Molecular carbon dioxide at high pressure and high temperature

5 figuresInternational audienceThe existence of intermediate states between the molecular and non-molecular forms of carbon dioxide has been actively debated in the recent past. Here we report new experiments at pressures up to 20 GPa and in the temperature range 300-950 K. They revealed the presence of a previously unobserved molecular phase, which we identified as the theoretically predicted high-temperature Cmca phase. Its relation and relative stability with respect to the other solid phases are discussed based on spectroscopic and melting data. We show that the existence of this strictly molecular phase challenges the interpretation of phases IV and II as intermediate forms between the molecular and covalent-bonded solids