Structural and dynamic heterogenization and phase transition in plastic crystal phase: Taking a representative molecular plastic crystal and an ionic one as examples

A plastic crystal (PC) has a crystalline phase characterized by disordered molecular (or ionic) orientations while maintaining a regular lattice of their positions. It is in a molten state from the perspective of orientation, while in a crystalline state from the perspective of position. Recent studies have reported heterogeneous dynamics within the PC phase, regardless of whether it is a molecular PC (MPC) or an ionic PC (IPC). By measuring the spin-lattice relaxation time (T1) and the spin-spin relaxation time (T2) using low-frequency NMR, it is possible to separately observe the reorientation motion of molecules (or ions) from T1 and their translational motion from T2. We present the results of relaxation time measurements using an MPC and an IPC samples. The heterogeneous dynamics is attributed to the existence of static heterogeneous regions in the PC phase: the core region and the surrounding region enveloping it. We demonstrate that molecules (or ions) within each region exhibit distinct translational motion behaviors, with the surrounding region particularly leading to a pre-melting phenomenon. This heterogenization may be described “a phase separation in solid phase”.