Abstract
Both carbon dioxide (CO(2)) and water (H(2)O) are triatomic molecules that are ubiquitous in nature, and both are among the five most abundant gases in the Earth's atmosphere. At low temperature and ambient pressure, both CO(2) and H(2)O form molecular crystals--dry ice I and ice I (h) Because water possesses distinctive hydrogen bonds, it exhibits intricate and highly pressure-dependent phase behavior, including at least 17 crystalline ice phases and three amorphous ice phases. In contrast, due to its weak van der Waals intermolecular interactions, CO(2) exhibits fewer crystalline phases except at extremely high pressures, where nonmolecular ordered structures arise. Herein, we show the molecular dynamics simulation results of numerous 2D polymorphs of CO(2) molecules in slit nanopores. Unlike bulk polymorphs of CO(2), 2D CO(2) polymorphs exhibit myriad crystalline and amorphous structures, showing remarkable polymorphism and polyamorphism. We also show that depending on the thermodynamic path, 2D solid-to-solid phase transitions can give rise to previously unreported structures, e.g., wave-like amorphous CO(2) structures. Our simulation also suggests intriguing structural connections between 2D and 3D dry ice phases (e.g., Cmca and PA-3) and offers insights into CO(2) polyamorphic transitions through intermediate liquid or amorphous phases.