Tetrahedral tumbling in the giant barocaloric ammonium sulfate

Ammonium sulfate is a deceptively simple hydrogen-bonded inorganic salt. It has attracted intense interest, originally for its ferroelectricity and recently for its barocaloric effect; both of these properties arise from a phase transition at -50 ºC. We recently undertook a comprehensive study of the dynamics of this system under pressure, using both inelastic and quasielastic scattering. Our QENS results indicate that the major hopping motion corresponds to tumbling of the tetrahedral ammonium ions, which intriguingly increases in frequency with applied pressure. However, definitive analysis has been hampered by the fact that we initially relied on a previously collected resolution function that proved inadequate, and by the lack of data at high Q, where different models differ most. Here we request a single day on OSIRIS to complete this experiment.