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: I4̅2d (to 279 GPa), P4_2/nmc (279–952 GPa), Pbcn (952–1018 GPa), and Pa3̅ (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 P4_2/nmc , Pbcn , and Pa3̅ phases have rather high bulk moduli (290–415 GPa). Phases with sixfold coordination of carbon atoms (Pbcn and Pa3̅ ) have higher coefficients of thermal expansion in comparison with the P4_2/nmc phase.