Understanding the barocaloric response of choline-based hybrid ionic plastic crystals [choline]2CoCl4 and [choline]2ZnCl4

Abstract

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 large pressure-driven entropy and temperature changes have been identified as viable solid-state refrigerants. The characteristic dynamic disorder of plastic crystals complicates experimental structural probes and requires molecular dynamics simulations to gain insight into the disorder. However, the dynamics of the disordered phase are seldom unambiguously characterized and rarely linked to how they drive the phase transition. In this work, we investigate the barocaloric properties of choline-based plastic crystals [choline]₂CoCl₄ and [choline]₂ZnCl₄. Through comprehensive characterization via variable pressure–temperature diffraction and ab initio molecular dynamics simulations we show that the microscopic mechanism for the observed crystallographic disorder and entropy changes can be attributed to hydrogen bonding competition in the disordered orthorhombic (Pmcn) and ordered monoclinic (P2₁/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 provides a comprehensive insight into the structural dynamics that drive competitive reversible adiabatic temperature and entropy changes at low working pressures.