A Novel Polymerization of Nitrogen in Beryllium Tetranitride at High

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A Novel Polymerization of Nitrogen in Beryllium Tetranitride at High Pressure Shuli Wei, Da Li, Zhao Liu, Wenjie Wang, Fubo Tian, Kuo Bao, Defang Duan, Bingbing Liu, and Tian Cui J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 26 Apr 2017 Downloaded from http://pubs.acs.org on April 28, 2017

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The Journal of Physical Chemistry

A Novel Polymerization of Nitrogen in Beryllium Tetranitride at High Pressure Shuli Wei, Da Li, Zhao Liu, Wenjie Wang, Fubo Tian, Kuo Bao, Defang Duan, Bingbing Liu and Tian Cui* State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China

ABSTRACT: The stable polymeric nitrogen and polynitrogen compounds have potential applications in high-energy-density materials. For beryllium nitrides, there is one known crystalline forms, Be3N2, at ambient conditions. In the present study, the structural evolutionary behaviors of beryllium polynitrdes have been studied up to 100 GPa using first-principles calculations and unbiased structure searching method combined with density functional calculations. One stable structural stoichiometry of beyrllium polynitride has been theoretically predicted at high pressures. It may be experimentally synthesizable at high pressures less than 40 GPa. It is therefore possible to synthesize BeN4 by compressing solid Be3N2 and N2 gas under high pressure and BeN4 may be quenching recoverable to ambient conditions. The predicted high-pressure P21/c-BeN4 compound contains a novel variety of polynitrogen, extended polymeric 3D puckered N10 rings network. To the best of our knowledge, this is the ﬁrst time that stable N10 rings network are predicted in alkaline-earth metal polynitrides. The decomposition of P21/c-BeN4 is expected to be highly exothermic, releasing an energy of approximately 6.35 kJ•g-1. The present results open a new avenue to synthesize polynitrogen compound and provide a key perspective toward the understanding of novel chemical bonding in nitrogen-rich compounds. Results of the present study suggest that it is possible to obtain energetic polynitrogens in main-group nitrides under high pressure.

INTRODUCTION The scientific research for new and efficient energy source has attracted great attention. In nitrogen gas, nitrogen exists in N2 molecules and the connection between nitrogen atoms is triple bonds N≡N. It has significant interest if one can transform the abundant N2 molecules into polynitrogen with single N−N and double N=N bonds. The large energy difference between the single N−N and double N=N or triple N≡N bonds can be used in industry.1-8 Solid nitrogens and polynitrogens contain single N−N and double N=N bonds and can be utilized as highenergy-density materials (HEDMs). The large energy difference between the single (∼167 KJ/mol) and double (∼419 KJ/mol) bonds in polynitrogens and the N≡N bonds (∼954 KJ/mol) in N2 molecules make polynitrogens release an enormously large amount of energy. For example, the estimated energy capacity of single-bonded polynitrogen is up to 4.6 eV/mol, three times more than that of the most powerful HEDMs known today.9 Besides, a single-bond phase of solid nitrogen with the theoretically predicted cubic gauche (cg-N) structure4 has been experimentally synthesized by Eremets et al. at high pressure and high temperature (110 GPa, 2000 K).10,11 The cg-N is predicted to have a more than three times higher energy storage capacity than the most powerful energetic materials.9,11,12 The realization of polynitrogens has been actively experimented in nitrides, due to the fact that the charged nitrogen species in such materials often show improved kinetic stability over pure polynitrogens. Metal azides have drawn considerable attention for their interesting chemical and physical properties. Recently, in order to synthesis a kind of potential high-

