Stability of dense liquid carbon dioxide

Area of Science:

• Geophysics
• Physical Chemistry
• Computational Materials Science

Background:

• Understanding the phase diagram of carbon dioxide (CO2) is crucial for planetary science, particularly for conditions within Earth's mantle.
• Previous studies have suggested potential phase separation of CO2 into constituent elements at high pressures.

Purpose of the Study:

• To computationally investigate the phase diagram and melting curve of liquid CO2 across a broad range of high-pressure and high-temperature conditions.
• To identify distinct liquid phases and characterize their structural and electronic properties.
• To examine the stability of CO2 and its potential decomposition at lower mantle conditions.

Main Methods:

• Ab initio calculations were employed to simulate the behavior of CO2.
• The study focused on predicting phase transitions, including liquid-liquid and liquid-solid equilibria.
• Gibbs free energy calculations were used to assess the stability and decomposition of CO2.

Main Results:

• Several distinct liquid CO2 phases were predicted up to 200 GPa and 10,000 K.
• A first-order liquid-liquid phase transition was identified with a critical point near 48 GPa and 3,200 K.
• A liquid-liquid-solid triple point was predicted around 45 GPa and 1,850 K.
• The predicted liquid-liquid transition involves molecular and polymeric CO2 phases but is not accompanied by metallization.
• Liquid CO2 was found to be stable and does not decompose into carbon and oxygen up to 200 GPa and 10,000 K.

Conclusions:

• The findings present a detailed phase diagram for CO2 under extreme conditions relevant to Earth's interior.
• The unique, non-metallizing liquid-liquid phase transition offers new insights into fluid behavior under high pressure.
• CO2 stability at deep mantle conditions challenges previous hypotheses of elemental phase separation.