First-Order Liquid-Liquid Phase Transition in Compressed Nitrogen

Abstract

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.

Electronic and structural properties of dense liquid and amorphous nitrogen

“…23 Vibrational frequencies in our simulated molecular fluid also compare well with experimental results 30 near 50 GPa and 2000 K. Furthermore, our simulations suggest the onset of molecular dissociation above 30 GPa at 6000 K in excellent agreement with previous experimental 20 and theoretical work. 22 …”

“…23 The structure of the emerging liquid continues to evolve with pressure; upon transformation, the liquid is predominantly 2-coordinated while further compression yields more 3-coordinated atoms. The band gap was found to close upon transformation to the polymeric liquid, indicating that the molecular-polymeric transition may coincide with an insulator-metal transition.…”

Melting line of polymeric nitrogen

“…Ab initio simulations of Boates and Bonev [8] reveal that dense liquid nitrogen may also have complex structure, similar to that found in the solid nitrogen. The transition from the molecular to the atomic structure can be interpreted as a break triple chemical bond in N 2 and formation of a network of ordinary chemical bonds connecting each N atom with three its nearest neighbors in the polymeric structure.…”

“…It should be noted that EOS for polymeric liquid [9] was calibrated on ab initio data [8] ranging from 2000 to 5000 K and hence the solution appear to be possible only within the limited range of temperatures and pressures. At temperatures above ~ 1750 K the predicted melting line crosses the (extrapolated) liquid-liquid phase separation line and therefore no above common tangent was found.…”

“…EOS for polymeric liquid nitrogen was calibrated on ab initio simulation data [8] and applied to prediction of fluid-fluid transition in strongly compressed nitrogen [9]. The 0 E  value was fitted to reproduce the melting pressure near the triple point at 1500 K [1].…”