Earth‐Abundant Oxides Promoting High‐Performance Mg 3 (Sb,Bi) 2 Thermoelectrics via a Metathesis Strategy

ABSTRACT The bright promise of Mg 3 (Sb,Bi) 2 as a next‐generation near‐room‐temperature thermoelectric material is challenged both by the need for reliable and economical synthesis and by performance degradation from detrimental Mg‐vacancy defects at grain boundaries and within grains. Here, we reveal that Earth‐abundant oxides (Fe 2 O 3 , ZrO 2 and TiO 2 ) can unlock superior thermoelectric performance in Mg 3 (Sb,Bi) 2 via a simple metathesis strategy. Incorporation of the oxides (at only 1–3 mol%) in Mg‐excess Mg 3 (Sb,Bi) 2 powders during spark‐plasma sintering initiates “Mg‐oxide” reduction reactions; the subsequent “transition metal‐Mg 3 (Sb,Bi) 2 ” reactions release a source of additional Mg, whose global diffusion refills cationic vacancies in the bulk matrix, enabling markedly enhanced power factors and figures of merit. Our fabricated 8‐pair Mg 3 Sb 0.75 Bi 1.25 /MgAgSb device attained remarkable conversion efficiency and output power density of 11.7% and 1.0 W cm −2 at a temperature gradient (Δ T ) of 315 K. Notably, a Mg 3 Sb 0.5 Bi 1.5 /Bi 0.5 Sb 1.5 Te 3 module demonstrated maximum cooling Δ T s competitive with state‑of‑the‑art Bi 2 Te 3 coolers at 200–373 K. The proposed metathesis strategy not only offers an eco‐friendly and cost‐effective route to defect engineering, promoting applications of Mg 3 (Sb,Bi) 2 TEs, but also provides key insights into the structure‐property relationships and thermoelectric performance optimization of other Mg‐based TEs and Zintl phases.