Abstract
The effects of Ni and Ce co-doping on the spin entropy in Ca3Co4O9+δ have been carefully studied. It is found that Ni and Ce co-doping is more effective than single rare earth element doping in improving the spin entropy of Ca3Co4O9+δ. The largest magnetothermopower ratio value of 70 % is achieved for Ca2.8Ce0.2Co3.9Ni0.1O9+δ, which is at least two times larger than that of the reported Ca2.5Ce0.5Co4O9+δ, Ca2.8Lu0.2Co4O9+δ, and Ca2.4Gd0.6Co4O9+δ materials. This enhancement proves the remarkable increase in the spin entropy of Ni and Ce co-doped Ca3Co4O9+δ. Co 2p X-ray photoemission spectroscopy shows the decrease in Co4+ concentration induced by Ni and Ce co-doping. The reduction in Co4+ concentration is suggested to result in the enhanced spin entropy from Co site. And coexistence of Ni2+ and Ni3+ provides a new hopping model for transporting spin entropy, contributing to the increase of the total spin entropy. The joint contributions from Co4+ concentration decrease and the new hopping model, both induced by Ni and Ce co-doping, make the spin entropy increase remarkably.
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