| Research Abstract |
As recent revived studies on perovskite-based oxides have confirmed, RE_<1-X>A_XMO_3-type crystals (RE=rare earth ions, M=Mn, Co) undergo a variety of phase transitions which originate from the interplay and competition of various physical interactions. There are many results of electric, magnetic and crystallographic properties in these systems, however, the thermal (phonon) transport properties such as the thermal conductivity κ, thermal diffusivity α, and thermal dilatation dL/L have not been reported. In this study, we systematically measured these thermal transport properties of La_<1-X>AE_XCoO_3 (AE=Sr, Ca, Ba) crystals and investigated the phonon scattering mechanisms. The remarkable results are as follows ;
(1)The thermal conductivity κ of the La_<1-X>Sr_XCoO_3 (X>0.2) system slightly enhances below the ferromagnetic transition temperature T_c, which is clearly understood on the basis of the increase in the electronic thermal conductivity κ_e. These results are contrast to those in La_<1-X>Sr_XMnO_3 (X>0.2), in which the relaxation of the Jahn-Teller (JT) distortion takes place below T_c.
(2)The thermal conductivity κ of LaCoO_3 is markedly enhanced at 30K, however, the peak drastically decreases due to the substitution by the Sr^<2+> ions for the La^<3+> with 0.5% or by the Cu^<2+> and Ni^<2+> ions for the Co^<3+> with only 2%. It was found that the introduction of the Co^<4+> JT ions by the substitution seriously influences to the phonon transport.
(3)The La_<1-X>Sr_XCoO_3 (0.1<X<0.15) system shows a promising candidate for the p-type thermoelecteric material around room temperatures, which is the phase boundary between the antiferromagnetic or spin-glass insulator and the ferromagnetic metal region.
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