Predicted High-Energy Density MN₈ Containing Anionic 18-Crown-6 Ring-Based Polynitrogen Monolayers Acting as Cryptand

In this study, we investigate the potential of the 18-crown-6-like two-dimensional (2D)-N₈ structure to accommodate electrons from metals without compromising its covalent nitrogen network. Employing the crystal structure prediction enhanced by evolutionary algorithm and density functional theory methodology, we successfully predicted the existence of 16 layered M@2D-N₈ complexes from a total of 39 MN₈ systems investigated at 100 GPa (M = s-block Na-Cs, Be-Ba and d-block Ag, Au, Cd, Hg, Hf, W, and Y). Among those, there are 13 quenchable M@2D-N₈ compounds that are dynamically stable at 1 atm. Orbital interactions and bonding analysis show that 2D-N₈ presents a flat localized π* band that can accommodate one or two electrons without breaking the 2D covalent nitrogen network. Depending on the metal-to-polynitrogen charge transfer (formally, 1-4 electrons), these N-rich phases are semiconducting or metallic under ambient conditions. Ab initio molecular dynamics simulations show that K(I)@2D-N₈ and Ca(II)@2D-N₈ are thermally stable up to 600 K, while the Hf(IV)@2D-N₈ compound is thermally not viable at 400 K because of the weakening of the N═N bonds due to a strong four-electron reduction. These metal 18-crown-6 ring-based polynitrogen compounds, as expected due to their high nitrogen content (eight nitrogen atoms per metal), could potentially serve as new high-energy density materials.