Christin Böhme

In this work, we systematically investigate the structure of Sr and Mg doped lanthanum gallate single crystals using x-ray diffraction (XRD) and transmission electron microscopy (TEM). XRD analysis revealed the presence of multiple domains with distinct lattice parameters. High-resolution TEM and integrated differential phase contrast imaging captured two regions, one with 90° symmetry and the other with non-90° symmetry. In addition, ordered oxygen vacancies in the non-90° region were revealed, while vacancies in the 90° region were randomly distributed. Energy-dispersive x-ray spectroscopy confirmed that the difference in the doping level of the two regions is within the detectability limits. However, the electron energy loss near-edge structure revealed detectable differences in their bonding character.

In this work, we investigated La0.6Sr0.4CoO3−δ / SrTi0.3Fe0.7O3−δ multilayer systems with different layer thicknesses. Reciprocal space mapping showed splitting of the reflections in the sample with 5 nm layers while for 15 nm thick layers reflections have been elongated. Using transmission electron microscopy, we investigated this phenomenon at the atomic scale and showed that the alternating 15 nm layers gradually increase their tensile out-of-plane strain, whereas the sample with 5 nm layers maintains a relatively stable strain state. In-plane strain relaxation is similar in both samples. Still, it differs in the strain relaxation mechanism, which involves the formation of amorphous regions in the 5 nm sample and the formation of edge dislocations in the 15 nm sample. Electron energy loss spectroscopy was employed to probe the oxidation states of Co, Fe, and Ti. In both samples, the oxygen vacancy concentration increases toward the surface.

The electron beam, during high-resolution transmission electron microscopy, was employed to induce a phase transition in La0.6Sr0.4CoO2.5 (LSC) from a brownmillerite ordering to an oxygen deficient perovskite structure. Prior to irradiation, a strongly alternating out-of-plane lattice parameter was observed, reflecting electrostatic interactions between AO and BO/BO2 planes in the brownmillerite ordering. During electron beam irradiation for one hour, the oxygen vacancy ordering vanished gradually, and a uniform cubic perovskite structure prevailed. To exclude beam-induced heating effects, in situ heating experiments were performed, revealing a stable brownmillerite ordering in the relevant temperature range (up to at least 500 °C). Thus, we conclude that the phase transition is caused by knock-on processes that affect oxygen vacancies in terms of a transition from structural vacancies toward extremely high concentrations of randomly distributed point defects in the ABO3 structure.