Difference in the Thermal Stability of Polymorphic Organic Crystals: A

Article Cite This: J. Phys. Chem. C 2019, 123, 16565−16576

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Difference in the Thermal Stability of Polymorphic Organic Crystals: A Comparative Study of the Early Events of the Thermal Decay of 2,4,6,8,10,12-Hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) Polymorphs under the Volume Constraint Condition Guangrui Liu,†,‡,§ Ying Xiong,† Ruijun Gou,‡ and Chaoyang Zhang*,†,§

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†

Institute of Chemical Materials, China Academy of Engineering Physics (CAEP), P.O. Box 919-311, Mianyang, Sichuan 621900, China ‡ College of Environment and Safety Engineering, North University of China, Taiyuan 030051, China § Beijing Computational Science Research Center, Beijing 100048, China S Supporting Information *

ABSTRACT: Polymorphism is universal in organic crystals like conventional energetic materials (EMs), and it may cause a difference in thermal stability, one of the most important properties of EMs. Nevertheless, deep insights into the differences of polymorphic EMs are lacking. 2,4,6,8,10,12-Hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) is the most powerful EM commercialized already and possesses three polymorphs stabilized at common condition, which exhibit many differences in properties and performances. However, the underlying mechanism responsible for these differences remains unclear. In this work, a self-consistent charge density functional tight binding scheme and molecular dynamics simulations are combined to reveal the difference in the decay mechanism of the three polymorphs of CL-20, with considerations of an extreme of volume constraint and two heating types. The lower thermal activity of ε-CL-20 is distinguished from β- and γ-CL-20 at a relatively low temperature of 1000 K, as only εCL-20 does not decompose in the time scale of the simulation of 20 ps. The lower thermal activity of ε-CL-20 is partly responsible for its lower impact sensitivity. Two relatively high temperatures of 1500 and 2000 K cannot differentiate their decay activities. Moreover, the unimolecular N−N breakage governs the first steps of the thermal decay of all of the three polymorphs. Five types of bond cleavages, including the NO2 partitions from the 5- and 6membered rings, the cleavage of C−N bonds of the 5- and 6-membered rings, and the C−C bond breakage, are observed in the decay. Interestingly, all of the five types of ignitions are not be observed in any case. Besides, we find that the low temperature disfavors the formation of the stable products of N2 and CO2. These results of the thermal decay of polymorphic EMs are expected to give deep insights into the complicated sensitivity mechanism of EMs. wurtzitane (CL-20) is also polymorphic.14 CL-20 possesses a cage-shaped molecular structure and outperforms the common traditional EMs like 2,4,6-trinitrotoluene, RDX, and HMX in some properties and performances, such as packing density, detonation velocity, detonation pressure, and oxygen balance.15−20 However, its practical applications still remain limited because of high cost and high sensitivity. The high sensitivity of an EM represents its high response degree to an external stimulation. Currently, the sensitivity mechanism, i.e., the mechanism of an EM against an external stimulation till the final combustion and/or detonation, is still unclear and requires extensive investigation. The high sensitivity of CL-20, e.g., the high mechanical sensitivity, is strongly related to the evolution after loading an external

1. INTRODUCTION Polymorphism is universal in organic crystals. For example, most CHON-containing energetic materials (EMs), such as 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX),1 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX),2−7 1,1-diamino-2,2dinitroethylene (FOX-7),8−10 and dihydroxylammonium 5,5′bistetrazole-1,1′-diolate (TKX-50),11−13 are polymorphic, and the polymorphic transition is usually induced by heat or stress. In principle, differences in properties and performances exist among the polymorphs, as their molecular and packing structures are different from one another. For EMs, thermal stability is one of the most important properties and attracts much attention. Thus, it is of interest to get insights into the difference in thermal stability among polymorphs, or the variation of thermal stability caused by polymorphic transition, to evaluate EMs. As the most powerful energetic material (EM) commercialized already, 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaiso© 2019 American Chemical Society

