Enhanced Thermoelectric Properties of La-Doped ZrNiSn Half-Heusler Compound

The effect of La doping on ZrNiSn half-Heusler (HH) compound has been studied to explore the composition variation and structural modifications for improvement of its thermoelectric performance. A series of La (x) Zr1-x NiSn (x = 0, 0.005, 0.01, 0.015, 0.02, 0.03) alloys were prepared by induction melting combined with plasma-activated sintering. Structural analysis using x-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) confirmed the resulting material to be a composite of HH, NiZr, and La3Sn4-type phases. The volume fraction for the phases other than HH ranged from 1.5% to 25% with increasing La content, as estimated by Rietveld analysis. The solubility of La in ZrNiSn is estimated to be 1.5%. Point defects may play a significant role in carrier and phonon transport. Interestingly, the thermoelectric transport properties exhibited a considerable increase in electrical conductivity sigma with La doping and a significant drop in thermal conductivity kappa, leading to a thermoelectric figure of merit (ZT) of 0.53 at 923 K, representing an improvement of about 37% compared with the undoped sample.