Atomic-resolution study of charge transfer and structural disorder in thermoelectric Ca$_{3}$Co$_{4}$O$_{9}$

Thermoelectric oxides have attracted increasing attention due to their high thermal power and temperature stability. In particular, Ca$_{3}$Co$_{4}$O$_{9}$, a misfit layered structure consisting of single layer hole-doped CoO$_{2}$ sandwiched between insulating Ca$_{2}$CoO$_{3}$ rocksalt layers, exhibits figure of merit (ZT) of $>$1 at 1000 K.$^{1 }$It was suggested that the Seebeck-coefficient can be further increased by controlling the spin- and valence-state of the Co-ions in the CoO$_{2}$ layers. This study combines aberration-corrected scanning transmission electron microscopy with electron energy loss spectroscopy (EELS) to examine the atomic and electronic structures of Ca$_{3}$Co$_{4}$O$_{9}$. Using annular dark and bright field imaging, it will be demonstrated that the CoO$_{2}$ layers are ordered, while the CoO columns in the Ca$_{2}$CoO$_{3}$ layer exhibit a modulation along (010). Atomic-column resolved EELS reveals that the Ca$_{2}$CoO$_{3}$ layers act as charge reservoirs providing mobile holes to the CoO$_{2}$ layers; the structural disorder in Ca$_{2}$CoO$_{3}$ is responsible for the low in-plane thermal conductivity. The temperature dependence of the Co-ion spin-state as the origin for the unusually high Seebeck coefficient of Ca$_{3}$O$_{4}$O$_{9}$ will be examined.$^{2}$ $^{1}$ K. Fujita, et al., \textit{Jpn. J. Appl. Phys.} \textbf{40} (2001), 4644--47$^{ }$ $^{2}$ Funded by: NSF CAREER Award DMR-0846748