Received: May 1, 2019 Revised: June 16, 2019 Published: June 19, 2019 16565

DOI: 10.1021/acs.jpcc.9b04126 J. Phys. Chem. C 2019, 123, 16565−16576

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The Journal of Physical Chemistry C

graphic parameters but also their molecular conformers.38−42 In principle, the ready mutual transformations of CL-20 are proved by the low transformation barriers. Moreover, the differences in the total energy and transformation barrier among the molecule conformers involved in the three polymorphs under common conditions are small. This favors the ready polymorphic transformation. This ready transformation among the molecular conformers is also a reason for the presence of a large amount of CL-20-based cocrystals, and various molecular conformers exist in these cocrystals, too.41 Under common conditions, ε-CL-20 is more thermodynamically stable than the other polymorphs, due to its advantage of high lattice energy.41 Meanwhile, ε-CL-20 is the most closely packed among all of the three forms and thus exhibits the highest power. Consequently, ε-CL-20 possesses the highest stability and the highest power, leading to a satisfaction of both high energy and high safety, which are highly desired for usual applications but generally conflict with each other, as the socalled energy−safety contradiction.43−45 The satisfaction also suggests that the contradiction does not exist always for the three polymorphs of CL-20. Besides, a similar case also occurs for HMX. In other words, ε-CL-20 is desired in practical applications, and it can be transformed into other forms with elevated sensitivity. Even though it has already been ascertained that ε-CL-20 possesses the lowest impact sensitivity among all of the polymorphs, the underlying mechanism has not been well understood. As aforementioned, revealing the thermal decomposition mechanism is helpful to understand the sensitivity mechanism; thus, a comparative study of the thermal decay details of all of the three CL-20 polymorphs is necessary because almost all related studies cover the ε-form only and never the other two. This motivates us to carry out comparative MD simulations of the thermal decay of all three polymorphs stabilized at ambient condition. Although the heat-induced polymorphic transformation of CL-20 can take place when heating to final decay, the original polymorphs can also stay until the decay when they are volumetrically constrained or heated at a very fast rate. Combining the self-consistent charge density functional tight binding (SCC-DFTB) scheme and MD simulations, this work compares the mechanisms of the early thermal decay of the three polymorphs in a case of constrained volume and shows a certain dependence of polymorphs at relatively low temperature and no significant difference in the decay at relatively high temperature. This suggests that the worsened impact sensitivity could result from the variation of molecular stacking, instead of that of thermal stability. These findings are expected to deepen insights into the properties and performance of polymorphic CL-20. Because, currently, insights into the variation of thermal stability after the polymorphic transition of EMs are lacking, we hope that this work will pave an avenue to evaluate EMs through MD simulations by considering some specified conditions.

mechanical stimulation. According to the widely accepted theory of hotspot, the thermal decomposition of the CL-20 molecules is inevitable prior to the hotspot formation and growth, as a premise of the final combustion or detonation. Hence, the thermal decomposition of CL-20 has attracted extensive attention, as lots of experiments and calculations have been performed since its discovery. Patil et al. found that, below 204 °C, the decomposition data does not obey an autocatalytic model and the N−NO2 homolysis dominates the initial step of the decomposition with the main intermediate of NO2, whereas, in the range of 250−400 °C, NO2, NO, CO2, HCN, CO, and N2O exist as main products.21 These results were also obtained by other experimental studies.22−25 Besides, molecular simulations were applied for detailing the primary steps of the thermal decay of CL-20. Five possible primary steps of the unimolecular decomposition were ascertained by Okvytyy et al. with density functional theory (DFT) calculations, including N−NO2 homolytic cleavage, HONO elimination, C−C and C−N bond breakage, and Hmigration.26 By means of ab initio molecular dynamics (AIMD) simulations, Isayev et al. studied the thermal decay of gaseous and solid CL-20. For the gaseous CL-20, only one distinct initial channel, the homolysis of the N−NO2 bond, was observed, with HCN, NO2, NO, CO, and OH as the main products; with regards to the solid decomposition, NO2, NO, N2O, and N2 were produced at the very early stage.27 Moreover, the MD simulations based on a reactive forcefield of ReaxFF-lg were performed to reveal the thermal decay mechanism of ε-CL-20 at various temperatures, with the N− NO2 cleavage observed as the initial reaction step, followed by the cage breakage through the C−N cleavage. Meanwhile, the simulations showed that the simulation temperatures do not change the initial reaction path, whereas they have a significant influence on the production rates of H2O and N2 and a little influence on those of CO2 and H2.28 In comparison, the two paths were also observed to trigger the molecular decomposition of the shocked CL-20, the NO2 fission, and the ring opening through the C−N bond cleavage.29 The low molecular stability of CL-20 is partly responsible for its high sensitivity. For example, in comparison, CL-20 is more impact sensitive than RDX and HMX, as a DFT calculation with B3LYP functionals and 6-311++G(d,p) of the dissociation energy of the weakest bond (BDE), i.e., the N−NO2, showed a lower BDE of 32.5 kcal/mol than those of RDX (33.8 kcal/mol) and HMX (37.1 kcal/mol).30 Besides the intrinsic low molecular stability, the ready polymorphic transformation of CL-20 is also partly responsible for the high sensitivity. It has been confirmed there are four polymorphs for CL-20: the β-, γ- and ε-forms at ambient conditions and the ζ-form at above 3.3 GPa.31,32 Another form of α-CL-20 is a hydrate of CL-20 only and not a polymorph.33 Crystallization conditions like the solvent, addition method, temperature, and agitation rate can significantly influence the CL-20 polymorphs and their transformations. For instance, the CL-20 polymorphs are transformed more readily in solutions than in solids. The transformations in solutions start at low temperatures, even at room temperature.34 By contrast, those in solids of β → γ and ε → γ are initiated above 400 K.35 In addition, experimental results show that β-CL-20 is initially precipitated from many solutions and then transformed into a more stable form of ε-CL-20.36,37 Besides, for these three polymorphs of CL-20, it is interesting to find that they are distinguished from one another by not only their crystallo-