energy-density material, and metal azides can be used as starting materials because of the potential lower synthesis pressure compared with pure nitrogen gas. Over the years, metal azides have been proposed in many experimental and theoretical works about LiN3 13-19, LiN5 20, NaN3 21-23, KN3 19,21,24-28, and CsN3 29,30, Ca-N system31. Besides, N2H,32,33 C-N system,34-36 S-N system37, P-N system38 and Xe-N system39 are also studied as high-energy materials. These successes led to an increasing interest in polynitrogen compounds and to the search of other stable polynitrongen forms in recent years. Moving forward, in the present study we investigated the closed-related Be-N system. Alkaline-earth metal beryllium is adjacent to alkali metal lithium in periodic elements table, besides, beryllium has one more valence electron than alkali metal element, and it can bring more chemical activity. Therefore, a study of beryllium polynitrides would help to theoretically investigate more promising high-energy material. Recently, Zhu31 explored theoretically the phase diagram of the Ca-N system at pressures ranging from 0-100 GPa, and discovered a series of new compounds in this family. Thus, one expects that mixing reactive beryllium with nitrogen will result in more diverse structures and higher energy densities of the products. Besides, the light weight of beryllium relative to Cu, Ag, and Pb, can help Be-N compounds have potential advantage in the storage of HEDMs. The experimental Be3N2 is the only energetically stable phase at ambient conditions.40 In the present work, the possible ambient-pressure BeN4 for the Be-N system in formation and stability have been well investigated. Because of the electron donors (Be), BeN4 is able to manipulate the bond strength of the polynitrogen anions

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such that the single N−N bonds can be stabilized at much lower pressures than that required for pure nitrogen. Significantly, the fascinating structure P21/c-BeN4 is predicted to be synthesized within the current capability of high-pressure synthesis and recovered at ambient conditions. It is predicted to become thermodynamically stable at pressures above 40 GPa. This study revealed the possibility of the formation of one new polynitrogen form - extended polymeric 3D puckered N10 rings network. Electrons acquired by polynitrogen anions in the P21/c-BeN4 crystals reduce the electrons sharing between nitrogen atoms which ultimately change the bonding order. The decomposition of P21/c-BeN4 is expected to be highly exothermic, releasing an energy of approximately 6.35 kJ·g-1 at 40 GPa, much lower than the pressure required for stabilizing polymeric cg-N (94 GPa to 127 GPa). The energy density of P21/c-BeN4 is mildly higher than those of the standard energetic materials TNT (4.3 kJ·g-1) and HMX (5.7 kJ·g-1).41 Thus, one expects that mixing reactive Be with nitrogen will result in more diverse structures and higher energy densities of the products. The structural stability, lattice dynamics, electronic structure, and bonding nature are studied, providing more insights into the mechanism of pressure-induced physical properties. The bonding nature in these polynitrogens is of great important to nitrogen chemistry and to the understanding of metal-nitrogen interactions. COMPUTATIONAL METHOD We performed a crystal structure prediction for the Be-N system via global minimization of free energy surfaces as implemented in the CALYPSO code,42,43 which has been validated with various known compounds, from elemental to binary and ternary compounds.44,45 The individual structure searching for BeNx (x =1/2, 2/3, 1, 3/2, 2, 3, 4 and 5) within 1-4 formula units (f.u.) in the simulation cell are implemented at 0, 20, 50, and 100 GPa. Total energy calculations, underlying structural optimizations, and electronic structure calculations were performed by density functional theory (DFT) with the VASP (Vienna ab initio simulation package) code.46 The PerdewBurke-Ernzerhof (PBE) exchange correlation functional within generalized gradient approximation (GGA) was adopted.47 The projector-augmented wave (PAW)48 pseudopotentials are taken from the VASP potential library. The 2s2 for Be and 2s22p3 for N were treated as valence states, respectively. An energy cut-off of 600 eV for the plane-wave basis sets and Monkhorst-Pack49 k-meshes (k-points grid 0.03 Å-1) were used to guarantee that total energies were well converged with energy differences within 1 meV/f.u. The convergence tests have been described elsewhere. The calculations of net charge are based on Bader analysis.50,51 The phonon frequencies for the structure was calculated using the supercell method with the PHONOPY code.52,53 RESULTS AND DISCUSSIONS

Figure 1. Relative enthalpies of formation of Be-N phases with respect to elemental beryllium and nitrogen solids. The convex hulls connecting stable phases (solid shapes) are shown by solid lines. Unstable/metastable phases are shown by open shapes. Inset: The stable pressure ranges of BeN.

their corresponding stable pressure ranges and were used as reference structures. Stability of Be-N compounds can be explored by the thermodynamic convex hull construction, as shown in Figure 1. Thermodynamic stability of the Be-N crystals obtained in the structure searches were examined using their enthalpies of formation with respect to the crystals of N2 and Be. The energetically stable phases at selected pressures: 0, 20, 40, and 100 GPa are shown by solid shapes connected by solid lines. The metastable phases are shown by open shapes and they are unstable if the kinetic barrier is sufficiently high decomposing into other binary stoichiometries and/or elements. Apparently, high metastability is a crucial asset for the applications of HEDMs. On decomposition, a metastable structure above the convex hull reduces to the products on the hull, and releases an amount of energy corresponding to their energy difference. At ambient pressure, the only stable stoichiometry is the experimentally known Be3N2.40 Among all predicted BeNx crystals, experimental Be3N2 is the most thermodynamically stable phase throughout the entire pressure range. Besides, our results show that the P21/c-BeN4 crystal is predicted to become energetically stable near 40 GPa, suggesting that it may be prepared by high-pressure synthesis.

