A novel insensitive cocrystal explosive BTO/ATZ: preparation and performance

Abstract

In order to explore new applications of novel nitrogen-rich energetic materials based on 1H,1′H-5,5′-bitetrazole-1,1′-diolate (BTO), aside from BTO based energetic-salts, a cocrystalline energetic material composed of BTO and 1-amino-1,2,3-triazole (ATZ) in a 1 : 2 molar ratio was synthesized, which is the first reported BTO based material with a cocrystal structure. The structure of the cocrystal was characterized using powder X-ray diffraction and single crystal X-ray diffraction, which indicate that the cocrystal is formed by intermolecular hydrogen bonding interactions, and is crystallized in the monoclinic system, space group C2/c, with a density of 1.697 g cm⁻³. The properties of the cocrystal including the thermal decomposition, sensitivity, and detonation performances are discussed in detail. Differential scanning calorimetry (DSC) and thermogravimetry/derivative thermogravimetry (TG/DTG) technologies were employed to determine the thermal decomposition behavior of the cocrystal, and the results are significantly different from the decomposition behavior of the co-formers. The enthalpy of formation was calculated as 1376.5 kJ mol⁻¹, which is obviously higher than that of RDX. Sensitivity studies showed that the cocrystal has an impact sensitivity of 24 J, and so is insensitive to impact stimulation. In addition, the detonation pressure (P) and detonation velocities (D) of the cocrystal were predicted by using K–J equations, and the results obtained are 8088 m s⁻¹ for detonation velocity and 28.1 GPa for detonation pressure, which are at the same level as RDX. Combining these advantages, this cocrystal possesses a promising future for use as a type of insensitive explosive. The discovery of the cocrystal contributes significantly to the expansion and application of the chemistry of 1H,1′H-5,5′-bitetrazole-1,1′-diolate.

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1. Introduction

In the past few decades, a basic strategy for exploring highly energetic explosives is to obtain ring or cage-like energetic compound structures by oxidizing and nitrifying C and N framework molecules with specific cyclic structures.^1–3 For example, researchers have successfully synthesized several representative energetic materials, including 2,4,6-trinitrotoluene (TNT), 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX), 1,3,5,7-tetranitro-1,3,5,7-tetraazcycloocant (HMX), and 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20).^4–7 However, accompanying the improvement of energetic properties, the sensitivities of explosives to mechanical stimulation and thermal stimulation are also increased, and the production process for these explosives can cause environmental pollution. Due to concerns about environmental impact, a major focus of today's researchers is to find “green” energetic materials with high safety, high energetic performance, and environmental friendliness.^8,9

Because they have many N–C, N–N and other energetic chemical bonds, energetic heterocyclic compounds with a high nitrogen content have many characteristics, such as high density, high heat of formation, acceptable sensitivity, and can easily achieve oxygen balance.^10,11 As a new type of high energy density materials (HEDMs), nitrogen-rich heterocyclic compounds have gradually become a hot topic, and have provided a powerful approach for the design of new energetic materials with special purposes.^12–14