2. METHODOLOGIES As aforementioned, we will focus on the mechanisms of the primary thermal decay of the three polymorphs of CL-20 at ambient condition in this work. Thereby, the SCC-DFTB MD simulations were performed to reveal the initial reaction steps at the atomic level and energy variations. An MD simulation, in principle, requires a model with adequate molecules and atoms 16566


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Figure 1. Supercells of the three polymorphs of CL-20 for MD simulations.

Table 1. Comparison of Relaxed and Experimental Cell Parameters of ε-CL-20 method 17

EXP this work DFTB16 GGA-PW9116 GGA-PBE16 ReaxFF-lg15 CPMD14

a, Å

b, Å

c, Å

α, deg

β, deg

γ, deg

density, g/cm3

8.852 8.719 8.661 9.122 9.370 9.035 9.095

12.556 12.226 12.244 12.937 13.123 12.837 12.607

13.386 13.617 13.449 13.418 13.796 13.655 13.741

90.0 90.0 90.0 90.0 90.0

106.8 109.2 108.7 106.5 109.4

90.0 90.0 90.0 90.0 90.0

2.04 2.12

111.4

for adequate free degrees to be much closer to practice. Considering this and the cost of the SCC-DFTB method, 3 supercells of 2 × 1 × 1, 2 × 1 × 1, and 1 × 2 × 1, each with 8 CL-20 molecules or 288 atoms, were established by enlarging the experimentally determined unit cells for β-, ε-, and γ-CL20, respectively, as illustrated in Figure 1. Prior to the MD simulations, the feasibility of the DFTB method for the CL-20 polymorphs at ambient condition was verified. It will be discussed in the next section. The principle of SCC-DFTB can be found elsewhere. Briefly, SCC-DFTB is based on a second-order expansion of the Kohn−Sham total energy in DFT with respect to charge density fluctuations. This method allows the description of total energies, atomic forces, and charge transfer in a self-consistent manner. It has been successfully tested on organic and bio-organic systems46,47 and shown to accurately predict reaction energies.48−50 Besides, three dispersion correction schemes, including Lennard-Jones, Slater Kirkwood, and DftD3, has been implemented in DFTB. We used in this work the Lennard-Jones scheme, in which the dispersion is included via a Lennard-Jones potential between each pair of atoms.51,52 In fact, this method has already extensively been ascertained to be viable for many other condensed EMs, including HN3, TATB, HMX, TKX-50, FOX7, and 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105), and thus has been employed to successfully reveal the reaction mechanisms of these EMs against heating, pressure, and shock.48−50,53−60 To relax the internal stress of the three supercells of CL-20, we first optimized the cell parameters and atomic positions using the conjugate gradient minimization method. Subsequently, an NVT (constant number of atoms, constant volume, and constant temperature) simulation at 300 K for 5 ps was performed with a time step of 0.25 fs to further relax the stress of each supercell. In our DFTB-MD simulations, two heating conditions were considered, including constanttemperature heating at 1000, 1500, and 2000 K and programmed heating from 300 to 2000 K at a rate of 85 K/ ps. The selection of the 1000, 1500, and 2000 K for the constant-temperature heating is based on an experimental fact that CL-20 is a thermally moderate EM (its differential scanning calorimetry temperature peak is ∼230 °C at a heating rate of 10 °C) and these temperatures are appropriate for

