Polymerization of Nitrogen in Ammonium Azide at High Pressures

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Polymerization of nitrogen in ammonium azide at high pressures Hongyu Yu, Defang Duan, Fubo Tian, Hanyu Liu, Da Li, Xiaoli Huang, Yunxian Liu, Bingbing Liu, and Tian Cui J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b08595 • Publication Date (Web): 19 Oct 2015 Downloaded from http://pubs.acs.org on October 24, 2015

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The Journal of Physical Chemistry

Polymerization of Nitrogen in Ammonium Azide at High Pressures Hongyu Yu†, Defang Duan†, Fubo Tian†, Hanyu Liu‡, Da Li†, Xiaoli Huang†, Yunxian Liu†, Bingbing Liu† and Tian Cui*,† †

State key Laboratory of Superhard Materials, Jilin University, Changchun, 130012, P. R. China

‡

Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, S7N

5E2, Canada *Corresponding authors: [email protected].

ACS Paragon Plus Environment

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ABSTRACT: By ab initio molecular dynamics simulations, ammonium azide (AA, NH4N3) is predicted to be an effective precursor to form polynitrogen. Our simulations at 60 and 90 GPa show that the critical temperatures for nitrogen polymerization are about 2200 and 1600 K, respectively. Compared with molecular nitrogen (110 GPa and 2000 K), the synthesis pressure of polymeric nitrogen in AA significantly lowers. In the obtained polymeric nitrogen compounds, there are kinds of nitrogen backbone: one-dimensional chains, branched chains, and five-member rings. By annealing simulations at 90 GPa, a one-dimensional pure nitrogen periodic chain is formed. Our finding might open a way for the practical application of polymeric nitrogen compounds as further depressurization simulations at 300 K confirm that both hydrogenpassivated polymeric networks and five-member rings can be preserved at ambient conditions.


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Three decades ago, McMahan and LeSar1 first predicted that solid nitrogen molecule (N2) dissociated into a simple cubic structure of monatomic nitrogen under 100 GPa by firstprinciples calculation. After that, polymeric nitrogen attracts a great deal of attention and is considered to be potential high energy density materials (HEDM). To date, numerous structures of polymeric nitrogen have been proposed theoretically.2-10 Among these, cubic gauche phase (cg-N) was synthesized firstly by Eremets et al.11,12 from molecular nitrogen at temperatures above 2000 K and pressures above 110 GPa. Subsequently, many experimenters also successively synthesize cg-N at high pressure and high temperature.13-17 However, under weak laser illumination at 42 GPa, cg-N will transform back into a molecular phase, indicating that this polymeric nitrogen phase is unstable at ambient conditions.11 The double bond (418 KJ/mol) in N3- is weaker than the triple bond (954 KJ/mol) in N2, which means that the N3- anions will create polymeric networks easier than N2 molecule under high pressure and temperature. Therefore, inorganic azides are considered to be more effective precursors for forming polymeric nitrogen than N2. The feasibility of this idea has been verified by the formation of a nonmolecular nitrogen state in NaN3 at about 50 GPa and room temperature16,18. Unfortunately, these polymeric forms could not be preserved under ambient conditions. Therefore, it is urgent to find a new way to synthesize the polymeric nitrogen which can be preserved at ambient conditions. It is worthy of note that the inactivation of hydrogen can improve the stability of polymeric nitrogen.19 Besides, there is one fact that some hydronitrogen compounds which contain N-N single bonds are stable at ambient conditions, for instance, hydrazine, triaziridine and trans-tetrazene (TTZ). The above conclusions suggest that once hydrogen-passivated polymeric nitrogen compounds are synthesized, they are very likely to be