3.1 Phase diagram and stability The calculated formation enthalpies ∆Hf for the energetically most favorable Be1-xNx structures in the pressure range from 0 to 100 GPa is shown in Figure 1. The ∆Hf of each Be-N structure was calculated by ∆Hf (Be1-xNx) = H(Be1-xNx) − (1−x)H(Be) − xH(N) (0 < x < 1) at T = 0 K. The known solid beryllium (hcp and bcc phases)54 and solid nitrogen (α-, Pbcn-, P2/c-, and cg-phases)55 were energetically most favorable in

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Figure 2. Phonon dispersion curves for the P21/c structure of BeN4 at ambient pressure.


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40 GPa is well within the current capability of high-pressure techniques. Moreover, this structure is mechanically stable at ambient conditions which may make an ambient-pressure recovery possible. The ambient-pressure decomposition of P21/c-BeN4 to Be3N2 and N2 is estimated to release 4.30 eV energy per formula unit (f.u.), which corresponds to an energy density of approximately 6.35 kJ·g-1. Such high energy content can establish that P21/c-BeN4 is indeed a high energy density material. It can replace modern typical modern high explosives, such as TATB, RDX, and HMX, which typically have energy densities ranging from 1 to 3 kJ·g-1.56 The Bader charge analysis50,51 reveals an almost complete electron transfer between beryllium and nitrogen atoms, contributing to better understand chemical bonding behaviors. The calculated phonon dispersion relations also confirmed the mechanical and dynamical stability of P21/c-BeN4, as shown in Figure 2. The lattice dynamical stability down to 0 GPa is clearly evidenced by the absence of any imaginary phonon mode. It thus allows for the possibility of stabilizing the beryllium polynitride P21/c-BeN4 under some kinetic regime at ambient conditions. 3.2 Structural features For BeN4, it is the most nitrogen-rich phase in beryllium nitrides. It is predicted to take the monoclinic P21/c space group at 40 GPa, as shown in Figure 3a and 3b. The lattice constants of predicted P21/c-BeN4 at ambient pressure are a = 3.649 Å, b = 3.556 Å, c = 5.229 Å and β = 99.0º. Three inequivalent atoms occupy the crystallographic: Be at 2b (0.5, 0.5, 0.5), N1 at 4e (0.889, 0.908, 0.391), and N2 at 4e (0.773, 0.794, 0.748) positions. The P21/c-BeN4 crystal is predicted to become energetically stable near 40 GPa, suggesting that it may be prepared by high-pressure synthesis. The potential energy storage capabilities for the predicted new structure have a close relationship with its structure motifs and the patterns of N atoms. Figure 3a shows crystal structure of P21/c-BeN4, in which the

