Structural evolution in high-pressure amorphous CO₂ from textit{ab initio} molecular dynamics

By employing $ab$ $initio$ molecular dynamics simulations at constant pressure, we investigated behavior of amorphous carbon dioxide between 0-100 GPa and 200-500 K and found several new amorphous forms. We focused on evolution of the high-pressure polymeric amorphous form known as a-carbonia on its way down to zero pressure, where it eventually converts into a molecular amorphous solid. During decompression, two nonmolecular amorphous forms with different proportion of three and four-coordinated carbons and two mixed molecular-nonmolecular forms were observed. Transformation from a-carbonia to the molecular state thus appears to proceed discontinuously via several intermediate stages suggesting that solid CO$_2$ might exhibit interesting polyamorphism. We also studied relations of the amorphous forms to their crystalline counterparts. The tetrahedral-like a-carbonia is most probably related to phase-V according to their structural properties, while presence of the mixed forms may reflect hypothetical existence of metastable three-coordinated polymeric phase that is composed of linear chains. Our molecular amorphous form seems to be related to phase-I according to molecular coordination and their relative bond orientations.