures

The anharmonic lattice dynamics of sodium under pressures are studied from first-principles calculations to elucidate its intriguing thermal expansion and melting behaviors. It is predicted based on quasi-harmonic approximation and phonon calculations that the face centered cubic and cI16 (a distorted body centered cubic structure) phases of sodium exhibit negative thermal expansions above 90 GPa. By calculating the Grüneisen dispersions of the different phases of sodium, it is found that the large negative Grüneisen parameters of the transverse acoustic modes are mainly responsible for the predicted negative thermal expansion. Moreover, the phonon spectra of sodium at elevated temperatures are calculated using the anharmonic third-order force constants and the perturbation theory. Phonon-phonon interactions related to the low frequency vibrational modes are found to be greatly affected by external hydrostatic pressures. Based on this theoretical study, we achieve deeper insight into the intriguing melting behavior of sodium under high pressures.