Mechanical Anisotropy of the Energetic Crystal of 1,1-Diamino-2,2

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Mechanical Anisotropy of the Energetic Crystal of 1,1Diamino-2,2- dinitroethylene (FOX-7): A Study by Nanoindentation Experiments and Density Functional Theory Calculations Xiaoqing Zhou, Zhipeng Lu, Qi Zhang, Dong Chen, Hongzhen Li, Fude Nie, and Chaoyang Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04612 • Publication Date (Web): 07 Jun 2016 Downloaded from http://pubs.acs.org on June 11, 2016

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The Journal of Physical Chemistry

Mechanical

Anisotropy

of

the

Energetic

Crystal

of

1,1-Diamino-2,2-

dinitroethylene (FOX-7): A Study by Nanoindentation Experiments and Density Functional Theory Calculations Xiaoqing Zhou,† Zhipeng Lu,†,‡ Qi Zhang,*,† Dong Chen,† Hongzhen Li,*,† Fude Nie, † and Chaoyang Zhang*,† †

Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang, Sichuan 621900, China ‡ Department of Mathematics and Physics, Officers College of CAPF, Chengdu, Sichuan, 610213, China

ABSTRACT The mechanical anisotropy of the wavelike π-stacked energetic crystal of 1,1-diamino-2,2-dinitroethylene (FOX-7) is investigated by nanoindentation experiments and density functional theory (DFT) calculations. The FOX-7 crystal exhibits distinct mechanical anisotropy when indented on different faces. The elastic modulus and hardness of the (020), (−101), and (002) faces change in a decreasing order. The indentation on the (020) face induces the largest depth and the highest pile-up around all three edges of the indenter without causing crack formation. By contrast, the indentations on the (−101) and (002) faces are similar and induce a small indentation depth, low pile-up with a small distribution, and crack formation. Mechanical anisotropy is essentially determined by the wavelike π stacking of FOX-7 along the (020) face with the support of intermolecular hydrogen bonds, i.e., the molecular orientations and intermolecular spaces along different faces vary distinctly. This is also supported by the DFT calculations on uniaxial compression and shear sliding. In this work, the nature of the wavelike π stacking responsible for the low impact sensitivity of FOX-7 is discussed and compared with that of other explosives with different packing structures.

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1. INTRODUCTION Crystal engineering is the understanding of the intermolecular interactions in crystal packing and the utilization of such understanding in designing new solids with desired physical and chemical properties1,2. For energetic materials (EMs), current studies show that their crystal packing is strongly responsible for their impact sensitivity3–6. π-π stacking can effectively serve as buffer against external mechanical stimuli via π-π sliding to dissipate energy, avoid hot spot formation and growth, and achieve low impact sensitivity. Thus, such π-π stacking should be considered in designing new highly energetic and insensitive materials7, in addition to stable energetic compound molecules. This process is an important part of the crystal engineering of EMs. Macroscopically, the impact sensitivity of EMs relates to their mechanical properties. For instance, softness, such as that of olefin at ambient conditions, usually benefits low-impact sensitivity8. The aforementioned π-π stacking, particularly face-to-face π-π stacking or graphite-like stacking, leads to softness9, thereby providing a typical example of a macro-property determined by micro-structure. Although this finding has already been verified theoretically with remarkable mechanical anisotropies and at least one easy sliding orientation5, it still lacks experimental evidence. Thus, we aimed to experimentally validate the mechanical anisotropy of a π-π-stacked explosive crystal and to gain deep insights into the packing–sensitivity relationships of EMs. The major difficulty in gathering evidence was obtaining single crystals of π-π-stacked EMs because no related report exists about it. In this work, we considered a wavelike stacked EM, 1,1-diamino-2,2-dinitroethylene (FOX-7), as an example to prepare single crystals and characterize their mechanical properties via nanoindentation tests on various crystal faces. These properties were also evaluated with density functional theory (DFT) calculations. FOX-7 was synthesized by

