Balancing the Competing Effects of Carrier and Phonon Transport Mechanisms to Enhance the Thermoelectric Properties of p‐Type <scp>T</scp> i ₂ <scp>Z</scp> r _{2− <i>x</i>} <scp>H</scp> f ₂ <scp>N</scp> b ₂

Boosting thermoelectric performance is challenging due to the intricate interplay between electrical and thermal transport properties. This study focuses on Zr vacancy‐filled p‐type Ti 2 Zr 2 Hf 2 Nb 2 Fe 5.6 Ni 2.4 Sb 8 ‐based thermoelectric materials to explore how Zr vacancies affect their structural and transport characteristics. Density functional theory calculations demonstrate that Zr vacancies induce proximal contraction/distal relaxation, strengthening lattice distortion while preserving the intrinsically intense phonon scattering in Ti 2 Zr 2 Hf 2 Nb 2 Fe 5.6 Ni 2.4 Sb 8 samples. The intensified asymmetric electron localization between adjacent anions and the cation vacancies softens local chemical bonds. Microscopic investigations reveal that Ti 2 Zr 2− x Hf 2 Nb 2 Fe 5.6 Ni 2.4 Sb 8 alloys with an optimal number of Zr vacancies balance the competing effects of carrier and phonon transport mechanisms by regulating multi‐scale defects. Introducing appropriate Zr vacancies optimizes both the Seebeck coefficient and κ L without greatly affecting electrical conductivity and weight mobility, achieving a 23% maximum power factor improvement and roughly 10% κ L reduction. The bipolar diffusion effect is effectively suppressed to negligible levels by energy filtering effects, thus ensuring high‐temperature stability. The maximum ZT of Ti 2 Zr 1.97 Hf 2 Nb 2 Fe 5.6 Ni 2.4 Sb 8 and Ti 2 Zr 1.95 Hf 2 Nb 2 Fe 5.6 Ni 2.4 Sb 8 is 30% higher than that of pristine samples without Zr vacancies. These findings are the first demonstration of vacancy engineering as a promising strategy in p‐type double half‐Heusler alloys to enhance their thermoelectric performance and decouple intertwined transport parameters.