Anomalous Pd substitution effects in the thermoelectric oxide NaCo₂₋ₓPdₓO₄

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We prepared a set of polycrystalline samples of the thermoelectric oxide NaCo₂₋ₓPdₓO₄ (x= 0, 0.05, 0.1 and 0.2), and investigated the Pd substitution effects on transport phenomena. The effects are so drastic that only 5-10% Pd ions reduce the resistivity and the Seebeck coefficient to one-third of the values for x=0, and increase the magnitude of the Hall coefficient by three times. A semi-quantitative analysis has revealed that the x=0.2 sample has much smaller effective mass and carrier concentration than the x=0.05 sample. This is difficult to explain within a rigid-band picture, and is qualitatively consistent with a strong-correlation picture applied to the Ce-based heavy fermion systems.