Discrete breathers in barocaloric ammonium salts

Barocaloric materials, which can be cycled between high- and low-entropy phases by applying pressure, provide an important, environmentally friendly alternative to vapour-compression refrigeration. However, a far greater understanding of the structural origins of the entropy change is needed in order to systematically improve these materials’ properties to the point of commercial value. One unexplored route to increasing the entropy is to take advantage of “discrete breathers”, intrinsically localised modes that can form as the result of anharmonic interactions between the molecular components of a crystal. Here we propose a detailed exploration of these modes in a simple barocaloric, ND4I, in which they have been theoretically predicted but never experimentally observed. Our results will demonstrate whether these modes can be used as a design tool for entropic materials. We will further compare them against molecular simulations as part of a wider ISIS-funded PhD project on molecular modelling of inelastic and quasielastic scattering data.