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Latypov in 199810, and it has been identified as a potential insensitive EM afterwards because it satisfies energy and safety requirements. The energy and sensitivity of this EM fall between those of 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX) and 1,3,5-triamino-2,4,6-trinitrobenzen (TATB). For example, the detonation velocities of HMX, FOX-7, and TATB decrease from 9198 m/s to 9000 and 8640 m/s in terms of EXPLO5 (Version 6.02) calculations based on BKW and Cowan-Fickett equations of state11,12, whereas their impact energies increase from 7 and 25 J to >78 J, thus showing the increasing safety. Besides, this charming EM attracts other extensive attention. With respect to the single FOX-7 molecule, some insight into its decomposition in ground, charged and excited states was performed. It covered the possible initial decay steps of the C-NO2, C-NH2 and C=C bond break, the C-NO2/C-ONO isomerization, and the intramolecular hydrogen transfer, exhibiting the good molecular stability of FOX-7.13-15 Furthermore, the heating decay mechanism of FOX-7 molecules on Al and α-Al2O3 crystal faces was theoretically researched with a consideration that FOX-7 was applied a component of mixed explosives.16,17 Moreover, many efforts were paid to the behaviours and properties of the FOX-7 under common and compression conditions, showing lots of important details of stress-induced structural variation and decomposition18-25, polymorphic transformation26-27,

morphologic modification28, and providing a sufficient base for understanding

and applying it. As to the nanoindentation technique, particularly depth-sensing nanoindentation, it has been proven to be a useful tool for evaluating the mechanical behavior of inorganic and engineering materials29–33. Its utilization in characterizing organic molecular crystals is also increasingly enhanced because such a technique facilitates increasing studies on structure–property relationships34–38.

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By adding a morphological modifier, we obtained large single crystals of FOX-7 with three main crystal faces grown naturally from the solution, namely, (020), (−101), and (002) faces. The mechanical properties of these faces were characterized by nanoindentation examinations, and the distinct mechanical anisotropy of FOX-7 was experimentally verified. The nanoindentation behavior, such as indentation depth, pop-in, pile-up, and crack formation, was understood via DFT calculations. The roots for the apparent behavior were the π-conjugated molecular structure and π-stacked crystal packing structure of FOX-7 with the support of intermolecular hydrogen bonds (HBs). Through this work, a strategy for insensitive EMs, i.e., HB-aided π-π stacking3,5, was then strengthened. Furthermore, the nanoindentation technique for EMs was validated39. 2. METHODOLOGIES 2.1 Crystal Preparation The preparation of single crystals of FOX-7 was a key component of this work. They were prepared by slowly evaporating a saturated solution at room temperature. The saturated FOX-7 solution was made by dissolving FOX-7 in acetone at 30 °C with a crystal growth modifier. The solution was stirred for 2 h, filtered, decanted into a 100 mL beaker, and then sealed with parafilm-coated coverings by opening some pores at the center to control evaporation. The entire beaker was housed in a constant temperature bath at 30 °C. After a month, the FOX-7 crystals were harvested from the beaker by reducing the temperature to room temperature over an extended period and exchanging the solutions with deionized water. The large single crystals of FOX-7 were obtained by isolating them from the solution. We should note that during harvest, crystal cracking caused by temperature differences must be avoided.

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Optically clear and untwined large single crystals of FOX-7 as well as normal and transverse crystal faces were selected for the subsequent experiments. The crystal faces were determined via synchrotron X-ray diffraction (SXRD) with an Agilent SuperNova single-crystal diffractometer, a molybdenum target (λ = 0.71073 Å), and a charge-coupled device detector. The required crystal faces were cut from the large single crystal using a low-speed (cut rate of 0.8 mm/min) diamond wire saw with deionized water as a lubricant to satisfy the smoothness requirement of crystal faces. Individual crystals were then cold-mounted using epoxy resin and carefully polished using increasingly fine alundum powders, followed by a final polishing step using 0.75 µm cerium oxide suspension. The final roughness was less than 10 nm, which was adequate to avoid any influences on the evaluation of mechanical properties, such as elastic modulus (E) and hardness (H). Additional details are available in S1 of the Supporting Information (SI). 2.2 Nanoindentation Experiments Nanoindentation was performed at ambient temperature using a triboindenter (Hysitron, Minneapolis, USA) with in situ atomic force microscopy (AFM) imaging capability. The machine continuously monitored the load, P, and depth of penetration, h, of the tip with a force resolution of approximately 1 nN and a displacement resolution of approximately 0.2 nm. A three-sided pyramidal Berkovich tip with a radius of 50–100 nm at the apex was used to determine E and H. Nanoindentation tests were performed on various faces of the FOX-7 single crystals. Prior to the testing, an optical microscope was used to find reasonably smooth areas and thereby avoid large roughness effects on the mechanical properties. For all quasi-static nanoindentation tests, a peak load, Pmax, of 8 mN, with loading and unloading rates of 16 mN/min, was employed. The hold time (at Pmax) was set to 15 s to minimize the effect of material creep. A series of 16 indents was

