Transformation pathways in high-pressure solid nitrogen: from molecular N$_2$ to polymeric cg-N

TL;DR

This study uses advanced simulations to elucidate the detailed transformation pathway of high-pressure solid nitrogen from molecular N$_2$ to the polymeric cg-N phase, revealing intermediate states and chain reorganization mechanisms.

Contribution

It provides the first detailed atomistic mechanism of nitrogen polymerization under high pressure, identifying key intermediate phases and chain conformations involved in the transformation.

Findings

Identified a stepwise transformation involving intermediate phases.

Discovered new molecular and chain-like nitrogen phases.

Elucidated the role of trans-cis chain conformations in polymerization.

Abstract

The transformation pathway in high-pressure solid nitrogen from N$_2$ molecular state to polymeric cg-N phase was investigated by means of \textit{ab initio} molecular dynamics and metadynamics simulations. In our study, we observed a transformation mechanism starting from molecular $Immm$ phase that initiated with formation of $trans$-$cis$ chains. These chains further connected within layers and formed a chain-planar state, which we describe as a mixture of two crystalline structures - $trans$-$cis$ chain phase and $planar$ phase, both with $Pnma$ symmetry. This mixed state appeared in molecular dynamics performed at 120 GPa and 1500 K and in the metadynamics run at 110 GPa and 1500 K, where the chains continued to reorganize further and eventually formed cg-N. During separate simulations, we also found two new phases - molecular $P2_1/c$ and two-three-coordinated chain-like $Cm$. The transformation mechanism heading towards cg-N can be characterized as a progressive polymerization process passing through several intermediate states of variously connected $trans$-$cis$ chains. In the final stage of the transformation chains in the layered form rearrange collectively and develop new intraplanar as well as interplanar bonds leading to the geometry of cg-N. Chains with alternating $trans$ and $cis$ conformation were found to be the key entity - structural pattern governing the dynamics of the simulated molecular-polymeric transformation in compressed nitrogen.