title: First-principles study of the defect chemistry and conductivity in sodium bismuth titanate
Creators: name: Koch, Leonie
Publisher: name: UNSPECIFIED
Year: 2021

description: The present thesis is an endeavor to understand the defect chemistry and conductivity in polymorphic sodium bismuth titanate (NBT) and NBT-based systems with the ultimate goal to design the next generation of ionic conductors for solid oxide fuel cells. Structural modifications such as non-stoichiometry and doping lead to unexpected high oxygen ionic conductivities in these A-site mixed perovskite structures. Particularly, the non-linear dependency of mobile oxygen vacancies on the defect concentration represents a challenge for scientists worldwide in terms of reliability and controllability. What are the fundamental mechanisms during oxygen vacancy migration and how can we use this knowledge to manipulate electric conductivities in our favor? These two central questions will guide us through the present thesis, where we will explore defect interaction, migration, and charge states in doped and undoped NBT. We discuss these results in the light of different chemical A-site orders, polar, and tilt distortions. For this purpose, we employ density functional theory calculations and focus our analysis on the following main aspects: Electrostatic, covalent, and elastic interactions between defects as well as their coupling to the host lattice. Such a comprehensive defect chemical understanding will allow deriving material properties of structurally and chemically more complex systems such as solid solutions. Furthermore, this knowledge is necessary to establish NBT in versatile industrial applications, ranging from low-loss piezoelectrics to high ionic conductors. After a broad overview of the present research and an introduction to the most relevant concepts in the field of ferroelectric perovskite oxides in the Chapters 1, 2, and 3, we introduce a macroscopic defect chemical model in Chapter 4. This approach is an over-simplification of the defect chemical complexity and neglects several degrees of freedom, for instance, displacement or electronic state occupation fluctuations. However, it illustrates how phase symmetries, dopant concentrations, and dopant types influence the conductivity in NBT. In Chapter 5, we present several novel material properties for the undoped and stoichiometric NBT structure, calculated mainly by density functional perturbation theory. These properties are necessary input parameters for all remaining chapters' calculations and serve as a benchmark of density functional theory approaches for A-site disordered perovskite oxides. A detailed comparison between a Mg-, Fe-, and Al-doping follows in the Chapters 6, 8, 9, and 10. In the former, we primarily deal with the electrostatic interaction between different defect types and local relaxation patterns. In the latter, we focus on charge transition states, elastic effects, and the covalent binding environments. All chapters show a delicate interplay between the electrostatic and the elastic interaction, leading to a reduction of the effective charge carrier concentration. The elastic contribution is particularly prevalent for small dopant types. We further identify the importance of polar and tilt distortions on the association energy between an oxygen vacancy and a neighboring B-site acceptor dopant. Especially in Chapter 6, we contrast our results to experimental impedance spectroscopy measurements. The formation of defect associates between an oxygen vacancy and an aluminium dopant in a ((Na,K)0.5Bi0.5)TiO3-BiAlO3 solid solution is addressed by a combination of nuclear magnetic resonance spectroscopy and ab-initio calculations. In the framework of Smarter Crystallography, we show that stable first-order defect associates only exist at small Al-dopant concentrations.