1.92 1.88

relative errors, % 3.8 2.6 2.1 4.5 5.6 6.0

simulations to check dominant primary steps within a time scale limit of several tens of picoseconds. Also, the programmed heating was adopted to resemble cook-off in practice as usual. All our MD simulations were performed by DFTB+ software with 3ob-3-1 parameter set,61 and each lasted for 20 ps with canonical ensembles and a time step of 0.2 fs. Besides, the MD trajectories were recorded per 10 steps (2 fs). In the simulations, the SCC tolerance was set to 1.0 × 10−6 e, and the temperature control was implemented by Berendsen thermostats62 with a coupling strength of 0.1. After the simulations, a series of FORTRAN scripts written by us63,64 were applied to trap the highly frequent primary reactions of the earlier decay of CL-20. To analyze the chemical species evolution in the DFTB-MD simulations, the atomic connectivity was determined by bond distance cutoffs and a lifetime criterion to avoid miscounting the short-term fluctuations in bonds above/within the cutoffs. Cutoffs do not actually break/form a bond. Two atoms were counted as bonded once their distance was less than 1.33 times of the normal covalent bond length and they survived at least 20 and 45 fs for bonds involving and not involving H atoms, respectively. These time scales were chosen in terms of the characteristic vibrational frequency of the related covalent bonds based on the fact that the bond between two atoms should maintain at least a vibrational period to complete one vibration.53,64

3. RESULTS AND DISCUSSION 3.1. Validation of the DFTB Method for the Polymorphs of CL-20. The validation of the SCC-DFTB method for the three polymorphs of CL-20 under common conditions is the base of this study. Thus, it is implemented by comparing the cell parameters derived from various methods. Previously, all of the simulations aiming at the condensed CL20 were performed on ε-CL-20, as it the most stable at ambient condition. As listed in Table 1, the cell parameters derived from the SCC-DFTB method are comparable to those from DFT methods, with a relative error in the experimental observation of 3.8%.27−30 It suggests the validation. Furthermore, we implement the validation of the DFTB method by comparing the cell parameters of relaxed and experimental observations. As illustrated in Table 2, the 16567


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The Journal of Physical Chemistry C Table 2. Comparison of Relaxed and Experimental Cell Parameters of Three Polymorphs of CL-20 crystal β-CL-20 ε-CL-20 γ-CL-20

+

DFTB Exp. DFTB+ Exp. DFTB+ Exp.

a, Å

b, Å

c, Å

α, deg

β, deg

γ, deg

density, g/cm3

relative error, %

19.512 19.352 17.440 17.704 13.843 13.231

12.567 13.006 12.227 12.556 15.025 16.340

11.586 11.649 13.619 13.386 15.119 14.876

90.0 90.0 90.0 90.0 90.0 90.0

90.0 90.0 109.2 106.8 112.8 109.2

90.0 90.0 90.0 90.0 90.0 90.0

2.05 1.99 2.12 2.04 2.01 1.92

3.0 3.8 4.7

Figure 2. PE evolution of the potential energy of the three polymorphs of CL-20 under heating.

