New insights on the high-pressure phase diagram of molecular CO2

TL;DR

This paper reports the discovery of a new high-pressure molecular phase of CO2, identified via x-ray diffraction, which challenges existing interpretations of CO2 phase transitions and stability.

Contribution

It experimentally confirms a theoretically predicted high-temperature phase of CO2 and discusses its relation to other known phases, providing new insights into CO2's phase diagram.

Findings

Discovery of a new molecular phase of CO2 at high pressure and temperature

Identification of the phase as the predicted Cmca structure

Discussion of the phase's stability and relation to other phases

Abstract

We report the discovery of a new molecular phase of carbon dioxide at high-pressure and high-temperature. Using x-ray diffraction, we identify this phase as the theoretically predicted high-temperature Cmca phase [Bonev et al., Phys. Rev. Lett., 91, 065501 (2003)]. Its relation with phase III, on one hand, and its relative stability with respect to phase IV, on the other hand, are discussed based on spectroscopic and melting data. The existence of this strictly molecular phase challenges the interpretation of phases IV and II as intermediate phases between the molecular and covalent-bonded forms of CO2.