Strain-driven transition of phonon scattering mechanisms in BiOCl monolayer: From three-phonon to four-phonon scattering

The layered BiOCl, a star material for photocatalysis, possesses a large bandgap, low-symmetry lattice, and strong phonon anharmonicity induced by lone-pair electrons, which offers a suitable platform for investigating the relationship between the high-order anharmonicity and structural symmetry. In this work, the thermal conductivity (κl) of the BiOCl monolayer was studied using first-principles calculations and the linearized Boltzmann transport equation, in which four-phonon scattering processes are explicitly considered. Our results indicate that the predicted κl is reduced from 7.1 to 5.2 W/(mK) after including four-phonon scattering (a 27% reduction). When introducing a tensile strain of 3%, the room temperature κl can be further reduced by 77% as compared to that of the strain-free case. Such a huge reduction primarily arises from the flattening transverse acoustic branch and the densified low-frequency phonons in the strained BiOCl monolayer, which enables qualitative changes in the dominant mechanism of phonon scattering: from three-phonon scattering to four-phonon scattering. These findings provide deeper insights into the thermal transport behavior of the BiOCl monolayer and underscore the significant potential of strain engineering as a power tool to tailor high-order anharmonic effects for symmetry-breaking 2D materials.