One promising backbone for tailoring new HEDMs is tetrazole. With four N atoms in the ring, the five-membered heterocyclic compound has a high nitrogen content of above 80%. Tetrazole derivatives tend to show high energetic performances, and can be controlled by selecting various substituents to occupy the position of the C-5 carbon atom. These tetrazole derivatives tend to show high energetic performances, due to their high density and high heats of formation.^15–18 Additionally, recently published studies have shown that the introduction of N-oxides in tetrazole can provide energetic materials with even higher densities, stabilities, and better oxygen balances.^19–21 Combining these principles in mind, a new excellent tetrazole derivative, 1H,1′H-5,5′-bitetrazole-1,1′-diolate (BTO), has become the focal point of research, after it was first reported by Tselinskii in 2001.²² BTO is a relatively strong acid, which helps it to dissociate into salts. Within the BTO molecule, there are hydroxyl groups that facilitate the formation of intra- or inter-molecular hydrogen bonds. A series of ionic salts based on BTO have been synthesized and reported, including the high-performing energetic material dihydroxylammonium 5,5′-bistetrazole-1,1′-diolate (TKX-50).^23–25 However, BTO always has two water molecules in its crystal structure, which affect its energetic properties. Although BTO based energetic salts have excellent detonation performances, some of them are too sensitive to mechanical stimuli, like dihydrazinium 1H,1′H-5,5′-bitetrazole-1,1′-diolate (IS, 9 J), 5-aminotetrazolium 1′-hydroxyl-1H,1′H-5,5′-bitetrazole-1-olate (IS, 4 J) and 1,5-diaminotetrazolium 1′-hydroxyl-1H,1′H-5,5′-bitetrazole-1-olate (IS, 2 J). The extreme sensitivity to impact, and the unavoidable problem of moisture absorbability, have limited the practical applications for some of these salts employed as energetic materials. 1-Amino-1,2,3-triazole (ATZ) is another promising energetic backbone because of its greater ring tension caused by the three contiguous nitrogen atoms, which result in a higher positive formation enthalpy (in the gas phase: 272 kJ mol⁻¹) than its isomer 1,2,4-triazole (in the gas phase: 194 kJ mol⁻¹).²⁶ Moreover, the existence of a large amount of intra- and inter-molecular hydrogen bonds in this compound helps to improve the structure stability of the triazole ring as well, which contributes to a reduction in sensitivity. However, the melting point of ATZ is terribly low, and it may become melted and exist as a liquid even at room temperature, which may also affect the thermal stability of ATZ based salts, and finally affect its application for practical usage.

Therefore, we have employed a co-crystallization technique to improve the performance of two promising energetic materials. Co-crystallization is a powerful and facile route to improve physical and chemical properties, and enhance energetic performance and sensitivity.^27–29 By regularly arranging two or more kinds of molecules together, where at least one is an energetic material, co-crystallization has become a new kind of crystal engineering, providing a promising method to obtain energetic materials with excellent comprehensive performances.³⁰ The co-formers in a co-crystal are mainly connected with each other by non-covalent interactions, like hydrogen bonding, van der Waals forces, and π–π stacking interactions, without changing the chemical structures.^31–33 It has become an effective method to modify the properties of energetic materials, such as detonation performances and sensitivities. A certain number of energetic cocrystals have been reported thus far, including CL-20/TNT, CL-20/HMX, and so on.^34–37

Herein, we report the synthesis of the first cocrystal based on BTO, the structure of BTO/ATZ (ATZ = 1-amino-1,2,3-triazole) with a molar ratio of 1 : 2, and its full characterization using NMR, FT-IR, Raman spectroscopy, and XPS spectroscopy. The structure was fully determined using powder X-ray diffraction and single crystal X-ray diffraction. Differential scanning calorimetry (DSC) and thermogravimetry/derivative thermogravimetry (TG/DTG) technologies were employed to determine the thermal decomposition behavior of the cocrystal, and the enthalpy of formation was calculated. The sensitivity to impact, and the detonation pressure (P) and detonation velocities (D) were determined. The results show that the cocrystal possesses a promising future for use as a type of insensitive explosive. The completely novel BTO based cocrystal opens up a new field of energetic materials based on 1H,1′H-5,5′-bitetrazole-1,1′-diolate.

2. Experimental section

2.1 Materials and preparation of the cocrystal

All chemical regents and solvents of analytical grade were bought from the reagents company and used as supplied. BTO and ATZ were synthesized according to the literature, and the chemical structures for the chosen materials can be seen in Fig. 1.

Fig. 1 Chemical structures of BTO and ATZ.

Crystallization experiments were carried out by dissolving BTO and ATZ in a molar ratio of 1 : 2 in 30 mL of anhydrous methanol, and the mixture was stirred at 45 °C. The solution was filtered after the solids were completely dissolved. The solvent was evaporated over a period of 7 days at room temperature, and a new colorless energetic cocrystal of BTO/ATZ was obtained.