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performed on each sample, and the average of the test values was reported. The indent spacing was set to at least 50 times the indentation depth to avoid interactions of deformed zones. Post-indentation images of the impressions were immediately captured using an atomic force microscope to avoid any time-dependent elastic recovery. The load–displacement (P–h) curves obtained were analyzed using the Oliver–Pharr method to extract the E of the crystal in the related orientation. However, H was not estimated with this method because pile-ups of the material against the indenter faces could lead to an overestimation. Thus, H was determined as Pmax/A, where A is the contact area estimated from the AFM images of the indentation impressions. The Poisson’s ratio of the sample was set to 0.3. Additional details of the data analyses are available in S2 of SI

40–45

.

The AFM examinations for general surface topographic imaging were conducted via tapping mode with DI Nanoman VS (Bruker). The amplitude set point and scan rate were set to 310 mV and 1 Hz, respectively. All scans were conducted at a 384 × 384 resolution. The integral and proportional gains were set to 1 and 5, respectively. 2.3 DFT Calculations All DFT calculations were performed using the CASTEP code46 with the Vanderbilt ultrasoft pseudopotential.47 The H1s, C2s2p, N2s2p, and O2s2p orbitals were taken as valence states, and the exchange-correlation functional was treated with the generalized gradient approximation following the Perdew, Burke, and Ernzerhof formulation.48 Structure optimization was performed using the Broyden–Fletcher–Goldfarb–Shanno scheme49 with 2 × 2 × 1 Monkhorst–Pack k-point sampling in reciprocal space and a plane-wave basis set with an energy cutoff of 850 eV. The density functional dispersion correction 2 (DFT-D2) method of Grimme50 was employed to correct long-range dispersion interactions. In the DFT-D2 method, dispersion corrections are calculated not only for 6


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the forces acting on the atoms but also for the stresses on the unit cell; thus, a simultaneous optimization of all degrees of freedom is permitted. The self-consistent convergence criteria of energy were set to 5 × 10−7 and 5 × 10−6 eV for electronic and ionic relaxation, respectively. To ascertain the reliability of the selected method, we employed it to conduct full structural relaxations of the primitive unit cells of FOX-7. The optimized cell parameters at 0 K were a = 7.0058 Å, b = 6.4775 Å, c = 11.2791 Å, and β = 91.005°, all of which agreed well with the X-ray values of a = 6.922 Å, b = 6.501 Å, c = 11.266 Å, and β = 90.485° at 100 K51. These results supported the reliability of the DFT calculations presented here. To account for the results of the indentation experiment, we conducted a comparative study of the strain energy under uniaxial stress along the (100), (010), and (001) directions of FOX-7 obtained via the DFT calculations and the experiments. The strain energy under uniaxial stress was calculated by compressing the crystals along one direction with a series of incremental strains with all other directions and atoms fully relaxed. The intermolecular HBs in the crystal were analyzed with Bader’s quantum theory of atoms in molecules52 and DFT calculations, along with Critic2, a code for analyzing the real-space quantum-mechanical interactions in periodic solids based on electron densities and other related scalar fields53,54. The core and valence electron densities of a crystal were obtained from the DFT calculations. The HB energy or bond dissociation energy of HB (EHB) was predicted using an empirical equation EHB = −(1/2)v proposed by Espinosa et al.55 and was then used to assess HB strength in terms of electron densities (ρ) at the bond critical points (BCPs). The potential energy densities (v) could then be obtained. 3. RESULTS AND DISCUSSION 3.1 Nanoindentation Characteristics 7



Figure 1. Large single crystals of FOX-7 grown naturally by adding a morphological modifier.

