Brian Boates

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.

Structural and optical properties of liquid CO2 for pressures up to 1 TPa

We report on the use of first-principles molecular dynamics calculations to examine properties of liquid carbon dioxide in the pressure-temperature range of 0-1 TPa and 200-100 000 K. The computed equations of state points are used to predict a series of shock Hugoniots with initial starting conditions that are relevant to existing and ongoing shock-wave experiments. A comparison with published measurements up to 70 GPa shows excellent agreement. We find that the liquid undergoes a gradual phase transition along the Hugoniot and have characterized this transition based on changes in bonding and structural properties as well as the conductivity and reflectivity of the fluid.