Dramatically enhanced thermoelectric properties of Ca3Co4Oy by large amount of RE substitution

report the synthesis and thermoelectric properties of [(Ca1-xREx)(2)CoO3-delta](0.62)CoO2 compounds (RE: rare earth element, RE = Y, Sm, Eu, Tb, Dy, Ho and Lu). From optimization of synthesis conditions, all the chosen RE in this study were able to be substituted for the Ca sites in the block layers up to x = 0.25. In the case of RE = Th, at 1150 K, only K-was systematically decreased with x, while both Seebeck coefficient (S) and resistivity (P) were almost constant for the charge of x. Consequently, the dimensionless figure of merit (ZT) was increased with x and reached 0.2 at 1150 K for x = 0.25. Moreover, interestingly, it was found that S and P at 300 K were enhanced with ionic radius of the doped trivalent RE ions in spite of the doping of equivalent RE ions. This is probably because local structures of oxygen sites around Co ion in the CoO2 layers largely depend on kinds of the doped RE.