Crystal structures and dynamical properties of dense CO2.

Structural polymorphism in dense carbon dioxide (CO) has attracted significant attention in high-pressure physics and chemistry for the past two decades. Here, we have performed high-pressure experiments and first-principles theoretical calculations to investigate the stability, structure, and dynamical properties of dense CO We found evidence that CO-V with the 4-coordinated extended structure can be quenched to ambient pressure below 200 K-the melting temperature of CO-I. CO-V is a fully coordinated structure formed from a molecular solid at high pressure and recovered at ambient pressure. Apart from confirming the metastability of CO-V (I-42d) at ambient pressure at low temperature, results of ab initio molecular dynamics and metadynamics (MD) simulations provided insights into the transformation processes and structural relationship from the molecular to the extended phases. In addition, the simulation also predicted a phase V'(Pna2) in the stability region of CO-V with a diffraction pattern similar to that previously assigned to the CO-V (P222) structure. Both CO-V and -V' are predicted to be recoverable and hard with a Vicker hardness of ∼20 GPa. Significantly, MD simulations found that the CO in phase IV exhibits large-amplitude bending motions at finite temperatures and high pressures. This finding helps to explain the discrepancy between earlier predicted static structures and experiments. MD simulations clearly indicate temperature effects are critical to understanding the high-pressure behaviors of dense CO structures-highlighting the significance of chemical kinetics associated with the transformations.