Synergistic Band and Interface Engineering via Dual Y/Ta Doping for High‐Performance n‐Type Mg 3 (Sb, Bi) 2 Thermoelectrics

ABSTRACT Bi‐rich n ‐type Mg 3 (Sb, Bi) 2 alloys are promising thermoelectric materials for medium‐to‐low temperature applications due to their simple synthesis, low cost, and environmental friendliness. However, maintaining high performance over a wide temperature range remains challenging. Here, we employ a dual‐doping strategy using Y and Ta to synergistically optimize electronic band structure and interfacial properties. Substituting Mg 2+ with Y 3+ tunes the Fermi level, increases carrier concentration, and preserves electrical conductivity. Ta forms metallic nanoinclusions that lower interfacial barriers, reduce grain‐boundary scattering, and enhance phonon scattering through multiscale interfaces. This cooperative approach enables simultaneous optimization of charge and phonon transport, outperforming single‐element doping. At the optimal composition, Mg 3.48 Bi 1.5 Sb 0.5 Y 0.01 Ta 0.01 achieves ZT = 0.68 at 322 K, while Mg 3.475 Bi 1.5 Sb 0.5 Y 0.01 Ta 0.015 reaches a peak ZT > 1.2 between 479 and 531 K and an average ZT of 1.05 over 300–600 K. A single‐leg device exhibits a predicted conversion efficiency of 14.58% at a temperature difference of 464 K. These results demonstrate the effectiveness of combining Y doping with Ta nanophase engineering to simultaneously modulate electron and phonon transport, offering a new strategy for sustainable, high‐performance n ‐type thermoelectrics.