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

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.