Unlike semiconducting TiCoSb, ZrCoSb and HfCoSb half-Heusler phases are semimetallic below room temperature and exhibit small Seebeck coefficients of ∼−10 μV/K at 300 K. However, upon substituting (doping) the Co and Sb sites with Pt and Sn, respectively, much larger thermopowers (S) are obtained. For ZrCoSb, S reaches −110 and +130 μV/K while resistivity ρ decreases from ∼5×104 μΩ cm in the undoped phase to 1–2×103 μΩ cm in the substituted phases at 300 K. The lowest thermal conductivity obtained in the substituted alloys is ∼3.0 W/m K at 300 K, which is among the lowest reported for this class of structural phases. There are indications that the thermoelectric properties have not been optimized in these multinary alloys.

Half-Heusler alloys ͑MgAgAs type͒ with the general formula MNiSn where M is a group IV transition metal ͑Hf, Zr, or Ti͒ are currently under investigation for potential thermoelectric materials. These materials exhibit a high negative thermopower (Ϫ40 to Ϫ250 V/K) and low electrical resistivity values ͑0.1-8 m⍀ cm͒ both of which are necessary for a potential thermoelectric material. Results are presented in this letter regarding the effect of Sb doping on the Sn site (TiNiSn 1Ϫx Sb x). The Sb doping leads to a relatively large power factor of ͑0.2-1.0͒ W/m K at room temperature for small concentrations of Sb. These values are comparable to that of Bi 2 Te 3 alloys, which are the current state-of-the-art thermoelectric materials. The power factor is much larger at TϷ650 K where it is over 4 W/m K making these materials very attractive for potential power generation considerations.

Electrical transport measurements have been performed on doped and undoped TiCoSb half-Heusler phases. The semiconducting properties are found to be more robust than those reported for MNiSn (M = Ti, Zr, Hf ). Undoped TiCoSb phases exhibit large n-type Seebeck coefficients and high resistivities that reach −500 µV K −1 at 300 K and ∼1500 cm at 4.2 K, respectively. A tendency towards carrier localization is seen in several disordered phases. The effects due to n-type and p-type dopants are readily manifested in the thermopower, from which moderately heavy electron and hole band masses are inferred. The unusual properties measured are consistent with the prediction of a wide bandgap for the TiCoSb phase. A resistivity minimum is observed at 500-600 K for undoped and V-doped TiCoSb. Consequently, the semiconducting gap has not been determined.

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Half-Heusler alloys with the general formula TiNiSn 1Ϫx Sb x are currently being investigated for their potential as thermoelectric ͑TE͒ materials. A systematic investigation of the effect of Sb doping on the Sn site and Zr doping on the Ti site on the electrical and thermal transport of the TiNiSn system has been performed. Unexpectedly, lattice thermal conductivity L appears to increase somewhat randomly with small amounts (xϽ5%) of Sb doping. Subsequently, an investigation of grain structure in these Sb-doped materials has been found to correlate with the anomalous behavior of L. Furthermore, effects of submicron grain sizes on L in ball milled and shock compressed samples are also presented.

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