Epitaxial strain-induced chemical ordering in la_0.5Sr _0.5CoO_3-δ Films on SrTiO₃

Fast ion conductors are at the foundation of a number of important technologies, ranging from fuel cells to batteries to gas separators. Recent results suggest that strained interfaces and thin films may offer new mechanisms for achieving enhanced ionic transport. In this work, we investigate strained 40-nm films of perovskite La_0.5Sr_0.5CoO _3-δ, which is an important material for solid oxide fuel cell cathodes and oxygen separation membranes. We demonstrate that a strained thin film of La_0.5Sr_0.5CoO_3-δ on SrTiO ₃ can have dramatically different anion and cation thermodynamics and kinetics than bulk La_0.5Sr_0.5CoO_3-δ. We use synchrotron X-ray diffraction to show that La_0.5Sr _0.5CoO_3-δ thin films form an ordered phase at 650 K. The ordered phase consists of La and Sr cations in planes parallel to the surface and is associated with coherent expansion in the c-direction of ∼5%. This chemical ordering is not observed in the bulk material and is ascribed to the interplay between the epitaxial strain imposed by the substrate, changes in oxygen vacancy content and cation mobility, and the ordering of oxygen vacancies.