2.2 Morphology characterization

The morphologies of the BTO/ATZ cocrystal and the co-formers were examined with a HITACHI (Japan) S-3400N-II Scanning Electron Microscope (SEM) at 5 kV and 10 mA.

2.3 NMR spectroscopy

¹H, ¹³C, and ¹⁵N NMR spectra were recorded with a Bruker AVANCE III 500 MHz NMR instrument. The chemical shifts quoted in ppm in the text refer to typical standards such as tetramethylsilane (¹H, ¹³C) or nitromethane (¹⁵N).

2.4 Structure characterization

Powder X-ray diffraction data of the title cocrystal and the raw materials were collected with a Bruker D8Advance diffractometer using Cu-K_α radiation (λ = 0.154439 nm). The voltage and current applied were 40 kV and 40 mA, respectively. The data were collected over an angle range from 5° to 50°, with a step size of 0.03° and a scan speed of 0.4 s per step.

X-ray single-crystal diffraction data collections for a single cocrystal of suitable quality were carried out using graphite-monochromated Mo-K_α radiation (λ = 0.071073 nm) on a Bruker CCD area-detector diffractometer, under a temperature of 298 K throughout the collection process. The structure was solved by direct methods using SHELXS97 (ref. 37) and refined anisotropically on F² by the full-matrix least-squares technique using the SHELXL97 program.³⁹ All non-hydrogen atoms were located using the differential Fourier map, and all hydrogen atoms were generated geometrically.

2.5 FT-IR spectroscopy

The FT-IR spectra of the cocrystal and the raw materials were recorded by Fourier transform techniques using KBr pellets on a Bruker Equinox 55 infrared spectrometer. Each spectrum was scanned in the range of 4000–400 cm⁻¹ with a resolution ratio of 4 cm⁻¹.

2.6 XPS spectroscopy

The C1s, N1s and O1s spectra of the cocrystal and the raw materials were recorded by using a Thermo Scientific Escalab 250Xi photoelectron spectrometer.

2.7 Thermal analysis

The differential scanning calorimetry (DSC) tests of the cocrystal and the raw materials were carried out using a Pyris-1 differential scanning calorimeter. The samples, which weighed about 0.5 mg, were placed in aluminum pans and heated at a heating rate of 5 °C min⁻¹ over a range from room temperature to 500 °C, under a dry nitrogen atmosphere with a flow rate of 20 mL min⁻¹.

The thermogravimetry (TG) test of the BTO/ATZ cocrystal was recorded using a Pyris-1 thermogravimetric analyzer by heating about 0.5 mg of the cocrystal sample at a heating rate of 5 °C min⁻¹ over a range from room temperature to 500 °C, under a dry nitrogen atmosphere with a flow rate of 20 mL min⁻¹.

2.8 Sensitivity test

The impact sensitivity test was carried out by using a BAM fall hammer apparatus with a 5 kg drop weight on a sample of about 20 mg. According to the previous result, the level of impact energy for the next trial was increased after “no reaction” and decreased after “ignition”.

2.9 Evaluation of the detonation properties

One of the important parameters in evaluating the energetic performance of a novel energetic compound is the enthalpy of formation. The enthalpies of formation for the cocrystal and the co-formers at 298 K were calculated using the atomization energy method,⁴⁰ for which the first step was optimizing the structure and frequency through the Gaussian 03 suite of programs.⁴¹ The geometries of BTO, ATZ and the cocrystal were optimized using the hybrid DFT-B3LYP method with the 6-311++g (3d, 2p) basis set.⁴²

The detonation properties of the cocrystal, including the detonation pressure (P) and detonation velocity (D) based on the calculated enthalpy of formation and crystal density, were determined according to a reported method.⁴³ In addition, the detonation properties of the co-formers were also calculated for comparison.

