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

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 this problem is investigated by molecular dynamics simulation for 2D 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 those of hexagon and pentagon are predominantly governed by translational motion 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. These findings promote 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.