Atomic-resolution in-situ cooling study of oxygen vacancy ordering in La0.5Sr0.5CoO3−δ thin films

The presence and potential ordering of oxygen vacancies play an important role in determining the electronic, ionic, and thermal transport properties of many transition metal oxide materials. Controlling the concentration of oxygen vacancies, as well as the structures of ordered oxygen vacancy domains, has been the subject of many experimental and theoretical studies. In epitaxial thin films, the concentration of oxygen vacancies and the type of ordering depend on the structure of the substrate as well as the lattice mismatch between the thin films and the substrate. However, the role of temperature or structural phase transitions in either the substrate or the epitaxial thin films in the oxygen vacancy ordering has remained largely unexplored. In particular, atomic-resolution imaging and spectroscopy analysis of oxygen vacancy ordering in thin films at temperatures below 300 K have not yet been reported. Here, we use aberration-corrected scanning transmission electron microscopy combined with in-situ cooling experiments to characterize the atomic/electronic structures of oxygen-deficient La0.5Sr0.5CoO3−δ thin films grown on SrTiO3 across its antiferrodistortive phase transition at 105 K. We demonstrate that atomic-resolution imaging and electron energy-loss spectroscopy can be used to examine variations in the local density of states as a function of sample temperature.