Two-dimensional dry ices with rich polymorphic and polyamorphic phase behavior.

Abstract

Both carbon dioxide (CO₂) and water (H₂O) 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 CO₂ and H₂O form molecular crystals--dry ice I and ice I _<i>h</i> 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, CO₂ 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 CO₂ molecules in slit nanopores. Unlike bulk polymorphs of CO₂, 2D CO₂ 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 CO₂ structures. Our simulation also suggests intriguing structural connections between 2D and 3D dry ice phases (e.g., <i>Cmca</i> and PA-3) and offers insights into CO₂ polyamorphic transitions through intermediate liquid or amorphous phases.