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

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 an energetic material. Its structure and properties are, however, still not well understood. We studied amorphous nitrogen by ab initio simulations, employing molecular dynamics and evolutionary algorithms. Amorphous nitrogen was prepared at a pressure of 120 GPa by quenching from a hot liquid, by pressure-induced amorphization of a 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, the 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 N2 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. arXiv:1804.09072v2 [cond-mat.mtrl-sci]

Multistep Dissociation of Fluorine Molecules under Extreme Compression

“…Apart from the exceedingly wide range of stable pressures, the structures of these phases show a striking feature, namely the coexistence of atomic, molecular, and polymeric F atoms in the same structure. Although similar abnormal structural features were reported in amorphous N 2 [32] and SO 2 [33] recently, it remains a surprise that all the three different stages of structural evolution of the light p-block elements can coexist in thermodynamically stable crystalline phases. Among four stable structures, Cmca is a pure molecular phase and Fddd is a true atomic phase.…”

Pressure-induced amorphization and existence of molecular and polymeric amorphous forms in dense SO 2

“…It will be of interest to study whether the two structurally distinct forms continue to exist also in liquid state, either in stable or metastable (supercooled) regions. For example, a transition between molecular and polymeric liquid has been proposed in nitrogen (40), and a cross-over between such forms was observed in recent computational study of amorphous nitrogen (41). Further experimental and theoretical work is necessary to accurately map the solid and liquid regions and uncover further details of the phase diagram of SO 2 .…”

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

“…A recent systematic study using ab initio molecular dynamics simulations and evolutionary search suggests that amorphous polymeric nitrogen may be prepared by quenching from melts and/or pressure‐induced amorphization of molecular phases. [ 69 ] The number of single bonds in amorphous nitrogen is predicted to be associated with external pressure. At 100 GPa, certain amorphous phases are calculated to have 80% of all nitrogen atoms in 3‐coordinaiton, or being connected by purely single bonds.…”

“…At 100 GPa, certain amorphous phases are calculated to have 80% of all nitrogen atoms in 3‐coordinaiton, or being connected by purely single bonds. [ 69 ] The formation of cg nitrogen and other dense crystalline phases, on the contrary, requires significant kinetic energy of the nitrogen atoms equivalent to temperature of a few thousand Kelvin. Such extreme conditions were latter achieved by using laser‐heated DAC.…”