Shape-Determined Kinetic Pathways in 2D Solid-Solid Phase Transitions

TL;DR

This study uses molecular dynamics simulations to reveal how shape and anisotropic motions influence the kinetic pathways of solid-solid phase transitions in 2D polygonal systems, highlighting shape-dependent defect organization and transition mechanisms.

Contribution

It uncovers the shape-determined kinetic pathways and defect patterns in 2D polygonal systems during phase transitions, emphasizing the role of anisotropic motion coupling.

Findings

Octagon transitions follow quasi-equilibrium pathways.

Pentagon and hexagon transitions are governed by rotational and translational motions.

Distinct defect stripe patterns are observed for different shapes.

Abstract

Solid-solid phase transitions are ubiquitous in nature, but the kinetic pathway of anisotropic particle systems remains elusive, where the coupling between translational and rotational motions plays a critical role in various kinetic processes. Here we investigate this problem by molecular dynamics simulation for two-dimensional ball-stick polygon systems, where pentagon, hexagon, and octagon systems all undergo an isostructural solid-solid phase transition. During heating, the translational motion exhibits merely a homogeneous expansion, whereas the time evolution of body-orientation is shape-determined. The local defects of body-orientation self-organize into a vague stripe for pentagon, a random pattern for hexagon, while a distinct stripe for octagon. The underlying kinetic pathway of octagon adheres to the quasi-equilibrium assumption, whereas the pathways of hexagon and pentagon are governed by translational and rotational motion, respectively. This diversity is originated from different kinetic coupling modes determined by the anisotropy of molecules, and can affect the phase transition rates. The reverse process in terms of cooling follows the same mechanism, with more diverse kinetic pathways attributed to the possible kinetic traps. Our findings promote the theoretical understanding of microscopic kinetics of solid-solid phase transitions as well as provide direct guidance for the rational design of materials utilizing desired kinetic features.