Transformation pathways in high-pressure solid nitrogen: From molecular N2 to polymeric cg-N

Abstract

The transformation pathway in high-pressure solid nitrogen from N 2 molecular state to polymeric cg-N phase was investigated by means of 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 P nma 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 P 2 1 /c and two-threecoordinated 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 transcis 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.

Solid Nitrogen and Nitrogen‐Rich Compounds as High‐Energy‐Density Materials

“…[ 48,49 ] The limit of oligomers, the 1D infinite chain, was predicted to form as well. [ 14,15,50 ] In chainlike structure, nitrogen atoms are connected by alternating single and double bonds, and therefore its region of stability is predicted to be after molecular N 2 but before that of cg nitrogen. [ 50 ]…”

“…In theory, the chain structures have been considered as an intermediate phase between the molecular N 2 and extended polymeric phase. [ 50 ] In 1992, Mailhiot et al made initial prediction of chainlike structure (ch phase) made of alternating trans and cis dihedral angles. [ 11 ] However, this structure was later found both mechanically and dynamically unstable up to the investigated 360 GPa.…”

“…[48,49] The limit of oligomers, the 1D infinite chain, was predicted to form as well. [14,15,50] In chainlike structure, nitrogen atoms are connected by alternating single and double bonds, and therefore its region of stability is predicted to be after molecular N 2 but before that of cg nitrogen. [50] Recent prediction of allotropes of nitrogen were made primarily using crystal structure prediction (CSP) methods based on first-principles calculations.…”

Structural evolution of amorphous polymeric nitrogen from ab initio molecular dynamics simulations and evolutionary search

“…The mechanism of creation of cg-N from the molecular phase was theoretically studied by ab initio metadynamics in Ref. [8]. Several recent * dominika.melicherova@fmph.uniba.sk works investigated the possibility of creating and stabilizing single N-N bonds in systems other than pure nitrogen [9][10][11][12][13][14].…”

Pressure-induced stability and polymeric nitrogen in alkaline earth metal N-rich nitrides (XN6, X = Ca, Sr and Ba): a first-principles study

“…3 Generally, an ideal HEDM should have the following characteristics: (i) high density, (ii) positive heat of formation, (iii) high detonation performance, (iv) low sensitivity towards external stimuli, (v) good thermal stability. In previous studies, for instance, scientists successfully synthesized cubic gauche nitrogen (cg-N) 6 containing only single bonds under the condition of (110 GPa, 2000 K), 7,8 it can be used as a potential high energy-density material. Therefore, N-rich compounds containing a large amount of nitrogen and nitrogen single bond and double bonds may become potential high energy-density materials.…”