Defect-Energy-Targeted Lattice Repair Delivers High Thermoelectric Performance in Magnesium Antimonide

Magnesium-based Mg₃(Sb,Bi)₂ has emerged as a premier candidate for waste-heat recovery. However, its performance is fundamentally capped by intrinsic Mg vacancies that severely scatter carriers. Here, we overcome this bottleneck via a defect-energy-targeted lattice repair strategy, substituting labile Mg sites with homologous alkaline-earth metals (Ca, Sr, Ba). Theoretical calculations reveal that the lower electronegativity of these dopants strengthens the local metal-Sb bonding, drastically raising the vacancy formation energy from ∼0.97 to ∼2.42 eV. This thermodynamic stabilization effectively "repairs" the lattice, suppressing vacancy generation and yielding a ∼35% boost in carrier mobility without compromising carrier concentration. Simultaneously, the heavy dopants induce mass fluctuations and strain fields that, coupled with dense dislocations, minimize the lattice thermal conductivity to ∼0.4 W m^-1 K^-1 at 773 K. The synergy of restored charge transport and suppressed heat propagation leads to a record-high figure of merit (<i>zT</i>) of ∼2.1 at 773 K and an outstanding average <i>zT</i> of ∼1.5 in Mg_3.2Ba_0.005Sb_1.5Bi_0.49Te_0.01. Remarkably, a single-leg device demonstrates a conversion efficiency of ∼14%, outperforming state-of-the-art n-type thermoelectrics. This work demonstrates that targeting defect energetics is a powerful, broadly applicable approach to breaking the performance ceilings of Zintl-phase thermoelectrics.