First-Principle Study on High-Pressure Behavior of Crystalline

Article pubs.acs.org/JPCC

First-Principle Study on High-Pressure Behavior of Crystalline Polyazido-1,3,5-triazine Fang Wang, Hongchen Du, Jianying Zhang, and Xuedong Gong* Department of Chemistry, Nanjing University of Science and Technology, Nanjing 210094, People’s Republic of China ABSTRACT: A detailed study on the structural, electronic, and thermodynamic properties of the solid polyazide 4,4′,6,6′-tetra(azido)azo-1,3,5-triazine (TAAT) under the hydrostatic pressure of 0−100 GPa was performed using the plane-wave density function theory method. The predicted crystal structure compares well with the experimental results at the ambient pressure. The results show that a pressure less than 40 GPa does not significantly change the crystal and molecular structures. When the higher pressure is applied, the molecular geometry, band structure, and density of states change regularly except at 48 and 90 GPa where the azide−tetrazole transformation occurs. At 48 GPa, the tetrazole rings are almost coplanar with the triazine rings, whereas it has a big deviation at 90 GPa. The azide cyclization for polyazido-1,3,5-triazine has not been observed in the gas phase or polar solvents. Moreover, the band gap reduction is more pronounced in the low-pressure range than in the high-pressure region. The band gap decreases to nearly zero at 70 GPa. This means the electronic character of the organic crystal has metallic properties. An analysis of density of states shows that the electronic delocalization in TAAT increases generally under the influence of pressure. This shows that an applied pressure may increase the impact sensitivity.

1. INTRODUCTION Nitrogen-rich heterocyclic azides have currently attracted significant attention as effective precursors for carbon nitride nanomaterials and potential high-energy density materials (HEDMs)1−9 due to their relative high heats of formation (HOFs) raised by the azido group which adds about 87 kcal/ mol to a hydrocarbon compound. Carbon nitrides with the bulk formulas C3N4 and C3N5 have been prepared from cyanuric azide (TAT)10 as well as 3,6-diazido-1,2,4,5-tetrazine (DiAT).11 However, these polyazides are very sensitive to shock and friction and thus make the handling difficult and hazardous. Fortunately, Huynh et al. have made new progress on improving the stability of polyazide heteroaromatic compounds. They have synthesized two novel azides, 4,4′,6,6′tetra(azido)azo-1,3,5-triazine (TAAT) and 4,4′,6,6′-tetra(azido)hydrazo-1,3,5-triazine (TAHT), for which the introduction of azo and hydrazo linkages not only increases their densities and HOFs but also dramatically decreases their sensitivity to impact and friction. Moreover, the pyrolyses of TAAT can generate novel nitrogen-rich carbon nitrides C2N3 and C3N5.11 Meanwhile, great efforts have been dedicated to find a new precursor for the preparation of nitrogen-rich carbon nitrides. The isomeric tetrazoles, which are formed by the azide cyclization of polyazides, are found to be potential replacements for the hazardous polyazides because the aromatic tetrazoles are less sensitive than covalent azides.12 Recent studies showed that DiAT favors the tetrazoles in polar solvents, whereas TAAT has not been observed to undergo any azide−tetrazole tautomerization or tetrazolo transformation in water or polar solvents.12−14 As a unique class of HEDMs, many fundamental and practical problems of the polyazide heterocycles are still not well understood because they possess a complex chemical © 2012 American Chemical Society

