Atomic and electronic structure of Ti substitution in Ca3Co4O9

We examine the role of Ti doping in the incommensurately layered thermoelectric oxide material Ca3Co4O9 (CCO). The measured Seebeck coefficient of S = 135 µV /K in Ti-doped CCO thin films of composition Ca3Co3.8Ti0.2O9 indicates no significant enhancement of S compared to pristine CCO, thus confirming prior experimental results. Using a combination of aberration-corrected scanning transmission electron microscopy, electron energy-loss spectroscopy and first-principles computations, we determine the atomic and electronic structures of Ti-doped CCO, including the preferred location of Ti dopants and valence states of Ti and Co atoms. Our findings on the structural, electronic and transport properties of the Ti-doped CCO are discussed in light of previously published results