Defense:Chengjie Mao- Phase transitions, structural fluctuations and temperature dependent thermal conductivities of energy materials from phonon anharmonicty

Anharmonic lattice dynamics underlie many key material properties, including thermal transport, ferroelectricity, and structural phase transitions. Understanding the couplings between phonons, electrons, and other microscopic degrees of freedom is essential for the design of next-generation energy materials. This research combines inelastic neutron and X-ray scattering with first-principles simulations to explore phonon anharmonicity and its role in energy transport across two representative systems: elemental bismuth and halide perovskites. In bismuth, we investigate the strong temperature-dependent phonon softening and broadening driven by anharmonic interactions and electron-phonon coupling. Using inelastic neutron scattering and anharmonic phonon calculations, we quantify the acoustic phonon energy shifts and lifetimes, revealing the microscopic origin of temperature dependence of lattice thermal conductivity and energy dissipation processes. In parallel, we study inorganic and hybrid halide perovskites, including CsSnBr3, CsPbBr3, Cs2AgBiBr6, and MAPbI3, which exhibit ultra-soft lattice dynamics and complex structural phase transitions. Diffuse scattering and phonon measurements uncover low-energy soft modes and domain formation linked to octahedral rotations and molecular dynamics. Our results show how anharmonic lattice behavior governs their thermal and optoelectronic properties, offering insights into long carrier lifetimes and low thermal conductivity. Together, these studies demonstrate how experimental and computational tools can jointly reveal the fundamental role of anharmonic phonons in controlling the behavior of quantum and energy-relevant materials.