Examining the structure and bonding in complex oxides using aberration-corrected imaging and spectroscopy

Our ability to directly characterize the atomic and electronic structures is crucial to developing a fundamental understanding of structure-property relationships in complex-oxide materials. Here, we examine one specific example, the misfit-layered thermoelectric material Ca${}_3$Co${}_4$O${}_9$, which exhibits a high Seebeck coefficient governed by spin-entropy transport as well as hopping-mediated electron transport. However, the role of oxygen and its bonding with cobalt in thermoelectric transport remains unclear. We use atomic-resolution annular bright-field imaging to directly image the oxygen sublattice and to combine our experimental data with multislice image calculations to find that the oxygen atoms in the CoO${}_2$ subsystem are highly ordered, while the oxygen-atomic columns are displaced in the Ca${}_2$CoO${}_3$ subsystem. Atomic-column-resolved electron energy-loss spectroscopy and spectrum image calculations are used to quantify the bonding in the different subsystems of incommensurate Ca${}_3$Co${}_4$O${}_9$. We find that the holes in the CoO${}_2$ subsystem are delocalized, which could be responsible for the $p$-type conductivity found in the CoO${}_2$ subsystem.

Figure 1

Atomic structure of the $({\mathrm{Ca}}_2{\mathrm{CoO}}_3{)}_2{\left({\mathrm{CoO}}_2\right)}_3$ approximant used for imaging and EELS calculations (a) in the [100] and (b) [110] orientations.

Figure 2

(a) and (b) show simultaneous Z-contrast and ABF images, respectively, of Ca${}_3$Co${}_4$O${}_9$ [100]. (c) ABF image with inverted contrast. Both Z-contrast and ABF images show the incommensurate structure of Ca${}_3$Co${}_4$O${}_9$ along the $c$ axis.

Figure 3

Simulated (a) HAADF and (b) ABF images of Ca${}_3$Co${}_4$O${}_9$ [100].

Figure 4

Intensity line profile from the HAADF and the inverted ABF images across one unit cell of Ca${}_3$Co${}_4$O${}_9$. The different apparent intensities of the CoO${}_2$ columns are artifacts of the incommensurate structure of Ca${}_3$Co${}_4$O${}_9$ and the straight line of the intensity profile.

Figure 5

(a) Z-contrast image of Ca${}_3$Co${}_4$O${}_9$ [110], (b) simultaneous ABF image, and (c) ABF image with inverted contrast. The bright-field image clearly shows the location of the oxygen-atomic columns and the asymmetry of these atomic planes in the Ca${}_2$CoO${}_3$layers. [Inset in (a)] EELS spectrum image showing the integrated O $K$ edge (red), Co $L$ edge (blue), and Ca $L$ edge (green).

Figure 6

Simulated (a) HAAF and (b) ABF images of Ca${}_3$Co${}_4$O${}_9$ [110].

Figure 7

Atomic-column-resolved EELS from different positions inside the Ca${}_3$Co${}_4$O${}_9$ unit cell.

Figure 8

Spectrum images showing the spatial distribution of the (a) O $K$-edge pre-peak $A$, (b) peak $B$, and (c) peak $C$ intensities. All the integrated intensities are normalized to the integrated intensity of the first 30 eV of the O $K$ edge.

Figure 9

Calculated spectrum image intensity distribution for the integrated O $K$ edge of Ca${}_3$Co${}_4$O${}_9$ [110] using a sample thickness of (a) 10 nm and (b) 30 nm.