Crystal structures and dynamical properties of dense CO2

Abstract

Structural polymorphism in dense carbon dioxide (CO₂) 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 CO₂ We found evidence that CO₂-V with the 4-coordinated extended structure can be quenched to ambient pressure below 200 K-the melting temperature of CO₂-I. CO₂-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 CO₂-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'(Pna2₁) in the stability region of CO₂-V with a diffraction pattern similar to that previously assigned to the CO₂-V (P2₁2₁2₁) structure. Both CO₂-V and -V' are predicted to be recoverable and hard with a Vicker hardness of ∼20 GPa. Significantly, MD simulations found that the CO₂ 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 CO₂ structures-highlighting the significance of chemical kinetics associated with the transformations.

Keywords: carbon dioxide; high pressure; material science; molecular dynamics.

Fig. 1.

Vibrational properties and structure changes under compression. (A) Phonon band structure of experimental P2₁2₁2₁ structure and temporal variation of the volume in the shock compression. (B) Distinctive intermediate structures found in the compression run. The arrows indicate significant changes in the chemical bonding.

Fig. 2.

Candidate structures and phonon structure. (A) Candidate structures of dense CO₂ obtained from shock compression. (B and C) Phonon band structure for Pna2₁-8fu structure at (B) 0 GPa and (C) 40 GPa.

Fig. 3.

Stability of candidate structure. (A) Calculated enthalpies of candidate structures of solid CO₂ as a function of pressure. A, Inset shows the Gibbs free energy difference of Cmca and I-42d structure at 300 K. (B) Equations of states of dense CO₂. (B, Inset) Expanded low-enthalpy region.

Fig. 4.

Calculated XRD data of several candidate structures compared with experiment data.

Fig. 5.

Experiential Raman and metastablity CO₂-V. Raman spectra of CO₂-V obtained during pressure unloading at ambient temperature (A) and low temperatures (B), showing the metastability of CO₂-V at ambient pressure below 200 K.

Fig. S1.

(A) Initial β-cristobalite–like I-42d structures of CO₂-V and the obtained structure in the 100th metastep. (B and C) Plot of evolution of energy with metastep Gaussian width = 3.0 (kbar ų)^1/2 and height = 20.0 kbar ų (B) and Gaussian width = 2.0 (kbar ų)^1/2 and height = 10.0 kbar ų (C).

Fig. S2.

The calculated phonon dispersion of I-42d at ambient pressure.

Fig. 6.

Ideal strength. The calculated tensile and ideal strength (hardness) of I-42d (A and B) and Pna2₁-8fu (C and D) CO₂-V structures.

Fig. 7.

The temporal evolution of the O-C-O angle of different CO₂-IV phases.