Characterization of Thermoelectric Ca3Co4O9 Microstructure Using Transmission Electron Microscopy

Ca 3 Co 4 O 9  (CCO), a misfit layered structure exhibiting large Seebeck coefficient at temperatures up to 1000 K has attracted increasing attention as a novel high-temperature thermoelectric material. In this work, we investigate bulk CCO as well as thin CCO films grown on SrTiO 3  (001) and Al 2 O 3  (0001) using pulsed laser deposition. Resulting crystal structure and quality of the samples is examined using high-resolution transmission electron microscopy in order to correlate with thermoelectric properties. HRTEM images show incommensurate stacks of CdI 2 -type CoO 2  layers alternating with rock-salt-type Ca 2 CoO 3  layers along the c-axis. A natural buffer layer about 10 nm thick was found present between CCO and SrTi 3  substrate accompanied by higher density of stacking faults. The CCO grown on Al 2 O 3  exhibited numerous misoriented grains, crystal defects and presence of Ca x CoO 2  phase.