Introduction of resonant states and enhancement of thermoelectric properties in half-Heusler alloys

Abstract

The inclusion of small concentrations of vanadium (less than 1%) was found to produce a substantial increase in the Seebeck coefficient of polycrystalline $n$-type half-Heusler alloys based on Hf${}_{0.75}$Zr${}_{0.25}$NiSn. Some degree of vanadium-induced thermopower enhancement was found to be present even at temperatures as high as 800 K. Electrical resistivity values of the vanadium-doped samples, on the other hand, exhibited only modest increases, thereby resulting in a 120% increase in the thermoelectric power factor of Hf${}_{0.75}$Zr${}_{0.25}$NiSn above room temperature. Such augmentation of the Seebeck coefficient, however, was diminished at all measured temperatures with the addition of a sufficient quantity of antimony to dope this class of compounds optimally. These observations are discussed in terms of carrier concentration, mobility, effective mass, and calculations of the effective gap size. When taken in conjunction with low temperature heat capacity measurements, these results indicate an increase in the density of states at the Fermi level that is consistent with the resonant state phenomena investigated earlier by theoretical work on semiconductors.