Superior carrier mobility and ultra-low lattice thermal conductivity endow high thermoelectric performance in Se–Er co-doped Mg₃Bi_1.4Sb_0.6

The cost-effective Mg₃Bi₂-based compounds are potential near room-temperature thermoelectric (TE) materials, yet the intrinsic low carrier concentration (n) and Mg vacancies severely hinder the further enhancement of TE properties. Herein, Er–Se co-doping shifts the conduction bands to the Fermi level (E_F), resulting in combined the heavy and light bands participating in carrier transport, significantly increasing the n and carrier mobility (μ) of n-type Mg₃Bi_1.4Sb_0.6 with adjusted Bi/Sb ratio. Consequently, a superior μ of ∼139.8 cm² V⁻¹ s⁻¹ is obtained in Mg_3.2Bi_1.4Sb_0.58Se_0.02 at room temperature. Benefiting from the improved n and μ, an excellent power factor (PF) of ∼17.94 μW cm⁻¹ K⁻² is achieved in Mg_3.18Er_0.02Bi_1.4Sb_0.58Se_0.02 at 523 K. Meanwhile, based on the enhancement of multiscale defects on the phonons scattering, an ultra-low lattice thermal conductivity (κ_l) of ∼0.26 W m⁻¹ K⁻¹ is obtained in Mg_3.16Er_0.04Bi_1.4Sb_0.58Se_0.02 at 523 K. Ultimately, the Mg_3.18Er_0.02Bi_1.4Sb_0.58Se_0.02 sample attains a high figure-of-merit (ZT) of ∼1.0 at 523 K, which improves by 354 % than that of pristine Mg_3.2Bi_1.4Sb_0.6. The work demonstrates an approach for designing high-performance n-type Mg₃Bi₂-based compounds with superior μ and ultra-low κ_l, elucidating the driving mechanism of high ZT.