Abstract: P-type Mg₃Sb₂-based Zintls have attracted considerable interest in the thermoelectric (TE) field due to their environmental friendliness and low cost. However, compared to their n-type counterparts, they show relatively low TE performance, limiting their application in TE devices. In this work, we simultaneously introduce Bi alloying at Sb sites and Ag doping at Mg sites into the Mg₃Sb₂ to cooperatively optimize the electrical and thermal properties for the first time, acquiring the highest ZT value of ∼0.85 at 723 K and a high average ZT of 0.39 in the temperature range of 323-723 K in sample Mg_2.94Ag_0.06Sb_1.9Bi_0.1. The first-principle calculations show that the co-doping of Ag and Bi can shift the Fermi level into the valence band and narrow the band gap, resulting in the increased carrier concentration from 3.50 × 10¹⁷ cm^-3 in the reference Mg₃Sb_0.9Bi_0.1 to ∼7.88 × 10¹⁹ cm^-3 in sample Mg_2.94Ag_0.06Sb_0.9Bi_0.1. As a result, a remarkable power factor of ∼778.9 µW m^-1 K^-2 at 723 K is achieved in sample Mg_2.94Ag_0.06Sb_0.9Bi_0.1. Meanwhile, a low lattice thermal conductivity of ∼0.48 W m^-1 K^-1 at 723 K is also obtained with the help of phonon scattering at the distorted lattice, point defects, and nano-precipitates in sample Mg_2.94Ag_0.06Sb_0.9Bi_0.1. The synergistic effect of using the multi-element co-doping/-alloying to optimize electrical properties in Mg₃Sb₂ holds promise for further improving the TE performance of Zintl phase materials or even others.

Key words: Thermoelectric materials, Band engineering, Nanostructuring, P-type Mg₃Sb₂, Ag and Bi Co-doping