Phonon Transport Properties and Magnetic Transition in Perovskite Cobaltites

最近对钙钛矿基氧化物的研究已经证实，RE_<1-X> a_xmo_3型晶体（RE=稀土离子，M=Mn, Co）由于各种物理相互作用的相互作用和竞争而发生各种相变。在这些体系中有许多电、磁和晶体学性质的结果，但热（声子）输运性质如导热系数κ、热扩散系数α和热膨胀dL/L尚未报道。在本研究中，我们系统地测量了La_<1-X>AE_XCoO_3 （AE=Sr, Ca, Ba）晶体的这些热输运性质，并研究了声子散射机制。显著的结果如下：(1) La_<1-X>Sr_XCoO_3 （X>0.2）体系的热导率κ在铁磁转变温度T_c以下略有增强，这可以通过电子热导率κ_e的增加得到明确的理解。这些结果与La_<1-X>Sr_XMnO_3 （X>0.2）的结果相反，其中Jahn-Teller （JT）畸变的弛豫发生在T_c以下。(2)在30K时，LaCoO_3的导热系数κ明显增强，但由于Sr^<2+>离子以0.5%取代La^<3+>, Cu^<2+>和Ni^<2+>离子仅以2%取代Co^<3+>，峰值急剧降低。发现Co^<4+> JT离子的取代引入对声子输运有严重影响。(3) La_<1-X>Sr_XCoO_3 （0.1<X<0.15）体系在室温下是反铁磁或自旋玻璃绝缘体与铁磁金属区的相界，是p型热电材料的理想选择。

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.