Subcritical Determination of the Frenkel Line in Liquid Nitrogen, the Emergent Final Picture, and a Universal Equation for the Coordination Number of Real Fluids

We performed a series of neutron scattering experiments on deeply subcritical liquid nitrogen at 90 K (0.7<i>T</i>_<i>C</i>). Our findings, when taken together with our previous results at 160 K (1.27<i>T</i>_<i>C</i>) and 300 K (2.4<i>T</i>_<i>C</i>), allow the Frenkel line phenomenon to be characterized in a reliable and consistent manner over an extremely broad temperature range, extending into the subcritical regime. Through an analysis of local order, we show how the fluid structure changes as the Frenkel line is crossed and present a new method for identifying the line. Our determination of coordination numbers shows a remarkable data collapse when plotted against density. This allows us to produce a universal relationship relating the coordination number to the density of a simple fluid, dictated by molecular/atomic size and its density on the melt line.