Magnetic, transport, and structural properties of Fe1-xIrxSi

Magnetic susceptibility, resistivity, Seebeck, Hall, and powder x-ray and neutron-diffraction measurements were used to characterize single crystals of FeSi and polycrystalline samples of Fe1-xIrxSi for x<0.2. The Rietveld refinement of low-temperature powder neutron-diffraction data on FeSi showed no change in the space group and no structural anomalies from 4 to 300 K. Magnetic and transport data from 4 to 700 K are consistent with the characterization of FeSi as a narrow-gap semiconductor (Eg=1200 K) with strong intrasite correlations for the states just below and above the gap. Fits to the magnetic susceptibility and resistivity data suggest that the magnetic (or direct) gap may be larger than the transport (indirect) gap. Electron mobilities in FeSi are very low (35 cm2/V s). The thermopower of FeSi has a large positive peak (500 V/K) at 50 K that is attributed to an unusually strong phonon-drag mechanism. Iridium acts as an electron donor in the Fe1-xIrxSi alloys. As the iridium doping level is increased, there is a rapid decrease in the low-temperature resistivity and a large negative (-140 V/K) phonon-drag contribution to the thermopower. For Peltier cooling applications, a maximum value for ZT of 0.07 was found for a Fe0.95Ir0.05Si alloy at 100 K.