Development of n-type Ti₂CoNiSb₂ double half-Heusler compound by the breaking of valence-balanced rule and achieving high thermoelectric performance via Bi/Cu co-doping

Half-Heusler (HH) thermoelectric materials exhibit excellent electronic transport properties but suffer from intrinsically high lattice thermal conductivity, which limits their thermoelectric performance. To address this challenge, double half-Heusler (DHH) compounds with 18 valence electrons have recently been proposed. However, the disordered nature of DHH phases often degrades their electronic transport properties, hindering the achievement of high thermoelectric conversion efficiency. In this work, we design a new n-type Ti₂CoNiSb₂ DHH compound with 18.5 valence electrons by combining TiCoSb and TiNiSb half-Heuslers, intentionally breaking the conventional valence balance. Furthermore, the effects of Cu and Bi doping at the Ni and Sb sites, respectively, are systematically investigated. Cu doping effectively enhances phonon scattering through point defects, while Bi doping significantly improves the weighted mobility. When Cu and Bi are co-doped, phonon scattering is further strengthened, particularly at high temperatures, and the weighted mobility is simultaneously increased. As a result, a peak figure of merit (zT) of ∼0.82 is achieved at 973 K in Ti₂CoNi_0.9Cu_0.1(Sb_0.925Bi_0.075)₂, nearly four times higher than that of the pristine Ti₂CoNiSb₂ (zT ∼0.22). This work highlights the effectiveness of co-doping strategies that simultaneously optimize thermal and electronic transport properties in DHH thermoelectric systems.