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

Area of Science:

• Materials Science
• Physical Chemistry
• Computational Physics

Background:

• Carbon dioxide (CO2) and water (H2O) are abundant triatomic atmospheric gases.
• Bulk CO2 exhibits limited crystalline phases due to weak intermolecular forces, unlike water's complex phase behavior.
• High pressure can induce nonmolecular ordered structures in CO2.

Purpose of the Study:

• To investigate the molecular dynamics simulation results of 2D polymorphs of CO2 molecules in slit nanopores.
• To explore the polymorphism and polyamorphism of CO2 in a 2D confined environment.
• To understand CO2 phase transitions and structural relationships between 2D and 3D phases.

Main Methods:

• Molecular dynamics simulations were employed to study CO2 behavior in nanopores.
• Numerous 2D polymorphs of CO2 molecules were analyzed.
• Thermodynamic paths were explored to observe solid-to-solid phase transitions.

Main Results:

• 2D CO2 polymorphs exhibit extensive crystalline and amorphous structures, demonstrating significant polymorphism and polyamorphism.
• Previously unreported structures, such as wave-like amorphous CO2, were observed.
• Intriguing structural connections between 2D and 3D dry ice phases were identified.

Conclusions:

• Confined 2D CO2 displays remarkable structural diversity, unlike its bulk counterpart.
• Phase transitions in 2D can yield novel CO2 structures.
• The study provides insights into CO2 polyamorphic transitions via intermediate phases.