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.

The resistivity of single-crystal pentatellurides, HfTe 5 and ZrTe 5 , has been measured as a function of temperature and applied magnetic field. At zero magnetic field these materials exhibit a peak in their resistivity ͑at T P ) as a function of temperature that corresponds to an, as yet, undetermined phase transition. The application of a transverse magnetic field ͑B Ќ to the current I͒ has a profound effect on the resistive peak in these materials, shifting the peak to slightly higher temperatures and producing a large enhancement of the resistivity at the peak, up to a factor of 3 in ZrTe 5 (T P ϭ145 K͒ and 10 in HfTe 5 (T P ϭ80 K͒. Larger magnetoresistance is observed at even lower temperatures, TϽ20 K. ͓S0163-1829͑99͒06035-X͔

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