Direct measurement of charge transfer and spin stat transitions in thermoelectric Ca$_{3}$Co$_{4}$O$_{9}$

The misfit-layered thermoelectric material Ca$_{3}$Co$_{4}$O$_{9}$ has been the focus of many recent studies due to its high thermal power and good high temperature stability. In particular, it has been suggested that the presence of a mixed valence state in the strongly correlated CoO$_{2}$ layer is essential for the high p-type thermoelectric properties in Ca$_{3}$Co$_{4}$O$_{9.}$ In this study, we combine aberration-corrected scanning transmission electron microscopy (STEM) with electron energy loss spectroscopy (EELS) to study the atomic and electronic structures of Ca$_{3}$Co$_{4}$O$_{9}$. We will show that the position of the O atomic columns in the CoO$_{2}$ layers are highly ordered and can therefore be directly imaged, while the CoO columns in the Ca$_{2}$CoO$_{3}$ 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 strongly correlated CoO$_{2}$ layers. In addition, we will present the results of our in-situ heating experiments of Ca$_{3}$Co$_{4}$O$_{9}$ [010] at 500 K, which show that the phase transition at 420 K is not accompanied by a structural transition but rather a transition of the Co-ion spin states.