Routes to barocaloric materials: importance of rotational dynamics in ammonium sulfate

Solid-state cooling using barocaloric materials is a promising avenue for eco-friendly, inexpensive and efficient cooling. However, in order to design barocaloric compounds it is essential to understand the mechanisms behind this group's large pressure-driven entropy change. To this end, we studied the rotational dynamics in the giant inverse-barocaloric ammonium sulfate. Using a newly developed low-background, high-pressure gas cell, quasi-elastic neutron scattering experiments under pressure have afforded detailed insight into the origin of the barocaloric effect. In the low-entropy phase, jump-rotations of the ammonium cations increase with pressure. This is the result of pressure destabilising the structure and driving the material to the high-entropy phase, where rotations are maximally activated. We argue that this mechanism is the result of competing hydrogen bond networks between the two phases; this feature can be a guide in the search for new caloric materials.