Stabilization of the High-Energy-Density CuN5 Salts under Ambient Conditions by a Ligand Effect.

A series of excellent works have demonstrated that high-nitrogen-content metal pentazolate (cyclo-N5 -) compounds could be stabilized by high pressure. However, under ambient conditions, low stability precludes their synthesis and application in the field of high-energy-density material. In this work, by using a constrained structure search method, we predicted two new structures as P212121-CuN5 and P21/c-CuN5 containing cyclo-N5 - with strong N-N and Cu-N bonds. In both structures, cyclo-N5 - form four coordination with the Cu+ ligand, which increases the structural stability by lowering the disturbance to the aromaticity of cyclo-N5 -. The calculated results show that the P212121-CuN5 and P21/c-CuN5 structures exhibit high dynamic and thermal stability up to 400 K, indicating that they can be stabilized under ambient conditions. The decomposing energy of P212121-CuN5 and P21/c-CuN5 can reach up to 2.40 and 2.42 kJ/g, respectively. Strikingly, the detonation velocity and the pressure of P212121-CuN5 is predicted to be up to 10.42 km/s and 617.46 kbar, respectively, indicating that they are promising high-energy candidates in the field of explosive combustion.

Introduction

The pentazolate anion (cyclo-N5–), behaving like a five-membered ring solely composed of nitrogen atoms, has attracted great interest because of its great potential application as a high-energy-density material.[1−4] Because of the large energy difference between triple-bond dinitrogen (N≡N, ∼954 kJ/mol) and single (N–N, ∼160 kJ/mol) or double (N=N, ∼418 kJ/mol) nitrogen, an enormous amount of energy could be released during the unique transformation of N–N bonds in cyclo-N5– to N≡N bonds. Moreover, the carbon- and hydrogen-free products (nitrogen only) make it a potential eco-friendly superior explosive material. However, the low stability of the metastable structure of cyclo-N5– compounds under ambient conditions impedes their synthesis and application.[5,6]

Recently, tremendous experimental and theoretical efforts have demonstrated that high pressure can be used to stabilize the pentazolate anion. Many metal pentazolate compounds (e.g., LiN5, NaN5, CsN5, AlN15, and MgN10) have been synthesized and predicted by high pressure, but few metal pentazolate compounds could be quenched down to ambient conditions.[7−15] On the other hand, a ligand effect plays an important role in pentazolate compounds under ambient conditions.[16] In previous studies, Xu et al. and Zhang et al. successfully synthesized some metal and metal-free pentazolate hydrates by H+ or a sole metal ligand, such as [Na(H2O)(N5)]·2H2O and (N5)6(H3O)3(NH4)4Cl salts.[17−19] Lu et al. reported hydrogen bonding network pentazolate compounds.[20−23] Wang et al. successfully obtained Na20N60 and Na24N60 nanocages in the zeolitic architecture.[24] Although the produced compounds exhibit good thermal stability, the amount of redundant ligand (such as H2O) used to stabilize them would weaken their explosive performance. Therefore, further works are needed to find new, water-free, stable, and high-performance pentazolate compounds under ambient conditions.

Cu+ is an excellent ligand in many compounds Li et al. reported a simple route for the synthesis of CuN5 compounds via compressing CuN6 at 50 GPa.[25] Lu et al. obtained three-dimensional (3D) framework Cu(N5)(N3) compounds by chemical synthesis.[26] Motivated by this, by directly using a constrained structure search method, here, we predicted that Cu+ could coordinate with cyclo-N5– in different coordinated configurations; two new phases such as P212121-CuN5 and P21/c-CuN5 are more stable under ambient conditions compared with others, owing to forming more coordination bonds. These two stabilized CuN5 structures exhibit lower energy and high dynamic, optical, and thermal stability (up to 400 K). The bond analysis reveals that cyclo-N5– maintains its strong N–N bonding while it binds with Cu under ambient conditions. The decomposing energy of P212121-CuN5 and P21/c-CuN5 can reach up to 2.40 and 2.42 kJ/g, respectively. Strikingly, the detonation velocity and pressure of P212121-CuN5 is predicted to be up to 10.42 km/s and 617.46 kbar, respectively, indicating that they are promising eco-friendly and high-energy candidates as high-power energetic materials.

