High-Pressure Structural Response of an Insensitive Energetic Crystal

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High-Pressure Structural Response of an Insensitive Energetic Crystal: 1,1-Diamino-2,2-dinitroethene (FOX-7) Zbigniew A Dreger, Adam I Stash, Zhi-Gang Yu, Yu-Sheng Chen, Yuchuan Tao, and Yogendra M. Gupta J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10010 • Publication Date (Web): 15 Nov 2016 Downloaded from http://pubs.acs.org on November 20, 2016

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The Journal of Physical Chemistry

High-Pressure Structural Response of an Insensitive Energetic Crystal: 1,1-Diamino-2,2-dinitroethene (FOX-7)

Zbigniew A. Dreger,1* Adam I. Stash,2 Zhi-Gang Yu,1 Yu-Sheng Chen,3 Yuchuan Tao,1 and Yogendra M. Gupta1 1

Institute for Shock Physics and Department of Physics and Astronomy, Washington State

University, Pullman, WA 99164-2816, USA 2

State Scientific Center of Russian Federation, Karpov Institute of Physical Chemistry,

103064 Moscow, Russia 3

ChemMatCARS, The University of Chicago, Advance Photon Source, Argonne, IL

60439, USA

*Corresponding author. E-mail address: [email protected], phone: (505) 335-4233.

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Abstract High-pressure structural response of an insensitive energetic crystal – 1,1-diamino-2,2dinitroethene (FOX-7) – was examined to gain insight into its structural and chemical stability and to obtain isothermal compression data. Using synchrotron single-crystal xray diffraction measurements, details of pressure-induced structural changes to 12.8 GPa were determined across three FOX-7 phases: α (P21/n), α′ (P21/n) and ε (P1). We found that the C-NO2 bond is the most compressible chemical bond, and high-pressure significantly reduces and homogenizes the length of H-bonds in the hydrogen bonds network. The α′- ε phase transition, at 4.5 GPa, significantly affects all molecular and crystal properties, whereas the α - α′ transition, at 2 GPa, is associated with subtle molecular and intermolecular changes. Anisotropic compressibility was observed over entire pressure range, consistent with the layered structure of the crystal. The equation-ofstate parameters were obtained, using the 3rd-order Birch-Murnaghan equation, below and above the α’- ε phase transition. It is shown that dispersion-corrected DFT-D calculations reproduce well pressure-induced changes in the unit cell parameters. The findings of this work provide new insights into the molecular and structural mechanisms governing the high-pressure stability/insensitivity of insensitive energetic crystals.

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1. Introduction 1,1-Diamino-2,2-dinitroethene (C2(NO2NH2)2, FOX-7 or DADNE) is an extremely promising insensitive high explosive (IHE) crystal with excellent performance, low sensitivity to initiation, and a relatively simple molecular structure.1-3 Because of these attractive characteristics, FOX-7 is viewed as a prototypical IHE crystal, and there have been considerable efforts to identify and understand its microscopic response at pressure and temperature conditions relevant to its initiation.4-15 In particular, detailed knowledge of the high-pressure response is essential for understanding its structural and chemical stability/insensitivity in terms of the relevant molecular and crystalline processes. Additionally, determination of the pressure dependence of the lattice parameters is required to develop accurate equation of state (EOS) models for describing the continuum response of FOX-7. Motivated by the above needs, recent single-crystal X-ray14 and powder neutron12 diffraction studies have examined changes in the FOX-7 structure under high pressure. These studies have established that FOX-7 can exist in three high-pressure phases: α, α′, and ε. The α phase (monoclinic, P21/n),2 stable at ambient conditions (Fig. 1), transforms to the α′ phase (> 2 GPa) and to the ε phase (> 4.5 GPa).12,14 Although the α′ phase was found to have the same space group as the α phase,12,14 detailed structural changes were not determined experimentally. The ε phase, examined at 5.9 GPa, was found to have the triclinic structure with P1 space group, and planar molecules and molecular layers.14 This result constituted the first determination of the high pressure structure of an IHE crystal and provided critical insight into structural factors stabilizing the structures of IHE crystals. 3 ACS Paragon Plus Environment


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Despite the identification of the polymorphic phases of FOX-7, previous studies covered a very limited pressure range and detailed structural information was obtained only at two pressures.4,12,14 Thus, extensive studies over an extended pressure range are required for understanding the FOX-7 response at conditions relevant to shock initiation. Careful determination of the crystal and molecular changes is needed to understand the structural factors governing the high-pressure stability. In particular, knowledge of atomistic details regarding, intra- and inter-molecular bonds is central for linking stability and structural factors. Furthermore, determination of pressure-induced changes of the unit cell parameters is required to obtain accurate equation of state parameters for developing continuum models. In this work, we present results from synchrotron single-crystal X-ray diffraction experiments to address the above objectives and provide a comprehensive insight into the structural response of FOX-7. Structural changes were examined across three FOX-7 phases (α, α′, and ε) and over a broad pressure range, to 12.8 GPa. By using singlecrystal diffraction measurements, we obtained detailed structural information, not available from previous powder diffraction experiments. In addition to the experimental effort, density functional theory (DFT) calculations were carried out: (i) to refine hydrogen atom positions to better determine the hydrogen bonding geometry, and (ii) to evaluate the use of DFT methods for determining pressure-induced changes in the unit cell parameters. The organization of the paper is as follows. In section 2, we briefly present the experimental, analytical, and computational methods used in this work. Section 3 presents results and discussion including: (i) experimental and computational results


