Atomic Structure Analysis of a Second Order Ruddlesden-Popper Ferrite-a High Resolution STEM Study

New energy materials based on mixed proton-, oxygen ion- and electron-conducting ceramics (triple conducting oxides, TCOs) offer attractive possibilities for future applications in protonic ceramic fuel cells, electrolyser cells or membranes for hydrogen separation. Fundamental research is of high interest when it comes to mass and charge transport as well as defect chemistry – properties of new TCOs, which are influenced by the crystal structure. In this work, we show an elemental analysis at atomic resolution of the second order Ruddlesden-Popper ferrite Ba1.1La1.9Fe2O7. We characterized the atomic structure by X-ray diffraction and high-resolution scanning transmission electron microscopy. Furthermore, we revealed the position of the elements in the crystal structure via high-resolution elemental maps using electron energy loss spectrometry and energy-dispersive X-ray spectrometry. This enables us to distinguish between La and Ba, both located at the A-sites within the A3B2O7 phase: Our experiments show that La favours the 9-fold coordination sites in the rock salt layer, whereas Ba prefers the 12-fold coordination sites within the perovskite block. Comparing the intensities of atom columns on one specific site, we recognized slight fluctuations in Ba and La concentrations, which point to cation diffusion within the crystallites. These new insights on cation ordering in Ba1.1La1.9Fe2O7 may further contribute to the understanding of mass and charge transfer properties