Cobalt‐Induced Strong Anharmonicity Coupled With Hierarchical Defect and Band Structure Engineering for Enhanced n‐ and p‐Type Thermoelectric Performance in ZrFe 0.5 Ni 0.5 Sb Double Half‐Heuslers

Enhancing the power factor while reducing thermal conductivity is crucial for establishing double half‐Heuslers (DhHs) as viable thermoelectric materials. This study investigates Co substitution in ZrFe 0.5 Ni 0.5 Sb at Fe and Ni sites, revealing site‐dependent effects on performance. In n‐type ZrFe 0.5‐ x Co x Ni 0.5 Sb, doping improves performance through electron donation and band structure changes, increasing the power factor while reducing lattice thermal conductivity (κ latt ) by over 40% through induced multiscale defect mechanisms. Insights from the recently developed Debye–Callaway model simulator reveal that lattice anharmonicities (sound velocity and Umklapp), point defect, grain boundary, and nanoinclusion scattering contribute 52%, 20%, 15%, and 13%, respectively. This highlights an unprecedented influence of bond changes on the observed z T enhancement from 0.001 (undoped) to 0.51 ( x = 0.3) and an increase in unileg thermoelectric generator output from 80 to 270 µW at Δ T = 773 K. In contrast, Co doping at the Ni site in ZrFe 0.5 Ni 0.5− y Co y Sb p‐type samples stiffens the bonds, degrading z T . Collectively, these insights are critical for guiding future strategies aimed at further optimizing thermoelectric performance through targeted microstructural and band structure engineering.