Reduced lattice thermal conductivity and strong four-phonon scattering in h-B12 assembled from boron clusters on a honeycomb lattice†

Two-dimensional (2D) honeycomb materials have emerged as promising candidates for thermal transport applications. However, these materials typically exhibit relatively high lattice thermal conductivity (κ_lat), which limits their application in fields requiring low κ_lat, such as thermoelectric applications. Herein, we propose that cluster assembly could be a viable strategy to effectively reduce κ_lat. Taking h-B₁₂, which is assembled from icosahedral B₁₂ clusters on a honeycomb lattice, as an example, we investigate its thermal transport properties by using first-principles calculations and iteratively solving the linearized Boltzmann transport equation. We reveal that at room temperature, considering both three-phonon (3ph) and four-phonon (4ph) scattering processes, the κ_lat of h-B₁₂ is 31.80 W m⁻¹ K⁻¹ (34.36 W m⁻¹ K⁻¹) along the x(y)-direction, which is lower than that of typical honeycomb materials, such as graphene. Detailed analysis indicates that the intensity of 4ph scattering in h-B₁₂ matches that of 3ph scattering at room temperature. Additionally, the strong 4ph scattering significantly suppresses the contribution of flexural acoustic (ZA) modes in thermal transport due to the mirror symmetry-induced selection rule, which is an important reason for the low κ_lat and the pronounced reduction in κ_lat with the inclusion of 4ph scattering. Our study paves the way for reducing the κ_lat of 2D materials via cluster assembly.