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regarding pressure effects on unit cell parameters, (ii) isothermal equation of state, and (iii) experimental results of pressure effects on molecular and intermolecular bonding. The main conclusions of this work are presented in section 4. 2. Methods Details of both experimental and computational approaches were provided in our recent work.14 Here, we summarize only the salient features of these approaches. 2.1. Experimental and analytical approach FOX-7, as a fine powder, was received from Dr. Joel R. Carney of Naval Surface Warfare Center-Indian Head Division (NSWC-IHD). Single crystals were grown from a solution of FOX-7 in dimethyl sulfoxide (DMSO) at room temperature. High-pressure was generated using a Merrill-Bassett type diamond-anvil-cell (DAC) equipped with type Ia diamond anvils. The anvils were mounted on tungsten carbide backing plates of Boehler-Almax design16 to provide a wide opening angle (~ 80°) for the incident and diffracted x-ray beams. A 4:1 methanol/ethanol mixture (4:1 M/M) was used as pressure transmitting medium and pressure was monitored using ruby fluorescence to an accuracy of ~ 0.05 GPa. High-pressure experiments were performed to 12.8 GPa, the approximate limit of hydrostatic compression for 4:1 M/M. Note, the hydrostatic compression was confirmed by monitoring the splitting between the R1 and R2 ruby lines. High-pressure single-crystal x-ray diffraction measurements were obtained at ChemMatCARS (Sector 15) of the Advanced Photon Source, Argonne National Laboratory. All measurements were performed at room temperature. The beam energy was 30 keV and the beam size at the sample was ~ (0.08 x 0.08) mm2. Data were collected using a Bruker D8 fixed-chi diffractometer equipped with an APEX II CCD 5 ACS Paragon Plus Environment


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detector. Data collection and integration were performed using the APEX II v. 2014.5-0 software suite.17 The structure determination and refinement were carried out using the SHELX software package.18 Because the crystals undergo twinning at the 4.5 GPa phase transition, the unit cell parameters of ε-phase were determined using the CELL_NOW program.18 The structure determination was performed using single domain data. The TWINABS (APPEX II) program was used to extract the single domain data. The structure motif was obtained using the XT program from the SHELX software package.18 For structure refinement, amplitudes of the two domains were integrated using the scale factor from BASF (SHELX). For all structures, the thermal parameters were refined isotropically. Position of the hydrogen atoms was refined using DFT calculations. 2.2. Computational approach Calculations were performed using the periodic plane-wave pseudopotential method implemented in the CAmbridge Serial Total Energy Package (CASTEP).19 The generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE)20 parameterization and a plane-wave basis set with norm-conserving pseudopotential and energy cutoff of 380 eV were used. The Brillouin zone of the reciprocal space of FOX-7 crystal was sampled by the Monkhorst-Pack grid:21 2 × 2 × 1 k points. The BroydenFletcher-Goldfarb-Shanno (BFGS) algorithm22 was utilized to optimize the unit cell parameters and atomic coordinates simultaneously, while maintaining the corresponding crystal structure symmetry. Van der Waals interactions were systematically taken into account via the dispersion correction to the Kohn-Sham DFT energy developed by Grimme23 as implemented in CASTEP. 6 ACS Paragon Plus Environment

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3. Results and Discussion 3.1. Unit cell parameters Pressure-induced changes in the unit cell lattice parameters and angles are presented in Figure 2. Pressure-volume (p-V) data, calculated from the lattice parameters, are presented in Figure 3. In addition, the tabulated data are provided in Table S1. Initially, all unit cell parameters decrease monotonically with pressure to ~ 4.27 GPa (Figure 2). Although the lattice parameters show distinct anisotropic compression, no measurable changes occur at the α - α’ transition (at 2 GPa). At 1 atm, the b-axis compressibility is about 2.5-2.6 times larger than the a and c axes compressibility (Table 1); and this ratio is essentially maintained to 4.27 GPa, irrespective of the large decrease in compressibilities. The observed anisotropy is in accord with the anticipated difference in the intermolecular forces between the layers (van der Waals) and within the layers (H-bonding). The unchanged anisotropy ratio implies that the balance between intra- and inter-layer interactions is retained over this pressure range. At compression above 4.27 GPa, FOX-7 transforms to the ε phase which has a triclinic structure14 and all unit cell parameters are changed. The c-axis is reduced by ~ 38%, the unit cell angles increase considerably, and the a and b axes increase the least (Figure 2). As a result and as shown in Figure 3 the normalized unit cell volume (volume/Z) is reduced by ~ 1.1 ± 0.1 % at the transition. This volume change is relatively small; hence, the crystal does not fracture upon transition. At pressures beyond the α′- ε phase transition, all lattice parameters decrease gradually, and the β and γ angles increase slightly. These continuous changes preserve the large anisotropic compressibility (see Table 1) and the ε phase structure to 12.8 GPa – the highest pressure examined in this 7 ACS Paragon Plus Environment


