Mechanism of Thermoelectric Performance Enhancement in CaMg2Bi2-Based Materials with Different Cation Site Doping

Chemical bonds determine electron and phonon transport in solids. Tailoring chemical bonding in thermoelectric materials causes desirable or compromise thermoelectric transport properties. In this work, taking an example of CaMg2Bi2 with covalent and ionic bonds, density functional theory calculations uncover that element Zn, respectively, replacing Ca and Mg sites cause the weakness of ionic and covalent bonding. Electrically, Zn doping at both Ca and Mg sites increases carrier concentration, while the former leads to higher carrier concentration than that of the latter because of its lower vacancy formation energy. Both doping types increase density-of-state effective mass but their mechanisms are different. The Zn doping Ca site induces resonance level in valence band and Zn doping Mg site promotes orbital alignment. Thermally, point defect and the change of phonon dispersion introduced by doping result in pronounced reduction of lattice thermal conductivity. Finally, combining with the further increase of carrier concentration caused by Na doping and the modulation of band structure and the decrease of lattice thermal conductivity caused by Ba doping, a high figure-of-merit ZT of 1.1 at 823 K in Zn doping Ca sample is realized, which is competitive in 1-2-2 Zintl phase thermoelectric systems.

Automated summary

Chemical bonds play a crucial role in determining electron and phonon transport in solids. This study investigates the effect of tailoring chemical bonding in the thermoelectric material CaMg2Bi2, which has both covalent and ionic bonds. The researchers find that doping with the element Zn at both the Ca and Mg sites weakens the ionic and covalent bonding. This doping also increases carrier concentration and reduces lattice thermal conductivity, resulting in a high figure-of-merit ZT value of 1.1 at 823 K in the Zn-doped Ca sample.