We successfully cultivated a number of large single crystals of FOX-7 by adding a morphological modifier; such crystals served as the basis of this study. Figure 1 shows that the crystals are bright yellow and shaped like a block. After SXRD crystal face determination and surface polishing, nanoindentation tests were performed on the three faces of (020), (−101), and (002), which were grown from a solution emergently. On the basis of these test results and related data analyses, we can discuss the nanoindentation characteristics of the FOX-7 crystal. 9 8 7

(020) (-101) (002)

6

P, mN

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5 4 3

Loading

2

Unloading

1 0 0

200

400

600

800

1000

h, nm Figure 2. P–h curves of the three faces of the FOX-7 crystal. The pop-ins are identified by the horizontal arrows.

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We focus on the P–h curves in Figure 2. The indentation behavior of the (020) face generally differs from those of the (−101) and (002) faces, which are close to each other. At the early stages of loading or at the stages of elastic deformation, the (020) face shows lower resistance to indenter penetration in comparison with the other two faces. The Ps of the (−101) and (002) faces increase at a rate that is approximately twice as fast as that of the (020) face; thus, the (020) face is softer than the (−101) and (002) faces. Several pop-ins subsequently occur on each curve. For the (020) face, the first significant displacement of 13 nm occurs at P = 0.34 mN. The displacements of the subsequent pop-ins are 10, 19, and 33 nm. These displacements are close to the integral multiples of the interplanar distance of (020) equal to 3.28 Å. In case of the (−101) face, the first significant displacement of a pop-in is found to be 30 nm at 1.70 mN. Several pop-ins with a displacement magnitude of 5.9 nm (close to the interplanar distance of (−101) of 5.94 Å) are also observed. With respect to the (002) face, the first significant displacement of 17 nm occurs at 0.62 mN. The subsequent displacements are 6, 44, 11, and 28 nm, which are close to the integral multiples of the interplanar distance of (002) of 5.66 Å. A pop-in displacement close to the integral multiples of interplanar distance is also found in saccharin36 and aspirin37. The phenomenon is attributed to the intermittent plastic flow caused by the sudden and collective slip of crystallographic planes, which occurs so as to relax the indentation-imposed stress36,37. The comparison of all three cases indicates unlike that of the other two faces, the first pop-in of the (020) face occurs correspondingly to a large h. By the end of the loading of 8 mN, the (020) face possesses the largest h. Finally, the (020) face exhibits the largest residual h upon unloading and thus undergoes the largest plastic deformation upon indentation. Table 1. E and H derived from nanoindentation tests. Mechanical properties

Crystal faces (020)

(-101)

(002) 9



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E, GPa

11.09±0.83

16.65±0.79

21.34±1.01

H, GPa

0.52±0.05

0.63±0.02

0.67±0.03

Note: All errors correspond to standard deviations on 16 indents (see S2 of SI).

As shown in Figure 2, the FOX-7 crystal features elasticity and plasticity, which are expected of common crystals. Therefore, we pay attention to the two mechanical properties of E and H (elasticity and plasticity, respectively), which are deduced from the indentation examinations on the three faces in Table 1. From the table, we can deduce the difference in elastic deformation as E(020)/E(−101)/E(002) = 1:1.50:1.92, which suggests the marked E anisotropy of the three faces with a difference exceeding 90%. With respect to plastic deformation, H(020)/H(−101)/H(002) = 1:1.21:1.29 shows H anisotropy but with small differences, particularly a small difference between the (−101) and (002) faces (6%). FOX-7 shows evident elastic anisotropy (i.e., the elasticity increases from the (020) face to the (−101) and (002) faces) and plastic anisotropy (i.e., the plasticity increases from the (002) face to the (−101) and (020) faces).

