Two-dimensional (2D) ${\mathrm{MoSi}}_2{\mathrm{N}}_4$ and its derivative materials have recently garnered significant attention due to their rich physical properties, including remarkable ambient stability, high carrier mobility, and exceptional thermal conductivity. In this study, we constructed a class of 2D sextuple-layer materials $ABZ$ ($A/B$ = C, Si, and Ge; $Z$ = N, P, and As) based on the structure of ${\mathrm{MoSi}}_2{\mathrm{N}}_4$ and identified four stable structures through first-principles density functional theory (DFT) calculations. Due to their strong covalent bonds, CSiN and ${\mathrm{C}}_2\mathrm{N}$ monolayers are wide-band-gap semiconductors with band gaps of 3.42 and 5.32 eV, respectively, as determined by the hybrid DFT calculations. Notably, both materials exhibit ultrahigh lattice thermal conductivities, reaching 318 and 694 ${\mathrm{W}\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ at room temperatures, respectively. Our analysis attributes the ultrahigh lattice thermal conductivity to their large phonon group velocities and long phonon lifetimes arising from their strong covalent bonds. Additionally, we propose that CSiN and ${\mathrm{C}}_2\mathrm{N}$ could transition from their polar $C_{3\mathrm{v}}$ phase to a metastable centrosymmetric $D_{3\mathrm{d}}$ phase, with transition barriers of 0.89 and 0.70 eV per formula unit, respectively. In contrast, the ${\mathrm{C}}_2\mathrm{P}$ and ${\mathrm{Si}}_2\mathrm{P}$ monolayers exhibit smaller band gaps of 2.77 and 1.39 eV, respectively. This work highlights the potential of ultrahigh thermal conductivity semiconductors and provides valuable insights into designing advanced 2D materials for future applications.

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• Band gap
• Lattice thermal conductivity
• Phonons
• 2-dimensional systems
• Wide band gap systems
• First-principles calculations