Barocalorics for green cooling: from understanding to design

Cooling is vital to modern civilisation on a rapidly warming planet, with uses from domestic air conditioning to computing clusters and transport and storage of food and medicine. Unfortunately, the currently dominant technology, vapour-compression refrigeration, is energy-inefficient and environmentally damaging. It relies on fluid refrigerants that are greenhouse gases and often ozone depletors, many of which have now been banned by a series of international treaties beginning with the Montreal Protocol (1987). A replacement is urgently needed. The most promising alternative technology instead uses barocaloric materials, which can be driven between low- and high-entropy phases by applying pressure. These materials can thus be used as refrigerants in a cooling cycle. Because both phases are solid, there is no risk of leakage or environmental contamination, and the refrigerant can easily be recycled. Until recently, such materials were thought to be rather rare. However, work over the past five years – including our own – has demonstrated that barocalorics can be found in a wide range of materials families, including metals, molecular crystals, molecular-ionic salts, and coordination frameworks of different dimensionalities. This discovery is certainly encouraging, but it brings its own challenges: in such a vast and chemically diverse parameter space, how are we to identify the materials with the best barocaloric properties, let alone to design new materials for this purpose? In fact, this problem is at the very forefront of modern crystal engineering. Almost all methods for interpreting scattering data in terms of atomic structure and dynamics rely on perfect crystalline order. Yet the high-entropy phases of barocaloric materials necessarily involve substantial structural disorder. Understanding this behaviour, therefore, will necessarily involve developing new theoretical and computational capacity. Here, we propose not only this, but to go one step further and design new materials with high-entropy states making them industrially applicable barocalorics. Both the structure and dynamics of a material contribute to its entropy. Neutron scattering is the ideal experimental method for understanding these, because it is exquisitely sensitive to both. To extract a maximum of information, these experiments must be supported by detailed atomistic modelling. Our team is expert in both the experimental and computational sides of this work, capable of leading the developments in sample preparation, characterisation, and simulation that will be needed. We will perform elastic, quasielastic and inelastic scattering studies of the most promising materials across a chemically and physically diverse range of families. Then we will use atomistic simulation, taking advantage of machine-learning-based potentials to achieve the length and time scales needed, to interpret our results. Thus in this project, first, we will develop the theoretical apparatus both to simulate the behaviour of barocaloric materials and to interpret experiment. This will be relevant to the study of disordered materials far beyond our own targets. Second, we will achieve an understanding of the atomistic origin of these materials' entropy encompassing all relevant sources – including configurational, rotational, and vibrational – and materials classes. Third and finally, our results will point the way to new materials that rival the properties of vapour-liquid refrigerants.