3. Results and discussion

3.1 Synthesis

The new colorless energetic cocrystal of BTO/ATZ was obtained by dissolving BTO and ATZ in a molar ratio of 1 : 2 in anhydrous methanol, and then stirring to dissolve and filter the mixture. Elemental analysis results for the cocrystal (confirmed by a Flash EA 1112 fully automatic trace element analyzer) are as follows: calcd: C 21.28, H 2.96, N 66.21; found: C 21.23, H 2.92, N 66.28. IR (KBr, cm⁻¹): 3467, 3167, 3135, 3081, 2476, 2410, 1996, 1770, 1661, 1577, 1470, 1419, 1400, 1343, 1318, 1305, 1259, 1224, 1190, 1167, 1101, 1084, 1052, 1018, 994, 872, 811, 721, 693, 672, 637, 516, 466, 430. ¹H-NMR (d₆-DMSO, 500 MHz): 9.54, 8.02, 7.82, 7.25, 7.15, 7.05, 2.56, 2.36. ¹³C-NMR (d₆-DMSO, 500 MHz): 135.73, 132.37, 124.68. ¹⁵N-NMR (d₆-DMSO, 500 MHz): 381.31, 362.18, 358.47, 328.86, 275.98, 254.80, 82.66. The results suggest that the cocrystal contained no impurities after crystallization.

3.2 Morphology

Crystal quality, including the crystal shape, surface, and defects, plays an important role in the safe storage and transport of an energetic material, and ultimately affects the detonation performance of the explosive.³³ SEM images of BTO, ATZ, and the cocrystal explosive are shown in Fig. 2, and there are obvious differences in morphology between them. The BTO co-former is a colorless tablet-like transparent crystal. ATZ shows an irregular block glomerocryst microstructure, with many defects such as dislocations and cracks observed on the crystal surface. Meanwhile, the cocrystal exhibits a colorless prism type crystal morphology with a uniform size, regular structure, and smooth surface. The SEM images of the crystal reflects its microscopic structure, and the differences imply the formation of a new structure. The results indicate that the cocrystallization method can change not only the crystal size but also the shape, and therefore provides a potential new method for synthesizing explosive materials with tunable properties. Considering the co-crystal's approximate spherical shape, which can increase the charge density to a great degree, it is a good candidate for practical application as an energetic material. Energetic materials with a uniform size, regular structure, and smooth surface have better detonation performance.

Fig. 2 SEM images of BTO (left), ATZ (middle), and the cocrystal (right).

3.3 Structure of the cocrystal

3.3.1 Powder X-ray diffraction. The powder X-ray diffraction (PXRD) pattern of the cocrystal is shown in Fig. 3, and is evidently different from those of the co-formers. As illustrated in the figure, in the 2θ range of 23° and 27°, the main sharp peaks for the BTO and ATZ co-formers do not appear in the diffraction pattern of the cocrystal. On the other hand, in the range of 16–22°, a series of sharp peaks appear in the pattern of the cocrystal, as well as at 30° and 35°. These differences easily distinguish the cocrystal from the raw materials, and indicate that the cocrystal is a new material rather than a product of crystal transformation or the product of a physical mixture. Additionally, the theoretical value and measured value of the PXRD pattern are consistent with each other, as shown in the ESI,† which also indicates that the powder product is no different from the single crystal product.

Fig. 3 Comparison of the PXRD patterns of the cocrystal and the pure materials.

3.3.2 Single-crystal X-ray diffraction. The results from the crystallographic data collection and structure refinements are summarized in Table 1, and were determined by choosing a BTO/ATZ cocrystal of suitable size (0.48 mm × 0.45 mm × 0.44 mm) for single-crystal X-ray diffraction (SXRD). The selected bond lengths and angles are listed in Table 2, while the hydrogen bond lengths and bond angles of the cocrystal are given in Table 3. As can be seen in Fig. 4, the cocrystal consists of BTO and ATZ in a 1 : 2 molar ratio, belongs to the monoclinic system, with space group C2/c, and has a density of 1.697 g cm⁻³.