behavior. Thus, it continues to inspire new research efforts to better understand their structure properties and decomposition mechanisms.7,14,15 It is known that intra- and intermolecular forces control diverse phenomena such as diffusion, aggregation, and detonation. It is also acknowledged that the performance of an explosive basically depends on the solid condition since the intra- and intermolecular interactions influence the strengths of some weak bonds that are referred to as initiators in the decomposition or detonation processes. After all, the crystal structure of a material determines its physical and chemical properties. Therefore, a desire to probe more fundamental questions related with the basic properties of solid TAAT as the nitrogen-rich material generates great significance in the crystalline phase properties of this energetic system. However, to the best of our knowledge, there has been no such investigation until now. High-pressure techniques have been applied extensively to study various materials since it can be used to tune the physical properties of materials. Many experimental efforts16−21 have been devoted to investigate the structures and properties of the energetic materials at different hydrostatic pressures. Results show that the changes in pressure may cause polymorphic transitions and changes in crystal structure and properties such as sensitivity, chemical reactivity, and performance. Besides experiments, computer simulation is an alternative approach to model the properties of solids under high pressure and can provide more detailed information on initiation and kinetics mechanism of energetic materials. Density functional theory (DFT) methods with pseudopotentials and a plane-wave basis Received: November 30, 2011 Revised: February 22, 2012 Published: February 22, 2012 6745

dx.doi.org/10.1021/jp211539z | J. Phys. Chem. C 2012, 116, 6745−6753

The Journal of Physical Chemistry C

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set have been applied to study solid polyatomic molecules22−27 owing to its reasonable accuracy and efficiency.22,28 Although it is known that DFT appears to be inadequate for the studies of crystalline energetic materials at low pressures because of low overlaps of electronic densities between molecules, there is evidence that DFT can better represent weakly bound molecular systems under sufficient degrees of compression with substantial overlap of electronic densities between molecules.26,29 It has been employed successfully to study the crystal structures and properties of many energetic materials such as hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), 1,3,5,7tetranitro-1,3,5,7-tetraazacyclooctane (HMX), 2,4,6,8,10,12hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20), hexanitrostilbene (HNS), and 2-diazo-4,6-dinitrophenol (DDNP) under varying degrees of compression.23,26,30−34 In this study, periodic DFT calculations were performed to study the high-pressure behavior of the crystalline TAAT. The variations in the structures and electronic properties were examined under hydrostatic compression in the range of 0−100 GPa.

directions a, b, and c. The full relaxation of the structure was performed to allow the atomic configurations, cell shape, and volume to change. In the geometry relaxation, the total energy of the system was converged to less than 5.0 × 10−6 eV, the displacement of atoms less than 5.0 × 10−4 Å, and the residual bulk stress less than 0.02 GPa.

3. RESULTS AND DISCUSSIONS To benchmark the performance of the theoretical approach, we applied two different functionals, LDA and GGA (generalized gradient approximation), to the bulk TAAT as a test. The LDA in the CA-PZ scheme and GGA in the PW91 (Perdew−Wang91)41 and RPBE (Revised Perdew−Burke−Ernzerhof)42 schemes were selected to fully relax the TAAT at ambient pressure without any constraint. Table 1 lists the calculated lattice parameters together with the experimental data. It is found that the errors of the LDA results are obviously smaller than those of the GGA results in comparison with the experimental values. The former reproduces the experimental lattice constants well, whereas the latter overestimates them. This shows that the accuracy of LDA is better than that of the GGA functional. Table 2 presents the bond lengths and bond angles along with the corresponding experimental data. Since TAAT is centrosymmetric in the structure, only half of the geometrical parameters are listed. Analysis of the results shows that the calculated geometrical parameters compare well with experimental values. This confirms that the computational results are reasonable. Consequently, LDA is used in subsequent calculations. 3.1. Crystal Structure. Studies in both experiments and computer simulations have shown that the external pressure may induce the molecular conformation changes, phase transitions, and the formation of more densely packed materials.16−20,23,24,27,31 This is because the binding forces that hold together molecules and atoms are changed by the compression. To show the effect of pressure on the crystal structure, Figure 3 depicts the relaxed lattice constants (a, b, c), unit cell volume (V), and density (ρ) in the pressure range of 0−100 GPa. The calculated lattice constants agree well with the experimental values at ambient pressure. With the pressure increasing, by and large, the lattice constants decrease. This is because the external pressure is large enough to overcome the intermolecular repulsion along the crystallographic directions and makes the crystal structure shrink. Consequently, V decreases significantly, and ρ increases correspondingly. When the pressure arrives at 90 GPa, ρ reaches the biggest value in the studied range of pressure, which increases by about 134% in comparison with that at ambient pressure. According to the well-known Kamlet−Jacobs formula,43 detonation velocity (D) and detonation pressure (P) increase tremendously with the increasing ρ and ρ2, respectively. Therefore, it is crucial to increase ρ for improving the performance of the explosive. For TAAT, one can increase its solid-state density to higher than 2.4 g/cm3 by compressing it under more than 10 GPa to improve its D and P. As is noted in Figure 3, the largest compression of the unit cell happens in the pressure region below 10 GPa. In the pressure range of 10−40 GPa and 55−80 GPa, the lattice parameters decrease slowly. The compressibility along the three directions is not equal at various pressures, which means the compressibility of TAAT is anisotropic. When the pressure increases from 0 to 10 GPa, the unit cell is compressed by 8.04%, 6.43%, and 12.70% along the directions of a, b, and c,