N atoms adopts the intriguing 3D puckered “N10” rings network along the (100) plane. The Be and N atoms are localized in different layers and construct an intriguing N-Be-N sandwiches structure (Figure 3b). The 3D puckered “N10” rings layers of P21/c-BeN4 contain two non-equivalent nitrogen atoms: every N2 atom bonded with neighbor two N1 atoms, and every N1 atom connected by one N1 atom and two equivalent N2 atoms (Figure 3c). The N atoms in this structure are sp3 hybridization containing a high content of single N–N bonds in P21/c-BeN4. The P21/c-BeN4 is entirely composed of covalent N-N σ bonds. Every Be atom is surrounded by 20 N atoms forming N-sharing 20-fold BeN20 puckered octahedrons (Figure 3d). To our best knowledge, this is the first time 3D puckered “N10” rings network being reported in literature. Additionally, the bond lengths of N-N are from 1.392 Å, 1.422 Å to 1.495 Å, which are very similar to that of the single bonds (1.45 Å). The single-bond feature of the “N10” rings is shown nicely by the calculated Mayer bond order (MBO)57 values of 1.08, 1.04 and 0.98 (Figure 3c), which are close to the single bonds (1.0). In order to indicate the outstanding single-bond properties in “N10” rings network, the average NN bond length in alkali metal polynitrides LiN3,14 LiN5,20 NaN322 and KN324 and alkaline-earth metal polynitrides CaN431 was computed at the same pressure 40 GPa, as shown in Figure 4. The pseudo-benzene “N6” molecules rings in LiN314 and KN324 have the similar N-N bond length 1.318 Å and 1.319 Å. The endothermic N3 molecules in NaN322 and KN324 have the similar N-N bond length 1.165 Å and 1.168 Å. Thus, we can conclude the same polynitride form has the similar N-N bond length at the same pressure. The average N-N bond length in cyclic N5 molecules of LiN520 and one-dimensional bent chains of CaN431 are 1.302 Å and 1.332 Å, respectively. By comparison, the average bond lengths of N-N in 3D puckered “N10” rings network is 1.425 Å, and it is close to single bonds (1.45 Å). Such high content of single N–N bonds in alkaline-

Figure 3. Crystal structure of predicted stable P21/c-BeN4 phase: (a) side view along a axis, (b) side view along b axis, (c) two nonequivalent N atoms in polymeric 3D puckered N10 rings network and (d) puckered arrangement of N10 ring surrounding Be. For P21/cBeN4 at 40 GPa, the lattice parameters are a = 3.649 Å, b= 3.556 Å, c = 5.229 Å and β = 99.0º, with Be at 2b (0.5, 0.5, 0.5), N1 at 4e (0.889, 0.908, 0.391), and N2 at 4e (0.773, 0.794, 0.748). The large and small spheres denote beryllium and nitrogen atoms, respectively.



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ergy range in the P21/c-BeN4 structure. A large band gap of nearly 2.8 eV appears between the valence and conduction bands. Under high pressure, the band gap of the P21/c-BeN4 phase was found to reduce in the Supporting Information. In addition, to study the impact of partial occupation of N-2p orbital on electronic properties, we calculate the charge of Be and N atoms based on Bader analysis, as shown in Table S1. The Bader method is chosen to analyze the charge transfer as implemented in the algorithm developed by Henkelman et al.49,50 It reveals that the Be atoms contribute almost 1.7e to N atoms forming charged “N10” rings network which suggests that P21/c-BeN4 has ionic characteristics in chemical bonds. In the predicted P21/c-BeN4 crystal, the Be atoms behave as electron donors whose concentration strongly influences the N-N bonding, contributing to better understand chemical bonding behaviors. 3.4 Chemical bonding Figure 4. The average N-N bond length in LiN3,14 LiN5,20 NaN3,22 KN324 and CaN431 at 40 GPa.

earth metal polynitride mainly result in a such high energy density of 6.35 kJ·g-1, helping investigate theoretically promising material for energy storage applications. This novel polymeric 3D puckered “N10” rings network form of nitrogen is firstly discovered in the alkaline-earth metal nitrides. And the pressure effect on beryllium azide might be of great impact in the rational design of polymeric nitrogen. The present study provides new insights to the understanding of polynitrogens and encourages experimental exploration of these promising high-energy materials in the future. 3.3 Electronic Structure To understand the nature of the chemical bonding and the formation mechanism of P21/c-BeN4, we have calculated its electronic band structure and projected density of states (PDOS) at ambient pressure, as shown in Figure 5. The P21/c-BeN4 is an insulating phase resulting from the ionic nature of the structure. The bonding in the charged “N10” rings network is covalent and stabilized by the electron transferred from nearby Be atoms. As can be seen from the PDOS, N-2p states contribute most to the valance band and the PDOS among the entire en

Figure 5. Band structure and projected density of states of P21/cBeN4 at 40 GPa. The s state of nitrogen is denoted by red circle. The p state of nitrogen is denoted by blue circle.

