The effect of optical-acoustic phonon coupling on the thermal conductivity of 2D MgI₂

2D MgI₂ has a large phonon band gap and strong coupling of optical and acoustic phonons, and it is difficult to accurately predict thermal conductivity by considering only three-phonon scattering. Thus, in this study, the effect of four-phonon scattering on the thermal conductivity of a 2D MgI₂ lattice was investigated using first-principles calculations combined with Boltzmann transport theory. The results show that with increasing temperature, four-phonon scattering induces an increase in the scattering of phonons at the optical and acoustic phonon coupling (2 THz), as well as in the vicinity of the optical phonon branch (4.5 THz), which leads to the enhancement of the anharmonicity of phonon transport and results in a decrease in the thermal conductivity of the 2D material. At 700 K, the thermal conductivity of MgI₂ decreases by over half, from 0.47 W m^-1 K^-1 to 0.23 W m^-1 K^-1, when considering both three- and four-phonon scattering, compared to considering only three-phonon scattering. This study confirms the need to consider the role of four-phonon scattering to enhance optical and acoustic phonon coupling to accurately predict the thermal conductivity of 2D materials with larger phonon band gaps.