Optimizing the Thermoelectric Performance of TiCoSb Half-Heusler Alloy through Isovalent Ir-d Orbital Engineering

Abstract

Thermoelectric materials, which enable the conversion of waste heat into electricity, are important for sustainable energy applications. Recently, the half-Heusler (HH) TiCoSb alloy has been experimentally synthesized in its pristine form, and various isovalent and aliovalent substitutions at Ti and Sb sites have improved its thermoelectric performance. However, a systematic understanding of isovalent d-electron engineering at the Y site (Co site) remains limited. Here, we report a systematic first-principles investigation of Y-site substitution in TiCo1−xIrxSb (x = 0.25, 0.75, and 1) to elucidate the role of d-orbital modulation on electronic structure and transport properties. The isovalent Ir substitution induces an indirect-to-direct band gap transition for partially substituted compositions and significantly reconstructs the conduction band edge. Notably, the TiCo0.25Ir0.75Sb alloy exhibits thermally accessible conduction band convergence at Γ (ΔE ≈ 0.02 eV), leading to a moderate enhancement in the density-of-states effective mass (mDOS∗≈0.66me) and improved Seebeck coefficient (255.12 μV K−1). Simultaneously, the large mass variance between Co and Ir introduces strong alloy-induced phonon scattering, significantly suppressing the lattice thermal conductivity. The combined electronic optimization and phonon scattering yield a maximum ZT of 0.97 at 1200 K and a maximum conversion efficiency of ∼12.5% (at ΔT = 900 K) for TiCo0.25Ir0.75Sb. Furthermore, Ir substitution improves mechanical robustness, promoting a brittle-to-ductile transition and maintaining a high melting temperature (∼3623 K), suggesting enhanced device reliability at elevated temperatures. These findings establish Y-site d-electron engineering via isovalent Ir substitution as an effective strategy for conduction band modulation, phonon scattering enhancement, and high-temperature thermoelectric performance optimization in TiCoSb-based HH alloys. This work provides an effective strategy of simultaneously improving thermoelectric performance and mechanical flexibility by Y-site d-electron engineering via isovalent Ir substitution in TiCoSb alloy, and it also indicates that ductile semiconductor TiCo0.25Ir0.75Sb can be used in flexible thermoelectric generators.