Four-Phonon-Limited Lattice Thermal Transport in a Two-Dimensional MBene

Accurate prediction of lattice thermal transport in low-dimensional materials requires a rigorous treatment of anharmonic phonon scattering beyond the conventional three-phonon framework. Here, we present a fully ab initio investigation of four-phonon-limited lattice thermal transport in monolayer TiB 2, a representative two-dimensional MBene. Harmonic and higher-order interatomic force constants obtained from density functional theory are combined with an iterative solution of the phonon Boltzmann transport equation to explicitly resolve the competing roles of three-phonon and four-phonon scattering over a broad temperature range. Analysis of the weighted phonon phase space and frequency-resolved scattering rates reveals a rapid activation of four-phonon channels with increasing temperature. While three-phonon processes dominate thermal resistance at low and intermediate temperatures and yield an approximate inverse-temperature scaling, κ ∼ T –1, explicit inclusion of four-phonon scattering reduces the room-temperature lattice thermal conductivity by approximately 27% and induces a systematic breakdown of conventional T –1 behavior at elevated temperatures. Despite this strong renormalization, the pronounced in-plane thermal conductivity anisotropy remains robust, reflecting intrinsic anisotropy in phonon dispersions and transport velocities. The predicted room-temperature lattice thermal conductivity is nearly 2 orders of magnitude lower than representative MXenes, more than an order of magnitude lower than typical transition-metal dichalcogenides, and orders of magnitude lower than graphene, underscoring the exceptionally suppressed heat transport in two-dimensional MBenes. These results establish quartic anharmonicity as essential for quantitatively reliable thermal transport predictions and provide a direct microscopic link between multiphonon phase-space growth, enhanced four-phonon scattering, and temperature-dependent transport scaling in low-dimensional metallic lattices.