Intricate Low Hysteretic Phase Transitions of an Organic Plastic Crystal: Temperature Reliant Mechanical Properties

Abstract

Plastic crystals (PCs) have recently gained renewed interest due to their potential applications in emerging sustainable technologies. PCs, typically characterized by high entropic reversible solid-to-solid phase transitions, facilitate effective energy conversion and can be used as barocaloric materials in refrigeration technologies. However, the intricate relationship between their structure and physical properties associated with the solid-to-solid phase transformations remains underexplored. Moreover, studies focusing on phase transition temperature, hysteresis loop, and mechanical properties are scarce in the literature. Here, we show an ordered crystalline (OC, Phase III) 2-adamantanol (2-Aol) with two extremely low hysteretic reversible solid-to-solid phase transitions, one at 50°C, to a semi-ordered plastic crystal (SOPC) phase (Phase II) and the other at 115.6°C, to a fully re-orientational disordered phase (RD, Phase I). Furthermore, the mechanical properties, obtained using a variable temperature nanoindentation technique, show a significant drop in elastic modulus and hardness upon the OC-to-SOPC (III→II) phase transition, confirming the influence of the reorientational motions of molecules in the high-temperature phase. © 2026 Wiley-VCH GmbH.