Magnesium‐Retention Assisted Synthesis of High‐Performance Zintl Thermoelectrics

For decades, performance optimization of Mg 3 (Bi, Sb) 2 ‐based thermoelectric materials has mainly relied on doping and alloying strategies, while the influence of synthesis‐process innovation on thermoelectric mechanisms and properties has remained largely unexplored. Here, we develop a magnesium‐retention assisted synthesis (MRAS) strategy that stabilizes Mg through a sacrificial Mg reservoir and an inert‐gas‐regulated melting environment, effectively suppressing Mg volatilization and chemical instability during high‐temperature synthesis. MRAS enables scalable fabrication of phase‐pure, compositionally uniform Mg‐based alloys without costly tantalum encapsulation. The resulting Mg‐rich Mg 3.15 Bi 1.5 Sb 0.49 Te 0.01 features interconnected Mg‐rich regions and multiscale defects that synergistically promote electrical transport and suppress lattice thermal conductivity, delivering an ultrahigh power factor of 41.7 µW cm −1 K −2 , a ZT of 1.2 ± 0.1 at 323 K, and a peak ZT of 1.8 ± 0.1 at 523 K. MRAS is also compatible with conventional compositional engineering: Ba doping at the Mg site further reduces thermal conductivity through point‐defect scattering while largely preserving the power factor, yielding a ZT around 1.9 ± 0.1 at 523 K in Mg 3.144 Ba 0.006 Bi 1.5 Sb 0.49 Te 0.01 . Moreover, successful extension to MgCu 2 and MgAgSb highlights the broad applicability of MRAS as a scalable synthesis platform for high‐performance Mg‐based materials.