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stable at ambient conditions. Considering the synthesis condition and stability of polymeric nitrogen compounds, AA may be one more-effective precursor. The high-pressure phases and hydrogen-bond interaction of AA have been widely studied experimentally20-23 and theoretically24-26. At ambient conditions, the crystallographic information of AA is determined by using neutron diffraction20: an orthorhombic structure with Pmna space group. This study also gives the result that there are strong hydrogen bonds in AA. By ab initio evolutionary structure searches, Hu and Zhang27 proposed a hydronitrogen solid with a composition of (NH)4, and predicted that AA transformed to (NH)4 at about 36 GPa. But, Raman scattering investigations reveal that AA undergoes a phase I-II transition at 3 GPa and room temperature, and phase II is thermodynamically stable at least up to 55 GPa21. So, it is considered that high temperature must be another important condition to synthesize polymeric nitrogen compounds from AA. In the present study, we use ab initio molecular dynamics (MD) simulations based on the density functional theory to explore the formation of polymeric nitrogen from AA and check the stability of polymeric networks. The projector-augmented wave method28 is adopted and exchange correlation function is treated within the Perdew-Burke-Ernzerhof generalized gradient approximation29. Taking into account long-range van der Waals interactions, the tested optB86bvdW function30 is adopted. The MD simulations are performed within the NPT (N - constant number of particles, P - constant pressure, and T - constant Temperature) ensemble31, as implemented in the Vienna ab initio simulation package (VASP) code32. A plane-wave basis-set cutoff energy of 520 and 700 eV are employed for MD simulations and geometry optimization, respectively. Brillouin zone integration is restricted to the Г point of supercell for MD


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simulations. The MD simulation time of 10–30 ps (with time step of 1 fs) are enough to get credible results.

Figure 1. (a) Our predicted crystal structures of phase P2/c, P21/c, and P21. Large spheres represent N and small spheres denote H atoms, respectively. (b) Calculated enthalpies of three new AA phases, TTZ, and hydronitrogen of the form (NH)4 relative to that of AA Pmna structure plotted as functions of pressure. P21/c and P21 structures are unstable when pressures below 40 and 20 GPa, respectively, which are not shown in this figure. The inset shows enthalpy difference between P2/c and Pmna at the low pressure region.

We optimize the primitive cell of AA (Pmna) at 0 K and a series of pressures (0.5, 60, and 90 GPa), then expand them to supercell of 28 AA molecules as initial structures for MD simulations at 300 K. At 0.5 GPa, the simulation result is consistent with initial structure. Two new structures with P2/c and P21 symmetry appear at 60 and 90 GPa, respectively. We separately choose them as initial structures and perform simulations at 1000 K, and find that the P2/c structure remains stable at 60 and 90 GPa. However, the P21 structure changes into P2/c structure at 60 GPa, and


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transforms to another new structure with P21/c symmetry at 90 GPa. Hence, for the simulations at 90 GPa, we have two alternative trajectories to explore the polymerization reaction, one with P2/c symmetry and the other in a P21/c structure. Three new phases P2/c, P21/c, and P21 are reported in Figure 1a, and corresponding lattice parameters are given in Table 1. Table 1. Structure parameters of three new phases of AA in this paper.

Structure Parameters (Å, deg) P2/c a = 2.8591 b = 2.8585 c = 10.1124 β = 91.75 P21/c

a = 2.6060 b = 7.0966 c = 8.2749 β = 118.08

P21

a = 6.4128 b = 8.2146 c = 2.5689 β = 91.75

Atom x

y

z

H1 H2 N1 N2 N3 H1 H2 H3 H4 N1 N2 N3 N4 H1 H2 H3 H4 H5 H6 H7 H8 N1 N2 N3 N4 N5 N6 N7 N8

-0.5100 -0.0965 -0.7862 -0.0000 -0.3020 0.6126 0.6798 0.4602 0.5075 0.5696 0.3151 0.4004 0.2164 0.4298 0.3391 0.2270 0.2743 0.6183 0.7537 0.8188 0.6734 0.0370 0.9521 0.0910 0.9720 0.0765 0.3224 0.3195 0.7167

0.3149 0.3150 0.0823 0.0000 0.2500 0.9361 0.7612 0.8624 0.7273 0.8246 0.4958 0.6018 0.3880 0.4615 0.1147 0.5679 0.0899 0.3514 0.8070 0.3558 0.7996 0.1477 0.0388 0.3176 0.9856 0.8525 0.7940 0.3083 0.5764

0.3731 -0.2134 -0.1868 -0.5000 0.5000 0.5129 0.8664 0.9691 0.2454 0.1560 0.0388 0.4228 0.6835 0.4580 0.2776 0.3911 0.5313 0.0003 0.0891 0.9302 0.8455 0.7760 0.2764 0.6287 0.9239 0.3115 0.7458 0.4138 0.9627


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Relative enthalpies of three new AA phases our predicted, TTZ, and (NH)4 as a function of pressure at 0 K are depicted in Figure 1b. It is shown that Pmna structure transforms into P2/c structure at about 1.5 GPa, and that P2/c structure is the most stable structure until 77 GPa. According to Figure 1a and Table 1, it is clear that all azide ions occupy equivalent crystallographic positions in P2/c structure. Besides, only one Raman peak corresponding to symmetric internal stretch of azide-anion was observed experimentally21, indicating that all azide ions occupy equivalent crystallographic positions in phase II. So we speculate that the P2/c structure is phase II. Obviously, the stable pressure region of P2/c structure (1.5-77 GPa) provides a reasonable explanation for recent experiments21,22 in which polymerization reaction have not been found until 55 GPa at room temperature.

