Band convergence and phonon engineering to optimize the thermoelectric performance of CaCd₂Sb₂

Alignment of valence bands has been demonstrated to be effective in promoting the thermoelectric performance of p-type AB2X2 zintl phases. In this work, the degeneracy of the valence bands is manipulated by alloying CaCd2Sb2 with CaMg2Sb2. It is found that the Γ(pxy) band and the Γ(pz) band were effectively converged in CaCd1.5Mg0.5Sb2. By further doping Ag at the Cd site, the carrier concentration can be maintained when the alloying concentration varies. The roomerature Seebeck coefficient increased from ∼150 μV K-1 in CaCd2Sb2 to ∼190 μV K-1 in CaCd1.5Mg0.5Sb2 when the carrier concentration was maintained at ∼2.5 × 1019 cm-3. In addition, Cd/Mg substitutional point defects with substantial atomic mass difference induced significant phonon scattering; thus, a lattice thermal conductivity as low as ∼0.5 W m-1 K-1 was achieved at 750 K. Eventually, a peak zT value of ∼1.3 was realized in CaCd1.494Ag0.006Mg0.5Sb2.