Probing oxygen vacancy concentration and homogeneity in solid-oxide fuel-cell cathode materials on the subunit-cell level

Abstract

Oxygen vacancy distributions and dynamics directly control the operation of solid-oxide fuel cells and are intrinsically coupled with magnetic, electronic and transport properties of oxides. For understanding the atomistic mechanisms involved during operation of the cell it is highly desirable to know the distribution of vacancies on the unit-cell scale. Here, we develop an approach for direct mapping of oxygen vacancy concentrations based on local lattice parameter measurements by scanning transmission electron microscopy. The concept of chemical expansivity is demonstrated to be applicable on the subunit-cell level: local stoichiometry variations produce local lattice expansion that can be quantified. This approach was successfully applied to lanthanum strontium cobaltite thin films epitaxially grown on substrates of different symmetry, where polarized neutron reflectometry revealed a strong difference in magnetic properties. The different vacancy content found in the two films suggests the change in oxygen chemical potential as a source of distinct magnetic properties, opening pathways for structural tuning of the vacancy concentrations and their gradients.

Reversible nano-structuring of SrCrO3-δ through oxidation and reduction at low temperature

“…The in-plane lattice parameters for the as-grown films (3.82 Å on LAO and 3.86 Å on LSAT) are nearly the same as those of their respective substrates, whereas the out-of-plane lattice constant (c) is B3.92 Å for both films on both substrates. The larger value of c in the as-grown films compared with that of bulk SrCrO 3 (3.819 Å) implicates the presence of V O through a phenomenon called chemical expansivity 8,9 . The drop in c upon air annealing is consistent with oxidation of R-SCO to P-SCO.…”

Picometre-precision analysis of scanning transmission electron microscopy images of platinum nanocatalysts

“…Registering two images, R and T, requires finding a deformation f of the image domain such that the composition T3f agrees with R. Figure 1a shows the conventional approach, in which f is a rigid translation 11,13,14 . A rigid f works for the unchanging pixel grid in Fig.…”

“…We demonstrate sub-picometre (pm) precision measurements of atom positions in aberration-corrected Z-contrast STEM images, more than five times better than previous methods 13,14 , based on NR registration and averaging of an image series. NR registered images of a Pt nanocatalyst show pm-scale contraction of atoms at a (1 11)/( 1 11) corner towards the particle centre and expansion of a (1 11) facet.…”

“…Precision is the statistical spread of repeated distance measurements in an image.) Precision smaller than the resolution is routinely achievable [11][12][13][14] : the best reported precision for TEM and STEM images is 1-3 picometre (pm) (refs 11,12) and 4-5 pm (refs 13,14), respectively. Precision is fundamentally limited by the signal-to-noise ratio (SNR) 15 ; however, in STEM serial acquisition of pixels translates instabilities in the sample and probe positions into displacements of the imaged atoms, limiting precision well above the SNR limit.…”

Lattice mismatch accommodation via oxygen vacancy ordering in epitaxial La0.5Sr0.5CoO3-δ thin films

“…11 O vacancy ordering occurs in the related bulk SCO and high x LSCO compounds, 6,7 and similar superstructures have been found in epitaxial LSCO. [8][9][10][11][18][19][20][21] These superstructures are indicators of O vacancy ordering. In the present set of STO(001)/LSCO samples, we have previously reported direct observation of O content modulation by atomic-resolution electron energy loss spectroscopic imaging.…”