Figure 2. Radial distribution functions (a) gNN(r) at 60 GPa and different temperatures, (b) gNN(r) at 90 GPa and different temperatures and (c) gHN(r) at 60 GPa and different temperatures. The arrows indicate the appropriate bond length for NN and NH, respectively.


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The radial distribution functions (RDF) gNN and gHN obtained from the MD simulations at 60 and 90 GPa, and different temperatures are presented in Figure 2. It is shown that there is a single peak at 1.16 Å corresponding to the N-N double bonds in N3- anions at 60 GPa and 300 K (Figure 2a). When the temperature increases to 2000 K, there is still only a single peak located at 1.17 Å, which illustrates that the double bonds in N3- are remaining stable. At 2200 K, it is interesting to note that the first single peak splits into a double-peak. The left peak corresponds to the remanent N-N double bonds and the right new peak indicates the formation of single bonds. Simultaneously, this figure shows a nice zero in the gNN at 2200 K between 1.65 Å and 1.9 Å, which we use to determine a befitting cut-off radius of N-N bond (1.65 Å). For the case of gNN at 90 GPa (Figure 2b), the first peak at 1.16 Å significantly shifts to 1.29 Å with temperature increasing from 1500 to 1600 K, which clearly exhibits the transformation from the double bonds to single N-N bonds. In addition, we integrate the radial distribution gHN at 2200 K and 60 GPa (Figure 2c), and find that the integral value is 1 at 1.27 Å, which is a reasonable N-H bond length.


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Figure 3. Snapshot of MD trajectory and polymeric networks at (a) 60 GPa and 2200 K, (b) 90 GPa and 1600 K, (c) 90 GPa and 300 K and (d) at 1 bar and 300 K. The center is the supercell of MD simulation, and the polymeric networks are exhibited around it. Large (blue) and small (red) spheres denote nitrogen and hydrogen atoms, respectively. The red arrows label the both ends of polymerization unit.

An instantaneous configuration at 60 GPa and 2200 K is depicted in the center of Figure 3a. The N-N and N-H bonds shown in this picture are decided by the above determined cut-off radius. As exhibited around the center, there are one-dimensional chains (N3, N4, N5, and N7) and two-dimensional structure units (N6(N)N8) in the instantaneous configuration. From the gNN and configuration of polymeric networks, one can see that there are abundant N-N single bonds in this supercell. Subsequently, a quenching simulation is performed at 300 K with the configuration got from 2200 K and 60 GPa as initial structure. Interestingly, we find that the two-dimensional chain is extended to be N8(N)N8, containing as many as 17 N atoms, meanwhile a new five-member ring (N5r) is created. At 90 GPa, a series of MD simulations at different temperatures are performed with the P21/c configuration as initial structure. We observe the polyreaction at 1600 K. The snapshot of the


