Chemical Fluctuations and In‐Situ Structural Order Unlock High‐Performance Mg ₃ Bi _1.4 Sb _0.6 ‐Based Thermoelectrics

Carrier mobility (µ_H) regulation is established as a core strategy for developing high-performance thermoelectric (TE) materials. However, a long-standing challenge lies in enhancing the overall TE performance for Mg₃Bi₂-based alloys through µ_H optimization while retaining the favorable effects of multi-scale defects on phonon scattering and strength-ductility. Herein, we achieve the dual enhancement of TE and mechanical performance of Mg_3.2- _xQ_x(Bi_0.7Sb_0.3)_1.99Te_0.01 (Q = Cu or Ag) through chemical fluctuations and structural order. Specifically, Cu/Ag doping shifts the Fermi level deeper into the conduction band and narrows the bandgap, boosting the electrical conductivity. In-situ synchrotron X-ray pair distribution function and atomic probe tomography characterizations demonstrate that interstitial Cu/Ag atoms induce chemical fluctuations and structural order, thus effectively improving µ_H while preserving strong phonon scattering. Meanwhile, multi-scale defects not only scatter multi-frequency phonons but also trigger multiple strengthening mechanisms, which concurrently reduce lattice thermal conductivity and improve mechanical properties. Ultimately, Mg_3.17Cu_0.03(Bi_0.7Sb_0.3)_1.99Te_0.01 and Mg_3.17Ag_0.03(Bi_0.7Sb_0.3)_1.99Te_0.01 demonstrate remarkable average zT values of 1.11 and 1.06 between 323 and 573 K, respectively, along with excellent compressive strengths of 402.3 and 386.9 MPa. This work demonstrates that chemical fluctuations and structural order establish a novel paradigm for the simultaneous optimization of zT_ave and mechanical reliability of TE materials.