Atomic Resolution Coordination Mapping in Ca₂FeCoO₅ Brownmillerite by Spatially Resolved Electron Energy-Loss Spectroscopy

Using a combination of high-angle annular dark field scanning transmission electron microscopy and atomically resolved electron energy-loss spectroscopy at high energy resolution in an aberration-corrected electron microscope, we demonstrate the capability of coordination mapping in complex oxides. Brownmillerite compound Ca 2 FeCoO 5, consisting of repetitive octahedral and tetrahedral coordination layers with Fe and Co in a fixed 3+ valency, is selected to demonstrate the principle of atomic resolution coordination mapping. Analysis of the Co-L 2,3 and the Fe-L 2,3 edges shows small variations in the fine structure that can be specifically attributed to Co/Fe in tetrahedral or in octahedral coordination. Using internal reference spectra, we show that the coordination of the Fe and Co atoms in the compound can be mapped at atomic resolution.