First-principles Study of Lattice Dynamics in Crystals

Abstract

Lattice dynamics is a key component in solid state physics. It helps the understanding of many physical properties like structural phase transitions and ferroelectricity. Density functional theory, as a first-principles method, is used to investigate the lattice dynamics in this thesis. Followed by an introduction of density functional theory and lattice dynamics, I first study the strain-suppresed polarization switching barriers in layered perovskites. It is shown that the epitaxial strain is strongly coupled with the free energy of different crystal structures, which enables us to tune the energy difference between stable and transition states. The concept of distortion symmetry group is also utilized here to model the switching process accurately. Second, the idea of free-carriers-induced ferroelectricity is introduced. Free charge carriers is typically detrimental to proper ferroelectricity, but it is not the case for hybrid improper ferroelectrics. This unexpected phenomenon will be explained by the electron-enhancement of oxygen octahedral rotation. Group theory analysis and Landau free energy are also carefully looked into in this system. Third, the nature of chemical bonding in transition metal dichalcogenides (TMD) is investigated using Wannier functions. My DFPT results indicate anomalous ionic charges of HfS2 in the in-plane direction, which is also confirmed by infrared and Raman spectrum from our collaborators. The study of Wannier functions attributes this robust ionicity to the hybridization of Hf and S orbitals. Finally, this dissertation is concluded by a brief comment of future opportunities and challenges in this research field.