Thermoelectric and Magnetic Properties of [(Ca1-xPbx)2CoO3.1]0.62CoO2 ($0\leqq x\leqq 0.03$)

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We have prepared polycrystalline [(Ca1-xPbx)2CoO3.1]0.62CoO2 ($0\leqq x\leqq 0.03$) samples by a conventional solid-state reaction method and investigated the effects of Pb substitution on the thermoelectric and magnetic properties of the samples. With Pb substitution, both electrical resistivity and Seebeck coefficient do not change markedly. This is attributed to the carrier concentration of the samples. Seebeck and Hall coefficient measurements reveal that the major charge carriers in the samples are holes, however, the carrier concentration does not change with increasing $x$. Neutron powder diffraction and magnetic susceptibility measurements also reveal that Pb ions are in a divalent state in a rock salt-type Ca2CoO3 block layer. The average valence state of Co ions in a CdI2-type CoO2 sheet is 3.1+ and that of Co ions in the block layer is 3.6+. The resulting dimensionless figure of merit for an $x=0.02$ sample at room temperature is 0.024, which is approximately equal to the corresponding value of a polycrystalline NaCo2O4 sample.
2007-05-15