Carbon dioxide exhibits a rich phase diagram at high pressure, with a number of molecular and polymeric phases. Phase III and VII carbon dioxide have remarkably similar structures, with both molecular phases adopting the <i>Cmca</i> space group and exhibiting closely related lattices. Using high-accuracy fragment-based electronic structure methods, we predict the crystalline structures and Raman spectra for these and other phases of carbon dioxide over a range of temperatures and pressures at the level of second-order Moller-Plesset perturbation theory (MP2). We demonstrate near-quantitative reproduction of the key Raman-active lattice phonon modes across multiple different phases. However, the simulated spectra reveal that the experimentally reported structure of phase III is inconsistent with its experimental Raman spectrum. On the other hand, we show that one can re-interpret phase III Raman spectra in terms of the phase VII crystal structure. In the end, based on the combination of structural and spectroscopic simulations, we argue that the phases III and VII are in fact identical.[1] We suggest that the phase III region of the phase diagram corresponds to conditions under which this polymorph is kinetically accessible, while the phase VII region is where it is the thermodynamically preferred phase.

Are carbon dioxide phases III and VII actually the same phase?

2019 Spring National Meeting

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