Fast Mapping of the Cobalt-Valence State in Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-dby Electron Energy Loss Spectroscopy

Abstract

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-L3 and Co-L2 white-lines as a reliable property for the determination of Co-valence states between 2+ and 3+. The determination of the Co-L2,3 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-M4,5 white-lines on the Co-L2,3 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°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°C.

Atomic Self‐reconstruction of Catalyst Dominated Growth Mechanism of Graphite Structures

“…A 4.5 value of Co L 3 /L 2 peak intensity ratio is achieved. Comparing with the reported spectra, [32,33] this value unravels a + 2 valence state, revealing that the activation of carbon atoms is actually promoted by the valent cobalt ions rather than metallic Co. This 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 transition process indicates that, the NPs inside carbon nanofiber were gradually carbonized under the combined action of dissociative carbon atoms and high temperature during the process of rising to the suitable temperature for catalytic graphitization (Figure 2e).…”

The effect of B‐site Y substitution on cubic phase stabilization in (Ba0.5Sr0.5)(Co0.8Fe0.2)O3−δ

“…Previous work on undoped BSCF reported Fe in a valence state close to +3 which is not modified by Y‐doping. Since the Co–L 2,3 edges are superimposed by the Ba–M 4,5 white‐lines, the white‐line distance technique was applied where the Co‐valence state can be quantitatively extracted from the distance between the Co–L 2 and Co–L 3 white‐lines (cf. Figure B).…”

“…The low Fe tolerance of the hexagonal phase can be understood by the reduced free volume of the crystal structure within the hexagonal unit cell. Since the valence state of the Co cation is increased in the hexagonal phase, large B‐site ions like Fe cannot be accommodated in the hexagonal crystal structure with its face‐sharing configuration of oxygen octahedrons. Figure also illustrates that the hexagonal phase precipitates at different nucleation sites like BCO lamellae, Co 3 O 4 precipitates and grain boundaries.…”

“…Oxidation states of the cations were evaluated using electron energy loss spectroscopy (EELS). Spectra were recorded with a post‐column Gatan (Gatan, Pleasanton, CA, USA) Imaging Filter (Tridiem 865 HR) using the white‐line distance technique . The convergence angle of the electron beam was set to 16.7 mrad with a spectrometer collection angle of 6.7 mrad.…”

“…However, long‐term experiments have shown a gradual decrease of the oxygen flux which was attributed to a partial decomposition of the desired cubic BSCF phase by the precipitation of a hexagonal phase at temperatures ≤825°C . It is commonly accepted that the formation of the hexagonal phase is strongly connected with changes of oxidation and spin state of the multivalent Co ion . The Fermi level in BSCF is located within a partly filled Co and O band as motivated by DFT calculations .…”