Realization of Band Convergence in p-Type TiCoSb Half-Heusler Alloys Significantly Enhances the Thermoelectric...

Band engineering is a promising approach that proved successful in enhancing the thermoelectric performance of several families of thermoelectric materials. Here, we show how this mechanism can be induced in the p-type TiCoSbhalf-Heusler (HH) compound to effectively improve the Seebeck coefficient. Both the Pisarenko plot and electronic band structure calculations demonstrate that this enhancement is due to increased density-of-states effective mass resulting from the convergence of two valence band maxima. Our calculations evidence that the valence band maximum of TiCoSb lying at the Γ point exhibits a small energy difference of 51 meV with respect to the valence band edge at the L point. Experimentally, this energy offset can be tuned by both Fe and Sn substitutions on the Co and Sb site, respectively. A Sn doping level as low as x = 0.03 is sufficient to drive more than ∼100% increase in the power factor at room temperature. Further, defects at various length scales, that include point defects, edge dislocations, and nanosized grains evidenced by electron microscopy (field emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM)), result in enhanced phonon scattering which substantially reduces the lattice thermal conductivity to ∼4.2 W m^-1 K^-1 at 873 K. Combined with enhanced power factor, a peak ZT value of ∼0.4 was achieved at 873 K in TiCo_0.85Fe_0.15Sb_0.97Sn_0.03. In addition, the microhardness and fracture toughness were found to be enhanced for all of the synthesized samples, falling in the range of 8.3-8.6 GPa and 1.8-2 MPa·m^-1/2, respectively. Our results highlight how the combination of band convergence and microstructure engineering in the HH alloy TiCoSb is effective for tuning its thermoelectric performance.