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Figure 3. 2D (a) and 3D (b) AFM images and depth profiles (c) drawn along the corners to the corresponding edges of the residual impressions after indentation at Pmax of 8 mN on the (020) (1), (−101) (2), and (002) (3) faces. The white lines on the 2D AFM images depict the positions where the line profiles are drawn. And the blue arrows on the 2D images point to places where cracks are formed.

We use AFM images and h profiles to further study the nanoindentation characteristics of the FOX-7 crystal. Figure 3 presents the AFM height topologies of the residual Berkovich indents made on the (020), (−101), and (002) faces with Pmax of 8 mN and their corresponding cross-sectional profiles. The distribution and profile of the pile-up strongly depend on the crystallographic orientation. The images of the indents on the (020) face show a significant material pile-up along all three edges of the indenter. Nevertheless, the same applied load on the (−101) or (002) face produces relatively flat indentation impressions and pile-ups. The pile-up is found only in one corner for the (−101) face and in one corner and the opposite edge of the indenter for the (002) face. The pile-up height on the (020) face is approximately 450 nm (Figure 3(c1)), whereas those on the 11



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(002) and (−101) faces are only approximately 180 and 80 nm, respectively. Cracks along the two edges of the intender are only observed on the (−101) and (002) faces. Significant mechanical anisotropy of the FOX-7 crystal with respect to plastic deformation and fracture behavior is thus found. Through the P–h curves, E and H, and the distribution and height of pile-ups, we can verify the strong mechanical anisotropy of FOX-7. 3.2 Understanding of Mechanical Anisotropy In principle, the nanoindentation characteristics of FOX are mostly determined by its crystal packing structure. Therefore, in this subsection, we analyze the nanoindentation characteristics based on the packing structure by performing DFT calculations.

Figure 4. Molecular structure (a), HBs denoted by purple dash (b), crystal packing of FOX-7 viewed along the a-axis (c) and along the a-axis with a small incline degree (d), and the three faces on which the nanoindentation tests are performed. The C, H, O, and N atoms are represented in gray, green, red, and blue, respectively. These representations are considered in the following figures.

The FOX-7 molecule is composed of an ethylene frame with two amino groups on one C atom and two nitro groups on another C atom (Figure 4(a)). The molecule is roughly planar with the largest torsion of CCNO of 36°, which is significantly greater that of the insensitive explosive 12


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molecule TATB (5°)56. All non-hydrogen atoms are not fully π-conjugated. This condition may possibly lead to a rather large sliding constraint during loading. Moderate intramolecular HBs of N-H…O also exist with HB acceptor-donor (A-D) distances of approximately 2.6 Å (Figure 4(b)). The wavelike-layered FOX-7 crystal is stacked along its b-axis, with weak interlayered π…π interactions and intralayered interactions, including strong intramolecular covalent bonds and rather weak but abundant intermolecular HBs with A-D distances of 2.9–3.2 Å (Figure 4(b)). The (020), (−101), and (002) faces (Figures 4(c) and 4(d)) make up the majority of the naturally grown faces in our crystal cultivation and serve as the orientations along which the indentation is performed.

Figure 5. Crystal packing of FOX-7 viewed along (top) and vertical to (bottom) the three indentation orientations of the (020) (a), (−101) (b), and (002) (c) faces, which are represented in orange.

The (020), (−101), and (002) faces are the indentation orientations and should be focused on. Microscopically, the movements of the FOX-7 molecules with certain deformations are vertical to these faces during indentation, thereby resulting in compression and sliding. Compression occurs mainly under the indenter, whereas sliding takes place mainly around the indentation. Accordingly, the packing structures vertical to these faces should be the root of the indentation properties of FOX-7. Figure 5(a) shows the (020) face parallel to the wavelike layers, i.e., the indentation on this 13



