Molecular dynamics study of a simple liquid at negative pressures

We investigate the effect of isochoric cooling of the Lennard-Jones liquid at the triple point i.e. ρ⁎ = 1.20 r0, T⁎ = 0.70 into the negative pressure region of crystal + vapor phase with temperatures varying from T⁎ = 1.00 to 0.0001. Along this line, initially, the liquid remains stable with respect to its crystallization, and is metastable with respect to liquid-vapor nucleation. The objective is to determine the limits of metastability extrapolated to sub-triple point temperatures along the line of crystal-liquid coexistence, and to explore the mechanism of return to thermodynamic equilibrium, i.e. crystal + vapor stability. It is found that a transient unstable phase is encountered at the reduced temperature T⁎ = 0.3250 for a N = 4000 atom system, and T⁎ = 0.45 for N = 32,000. The results indicate that T⁎ = 0.35 and T⁎ = 0.50 are the spinodal limits for simple liquids at the triple point density ρ⁎ = 1.2 for both systems respectively. The pressure line changes its slope from negative to positive as the new phase i.e. crystal + hole (vapor) begins to form. The transformation mechanism from supercooled liquid to crystal + vapor is two-stage; first “metastable liquid” at negative pressure to “liquid + hole” at zero pressure, then from “liquid + hole” to “crystal + hole”.