Molecular Mechanisms of Heat Transport across Phase Transitions in Complex Crystals

Abstract

Understanding and controlling heat transfer in solids is a fundamental challenge with broad implications for thermal management, energy efficiency, and the development of next-generation climate technologies. While the phonon gas model has long provided a framework for describing thermal conduction in crystalline solids, its applicability to many functional materials becomes limited by their chemical and structural complexity. In these systems, anharmonicity, large unit cells with anisotropic bonding environments, and stimuli-responsive behavior can give rise to transport regimes that even modern theoretical frameworks may struggle to predict or rationalize. This dissertation investigates the chemical factors that govern thermal transport, combining structural, thermophysical, spectroscopic, and mechanical characterization to examine the influence of molecular-level and extended structure. Through a series of structurally complex crystals that undergo solid-state phase transitions, we interrogate the elusive relationship between local dynamics and thermal conduction by tracking the evolution of vibrational mode character. This body of work ultimately seeks to leverage chemistry as a powerful lens to elucidate how structure shapes thermal transport in complex systems. To contextualize these studies, Chapter 1 surveys vibrational thermal transport models and introduces systems with alkyl chain bilayers that undergo phase transitions between ordered and dynamically disordered states. Chapter 2 establishes a methodological foundation for performing high-quality end-to-end thermal conductivity measurements, with particular emphasis on our development of single crystal deposition techniques to enable reliable thermophysical characterization across this family of layered materials. Building on this foundation, Chapter 3 examines thermal transport mechanisms in a two-dimensional metal–halide perovskite across its order–disorder phase transition. Our results reveal a crossover in energy transport from phonon propagation-dominated to wave-like tunneling regimes, demonstrating how the vibrational transport mechanism can abruptly shift even as the total thermal conductivity remains largely stable. Chapter 4 investigates the manipulation of thermal conductivity through systematic structural modification of layered perovskites, evaluating the roles of alkyl chain length, inorganic layer composition, and non-covalent chemical interactions at the organic–organic interfaces. Chapter 5 extends the transport picture to dialkylammonium halide salts as analogous bilayer systems with distinct chemical interaction networks, but comparable local chain environments and phase-change thermodynamics. Despite the similarities, we observe different thermal conductivity changes across their transitions, highlighting how the extended material framework directly impacts transport behavior in ways not fully captured by the molecular picture. Together, these results help us establish a conceptual framework for understanding thermal transport in chain-bilayer systems and point towards design rules for enhanced control over heat in complex phase-change crystals.