relative errors of densities of β-, ε-, and γ-CL-20 are 3.2, 3.8, and 4.7%, respectively, showing the reliability of the DFTB method applied to the three polymorphs of CL-20 at ambient condition. In combination with previous successful applications of the method on the decay mechanisms of EMs, such as HN3, TATB, HMX, TKX-50, and so forth,48−50,53−60 we believe that our simulations of this work should be reliable. 3.2. Evolution of Potential Energy (PE). The PE evolution of an energetic material loaded by external stimuli reflects its variation tendency under loading; thus, it is usually employed to describe reaction stages, such as endothermic stage, exothermic stage, and equilibrium stage. The endothermic stage is also called the delay interval or induction interval. Similar to almost all simulated results of heated EMs, the PE first increases, subsequently reduces, and finally reaches equilibrium.28,53,58 It does so for the three polymorphs of CL-20. Figure 2a illustrates the PE evolution under the programmed heating from 300 to 2000 K. With the increasing temperature, the PE of the three polymorphs successively increases and three PE curves overlap one another till ∼17 ps when the PE becomes fluctuant and the curves become differentiable. In comparison, γ-CL-20 first tends to decrease its PE, showing the lowest thermal stability, whereas ε-CL-20 shows the highest thermal stability as it exhibits the highest PE and the latest PE reduction; β-CL-20 mediates between the other two forms. Figure 2b−d demonstrates the cases of constant-temperature heating at 1000, 1500, and 2000 K, respectively. Under the constant-temperature heating, the PE of each case first immediately increases to a peak, followed by a reduction. A similar tendency was also found in a recent ReaxFF simulation on ε-CL-20.28,65 At 1000 K, wholly, the PE changes mildly, as

demonstrated in Figure 2b. The PE reduction takes place after 10 ps for β-CL-20 and 16 ps for γ-CL-20, and it does not occur within the time scale of simulation of 20 ps for ε-CL-20. This suggests the highest thermal stability of ε-CL-20, followed by γ-CL-20 and β-CL-20. Comparing the PE evolutions in Figure 2a,b, we find an inconsistency of the thermal stability between γ-CL-20 and β-CL-20, suggesting a temperature dependence of the thermal decomposition of the CL-20 polymorphs. At the earlier stages of the thermal decomposition of CL-20 heated at 1500 K (Figure 2c) and 2000 K (Figure 2d), the PE of γ-CL20 decreases the fastest, followed by β-CL-20 and ε-CL-20. It agrees with the above case of the programmed heating. Summarily, from the viewpoint of the PE evolution, ε-CL-20 always exhibits the highest thermal stability. 3.3. Initial Decomposition Reaction Steps of CL-20. It is of interest to clarify the initial steps of the decomposition to understand the properties and performances of EMs.53,57 In our simulations, five steps are observed, as shown in Figure 3: the NO2 partitions from the 5-membered ring (A) and the 6membered ring (B), the cleavage of C−N bonds of the 6membered ring (C) and the 5-membered ring (D), and the C−C bond breakage (E). Previously, Okovytyy et al. assumed that there are five possible types of steps initiating the CL-20 decay including the homolytic cleavage of the N−N bond, Hmigration for the HONO elimination, and C−C and C−N bond breakage leading to ring opening.26 Maybe, an isomerization of NO2 to ONO is another type of the initial step, as it is a possible thermal decomposition path for NO2-containing compounds.66 In the present work, the H-migration and the NO2 to ONO isomerization are not found. Besides, in our simulations, all of the CL-20 molecules are decayed in a 16568


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partition, dominates the initial steps of the thermal decay of βCL-20; the C−N (C and D) and C−C (E) bond breakages have similar probabilities (Figure 4a−d). In a previous study of shocked ε-CL-20, the ring opening was observed to trigger molecular decay at all four shock conditions, whereas the sufficient NO2 fission was observed at shock velocities Us = 8 and 9 km/s and strongly inhibited at Us = 10 and 11 km/s.29 This suggests a dependence of Us on the initial decomposition step, different from the case of thermal decay. The dominance of the N−N cleavage is also found for both ε- and γ-CL-20. For ε-CL-20, no reaction takes place at 1000 K; the CL-20 molecules are most decayed from the NO2 partition and little from the C−N bond breakage; and the C− C bond breakage occurs only once (Figure 4e−h). With regard to γ-CL-20, all of the thermal decomposition proceeds from the N−N scission at 1500 K and under the programmed heating from 300 to 2000 K and the C−C bond breakage never happens (Figure 4i−l). In Figure 4, we can conclude that primary decomposition of all of the three polymorphs of CL-20 proceeds in a unimolecular manner under various heating conditions; the NO2 partition governs the initial decomposition step, the C−N cleavage happens sometimes, and the C−C breakage seldom occurs. In fact, the dominance of the NO2 partition in the primary decay was also found in the CL-20/HMX cocrystal.67 Moreover, all of the five types of ignitions are not be observed in any case. Besides, the secondary reactions of CL-20 are concerned. Figure 5 shows the case of β-CL-20. In the case of the programmed heating from 300 to 2000 K, the breakage of C− N and C−C bonds follows the NO2 partition, as the secondary reactions, whereas the NO2 partition follows the C−C bond breakage. At 1000 K, the breakage of C−N bonds dominates the secondary reactions, with a little C−C cleavage. At relatively high temperatures of 1500 and 2000 K, the NO2

