Theoretical Investigation of 2D Metal Nitrides <i>M</i> ₃ <i>N</i> ₂ (M = Be, Mg) with Ultralow Lattice Thermal Conductivity for Thermoelectric Applications

In this work, we systematically explore the structural and electronic attributes of planar two-dimensional (2D) M3N2 structures and probe their consequences on thermoelectric applications by solving the Boltzmann transport equation (BTE) within a first-principles formalism. A comprehensive evaluation of their mechanical response confirms the ductile nature of these metal nitrides and demonstrates their mechanical flexibility through computed elastic constants. Both of them feature a direct band gap, and the intrinsic band anisotropy with higher acoustic phonon-limited carrier mobility (103 cm2 V–1 s–1) is expected to substantially enhance electronic transport. Inherent morphology in the electronic band structure is further narrated by an analytical tight-binding model. Remarkably, despite containing intrinsically light elements such as Be, Mg, and N, the thermal conductivity of M3N2 is substantially lower (2–7 W/m·K) than that of MoS2. A detailed analysis of the M3N2 sheet unveils that their suppressed thermal conductivity stems from the unconventional atomic arrangement combined with the pronounced electronegativity disparity between constituting elements, which collectively induce strong phonon anharmonicity and enhance scattering rates. The room-temperature power factors obtained at optimal doping levels result in a peak thermoelectric figure of merit of 0.86 at 300 K, rising to nearly unity at 700 K. The results obtained in this study not only advance the fundamental understanding of heat transport in low-dimensional materials but also provide an instructive foundation for the rational design and optimization of thermal-functional, high-performance thermoelectric materials.