Magnesium-Retention Assisted Synthesis of High-Performance Zintl Thermoelectrics

For decades, performance optimization of Mg₃(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_3.15Bi_1.5Sb_0.49Te_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_3.144Ba_0.006Bi_1.5Sb_0.49Te_0.01. Moreover, successful extension to MgCu₂ and MgAgSb highlights the broad applicability of MRAS as a scalable synthesis platform for high-performance Mg-based materials.