Microstructural Observations of Reversible Phase Transformations in Barocaloric Plastic Crystal Systems

Plastic crystals exhibit a degree of structural order between an ordered crystal, in which molecules have fixed position and orientation, and liquid, in which molecules can rotate and translate in every direction. Small molecules within plastic crystals are fixed on a rigid lattice, but still maintain rotational degrees of freedom. Such plastic crystals typically crystallize in cubic systems (FCC or BCC) at high temperature, and undergo a first order phase transition associated with a large entropy change to lower symmetry phases upon cooling. The transition between ordered crystals and plastic crystals are associated with introduction of rotational degrees of freedom and thus, large changes in entropy, with a pressure-dependent <i>T</i>_cr due to the Clausius-Clapeyron coupling between pressure and volume change at the transition. This pressure and volume change dependence forms the basis for giant barocaloric effects, which can be used in heat pump or refrigeration cycles, and can also be considered to represent a tunable thermal energy storage media, where pressure is used as an independent variable to control the temperature at which heat is stored or discharged. In both cases, the overall efficiency of the refrigeration cycle or the energy storage scheme is dominated in large part by the reversibility of the plastic crystal transition. Thus, understanding the origin of the undercooling in plastic crystal transformations and developing strategies to reduce undercooling is a critical element of advancing these systems towards practical use.<br/><br/>Here, we use calorimetry and optical microscopy to investigate the reversibility of the plastic crystal transformation, and its microstructural evolution through the heating and cooling transformations. We demonstrate that the transformation on heating (low to high symmetry) exhibits a dense concentration of nucleation points and significant phase coexistence, which remains stable when held under isothermal conditions. In contrast, the transformation on cooling is nucleation-limited, resulting in abrupt propagation of a phase boundary throughout the sample volume after nucleation of an under-cooled volume. This cooling transformation does not appear to conserve the original microstructure of the rotationally ordered plastic crystal phase. We report specifically on the role of plastic crystal chemical structure and the impact of introducing small concentrations of other plastic crystal molecules below the solubility limit (&lt;5 wt%) on the degree of undercooling and microstructure of the system. These results reveal the fundamental mechanisms occurring during the transition, including its asymmetric nature, and demonstrate a pathway towards dramatically decreasing undercooling and thereby increasing efficiency in plastic crystal systems.