The Effects of Lattice Anharmonicity on Phase Transitions and Thermal Conductivity Through Phonon Measurements in Energy Materials

Abstract

Advances in neutron scattering instrumentation and first-principles calculations (due to increasing computational power) now allow for detailed studies of phonon anharmonicity, even in relatively complex systems. The work in this thesis leverages such improvements to gain unprecedented insights into the lattice dynamics of ultra-low thermal conductivity materials, structural phase transitions, phonon renormalization with temperature, and the impacts of anharmonic lattice vibrations on electrical properties. These phenomena are studied in a number of important materials for energy conversion technologies and the resultant fundamental insights will prove valuable to engineering advanced materials for improved performance.A spectacular and highly unusual softening of a whole manifold of acoustic and optical phonon modes was uncovered in thermoelectric materials SnSe and SnS as they approach structural phase transitions. This was achieved though detailed phonon measurements which will be invaluable to advancing anharmonic phonon theories. These measurements resolved a debate in the literature regarding the mechanism of transition. The experimentally observed drastic phonon renormalization with temperature, which is neglected in most calculations, has significant implications on thermal conductivity. Such effects were probed with first-principles simulations benchmarked against experimental data and demonstrated that phonon renormalization is vitally important to understanding thermal conductivity in highly anharmonic materials, such as those close to lattice instabilities.Highly unusual two-dimensional lattice dynamics was uncovered in cubic CsPbBr3. Through detailed neutron and x-ray measurements alongside first-principles simulations we were able to specifically identify how these fluctuations arise from overdamped zone-boundary phonons associated with lead-halide octahedral rotations. We showed that they significantly alter the electronic structure which has been largely ignored in the literature. These results are highly relevant to optimizing thermal and electronic properties, and their coupling, in these technologically-pertinent materials.There is ongoing debate surrounding the vibrational properties of fillers in skutterudites and how they impact thermal conductivity. From studying the skutterudite Yb-filled CoSb3, I have shown that the thermal conductivity is reduced through a complex interplay of phonon-disorder and phonon-phonon scattering which is highly-temperature dependent. This was demonstrated through the benchmarking of advanced machine learned molecular dynamics against detailed experimental measurements. I further found that filling leads to a broadening of the entire phonon density of states (PDOS) and a softening of higher energy modes. The shape of the Yb modes in the PDOS depends significantly on filler concentration and leads to an effective stiffening with additional filling.