Figure 3. Five initial thermal decomposition steps of CL-20 found in this work.

unimolecular way, without bimolecular reaction observed as the first step. The percentage statistics of each step under four heating conditions are listed in Figure 4. For β-CL-20, A increases and B decreases with the increasing temperature, C is never observed, D occurs under all three constant-temperature heating conditions, and E takes place at relatively high temperatures. If we divide the above five types of steps into three classes in terms of the types of bonds broken, we can readily find that the N−N cleavage (A and B), or the NO2

Figure 4. Percentages (%) of the five initial thermal decomposition steps of CL-20 under four heating conditions. 16569


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Figure 5. Primary and secondary decomposition steps of β-CL-20 under four heating conditions. A fraction and a bond above an arrow represent the ratio of the number of the CL-20 molecules proceeding in a primary step to decay to the total and the bond broken in the step, respectively. If the ratio is equal to 1, it will not be marked. These presentations are also considered in the following figures.

Scheme 1. Reaction Showing a Ready N−N Cleavage as a Consequence of C−C Breakage

heating from 300 to 2000 K and heating at 1500 K, it is the only first step for decomposition. As shown in Figure 7, the NO2 partition is mainly followed by the C−N breakage, with a little N−N or C−C cleavage. From the above discussion, we can know that the primary decomposition steps of the three CL-20 polymorphs are largely governed by the N−N breakage, partly by the C−N breakage, and a bit by the C−C breakage, which are responsible for the NO2 partition, the ring opening, and the cage opening, respectively. All of the CL-20 is decayed in a unimolecular manner first, and the secondary reactions are almost unimolecular, appear in above simulations too. 3.4. Evolution of the Small Molecular Intermediates and Products during the CL-20 Thermal Decomposition. The evolution of small molecular intermediates and products during the thermal decay of an EM is also responsible for its properties and performances. With the help of some previous results,21,24,27,29,65 we checked 23 molecular fragments (C6H6N12O12, C6H6N11O10, C6H6N10O8, C2H2N4O4, N2O5, N2O4, N2O3, N2O2, N2O, N2, NO3, NO2, NO, CO2, CO, H2O2, H2O, HONO, HO, HNO3, HCO2, HCNO, and HCN), among which the 9 most common molecules with the highest contents were employed for discussion, including N2O,

partition becomes dominant in the secondary reactions. This should be attributed to the fact that the NO2 partition contributes to an increase of molecular numbers and thus an enhancement of the entropic effect (TΔS). A similar case was also exhibited in the thermal decomposition of another impactinsensitive EM, 2,2-dinitroethylene-1,1-diamine (FOX-7).60 Interestingly, it is found that the NO2 partition, as a secondary reaction, always follows the C−C cleavage. It should be rooted by the fact that the NO2 partition facilitates the formation of a double bond between C and N atoms, with a high thermodynamic advantage, as demonstrated in Scheme 1. The case of ε-CL-20 is similar to that of β-CL-20, with the main difference of no reaction at 1000 K as aforementioned. As demonstrated in Figure 6, the first step of the thermal decay of ε-CL-20 is dominated by the N−N bond cleavage, with a little breakage of the C−N and C−C bonds. Following the NO2 partition, the C−N breakage occurs as the main secondary reaction, with a little C−C breakage or NO2 partition; the primary step of C−N or C−N scission is generally followed by the NO2 partition. Same as β-CL-20, the NO2 partition still appears as the secondary reaction after the C−C breakage in εCL-20. The NO2 partition also governs the initial decay of γCL-20; particularly, in the two cases of the programmed 16570


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Figure 6. Primary and secondary decomposition steps of ε-CL-20 under three heating conditions.

