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. To address this inquiry, a series of M@2D-N8 complexes were examined, with M representing a metal likely to exhibit a range of oxidation states. Because of the layered structure of the parent bulk 2D-N8 structure, several polymorphs were additionally studied to ensure the correct global minimum energy structures were located on their respective potential energy surface. Employing our 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, which are dynamically stable at 1 atm. Orbital interactions and bonding analysis show that the 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 to 4 electrons), these N-rich phases are semiconducting or metallic at ambient conditions. Ab initio molecular dynamics simulations show that the 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 4-electron reduction. These metal 18-crown-6 ring-based polynitrogen compounds, as expected due to their high nitrogen content (8 nitrogen atoms per metal), could potentially serve as new high-energy density materials. Metallic 2D-N8 intercalation compounds, specifically Kx@2D-N8 with x = 0.5 and 0.75, are predicted to be stable at ambient pressure. The stoichiometric number of alkali has the capability to modulate the Fermi level, thereby...