Towards a Molecular-Level Understanding of Entropy Changes in Layered Barocaloric Materials via Quasielastic Neutron Scattering

Solid-state, barocaloric materials—materials which undergo thermal changes in response to applied pressure—have emerged as a promising alternative to conventional yet environmentally-hazardous hydrofluorocarbon refrigerants. In particular, materials featuring bilayers of long hydrocarbon chains—including hybrid organic-inorganic perovskites and symmetric di-<i>n</i>-alkylammonium salts—have recently been reported to drive sizable entropy changes (&gt; 100 J kg^–1 K^–1) at relatively low driving pressures (&lt; 200 bar). These substantial barocaloric effects arise from the solid-state ordering and disordering of hydrocarbon chains, suggesting that the manipulation of hydrocarbon-disordering thermodynamics is a particularly promising direction for barocaloric material design. Critical to the development of such design principles is an understanding of the molecular mechanisms that drive the entropy changes in these materials—that is, what are the types and ranges of motion that disordering hydrocarbon chains undergo? <br/><br/>Here, we present experimental neutron scattering investigations into the molecular motions of hydrocarbon chains in hybrid perovskites and di-<i>n</i>-alkylammonium salts before and after their respective order-disorder transitions. Supplementing quasielastic neutron scattering (QENS) experiments with machine-learning molecular dynamics (MLMD) simulations, we show that the same length hydrocarbon chain accesses fundamentally different geometries and amplitudes of motion across the order-disorder transition when confined in a hybrid perovskite <i>vs.</i> di-<i>n</i>-alkylammonium salt architecture. In doing so, we establish a molecular basis for understanding why molar entropy changes are nearly twice as large in a di-<i>n</i>-alkylammonium salt than in a hybrid perovskite. Ultimately, this work advances a molecular-level understanding of chain disordering-driven entropy changes and offers insights towards the precise and rational design of barocaloric materials.