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Probing Hidden Phase Transitions in Plastic Crystal Succinonitrile
Hiroshi Abe1, Keiko Nishikawa2, Kozo Fujii2
1Department of Materials Science and Engineering, National Defense Academy, Yokosuka 239-8686, Japan.
The Journal of Physical Chemistry Letters
|April 1, 2026
Summary
A hidden phase transition in succinonitrile (SN) plastic crystals (PCs) was discovered using slow heating. This transition, involving symmetry reduction, is only observable with ultralow-rate synchrotron X-ray scattering.
Area of Science:
- Materials Science
- Crystallography
- Phase Transitions
Background:
- Succinonitrile (SN) exists as trans (0 D) and gauche (5.45 D) conformers.
- Plastic crystals (PCs) are known for their unique orientational disorder.
- Understanding phase transitions in molecular crystals is crucial for materials development.
Purpose of the Study:
- To investigate the phase behavior of succinonitrile (SN) under slow heating conditions.
- To characterize the structural changes associated with a potential hidden phase transition.
- To determine the influence of heating rate on the observed phase transitions.
Main Methods:
- Synchrotron small- and wide-angle X-ray scattering (SWAXS) was employed.
- Measurements were conducted during slow heating (0.2 K/min) and conventional heating (∼5 K/min).
- Analysis focused on Bragg reflection splitting and diffuse scattering patterns.
Main Results:
- A previously hidden phase transition from a cubic plastic crystal (PC) phase to a lower-symmetry ferro-like ordered (fC) phase was observed.
- The PC-fC transition involves a reduction in crystal symmetry.
- This transition was exclusively detected at ultralow heating rates, with additional diffuse scattering observed.
Conclusions:
- Succinonitrile exhibits a hidden phase transition that is highly sensitive to heating rate.
- The ferro-like ordered (fC) phase is stable across thermal cycles.
- Ultralow-rate synchrotron SWAXS is essential for revealing subtle phase transitions in plastic crystals.
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