Formation of Two-Dimensional Crystals with Square Lattice Structure from the Liquid State

Formation of two-dimensional (2D) crystals with a square lattice structure from the liquid state is studied by molecular dynamics simulation. Models contain 6400 particles interacted via the square interatomic potential proposed by Rechtsman et al. (Phys. Rev. E, 2006, 73, 011406). Evolution of the structure and various thermodynamic properties upon cooling from the melt is analyzed in detail. Crystallization of the system appears to be a first-order-like phase transition. Structural properties of 2D crystals formed by cooling from the melt are analyzed via the radial distribution function, coordination number distribution, ring statistics, bond-orientation order, interatomic distance and bond-angle distributions, and 2D visualization of atomic configurations. We find that the main structural defects in the obtained 2D crystals are vacancies of various sizes and shapes, double triangles splitting from a strongly distorted square, rings of various sizes differing from 4-fold rings, and distorted squares. The atomic mechanism of solidification of the system is studied via analysis of spatiotemporal arrangements of high bond-orientation-order atoms occurring during the cooling process.