Coupled defect engineering and secondary phase scattering via rare-earth sulfides for high performance n-type Mg₃(Sb, Bi)₂

n-type Mg₃(Sb, Bi)₂-based thermoelectric materials are promising for mid-temperature applications, yet further performance improvement is still hindered by the strong interdependence among electronic and thermal transport parameters. Here, by using n-type Mg_3.2Sb_1.5Bi_0.5 as the matrix, we separately incorporated Pr₂S₃ and Gd₂S₃ via mechanical alloying, enabling a coupled optimization that combines donor electron supply with second-phase induced phonon scattering. First-principles calculations indicate that rare-earth sulfide incorporation narrows the band gap, flattens the band-edge dispersion, and induces pronounced avoided crossings in the phonon dispersion. Experimentally, Pr/Gd- and S-enriched second-phases are observed, accompanied by interfacial strain and dislocation networks, thereby establishing multiscale phonon scattering centers. Transport measurements show that an anomalous decrease in thermal conductivity emerges at the heavily doped composition, suggesting strengthened phonon scattering associated with second-phase precipitation and interfacial defects. As a result, the (Pr₂S₃)_0.025(Mg_3.2Sb_1.5Bi_0.5)_0.975 sample achieves a peak zT of 1.66 at 723K, and finite-element simulations further predict an energy conversion efficiency approaching 12% under a temperature difference of 423K, while maintaining favorable mechanical properties. This work demonstrates an engineering-viable rare-earth sulfide modification route for n-type Mg₃(Sb, Bi)₂-based thermoelectric materials and clarifies the associated structure-property correlations.