Stability of dense liquid carbon dioxide

We present ab initio calculations of the phase diagram of liquid CO2 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 CO2 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 CO2 into its constituent elements at lower mantle conditions is examined by evaluating their Gibbs free energies. We find that liquid CO2 does not decompose into carbon and oxygen up to at least 200 GPa and 10,000 K.