Pressure dependence of rotational dynamics in barocaloric ammonium sulfate

Solid-state cooling using barocaloric materials is a promising avenue for eco-friendly, inexpensive and highly efficient cooling. To design compounds ready deployment, it essential to understand their thermodynamic behaviour under working conditions. this end, we have studied the rotational dynamics in molecular-ionic crystal ammonium sulfate pressure, providing detailed insight into origin of its large effect. Using quasielastic neutron scattering experiments, show that rotation cations facilitated by pressure low-entropy phase, with "hopping" motion increasing frequency as pressure-induced phase transition approached. We explain unusual terms competing hydrogen-bond networks represented two phases. This work includes first results recently developed low-background, high-pressure gas cell scattering, showcasing power obtaining high-precision measurements molecular pressure.