Results and Discussion

Crystal Structure and Stability

Through a direct constrained structure search at 0 GPa, as summarized in Table , here we reported two CuN5 compounds, named as P212121-CuN5 and P21/c-CuN5, possessing much lower energy than that in the previously reported CuN5 (P21/m-CuN5, obtained at high pressure),[25] as illustrated in Figure . The lattice constants and atomic coordinate positions are shown in Table S1. Compared with P21/m-CuN5, each Cu atom in P212121-CuN5 and P21/c-CuN5 binds with four different cyclo-N5– rings and forms a tetrahedron linked by Cu–N coordinative bonds. The lengths of Cu–N bonds are 1.79, 2.02, 2.03, and 2.10 Å in the P212121-CuN5 phase, while 1.98, 2.02, 2.03, and 2.11 Å in the P21/c-CuN5 phase. In comparison with the bond length of Cu–N in known Cu(N5)(N3) (2.03 and 2.54 Å),[26] the Cu+ in P212121-CuN5 and P21/c-CuN5 forms a stronger coordination bond with N atoms. The four N atoms in a planar cyclo-N5– are bonded with the neighboring Cu atom. The bond length of N–N on average is around 1.33 Å in both P212121-CuN5 and P21/c-CuN5 phases, which is between the N–N single bond (1.45 Å) and the N=N double bond (1.25 Å), and close to that in P21/c-LiN5 (1.32 Å), [Na(H2O)(N5)]·2H2O (1.33 Å) and [Mg(H2O)6(N5)2]·4H2O (1.32 Å),[13,17] indicating its uniqueness for cyclo-N5– rings.

Table 1

The Coordination Unit and Energy of CuN5 Compounds

compounds	coordination unit	energy (meV/formula)
P212121-CuN5	Cu tetrahedron, η4-N5	0.00
P21/c-CuN5	Cu tetrahedron, η4-N5	25.78
P4/1-CuN5	Cu triangle, η3-N5	63.72
P21/m-CuN5	Cu linear, η2-N5	424.53

Figure 1

The structure of (a) P212121-CuN5 and (b) P21/c-CuN5 under ambient condition, respectively. The bond length of Cu–N and N–N are labeled. The blue balls are Cu atoms, and the gray balls are N atoms.

As shown in Table , the coordination numbers of the Cu+ ion in P41-CuN5 and P21/m-CuN5 (Figure S1) are 3 and 2, respectively, which are both smaller than the coordination number of the Cu ion in P212121-CuN5 and P21/c-CuN5. Although the length of Cu–N bonds in P41-CuN5 and P21/m-CuN5 is 0.08 and 0.17 Å shorter than P212121-CuN5 on average, respectively, the energy of P41-CuN5 and P21/m-CuN5 is 63.73 and 424.53 meV per formula, respectively, higher than that of P212121-CuN5. Moreover, P21/c-CuN5 are composed of a similar coordination unit and show different atomic configurations with P212121-CuN5; the energy of P21/c-CuN5 is 25.78 meV per formula, higher than P212121-CuN5, and still lower than P41-CuN5 and P21/m-CuN5. We could conclude that the high coordination number and right coordination configuration would lower the energy of CuN5 compounds, and the ligand effect could play an important role in stabilizing CuN5 under ambient condition.

The dynamic stabilities are further evaluated by calculating their phonon spectra. Figure a,b shows the calculated phonon dispersion along high-symmetry directions and phonon density of states (DOS) for P212121-CuN5 and P21/c-CuN5, respectively. All acoustic branch frequencies are positive, ensuring that both the structures of P212121-CuN5 and P21/c-CuN5 are dynamically stable. For both structures, we can see that the atomic motions are separated in phonon PDOS and the metal atom modes are right below the cyclo-N5– ring modes; the high-energy modes (>20 THz) consist of N–N vibrations modes, which indicates that the cyclo-N5– ring is stable by the strong N–N bonds. The overlaps between Cu atom modes and N–N modes show lower energy modes (<10.6 THz), which are resulted from the translational motion of the Cu atom and cyclo-N5–.

