Enhancing thermoelectric performance of p-type Mg(Mg,Zn)2Sb2 through Mg-site-specific substitution

Zintl-phase thermoelectrics with CaAl 2 Si 2 -type structures exhibit desirable “phonon-glass electron-crystal” behavior. However, the development of environmentally friendly p-type counterparts remains challenging. While n-type Mg 3 Sb 2 -based compounds have achieved high figures of merit ( zT > 2.0), their integration with non-Zintl systems is hindered by thermal and chemical mismatches. Moreover, many existing p-type Zintl compounds rely on toxic (Cd, Pb) or scarce (Yb, Ge) elements, restricting their scalability and sustainability. Here, we report a scalable, rare-earth- and toxicity-free p-type Zintl compound, Ca 0.5 Mg 1.297 Ag 0.003 Zn 1.2 Sb 2 , developed via targeted Mg-site substitution. First-principles calculations reveal that Ca substitution at Mg1 sites induces beneficial electronic structure modifications, including bandgap widening, valence band sharpening, and an indirect-to-direct bandgap transition, collectively enhancing carrier mobility (50 cm 2 ·V -1 ·s -1 ) and strengthening carrier-phonon scattering at elevated temperature. Simultaneously, trace Ag doping at Mg2 sites precisely tunes the carrier concentration by nearly one order of magnitude, achieving improved electrical conductivity while maintaining ultralow thermal conductivity. As a result, the optimized compound achieves a peak zT of 0.81 at 773 K and an average zT of 0.50 over 300–773 K. These results demonstrate a chemically stable, sustainable, and high-performing Zintl platform, offering a promising strategy for advancing eco-friendly thermoelectric technologies.