Polycrystalline K-filled CoSb_(3) are synthesized successfully. The uplimit for K filling is at least 0.45, being higher than those of either alkaline-earth (AE) or rare-earth (RE) metals but being in consistent with our earlier theoretical prediction. The measured transport properties (300-800 K) show that K filling does not lower thermal conductivity much in comparison with AE or RE filling due to the relatively low mass of K atom. However, it improves electrical conductivity, retains large Seebeck coefficient, and leads to a reasonably good thermoelectric performance for the filled skutterudites. The maximum figure of merit ZT reaches 1 at 800 K for K_(0.38)Co_(4)Sb_(12).
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