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