Weak Chemical Bonds and High Electrical Conductivities Lead to High Thermoelectric Performance in Zintl Compounds Sr₂CdX₂ (X = As, Sb, and Bi)

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 (<i>Cmc</i>2₁) 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 <i>x</i>- and <i>z</i>-axis, respectively. Consequently, the highest <i>ZT</i> values of Sr₂CdAs₂ (1.39), Sr₂CdSb₂ (1.97), and Sr₂CdBi₂ (1.74) are achieved along the <i>x</i>-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.