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

Bi-rich n-type Mg₃(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 Mg²⁺ with Y³⁺ 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_3.48Bi_1.5Sb_0.5Y_0.01Ta_0.01 achieves ZT = 0.68 at 322 K, while Mg_3.475Bi_1.5Sb_0.5Y_0.01Ta_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.