High-Performance CaMg₂Bi₂-Based Thermoelectric Materials Driven by Lattice Softening and Orbital Alignment via Cadmium Doping

Zintl compounds such as n-type Mg-3(Sb,Bi)(2) show promising thermoelectric applications benefiting from their high valley degeneracy and low lattice thermal conductivity. However, the heavier p-type AMg(2)X(2) (A = Ca, and Yb; X = Bi and Sb) Zintl counterparts even exhibit a higher kappa(lat) due to strong chemical bonding. Reducing kappa(lat) of AMg(2)X(2) is an important route for improving thermoelectric performance. Herein, it is found that Cd doping at the Mg site in CaMg2Bi2 can weaken intralayer covalent bonds and soften acoustic phonons, as well as fill the optical phonon gap. These effects result in large atomic displacement, low phonon group velocity, and strong lattice vibration anharmonicity. Doping 10% Cd leads to a reduction of 56% in the kappa(lat) of CaMg2Bi2. Moreover, Cd doping promotes orbital alignment and thus increases the density-of-states effective mass and Seebeck coefficient. Eventually, in conjunction with carrier concentration optimization by Na doping and band structure engineering by Ba doping, a high ZT of approximate to 1.3 at 873 K in (Ca0.85Ba0.15)(0.995)Na0.005Mg1.85Cd0.15Bi2 sample is realized. This work highlights the significant role of manipulating chemical bonding in suppressing phonon propagation of semiconductors.