Two stage order-disorder transformation in simple monatomic supercritical fluid: superstable tetrahedral local order

TL;DR

This paper reveals that supercritical fluids exhibit a two-stage transition from superstable tetrahedral local order to disordered states, with significant implications for understanding their structural dynamics at high temperatures and densities.

Contribution

It demonstrates the existence of superstable tetrahedral local order in supercritical fluids and describes a two-stage melting process distinct from solid-liquid transitions.

Findings

Superstable tetrahedral local order persists at high temperatures and critical densities.

Solid-like local orders (hcp, fcc) vanish at lower temperatures than tetrahedral order.

Structure relaxation times are significantly larger than in weakly interacting gases.

Abstract

The local order units of dense simple liquid are typically three dimensional (close packed) clusters: hcp, fcc and icosahedrons. We show that the fluid demonstrates the superstable tetrahedral local order up to temperatures several orders of magnitude higher than the melting temperature and down to critical density. While the solid-like local order (hcp, fcc) disappears in the fluid at much lower temperatures and far above critical density. We conclude that the supercritical fluid shows the temperature (density) driven two stage "melting" of the three dimensional local order. We also find that the structure relaxation times in the supercritical fluid are much larger than ones estimated for weakly interactive gas even far above the melting line.