| dc.description.abstract | The many exotic properties found in complex oxides have great potential for device applications. Complex oxides within the perovskite structure are particularly attractive, as they display a large diversity of chemical compositions and functionality, and are often referred to as inorganic chameleons. Major progress has been made in understanding the complex behaviour of functional perovskite oxides during the last decades. However, there is still a need for fundamental knowledge on how functional properties could be engineered and combined for novel devices. Theoretical studies are important to enable rational design of new device concepts. Density functional theory (DFT) is a particularly powerful tool which provides a theoretical microscope to accurately describe ground state structures. From the structural ground state, functional properties can be predicted. In perovskite oxides, in particular, the distortions of oxygen octahedra are crucial for their functionality. Oxygen positions are challenging to detect experimentally with common methods like X-ray and electron diffraction, but can be accurately predicted by DFT. A big challenge for the field of complex oxides is also to gain control over oxygen stoichiometry. Thus, studies on how oxygen point defects affect the crystal and impact properties is imperative for further developments in this field, in which DFT is an excellent tool.
This thesis was aimed at expanding the fundamental knowledge of the structural and functional behaviour of several important material classes within complex perovskite oxides, how these can be controlled with epitaxial strain and in heterostructures, and their interplay with oxygen vacancies. This has been done using density functional theory. There are four parts of this thesis. In the first part, the interplay between oxygen vacancies, A-site cation size/tolerance factor, epitaxial strain, ferroelectricity and magnetism was investigated in the perovskite manganite series, AMnO3 A=Ca2+, Sr2+, Ba2+). Some general trends were established considering the stability of oxygen vacancies: increasing the volume through either chemical pressure or tensile strain generally lowers the formation energy of neutral oxygen vacancies consistent with their established tendency to expand the lattice. It was also found that increased volume favours polar distortions, both because competing rotations of the oxygen octahedra are suppressed and because Coulomb repulsions associated with cation off-centering is reduced. Ferroelectric polarization was found to favour ferromagnetic over antiferromagnetic ordering because antiferromagnetic superexchange is suppressed as the polar distortion bends the Mn-O-Mn bond angles away from the optimal 180º. Polar distortions were also found to compete with the formation of oxygen vacancies, which have a higher formation energy in the polar phases. Conversely one should expect that the presence of oxygen vacancies suppresses the onset of polarization. In contrast, oxygen vacancy formation energies are lower for ferromagnetic than antiferromagnetic orderings of the same structure type.
Among the II-IV manganites considered in the first part of the work, SrMnO3 is most sensitive to strain-induced phase transitions, owing its intermediate tolerance factor. SrMnO3 was therefore chosen as the model system to explore the structural and functional response of (111)-strain compared with (001)-strain. In the second part of the work, the presence of structural Goldstone-like phonon modes in (111)-strained SrMnO3 was predicted. Goldstone modes are massless particles resulting from spontaneous symmetry breaking. Under compressive strain the coupling between two in-plane rotational instabilities give rise to a Mexican hat shaped energy surface characteristic of a Goldstone mode. Conversely, large tensile strain induces in-plane polar instabilities with no directional preference, giving rise to a continuous polar ground state. Such phonon modes with U(1) symmetry could emulate structural condensed matter Higgs modes. The mass of this Higgs boson, given by the shape of the Mexican hat energy surface, can be tuned by strain through proper choice of substrate.
In the two first parts of this thesis, a strong coupling been oxygen vacancy stability and epitaxial strain was demonstrated. This is due to the fact that oxygen vacancies tend to expand the lattice when formed, known as chemical expansion. In the third part of this thesis, the microscopic origin of chemical expansion due to oxygen vacancy formation was explored in perovskite oxides. The II-IV manganite and II-IV titanate series were used as model systems, having either Ca2+, Sr2+, Ba2+ on the A-site. In particular, the effect of electron localization was elucidated by systematically varying the Hubbard U, and it was found that the localization behaviour is significantly different in the manganites and titanates. The chemical expansion was explicitly calculated for all compounds and it was found that increased electron correlation on the B-site in the lattice yields increased chemical expansion in the manganites and reduced chemical expansion in the titanates. The opposite behaviour of the manganites and titanates arise from different electrostatic screening of oxygen vacancies. This can be attributed to differences in electronic energy levels, specifically that Mn-O bonds are more covalent than Ti-O bonds. Trends with respect to Hubbard U were also rationalized by considering the U/W ratio, where W is the relevant electronic bandwidth. The interplay between ferroelectricity and chemical expansion was discussed for the Ba-containing compounds.
Whereas the two first parts of this thesis explored the isolated effect of epitaxial strain on properties, the last study explored the possibility of utilizing ABO3 hexagonal polytypes as structural and functional control knobs in superlattices with perovskites. The tunability of a metal-insulator transition (MIT) was demonstrated in superlattices of corner-sharing metallic LaNiO3 and face-sharing band-insulator BaNiO3. The important criteria for inducing the MIT in these superlattices include confinement of the number of corner-sharing LaNiO3 layers and the electronic band filling. By confining the number of corner-sharing LaNiO3 layers, this reduces the number of electronic bands at the Fermi level. An appropriate band filling then further induces a charge disproportionation that manifests in an octahedral breathing distortion, which is confined to the corner-sharing (and interface) layers.
Our findings suggest a rich and complex phase diagram within the II-IV manganites, in which defect chemistry, polarization, structure and magnetism can be modified using chemical potential, epitaxial strain and electric or magnetic fields. The interface geometry is important for how strain is accommodated by rotational and polar modes, and importantly has large implications for the energy landscape of the different modes with strain. Our approach to understand the important phenomenon of chemical expansion, based on electronic structure and chemical bonding, paves the way for rational design of chemical expansion in oxide material for energy technology, sensors and actuators. Lastly, an additional control knob for tailoring properties within heterostructures is proposed, which could expand the possibilities of structural control across an interface beyond octahedral corner-sharing. | nb_NO |
