Metallic n-Type Mg₃ Sb₂ Single Crystals Demonstrate the Absence of Ionized Impurity Scattering and Enhanced Thermoelectric Performance.

Abstract

Mg₃ (Sb,Bi)₂ alloys have recently been discovered as a competitive alternative to the state-of-the-art n-type Bi₂ (Te,Se)₃ thermoelectric alloys. Previous theoretical studies predict that single crystals Mg₃ (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 Mg₃ (Sb,Bi)₂ single crystals. Here, the first thermoelectric properties of n-type Te-doped Mg₃ Sb₂ 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 Mg₃ Sb₂ 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 Mg₃ (Sb,Bi)₂ solid solutions can exhibit higher zT compared to polycrystalline samples.