A MD simulation study on the influence of pressure on sodium crystal melting

The molecular dynamics (MD) method and EAM/FS potential are used to study the influence of pressure (1 kPa ∼ 500 MPa) on the non-equilibrium melting process of alkali metal sodium crystals. The changes in macroscopic physical parameters such as potential energy, volume, and heat capacity (C_p) with pressure are obtained, and the evolutions of atomic clusters during the melting process are analyzed by RDF and PTM methods. The non-equilibrium melting temperature (T'_m) and melting time (Δt_m) under different pressures are gotten by macroscopic physical parameters analysis. The pressure has a significant impact on the melting behavior of sodium, particularly at pressures below 150 MPa. The T'_m increases with the increase of pressure, ranging from 423 K to 464 K within the pressure range of 1 kPa to 500 MPa, which is consistent with the experimental data. Besides, the Δt_m first decreases and then increases with increasing pressure, and the shortest melting time is 7.6 ps at 90 MPa. Both RDF analysis and PTM analysis can describe the phase transition process of sodium melting, and also indicate that T'_m increases with increasing pressure. The PTM analysis method can effectively study the transformation of different sodium atom clusters (BCC, HCP, FCC, and Other clusters) during the sodium melting process. It is found that as the temperature increased, some BCC clusters directly transform into Other clusters, while the others first transform into intermediate FCC and HCP clusters, and then into Other clusters. The range of T'_m of sodium is determined to be 425 K ∼ 467 K using PTM analysis method, which is consistent with the results of C_p analysis. There is a good correspondence between the changes in macroscopic physical parameters and the evolution of the atomic clusters during the melting process of sodium crystals. Both of these changes can reveal the melting process of sodium. The thermodynamic parameters related to sodium melting obtained through the MD simulations can provide theoretical support for the applications and simulations of solid-liquid phase transition of sodium under relatively low-pressure conditions.