On the defective origin of conductive and structural properties of oxides: insights from first principles

Doktorsavhandling, 2017

Oxides are versatile materials with applications in many different research fields; especially those related to clean energy technologies, such as fuel cells, batteries and solar panels. Many functional properties of these materials derive from lattice imperfections, or defects, and a lot of effort has been put into fine tuning these materials by modifying their structure on the atomic scale. This lays the foundation for this thesis, where the aim has been to explain the underlying mechanisms of properties in a selection of oxides, mainly those with the perovskite structure ABO3, by studying a variety of different defect species: oxygen vacancies, protons and hydride ions, acceptor dopants, electron and hole polarons, and grain boundaries (GBs). The methodology employed here constitutes a set of first-principles methods, mainly different flavors of density-functional theory (DFT) but also perturbation theory within the G0W0 approximation, assisted by thermodynamic modeling.   The major part of the thesis is focused on defects in BaZrO3, an oxide that when acceptor-doped and exposed to water acts as one of the most promising ceramic proton conductors by combining high bulk conductivity with high stability. The total proton conductivity in this material is, however, limited by high impedance at GBs due to the formation of space charges at these interfaces. This issue is addressed in four of the appended papers, which include studies on several different GBs as well as a comparison with GBs in the similar perovskite BaCeO3. Acceptor-doped BaZrO3 is also a p-type conductor under oxidizing conditions and this serves as the motivation for two papers, which address the methodology required for a proper theoretical description of holes in these materials. The final paper on BaZrO3 sets out to describe the chemical expansion the material exhibits upon hydration. The work extends beyond conventional ceramic proton conductors with a study on the novel oxyhydride material BaTiO3-xHx, where the electronic and vibrational properties of hydride ions are investigated. Finally, the thesis is concluded with a study of the oxygen vacancy in a set of binary and ternary oxides, where the aim is to show the general behavior this defect displays in these materials despite the fact that both the electronic and ionic structure varies significantly between the different compounds.