First-Principles Study of Doping and Defect Effects on Ferroelectric/Electromechanical Properties of Metal Oxides

Abstract

In this thesis, we present a comprehensive first-principles study based on DFT to investigate the effects of doping and defects on the ferroelectric and electromechanical properties of metal oxides. The research focuses on identifying the atomistic mechanisms to overcome the physical limitations of two key systems: HfO2/ZrO2-based ferroelectric memory devices and manganese oxide-based catalyst systems for proton exchange membrane water electrolyzers. First, we propose a structural engineering strategy to address the high switching energy barriers and chronic fatigue issues in conventional HfO2- and ZrO2-based ferroelectrics. Using NEB calculations, we demonstrate that the 2Fe+VO defect complex effectively stabilizes the intermediate states during polarization reversal. This stabilization leads to a dramatic reduction in energy barriers to 26.96 meV/f.u. and 11.67 meV/f.u. for HfO2 and ZrO2, respectively. We show that the strong preference of iron ions for an octahedral coordination environment induces the stable polar monoclinic Pc phase. This mechanism provides a promising methodology for achieving low-voltage operation and enhancing the long-term reliability of next-generation non-volatile memory applications. Second, we elucidate the charge and orbital mechanisms by which neodymium (Nd) doping stabilizes active Mn3+ ions in manganese oxide catalysts under harsh acidic OER conditions. Through DFT-based Bader charge analysis, we identified that Nd ions act as robust electron injectors, significantly enriching the electron density of neighboring Mn atoms. This electron injection facilitates 4f-2p-3d gradient orbital coupling, which effectively suppresses the thermodynamic disproportionation of Mn3+ into Mn2+ and Mn4+, as well as the detrimental overoxidation that leads to Mn leaching. Furthermore, by quantitatively evaluating the changes in Jahn-Teller distortions upon Nd incorporation, we prove that the electronic redistribution-driven local environment control is the decisive factor in securing the thermodynamic stability of the active centers. In conclusion, this work demonstrates that atomistic-level modulation of electronic structures and lattices using dopants and defects is a powerful tool for overcoming the performance limits of metal oxides. The mechanisms identified in this study provide a fundamental theoretical framework for the design of high-performance ferroelectric devices and sustainable energy conversion materials.