Understanding temperature-pressure dependence of molecular dynamics influencing thermal hysteresis in barocaloric plastic crystal composites

Plastic crystals (PCs) are emerging as highly promising phase change materials for applications in solid-state energy storage and barocaloric heating & cooling technologies. This is attributed to their pressure-driven solid-solid first order phase transitions (FOPT), characterized by significant entropy changes associated with molecular rotations. However, notable drawbacks associated with these materials include low thermal conductivity and significant thermal hysteresis. We have recently demonstrated that the thermal hysteresis of the plastic crystal neopentyl glycol (NPG) can be reduced to varying degrees by forming composites with inorganic inclusions. Our prior quasielastic neutron scattering (QENS) experiments have been essential in determining that inclusions play a pivotal role in altering molecular reorientations approaching phase transitions in NPG composites, leading to a notable reduction in thermal hysteresis. Here we propose to explore the effect of both the inclusion type and hydrostatic pressure on the molecular dynamics in NPG composites. The results will provide a more complete picture of the physical processes underlying thermal hysteresis in BC PCs, representing a crucial step towards the systematic design of novel barocaloric materials with practical applications in advancing solid-state heating & cooling technologies.