2. COMPUTATIONAL METHODS The first-principle calculations were performed using the DFT method with Vanderbilt-type ultrasoft pseudopotentials and a plane-wave expansion of the wave functions35 as implemented in the CASTEP code.36 The self-consistent ground state of the system was determined by using a band-by-band conjugated gradient technique to minimize the total energy of the system with respect to the plane-wave coefficients. The electronic wave functions were obtained by the Pulay density-mixing scheme,37 and the structures were optimized by the BFGS method.38 The local density approximation (LDA) with Ceperley−Alder exchange-correlation potential39 parametrized by Perdew and Zunger40 was employed. The cutoff energy of plane waves was set to 380.0 eV. Brillouin zone sampling was performed by using the Monkhost−Pack scheme with a k-point grid of 3 × 3 × 3. The values of the kinetic energy cutoff and the k-point grid were determined to ensure the convergence of total energies. The initial crystal was taken from Huynh et al.13 and used for the computations. TAAT crystallizes in a triclinic space group P-1 and contains two irreducible molecules with a total of 52 atoms per unit cell (Figure 1). Figure 2 shows two conformers exist in TAAT crystal. It is found that the geometries of the two conformers in the crystal structure are roughly the same as those in the gas phase.12 The periodic nature of the crystal was considered by using periodic boundary conditions in all three

Figure 1. Unit cells for crystalline TAAT. 6746



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Figure 2. Structures of two conformers (a1) and (a2) of TAAT.

Table 1. Experimental and Calculated Lattice Parameters for TAAT at Ambient Pressure method

a (Å)

b (Å)

c (Å)

α (deg)

β (deg)

γ (deg)

mean errora

exp. LDA PW91 RPBE

8.703 8.662 (−0.47) 9.510 (9.28) 9.502 (9.18)

9.140 9.171 (0.34) 9.699 (6.12) 9.715 (6.30)

10.198 9.475 (−7.09) 12.587 (23.43) 12.559 (23.15)

104.092 106.727 (2.53) 103.056 (−1.00) 103.150 (−0.91)

98.730 99.671 (0.95) 97.144 (−1.61) 97.145 (−1.61)

112.543 111.287 (−1.12) 109.659 (−2.56) 109.531 (−2.68)

2.08 7.33 7.30

a

Values in parentheses correspond to the percentage differences relative to the experimental data; the mean errors indicate the mean absolute values of differences.

Table 2. Experimental and Calculated Bond Lengths (Å) and Angles (°) for TAAT bond length C1−N4 C1−N10 C2−N4 C2−N6 C2−N5 N5−N5′ C3−N6 C3−N10 C3−N7 N7−N8 N8−N9 N9−N10

bond angle

LDA

exp.