To detect the interatomic interaction in the P21/c-BeN4 compound, we calculated the integrated crystal orbital Hamilton populations (ICOHPs) for N−N and N−Be pairs at 40 GPa. The ICOHP has a tendency to scale with bond strength (metallic or covalent) in compounds by counting the energyweighted population of wave functions between two atomic orbitals for a pair of selected atoms. In the P21/c-BeN4 structure, the values of ICOHP between N−N and N−Be pairs are −4.709 and −0.518 per pair, illustrating that the N−N interaction is much larger than that of N−Be. The major contribution of N−N interaction originates from 2p−2p, which corresponds to the decomposed ICOHP of – 3.99 eV per pair. More information on the N−N and N−Be interaction can be found in the Supporting Information. (Table S2). The calculated ICOHP shows that the N−Be bonds between the nearest neighboring N−Be pairs have smaller ICOHP values than N−N, indicating weaker covalent bonding, which is consistent with the observation that Be atoms are strongly ionic and bind with N atoms mainly by electrostatic forces, as shown by the Bader charge analysis.

Figure 6. The ELF distributions of 3D puckered “N10” rings network along (1 0 0) plane in P21/c-BeN4. The value of isosurface is 0.80.


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Subsequently, in order to understand the bonding character of P21/c-BeN4, we calculated the electronic localization function (ELF)58 to study its bonding feature, shown in Figure 6. The ELF is a measure of relative electron localization in extended structures, and large ELF values usually occur in regions where there is a high tendency of electron pairing, such as cores, bonds, and lone pairs. For P21/c-BeN4 structure, the strong covalent bonding between nitrogen atoms as well as the lone pair electrons is revealed clearly by the ELF shown in Figure 6. Figure 6 shows large areas with big ELF values between N and N atoms, which are typical of strong covalent bonding. In the 3D puckered “N10” rings network of P21/c-BeN4 shown in Figure 3c, all N1 and N2 atoms are in the sp3 hybridization. For N1 atoms, three sp3 hybridization orbitals form three N–N σ bonds with neighboring one N1 atom and two N2 atoms, the rest of the sp3 hybridization orbital of this N1 atom is filled and form one lone pair. These signal the semiconducting properties of P21/c-BeN4. For every N2 atom, two sp3 hybridization orbitals form two σ bonds with the sp3 hybridization orbitals of two neighboring N1 atoms at each side. The remaining two sp3 hybridization orbitals of N2 atoms are filled and form lone pairs. The covalent bondings between different inequivalent N atoms as well as the lone pairs on the side N atoms are also revealed by the ELF (Figure 6). Therefore, all the bonding states and lone pair states are filled and all the anti-bonding states are unoccupied in P21/c-BeN4, leading to a semiconducting state. The single-bond feature can be inferred from the similar bond lengths and strengths of these N-N bonds. As a potential highenergy-density material, the dissociation of P21/c-BeN4 is highly exothermic because of the existence of weak nitrogen bonds in the 3D puckered “N10” rings network. 3.4 Energy density of the polymeric BeN4 phase The predicted novel polymeric P21/c-BeN4 phase, featuring charged 3D puckered “N10” rings network, are expected to be favorable for HEDM. The nitrogen atoms are in the sp3 hybridization in “N10” rings network indicates that each N atom forms two or three σ bonds with its neighboring N atoms. It contains an exceptionally high content of the single N-N

bonds. In Figure 7, we show their energy densities at ambient pressure in relation to the pressure required to stabilize them. After decomposition, the final products of P21/c-BeN4 phase are Be3N2 and N2, BeN4 → (1/3)Be3N2 + (5/3)N2. The resulting energy density is 6.35 kJ·g-1 for the P21/c-BeN4 phase. The value is three times larger than the 2.2 kJ g-1 of the CO/N2 mixture (CNO)59 and also larger than that of N2H (4.4 kJ·g1 33 ), B3N5 (3.44 kJ·g-1),36 and LiN5 (2.72 kJ·g-1).20 The energy density of P21/c-BeN4 is also higher than those of the standard energetic materials TNT (4.3 kJ·g-1) and HMX (5.7 kJ·g-1).41 Although the energy density of of cg-N (9.7 kJ·g-1) is higher than the 6.35 kJ·g-1 of P21/c-BeN4 phase, the stabilization pressure for the P21/c-BeN4 is only 40 GPa, much lower than the pressure required for stabilizing polymeric cg-N (94 GPa~127 GPa).10,11 In order to explore the potential high energy density in P21/c-BeN4 comparing with cg-N, the crystal structures and chemical bonding of them were shown in Figure 7. The P21/c-BeN4 has similar coordination in the N1 atoms comparing with cg-N, suggesting an exceptionally high content of the single N-N bonds, and it can be as a kind of high energy-density material. CONCLUSIONS In summary, the first-principles calculations were employed to explore the high-pressure polymeric nitrogen phase of Be-N up to 100 GPa. One new monoclinic P21/c-BeN4 is predicted to become energetically stable near 40 GPa, suggesting that it may be obtained by high-pressure synthesis. Moreover, this new predicted high-pressure structure can be mechanically stable at ambient conditions which may make an ambientpressure recovery possible. For the first time, we identify one novel phase featuring charged 3D puckered “N10” rings network in P21/c-BeN4 structure. The nitrogen atoms are in the sp3 hybridization in “N10” rings network indicates that each N atom forms two or three σ bonds with its neighboring N atoms. The P21/c-BeN4 has similar coordination in the N1 atoms comparing with cg-N, suggesting an exceptionally high content of the single N-N bonds, which, if realized in controlled manner, may find applications as high energy carriers. On decomposition, P21/c-BeN4 is expected to release an enormously large amount of energy (6.35 kJ·g-1), and thus may find applications as a high energy material. The present study provides new insights to the understanding of polynitrogens and encourages experimental exploration of these promising materials in the future.

