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 Ca3Co4O9, 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 CoO2 subsystem are highly ordered, while the oxygen-atomic columns are displaced in the Ca2CoO3 subsystem. Atomic-column-resolved electron energy-loss spectroscopy and spectrum image calculations are used to quantify the bonding in the different subsystems of incommensurate Ca3Co4O9. We find that the holes in the CoO2 subsystem are delocalized, which could be responsible for the p-type conductivity found in the CoO2 subsystem.