All-nitrogen cages and molecular crystals: Topological rules, stability, and pyrolysis paths

TL;DR

This study investigates the stability and pyrolysis mechanisms of nitrogen cages using ab initio simulations, revealing a topological rule for stability and identifying N4 and N6 as the most stable structures.

Contribution

It introduces a simple topological rule for nitrogen cage stability and provides detailed insights into their thermal stability, electronic properties, and potential as high-energy nanosystems.

Findings

Adjacent hexagons cause instability in nitrogen cages.

Smaller nitrogen clusters are more stable than larger ones.

N4 and N6 cages are the most stable and form insulating crystals.

Abstract

We have combined ab initio molecular dynamics with the intrinsic reaction coordinate in order to investigate the mechanisms of stability and pyrolysis of N$_{4}$-- N$_{120}$ fullerene-like nitrogen cages. The stability of the cages was evaluated in terms of the activation barriers and the activation Gibbs energies of their thermal-induced breaking. We found that binding energies, bond lengths, and quantum-mechanical descriptors failed to predict the stability of the cages. However, we derived a simple topological rule that adjacent hexagons on the cage surface resulted in its instability. For this reason, the number of stable nitrogen cages is significantly restricted in comparison with their carbon counterparts. As a rule, smaller clusters are more stable, whereas earlier proposed rather large cages collapse at room temperature. The most stable all-nitrogen cages are N$_{4}$ and N$_{6}$ clusters, which can form the van-der-Waals crystals with the densities of 1.23 and 1.36 g/cm$^{3}$, respectively. Examination of their band structures and densities of electronic states shows that they are both insulators. Their power and sensitivity are not inferior to the modern advanced high-energy nanosystems.