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corresponding MD simulation shows that three kinds of periodic chains ((N2(N2)N5)n, (N4(N)N5)n, and (N2(N)N4(N2)N)n) and seven N5r-X (X = N or N chain) rings are formed (see Figure 3b). Further, annealing simulations are performed from 1600 to 300 K and the annealed structure is shown in Figure 3c. By annealing simulations, one periodic chain (N2(N2)N5)n successfully evolves into a pure N chain (N7)n, which strongly reveals that AA could be used as an effective precursor to synthesize polymeric nitrogen. During the early stages of annealing process, another periodic chain (N2(N)N4(N2)N)n is broken and transforms into a longer chain with 13 N atoms (N4(N)N8). In the following simulation, this chain connects to chain (N4(N)N5)n and forms a highly branched structure (N4(N2)N(N13)N3)n, where N13 is the branch chain N4(N)N8 (see Figure 3c). To check the stability of these polymeric networks at 300 K roughly, the pressure on the annealed simulation cell is released with a rate of 10 GPa/ps until atmospheric pressure (see Figure 3d). It is found that the unit of pure N chain (N7)n captures a H atom of nearby [NH4]+ at 20 GPa, then dissociates into two N2 molecules and one HN3 unit at 10 GPa, indicating that the chainlike polymeric nitrogen which contains only N atoms is unstable at low pressure. However, the other hydrogen-passivated periodic chain remains stable until 10 GPa and transforms into a long chain with 23 N atoms at 5 GPa. To get accurate results at atmospheric pressure, we perform MD simulation with 10 ps. In this simulation, the long chain is broken up into two chains which can be preserved at ambient conditions successfully. To further examine the stability at atmospheric pressure, we perform simulations at higher temperatures (380, 450 and 600 K), and find that all structures remain stable. Compared with the instability of pure N polymers, the hydrogen-passivated polymeric networks are stable. Furthermore, it is worthy to mention that N5r-X units containing few H (even pure N5r) exhibit


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extraordinary stability at all conditions. The 5 N atoms are almost in the same plane at all conditions. The bond lengths between N atoms are almost equal in pure N5r ring, but in N5r-X structures the bond lengths are different. In addition, we also perform simulations starting with P2/c symmetry at 90 GPa and find that the polymerization begins at about 1800 K. Analogous to the simulations results starting with P21/c symmetry, highly branched periodic chain, onedimensional chains and N5r-X rings are all found. Except for inactivation, H atoms also play an important role in the formation of polymeric networks, which can be verified by the hydrogen hopping and proton transfer. In order to understand the behaviors of hydrogen, we introduce the δ function proposed by Ikeda et al.33, which defined as δn(t) = r(NnHn) - r(NmHn),

(1)

where NnHn is the N-H bond assigned at the beginning of considered time interval in this work, r(NmHn) is the distance between the nth hydrogen and the mth nitrogen atom. This atom Nm is selected during the course of time t according to the criterion r(NmHn) = min r(NkHn) (k ≠ n).

(2)

If the initial N-H bond is maintained, δn always keep a negative value. If, on the contrary, the hydrogen hops to Nm, δn will take a positive value.


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Figure 4. Hydrogen hopping in P21/c symmetry at 90 GPa. (a) Temperature induced proton hopping as measured by the parameter δ defined in Eq. (1). The insert displays the probability density of parameter δ from -0.5 Å to 0.5 Å. (b) The probability density of δ parameter changes as simulation performed at 1600 K.

Figure 4a illustrates the distribution of δ in the P21/c structure at 90 GPa and different temperature. At 1000 K, only one peak appears and locates in negative area, indicating that the N-H bonds in [NH4]+ are remaining stable during the MD simulation. At 1500 K close to the polyreaction temperature, the intensity of first peak declines slightly and two new peaks appear in the positive area. One peak at about 1.1 Å indicates that a few H atoms dissociate from [NH4]+ and bond to N3- anions due to the hydrogen-bond interaction. The other peak at about 2.3 Å reveals that some new formed N-H bonds are broken and H atoms undergo a further transfer. Under 1600 K, there are three peaks and a heavy tail, showing that N-H bonds in [NH4]+ ions are broken frequently. Therefore, we speculate that the incessant forming and breaking of N-H


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bonds are beneficial to break the N-N double bonds and activate the N atom in azide ion. Moreover the [NH4]+ which lost proton also becomes activated units. Then, some N3 units combine with other N3 units or activated NH3 units to form polymeric nitrogen. After this reaction, the new polymer undergoes a further polymerization and evolution. In addition, the probability density is not zero when δ equal 0 (see insert in Figure 4a), meaning that some hydrogen-bond symmetrization occurs in certain regions. By exploring the transformation of H atoms at 1600 K, we further verify that H atoms play an important role in the formation of polymer. Specifically, we calculate the δ distributions at time intervals 0-5, 10-15, 20-25, and 25-30 ps, as shown in Figure 4b. It can be seen that the first peaks initially decrease and then increase, indicating that the frequency of the breaking and forming of N-H bonds increases at first and then decreases. Besides, by observing the snapshots of MD trajectory, it is found that the forming of N-N bonds is synchronized with the transformation of N-H bonds. In conclusion, we find kinds of nitrogen backbone polymeric networks in AA under high temperatures and high pressures: one-dimensional chains, branched chains, and ring-shaped N5r units. In addition, a one-dimensional purely nitrogen chain (N7)n is also obtained from annealing simulations and preserved in a large pressure range (20-90 GPa) at 300 K. More interestingly, the hydrogen-passivated polymeric networks and the structures containing five-member ring are stable under ambient conditions. Further analysis reveals that H atoms play an important role in the formation of polymeric nitrogen networks. The present work sustains the standpoint that AA or other azides provide a new way to synthetic polymeric nitrogen which might probably be preserved under ambient conditions.