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face is vertical to the wavelike-layered stacking direction. This orientation possesses good compressibility because of the large intermolecular distance or the large interlayered distance of 3.28 Å (π…π stacking) along this orientation. Nevertheless, above a certain extent of compression, the layers are disrupted by the break of intermolecular HBs. The sliding vertical to this face is constrained because of the absence of layered stacking along the orientation, which is another reason for the layer disruption. Figures 5(b) and 5(c) show that the cases of the (−101) and (002) faces are similar because they are both perpendicular to the wavelike layers. The indentations on these two faces are parallel to the wavelike-layered stacking direction. Given the shorter intermolecular distances, the compression on these two faces becomes more difficult than that on the (020) face. As demonstrated in the bottom sections of Figures 5(b) and 5(c), the sliding vertical to these two faces is partly allowed. The preceding discussion indicates that the crystal packing vertical to the (020) face of FOX-7 is much distinct from those to the (−101) and (002) faces, thus setting a base for mechanical anisotropy, i.e., the compression on the (020) face is easy, and the sliding along the same direction is difficult. The cases are contrary to the case of the (−101) and (002) faces. We can then theoretically deduce the mechanical properties of the FOX-7 crystal. Owing to the easy compression on the (020) face, it exhibits greater elastic deformation on the P–h curve in Figure 2 and lower E and H in Table 1 in comparison with the (−101) and (002) faces. With the sliding constraint vertical to the (020) face, the pile-ups with the largest height appear around all three edges of the indenter (Figures 3a1 to 3c1), whereas the sliding vertical to the (−101) or (002) face is partly allowed, thereby resulting in a lower pile-up occurring around one of the indenter edges (Figures 3b2 to 3c3).

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0.16 0.14 0.12

(100) (010) (001)

0.10 Energy, eV/cell

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0.08 0.06 0.04 0.02 0.00

-0.02

0.00

0.01

0.02 Strain

0.03

0.04

Figure 6. Strain energy as a function of the strain of FOX-7 under uniaxial stress on (100), (010), and (001) faces.

Figure 7. Molecular arrangement and HBs of one single layer in the FOX-7 crystal viewed along the [010] direction (the b-axis). The bold blue and orange lines indicate the (−101) and (001) planes, respectively. The blue and orange arrows indicate the direction of uniaxial stress. The intermolecular HBs are denoted by green dashes. The HBs of one molecule between (−101) planes are denoted by black dashes, whereas those between (001) planes are denoted by blue dashes. The intralayered HB dissociation energies of (−101) and (001) planes are approximately 1.18 and 2.28 kJ/(mol·Å2), respectively. Table 2. Geometry parameters, positions of BCPs, electronic density (ρBCP, in e/bohr3) and its Laplacian (∇2ρBCP, in e/bohr5) at BCPs, and dissociation energy (EHB, in kJ/mol) of intermolecular HBs in FOX-7. λ is the change in the electron density at the BCP relative to the corresponding procrystal. 2 … ∇ ρBCP λ D-H…A D-H, Å H…A, Å ∠(D-H A), º ρBCP EHB 15



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N4-H3…O4 N3-H1…O1 N3-H2…O4 N4-H4…O2 N3-H1…O3 N4-H3…O1

0.904 0.945 0.943 0.849 0.945 0.904

2.143 2.145 2.291 2.341 2.446 2.390

143.7 152.5 132.9 138.1 118.1 148

0.0149 0.0154 0.0123 0.0096 0.0094 0.0090

-36% -33% -33% -42% -35% -42%

0.0645 0.0584 0.0490 0.0435 0.0406 0.0376

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13.869 13.602 10.296 8.034 7.590 7.061