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study. The structural constancy of the ε phase over a broad range of pressures emphasizes the importance of this phase for understanding the insensitivity/stability of FOX-7 at pressures relevant to shock initiation. 3.1.1. DFT calculations Single crystal experimental results over a broad pressure range provide excellent benchmark data for examining the applicability of DFT methods to FOX-7. Predictions of compression induced volume and lattice parameter changes serve as a good test for assessing the accuracy of DFT methods to describe response of solids, in general, and HE crystals, in particular, under extreme conditions. There have been several previous DFT studies to examine pressure effects on the FOX-7 crystal structure.12, 24-28 Both the local density approximation (LDA) and the generalized gradient approximation (GGA) were used for the exchange-correlation potentials. The calculated ambient and high-pressure unit cell parameters were clearly underestimated by LDA and overestimated by GGA, due to the insufficient treatment of van der Waals interactions using these standard approximations. Recently, it has been shown that the dispersion-corrected DFT methods can effectively address this deficiency.26 In particular, the empirical dispersion correction proposed by Grimme23 (DFT-D) was used to examine data from powder x-ray4 (to 3.86 GPa) and neutron12 (to 4.14 GPa) diffraction experiments. An improved agreement between the calculated and experimental unit cell parameters was reported for both x-ray26-28 and neutron12 diffraction data. Here, we present the use of the DFT-D approach for simulating our single crystal results. Pressure-induced changes in the unit cell parameters in the α, α’, and ε phases 8 ACS Paragon Plus Environment

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were examined over the pressure range, to 12.8 GPa. The calculated results are presented in Figure 3, along with the experimental data. An examination of Figure 4 and Table S2a, b reveals several characteristic features. Both below and above 4.5 GPa, the pressure trends and experimental data are reproduced well by the DFT-D method, for all parameters. The calculated unit cell volumes and angles are within 1.2% of the measured values for all pressures. The deviations are slightly larger for the lattice parameters than for the volume, with a maximum deviation of -2.1%. The calculated length along the axis normal to the molecular layers (b-axis below 4.5 GPa, and a-axis, above 4.5 GPa) deviates more from the experimental value than the lengths along the other two axes. This result indicates that, even with the DFT-D method, the weak inter-layer interactions are not reproduced as well as the strong in-layer interactions in the calculations. With increasing pressure, the interactions between the layers become stronger and the agreement between the calculated and experimental values improves significantly (Table S2). Overall, our results demonstrate that pressure-induced changes in the FOX-7 unit cell parameters are reproduced well by the DFT-D calculations across different phases and over a broad pressure range. This result lends confidence for the use of the DFT-D method for predicting the FOX-7 response at high pressures. 3.1.2. Isothermal compressibility The pressure-volume (p-V) data provide a thermodynamic relationship fundamental to the equation of state (EOS) models used for continuum modeling. Although the FOX-7 single crystal compression data display strong anisotropy, as discussed above, we have used an isotropic description to provide consistency with the literature on energetic 9 ACS Paragon Plus Environment


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crystals. The p-V data from our work were fit to a semiempirical EOS to determine the bulk modulus and its first derivative for isothermally compressed FOX-7. We used the third-order Birch-Murnaghan (3-BM) EOS, often used to describe compressibility over a broad range of pressures and commonly used to compare thermodynamic parameters between different HE crystals.29 The 3-BM EOS equation is as follows:30 =

−

× 1 + − 4

− 1 ,

where V0 and V are the unit cell volumes at 1 atm and pressure p, and and denote the 1 atm isothermal bulk modulus and its pressure derivative, respectively. The resulting p-V fit is shown in Figure 5 and in Figure S1. Below 4.5 GPa, our results are presented along with results obtained from the x-ray4 and neutron12 powder diffraction experiments to 3.86 GPa and 4.14 GPa, respectively. Our results and neutron diffraction12 results are essentially indistinguishable, whereas the curve for x-ray results from Ref. 4 has a clearly different curvature. The and parameters for these curves are listed in Table 2; the bulk moduli from the three studies are comparable, within ± 10 %. However, the parameters, related to nonlinear compressibility, from our results and neutron diffraction results are close, but very different from the powder x-ray diffraction results.4 This discrepancy is likely, in part, due to nonhydrostatic compression, because no pressure transmitting medium was used in that study.4 To obtain EOS parameters above 4.5 GPa, the ε phase volume at 1 atm is required. Because this value is unknown, the p-V data were fit by assuming this value to be half the volume of the α phase at 1 atm (ε = α /2, because the unit cell volume is halved at the α′- ε phase transition. The B0 and B′0 parameters obtained are shown in


