On the transition from gas-like to liquid-like behaviour in supercritical N2

TL;DR

This study investigates the transition from gas-like to liquid-like behavior in supercritical nitrogen at 300 K, identifying the Frenkel line through combined experimental and simulation methods.

Contribution

It provides a detailed characterization of the Frenkel line in supercritical N2 using diffraction, spectroscopy, and molecular dynamics simulations, establishing reliable markers for this transition.

Findings

Narrow transition at ~150 MPa identified as Frenkel line

Both diffraction and spectroscopy effectively characterize the transition

Simulation supports experimental observations

Abstract

We have studied in detail the transition from gas-like to rigid liquid-like behaviour in supercritical $N_2$ at 300 K (2.4 $T_C$). Our study combines neutron diffraction and Raman spectroscopy with empirical potential structure refinement and ab-initio molecular dynamics simulations. We observe a narrow transition from gas-like to rigid liquid-like behaviour at ca. 150 MPa, which we associate with the Frenkel line. Our findings allow us to reliably characterize the Frenkel line using both diffraction and spectroscopy methods, backed up by simulation, for the same substance. We clearly lay out what parameters change, and what parameters do not change, when the Frenkel line is crossed.