DFT calculations on the mechanical properties of the FOX-7 crystal are performed to further understand the experimental observations and to verify the theoretical deduction. In principle, calculations on compression and sliding along crystal axes are more feasible than those along other orientations. For example, regarding the (−101) face, it should construct a large cell to strictly reflect crystal packing and calculate related properties. Thus, two strategies are employed to reveal the compressibility differences in the three faces. The compressibility on the (020) and (002) faces can be readily distinguished by the strain energy increase rates shown in Figure 6, and that on the (100) face is obtained. As demonstrated in Figure 6, the elastic moduli of the (010), (001), and (100) faces increase as the corresponding strain energies increase rapidly. This result is in agreement with the experimental determination of E of the (020) and (002) faces in Table 1. The compressibility of the (−101) face cannot be evaluated via the calculated results in Table 1. Although it can be strictly calculated as those of the (020) and (002) faces, a long computer time should be required. Thus, the strength of intermolecular interactions along a given orientation is adopted to the compressibility. Strong interactions suggest high compressibility. As stated above, the intralayered intermolecular interactions of FOX-7 are dominated by HBs. Owing to the weaker strength of intermolecular interactions relative to intramolecular covalent bonds, the degree of shortened intermolecular distances is larger than that of shrunk molecular sizes when a crystal is compressed. The early compressions on the (−101) and (001) planes strongly depend on intermolecular HBs because these two faces are both vertical to the wavelike layers. As illustrated in Figure 7, the intralayered HB dissociation energy of the (−101) face (1.18 kJ/(mol·Å2)) is less 16


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than that of the (002) face (2.28 kJ/(mol·Å2)) (these values are derived from Table 2); the compression on the (002) face is obviously difficult. This result is in accordance with the high value of E of the (002) face. The compressibility of the three faces is understood via the calculations, i.e., as a decreasing order of the (020), (−101), and (002) faces or an increasing order of E in Table 1. The compression anisotropy observed in FOX-7 is similar to that observed in other layered materials, such as Mn 2,2-dimethylsuccinate [Mn(C6H8O4)(H2O)]57 and Cu1.5(H2O)[O3PCH2CO2]58, which are both 2D layered. However, the interlayered interactions of these two materials significantly differ from each other. The interlayered interactions of Mn 2,2-dimethylsuccinate are weak van der Waals interactions, which result in high compressibility vertical to the layers, with E being 9.4 GPa. For Cu1.5(H2O)[O3PCH2CO2], the neighboring layers are hydrogen-bonded, with a larger E of 34.5 GPa. The compressibility or E strongly depends on the intermolecular interactions along the compression direction. Regarding the pop-ins on the P–h curves in Figure 2, they can be considered as the discrete responses of the crystal against compression or the plastic break of packing layers. As demonstrated in Figure 4, along all the three faces, each molecular layer is held by strong intralayer covalent bonds and HBs. When indenting the three faces (or the molecular layers), the mechanical loading leads to elastic compression first, followed by the break of the molecular layers caused by break intermolecular interactions. As the break proceeds, stress releases partly while the indenter tip penetrates continuously. Consequently, the displacements of pop-ins are the integral multiples of interplanar distances. The break then stops, and the molecular layers are elastically pressed. Beyond a certain extent, the break resumes, and the pop-in occurs again. The P–h curve shows that the molecular layers undergo successive compression and breaking when loading, which are determined by the intermolecular interactions along the loading direction. 17



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Figure 8. Scheme cited from Ref. 3 denoting the sliding characteristic of the FOX-7 crystal. (a) is the sliding barrier, and (b) shows the sliding constrained partly along the c-axis or vertical to the (002) face. The double arrows in (b1) and (b3) denote the steric hindrance during sliding. (b2) without the double arrows shows the free crystal.

With respect to the pile-up and crack in Figure 3, they can be understood on the basis of the sliding characteristic of FOX-7. Sliding characteristics have been discussed previously3. To conveniently reveal the pile-up and crack formation mechanism, we cite the previous result3, as illustrated in Figure 8. As demonstrated by a narrow belt in Figure 8(a), the interlayered sliding of FOX-7 is strictly allowed in a special direction, that is, along its a-axis only. Figure 8(b) shows that the sliding constraints are caused by the conflicts of the wave crests and the hollows of the neighboring layers along the (020) face. On the basis of this knowledge of sliding, the differences among the three faces in terms of pile-ups and cracking (Figure 3) can be understood. During the indentation on the (020) face, no sliding can occur because the press is vertical to the layers. Thus, the pile-ups appear around all the sides of the indentation. During the indentation on the (−101) or (002) face, the loading is parallel to the layer, which results in the sliding and pile-ups around the indentation parts. Given that no sliding occurs for a stress release when pressing the (002) face, the highest pile-up is extruded around the indentation, and a crack appears. As a result of the sliding, 18


