Phase diagram engineering advances the thermoelectric performance of Zintl phase EuZn₂Sb₂

EuZn₂Sb₂ shows promise for mid-temperature thermoelectrics but suffers from inconsistent performance due to an incomplete phase diagram, significantly impeding its further development. Herein, the 773 K isothermal section of the Eu-Zn-Sb ternary system has been established, unambiguously identifying the multiple chemical states of EuZn₂Sb₂. Through phase-boundary mapping, the defect chemistry and thermoelectric properties across different chemical states were elucidated. Ultimately, chemical state III (EuZn₂Sb₂ + ZnSb + Zn₄Sb₃ three-phase region) achieved a reproducible zT of ∼0.8 at 723 K. Further Yb and Cd alloying synergistically improved band degeneracy and introduced substantial phonon scattering by mass/stain fluctuations, yielding a high peak zT of ∼1.2 at 773 K and an average zT of ∼0.68 between 323 and 773 K. Additionally, the EuZn₂Sb₂/Mg₃(Sb, Bi)₂ two-pair thermoelectric module was fabricated, achieving an efficienty of ∼9% at the temperature difference of 473 K and exhibited no degradation after being stored in air for 14 months. This work not only provides a new perspective on the intrinsic relationship between phase diagram and thermoelectric performance, but also lays the experimental and theoretical foundation for designing high-performance thermoelectric materials via phase diagram engineering.