Electronic structure of transition metal-doped XNiSn and XCoSb (X = Hf,Zr) phases in the vicinity of the band gap

Half-Heusler alloys of the compositions X1-aAaNi1-bBbSn and X1-aAaCo1-bBbSb (X = Hf,Zr) were synthesized with transition metals (A, B) substituted at the X and Ni/Co sites with the values of a and b between 0 and 0.15. Thermopower and electrical resistivity measurements from room temperature to 1100 K were performed to investigate resultant modifications to the electronic band structure in the vicinity of the band gap. As a result of these substitutions, thermopower was typically reduced across the entire temperature spectrum, in some cases changing sign. In the case of XNiSn-type alloys, electrical resistivity curves were indicative of semiconducting behavior, except in the case of samples in which Sb was introduced to the Sn site as a dopant. XCoSb-type alloys, however, were found to exhibit metallic resistivity behavior for all substitutions investigated. Hall effect measurements were performed to confirm the dominant carrier type and carrier concentration. The effects of transition metal substitution on the locations of a dopant band and the pinned Fermi level were discussed in the light of recent first-principles electronic structure calculations for half-Heusler alloys. For some of the semiconducting alloys, the band gaps that were determined from the high temperature region of the resistivity curves were found to be in closer agreement with those obtained from calculations than previously reported.