Persistent local order heterogeneity in the supercritical carbon dioxide

TL;DR

This study reveals that supercritical carbon dioxide exhibits persistent medium-range order heterogeneity, challenging the view of its uniformity, with implications for planetary science and industrial applications.

Contribution

It uncovers the existence of stable local order heterogeneity in supercritical CO2, including shell-specific interactions and their role in diffusion mechanisms.

Findings

Presence of shell-specific gas-like and liquid-like interactions.

Persistence of heterogeneity across a wide temperature range.

Implications for planetary atmospheres and industrial processes.

Abstract

The supercritical state is currently viewed as uniform and homogeneous on the pressure-temperature phase diagram in terms of physical properties. Here, we study structural properties of the supercritical carbon dioxide, and discover the existence of persistent medium-range order correlations which make supercritical carbon dioxide non-uniform and heterogeneous on an intermediate length scale, a result not hitherto anticipated. We report on the carbon dioxide heterogeneity shell structure where, in the first shell, both carbon and oxygen atoms experience gas-like type inter- actions with short range order correlations, while within the second shell oxygen atoms essentially exhibit liquid-like type of interactions with medium range order correlations due to localisation of transverse-like phonon packets. We show that the local order heterogeneity remains in the three phase-like equilibrium within very wide temperature range. Importantly, we highlight a catalytic role of atoms inside the nearest neighbor heterogeneity shell in providing a mechanism for diffusion in the supercritical carbon dioxide on an intermediate length scale. Finally, we discuss important implications for answering the intriguing question whether Venus may have had carbon dioxide oceans and urge for an experimental detection of this persistent local order heterogeneity.