Hybrid Mechanism of Nucleation and Cooperative Propagation in a Single-Crystal-to-Single-Crystal Transition of a Molecular Crystal

Martensitic transition is a type of solid-phase transition that involves a collective, rapid propagation of crystal structure change. In molecular crystals, such a transition is rarely observed, as most systems exhibit a nucleation and growth mechanism. Thus, the process and mechanism of martensitic transition are underexplored. Here we use in situ microscopy of organic single crystals complemented with interaction energy calculations to provide new insights on martensitic transition. We separate the transition process into three distinct steps where each step corresponds to propagation of structural change in a specific direction. We analyze an initiation stage and two propagation stages from hysteresis and propagation speed during cyclic transitions. We discover the dichotomous role of defects in facilitating the initiation step and hindering the propagation steps of transition. We conclude that the organic martensitic transition shows mixed mechanisms of nucleation and cooperativity. This study presents new experimental evidence of a rare phenomenon that will contribute to expanding the understanding of martensitic transition in molecular crystals