Enhancement ofthermoelectric properties of La_1-XAE_XCoO₃ atX~0.10 (AE=Ba, Sr, Ca)

H. Fujishiro¹, Y.Fujine¹, Y. Kashiwada¹, M. Ikebe¹, J.Hejtmanek²

¹ Faculty of Engineering,Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan

² Institute of Physics ofASCR, Cukrovarnivka 10, 16200 Praha 6, Czech Republic

The Co-based oxides such asNaCo₂O₄ and Ca₃Co₄O₉have attracted much attention as thermoelectric materials with a high figure ofmerit Z (=S²/kr; S the Seebeck coefficient, k the thermal conductivity, r the electrical resistivity). The highdegeneracy of Co spin state has been pointed out as a possible origin of largeS values. In this paper, we present the thermoelectric properties and themagnetism of the La_1-XAE_XCoO₃ (AE=Ba, Sr, Ca)system from 10K to 300K. In the parent material LaCoO₃, the spinstate of Co³⁺ shows the low-spin (LS; S=0) and intermediate-spin(IS; S=1) configuration and the ferromagnetic metallic (FM-M) state appears forX>0.2 by the introduction of Co⁴⁺ spins (IS, S=1/2). For all theAE ion doping, the Z value increases with increasing hole concentration X andthen shows a maximum at X=0.10~0.15. The larger Z value (for example, Z=210 x10^-6 (K^-1) at 220 K for AE=Ba and at X=0.10) mainly comesfrom the larger S and relatively small rvalues due to the appropriate carrier density at which the FM moment starts toappear. The effect of the Co site substitution by the transition metals on thethermoelectric properties was also investigated.