Thermoelectric Properties of Two NaxCoO2 Crystallographic Phases

概要

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We measured and analyzed the electrical resistivity ($\rho$), Hall coefficient ($R_{\text{H}}$), Seebeck coefficient ($S$) and magnetic susceptibility ($\chi$) of $\gamma$-phase Na0.6CoO2 and $\alpha$-phase Na0.9CoO2 crystals prepared by solution growth using NaCl-Na2CO3-B2O3 fluxes. The $\rho$ of $\alpha$-phase Na0.9CoO2 ($\rho_{\alpha}$) is much higher than that of $\gamma$-phase Na0.6CoO2 ($\rho_{\gamma}$), while the magnitude of $S$ of $\alpha$-phase Na0.9CoO2 ($S_{\alpha}$) exceeds that of $\gamma$-phase Na0.6CoO2 ($S_{\gamma}$). The origin of these differences is primarily the difference in carrier density. In addition, $\rho_{\alpha}$ has a minimum at around 20 K, while $\rho_{\gamma}$ exhibits a metallic temperature-dependence throughout the measured temperature range. Since $\alpha$-phase Na0.9CoO2 exhibits an antiferromagnetic transition at the same temperature 20 K, the increase in $\rho_{\alpha}$ at low temperatures implies the carrier localization effect caused by forming a long-range order among magnetic moments in the conductive CoO2 layer.
Published by the Japan Society of Applied Physics through the Institute of Pure and Applied Physicsの論文
2003-12-15