Evaluating The Vibrational Character of Thermal Carriers across Order–Disorder Phase Transitions

Materials containing aligned alkyl chains have shown considerable promise for energy applications, including solid-state cooling through caloric effects. Understanding how heat flows through these chains—particularly during phase transitions—can enable precise manipulation of their thermal transport properties, which are critical to optimizing efficiency in practical applications. Many of these materials offer tunable interchain chemistries and thermally responsive structural dynamics, which can offer molecular-level insights—and thus control—of heat conduction mechanisms. Here, we examine thermal transport in two classes of two-dimensional (2-D) layered crystals whose alkyl chains undergo thermally induced order–disorder transitions: (1) hybrid perovskites, and (2) organic salts. Variable temperature frequency-domain thermoreflectance (FDTR) measurements of analogous 2-D perovskite and organic salt structures allow us to describe the role of chemical interactions and chain confinement in dictating thermal conductivity. Microscopy and nanomechanical characterizations similarly describe the influence of structural features and microstructure. Through quasielastic and inelastic neutron scattering, we highlight how the vibrational nature of thermal energy carriers changes as the alkyl chains in these materials transition from ordered to dynamically disordered states. These complementary techniques allow us to detail the key chemical levers that tune thermal transport in phase change materials. Our investigations reveal a comprehensive picture of the mechanisms underlying thermal conduction across phase changes of confined alkyl chains, and how interlayer chemistry regulates heat flow in complex crystals.