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Stability of dense liquid carbon dioxide
- Boates, Brian, Teweldeberhan, Amanuel M., Bonev, Stanimir A.
- Proceedings of the National Academy of Sciences of the United States of America 2012 v.109 no.37 pp. 14808-14812
- Gibbs free energy, carbon, carbon dioxide, melting, oxygen, physical phases, separation, temperature
- We present ab initio calculations of the phase diagram of liquid CO ₂ and its melting curve over a wide range of pressure and temperature conditions, including those relevant to the Earth. Several distinct liquid phases are predicted up to 200 GPa and 10,000 K based on their structural and electronic characteristics. We provide evidence for a first-order liquid–liquid phase transition with a critical point near 48 GPa and 3,200 K that intersects the mantle geotherm; a liquid–liquid–solid triple point is predicted near 45 GPa and 1,850 K. Unlike known first-order transitions between thermodynamically stable liquids, the coexistence of molecular and polymeric CO ₂ phases predicted here is not accompanied by metallization. The absence of an electrical anomaly would be unique among known liquid–liquid transitions. Furthermore, the previously suggested phase separation of CO ₂ into its constituent elements at lower mantle conditions is examined by evaluating their Gibbs free energies. We find that liquid CO ₂ does not decompose into carbon and oxygen up to at least 200 GPa and 10,000 K.