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Table 2, and the fitting curve in Figure S1. As seen, the bulk modulus of 15.5 ± 1.6 GPa for the ε phase is considerably larger than for the α phase. More importantly, this value provides a lower limit since the ε phase volume at 1 atm will be lower than for the α phase due to the 1.1% volume reduction at the α′- ε phase transition.31 Thus, FOX-7 is clearly transformed to a significantly stiffer crystal at pressures above 4.5 GPa. 3.2. Intra- and inter-molecular interactions Pressure effects on intra- and inter-molecular distances provide key insights into the role of these interactions on the chemical and structural stability of the compressed crystal. In FOX-7, the molecules are arranged into molecular layers and held together by weak van der Waals interactions and relatively strong hydrogen bonds. Thus, the FOX-7 stability is governed by the balance between these two types of interactions and the intramolecular strains. 3.2.1. Intramolecular bonds FOX-7 molecule has five different types of intramolecular bonds. In Figure 6, we present pressure-induced changes for selected bonds – C-NO2, C-NH2, and N-O bonds – to illustrate the trends. Results in Figure 6 represent the average values for the corresponding bonds length. Linear fits to the data show that all bonds display small but consistent shortening with increasing pressure below 4.5 GPa. The estimated reduction in lengths were ~ 1.0, 0.5 and 0.1% for the C-NO2, C-NH2, and N-O bonds, respectively.32 The largest change in the C-NO2 bond suggests that this bond is the most compressible and the observed shortening may imply its strengthening. Although the molecular mechanisms of FOX-7 decomposition under static or dynamic compression are unknown,



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it is anticipated that the C-NO2 bond homolysis could be an important step in this process.33 Therefore, the observed considerable shortening of this bond may play a part in the increased chemical stability of FOX-7 at high pressures. Due to the α′- ε phase transition above 4.5 GPa, FOX-7 molecules in the unit cell assume two different conformations.14 As shown in our previous work,14 these two molecules are ‘flatter’ and have different bonds length than the molecules in the α′ phase. Because detailed changes in the geometry of the molecules at the α′- ε phase transition were reported previously,14 only pressure effects on the bond length are considered here. Pressure induced changes in the bonding of two conformers, molecules 1 and 2, are shown in Figure 6. We can see that all bond lengths change at the transition by shortening the bond in one molecule and lengthening the corresponding bond in the other molecule. The experimental data in the ε phase show large distribution and uncertainties, because of the lower accuracy of structure refinement for a twinned crystal. Hence, linear fits used to assess the pressure dependence of intramolecular bonds in the ε phase should be viewed with care. 3.2.2. Inter- and in-layer molecular distances Pressure effects on intermolecular distances are presented in Figures 7 and 8. Changes in the inter-layer distance shown in Figure 7a were derived from the spacing of (020) planes for pressures below 4.5 GPa and from the spacing of (200) planes for pressures above 4.5 GPa. As pressure increases, the distance reduces gradually by ~ 9% to 4.27 GPa. However, it increases by ~ 2.4% at the α′- ε transition due to the transformation of the wave-shaped layers into the planar layers. The pressure-induced changes in the interlayer angle, which define changes in the planarity of layers, are shown in Figure 7b. As 12 ACS Paragon Plus Environment

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pressure increases, the angle gradually increases from 35.5° to 42.3°, and then rapidly decreases to ~ 5° upon the α′- ε transition. All these changes likely reduce the repulsive forces between layers, enabling further gradual decrease in the inter-layer distance with pressure in the ε phase. Additionally, formation of well-spaced planar layers of molecules may have important implications for the stability/insensitivity of FOX-7 under shock compression. The arrangement of planar layers provides a potential channel for dissipating the energy imparted to the crystal from the shock wave through interlayer slip planes. Thus, FOX-7 in the ε phase would require higher stresses or energies for shock initiation. Pressure effects on intermolecular distances within FOX-7 layers were examined by determining the distance between molecular cores and between the closest atoms. The first provides information on changes in the relative arrangement of the molecules, whereas the second provides changes in the hydrogen bonding. Distance between the molecular cores was determined as a distance between the corresponding carbon-atoms of the neighboring molecules. As shown in Figure 8a and b, molecules in the layers form ‘ribbons’ of head-to-tail oriented molecules. Because of this arrangement, we examined changes in the distance between molecules in the same ribbon (molecules A and B) and between molecules from adjacent ribbons (molecules A and C). The results are presented in Figure 8c, with the labeling of molecules and C-atoms shown in Figure 8a. At ambient conditions, the distance between molecules located in the adjacent ribbons is longer than between molecules in the same ribbon. Also, we note that the C1-C1 and C2-C2 distances determined between molecules located in the adjacent ribbons (molecules A and C) are the same, but between molecules located in the same



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ribbon (molecules A and B) are different. This result shows that the A and B molecules are in different planes, whereas the A and C molecules are in the same plane. Several noticeable changes are observed in distances between molecules in the same ribbon and adjacent ribbons with pressure. Below 4.5 GPa, the distance between A and C molecules decreases more (3.4 %) than between A and B molecules (2.5%). These changes are also much smaller than the changes between the molecular layers (9%), confirming a large anisotropy between in-layer and inter-layer interactions. We also note a deviation from the gradual decrease of distances around 2 GPa, likely a manifestation of intermolecular changes due to the α-α′ phase transition. Pressure increase above 4.5 GPa reorients the molecules in the layers to form planar layers, Figure 7b.14 However, as seen in Figure 8b, this change increases the distance between molecules in the ribbons to values similar to the distance between molecules in the adjacent ribbons. This transition likely makes in-layer interactions more isotropic than in the preceding phases. 3.2.3. Hydrogen bonds The closest molecular distances in FOX-7 layers are controlled by hydrogen bonds (Hbonds). Therefore, a determination of changes in these bonds is critical for understanding FOX-7 molecular and crystal stability. As shown in Figure 9a, each FOX-7 molecule has two intramolecular H-bonds, and is bonded with six neighboring molecules through six pairs of intermolecular H-bonds. To follow pressure changes in these bonds, they were labeled with numbers: 4 and 8 for intramolecular bonds, and 1-3 and 5-7 for intermolecular bonds. The assignment of bonds to the numbers is given in Figure S2 and Table S3.


