The structural dynamics and barocaloric response of organic ionic plastic crystals

Refrigeration systems account for 17% of global electricity consumption. As global temperatures rise, the demand for sustainable cooling technologies grows. Conventional refrigerants are highly efficient but have significant environmental impacts, necessitating greener alternatives. Barocaloric materials, which undergo large entropy and temperature changes under pressure, offer a promising solid-state solution.

Organic ionic plastic crystals (OIPCs) exhibit multiple solid-solid phase transitions that can be driven by pressure, making them attractive barocaloric materials. OIPCs have tunable interionic interactions, and their characteristic phase transitions from highly orientationally disordered to ordered phase allow for large entropy changes. However, the microscopic mechanisms governing their phase behavior remain unclear.

(Cyanomethyl)trimethylammonium hexafluorophosphate ([N_111CN][PF₆]) undergoes a four-step disorder-order transition with notable thermal hysteresis. Understanding the nature of the disorder in its high-temperature phase is crucial for optimising its barocaloric performance.

This research compares ordered models of the disordered phase to explore its structural flexibility. Ab initio molecular dynamics simulations are employed to explore how the dynamic changes during the phase transition. Phase I is more disordered than other three phases, and the disordered events are likely result from the rotation of the anions [PF]₆^-. These insights will advance the understanding of OIPCs as next-generation solid-state refrigerants. Furthermore, once a reliable metric has been established to quantify such disorders, large scale parallel simulations will be conducted to generate data to train a machine learning model.