Fast Mapping of the Cobalt-Valence State in Ba0.5Sr0.5Co0.8Fe0.2O3-d by Electron Energy Loss Spectroscopy

A fast method for determination of the Co-valence state by electron energy loss spectroscopy in a transmission electron microscope is presented. We suggest the distance between the Co-L-3 and Co-L-2 white-lines as a reliable property for the determination of Co-valence states between 2+ and 3+. The determination of the Co-L-2,L-3 white-line distance can be automated and is therefore well suited for the evaluation of large data sets that are collected for line scans and mappings. Data with a low signal-to-noise due to short acquisition times can be processed by applying principal component analysis. The new technique was applied to study the Co-valence state of Ba0.5Sr0.5Co0.8Fe0.2O3-d (BSCF), which is hampered by the superposition of the Ba-M-4,M-5 white-lines on the Co-L-2,L-3 white-lines. The Co-valence state of the cubic BSCF phase was determined to be 2.2+ (+/- 0.2) after annealing for 100 h at 650 degrees C, compared to an increased valence state of 2.8+ (+/- 0.2) for the hexagonal phase. These results support models that correlate the instability of the cubic BSCF phase with an increased Co-valence state at temperatures below 840 degrees C.