Thermal Transport Mechanisms of Phonons, Diffusons, Electrons, and Ions in Functional Materials

The design of advanced functional materials relies on a fundamental understanding of heat transport processes, which are governed by multiple thermal carriers including phonons, diffusons, electrons, and mobile ions. Each of these carriers interacts with the underlying atomic structure through mechanisms closely related to lattice dynamics and anharmonicity. In this thesis, I systematically investigate thermal transport behavior in solid-state materials—ranging from two-dimensional crystals to layered and bulk compounds—with a focus on identifying the distinct contributions from phononic, diffusonic, electronic, and ionic channels. Employing a theoretical framework which incorporates first-principles anharmonic lattice dynamics into a unified heat transport theory, I explore how intrinsic structural features and anharmonic interactions influence thermal conductivity. The findings provide insights into carrier scattering processes, transport anisotropy, and temperature-dependent conductivity trends. Firstly, I focus on the impact of phonon transport on thermal properties by employing the two-channel model, which distinguishes between propagating phonon modes and diffusive (locally confined or non-propagating) vibrational modes. In Chapter 3, I examine the potential utilization of the diverse crystal structure found in Cu2Te for on/off thermal conductivity switching. In Chapter 4, I find that the nitride perovskite LaWN3 displays strong anharmonic lattice dynamics manifested into a low lattice thermal conductivity and a non-standard temperature dependence. In Chapter 5, I investigate the thermal properties of Ag8SnSe6. Despite being a crystal, Ag8SnSe6 exhibits an exceptionally-low and nearly temperature independent lattice thermal conductivity.
Secondly, another key heat carrier, electrons, is introduced and investigated. In Chapters 6, I focus on the effects of high order phonon scattering and electron-phonon scattering on thermal properties in two-dimensional 2H-TaS2. In Chapters 7, the early concept of a “phonon-glass-electron-crystal” for enhancing the thermoelectric figure of merit (ZT) is explored theoretically in layered Ge-Se crystals, where phonon transport exhibits wave-like behavior.
Finally, I employ a hybrid approach combining the Green-Kubo formalism and molecular dynamics based on machine-learning potential to predict the thermal conductivity of Li3OCl. I systematically analyze the contributions of atomic vibrations, ion transport, and vibrationsions couplings to its thermal conductivity. It is found that as temperatures increase, Li3OCl transitions into a quasi-liquid state, resulting in a substantial ion migration that contributes non-negligibly to thermal conductivity. These findings are expected to enhance the fundamental understanding of thermal properties in solids and promote the practical applications of functional materials.