Table 1 Crystal data and structure refinement details for the cocrystal

	BTO/ATZ
a Note: w = 1/[s^2(Fo^2) + (0.0772P)^2 + 0.4851P], where P = (Fo^2 + 2Fc^2)/3.
Empirical formula	C6H10N16O2
Formula mass	338.30
Temperature (K)	298(2)
Crystal system	Monoclinic
Space group	C2/c
Z	4
a (Å)	21.5319(19)
b (Å)	3.7406(3)
c (Å)	19.5090(18)
α (°)	90
β (°)	122.552(2)
γ (°)	90
Cell volume (Å^3)	1324.5(2)
Dc (g cm^−3)	1.697
F(000)	696
Size (mm)	0.48 × 0.45 × 0.44
θ (°)	3.79–25.01
h	−24 to 24
k	−4 to 4
l	−23 to 23
Reflections collected	2986
Unique reflections	1156
Rint	0.0483
Data/restraint/parameter	1156/0/109
Goodness-of-fit on F^2	1.064
R1, [I > 2σ(I)]	0.0412
wR2, [I > 2σ(I)]	0.1141^a
R1, (all data)	0.0499
wR2, (all data)	0.1227^a
Δρmax (e Å^−3)	0.531
Δρmin (e Å^−3)	−0.369
CCDC	1436344

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Table 2 Selected bond lengths and bond angles for the cocrystal^a

Bond	Length/Å	Bond	Length/Å	Bond	Angle/°	Bond	Angle/°
a Note: #1 −x + 1, −y, −z + 1.
N1–O1	1.319(2)	N7–N6	1.320(2)	N6–N7–C2	111.73(16)	O1–N1–N2	122.39(14)
N1–C1	1.339(2)	N7–C2	1.342(3)	N6–N5–C3	112.82(16)	C1–N1–N2	108.81(15)
N1–N2	1.339(2)	N5–N6	1.316(2)	N6–N5–N8	120.04(17)	N5–N6–N7	104.12(16)
N4–C1	1.328(3)	N5–C3	1.340(3)	C3–N5–N8	126.94(16)	C1–N4–N3	106.00(15)
N4–N3	1.342(2)	N5–N8	1.391(2)	O1–N1–C1	128.79(15)	N4–C1–N1	108.15(16)
N2–N3	1.446(4)	C2–C3	1.355(3)	N4–C1–C1A#1	127.40(2)	N3–N2–N1	106.06(15)
C1–C1A#1	1.446(4)			N1–C1–C1A#1	124.40(2)	N2–N3–N4	110.98(16)
				N7–C2–C3	106.31(18)	N5–C3–C2	105.02(17)

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Table 3 Hydrogen bond lengths (Å) and angles (°) of the cocrystal

D–H⋯A	Length (D–H)	Length (H⋯A)	Length (D⋯A)	Angle (D–H⋯A)	Symmetry code
O1–H1⋯N7	0.820	1.800	2.588	160.63	x + 1/2, y − 1/2, z
O1–H1⋯N6	0.820	2.640	3.444	167.09	x + 1/2, y − 1/2, z
N8–H8A⋯O1	0.890	2.064	2.952	175.08	−x + 1, −y + 1, −z + 1
N8–H8B⋯O1	0.890	2.637	3.098	113.22	−x + 1, −y, −z + 1
N8–H8B⋯N4	0.890	2.145	2.961	152.02	None

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Fig. 4 The molecular structure of the BTO/ATZ cocrystal.