1.336 1.329 1.325 1.329 1.412 1.266 1.341 1.330 1.389 1.260 1.171 3.324

1.340 1.329 1.322 1.323 1.448 1.197 1.341 1.328 1.391 1.265 1.109 3.221

C1−N4−C2 C1−N10−C3 C2−N6−C3 N4−C1−N10 N4−C2−N6 N6−C3−N10 N1−C1−N4 N1−N2−N3 C2−N5−N5′ C1−N4−C2−N6 N3−N2−N1−C1 N5−C2−N4−C1 N9−N8−N7−C3 N2−N5−N5′−C2′

respectively; therefore, the structure is much stiffer in a and b than in c directions with the sequence of c > a > b. This can be understood from the perspective view of crystalline TAAT along three directions shown in Figure 4. In TAAT crystal, the triazine conformer a2 is approximately vertical to the c-axis, and the intermolecular distance along the c-direction is the farthest; consequently, the repulsion interaction is somewhat weaker along the c-axis, and the crystal has larger compressibility along the c-direction than along the a and b directions. Furthermore, it is worth noting that curves a, b, and c have sudden changes in magnitudes at about 48 and 90 GPa, which suggests large changes in crystalline form have taken place. At 48 GPa, the value of c is anomalously larger than that at 40 GPa, whereas a and b decrease sharply. Between 48 and 54 GPa, the values of a, b, and c change little, and c is the largest. Between 55 and 80 GPa, a, b, and c decrease gradually. There is also another break around 90 GPa. In comparison with the lattice parameters at 80 GPa, c drops sharply, and b rises greatly. However, the crystal keeps the triclinic P-1 space group under all hydrostatic compression. The variations of crystal structure are presented in Figure 5. It is found that molecular structure transformation occurs at 48 and 90 GPa and therefore results in large changes

LDA

exp.

113.45 114.03 112.90 125.15 127.56 125.52 115.63 171.43 114.53 4.12 173.47 178.72 176.07 180.00

112.05 113.90 111.88 126.37 129.09 126.57 113.71 171.97 113.09 3.52 171.88 179.45 176.35 180.00

in crystal parameters. Detailed structure transformations will be illustrated in the following section. 3.2. Molecular Structure. For heterocyclic polyazides, such as DiAT and triazidotri-s-triazine, azide−tetrazole isomerization that defined as a 1,5-dipolar cyclization has been subjected to many studies.12,14,44 However, it was not observed for TAAT in the gas phase or polar solvents. In this work, the azide−tetrazole transformation has been observed for the conformer a1 but not a2 of TAAT during hydrostatic compression. To study the azide−tetrazole isomerization in detail, some selected geometrical parameters of conformer a1 under various pressures are presented in Figure 6. It is evident that when the pressure increases from 0 to 40 GPa the applied compression mostly squeezes out the intermolecular space and causes only a little change in molecular structure. For instance, bonds C3−N7 and C3−N10 shorten slightly, and the N7−N8 and N8−N9 in the azido group almost have no variety. In this pressure region, the bond angle of N7−N8−N9 approximates to the experimental value of 172.54°, and the interatomic distance between N9 and N10 is more than 3.2 Å. As the pressure arrives at 48 GPa, the linear azido group N7−N8−N9 bends about 60°, and the distance between N9 and N10 6747



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Figure 3. Lattice constants (a, b, c), density, and unit cell volume of TAAT as a function of hydrostatic pressure.

