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

We have studied the transport properties in a family of purr and doped inte rmetallics of the form MNiSi (M=Zr, Hf), the structures known as the half-H eusler alloys. We have shown that the transport is very sensitive to struct ural arrangements of the constituent atoms, and this can be manipulated by annealing, isostructural alloying, and doping. The unusual transport proper ties 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 transi tion upward towards 200 K. The high-temperature transport is dominated by t he presence of heavy electrons that are responsible for surprisingly large values of thermopower. Minute amount of antimony (nl-type doping) have a sp ectacular influence on the nature of transport and drive the electrical res istivity and Hall effect to be metal-like at all temperatures. Sb-doped all oys display very high thermoelectric power factors, but the thermal conduct ivity is still too high to make the material a prospective thermoelectric. [S0163-1829(99)07013-7].