Anomalous behavior of lattice thermal conductivity in two-dimensional carbon chalcogenides

Phonon thermal transport properties of two-dimensional carbon monochalcogenides are computed using a first-principles approach and the Boltzmann transport theory. At room temperature (300 K), the three-phonon thermal conductivity (${\ensuremath{\kappa}}_{3\mathrm{ph}})$ of CS (34.98 W/m K) is the highest, followed by CTe (26.40 W/m K) and CSe (10.26 W/m K), which contradicts the typical behavior of lattice thermal conductivity with a mass of the chalcogen. Notably, the inclusion of four-phonon scattering drastically reduces the thermal conductivity $({\ensuremath{\kappa}}_{3+4\mathrm{ph}})$ for CTe by 40%, while CS and CSe remains largely unaffected, yet the anomalous thermal conductivity trend persists. Computed phonon mean free paths for CTe are significantly larger than the CSe and CS, which may restrict the scattering events in the CTe monolayer. Furthermore, the electron localization function implies that the buckling structure restricts the delocalization of electrons and thus a nonbonding lone pair of electrons in the CSe induces strong phonon anharmonicity and reduces lattice thermal conductivity than the CS and CTe monolayers. The present findings provide insights into the origin of the unusual behavior of lattice thermal conductivity in monolayer carbon monochalcogenides.