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the interlayered dislocations are observed as cracks that are vertical to the (−101) and (002) faces. Overall, the compression on the (020) face acts uniformly and induces no slide cracking, whereas the press on the (−101) or (002) face is uneven and causes sliding along the preferred (100) plane; therefore cracks, as sliding planes, frequently act as cleavage planes57,58. 3.3 Correlation between the Nanoindentation Characteristics of FOX-7 and Its Impact Sensitivity As for an EM, the safety (e.g., impact sensitivity) of FOX-7 is important to elucidate on the basis of its crystal packing3–6. Considering that the impact sensitivity of an EM is strongly related to its mechanical properties, a correlation between the nanoindentation characteristics of FOX-7 and its impact sensitivity should exist. The preceding discussion indicates the distinct mechanical characteristics of FOX-7, such as fair compressibility of the (002) face and a certain extent of sliding when indenting the (−101) and (002) faces. Such compressibility and sliding capability are advantageous as buffers against external mechanical stimuli and energy dispersion3. The result is rather low impact sensitivity. Nevertheless, such compressibility and sliding capability are constrained in a limited range. For example, the sliding of FOX-7 is constrained only along the direction with a high sliding barrier, which is more difficult than that of a more impact-insensitive explosive TATB56, in which sliding occurs freely along a plane instead of occurring in one direction3. FOX-7 also possesses higher strain energy than TATB at the same strain, which suggests that FOX-7 is stiffer than TATB. Given an insignificant molecular stability difference between the two explosives4, the difference in crystal stacking is considered a main root for the difference in their impact sensitivities. Compared with a non-π-stacked explosive, such as HMX, with poor compressibility and sliding capability, FOX-7 shows a certain compressibility and sliding capability that make it more insensitive. From the nanoindentation characteristics of FOX-7, we can deduce its

19



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impact sensitivity. Meanwhile, its rather low impact sensitivity mechanism can be revealed by analyzing its stacking structures via theoretical calculations. 4. CONCLUSIONS We successfully obtained large single crystals of FOX-7 by adding a morphological modifier. The mechanical properties of FOX-7 were characterized by nanoindenting its (020), (−101), and (002) crystal faces. FOX-7 was mechanically anisotropic. The indention on the three faces resulted in distinct differences in P–h curves, with deduced E and H, pile-ups (including their distribution and height), and cracks. According to the DFT calculations, these mechanical characteristics were essentially attributed to the wavelike π-stacking of FOX-7, which caused various levels of compressibility and sliding capability along different directions. The wavelike π-stacking of FOX-7 responsible for its rather low impact sensitivity was also discussed, with its limited compressibility and sliding capability relative to other insensitive explosives, such as TATB, identified as the root cause. On this basis and in view of the molecular and stacking structures of explosives, we look forward to opportunities to design new explosives with HB-aided π-stacked structures and excellent compressibility and sliding capability. Their applications will be guaranteed once we deal with the energy problem. As a matter of fact, the newly-synthesized energetic salts59-61 have already provided a good prospective of impact insensitive EMs. ■ ASSOCIATED CONTENT Supporting Information Sample preparation and analyses of Nanoindentation Data. This material is available free of charge via the Internet at http://pubs.acs.org. ■ AUTHOR INFORMATION Corresponding Author 20


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*Email: Q. Zhang, [email protected]; H. Z. Li, [email protected]; and C. Y. Zhang, [email protected]. Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENT We thank Dr. Shiliang Huang from ICM for help in indexing the primary crystal faces of FOX-7 by single-crystal X-ray diffraction, and Mr. Lingang Lan and Prof. Ming Li from ICM for their helps in nanoindentation experiments. We greatly appreciate the financial support from the National Natural Science Foundation of China (11302199 and U1530262).

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Table of Contents Graphic Mechanical Anisotropy of FOX-7

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Mechanical Anisotropy of the Energetic Crystal of 1,1-Diamino-2,2

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