Synergistic carrier and phonon engineering in n-type Mg3(Sb,Bi)2 via silver Co-doping

Abstract

Mg₃Sb₂-based Zintl compounds are promising for sustainable mid-temperature thermoelectrics, yet further performance gains are often impeded by the strong coupling among electrical and thermal transport parameters. Here, we propose an Ag-enabled dual strategy to synergistically engineer electron and phonon transport in n-type Mg₃Sb_1.5Bi_0.49Te_0.01. Minute Ag incorporation on the Mg sublattice enhances carrier transport by reducing defect-related scattering; notably, HAADF-STEM/TEM analyses reveal a pronounced decrease in dislocation density and improved lattice ordering, which together contribute to an approximately twofold increase in Hall mobility. In parallel, Ag- and Bi-rich nanoprecipitates form within the matrix and along grain boundaries, respectively, creating a multiscale defect structure that drastically reduces the lattice thermal conductivity. After further tuning the carrier concentration via Te content, the optimal composition of Mg_3.19Ag_0.01Sb_1.5Bi_0.485Te_0.015 achieves a peak ZT of 1.78 at 673 K and a high average ZT of 1.28 over 298–673 K. This work demonstrates that combining aliovalent doping with microstructure engineering can effectively decouple transport properties and markedly improve the thermoelectric performance of n-type Mg₃Sb₂-based materials.