Unraveling Thermal Hysteresis in Plastic Crystals—A Pathway to Efficient Solid-State Refrigerants

Plastic crystals are molecules which exhibit a solid-solid phase transition from a translationally and rotationally ordered phase to a rotationally disordered phase, driven by changes in pressure or temperature. This transition is reversible and accompanied by a significantly large isothermal entropy and adiabatic temperature change, posing plastic crystals as a promising alternative for current refrigerants. The thermal hysteresis associated with the plastic crystal transition imposes energetic losses cycle to cycle, diminishing their efficiency for practical application. The magnitude of the thermal hysteresis varies widely across known plastic crystals and has yet to be correlated with a particular parameter. Developing an understanding of the underlying origin of the thermal hysteresis in plastic crystals is critical for developing strategies to reduce the hysteresis for direct application.

In the present work, we explore the origins of hysteresis in three plastic crystals through calorimetric and optical microscopy observations and analyses in an attempt to isolate individual potential contributors to energy dissipation at the solid-solid transformation. Neopentyl glycol was chosen as a model system for the direct order-to-disorder transition, in which it simultaneously undergoes a symmetry change, a volume change, and the onset of complete freedom of rotation. 2-Methyl-2-nitropropane provides a way to decouple rotational freedom, in which complete rotational freedom is gained over two solid-solid phase transitions. Finally, 1-chloro-2,2-dimethylpropane is investigated because it has previously shown the existence of two distinct plastic crystal phases, differentiated solely by crystal symmetry. We highlight the finding that cases in which the order-to-plastic transition is split over multiple steps show significantly smaller magnitudes of thermal hysteresis than those in which the rotational, symmetric, and volumetric changes occur simultaneously. We explore the underlying origin of this phenomenon by evaluating the physical changes taking place during each transition and compare them to the magnitude of thermal hysteresis determined via calorimetry.

To expand the understanding of the origin of hysteresis in plastic crystals, we examine the differences in the magnitude of thermal hysteresis in a variety of other organic compounds which undergo a direct plastic crystal transition. Here, we focus on the molecular contributions to energy dissipation, focusing on both physical and chemical characteristics of the molecules. These include size, sphericity, and energy considerations, including the strain energy required to transform, as well as the intermolecular binding energies determined via energy decomposition analysis. We utilize machine learning methods, including SISSO and PCA, to help reduce the dimensionality of our data in hopes of discovering correlations between particular physical and chemical properties and the magnitude of thermal hysteresis in plastic crystal compounds.