Low Thermal Conductivity and High Thermoelectric Figure of Merit of Two-Dimensional Ba2ZnAs2 and Ba2ZnSb2

Thermoelectric (TE) technology can effectively alleviate energy shortage and environmental pollution problems and has thus attracted extensive attention. In this work, we designed two unexplored two-dimensional materials, Ba₂ZnAs₂ and Ba₂ZnSb₂, and investigated their stability, mechanical characteristics, and TE properties using first-principles calculations and by solving the Boltzmann transport equation. We revealed that the two materials possess high stability and moderate cleavage energies of 0.84 and 0.76 J m⁻². Moreover, they are indirect semiconductors with band-gaps of 1.26 and 0.97 eV and show flat energy dispersion near the valence band maximum, resulting in a high p-type Seebeck coefficient of approximately 0.72 and 0.29 mV K⁻¹ at 300 K. Furthermore, they have significant anisotropic TE power factor along the a- and b-axis, with maxima of 1.19 and 0.75 mW m⁻¹ K⁻² at 300 K. Owing to the strong coupling between the acoustic and optical phonons, as well as the low frequency for low-lying phonons, the materials have high phonon scattering rates and low lattice thermal conductivities of 0.54/0.52 and 0.81/0.43 W mK⁻¹ along the a-/b-axis. Ultimately, Ba₂ZnAs₂ and Ba₂ZnSb₂ can deliver high-performance TE transport with high figures-of-merit of 0.32 and 0.19 at 300 K, which increase further to 1.67 and 0.91, respectively, at 700 K.