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At ambient pressure, the two intramolecular bonds are the shortest H-bonds in FOX-7 and have similar lengths, see Figure 9b. In contrast, the length of intermolecular bonds varies considerably, comprising the range of 0.44 Å. Pressure increase to ~ 4.5 GPa barely changes the length of intramolecular H-bonds, but reduces substantially the length of almost all intermolecular H-bonds. Furthermore, the bonds connecting molecules in the adjacent ribbons (3, 5, and 6) are reduced more than the bonds connecting the molecules in the same ribbon (1, 2, and 7). We note irregular changes in the bonds length around 2 GPa, likely associated with the α - α′ phase transition. Around 4.5 GPa, the change in planarity of molecular layers affects the length of individual Hbonds, but maintains the same motif of the H-bonds network. The abrupt changes are observed in the length of all hydrogen bonds. Half of the bonds, including two intramolecular bonds, show significant decrease in the bond length, whereas the other half shows some increase. Thus, the resulting average length of H-bonds decreases in the ε phase. There is further decrease in the length of all H-bonds, as pressure increases beyond 4.5 GPa. At the highest pressure, 12.8 GPa, the intermolecular H-bonds show noticeably smaller distribution in length (0.16 Å), compared to distribution at ambient pressure (0.44 Å), suggesting ordering of intermolecular H-bonds toward an isotropic network. This result corroborates the observed homogenization of intermolecular distances in molecular layers in the ε phase. Significant decrease in the H-bonds length due to compression and the α′- ε phase transition, implies strengthening of these bonds and ordering of H-bonds network with increased pressure. Thus, the changes in hydrogen bonds will enhance the crystal stability through the increase of in-layers interactions.



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4. Summary and Conclusions High-pressure synchrotron single-crystal X-ray diffraction measurements were obtained for an insensitive energetic crystal, FOX-7, to 12.8 GPa. Molecular, intermolecular and crystalline changes were examined to understand their role on the structural and chemical stability/insensitivity of FOX-7. Also, pressure-induced changes in unit cell parameters were used to provide equation of state parameters and to evaluate DFT predictions of the pressure effects. The main findings of this work are: 1. The C-NO2 bond is the most compressible chemical bond. Since the C-NO2 homolysis could be an important step in the decomposition process, the observed shortening of this bond may contribute to the increased chemical stability of FOX-7 at high pressures. 2. Hydrogen bonding network maintains the same motif irrespective of the phase transformations. However, significant shortening and homogenization of hydrogen bond lengths were observed with increasing pressure. Such changes can stabilize the crystal by strengthening the intermolecular interactions in molecular layers. 3. Anisotropic interactions in the crystal are maintained over the entire pressure range examined, in accord with the layered-structure. The experimental anisotropy is well reproduced by the dispersion-corrected DFT-D calculations, demonstrating the applicability of this method to account for both the van der Waals and hydrogen bonding interactions. 4. The parameters of isothermal equation of state were determined for pressures below and above α′- ε phase transition, yielding a bulk modulus that was considerably higher in the ε phase. The FOX-7 crystal becomes stiffer after the phase transition at 4.5 GPa.


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We have shown that the high-pressure structural and chemical stability of an IHE crystal, FOX-7, is controlled mainly by the accommodation of external compression through the anisotropic compressibility of the unit cell, phase transformation of the structure to planar layers, and increase in the hydrogen and C-NO2 bonding strength. These findings will be useful for understanding the shock response of IHE crystals. Supporting Information Table S1: Pressure dependence of unit cell parameters and volume. Table S2: Comparison of experimental and calculated (DFT-D) unit cell parameters at selected pressures. Figure S1: Fit of p-V data using the Birch-Murnaghan equation. Figure S2: FOX-7 atoms labeling. Table S3: Hydrogen bonds labeling. Figure S3: Pressure effect on hydrogen bonds length. This material is available free of charge via the Internet at http://pubs.acs.org/. Acknowledgements Dr. Joel R. Carney from Naval Surface Warfare Center - Indian Head Division (NSWCIHD) is thanked for providing the FOX-7 powder. Dr. Przemyslaw Dera from University of Hawaii at Manoa is thanked for helping in the initial experimental and analytical efforts on the FOX-7 structure. ZAD, YT and YMG acknowledge support from DOE/NNSA (DE-NA0002007) and ONR (N000014-16-1-2088). Experiments were performed at the ChemMatCARS Sector 15 of Advanced Photon Source, Argonne National Laboratory. The ChemMatCARS Sector 15 is principally supported by the Divisions of Chemistry (CHE) and Materials Research (DMR), National Science Foundation, under grant number NSF/CHE-1346572. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy 17 ACS Paragon Plus Environment


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(DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357.


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Latypov, N. V.; Bergman, J.; Langlet, A.; Wellmar, U.; Bemm, U. Synthesis and Reactions of 1,1-diamino-2,2-dinitroethylene. Tetrahedron 1998, 54, 11525-11536.

