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

Abstract

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.