A combined INS and DFT exploration of the lattice dynamics in barocaloric Ammonium Sulfate

As global warming prompts more people to use air conditioning, it is imperative that new environmentally-friendly cooling technologies are developed that break the vicious heating cycle. Ammonium sulfate is a giant inverse barocaloric material, which is cheaply and commercially available, with potential for cooling applications. Its efficient technological exploitation requires an understanding of the mechanism driving the entropy change and heat flow through the material. 

To resolve the question of the origin of the entropy change, we have performed a comprehensive investigation combining neutron scattering with DFT simulations. Our total scattering measurements indicate that the previously favoured simple Boltzmann model of the entropy, where each molecular ion is disordered across the mirror plane in the high temperature phase, is untenable, while our QENS results demonstrate that pressure facilitates the rotation of the ammonium cations, destabilising the crystallographic structure, driving the high-entropy phase transition.

In this presentation we will report an inelastic neutron scattering and density functional theory investigation under working temperature and pressure conditions, revealing that the subtle differences between the Pna2₁ and Pnam structures produce a distinct change in the phonon spectrum, particularly in terms of the ammonium ion librational modes. Going from low to high temperature these modes become more dispersive, lower in energy and have more negative Gruneisen parameters, indicative of a flatter energy surface in their high entropy phase. The demonstrated benefits of a flat energy landscape due to competing hydrogen bonds may provide a new route to caloric materials.