(2)

Bemm, U.; Östmark, H. 1,1-Diamino-2,2-Dinitroethylene: A Novel Energetic Material with Infinite Layers in Two Dimensions. Acta Crystallogr. Sect. C 1998, 54, 1997-1999.

(3)

Lochert, I. J. FOX-7 - A New Insensitive Explosive. DSTO-TR-1238; Defense Science and Technology Organization, Aeronautical and Maritime Research Laboratory: Victoria, Australia, 2001; pp 1-23.

(4)

Peiris, S. M.; Wong, C. P.; Zerilli, F. J. Equation of State and Structural Changes in Diaminodinitroethylene under Compression. J. Chem. Phys. 2004, 120, 8060-8066.

(5)

Evers, J.; Klapötke, T. M.; Mayer, P.; Oehlinger, G.; Welch, J. α- and β-FOX-7, Polymorphs of a High Energy Density Material, Studied by X-ray Single Crystal and Powder Investigations in the Temperature Range from 200 to 423 K. Inorg. Chem. 2006, 45, 4996-5007.

(6)

Crawford, M.-J.; Evers, J.; Goebel, M.; Klapötke, T. M.; Mayer, P.; Oehlinger, G.; Welch, J. M. Gamma-FOX-7: Structure of a High Energy Density Material Immediately Prior to Decomposition. Propellants, Explos., Pyrotech. 2007, 32, 478-495.

(7)

Pravica, M.; Liu, Y.; Robinson, J.; Velisavljevic, N.; Liu, Z. X.; Galley, M. A. High-Pressure Far- and Mid-Infrared Study of 1,1-diamino-2,2-dinitroethylene. J. Appl. Phys. 2012, 111, 103534-9.



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(8)

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Bishop, M. M.; Chellappa, R. S.; Pravica, M.; Coe, J.; Liu, Z.; Dattlebaum, D.; Vohra, Y.; Velisavljevic, N. 1,1-Diamino-2,2-dinitroethylene under High PressureTemperature. J. Chem. Phys. 2012, 137, 174304-8.

(9)

Dreger, Z. A.; Tao, Y.; Gupta, Y. M. Polymorphs of 1,1-Diamino-2,2- dinitroethene (FOX-7): Isothermal Compression Versus Isobaric Heating. Chem. Phys. Lett. 2013, 584, 83-87.

(10) Dreger, Z. A.; Tao, Y.; Gupta, Y. M. High Pressure Vibrational and Polymorphic Response of 1,1-Diamino-2,2-Dinitroethene (FOX-7) Single Crystals: Raman Spectroscopy. J. Phys. Chem. A 2014, 118, 5002-5012. (11) Tao, Y.; Dreger, Z. A.; Gupta, Y. M. High-Pressure Stability of 1,1-Diamino-2,2dinitroethene (FOX-7): H/D Isotope Effect. Chem. Phys. Lett. 2015, 624, 59-63. (12) Hunter, S.; Coster, P. L.; Davidson, A. J.; Millar, D. I. A.; Parker, S. F.; Marshall, W. G.; Smith, R. I.; Morrison, C. A.; Pulham, C. R. High-Pressure Experimental and DFT-D Structural Studies of the Energetic Material FOX-7. J. Phys. Chem. C 2015, 119, 2322-2334. (13) Bishop, M. M.; Velisavljevic, N.; Chellappa, R.; Vohra, Y.K.; High PressureTemperature Phase Diagram of 1,1-Diamino-2,2-dinitroethylene (FOX-7). J. Phys. Chem. A 2015, 119, 9739-9747. (14) Dreger, Z. A.; Stash, A. I.; Yu, Z-G.; Chen, Y-S.; Tao, Y.; Gupta Y. M. HighPressure Crystal Structures of an Insensitive Energetic Crystal: 1,1-Diamino-2,2dinitroethene. J. Phys. Chem. C 2016, 120, 1218-1224.


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(15) Dreger, Z. A.; Tao, Y.; Gupta, Y. M. Phase Diagram and Decomposition of 1,1Diamino-2,2-dinitroethene Single Crystals at High Pressures and Temperatures. J. Phys. Chem. C 2016, 120, 11092-11098. (16) Boehler, R.; De Hantsetters, K. New Anvil Design in Diamond Cell, High Pressure Res. 2004, 24, 391-396. (17) Bruker AXS Inc. Madison, APEX 2, Wisconsin, YSA, 2014. (18) Sheldrick, G. M. SHELX, Program for Crystal Structure Solution, University of Gottingen, Germany, 2013. (19) Clark, S. J.; Segall, M. D.; Pickard, C. J.; Hasnip, P. J.; Probert, M. J.; Refson, K.; Payne, M. C. First Principles Methods using CASTEP. Z. Kristallogr. 2005, 220, 567-570. (20) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868. (21) Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B 1976, 13, 5188-5192. (22) Pfrommer, B. G.; Cote, M.; Louie, S. G.; Cohen, M. L. Relaxation of Crystals with the Quasi-Newton Method. J. Comput. Phys. 1997, 131, 233-240. (23) Grimme, S. Semiempirical GGA-type density functinal constructed with a longrange dispersion correction. J. Comput. Chem. 2006, 27, 1787-1799. (24) Zhao, J. J.; Liu, H. High-Pressure Behavior of Crystalline FOX-7 by Density Functional Theory Calculations. Comp. Mater. Sci. 2008, 42, 698-703. (25) Wu, Q.; Zhu, W. H.; Xiao, H. M. DFT Study on Crystalline 1,1-Diamino-2,2dintroethylene Under High Pressures. J. Mol. Model. 2013, 19, 4039-4047.



