Bandgap features and thermoelectric properties of Ti-based half-Heusler alloys

Electronic transport properties of narrow-gap TiNiSn and presumed wider-gap TiCoSb half-Heusler alloys are investigated by systematically doping the three sublattice sites. The two alloys are found to exhibit different doping trends. While all three sites in TiCoSb can be doped to enhance semimetallic behavior, only the Ti and Ni sites in TiNiSn can be efficiently doped. Meanwhile, several 3d dopants are found to lead to more localized electronic properties. These findings, together with results on Hall effect and thermopower measurements, have shed light on the bandgap structure of these metal-based semiconductors. Power factor and dimensionless figure of merit ZT reaching /spl sim/5.7/spl times/10/sup -3/ W/m-K/sup 2/ and /spl sim/0.5 at 680 K, respectively, are obtained in the Sb-doped (TiHf)NiSn system. The quite favorable thermoelectric parameters obtained in these low-mobility alloys are attributed to the existence of a moderately heavy electron band mass.