Prediction of a high-pressure phase transition and other properties of solidCO2at low temperatures

An optimization procedure is used to minimize the Gibbs free energy of solid ${\mathrm{CO}}_2$ at pressures 0≤P≤16 GPa, with respect to the unit-cell parameters, center-of-mass coordinates, and orientations of all the independent molecules. A lattice-dynamics procedure is used to calculate the libron frequencies. The observed Pa3 structure is predicted at low pressures, and a transition to an orthorhombic Cmca phase is found at P=4.3 GPa, with a volume change ΔV=0.3 ${\mathrm{cm}}^3$/mole. The calculated second virial coefficients, sublimation energy, molar volume, pressure-volume relation, and libron frequencies in the Pa3 phase are in good agreement with experiment. In the Cmca phase the calculated libron frequencies agree fairly well with recent Raman scattering results. An analysis of the potential-energy surface shows that the large observed hysteresis associated with the transition may be the result of large barriers between the phases.