Probing Hidden Phase Transitions in Plastic Crystal Succinonitrile.

A hidden phase transition in plastic crystal (PC) succinonitrile (SN) was revealed during slow heating. SN exhibits trans (0 D) and gauche (5.45 D) conformers. Synchrotron small- and wide-angle X-ray scattering (SWAXS) measurements detected the splitting of Bragg reflections near the melting point, indicating a symmetry reduction. Upon heating, the cubic PC phase transformed into a lower-symmetry crystal, namely a distinct ferro-like ordered phase (hereafter fC), which remained stable over the repeated thermal cycles. During the gradual PC-fC transition, additional diffuse scattering was observed in addition to orientational short-range-order diffuse scattering. Such a phase transition remains inaccessible under conventional heating rates (∼5 K/min) and is exclusively revealed by ultralow-rate (0.2 K/min) synchrotron SWAXS.