Quantifying Ordering Phenomena at the Atomic Scale in Rare Earth Oxide Ceramics via EELS Elemental Mapping

Oxide ceramics qualify for multiple applications for future energy technologies: small changes inthe concentration of the elements already lead to changes of the macroscopic properties such aselectronic and ionic conductivity and catalytic activity. Precise characterization techniques, e.g.EELS or EDX in STEM, help to uncover these secrets of the nanoworld. Our investigations on newoxide ceramics – promising materials as triple conducting oxides (proton-, oxygen ion- andelectron-conducting) – revealed interesting correlations visible within high-resolution EELSelemental maps: perovskite- and Ruddlesden-Popper phases containing rare earth elementsexhibit variations of their element distributions within equivalent crystal sites. This behaviour isvividly observable in lanthanum barium ferrate, a second order Ruddlesden-Popper phase whereboth La and Ba occupy the A-sites within the crystal. Our experiments show that La favours sitesin the rock salt layer, whereas Ba prefers the perovskite block. Moreover, the Ba/La distributionvaries from atomic column to atomic column within both rock salt and perovskite layers.Unfortunately, acquiring elemental maps at atomic scale is always prone to channelling effects,which lead to additional intensities stemming from neighbouring atomic columns – a circumstancewhich renders a straightforward, reliable quantification impossible. We address this issue by usinginelastic multislice calculations based on Slater-type orbitals in order to overcome the problem withunknown neighbouring off-axis intensities. After subtracting the additional off-axis intensity wesuccessfully performed a column-by-column quantification: through taking advantage of the largechanges in the elemental distribution from column to column we introduced a quantificationtechnique which substitutes inelastic scattering cross sections during the quantification step byparameters obtained from the actual experiment [1]. We revealed that (in terms of crystalstructure) equivalent atomic columns within either the rock salt layer or the perovskite layer do notexhibit distinct La/Ba ratios but a broad variation in concentration.