Molecular scale crystallization dynamic characteristics and melting mechanism of carbon dioxide

Abstract

Clarifying the microscopic phase transition characteristics of carbon dioxide (CO₂) crystallization and melting is of significant theoretical importance for regulating the solid-liquid phase transition behavior of CO₂ in natural gas liquefaction process. This study employs molecular dynamics simulations to investigate the microscopic processes of the crystallization growth of supercooled liquid CO₂ under different pressure conditions, as well as the dynamic characteristics of the melting phase transition in solid CO₂. The results indicate that during crystallization, the CO₂ molecules undergo a liquid-to-solid reconfiguration, with carbon atoms forming a lattice arrangement dominated by face-centered cubic (FCC), accompanied by the coexistence of metastable body-centered cubic (BCC) and hexagonal close-packed (HCP) configurations, suggesting a competitive mechanism among multiple crystal phases during crystallization. Additionally, the melting behavior of solid CO₂ is influenced by the volume of the amorphous atomic regions and the void region, with the melting temperature exhibiting pressure independence and an average value of 215 ± 7.46 K, deviating by only 0.73 % from the equilibrium melting temperature. During melting, the regions containing amorphous atoms gradually expand, and the outward expansion of these disordered regions disrupts the surrounding ordered crystal structure, thereby accelerating the melting process.