Librational contributions to entropy in barocaloric quinuclidinium salts

Barocaloric materials are a promising new environmentally friendly refrigeration technology, allowing heat to be pumped using phase transitions associated with large entropy changes. However, the atomic origins of these entropy changes are poorly understood at present, limiting the opportunities to engineer these materials. We have recently shown that phonons, in particular low-energy molecular librational modes, give major contributions to the entropy change even for materials with apparently pure order-disorder transitions. Here we will use TOSCA to measure the phonon density of states of a new family of barocaloric materials, the quinuclidinium salts, to investigate their large entropy changes, focusing on the low-energy librations at low temperature. We have separately applied for MAPS time to understand the changes in the hydrogen bonding network at high energy transfers. Our results will not only be invaluable for understanding this particular material and its immediate family, but will demonstrate the power of inelastic neutron scattering for understanding molecular barocalorics, even when deuteration is impractical.