Electrical resistivity (ρ) and thermoelectric power (S) were measured for NaxCo2O4 samples with various Na contents (x). A highly precise control of Na nonstoichiometry was facilitated by optimizing the synthesis procedure. With increasing x, the absolute value of ρ monotonically decreased, while the value of S increased. For the x=1.5 sample, S reached a value as high as ∼120 μV/K (at 300 K).

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