<i>In Situ</i> Observation of Oxygen Vacancy Dynamics and Ordering in the Epitaxial LaCoO<sub>3</sub> System

Vacancy dynamics and ordering underpin the electrochemical functionality of complex oxides and strongly couple to their physical properties. In the field of the epitaxial thin films, where connection between chemistry and film properties can be most clearly revealed, the effects related to oxygen vacancies are attracting increasing attention. In this article, we report a direct, real-time, atomic level observation of the formation of oxygen vacancies in the epitaxial LaCoO<sub>3</sub> thin films and heterostructures under the influence of the electron beam utilizing scanning transmission electron microscopy (STEM). In the case of LaCoO<sub>3</sub>/SrTiO<sub>3</sub> superlattice, the formation of the oxygen vacancies is shown to produce quantifiable changes in the interatomic distances, as well as qualitative changes in the symmetry of the Co sites manifested as off-center displacements. The onset of these changes was observed in both the [100]<sub>pc</sub> and [110]<sub>pc</sub> orientations in real time. Additionally, annular bright field images directly show the formation of oxygen vacancy channels along [110]­pc direction. In the case of 15 u.c. LaCoO<sub>3</sub> thin film, we observe the sequence of events during beam-induced formation of oxygen vacancy ordered phases and find them consistent with similar processes in the bulk. Moreover, we record the dynamics of the nucleation, growth, and defect interaction at the atomic scale as these transformations happen. These results demonstrate that we can track dynamic oxygen vacancy behavior with STEM, generating atomic-level quantitative information on phase transformation and oxygen diffusion.