High and Anomalous Thermal Conductivity in Monolayer MSi₂Z₄ Semiconductors

The lattice thermal conductivity (κ) of a newly synthesized two-dimensional (2D) MoSi₂N₄ family and its associated abnormality are anatomized by ab initio phonon Boltzmann transport calculations. κ of MoSi₂N₄ and WSi₂N₄ is found to be over 400 W m^-1 K^-1 at 300 K. κ of MoSi₂Z₄ (Z = N, P, As) obeys Slack's rule of thumb, decreasing by 1 order of magnitude from Z = N to Z = As with 46 W m^-1 K^-1. However, in MSi₂N₄ (M = Mo, Cr, W, Ti, Zr, Hf), the variation of κ with respect to M is anomalous, that is, deviating from Slack's classic rule. For M in the same group, κ of MSi₂N₄ is insensitive to the average atomic mass, Debye temperature, phonon group velocity, and bond strength owing to the similar phonon structure and scattering rates. MSi₂N₄ with heavy group-VIB M even possesses a 3-4 times higher κ than that with light group-IVB M due to its much stronger M-N and exterior Si-N bonds and thus 1 order of magnitude lower phonon scattering rates. Nevertheless, this abnormality could be traced to an interplay of certain basic vibrational properties including the bunching strength and flatness of acoustic branches and their nearby optical branches, which lie outside of the conventional guidelines by Slack. This work predicts high κ of 2D MSi₂Z₄ for thermal management and provides microscopic insights into deciphering the anomalous κ of layered 2D structures.