NaNO3 monolayer: A stable graphenelike supersalt with strong four-phonon scattering and low lattice thermal conductivity insensitive to temperature

Abstract

Since the discovery of graphene, numerous efforts have been made to seek new two-dimensional (2D) graphenelike materials with intriguing properties. Here,we report a stable graphenelike supersalt ${\mathrm{NaNO}}_{3}$ monolayer composed of superhalogen ${\mathrm{NO}}_{3}$. The 2D ${\mathrm{NaNO}}_{3}$ monolayer is found to possess an ultralow isotropic Young's modulus (2.983 N/m) and a low thermal conductivity (1.65 W/mK @300 K), which is much lower than that of 2D NaCl monolayer (3.72 W/mK @300 K). We attribute this behavior to the strong anharmonicity induced by the weak bonding between Na and ${\mathrm{NO}}_{3}$ cluster and the large atomic displacements of O in the ${\mathrm{NO}}_{3}$ cluster. A giant four-phonon scattering leads to a 60% reduction in lattice thermal conductivity at 300 K compared to that of three-phonon scattering. Furthermore, temperature-induced phonon hardening causes the lattice thermal conductivity to be nearly temperature independent in the studied temperature range (100--350 K), displaying significant difference from the atom-based 2D materials.