Thermal Properties of 2D Dirac Materials MN₄ (M = Be and Mg): A First-Principles Study

Recently, a novel two-dimensional (2D) Dirac material BeN₄ monolayer has been fabricated experimentally through high-pressure synthesis. In this work, we investigate the thermal properties of a new class of 2D materials with a chemical formula of MN₄ (M = Be and Mg) using first-principles calculations. First, the cohesive energy and phonon dispersion curve confirm the dynamical stability of BeN₄ and MgN₄ monolayers. Besides, BeN₄ and MgN₄ monolayers have the anisotropic lattice thermal conductivities of 842.75 (615.97) W m^-1 K^-1 and 52.66 (21.76) W m^-1 K^-1 along the armchair (zigzag) direction, respectively. The main contribution of the lattice thermal conductivities of BeN₄ and MgN₄ monolayers are from the low frequency phonon branches. Moreover, the average phonon heat capacity, phonon group velocity, and phonon lifetime of BeN₄ monolayer are 3.54 × 10⁵ J K^-1 m^-3, 3.61 km s^-1, and 13.64 ps, which are larger than those of MgN₄ monolayer (3.42 × 10⁵ J K^-1 m^-3, 3.27 km s^-1, and 1.70 ps), indicating the larger lattice thermal conductivities of BeN₄ monolayer. Furthermore, the mode weighted accumulative Grüneisen parameters (MWGPs) of BeN₄ and MgN₄ monolayers are 2.84 and 5.62, which proves that MgN₄ monolayer has stronger phonon scattering. This investigation will enhance an understanding of thermal properties of MN₄ monolayers and drive the applications of MN₄ monolayers in nanoelectronic devices.