Neutron studies of the structure and dynamics of barocaloric Ammonium Sulfate

Conventional vapour-compression refrigeration relies on refrigerants that contribute both to global warming and ozone depletion. Barocaloric materials, in which a large isothermal entropy change is associated with a pressure-induced phase transition, offer an eco-friendly alternative.

Recently a giant barocaloric effect has been demonstrated near the ferrielectric phase transition of ammonium sulfate [1], which is cheaply and widely available. The transition is associated with a symmetry change from Pnma to Pna2₁ at ~224 K at ambient pressure, while small changes in hydrostatic pressure drive giant inverse barocaloric effects near the ferrielectric phase transition.

The phase transition in (NH₄)₂SO₄ has been extensively studied, including crystallographic, Raman, reflectivity, NMR and infrared investigations, yet its exact nature is surprisingly still a matter of debate. We will present our neutron scattering experiments (total scattering, INS and QENS) probing the structure and dynamics of this material, which aim to identify the characteristics responsible for its exceptional barocaloric behaviour; and hence create a roadmap to develop new barocaloric materials and optimise their properties.

[1] Lloveras et al., Nat. Comm. 6, 8801 (2015)