Direct Observation of Thermal Hysteresis in the Molecular Dynamics of Barocaloric Neopentyl Glycol.

Barocalorics (BCs) are emerging as promising alternatives to vapor-phase refrigerants, which are problematic as they exacerbate climate change when they inevitably leak into the atmosphere. However, the commercialization of BC refrigerants is significantly hindered by hysteresis in the solid-solid phase transition that would be exploited in a refrigeration cycle. Here, we provide new insight into the hysteresis that is a critical step toward the rational design of viable BCs. By studying the benchmark BC plastic crystal, neopentyl glycol (NPG), we observe directly the liberation of the hydroxyl rotational modes that unlock the hydrogen bond network, distinguishing for the first time the molecular reorientation and hydroxymethyl rotational modes. We showcase the use of high-resolution inelastic fixed-window scans in combination with quasielastic neutron scattering measurements to build a comprehensive microscopic understanding of the NPG phase transition, directly tracking the molecular dynamics of the phase transition. Hysteresis previously observed in calorimetric studies of NPG is now observed directly as hysteresis in molecular rotational modes and hence in the formation and disruption of hydrogen bonding. Furthermore, by tracking the thermal activation of three main reorientation modes, we suggest that their fractional excitations may resolve an outstanding discrepancy between the measured and calculated entropy change. These results allow for a direct study of the molecular dynamics that govern the thermal hysteresis of small-molecule energy materials. They will be broadly applicable as many promising BC material families possess first-order transitions involving molecular reorientations.