Figure 7. Primary and secondary decomposition steps of γ-CL-20 under four heating conditions.

Figure 8 illustrates the evolution of some important small molecules and CL-20 in the case of programmed heating from 300 to 2000 K. The three polymorphs of CL-20 exhibit similar

NO3, NO2, NO, H2O, HONO, HNO3, HCN, and CO. These molecules almost always appear in the decay of CHONcontaining EMs. 16571


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Figure 8. Evolution of some important small molecules, as well as CL-20, during the thermal decay of the three CL-20 polymorphs under the programmed heating from 300 to 2000 K.

Figure 9. Evolution of some important small molecules, as well as CL-20, during the thermal decay of the three CL-20 polymorphs heated at 1000 K.

evolution with a small difference in thermal stability. ε-CL-20 possesses the highest thermal stability and is the most slowly decayed, followed by β- and γ-CL-20 (Figure 8a). The CL-20 molecules in the three polymorphs disappear at ∼15 ps (Figure 8a), when the number of NO2 arrives at a peak (Figure 8b), and some N-containing small molecules are found to increase, including NO, NO3, and N2O (Figure 8b−e). That is, the reduction of NO2 goes with the increase of NO, NO3, and N2O together. This verifies again that the NO2 partition dominates the primary decay of all of the polymorphs and NO2 is converted into NO, NO3, and N2O. Meanwhile, a small amount of HCN, HONO, and HNO3 is formed (Figure 8f−i). At the end of the time scale of the simulation of 20 ps, γ-CL-20 produces more N2O, CO, and H2O, whereas ε-CL-20 produces more NO3, HCN, and HNO3. Figure 9 shows the case of these polymorphs heated at 1000 K. At this relatively low temperature and in the time scale of the simulation of 20 ps, β-CL-20 is almost completely decayed, most molecules in γ-CL-20 are decomposed, and no decay is observed in ε-CL-20. This suggests the highest thermal stability. Due to the highest decay degree of β-CL-20 in the

20 ps simulation, it produces the largest amounts of NO2, NO2, N2O, and HNO3. With the temperature increasing to 1500 K, all of the CL-20 molecules in any polymorph disappear shortly in 3 ps (Figure 10). Similar to the case of 1000 K, it takes the longest time to completely consume ε-CL-20, verifying its highest thermal stability again. Therein, ε-CL-20 produces more slowly the main intermediates and stable products like NO, NO2, NO3, N2O, H2O, HNO3, and HCN, whereas it seems that no CO is formed in β-CL-20 and a little more HONO is produced in εCL-20 in contrast to other two polymorphs. This suggests the complexity of the CL-20 decay at high temperature. As can be seen in Figure 11 at 2000 K, the CL-20 molecules disappear around ∼0.3 ps faster relative to 1000 and 1500 K (Figure 11a). By the end of the simulation time scale of 20 ps, β- and γ-CL-20 produce more NO (Figure 11c) and less CO (Figure 11f) than ε-CL-20; γ-CL-20 produces less HCN (Figure 11g) and more N2O (Figure 11e) and H2O (Figure 11h) than β- and ε-CL-20; and there is no evident difference in the formation of other small molecules for the remaining cases. Totally, the decay rates of CL-20 molecules in the three polymorphs at 2000 K are close to one another. Comparing 16572