Figure 4. Perspective view of crystalline TAAT along various directions.

decreases remarkably by 1.58 Å. Meanwhile, bonds N8−N9, C3−N10, and C1−N10 increase by 0.13, 0.07, and 0.16 Å, respectively. Additionally, the tetrazole rings are almost coplanar with the triazine rings. These suggest that tetrazole rings have formed, and azide−tetrazole transformation has happened as can be seen clearly in Figure 5. In the pressure range of 48−54 GPa, crystalline TAAT still exists in the tetrazole forms, and the geometry changes little. As the pressure increases to 55 GPa, the bond angle N7−N8−N9 turns out to be 116°, and the distance between N9 and N10 is 2.26 Å; thus, the five numbered tetrazole ring was destroyed. From 55 to 70 GPa, the molecular structure remains the azide form and varies slightly. When 80 GPa is applied, N8−N9 significantly increases by 1.09 Å, and the azido group tends to deviate the triazine ring by 109.6°, which indicates a large geometrical distortion has occurred. As the pressure increases from 85 to 90 GPa, the distance between N9 and N10 decreases largely from 2.44 to 1.36 Å, with the bond angle N7−N8−N9 bending about 25°, which suggests that the bond N9−N10 has formed and the tetrazole ring appears again at 90 GPa, with the dihedral angle N6−C3−N7−N8 of 124.6°, whereas at 92 GPa the bond length N9−N10 and bond angle N7−N8−N9 increase greatly which shows the tetrazole ring is lost. The crystal TAAT keeps the azide form until 100 GPa. From 90 to

100 GPa, the bridged bond N5N5′ anomalously elongated from 1.410 to 2.383 Å. This suggests that the bridged bond has ruptured, and the interatomic force between N5 and N5′ is very small. The abnormal behavior has happened to resist the higher external pressure more than 100 GPa. Because both the external compression and tetrazole form contribute to more densely compact crystal structure, ρ at 90 GPa achieves the maximum value in the studied pressure range as illustrated in Figure 3. To gain insight into electron density redistribution in the cyclization by external compression, we examined the variations of Mulliken charges and bond order in conformer a1. Only the charge rearrangement during molecular transformation at 48 GPa is analyzed since another cyclization at 90 GPa has similar variations. In comparison with the azide form at 40 GPa, the charge on N9 becomes more positive from 0.02 to 0.21 e, and the ring nitrogen atom N10 attached to N9 loses a considerable amount of charges and becomes less negative from −0.39 to −0.13 e at 48 GPa. Moreover, the atom N8 carries positive charge (0.20 e) before the cyclization, but after the formation of the tetrazole ring, it bears a little negative charge (−0.02 e). The sum of positive and negative charges on N7, N8, and N9 is −0.07 e at 40 GPa, whereas it becomes −0.03 e when the tetrazole ring is built. This means that the electron is 6748



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Figure 5. Perspective view of the unit cell of crystalline TAAT at different pressures.

Figure 6. Geometrical parameters of crystal TAAT as a function of pressure.

ity.34,45−47 In this paper, the self-consistent band structure along different symmetry directions of the Brillouin zone for the optimized structure of bulk TAAT has been calculated at different pressures and illustrated in Figure 7. To be specific, only the energy bands between −0.8 and 4.0 eV were displayed. Simultaneously, the energy gap (ΔEg) between the highest occupied crystal orbital and the lowest unoccupied orbital is shown in Figure 8. By inspection of the figures, we can observe the following features: (i) In the studied pressure region, the electronic bands shift toward the lower energy regions from 0 to 40 GPa and shift to higher energy regions from 48 to 90 GPa. The abnormal changes are caused by the transformation in configuration of TAAT around 48 and 90 GPa. (ii) In the pressure region below 10 GPa, the energy bands are flat and fluctuate little because TAAT is a molecular crystal and therefore the molecular interactions are weak. When the pressure increases from 40 to 100 GPa, generally, the energy bands become less and less flat across the Brillouin zone, and the bandwidth increases remarkably, due to the increment of molecular interactions caused by large compression. (iii) Most