ASSOCIATED CONTENT Supporting Information Calculated Bader charges and integrated crystal orbital Hamiltonian populations for P21/c-BeN4. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author *Tian Cui Email:[email protected]

Author Contributions Figure 7. Calculated energy densities of the P21/c-BeN4 at ambient pressure in relation to the pressure required to stabilize them. The energy densities of cg-N,10,11 CNO,59 N2H,33 B3N5,36 LiN520, XeN639 as well as the standard energetic materials (HMX and TNT) 41 are also shown for comparison.

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes



The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Nos. 11404134, 51632002, 51572108, 11634004, 11574109, 11674122), Program for Changjiang Scholars and Innovative Research Team in University (No. IRT_15R23), National Found for Fostering Talents of basic Science (No. J1103202), and China Postdoctoral Science Foundation (2014M561279, 2016T90246). Parts of calculations were performed in the High Performance Computing Center (HPCC) of Jilin University.

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TOC Graphic


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Figure 1. Relative enthalpies of formation of Be-N phases with respect to elemental beryllium and nitrogen solids. The convex hulls connecting stable phases (solid shapes) are shown by solid lines. Unstable/metastable phases are shown by open shapes. Inset: The stable pressure ranges of BeN. 56x39mm (300 x 300 DPI)



Figure 2. Phonon dispersion curves for the P21/c structure of BeN4 at ambient pressure. 99x72mm (300 x 300 DPI)


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Figure 3. Crystal structure of predicted stable P21/c-BeN4 phase: (a) side view along a axis, (b) side view along b axis, (c) two non-equivalent N atoms in polymeric 3D puckered N10 rings network and (d) puckered arrangement of N10 ring surrounding Be. For P21/c-BeN4 at 40 GPa, the lattice parameters are a = 3.649 Å, b= 3.556 Å, c = 5.229 Å and β = 99.0º, with Be at 2b (0.5, 0.5, 0.5), N1 at 4e (0.889, 0.908, 0.391), and N2 at 4e (0.773, 0.794, 0.748). The large and small spheres denote beryllium and nitrogen atoms, respectively. 180x84mm (300 x 300 DPI)



Figure 4. The average N-N bond length in LiN3,14 LiN5,20 NaN3,22 KN324 and CaN431 at 40 GPa. 62x49mm (300 x 300 DPI)


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Figure 5. Band structure and projected density of states of P21/c-BeN4 at 40 GPa. The s state of nitrogen is denoted by red circle. The p state of nitrogen is denoted by blue circle. 80x61mm (600 x 600 DPI)



Figure 6. The ELF distributions of 3D puckered “N10” rings net-work along (1 0 0) plane in P21/c-BeN4. The value of isosurface is 0.80. 99x91mm (300 x 300 DPI)


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Figure 7. Calculated energy densities of the P21/c-BeN4 at ambient pressure in relation to the pressure required to stabilize them. The energy densities of cg-N,10,11 CNO,59 N2H,33 B3N5,36 LiN520, XeN639 as well as the standard energetic materials (HMX and TNT) 41 are also shown for comparison. 58x42mm (300 x 300 DPI)


A Novel Polymerization of Nitrogen in Beryllium Tetranitride at High

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