In the crystal structure, it is obvious that the water molecules disappeared, and were replaced by the ATZ triazole ring. As shown in Fig. 5 and Table 3, a series of strong intermolecular O–H⋯N and N–H⋯O hydrogen bonds are formed, and each molecule is both the donor and the acceptor of the hydrogen bonds.^27,44 For instance, the O1–H1⋯N7 intermolecular hydrogen bond that occurs between the hydroxyl oxygen atom of BTO and the N atom of the ATZ ring is the strongest intermolecular interaction in the cocrystal, with a distance of 1.800 Å (H⋯acceptor). Another N8–H8A⋯O1 hydrogen bond that lies between the amino group of ATZ and the hydroxyl group of BTO is also a strong hydrogen bond, with a distance of 2.064 Å (H⋯acceptor). The strong intermolecular hydrogen bonds are the primary driving forces behind the formation of the cocrystal, and combined with the influence of other weak hydrogen bonds, such as N8–H8B⋯N4, the good layered structure is finally formed. In addition, typical face-to-face π-stacking is observed between the ATZ triazole rings and BTO tetrazole rings from different layers. The large number of layered structures contributes to building the molecules into huge 3D networks on a larger scale, in a zigzag chain arrangement (see Fig. 6). The layered structures can buffer stimuli efficiently, and thereby reduce the sensitivity to stimuli such as impact and friction, and contribute to the stability of the crystal structure.

Fig. 5 Hydrogen bonds between the BTO and ATZ molecules.

Fig. 6 3D-packing structure of the cocrystal.

3.4 Infrared spectroscopy

IR spectroscopy works well in determining the predominant phase and can be used for the characterization of cocrystals. The IR spectra of ATZ, BTO, and the BTO/ATZ cocrystal are presented in Fig. 7. The assignments for the most characteristic vibrational bands are listed in Table 4. It can be concluded from the table that a group of low intensity bands assigned to the N–H stretching vibration of ATZ increased from being in the region of 3146.87–3314.61 cm⁻¹ to 3314.61–3428.23 cm⁻¹. Meanwhile, the O–H stretching of BTO is in the region of around 3347.23 cm⁻¹. Simultaneously, some peaks are also present for the bonds of both ATZ and BTO. These can be attributed to hydrogen-bonding, which changes the symmetry characteristics of the cocrystal structure.

Fig. 7 IR spectra of BTO, ATZ and the BTO/ATZ cocrystal.

Table 4 Assignments of the major bands for the IR spectra of ATZ, BTO, and the BTO/ATZ cocrystal

Assignment	ATZ	Cocrystal	BTO	Assignment
N–H stretching vibration	3428.2	3135.3	—	N–H stretching vibration
C=C stretching vibration	1643.9	1661.7	—	C=C stretching vibration
N=N asymmetric stretching vibration	1482.6	1470.9	1414.5	N=N asymmetric stretching vibration
C=N symmetric stretching vibration	—	1419.3	1377.6	C=N asymmetric stretching vibration
O–H stretching vibration	—	3467.49	3347.23	O–H stretching vibration

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3.5 XPS spectra

The XPS spectra of the cocrystal and the co-formers are shown in Fig. 8 and Table 5. The results indicate that the binding energy for C1s, N1s and O1s of the cocrystal are different from those of the co-formers, and also suggested that the cocrystal is a new material rather than the product of crystal transformation or the product of a physical mixture.

Fig. 8 The C1s, N1s and O1s spectra of the cocrystal and co-formers.

Table 5 The binding energies of C1s, N1s and O1s of the cocrystal and co-formers

Binding energy (eV)	C1s	N1s	O1s
BTO	288.04	401.25	534.48
ATZ	284.80	400.54	532.32
Cocrystal	284.80	401.09	532.81

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3.6 Thermal analysis

The differential scanning calorimetry (DSC) and thermogravimetry/derivative thermogravimetry (TG/DTG) measurements for determining the phase-transition temperature and thermal decomposition behavior of the cocrystal are illustrated in Fig. 9 and 10. All tests, using samples of 0.5 mg, were performed in covered Al containers and at a heating rate of 5 °C min⁻¹.

Fig. 9 DSC curves for BTO, ATZ and the BTO/ATZ cocrystal.

Fig. 10 TG/DTG curve for the BTO/ATZ cocrystal.