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(26) Sorescu, D. C.; Rice, B. M. Theoretical Predictions of Energetic Molecular Crystals at Ambient and Hydrostatic Compression Conditions Using Dispersion Corrections to Conventional Density Functionals (DFT-D). J. Phys. Chem. C 2010, 114, 6734– 6748. (27) Appalakondaiah, S.; Vaitheeswaran, G.; Lebegue, S. Structural, Vibrational, and Quasiparticle Band Structure of 1,1-Diamino-2,2-dinitroethelene from Ab Initio Calculations. J. Chem. Phys. 2014, 140, 014105-1-7. (28) Averkiev, B. B.; Dreger, Z. A.; Chaudhuri, S. Density Functional Theory Calculations of Pressure Effects on the Structure and Vibrations of 1,1-Diamino2,2-dinitroethene (FOX-7). J. Phys. Chem. A 2014, 118 10002-10010. (29) Peiris, S. M.; Gump, J. C. In: Static Compression of Energetic Materials; Peiris, S. M.; Piermarini, G. J. Eds.; Springer-Verlag: Berlin Heildelberg, 2008; ch. 3, pp 99126. (30) Birch, F. The Effect of Pressure upon the Elastic Parameters of Isotropic Solids, According to Murnaghan’s Theory of Finite Strain. J. Appl. Phys. 1938, 9, 279288. (31) For example, if the volume of ε phase at 1 atm is reduced by 1%, the 3-BM equation parameters would be: = 17.5 ± 1.7 GPa and = 5.8 ± 0.8. (32) The estimated shortening of C-C bond length was ~ 0.3-0.4%. The N-H bond length was kept invariant at 1.02 Å over the pressure range studied. (33) Bellamy, A. J. In: Structure and Bonding; Mingos, D. M., Ed.; Springer-Verlag: Berlin, 2007; Vol. 125, pp1-33.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure Captions Figure1. Crystal structure of FOX-7 at ambient pressure: (a) Projection of structure on the (101) plane, showing arrangment of molecules in the layer and (b) projection of structure on the (011) plane, showing arrangment of wave-shaped layers. Molecules are represented by the stick model. Hydrogen bonds are denoted by blue-dashed lines. Legend: Carbon, grey; Nitrogen, blue; Oxygen, red; Hydrogen, white. Crystal unit cells are represented by white lines. Figure 2. Pressure dependence of the FOX-7 unit cell parameters: (a) lattice parameters, the inset shows an enlarged view of the b and c axes results above the 4.5 GPa phase transition, (b) normalized lattice parameters, and (c) unit cell angles. Note, the a and b axes exchange the labels at the transition as a result of change in the space group. The error bars are within the size of symbols. Figure 3. Pressure dependence of the normalized unit cell volume (V/Z) of FOX-7. Open circles represent experimental results. The curves are fits to the experimental data: third order Birch-Murnaghan (3-BM) equation (< 4.5 GPa) and second order polynomial (> 4.5 GPa). The vertical dotted lines denote boundaries between different phases. The inset shows the V/V0 results. Figure 4. Comparison of measured and calculated pressure effects on the unit cell parameters and volume of FOX-7: (a) lattice parameters, (b) unit cell angles, and (c) unit cell volume/Z. Symbols represent the experimental data. Note, the a and b axes exchange the labels at the transition as a result of change in the space group. Curves were obtained from the DFT-D calculations.



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Figure 5. Comparison of pressure dependence of unit cell volume below 4.5 GPa . Open symbols represent the experimental results. The solid curves represent the fits to the third-order Birch-Murnaghan equation of state (EOS), and the fitting parameters are listed in Table 2. The blue triangles and blue solid curve represent data obtained from powder x-ray diffraction, Ref. 4. The green squares and green solid curve represent data obtained from powder neutron diffraction, Ref. 12. The red circles and red solid curve represent present results (single crystal x-ray diffraction). Figure 6. Pressure effect on intramolecular bonds lengths in FOX-7. The vertical dashed line marks the pressure for the α′- ε transition. (a) C-NO2 and C-NH2 bonds; results averaged from two corresponding bonds. (b) N-O bond; results averaged from four coresponding bonds. The experimental data were fitted with linear curves. For the ε phase, results are shown for two molecular conformers denoted as Mol-1 (molecule 1, solid circles) and Mol-2 (molecule 2, open circles). Note that the error bars are significantly larger for molecules in the ε phase, because of the lower accuracy in the structure refinement due to twinning. Figure 7. Pressure effects on intermolecular parameters. (a) Pressure effect on the distance between the FOX-7 molecular layers. Below 4.5 GPa, the distance is derived from the spacing of (020) planes. Above 4.5 GPa, distance is derived from the spacing of (200) planes. Curves are second order polynomials fits to the experimental data. (b) Pressure effect on the angle between molecules in the same layer. Insets represent shapes of molecular layers below and above the α′- ε phase transition. The angle is indicated in


