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 (κL). 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 Wm1- 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 κL 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.