An ab initio Method to Predict Phase Transitions in crystalline CO2

A crystal can exist in different phases and can change from one crystal symmetry to another when varying ranges of temperatures and pressures are applied. Solid Carbon dioxide (CO2) has been widely researched over decades mainly due its structural simplicity and importance in terrestrial chemistry, planetary chemistry, its abundance and its rich polymorphism. Various Density Functional Theory (DFT) methods have been devised and yet there exists challenges in computationally evaluating the interactions that exist in crystals using DFT. Here we consider an ab initio method which involves the calculation of Gibbs free energies and hence predicting the phase transition diagrams of solid CO2 and its phases I,II and III using Density Functional Perturbation theory (DFPT). The DFPT method is able to model accurately the crystal structure, at any given temperature and pressure which is based on the electronic interactions and forces acting on the lattice structures. We predict that for low values of temperatures, as pressure increases CO2 changes its crystalline phase from I to III and for higher values of temperature and pressure we observe a transition from phase III to I. The results obtained are in excellent agreement with the experimental results.