Phonon density of states of barocaloric [choline]2ZnCl4 ionic plastic crystals via variable temperature inelastic neutron scattering studies

Ionic plastic crystals (IPCs) are solids which undergo order-disorder structural phase transitions leading to significant orientational disorder in the crystal structure in the high-temperature phase. As a result, these materials have a rich landscape of temperature driven solid-solid phase transitions. Recent discoveries of colossal barocaloric effects in molecular plastic crystals (MPCs) suggest that PCs in general could have a huge impact on the future of solid-state cooling technology. We have recently synthesised an IPC based on the choline cation [(CH3)3(NCH2CH2OH)]ZnCl4 (CHOZn) which exhibits a pressure-induced entropy change of ~130 J/kg K across its order-disorder phase transition. Crystallography studies in tandem with ab initio molecular dynamics (AIMD) simulations show that hydrogen bonding plays a crucial role in the order-disorder phase transition. The phonon density of states calculated from the AIMD simulations, for both the ordered and disordered phases, show that the major contribution to the total entropy change is the change in vibrational entropy and is dominated by the weak intermolecular interactions (soft phonons). To compare against the phonon density of states calculated from AIMD simulations, we propose to experimentally determine the phonon density of states for CHOZn using inelastic neutron scattering experiments on the MARI spectrometer and observe its evolution with temperature across the temperature-driven phase transition.