Transport properties of pure and dopedMNiSn (M=Zr, Hf)

We have studied the transport properties in a family of pure and doped intermetallics of the form MNiSi (M=Zr, Hf), the structures known as the half-Heusler alloys. We have shown that the transport is very sensitive to structural arrangements of the constituent atoms, and this can be manipulated by annealing, isostructural alloying, and doping. The unusual transport properties are viewed in the context of a semimetal-semiconductor transition that in pure alloys sets in near 150 K. Doping with indium can shift the transition upward towards 200 K. The high-temperature transport is dominated by the presence of heavy electrons that are responsible for surprisingly large values of thermopower. Minute amount of antimony (n-type doping) have a spectacular influence on the nature of transport and drive the electrical resistivity and Hall effect to be metal-like at all temperatures. Sb-doped alloys display very high thermoelectric power factors, but the thermal conductivity is still too high to make the material a prospective thermoelectric.