Enhanced Thermoelectric Efficiency in P-Type Mg3Sb2: Role of Monovalent Atoms Codoping at Mg sites

Due to natural abundance, low cost, and compatibility with sustainable green technology, Mg₃Sb₂-based Zintl compounds are comprehensively explored as potential thermoelectric materials for near-room temperature applications. The effective use of these materials in thermoelectric devices requires both p and n-type Mg₃Sb₂ having comparable thermoelectric efficiency. However, p-type Mg₃Sb₂ has inferior thermoelectric efficiency efficiency compared to its n-type counterpart due to low electrical conductivity (∼103Sm-1). Here, we show that codoping of monovalent atoms (Li-Ag, and Na-Ag) at the Mg site of Mg₃Sb₂ produces a synergistic effect and boosts the electrical conductivity, which enhances the thermoelectric properties of p-type Mg₃Sb₂. While, Ag prefers to occupy the Mg2 site, Li and Na are favorable at the Mg1 site of Mg₃Sb₂ lattice. Compared to Li-Ag codoping, Na-Ag codoping in Mg₃Sb₂ is found to be more effective for increasing the charge carrier concentration and significantly augmenting the electrical conductivity. The dominance of the three-phonon scattering mechanism in Li and Li-Ag doped Mg₃Sb₂ and the four-phonon scattering process for the Na and Na-Ag doped Mg₃Sb₂ are confirmed. Due to the simultaneous increase in electrical conductivity and decrease in thermal conductivity, the zT value ∼0.8 at 675 K achieved for Mg_2.975Na_0.02Ag_0.005Sb₂ is the highest value among p-type Mg₃Sb₂. Our work shows a constructive approach to enhance the zT of p-type Mg₃Sb₂ via monovalent atoms codoping at the Mg sites.