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AUTHOR INFORMATION Corresponding Author *Email: [email protected]. Tel./Fax: +86-431-85168825 Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by the National Basic Research Program of China (No. 2011CB808200), National Natural Science Foundation of China (Nos. 51572108, 11204100, 11404134, 11504127, 11574109), Program for Changjiang Scholars and Innovative Research Team in University (No. IRT1132), National Found for Fostering Talents of basic Science (No. J1103202). China Postdoctoral Science Foundation (2012M511326, 2013T60314, and 2014M561279). Part of calculations were performed in the High Performance Computing Center (HPCC) of Jilin University. REFERENCES (1) McMahan, A. K.; LeSar, R. Pressure Dissociation of Solid Nitrogen Under 1 Mbar. Phys. Rev. Lett. 1985, 54, 1929-1932. (2) Mailhiot, C.; Yang, L. H.; McMahan, A. K. Polymeric Nitrogen. Phys. Rev. B 1992, 46, 14419-14435. (3) Martin, R. M.; Needs, R. J. Theoretical Study of the Molecular-to-nonmolecular Transformation of Nitrogen at High Pressures. Phys. Rev. B 1986, 34, 5082-5092. (4) Lewis, S. P.; Cohen, M. L. High-pressure Atomic Phases of Solid Nitrogen. Phys. Rev. B 1992, 46, 11117-11120.


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(31) Klimeš, J.; Bowler, D. R.; Michaelides, A. Van Der Waals Density Functionals Applied to Solids. Phys. Rev. B 2011, 83, 195131. (32) Hernández, E. Metric-tensor Flexible-cell Algorithm for Isothermal–isobaric Molecular Dynamics Simulations. J. Chem. Phys. 2001, 115, 10282-10290. (33) Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for Open-shell Transition Metals. Phys. Rev. B 1993, 48, 13115-13118.


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TOC GRAPHICS


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Figure 1. (a) Our predicted crystal structures of phase P2/c, P21/c, and P21. Large spheres represent N and small spheres denote H atoms, respectively. (b) Calculated enthalpies of three new AA phases, TTZ, and hydronitrogen of the form (NH)4 relative to that of AA Pmna structure plotted as functions of pressure. P21/c and P21 structures are unstable when pressures below 40 and 20 GPa, respectively, which are not shown in this figure. The inset shows enthalpy difference between P2/c and Pmna at the low pressure region. 119x174mm (300 x 300 DPI)



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Figure 2. Radial distribution functions (a) gNN(r) at 60 GPa and different temperatures, (b) gNN(r) at 90 GPa and different temperatures and (c) gHN(r) at 60 GPa and different temperatures. The arrows indicate the appropriate bond length for NN and NH, respectively. 106x137mm (300 x 300 DPI)


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Figure 3. Snapshot of MD trajectory and polymeric networks at (a) 60 GPa and 2200 K, (b) 90 GPa and 1600 K, (c) 90 GPa and 300 K and (d) at 1 bar and 300 K. The center is the supercell of MD simulation, and the polymeric networks are exhibited around it. Large (blue) and small (red) spheres denote nitrogen and hydrogen atoms, respectively. The red arrows label the both ends of polymerization unit. 90x51mm (300 x 300 DPI)



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Figure 4. Hydrogen hopping in P21/c symmetry at 90 GPa. (a) Temperature induced proton hopping as measured by the parameter δ defined in Eq. (1). The insert displays the probability density of parameter δ from -0.5 Å to 0.5 Å. (b) The probability density of δ parameter changes as simulation performed at 1600 K. 120x174mm (300 x 300 DPI)


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TOC GRAPHICS 44x28mm (300 x 300 DPI)


Polymerization of Nitrogen in Ammonium Azide at High Pressures

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