Figure 2

Phonon spectra and phonon density of states of (a) P212121-CuN5 and (b) P21/c-CuN5 at 0 GPa, respectively. Evolution of total energy as a function of time step at 400 K and snapshots of the initial and final structures of (c) P212121-CuN5 and (d) P21/c-CuN5 in AIMD simulations at 0 and 10 ps, respectively.

Since the stability under ambient conditions is one of the key factors for potential high-density-energy materials, we further examined the stability of the P212121-CuN5 and P21/c-CuN5 structures at a fine temperature by molecular dynamics (MD) simulations. The calculations were performed at 300, 400, 600, and 800 K for 10 ps with a time step of 2 fs. Both predicted structures of P212121-CuN5 and P21/c-CuN5 did not suffer a large shape change up to 400 K, which is more stable than P21/m-CuN5 (300 K).[25] The closest averaged lengths of the N–N, Cu–N, and Cu–Cu bonds in P212121-CuN5 and P21/c-CuN5 (Figure S2) are statistically analyzed from the last 4 ps, estimated to be around 1.34(1.34), 1.97(1.99), and 3.712(3.71) Å. The bond lengths have just a small shift from the calculated results at 0 K, indicating their good structural stability. When temperature increases up to 600 K, the Cu–N bonds start to decompose (Figure S3), which is different from the decomposition path of phenylentazoles (PhN5). In the latter case, the decomposition starts from the breaking of the N5– ring.[27,28]

The synthesis method is optional, especially since the pentazolate salts ([Na(H2O)(N5)]·2H2O and Cu(N5)(N3)) are now available, we need not to break the C–N bond in PhN5, P212121-CuN5, and P21/c-CuN5, which could be directly synthesized via removing the N3– group from Cu(N5)(N3). The high-pressure and laser heating are also very popular synthesis methods, which have been successfully applied to the synthesis of CsN5 and LiN5.[12,14] Li et al. reported that P21/m-CuN5 could be synthesized via compressing CuN6 at 50 GPa; here we calculated the enthalpy of P212121-CuN5-CuN5 and P21/c-CuN5 by setting known P21/m-CuN5 as the reference zero point. The results indicate that both the enthalpy of P212121-CuN5 and that of P21/c-CuN5 have negative values when pressure increases up to about 40 GPa (Figure S4), which indicates that these compounds have the possibility to be synthesized by releasing pressure from P21/m-CuN5.

Optical Stability and Decomposition Route

Here, we further explored their optical stability and decomposition mechanism. The electronic structures and DOS for P212121-CuN5 and P21/c-CuN5 were calculated at the HSE06 level and are depicted in Figure , P212121-CuN5 and P21/c-CuN5 were both insulators with a band gap of 4.12 and 3.98 eV, respectively. The N 2p orbit makes main contributions to the conduction band minimum (CBM) by partial DOS calculations, whereas the Cu 3d electron mainly contributes to the valence band maximum (VBM). This is similar to many conventional semiconductors such as GaN in which the VBM states mainly consist of the p orbitals of the anions and the CBM states mainly consist of the s and p orbitals of the cations. The band gaps of P212121-CuN5 and P21/c-CuN5 are both large and the bands near the VMB and CMB are both flat, which indicates a large electron effective mass and that the electron transition from the valence band to the conduction band is difficult. Combined with our optical calculations and analysis, the large band gap induces weak optical absorption above 300 nm along each direction, indicating their high optical stability. Furthermore, as shown in Figure S5, the band gap decreases when pressure increases, so P212121-CuN5 and P21/c-CuN5 have higher optical stability at lower pressure.[29]

Figure 3

Calculated electronic band structures and partial density of states (PDOS) of (a) P212121-CuN5 and (b) P21/c-CuN5 at 0 GPa, respectively. Calculated absorption coefficients of (c) P212121-CuN5 and (d) P21/c-CuN5 for incident light polarized along the a, b, and c directions.

