The role of hydrogen bond competition in the colossal barocaloric response of choline-based hybrid ionic plastic crystals [choline]2CoCl4 and [choline]2ZnCl4

Traditional refrigeration methods based on vapour compression are environmentally damaging, prompting a need for greener alternatives. Hybrid ionic plastic crystals that exhibit order-disorder structural phase transitions around room temperature with colossal pressure-driven entropy and temperature changes have been identified as viable solid-state refrigerants. The dynamic disorder characteristic of plastic crystals complicates experimental structural probes and requires molecular dynamics simulations to gain insight into the disorder. Thus far however, the dynamics of the disordered phase are seldom unambiguously characterised and rarely linked to how they drive the phase transition. In this work we investigate novel choline-based plastic crystals, [choline]2CoCl4 and [choline]2ZnCl4, and through comprehensive characterisation via variable pressure-temperature diffraction and ab initio molecular dynamics simulations we show that the microscopic mechanism for the colossal entropy changes and the disorder observed can be attributed to hydrogen bonding competition in the disordered orthorhombic (P mcn) and ordered monoclinic (P 21/c) phases. Our study illustrates the potential of choline-based plastic crystals as viable candidates for barocaloric refrigeration and shows that the combination of theoretical and experimental approaches presented here can lead to an unprecedented insight into the structural dynamics which drive colossal entropy changes and competitive reversible adiabatic temperatures at low working pressures.