Defect-Assisted Ultrahigh zT of TaFeSb Based Half-Heuslers

The half-Heusler compound TaFeSb has been reported to exhibit promising $p$ -type thermoelectric properties. Here, a high and reproducible thermoelectric figure of merit (zT) of 1.55 with Ti substitution has been reported. It is demonstrated that the zT enhancement is not solely an effect of carrier optimization; the role of antisite disorder is also shown to be pivotal. To demonstrate this, samples are prepared with controlled stoichiometry to tune the disorder. The thermoelectric power factor was found suppressed in samples with only a nominal disorder, which also enhanced the lattice thermal conductivity, leading to a low zT of $\sim 1$ . However, with Fe deficiency, which is shown to increase the antisite disorder, we observed an increase in the Seebeck effective mass, leading to enhanced power factor and reduced lattice thermal conductivity, resulting in a zT of 1.35, i.e., a $\simeq 35$ % enhancement over the ordered sample. A further increase in zT is observed for the arc-melted sample, where disorder is more pronounced, resulting in a zT of 1.55. These findings provide valuable insights for optimizing the thermoelectric performance of half-Heusler materials through controlled defect engineering.