The decomposition route is important for P212121-CuN5 and P21/c-CuN5. The electron localization function (ELF) projected on the cycle-N5– plane (Figure a,b) demonstrates that valence electrons strongly localize on N–N bonds and the lone pairs are around nitrogen atoms, but weaker electrons localize between Cu–N bonds, which indicates that the strength of Cu–N bonds might be much weaker than that of N–N bonds. The Bader charge analysis[30] shows that there are 0.78 electron transferred from each Cu atom to neighbor cycle-N5– rings, which indicates that cycle-N5– is stabilized by the ligand effect of Cu atoms. As shown in Figure c,d, the charge distribution on cycle-N5– in P212121-CuN5 and P21/c-CuN5 is more homogeneous than P21/m-CuN5 and P41-CuN5 (Figure S6a,b), indicating that P212121-CuN5 and P21/c-CuN5 with more Cu coordination numbers have lower disturbance to the aromaticity of cyclo-N5–. To further characterize the chemical bonds in P212121-CuN5 and P21/c-CuN5 more quantitatively, we calculated the crystal orbital Hamilton population (-pCOHP) curves implemented in the LOBSTER program.[31] The calculations results (Figure e,4f) indicate that the nitrogen p-wave electrons make contributions to the strong covalent bonding interaction in cycle-N5– rings, but for the Cu–N bond, the electron occupation part exists on a small antibonding orbital. The integrated COHP values of N–N bonds in P212121-CuN5, P21/c-CuN5, P41-CuN5, and P21/m-CuN5 are −14.27, −14.22, −13.12, and −13.15, respectively. Comparing the COHP of the N–N bond in P21/m-CuN5 and P41-CuN5 (Figure S6c,d), the lower disturbance to the aromaticity of cyclo-N5– in P212121-CuN5 and P21/c-CuN5 makes less electron occupation on the antibonding orbital and shows more stable properties. Combined with the calculated results of ab initio molecular dynamics (AIMD), we can conclude that the decomposition of P212121-CuN5 and P21/c-CuN5 first occurs at Cu–N bonds, and the more coordinal bonds of Cu–N would enhance the stability of pentazolate salts by increasing the aromaticity of cycle-N5– ring.

Figure 4

Sectional view of ELF along the M and cyclo-N5– plane of (a) P212121-CuN5 and (b) P21/c-CuN5; the atomic Bader charge of (c) P212121-CuN5 and (d) P21/c-CuN5 at 0 GPa; -pCOHPs of the Cu–N bond and N–N bond in (e) P212121-CuN5, and (f) P21/c-CuN5, respectively. The red dashed lines represent the energy level of the top of the valence bands.

Explosive Performance

The lower energy of P212121-CuN5 and P21/c-CuN5 increased the stability of P21/m-CuN5[25] but decreased explosive properties. Here, we calculate the chemical energies when they dissociate into stable Cu3N2 solid and dinitrogen gas under ambient conditions. Taking into consideration the energy reduction for dinitrogen gas relative to the α nitrogen phase (around 0.25 eV/atom),[32] the chemical energy densities (Ed) of dense P212121-CuN5 and P21/c-CuN5 are estimated to be about 2.40 and 2.42 kJ/g in the dissociation of CuN5 (s) → Cu3N (s) + N2 (g) by referring the energy of their most stable phase at 0 GPa.[33,34] As shown in Table , we further use the empirical Kamlet–Jacobs equation[9,15,35] by Vd = 1.01(NM0.5Ed0.5)0.5(1 + 1.30ρ) and Pd = 15.58ρ2NM0.5Ed0.5 to calculate the detonation velocity and pressure (Vd, Pd) to evaluate the explosive performance. The release energies of P212121-CuN5 and P21/c-CuN5 are a bit lower than those of TNT and HMX, but the Vd and Pd (10.42 km/s and 617.46 kbar for P212121-CuN5, 9.72 km/s and 517.46 kbar for P21/c-CuN5) are higher than those of TNT, HMX[32,35] and even recently reported MgN10.[9] These high performances indicate the potential application of P212121-CuN5 and P21/c-CuN5 in high explosives.

