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The full-Heusler (FH) inclusions in the half-Heusler (HH) matrix is a well-studied approach to reduce the lattice thermal conductivity of ZrNiSn HH alloy. However, excess Ni in ZrNiSn may lead to the in situ formation of FH and/or HH alloys with interstitial Ni defects. The excess Ni develops intermediate electronic states in the band gap of ZrNiSn and also generates defects to scatter phonons, thus providing additional control to tailor electronic and phonon transport properties synergistically. In this work, we present the implication of isoelectronic Ge-doping and excess Ni on the thermoelectric transport of ZrNiSn. The synthesized ZrNi1.04Sn1–x Ge x (x = 0–0.04) samples were prepared by arc-melting and spark plasma sintering, and were extensively probed for microstructural analysis. The in situ evolution of minor secondary phases, i.e., FH, Ni–Sn, and Sn–Zr, primarily observed post sintering resulted in simultaneous optimization of the electrical power factor and lattice thermal conductivity. A ZT of ∼1.06 at ∼873 K was attained, which is among the highest for Hf-free ZrNiSn-based HH alloys. Additionally, ab initio calculations based on density functional theory (DFT) were performed to provide comparative insights into experimentally measured properties and understand underlying physics. Further, mechanical properties were experimentally extracted to determine the usability of synthesized alloys for device fabrication.

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 0.85 Fe 0.15 Sb 0.97 Sn 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.