Thermodynamic stability of layered structures in compressed CO2

The crystal structures of the recently discovered nonmolecular phases of CO2 are still the subject of intense debate. Based on density-functional theory calculations, we show that a layered structure (space group P4(2)/nmc) with carbon in tetrahedral coordination is thermodynamically stable between 200 and 900 GPa. The Raman spectrum for this phase agrees with that measured for CO2-VI. Contrary to SiO2, where octahedral coordination for silicon starts with stishovite at about 10 GPa, we find that structures with carbon in octahedral coordination are unlikely to be thermodynamically stable in CO2 at pressures currently reachable in the laboratory. We attribute the exceptional stability of tetrahedral structures in CO2 to the small atomic size of the carbon atom, which allows it to occupy the tetrahedral interstitial sites of the close-packed oxygen sublattices.