Weak Chemical Bonds and High Electrical Conductivities Lead to High Thermoelectric Performance in Zintl Compounds...

Zintl semiconductors are known to exhibit exceptional thermoelectric properties, due to their high electron transport and low lattice thermal conductivity (κ_L). In this work, thermoelectric properties of Zintl compounds Sr₂CdX₂ (X = As, Sb, Bi) were studied by using first-principles calculations in conjunction with Boltzmann transport theory. Both Sr₂CdSb₂ and Sr₂CdBi₂ adopt the same structure as Sr₂CdAs₂ (Cmc2₁) and are synthesizable. All of these compounds demonstrate low lattice thermal conductivities, ranging from 2.3 to 0.5 Wm^1- K^-1 at 300 K, attributed to their low sound velocity and significant three-phonon scattering, which originate from the weak chemical bonds and strong interaction between acoustic phonons and low-lying optical phonons. The low polar-optical phonon scattering is a result of the relatively small contributions of ions to dielectric constants. Notably, the electrical conductivities (σ) and κ_L of these compounds exhibit significant anisotropy, with the highest and lowest values being along the x- and z-axis, respectively. Consequently, the highest ZT values of Sr₂CdAs₂ (1.39), Sr₂CdSb₂ (1.97), and Sr₂CdBi₂ (1.74) are achieved along the x-direction at 800 K under n-type doping, respectively, which is comparable or even superior to the well-studied thermoelectric material Mg₃Sb₂, positioning them as promising candidates for thermoelectric applications.