Large increase in the spin entropy of thermoelectric [Ca.sub.3][Co.sub.4][O.sup.9+δ] induced by Ni and Ce co-doping

• Published in: Journal of materials science 2015-02, p.1746
• Main Authors: Tang, G.D, Yang, W.C, Jiang, Y.Y, Wu, Z.C, Wang, Z.H
• Format: Article
• Language: English
• ISSN: 0022-2461; 1573-4803
• DOI: 10.1007/s10853-014-8737-7

The effects of Ni and Ce co-doping on the spin entropy in [Ca.sub.3][Co.sub.4][O.sub.9+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 [Ca.sub.3][Co.sub.4][O.sub.9+δ]. The largest magnetothermopower ratio value of 70 % is achieved for [Ca.sub.2.8][Ce.sub.0.2][Co.sub.3.9][Ni.sub.0.1][O.sub.9+δ], which is at least two times larger than that of the reported [Ca.sub.2.5][Ce.sub.0.5][Co.sub.4][O.sub.9+δ], [Ca.sub.2.8][Lu.sub.0.2][Co.sub.4][O.sub.9+δ], and [Ca.sub.2.4][Gd.sub.0.6][Co.sub.4][O.sub.9+δ] materials. This enhancement proves the remarkable increase in the spin entropy of Ni and Ce co-doped [Ca.sub.3][Co.sub.4][O.sub.9+δ].Co 2p X-ray photoemission spectroscopy shows the decrease in [Co.sup.4+] concentration induced by Ni and Ce co-doping. The reduction in [Co.sup.4+] concentration is suggested to result in the enhanced spin entropy from Co site. And coexistence of [Ni.sup.2+] and [Ni.sup.3+] provides a new hopping model for transporting spin entropy, contributing to the increase of the total spin entropy. The joint contributions from [Co.sup.4+] concentration decrease and the new hopping model, both induced by Ni and Ce co-doping, make the spin entropy increase remarkably.