First-order liquid-liquid phase transition in compressed nitrogen.

We report results of first-principles molecular dynamics simulations, which predict a first-order phase transition from molecular to polymeric liquid nitrogen. The liquid-liquid phase boundary is near 88 GPa along the 2000 K isotherm and has a critical point between 4000 and 5000 K and 50 to 75 GPa. At higher temperatures, molecular nitrogen undergoes temperature-driven dissociation to an atomic liquid. The nature of the liquid-liquid transition and the structure of the new polymeric phase are characterized, and ways to experimentally confirm our findings are proposed.