UC Berkeley

Materials with strong coupling between structural and electronic degrees of freedom (charge, orbital and spin) are of interest for a wide array of functional applications. Localized electronic states resulting from point defects and bound-excitons in ionic materials influence functional behavior of these materials. We explore the consequences of such couplings on defect properties and excited states, for cases where intrinsic charge and structural disorder give rise to a variety of local environments. Using Density Functional Theory (DFT) with Hubbard U correction and hybrid density functional theory levels of theory, we study how local distortions and chemical disorder influence the localized electronic states determining bulk properties such as defect thermodynamics and scintillation. The work presented in this thesis focuses on two illustrative cases.We investigate the energetics underlying charge ordering, defect formation and clustering in iron oxides. For magnetite (Fe3O4), we study the energetics underlying the ordering of Fe2+ and Fe3+ cations, and the effect of this ordering on point defect energetics and electronic structure. The calculations reveal strong variations in anion and cation vacancy formation energetics and stable charge states with changes in local environment. We also extend this study to oxygen-iron vacancy complexes in magnetite (Fe3O4) and hematite (Fe2O3) and find a significant thermodynamic driving force for oxygen and iron vacancies to bind. The results have potentially important implications for ionic transport, as the low energy defect configurations in certain local chemical environments can lead to percolating networks of low-energy migration pathways. In the second topic, we consider the nature of localized exciton states in Tl2LiYCl6, a promising candidate for commercial scintillator applications such as γ-ray detection. The driving forces for structural distortions away from the idealized cubic elpasolite crystal structure are discussed as originating from maximizing the energy lowering interactions of Tl and Cl orbitals. These distortions influence the bulk electronic structure and lead to different stable and metastable exciton configurations. Comparison of electron-hole binding energies of exciton configurations show that the Tl+ centered exciton exhibits the best agreement with the broad peak, as observed in the measured emission spectra. Collectively, this work explores complexities of simulating localized defect and exciton electronic states in materials with variations in the coupled local structural and chemical environments. This research motivates further studies into developing approaches for sampling defect and exciton configurations generated by coupling of localized electronic states with structural and chemical order in the bulk.