Atomistic modeling of martensitic phase transition in hexamethylbenzene

Materials exhibiting martensitic phase transitions are essential for applications in shape memory alloys, actuators, and sensors. Hexamethylbenzene (HMB) has long been considered a classical example of ferroelastic organic crystals since Mnyukh's pioneering work in the 1970s. However, the atomistic mechanism underlying this phase transition has not been clarified. In this work, we present a molecular dynamics (MD) simulation to directly model the phase transition mechanism in HMB. For the first time, we report simulation results that accurately reproduce both the transition temperature and hysteresis loop observed in previous experimental studies. By analyzing the MD trajectories and potential energy surface, we identified that a low-barrier atomic sliding mode along the close-packed (11) plane of the low-temperature phase is key to triggering 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 insights into martensitic phase transition mechanisms in organic crystals and guide the development of new organic martensites.