Strongly correlated properties and enhanced thermoelectric response in Ca₃Co₄-_xM_xO₉ (M = Fe, Mn, and Cu)

We report the strongly correlated, electrical transport, magnetic, and thermoelectric properties of a series of Fe, Mn, and Cu doped Ca ₃Co₄O₉. The results indicate that Fe/Mn substitutes for Co in CoO₂ layers whereas Cu substitutes for Co in Ca₂CoO₃ layers. Because of the different doping sites, the electronic correlations increase remarkably in Fe and Mn doped series while remaining unchanged in Cu doped series. Correspondingly, the transport mechanism, magnetic properties, and some characteristic parameters along with transition temperatures all exhibit two distinct evolutions for Fe/Mn doping and Cu doping. The thermoelectric characteristics are improved in each series. Nevertheless, the improvement of thermoelectric performance is most significant in Fe doped samples due to the unexpected changes in thermopower and resistivity. The unusual thermopower behavior can be well described by the variations of electronic correlation. Possible approaches for further improvement of the thermoelectric performance in Ca₃Co ₄O₉ and other relevant strongly correlated systems are also proposed at the end.