transferred from the azido group to the triazine ring. Additionally, when the cyclization happens, bending of the angle N7−N8−N9 promotes an electron transfer from the bond N7−N8 to C3−N7, concomitant with the attack of the lone pair on N10 to the bond of N8−N9. This is reflected from the charge transfer from N10 to N9 and the decrease in the positive charge on C3. Due to the electron transfer, the bond C3−N7 becomes stronger because the bond order increases from 0.83 to 1.03, and the bond N8−N9 turns to be weaker for the bond order decreases from 1.40 to 0.82. As a result, a conjugation tetrazole system is built. Under hydrostatic compression, the charge redistribution of the solid TAAT during cyclization is found to be similar to that of the azide− tetrazole isomerism of polyazido triazine in the gas phase.12,44 3.3. Electronic Structure. Additional insight into the pressure effect on the electronic structure of solid TAAT has been obtained from the band structure and density of state (DOS). Researchers have showed that the electronic band structure of a solid material relates strongly to explosive characters and physical properties such as impact sensitiv6749



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Figure 7. Band structures of bulk TAAT under different pressures. The Fermi energy is shown as a dashed horizontal line.

qualitatively interpret experimental spectroscopic data. A better understanding of band structure is partial density of state (PDOS), which is used to investigate the constitution of energy bands. PDOS analysis gives a qualitative handle on the band nature, such as electron hybridization and the origin of main feature peaks in optical spectra.30,34,47 Therefore, to obtain further information about the bond nature of the crystal TAAT, the DOS and PDOS are calculated and displayed in Figure 9. Several features are summarized as follows. First of all, the curves of DOS are characterized by distinct peaks under low pressure. However, with the increasing pressure, the characteristic peaks in the DOS spectrum become more and more wide and dispersed; furthermore, they have a tendency of shifting to the lower energy. This means that the band splitting and band dispersion increase accompanied by a broadening of DOS, which is caused by the enhanced intermolecular interactions with the increasing pressure. Consequently, electronic delocalization in bulk TAAT gradually increases. At 70 GPa, the band gap reduces to nearly zero, and the DOS becomes almost continuum near the Fermi energy level, which indicates the electrons have become completely delocalized and can move freely in the valence and conduction bands. Thus, the molecular crystal TAAT turns into metal, which is in good agreement with the band structure analysis. Second, at the low or high pressure region, the DOS is mainly contributed from the p states from −5 eV up to the Fermi level, which suggests that the p states play a very important role in the chemical reaction of solid TAAT. Finally, with respect to the PDOS, it is seen that at 0 GPa the upper valence bands are predominated by the p states of the C and N atoms in ring, NN bridge, and −N3 groups. The lower conduction bands have the same constitution. Moreover, the N atoms in the NN bridge make more important contributions to both the valence and conduction bands than others. This means that the N atom in the bridged NN acts as an active center, which is consistent with the molecular structure analysis that the bridged bond has broken at 100 GPa. As the pressure increases, all PDOS become more and more dispersed in the valence bands, and those in the conduction bands have a tendency to shift to the lower energy. This shows that the compression greatly increase the

Figure 8. Band gap of crystal TAAT as a function of pressure.

importantly, the band gap reduces continuously from 1.65 to 1.00, 0.76, 0.74, 0.58, and 0.03 eV with the pressure ranging from 0 to 10, 20, 40, 48, and 70 GPa. The rapid closure of the band gap implies that the energetic molecular crystal TAAT undergoes an electronic phase transition from a semiconductor to a metallic system subjecting to the high degree of compression. This also indicates that the probability of electron transitions from the occupied valence bands to the empty conduction bands increases significantly. According to the principle of easiest transition (PET),48 it can be concluded that the decreasing band gap leads to the increasing impact sensitivity of crystal TAAT. This is supported by the previous works that an applied pressure increases the sensitivity of energetic materials.45,46,49 However, at 90 GPa, the band gap rises anomalously to 0.63 eV. This is caused by the structural transformation at 90 GPa, and the lowest conduction band shifts to a higher energy region resulting in a larger band gap. When 100 GPa is exerted, the band gap decreases to zero. In addition, the decrease in ΔEg (0.65 eV) from 0 to 10 GPa is the maximum energy reduction and is more pronounced in comparison with the high pressure region. This indicates that a subtle change in the electronic structure happens despite no obvious molecular geometry variation. It is known that the density of state (DOS) is helpful for analyzing the changes in electronic structure and can be used to 6750



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Figure 9. Total and partial densities of states (DOS) of crystal TAAT at different pressures. The Fermi energy is shown as a dashed horizontal line.

