Equations of State of Solid CO₂ Phases at Megabar Pressures

The stability of CO₂ phases at pressures up to 1600 GPa is confirmed using evolution methods for predicting crystal structures. Stable CO₂ phases are as follows: \(I\bar {4}2d\) (to 279 GPa), \(P{{4}_{2}}{\text{/}}nmc\) (279–952 GPa), Pbcn (952–1018 GPa), and \(Pa\bar {3}\) (above 1018 GPa). The equations of state for stable CO₂ phases up to pressures of about 1600 GPa are calculated for the first time using ab initio methods and high-temperature calculations within the quasi-harmonic approximation. It is shown that high-pressure \(P{{4}_{2}}{\text{/}}nmc\) , \(Pbcn\) , and \(Pa\bar {3}\) phases have rather high bulk moduli (290–415 GPa). Phases with sixfold coordination of carbon atoms (Pbcn and \(Pa\bar {3}\) ) have higher coefficients of thermal expansion in comparison with the \(P{{4}_{2}}{\text{/}}nmc\) phase.