Table 2

Detonation Properties of P212121-CuN5 and P21/c-CuN5, Estimated by Kamlet–Jacobs Empirical Equations, Compared to the Values of the Known Explosives of MgN10, TNT, and HMX[9,32,35]

compounds	ρ (g/cm3)	Ed (kJ/g)	Vd (km/s)	Pd (kbar)
P212121-CuN5	2.96	2.40	10.42	617.46
P21/c-CuN5	2.70	2.42	9.72	517.46
MgN10	2.06	3.48	11.06	586
TNT	1.64	4.30	6.90	190
HMX	1.90	5.70	9.10	393

Conclusions

In summary, we reported two novel stable CuN5 structures under ambient condition, named as P212121-CuN5 and P21/c-CuN5. Both ab initio MD and phonon spectrum simulations confirm their stability under ambient condition. By the chemical bond analysis, we reveal that the stability of cycle-N5– is benefited from a ligand effect by a higher Cu+ coordination number. These nitrogen-rich salts show high energy density (2.40 kJ/g for P212121-CuN5 and 2.42 kJ/g for P21/c-CuN5), while they are decomposed into stable Cu3N2 and nitrogen gas. Both P212121-CuN5 and P21/c-CuN5 have higher detonation velocity and pressure than those of TNT or HMX. In particular, the detonation velocity and pressure of P212121-CuN5 are about twice those of TNT, indicating good application prospects in the field of eco-friendly powerful explosives.

Calculational Methods

To explore the most stable structures of pentazolate compounds under ambient conditions, a molecular crystal search method was carried out to search those high-energy compounds using the particle swarm optimization (PSO) algorithm as implemented in the CALYPSO code.[36−39] Through this method, the bond connectivity of N atoms in cyclo-N5– rings could be restrained during the evolution of the structures. Using this method, we directly searched the most metastable structures of CuN5 compounds under 0 GPa with four to six times CuN5 units; the total 1400 structures were considered and ranked according to their calculated energy.

The local structural relaxations and electronic structures calculations are carried out in the framework of density functional theory (DFT), implemented by the Vienna ab initio simulation package (VASP).[40,41] The projector augmented wave method[42] is used to describe the pseudopotential. The exchange-correlation function in DFT calculations is implemented by generalized gradient approximation using the Perdew–Burke–Ernzerhof functional.[43] In the electron–ion interaction, 3s23p63d9 and 2s22p3 are treated as valence electrons for Cu and N atoms, respectively. A plane-wave basis cut-off energy is set to 700 eV for the span of the one-electron wave function. The conjugate gradient scheme is used to optimize the atomic positions and lattice constants until the residual Hellmann–Feynman forces on each atom were less than 0.01 eV/Å, and the energy is converged until the change is smaller than 1.0 × 10–5 eV/atom. Brillouin zone integrations were sampled using a Monkhorst–Pack[44] k-point mesh with a resolution of 2π0.03Å–1. To get an accurate electronic structure, we used hybrid functional implemented by the framework of Heyd–Scuseria–Ernzerhof (HSE06).[45] The absorption spectrum could be calculated bywhere E is the energy of the incident light, and and are the real and imaginary part of the frequency-dependent dielectric function, respectively.[46] The expensive ab initio molecular dynamics (AIMD) simulations were performed at a finite temperature at the NVT ensemble to analyze the dynamic properties. The phonon spectrum calculations were also carried out using a finite displacement approach with the PHONOPY code.[47,48] The qvasp was used to pre- and postprocess VASP calculated data.