Metallic n‐Type Mg 3 Sb 2 Single Crystals Demonstrate the Absence of Ionized Impurity Scattering and Enhanced Thermoelectric Performance

Mg 3 (Sb,Bi) 2 alloys have recently been discovered as a competitive alternative to the state‐of‐the‐art n‐type Bi 2 (Te,Se) 3 thermoelectric alloys. Previous theoretical studies predict that single crystals Mg 3 (Sb,Bi) 2 can exhibit higher thermoelectric performance near room temperature by eliminating grain boundary resistance. However, the intrinsic Mg defect chemistry makes it challenging to grow n‐type Mg 3 (Sb,Bi) 2 single crystals. Here, the first thermoelectric properties of n‐type Te‐doped Mg 3 Sb 2 single crystals, synthesized by a combination of Sb‐flux method and Mg‐vapor annealing, is reported. The electrical conductivity and carrier mobility of single crystals exhibit a metallic behavior with a typical T −1.5 dependence, indicating that phonon scattering dominates the charge carrier transport. The absence of any evidence of ionized impurity scattering in Te‐doped Mg 3 Sb 2 single crystals proves that the thermally activated mobility previously observed in polycrystalline materials is caused by grain boundary resistance. Eliminating this grain boundary resistance in the single crystals results in a large enhancement of the weighted mobility and figure of merit zT by more than 100% near room temperature. This work experimentally demonstrates the accurate understanding of charge‐carrier scattering is crucial for developing high‐performance thermoelectric materials and indicates that single‐crystalline Mg 3 (Sb,Bi) 2 solid solutions can exhibit higher zT compared to polycrystalline samples.