Structural evolution of amorphous polymeric nitrogen from \textit{ab initio} molecular dynamics simulations and evolutionary search

TL;DR

This study uses ab initio simulations and evolutionary algorithms to explore the structural evolution, properties, and potential energy storage capacity of amorphous polymeric nitrogen formed under high pressure and decompressed to ambient conditions.

Contribution

It provides a detailed analysis of the structural changes and properties of amorphous nitrogen during decompression, combining multiple simulation methods to enhance understanding.

Findings

Amorphous nitrogen forms similar structures regardless of preparation method.

Decompression leads to formation of longer chains and N2 molecules.

Amorphous nitrogen can store significant energy at ambient pressure.

Abstract

Polymeric nitrogen with single bonds can be created from the molecular form at high pressure and due to large energy difference between triple and single bonds it is interesting as energetic material. Its structure and properties are, however, still not well understood. We studied amorphous nitrogen by \textit{ab initio} simulations, employing molecular dynamics and evolutionary algorithms. Amorphous nitrogen was prepared at pressure of 120 GPa by quenching from hot liquid, by pressure-induced amorphization of molecular crystal and by evolutionary search. All three amorphous forms were found to be structurally similar. We studied in detail the structural evolution of the system upon decompression from 120 GPa to zero pressure at 100 K. At pressures above 100 GPa, system consists mainly of 3-coordinated atoms (80 \%) connected by single bonds while some short chains made of 2-coordinated atoms are also present. Upon decompression, the number of 3-coordinated atoms rapidly decreases below 60 GPa and longer chains are created. At 20 GPa the system starts to create also N$_2$ molecules and the ultimate structure at $p=0$ contains molecules inside a polymeric network consisting dominantly of longer chains made of 2-coordinated atoms. Besides structure, we also study vibrational and electronic properties of the system and estimate the amount of energy that could be stored in amorphous nitrogen at ambient pressure.