Enhanced Thermoelectric Properties of p-Type CaMg₂Bi₂ via a Synergistic Effect Originated from Zn and Alkali-Metal Co-doping

Bi-based Zintl phase CaMg2Bi2 is a promising thermoelectric material. Here, we report that the high-concentration point defects induced by equivalent Zn doping on the Mg site significantly enhance phonon scattering and then suppress lattice thermal conductivity by 50% at room temperature. Subsequently, partial substitution of divalent calcium ions with alkali-ion doping (Li, Na, K) not only optimizes the electrical transport properties by increasing the carrier concentration but also further reduces the lattice thermal conductivity through crystal disorder. Finally, the synergistic effect of Zn and Li co-doping leads to a high ZT of ∼1.0 at 873 K and an average ZT of 0.6 between 300 and 873 K for Ca0.995Li0.005Mg1.9Zn0.1Bi1.98. This work demonstrates an instructive method to reduce the lattice thermal conductivity via doping at the Mg site, which has never been reported in the CaMg2Bi2 system. Moreover, high-performance Ca0.995Li0.005Mg1.9Zn0.1Bi1.98 alloy does not contain any toxic elements and expensive rare earth elements, which is of great significance for the development of environment-friendly thermoelectric materials.