From the curves it becomes evident that the thermal decomposition process of the BTO/ATZ cocrystal is different from those of the co-formers, and can be divided into three stages: one endothermic melting stage and two exothermic decomposition stages. The differences in the thermal behavior of these substances also suggest the formation of a new cocrystal. The cocrystal is found to melt at 133.6 °C, exhibiting a sharp endothermic process with a peak temperature of 145.4 °C. The melting point of the cocrystal is much higher than that of ATZ (49.8 °C), which can be attributed to the changes in crystal packing caused by the strong intermolecular hydrogen bonds. It is obvious that the problems associated with the low melting point of ATZ have been overcome. Meanwhile, for the BTO co-former, the endothermic process that occurs from 69.5 to 115.5 °C is in response to the loss of water from the crystal structure with increasing temperature. Both the ATZ and BTO co-formers exhibit one exothermic process with a peak temperature of 184.6 °C and 225.2 °C, respectively. Meanwhile the cocrystal has two exothermic peaks, which is different from many other cocrystals. The first exothermic decomposition process occurs in the range of 154.4 to 178.8 °C, with a peak temperature of 163.7 °C, followed by a decomposition process occurring between 218.5 and 272.3 °C, which reaches a peak at 243.3 °C. The enthalpies of the two main exothermic decomposition processes are 87.9 kJ mol⁻¹ and 83.3 kJ mol⁻¹. The decomposition behavior of the cocrystal may be explained as the destruction of the crystal structure with increasing temperature.

In addition, the TG/DTG curve also shows the two main mass loss stages, corresponding to the decomposition processes of the cocrystal. The first mass loss process occurs in the range of 148 and 177 °C, and reaches its highest rate at 157.6 °C, with a mass loss percentage of 13.1%. Then a main mass loss stage happens from 203 to 258 °C, with a mass loss of 36.7%, in which the highest mass rate is reached at 238.9 °C. The final stage is a slow process of thermal decomposition with continuous mass loss, with a final residual mass of 22.2%.

3.7 Impact sensitivity properties

Impact sensitivity is always used to evaluate the safety of energetic materials, and it is largely dependent on the physical and chemical properties of energetic materials. Our results reveal that the cocrystal is relatively insensitive to mechanical stimuli, having an impact sensitivity of 24 J. The results indicate that the cocrystal is insensitive to mechanical stimuli due to its crystal structure formed by cocrystallization. Compared with types of physical mixtures, the intermolecular interactions (mainly the hydrogen bonds between BTO and ATZ molecules) help to decrease the volume and number of cavities in the cocrystal structure, which results in the reduction of so-called “hot spots” forming when mechanical stimuli is applied, leading to an increase in the crystal stability. This is in line with previous reports, and further indicates that cocrystallization is an effective method to alter the sensitivity of energetic materials for application in the fields of military and civilian explosives.

3.8 Prediction of detonation properties

The process of cocrystallization not only changes the crystal structure and thermal decomposition behavior of the energetic co-formers, but can also lead to some changes in density, an important physical property of energetic materials which can be used to evaluate the detonation properties. The BTO/ATZ cocrystal has a crystallographic density of 1.696 g cm⁻³, which is lower than that of BTO (the crystal density of BTO dihydrate is 1.769 g cm⁻³), but is significantly higher than that of ATZ (1.484 g cm⁻³).

The enthalpy of formation is one of the most important parameters of energetic materials. The enthalpies of formation (ΔH_f) for the cocrystal and co-formers at 298 K were calculated using the Gaussian 03 suite of programs. The results are listed in Table 6. The enthalpy of formation of the cocrystal is about six times of that of BTO, and about four times of that of ATZ.