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Figures 8−10, we find that the thermal decomposition reactivities of the three polymorphs are distinguished at the relatively low temperature, i.e., in the time scale of the simulation of 20 ps, β- and γ-CL-20 begin the decay, without any reaction found in ε-CL-20. Moreover, we are aware that N2 and CO2, as the main products of the decay of common CHNO-containing EMs, almost do not appear in the above simulations. Previous AIMD27 and ReaxFF MD28,65 simulations on heated ε-CL-20 showed that high temperature facilitates the fast formation of the two products. In combination with the multiscale shock simulation technique, a recent SCC-DFTB MD simulation on shocked ε-CL-20 did it too.29 We believe that no N2 or CO2

appearing in our simulations may be attributed to the low temperature and short simulation time. To verify this, two additional simulations were carried out. An MD simulation on the single cell of γ-CL-20 was performed for 54 ps. As illustrated in Figure 12, the two products appear in certain amounts and tend to increase, verifying that the high temperature indeed favors their formation. Another addition simulation is to elongate the simulation of γ-CL-20 at 2000 K to 40 ps. As a result, no N2 and some CO2 are produced, as shown in Figure S1 of the Supporting Information. This suggests that high temperature is more efficient to produce these two stable products, in agreement with previous simulations using other methods.27−29,65 16573


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Figure 12. Evolution of some important small molecules during the thermal decay of γ-CL-20 heated at 3000 K.

3.5. Thermal Stability as a Possible Factor Responsible for the Difference in the Impact Sensitivities of the Three Polymorphs. An experimental measurement showed that the impact sensitivities of the three polymorphs of CL-20 are different from one another, i.e., the drop heights H50 measured with a 2 kg hammer and 30 mg of the sample of each of ε-, γ-, and β-CL-20 are 53.1, 36.9, and 13.5 cm, respectively.68 That is to say, ε-CL-20 is the most impact insensitive, whereas β-CL-20 is the most impact sensitive. The factors responsible for the impact sensitivity are multiple and can be molecular stability, molecular packing mode, crystal perfection, crystal shape and size, and so forth. From the above discussion, we can know that ε-CL-20 is thermally least reactive. For example, we do not observe any chemical reaction when heating ε-CL-20 at 1000 K for 20 ps (Figure 9a). This can partly be responsible for the lowest impact sensitivity of εCL-20. Our recent results showed a small difference in the molecular volume of CL-20 molecules in various polymorphs and cocrystals, and the highest packing density of ε-CL-20 is attributed to its highest packing coefficient.41 As a matter of fact, a compact packing facilitates a slow temperature increase and thus a slow hotspot formation, i.e., a low impact sensitivity. Another recent work showed that the difference in the temperature increase between perfect and defected RDX can only be exhibited by adiabatic heating.69 A usual difference between perfect and defected crystals is that the latter possesses more free volume. For the three polymorphs of CL-20, the most compact ε-CL-20 possesses the least free volume and thus the lowest sensitivity. Of course, many other factors other than molecular stacking can also be responsible for its lowest sensitivity.

dominates the thermal decay of all of the three polymorphs. In total, five types of bond cleavages are observed in the decay: the NO2 partitions from the 5-membered ring and the 6membered ring, the cleavage of C−N bonds of the 6membered ring and the 5-membered ring, and the C−C bond breakage. Interestingly, all of the five types of ignitions are not be observed in any case. Besides, we find that the low temperature is disadvantageous to the formation of the stable products of N2 and CO2. These results of the thermal decay of various polymorphs with the same energetic composition are expected to deepen insights into the complicated sensitivity mechanism of EMs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b04126.

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Evolution of some important small molecules during the thermal decay of γ-CL-20 heated at 2000 K (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 86-816-2493506. ORCID

Chaoyang Zhang: 0000-0003-3634-7324 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the support of the Science Challenge Project (TZ-2018004) and the National Natural Science Foundation of China (21875227, 21673210, and U1530262).

4. CONCLUSIONS In summary, we carry out SCC-DFTB MD simulations to compare the thermal decomposition behaviors of the three polymorphs of CL-20 stabilized at ambient condition. In the simulations, an extreme of volume constraint and two heating types (programmed heating and constant-temperature heating) are considered. At a relatively low temperature like 1000 K, the thermal stability of ε-CL-20 and the other polymorphs is distinguished, as no reaction takes place in ε-CL-20, whereas the CL-20 molecules in the other two polymorphs begin to decompose; on the other hand, at 1500 and 2000 K, the thermal reactivity can hardly be differentiated. The lower thermal activity of ε-CL-20 is partly responsible for its lower impact sensitivity. Moreover, the unimolecular N−N breakage

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