Thermoelectric properties of Bi-doped half-Heusler alloys

Based on our preliminary promising results for Sb-doped ZrNiSn-type half-Heusler intermetallics, we made a new series of Bi-doped samples Zr/sub 0.5/Hf/sub 0.5/NiSn/sub 1-x/Bi/sub x/ with concentrations x in the range 0/spl les/x/spl les/0.02. While the isoelectronic alloying of Zr and Hf reduces the lattice thermal conductivity, doping on the Sn site with Hf controls the carrier density and thus the nature of transport. This confirms the trend we reported previously for Sb, namely, that a small amount of the group V semimetal substituted on the Sn-site has a spectacular effect on all transport properties. Specifically, the electrical resistivity is much reduced due to an order of magnitude higher carrier density, and the thermopower is exceptionally large (/spl sim/-250 /spl mu/V/K for x=0.01), comparable to the thermopower of the parent (undoped) compound. Doping on the Sn site with Sb or Hi is thus an effective way to achieve high power factors in these half-Heusler intermetallics. The optimal operational range of the intermetallics is above room temperature.