Multi-valley-driven thermoelectric transport in half-Heusler TiCoX ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>X</mml:mi> </mml:mrow> </mml:math> = P, As, Sb)

Abstract Half-Heusler (HH) compounds are promising thermoelectric (TE) materials for medium-to-high temperature applications, yet the coupled electron–phonon transport in TiCo-based HHs remains poorly understood despite reports of Z T > 1 . In this work, we have found that TiCoX ( X = P, As, Sb) exhibits a ninefold valley degeneracy near the valence-band maximum, enabling intrinsically enhanced electrical transport. Owing to long phonon lifetimes, TiCoP, TiCoAs, and TiCoSb possess relatively high lattice thermal conductivities of 29.45, 46.59, and 16.17 W (mK) − 1 at 300 K, yielding optimal p-type (n-type) Z T values of 0.79 (0.70), 0.87 (0.08), and 0.80 (0.27) at 800 K. At the same time, comparison with experiments reveals that elemental doping enhances carrier mobility and the Seebeck coefficient via an increased density-of-states effective mass, while Hf alloying suppresses lattice thermal conductivity by up to 82 % and enhances Z T by 204 % . Our results potentially establish a synergistic doping–alloying strategy for TiCo-based HH TEs, offering a promising route for high-efficiency waste heat recovery in industrial energy and chemical applications.