Ab initio determination of the phase diagram of CO2 at high pressures and temperatures
Abstract
The experimental study of the CO2 phase diagram is hampered by strong kinetic effects leading to wide regions of metastability and to large uncertainties in the location of phase boundaries. Here we determine the CO2 phase boundaries by means of ab initio calculations of the Gibbs free energy of several molecular and non-molecular solid phases of CO2. Temperature effects are included in the quasi-harmonic approximation. Contrary to previous results, we find that the boundary between non-molecular phases and phase V has a positive slope and starts at 21.5 GPa at T = 0 K. A triple point between phase IV, V, and the liquid phase is found at 35 GPa and 1600 K, indicating a broader region of stability for the non-molecular phases than previously thought. The experimentally determined boundary line between CO2-II and CO2-IV phases is reproduced by our calculations, indicating that kinetic effects are not relevant in that transition.