The effects of increased Co-ion spin states on the Seebeck coefficient 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$_{9 }$(CCO), a misfit layered structure consisting of single layer hole-doped CoO$_{2}$ sandwiched between insulating Ca$_{2}$CoO$_{3}$ rocksalt layers, exhibits a high Seebeck coefficient at 1,000 K. It was previously suggested that the Seebeck-coefficient can be further improved by stabilizing an increased Co-ion spin state in the CoO$_{2}$ layers. Here we report a significant increase in the room-temperature in-plane Seebeck coefficient of 40 nm thick CCO films grown by pulsed laser deposition on SrTiO$_{3}$ substrates. We combine aberration-corrected Z-contrast imaging, atomic-column resolved electron energy-loss spectroscopy, and density-functional calculations to show that the increase is caused by CoO$_{2}$ stacking faults with Co$^{4+}$-ions in a higher spin state compared to that of bulk CCO. The higher Seebeck coefficient makes the CCO system suitable for many high-temperature waste-heat-recovery applications. The role of dopants, such as Bi and Ti will also be explored.