First-Principles Studies of Complex Oxide Materials

Abstract

This thesis uses first-principles methods to study complex oxide materials. The first part of the thesis deals with complex oxide materials that have applications as lithium-ion battery electrodes. In the second part, a new method for the calculation of vibrational properties of correlated materials, specifically transition metal oxides, is developed. After introducing the relevant background and computational methods in Chapters~1 and~2, three chapters are devoted to the study of Wadsley--Roth crystallographic shear phases. This family of niobium-based oxides has attracted significant attention due to their promise as high-rate lithium-ion battery electrodes. Chapter 3 is devoted to the study of the electronic structure and magnetism of niobium suboxides. An electronic structure analysis establishes the coexistence of flat and dispersive energy bands, corresponding to localised and delocalised electron states. These states are shown to be inherent features of the crystal structures. A localisation-delocalisation transition occurs as the structural capacity for localised electrons is exceeded. The results shed light on the experimentally observed electrical and magnetic properties of the niobium suboxides. Chapter 4 examines cation disorder and lithium insertion mechanism of crystallographic shear phases, making use of an enumeration approach to generate sets of cation configurations and lithium-vacancy configurations. A three-step lithium insertion mechanism is revealed, discernible in the evolution of lattice parameters and the voltage profile. A predicted theoretical voltage curve is in good agreement with available experimental data. A distinctive change in the local structure is also discovered: transition metal oxygen octahedra become more symmetric on lithium insertion. The electronic structure behaves as expected for crystallographic shear phases, given the results of the previous chapter: small amounts of localised electrons are present during initial lithium insertion, but on further lithiation, metallicity results. Chapter 5 investigates the lithium diffusion mechanism of niobium tungsten oxide shear structures. Building on the results of the previous two chapters, transition state searches and molecular dynamics simulations were used to obtain hopping barriers and diffusion coefficients. Overall, a quasi-1D diffusion mechanism is observed with low activation barriers (80 - 300 meV) and high diffusion coefficients (10⁻¹² - 10⁻¹¹ m²s⁻¹). Structure-property relationships for crystallographic shear phases are discussed in detail in relation to battery performance. Chapter 6 develops a robust and efficient method to calculate phonons in correlated materials with DFT+DMFT. The method combines a DFT+DMFT force implementation with the direct method for lattice dynamics, using non-diagonal rather than diagonal supercells. In addition, a fixed self-energy approximation is proposed. The method is tested for a set of typical correlated materials, and shown to drastically reduce computational costs compared to previous work.