X-ray imaging of ionically-controlled nanoscale phase separation in cobaltites*

Controlling ionic distributions and stoichiometry provides a unique way for manipulating functional properties in perovskite oxides (ABO₃). This talk will focus on Gd/La_0.67Sr_0.33CoO₃ (LSCO) heterostructures due to high oxygen ion conductivity, as well as the coupled magnetic and electronic properties of LSCO, which are strongly dependent on the oxygen stoichiometry. This enables the ionic control of the functional properties of LSCO thin films through the presence of oxygen getter layers such as Gd. We utilize x-ray nanodiffraction to directly image the nanoscale morphology of LSCO thin films as they are progressively transformed from the equilibrium perovskite phase to the metastable brownmillerite (ABO_2.5) phase with increasing Gd thickness. Our studies show the coexistence of perovskite and brownmillerite phases with a critical oxygen vacancy concentration threshold which leads to the formation of extended brownmillerite filaments. In addition to lateral phase separation, we observed phase separation along the thin film thickness due to pinning of perovskite or brownmillerite by substrate/LSCO or Gd/LSCO interface respectively. Our measurements highlight local structural effects and provide insights into nanoscale strain behavior in Gd/LSCO heterostructures