The origin of anomalous mass-dependence of thermal conductivity in Janus XBAlY (X = Se, S, Te; Y = S, Se, O; X ≠ Y) monolayers

Owing to the unique asymmetric geometry, Janus monolayer compounds exhibit various exotic thermal properties and have promising applications in thermal management. In this study, we combine machine learning potentials and the phonon Boltzmann transport equation to perform a comparative study of the thermal transport properties in Janus XBAlY (X = Se, S, Te; Y = S, Se, O; X ≠ Y) monolayers. Our findings unveil a thermal conductivity (κp) ranking as SeBAlS > TeBAlO > SBAlSe, contradicting the conventional expectation that a higher κp is typically observed when the average atomic mass is smaller. At room temperature, the κp of SeBAlS is 174 Wm−1 K−1, which is 4.8 times that of SBAlSe when considering three-phonon scattering processes. Moreover, the consideration of four-phonon scatterings does not alter such ranking. The anomalous κp phenomenon was explained through a detailed analysis of the phonon–phonon scattering mechanism, phonon bandgap, phonon anharmonicity, and chemical bond strength. This study highlights the intricate relationship between atomic mass, bonding characteristics, and thermal properties, offering insights for designing Janus materials with tailored thermal conductivity.