Solid–liquid transitions of sodium chloride at high pressures

We investigate solid–liquid transitions in NaCl at high pressures using molecular dynamics simulations, including the melting curve and superheating/supercooling as well as solid–solid and liquid–liquid transitions. The first-order B1–B2 (NaCl–CsCl type) transition in solid is observed at high pressures besides continuous liquid structure transitions, which are largely analogous to the B1–B2 transition in solid. The equilibrium melting temperatures $(Tm)$ up to megabar pressure are obtained from the solid–liquid coexistence technique and the superheating–supercooling hysteresis method. Lindemann’s vibrational and Born’s mechanical instabilities are found upon melting. The Lindemann frequency is calculated from the vibrational density of states. The Lindemann parameter (fractional root-mean-squared displacement) increases with pressure and approaches a constant asymptotically, similar to the Lennard-Jones system. However, the Lindemann melting relation holds for both B1 and B2 phases to high accuracy as for the Lennard-Jonesium. The B1 and B2 NaCl solids can be superheated by $0.18Tm$ and $0.24Tm$⁠, and the NaCl liquid, supercooled by $0.22Tm$ and $0.32Tm$⁠, respectively, at heating or cooling rates of 1 K/s and 1 K/ps. The amount of maximum superheating or supercooling and its weak pressure dependence observed for NaCl are in accord with experiments on alkali halides and with simulations on the Lennard-Jones system and Al.