Through inspection of Figure 10, it is also seen that the increments of H and G increase as the pressure increases. This is because the high-pressure compression leads to the increasing intermolecular interactions and the successively rising energy of the unit cell. However, for S and Cp, the increments decrease with the increasing pressure. The difference between the values at different pressures decreases with the increasing pressure for H and Cp but increases for G and S. At 298.15 K, the change in free energy (ΔG) for the azide− tetrazole transformation that occurred at 48 and 90 GPa is about 2.5 kcal/mol. The small ΔG suggests the tetrazole ring may be facile to build as was found in the molecular transformation according to the fundamentals of chemical thermodynamics. However, investigations on the azide− tetrazole isomerism have shown that TAAT does not form any tetrazoles in the gas phase or in solution.12,13

probability of electronic excitations, and then the crystal TAAT becomes more energetic, which is also supported by Kuklja’s work that the increased number of excited states due to external compression gives rise to increased sensitivity.50 Additionally, azide−tetrazole transformation can also be seen in the variation of PDOS. For instance, in the upper valence bands, when the pressure changes from 0 to 48 GPa, the PDOS of the N atom in −N3 move toward the Fermi level and overlap with that of the N atom in ring. This indicates hybridization occurs between the N atom in ring and the −N3 group according to the principle of energy matching; thus, the azido group cyclizes, and the fivemembered tetrazole ring forms. 3.4. Thermodynamic Properties. The calculated thermodynamic functions including enthalpy (H), entropy (S), free energy (G), and heat capacity (Cp) as a function of temperature for crystalline TAAT at 0, 40, 48, 70, and 90 GPa are depicted in Figure 10. Obviously, H of the solid monotonically increases with the increase of temperature from 0 to 600 K. This is because the vibrational motion is intensified at higher temperature and makes more contributions to the enthalpy. The same is true for S and Cp. However, G decreases as the temperature increases.

4. CONCLUSIONS In this study, the structure and electronic properties of the energetic polyazide 4,4′,6,6′-tetra(azido)azo-1,3,5-triazine (TAAT) in the range of 0−100 GPa have been investigated using the density function theory method. The predicted crystal 6751



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Figure 10. Thermodynamic properties of crystalline TAAT as a function of temperature.

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structure with the LDA/CA-PZ functional agrees well with the experimental data at the ambient pressure. The results show that a pressure less than 40 GPa does not significantly change the crystal structure and geometric parameters. With the further increasing pressure, the molecular structure, band structure, and density of states change regularly except at 48 and 90 GPa where the structural transformation occurs; that is, the azido group cyclizes, and the tetrazole ring is built. At 48 GPa, the formed tetrazole rings are almost coplanar with the triazine rings, whereas a big deviation exists at 90 GPa. The compressibility of TAAT is anisotropic. The band gap drops to nearly zero at 70 GPa, which means the the electronic character of the crystal changes toward a metallic system. Moreover, the band gap reduction is more pronounced in the low-pressure range than in the high-pressure region. An analysis of density of states shows that the electronic delocalization increases under the influence of pressure. This shows that an applied pressure may increase the impact sensitivity. This work can provide useful information in understanding the highpressure behavior for the polyazido-1,3,5-triazines and may offer enhanced opportunities to discover some new structures that are generally inaccessible at the ambient pressure.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: + 86-25-84315947-803. E-mail address: gongxd325@ mail.njust.edu.cn. Notes

The authors declare no competing financial interest.

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REFERENCES

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ACKNOWLEDGMENTS

We greatly thank the National Natural Science Foundation of China (NSAF grant no. 11076017) for the support of this work. 6752



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