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C: Energy Conversion and Storage; Energy and Charge Transport
Pressure Stabilized High-Energy-Density Alkaline-Earth Metal Pentazolate Salts Kang Xia, Xianxu Zheng, Jianan Yuan, Cong Liu, Hao Gao, Qiang Wu, and Jian Sun J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b12527 • Publication Date (Web): 01 Apr 2019 Downloaded from http://pubs.acs.org on April 1, 2019
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The Journal of Physical Chemistry
Pressure Stabilized High-Energy-Density Alkaline-Earth Metal Pentazolate Salts Kang Xia,† Xianxu Zheng,‡ Jianan Yuan,† Cong Liu,† Hao Gao,† Qiang Wu,‡ and Jian Sun∗,† National Laboratory of Solid State Microstructures, School of Physics and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China, and National key laboratory for shock wave and detonation physics, Institute of fluid physics, China academy of engineering physics, Mianyang 621900, Sichuan, China E-mail:
[email protected] Abstract Polynitrogen compounds especially pentazolate anion complexes recently have attracted substantial attention due to their promising potential as high-energy-density materials. Here, using a machine-learning accelerated crystal structure search method and first-principles calculations, we predict a new hybrid compound by inserting a large fraction of nitrogen into alkaline-earth metals. It is a new stoichiometric type MN10 (M = Be, Mg), which possesses a metal-centering octahedral pentazolate framework with the space group F dd2. This type of ionic-like molecular crystal is found to be energetically more favorable than the mixtures of M3 N2 or MN4 compounds and pure nitrogen, and is possible to be synthesized at relatively low pressures (around 12 GPa for MgN10 ). ∗
To whom correspondence should be addressed National Laboratory of Solid State Microstructures, School of Physics and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China ‡ National key laboratory for shock wave and detonation physics, Institute of fluid physics, China academy of engineering physics, Mianyang 621900, Sichuan, China †
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The ab initio molecular dynamics simulations show that they are metastable and can be quenched to ambient conditions once synthesized at high pressure. Moreover, decomposition of this polymeric MN10 structure can release a large amount of energy and has a high performance in detonation. The detonation velocity and pressure of BeN10 is about twice and four times as that of TNT, respectively.
Introduction Pure nitrogen has been considered as a high-energy-density material (HEDM) because of its large energy difference between triple-bond dinitrogen (N≡N) and singly or doubly bonded nitrogen. The strongest N≡N bond containing average energy of ∼954 kJ/mol, 1 makes molecular N2 to be one of the most inert materials under ambient condition. While much weaker single (N–N) and double (N=N) bonds with average energies of ∼160 and 418 kJ/mol respectively, have been found to exist in metastable polymeric nitrogen frameworks. 2,3 In the unique transformation to N≡N bonds, the singly or doubly bonded polynitrogens should release an enormous amount of energy. Pursuing these potential high-energy-density materials can help to single out high-performing propellants or explosives, which are environmentally friendly and even more powerful than cyclotetramethylene tetranitramine (HMX). 4,5 Calculations have predicted plentiful high-energy-storage polynitrogen networks formed with single or double bonds, such as N4 , N5 , N6 , N8 , N20 , or even N60 clusters . 6–14 However, it is still a real challenge to synthesize stable nitrogen-rich HEDMs for the easy break of weakly single or double nitrogen bonds. 15–17 The strong C–N bonds in the conjugated structures can easily destroy the relatively weak N–N or N=N bonds. 18 Nevertheless, a white N5 − salt with a cubic crystalline structure, (N5 )6 (H3 O)3 (NH4 )4 Cl, has been recently synthesized, and is announced to be stable up to 117 ◦ C. 19 Structural characteristics estimate that chloride, ammonium and, the interactions of hydrogen bonds mainly stabilize pentazolate anion (N5 − ) which are further systematically discussed in a series of high-energy metal hydrates. 20 Yet, it seems not proper to treat these complexes as high explosives for their low nitrogen contents. 2
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High pressure has been found to be another effective way of stretching dinitrogen’s triple bonds into single or double bonds . 2,21–23 Especially, the storage of purely single bonds in the high-pressure cubic gauche polynitrogen frame (cg-N), has been theoretically predicted to be a potential powerful explosive. 2,21 Many efforts have been made to obtain this kind of high energy material since then. Armophous non-molecular products of small clusters connected with N–N single bonds, have been experimentally obtained when recovered to ambient pressure and temperatures 110 GPa and temperatures >2000 K at last. 4 Converting into dinitrogen, the cg-N can release five times more energy than HMX explosive. Hence, more polynitrogen structures were predicted under extremely high pressures (>200 GPa), such as layered P ba2, 24 cage-like diamondoid N10 , 25 and metallic P 4/nbm nitrogen. 26 These HEDM nitrigen polymeric structures exhibit may extreme mechanic property, such as superhardness, which was also predicted in boride materials. 27 Nevertheless, such high-pressure stabilization of polymeric nitrogen predicts expense of trial-and-error synthesis, and is impractical for the real word. The relatively low-pressure (
