Atomistic Modeling of Martensitic Phase Transition in Hexamethylbenzene

TL;DR

This study uses molecular dynamics simulations to uncover the atomistic mechanism of the martensitic phase transition in hexamethylbenzene, revealing a low-barrier atomic sliding mode as the trigger.

Contribution

First direct MD simulation of HMB's phase transition accurately reproduces experimental transition temperature and hysteresis, elucidating the atomic sliding mechanism.

Findings

Simulation reproduces transition temperature and hysteresis.

Identifies atomic sliding along the (11̅1) plane as key.

Softening of shear modulus observed near transition.

Abstract

Materials exhibiting a martensitic phase transition are essential for applications in shape memory alloys, actuators and sensors. Hexamethylbenzene (HMB) has long been considered as a classical example of ferroelastic organic crystals since Mnyukh's pioneering work in 1970s. However, the atomistic mechanism underlying this phase transition has never been clarified. In this work, we present a direct molecular dynamics simulation to investigate the phase transition mechanism in HMB. For the first time, we report a simulation results that can accurately reproduce both the transition temperature and hysteresis loop observed in previous experimental studies. By analyzing the MD trajectories, the potential energy surface, we identified that a low-barrier atomic sliding mode along the close-packed (11$\overline{1}$) plane of the low-temperature phase is the key to trigger the phase transition at the critical temperature window. This is further confirmed by the observed continuous softening of shear modulus around the transition window. Our results demonstrate that the integration of various atomistic modeling techniques can provide invaluable insight into the martensitic phase transition mechanisms in organic crystals and guide the development of new organic martensites.