A novel all-nitrogen molecular crystal N16 as a promising high-energy-density material

All-nitrogen solids, if successfully synthesized, are ideal high-energy-density materials because they store a great amount of energy and produce only harmless N-2 gas upon decomposition. Currently, the only method to obtain all-nitrogen solids is to apply high pressure to N-2 crystals. However, products such as cg-N tend to decompose upon releasing the pressure. Compared to covalent solids, molecular crystals are more likely to remain stable during decompression because they can relax the strain by increasing the intermolecular distances. The challenge of such a route is to find a molecular crystal that can attain a favorable phase under elevated pressure. In this work, we show, by designing a novel N-16 molecule (tripentazolylamine) and examining its crystal structures under a series of pressures, that the aromatic units and high molecular symmetry are the key factors to achieving an all-nitrogen molecular crystal. Density functional calculations and structural studies reveal that this new all-nitrogen molecular crystal exhibits a particularly slow enthalpy increase with pressure due to the highly efficient crystal packing of its highly symmetric molecules. Vibration mode calculations and molecular dynamics (MD) simulations show that N-16 crystals are metastable at ambient pressure and could remain inactive up to 400 K. The initial reaction steps of the decomposition are calculated by following the pathway of the concerted excision of N-2 from the N-5 group as revealed by the MD simulations.

Automated summary

All-nitrogen solids have great potential as high-energy-density materials, but currently can only be obtained through high-pressure synthesis. Molecular crystals, compared to covalent solids, have higher stability during decompression. This study demonstrates that the design of a novel N-16 molecule and its crystal structure can lead to the formation of an all-nitrogen molecular crystal with favorable properties.