Targeting Interstitial Atoms in n-type Mg3(Bi, Sb)2 Single Crystal for Robust Ambient Energy Conversion

Abstract

Mg₃(Bi, Sb)₂ thermoelectrics are a promising alternative to Bi₂Te₃ alloys for ambient energy conversion. However, their practical application is hindered by the challenge of simultaneously achieving high performance and robust durability under ambient conditions, a difficulty that is especially pronounced in polycrystalline materials. This work presents the growth of a series of Mn-containing Mg₃(Bi, Sb)₂ single crystals, i.e., Mg_3.05Mn_xBi_1.488Sb_0.5Te_0.012 (0≦x≦0.15), which demonstrate both exceptional chemical stability and superior thermoelectric performance under ambient conditions. The enhanced robustness is firstly attributed to the elimination of excess Mg, which typically accumulates at grain boundaries in polycrystalline materials. Although this excess Mg was previously considered essential for high thermoelectric performance, it critically undermines the material’s stability due to its extreme moisture sensitivity. More importantly, with Mn atoms residing in chemically vulnerable interstitial sites instead of Mg, characterized by the High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM), robust Mg₃(Bi, Sb)₂ materials can be achieved; for instance, a single crystal of Mg_3.05Mn_0.1Bi_1.488Sb_0.5Te_0.012 exhibited an outstanding room-temperature figure of merit (zT~1.05) and retained its power factor (PF~43 μW·cm^-1·K^-2) even after 44 hours of water soaking. Remarkably, a cooling module based on this single-crystalline material demonstrated reliable operation in air for 300 hours as well, reflecting a 15,000% enhancement in ambient durability relative to conventional Mg₃(Bi, Sb)₂ single-crystal thermoelectrics.