Direct measurement of charge transfer in thermoelectric Ca3Co4O9

The misfit-layered cobalt oxide Ca3Co4O9 exhibits outstanding physical properties including high thermoelectric power, low thermal conductivity, low resistivity, and high thermal stability. We utilize atomic-resolution Z-contrast imaging in conjunction with electron energy-loss spectroscopy (EELS) in an aberration-corrected scanning transmission electron microscope (STEM) to characterize the local atomic and electronic structure of Ca3Co4O9. We will show that the position of the O atoms in the CoO2 layers can be directly imaged, and that the CoO columns in the rocksalt layer exhibit a strong modulation in the (010) direction. Further, we measure the local Co valence and find significant hole transfer from the rocksalt CoO to the hexagonal CoO2 layers. Our results are confirmed by self-consistent multiple-scattering calculations and we conclude that this hole transfer increases the mobile hole concentration and breaks the electron-hole symmetry in the CoO2 layers, thereby enabling the high thermoelectric power in the strongly correlated CoO2 subsystem.