Crystal structures and dynamical properties of dense CO2

Area of Science:

• High-pressure physics and chemistry
• Materials science
• Solid-state chemistry

Background:

• Dense carbon dioxide (CO2) exhibits complex structural polymorphism under high pressure.
• Understanding these phases is crucial for high-pressure physics and chemistry.
• Previous studies have identified several CO2 phases, but their stability and transformation mechanisms remain areas of active research.

Purpose of the Study:

• To investigate the stability, structure, and dynamical properties of dense carbon dioxide (CO2) under high pressure.
• To explore the transformation processes between molecular and extended phases of CO2.
• To clarify discrepancies between theoretical predictions and experimental observations of CO2 structures.

Main Methods:

• High-pressure experiments.
• First-principles theoretical calculations.
• Ab initio molecular dynamics (MD) and metadynamics simulations.

Main Results:

• Evidence for CO2-V with a 4-coordinated extended structure, quenchable to ambient pressure below 200 K.
• Confirmation of CO2-V (I-42d) metastability at ambient pressure and low temperature.
• Prediction of a new phase, CO2-V' (Pna21), with a similar diffraction pattern to a previously assigned CO2-V structure.
• Both CO2-V and -V' are predicted to be hard (Vicker hardness ~20 GPa) and recoverable.
• MD simulations revealed large-amplitude bending motions in CO2 phase IV at high temperatures and pressures, explaining discrepancies in earlier static structure predictions.

Conclusions:

• The metastability of CO2-V at ambient conditions is confirmed.
• A new recoverable CO2 phase, CO2-V', is predicted.
• Temperature effects and chemical kinetics are critical for understanding high-pressure CO2 behavior and phase transformations.
• MD simulations provide crucial insights into the dynamics and transformation pathways of dense CO2 structures.