Thermal transport properties of 2D narrow bandgap semiconductor Ca₃N₂, Ba₃P₂, and Ba₃As₂: Machine learning potential study

By combining neuroevolution potential (NEP) with phonon Boltzmann transport theory, we systematically investigate the thermal transport properties of three two-dimensional (2D) narrow bandgap semiconductors: Ca₃N₂, Ba₃P₂, and Ba₃As₂. The room-temperature lattice thermal conductivities (κ_L) of Ca₃N₂, Ba₃P₂, and Ba₃As₂ considering only three-phonon scattering are 6.60 W/mK, 11.90 W/mK, and 8.88 W/mK, respectively. When taking into account the higher-order phonon (four-phonon) scattering processes, the κ_L of these three materials decrease to 6.12 W/mK, 9.73 W/mK and 6.77 W/mK, respectively. Among these systems, Ba₃As₂ undergoes the most pronounced suppression with a reduction of 23.8%. This is mainly due to the greater scattering phase space which enhances the four-phonon scattering. Meanwhile, it is revealed that unlike the traditional evaluation using the P₄/P₃ ratio as an indicator of the strength of four-phonon interactions, the thermal conductivity of Ba₃P₂ exhibits weaker four-phonon suppression behavior compared to Ba₃As₂, despite hosting a higher P₄/P₃ ratio. That is to say, the strength of four-phonon scattering cannot be evaluated solely by the ratio of P₄/P₃. These results presented in this work shed light on the thermal transport properties of such new 2D semiconductors with narrow bandgaps.