Table 6 Detonation properties for the BTO/ATZ cocrystal compared with other explosives

Samples	Density/g cm^−3	ΔHf/kJ mol^−1	D/km s^−1	P/GPa	IS/J
BTO/ATZ	1.697	1376.5	8.088	28.1	24
BTO·2H2O	1.769	205.9	8.554	32.1	40
ATZ	1.484	364.1	7.142	19.9	—
TNT	1.648	95.3	6.881	19.5	15
RDX	1.80	83.8	8.783	35.2	7.4
CL-20/TNT^45,46	1.76	136.9	8.426	32.3	5.9

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Based on the densities and enthalpies of formation, the critical detonation parameters of energetic compounds including the detonation velocity (D) and pressure (P) are predicted by the empirical Kamlet–Jacobs equations:

D = 1.01(NM^0.5Q^0.5)^0.5(1 + 1.30ρ) (1)

P = 1.558ρ²NM^0.5Q^0.5 (2)

where D is the detonation velocity (km s⁻¹); P is the detonation pressure (GPa); N is the number of moles of detonation gas per gram of explosives; M is the average molecular weight of these gases; Q is the heat of formation (kJ mol⁻¹); ρ is the loaded density of the explosives (g cm⁻³). ΔH_Q (kJ mol⁻¹) is the heat of detonation, which is calculated using Q. The results are listed in Table 6 and Fig. 11.

Fig. 11 Comparison of the energetic performances of BTO/ATZ, TNT, RDX, and CL-20/TNT.

From Table 6 and Fig. 11, it can be seen that the calculated detonation velocity and detonation pressure of the BTO/ATZ cocrystal are determined as 8.088 km s⁻¹ and 28.1 GPa. The detonation velocity of BTO/ATZ is roughly at the same level as RDX, only 7% lower than that of the latter. Meanwhile, the impact sensitivity of the cocrystal is 24 J, which is about three times of that of RDX (7.4 J), and helps in increasing storage and transportation safety. The situation is similar to that of the CL-20/TNT crystal: the detonation velocity of the BTO/ATZ cocrystal is about 4% lower, but the impact sensitivity is three times more than that of the CL-20/TNT cocrystal (5.9 J). Therefore, the BTO/ATZ cocrystal has a good detonation performance, and high safety, combined with an extremely high heat of formation, which leads to a high heat of detonation. This material is therefore a promising candidate for application in the field of the insensitive cocrystal explosives.

4. Conclusions

We have reported the first BTO based cocrystal explosive BTO/ATZ, which was designed and synthesized using the co-formers 1H,1′H-5,5′-bitetrazole-1,1′-diolate and 1-amino-1,2,3-triazole in a molar ratio of 1 : 2. The cocrystal structure fully overcomes the disadvantages of BTO and ATZ. The water molecules in BTO dehydrate are replaced by the ATZ triazole molecules, and the melting point of the cocrystal is more than 90 °C higher than that of AZT triazole. The structure was also characterized using powder X-ray diffraction and single-crystal X-ray diffraction, and the results show that the cocrystal belongs to the monoclinic system, with space group C2/c, has a density of 1.697 g cm⁻³, and the structure is mainly supported via hydrogen bonding. Thermal decomposition of the cocrystal was determined by DSC and TG/DTG, and the curves reveal that exothermic decomposition occurs in the temperature range of 154–272 °C with a melting point of 134 °C, which is significantly different from the decomposition behavior of the co-formers. Moreover, predicted detonation performance parameters of the cocrystal are 8088 m s⁻¹ for the detonation velocity, 28.1 GPa for the detonation pressure, and a significantly high heat of formation of 1376.5 kJ mol⁻¹, which leads to a high heat of detonation. While the detonation performance is at the same level of regular explosives, the BTO/ATZ cocrystal's heat of formation is enormously improved, and thus the impact sensitivity is overwhelmingly enhanced. Therefore the storage safety is ensured and the incident rate is decreased. This provides the cocrystal with a promising future for use as a type of insensitive explosive. Additionally, this report of the first BTO based cocrystal contributes significantly to the expansion and application of the chemistry of 1H,1′H-5,5′-bitetrazole-1,1′-diolate.

Acknowledgements

For this paper, the authors are indebted to the National Natural Science Foundation of China, and the project of State Key Laboratory of Science and Technology (ZDKT 16-04 & YBKT16-15) for their support.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1436344. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra14510h

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