Structural and Dynamical Properties of Oxygen Vacancy-Enriched Perovskite Oxides

Perovskites are a class of materials that are being extensively investigated for various applications, particularly in energy conversion technologies. One major limitation of these technologies is their high operating temperatures, which usually bottleneck device performance. However, integrating materials that can operate at lower temperatures can greatly improve the efficiency of these systems. Both the LSC (lanthanum strontium cobaltite) and LSGM (lanthanum strontium magnesium gallate) materials satisfy this requirement. The mixed electronic-ionic conductivity of LSC makes it a great candidate for cathode applications in such devices, while the ability of LSGM to conduct only oxygen ions makes it an ideal material for use as an electrolyte.
This dissertation investigates the behaviour of oxygen vacancies in the LSC and LSGM systems through four separate studies:
1. A study on the interaction between an electron beam and brownmillerite LSC material. It was revealed that an electron beam triggers a phase transition of brownmillerite LSC toward an oxygen-deficient perovskite structure. During this transition, the overall oxidation state of the material appears to be preserved, and only the distribution of oxygen vacancies changes from well-ordered to random.
2. A second part of this thesis focuses on the complex interplay between the oxygen vacancies and the crystallographic growth orientation of the LSC material. Brownmillerite-type ordering of oxygen vacancies is observed in (100) and (110) orientations. However, in (111) orientation, a non-uniform oxygen vacancy distribution was detected. Specifically, an approximately 10 nm thick region spanned along the interface with the substrate, where no vacancy ordering was present. Beyond this interfacial region, brownmillerite-type ordering emerged.
3. A study investigating the strain relaxation mechanism in LSC–STF multilayers with varying layer thicknesses and its correlation to the oxygen vacancies. Reciprocal space mapping and atomic-resolution electron microscopy displayed distinct differences in the state of the two systems. Notably, different strain relaxation mechanisms between the samples are reported. In both systems, strain relaxation was accompanied by an oxygen vacancy concentration gradient directed toward the surface.
4. A comprehensive comparison of two LSGM domains, where one exhibits orthorhombic symmetry, while the other has monoclinic symmetry. Although both domains are stoichiometrically identical, the monoclinic domain displays novel oxygen vacancy ordering that is not present in the orthorhombic domain. This highlights the significant influence of crystal symmetry on oxygen vacancy behaviour.