Optical-mode-dominated thermal conductivity in monolayer Ti2SiCO2

Understanding phonon transport in two-dimensional materials with complex lattice dynamics is essential for their integration into electronic and optoelectronic devices. Using the Boltzmann transport equation combined with second-, third-, and fourth-order interatomic force constants obtained from first-principles calculations, we systematically investigate the lattice thermal conductivity of monolayer Ti2SiCO2. At room temperature, the thermal conductivity with four-phonon scattering is 45.63 W m−1 K−1, about 20% lower than the three-phonon scattering prediction. Remarkably, unlike most two-dimensional materials in which heat transport is dominated by acoustic phonons, optical phonons are found to provide the dominant contribution to the total thermal conductivity in Ti2SiCO2. This unconventional behavior originates from low-frequency optical modes that strongly hybridize with longitudinal acoustic phonons, leading to pronounced acoustic-mode softening and suppressed group velocities. Frequency-resolved analysis shows that four-phonon scattering primarily suppresses contributions from very low-frequency acoustic phonons, while having a negligible impact on higher-frequency acoustic modes and optical phonons. Mode-resolved scattering-rate analysis further reveals that, as the acoustic phonon frequency approaches zero, three-phonon scattering rates rapidly diminish due to limited phase space, whereas four-phonon scattering rates remain nearly constant and become dominant. These results highlight the critical role of four-phonon processes and low-frequency optical modes in governing thermal transport in Ti2SiCO2, providing new insights into phonon engineering in two-dimensional materials with complex vibrational spectra.