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.