Realizing Ultrahigh Near-Room-Temperature Thermoelectric Figure of Merit for N-Type Mg₃(Sb,Bi)₂ through Grain Boundary Complexion Engineering with Niobium

Despite decades of extensive research on thermoelectric materials, Bi₂Te₃ alloys have dominated room-temperature applications. However, recent advancements have highlighted the potential of alternative candidates, notably Mg₃Sb₂-Mg₃Bi₂ alloys, for low- to mid-temperature ranges. This study optimizes the low-temperature composition of this alloy system through Nb addition (Mg_3.2-xNb_x(Sb_0.3Bi_0.7)_1.996Te_0.004), characterizing composition, microstructure, and transport properties. A high Mg₃Bi₂ content improves the band structure by increasing weighted mobility while enhancing the microstructure. Crucially, it suppresses detrimental grain boundary scattering effects for room-temperature applications. While grain boundary scattering suppression is typically achieved through grain growth, our study reveals that Nb addition significantly reduces grain boundary resistance without increasing grain size. This phenomenon is attributed to a grain boundary complexion transition, where Nb addition transforms the highly resistive Mg₃Bi₂-rich boundary complexion into a less resistive, metal-like interfacial phase. This marks the rare demonstration of chemistry noticeably affecting grain boundary interfacial electrical resistance in Mg₃Sb₂-Mg₃Bi₂. The results culminate in a remarkable advancement in zT, reaching 1.14 at 330 K. The device ZT is found to be 1.03 at 350 K, which further increases to 1.24 at 523 K and reaches a theoretical maximum device efficiency (η_max) of 10.5% at 623 K, underscoring its competitive performance. These findings showcase the outstanding low-temperature performance of n-type Mg₃Bi₂-Mg₃Sb₂ alloys, rivaling Bi₂Te₃, and emphasize the critical need for continued exploration of complexion phase engineering to advance thermoelectric materials further.