Ab Initio Molecular Dynamics Study On Ammonium Azideand Ammonium Nitrate Under High Pressure

Energetic materials energetic compound are also called high energy density materials（HEDM）, which have been widely used in civilian blasting, aerospace and defense and military engineering, etc. Common energetic materials include TNT（C7H5N3O6）, HMX（C4H8N8O8） and ammonium perchlorate（ NH4 Cl O4）. However, the above energetic materials have a more serious pollution for the environment in the process of production or application, so the study of environmentally friendly energetic materials is important and urgent. With the development of technology and industry, the traditional energetic materials are increasingly unable to meet the needs of some areas, so people pay more attention to ultra-highenergetic materials, such as nitrogen substances, highly active energy storage materials and metal hydrogen, etc.About three decades ago, Mc Mahan and Le Sar first predicted that solid nitrogen molecule（N2） dissociated into a simple cubic structure of monatomic nitrogen under 100 GPa by first-principles calculation, which was the beginning of research about nitrogen substances. Until 2004, cubic gauche phase（cg-N） was synthesized firstly in experiment from molecular nitrogen at temperatures above 2000 K and pressures above 110 GPa, but it is regrettable that cg-N is only stable to 42 GPa, the experiment has failed to get the normal temperature and pressure stable polymeric nitrogen. The azide root（N3-） in ammonium azide（NH4N3） is believed to effectively reduce nitrogen polymerization synthesis conditions. Meanwhile the presence of H elements can passivate the formed polymerization. Choosing ammonium azide as a precursor, the obtained polymerization nitrogen is very likely to be stable at ambient conditions.Through first-principle MD simulations at room temperature and high pressures, we studied detailedly NH4N3 and found three new structures: P2/c、P21/c and P21. By calculating the zero-temperature enthalpy curves, we found that Pnma structure translate into P2/c structure at about 1.5 GPa, and had been stable until 77 GPa. The experiments in the pressure range of 70 GPa and at room temperature have not been able to obtain polymerization nitrogen from NH4N3. So, we speculate that high temperature is an important condition to synthesize polymeric nitrogen compounds from NH4N3 and perform the MD simulation under high temperature and high pressure.In the instantaneous configuration at 60 GPa and 2200 K, we observe a polymeric chain which is up to 15 nitrogen atoms. Compaired with the pressure needed for polymerization reaction in molecular solids nitrogen（above 100 GPa）, the polymerization pressure of nitrogen atoms in the ammonium azide reduce significantly. In the snapshot of the MD simulation at 90 GPa and 1600 K, we find three kinds of periodic chains and a large number of five-membered ring structures（N5）. In the subsequent annealing simulation, we find a more complex periodic chain. When the pressure on the annealed simulation cell is gradually released to atmospheric pressure, the periodic chains are broken, but because of the passivation of H, the hydrogen-passivated polymeric networks, which is up to 12 N atoms, can be preserved at ambient condition successfully. At the same time, N5 structures show high stability throughout the simulation process, so we speculate that the N5 structure could become practical energetic materials. Besides, by analyzing the δ function, which reflects the jumping of H atoms, It can be seen that the existence of H elements, not only can improve the stability of the polymeric nitrogen, but also play an important role in the formation of polymeric nitrogen. Our finding about ammonium azide might help to open a way for the practical application of polymeric nitrogen compounds.Ammonium nitrate（NH₄NO₃） has the properties of higher nitrogen content and stable oxidative, but it is again aroused general concern because the product will not cause pollution. Currently, the large number of experimental and theoretical workers studies the high-voltage behavior of ammonium nitrate, but the phase transition of the crystal under high pressure remains controversial. In 2011, by XRD and Raman scattering, Davidson et al., found that under the non-hydrostatic conditions and the pressure up to 20 GPa, NH₄NO₃ occurs structural phase transition（phase IV �' IV ’）, but under hydrostatic conditions, even if the pressure increases to 35 GPa, there is no phase transition. We adopt first-principles molecular dynamics（MD） simulation to make a detailed study of ammonium nitrate in the crystal structure and phase change mechanism under high pressure.This article identifies the metastable phase IV’ of ammonium nitrateproposed by the experiments, determines the crystal structure, discoversa new phase（phase X） at higher pressures and clearly displays thetanglesome rotation of ammonium cation. Combining the changes of the“H atom cloud” with pressure, we give the mechanism of phase IV-IV’transition under high pressure. Through the NVT ensemble simulations,we reveal that lattice strains play an important role in formation andstabilization of phase IV’. Non-hydrostatic pressure facilitates thegeneration of lattice distortion, so we can explain the proposedconclusion in experiment that phase IV-IV’ transition is induced byshear stress under nonhydrostatic pressure. Finally, the phase diagram of AN is determined at pressure range of 5–60 GPa and temperature range of 250–400 K.