Significantly Enhanced Thermoelectric Performance of p-Type Mg₃Sb₂ via Zn Substitution on Mg(2) Site: Optimization of Hole Concentration Through Ag Doping

n-Type Mg₃Sb₂-based thermoelectric materials have recently garnered significant interest due to their superior thermoelectric efficiency. Yet, the advancement of p-type Mg₃Sb₂ for thermoelectric applications is impeded by its lower dimensionless figure of merit (zT). In this study, we demonstrate the improved thermoelectric performance of p-type Mg₃Sb₂ through the strategic optimization of Zn content and Ag doping on the Mg/Zn(2) site. Initially, samples of Mg_3-xZn_xSb₂ (x = 0, 0.5, 1.0, and 1.5) were synthesized via elemental reactions within a Pyrex tube, followed by densification through hot pressing. X-ray diffraction analysis confirmed that the Mg_3-xZn_xSb₂ phases retain the same P$\overline{3}$m1 space group as the pristine Mg₃Sb₂ phase. The strategic substitution of Zn improved the power factor via band convergence and reduced lattice thermal conductivity by introducing point defect phonon scattering. This led to a peak zT of 0.5 at 725 K, with an average zT of 0.25 across the 325-725 K range. Enhancement in carrier concentration was achieved by doping Ag onto the Zn site, culminating in a peak zT of 0.95 at 725 K and an average zT of 0.46 between 325 and 725 K for the Mg₂Zn_0.97Ag_0.03Sb₂ sample. This performance surpasses that of most p-type Mg₃Sb₂-based materials, markedly advancing the potential for Mg₃Sb₂-based materials in midtemperature heat recovery thermoelectric generators.