Ultra-low lattice thermal conductivity and thermoelectric performance of monolayer Mg₃As₂ and Mg₃SbAs

Developing magnesium-based Zintl-phase compounds has been highlighted as a promising approach to promote the widespread application of thermoelectric technology. Using a combination of first-principles calculations and Boltzmann theory, we demonstrate that monolayer Mg₃As₂ achieves a peak zT value of 1.82 at 500 K, marking a significant improvement in moderate-temperature performance within the Mg₃X₂ (X = Sb, Bi, As) family. The reason behind this is attributed to a combination of high electrical conductivity, stemming from the narrow bandgap, and an ultra-low lattice thermal conductivity of 0.4 W m^-1 K^-1 at 300 K, which arises from the short phonon relaxation time, low group velocity, and significant low-frequency Grüneisen parameter. Interestingly, however, we observed that Mg₃As₂ exhibits significant bipolar effects, which inhibit its thermoelectric performance at high temperatures. When Sb is introduced to form Mg₃SbAs, the valley degeneracy increases, the Seebeck coefficient improves, and the bandgap widens, thereby suppressing the bipolar effect. As a result, the optimal thermoelectric performance shifts to a higher temperature range, with a peak zT of 1.77 at 900 K. Additionally, carrier scattering in both materials is dominated by acoustic deformation potential (ADP) scattering, with Mg₃As₂ exhibiting an ADP-scattering-derived mobility of 10⁵ cm² V^-1 s^-1, while Mg₃SbAs exhibiting 10⁴ cm² V^-1 s^-1, and ADP scattering intensifies with temperature, influencing carrier mobility and electrical conductivity. Our results demonstrate that, within the Mg₃X₂ (X = Sb, Bi, As) system, doping can be used to modulate the bandgap and the intensity of bipolar effects, thereby tailoring the material's optimal operating temperature.