Page 25 of 37

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the inset. The vertical dashed line marks the pressure for the α′- ε transition. The error bars are comparable to the size of symbols. Figure 8. Pressure effect on intermolecular distances in FOX-7 layers. The vertical dashed lines denote pressures for the α - α′ and α′- ε transitions. The distances are measured between carbon atoms. (a) View of molecular arrangements in the layer; molecules A and B belong to the same ‘ribbon’, whereas A and C belong to adjacent ‘ribbons’. The dashed lines are used to delineate the ‘ribbon’. (b) View showing the wave-shaped layer, and (c) Pressure dependence of intermolecular distances between molecules in the same ‘ribbon’: C1(A) – C1(B) or C2(A) – C2(B), and between molecules in adjacent ‘ribbons’ C1(A) – C1(C) or C2(A) – C2(C). The error bars are comparable to the size of symbols. Figure 9. (a) Comparison of hydrogen bond networks below and above 4.5 GPa. The hydrogen bonds are depicted by dashed blue lines. Numbers denote the corresponding bonds in two phases. The assignment of bonds to numbers is given in Table S3. (b) Pressure effect on length of hydrogen bonds (O⋅⋅⋅H) length. The vertical dashed lines denote pressures for the α - α′ and α′- ε phase transitions. Experimental data (points) are connected with lines to guide the eye. Two vertical bars represent the avaraged error bars below and above 4.5 GPa. For pressure > 4.5 GPa, the results are shown for the layer centered at x = ~ 1/4. The results for the layer centered at x = ~ 3/4 are shown in Figure S3.



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Page 26 of 37

Table 1. Compressibility of FOX-7 crystal along the a, b, and c axes at different pressures.

Pressure GPa

Linear compressibility (10-2 GPa-1)a Below 4.5 GPa

0

βa 1.63

βb 4.28

βc 1.69

4.27

0.46

1.06

0.43

Above 4.5 GPa 5.90 12.8 a

βb 0.33 0.15

βa 1.14 0.70

βc 0.27 0.17

Linear compressibility is defined as βi = - (1/i0)(∂i/∂p), where ‘i’ stands for the a-, b- or

c-axis of the unit cell. Index 0 corresponds to the values at 1 atm for pressures below 4.5 GPa and to the values at 5.9 GPa for pressures above 4.5 GPa. Note, the labeling of the a and b axes switches at the α′ - ε phase transition.


Page 27 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Bulk moduli (B0) and its pressure-derivative (B′0) for FOX-7 using the thirdorder Birch-Murnaghan equation.

V0 (Å3)

B0 (GPa)

B′0

Below 4.5 GPa Peiris et al. Ref [4] Hunter et al. Ref [12] This work

514.60 ± 3.30

9.5 ± 1.3

22.4 ± 4.6

521.07 ± 0.02

11.3 ± 0.6

12.7 ± 1.3

522.92 ± 0.06

10.1 ± 0.7

14.3 ± 1.7

Above 4.5 GPa This worka a

261.46 ± 0.03

15.5 ± 1.6

6.3 ± 0.9

For the ε phase, the volume at p = 1 atm (V0) is adapted from the α phase, but it is

divided by two because of the unit cell volume being halved above 4.5 GPa.



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Page 28 of 37

Figure 1, Dreger et al

A (a)

a c

(b) b c

a b c


Page 29 of 37

Normalized LP (a. u.)

Lattice parameters (LP) (Å)

Figure 2. Dreger et al.

12

(a)

c

c

b

11

6

8

a

7

0.90

12

a

6 1.00

10

b~c

b

c/c0

b/b0

b/b0 a/a0

a/a0

0.64

c/c0

0.60

(b)

120 Angle (degree)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(c)



115 110



95





90 0

2

4

6

8

10 12 14

Pressure (GPa)




1.0

V/V0

130

0.9

0.8

3

Unit cell volume/Z (Å )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

120 0.7 0 2 4 6 8 10 12

Pressure (GPa)

110



'



100 P21/n (Z=4)

P1 (Z=2)

0

6

90 2

4

8

10 12 14

Pressure (GPa)


Page 30 of 37

Page 31 of 37

Lattice parameters (Å)


12

(a)

c

11 b~c

a

7

b

a

6 120

(b)



o

Angle ( )

115 110



95





90 3

Unit cell volume/Z (Å )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(c)

130 120 110 100 0

2

4

6

8

10 12 14

Pressure (GPa)




520 3

Unit cell volume (Å )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 37

Ref [4] Ref [12]

500

This work

480

460

440 0

1 2 3 4 Pressure (GPa)


5

Page 33 of 37

Figure 6, Dreger et al.

1.55 Intramolecular bond length (Å)

(a)

C-NO2 (Mol-2)

1.50 1.45

C-NO2

C-NO2 (Mol-1)

1.40 1.35

C-NH2 C-NH2 (Mol-2)

1.30 1.25

C-NH2 (Mol-1)

1.20 0 Intramolecular bond length (Å)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2

4 6 8 10 12 14 16 Pressure (GPa)

(b)

1.35

N-O (Mol-2)

1.30 N-O

1.25

N-O (Mol-1)

1.20 0

2

4 6 8 10 12 14 16 Pressure (GPa)




Interlayer distance (Å)

3.4

(a)

3.2 0.07 Å (2.4 %)

3.0

2.8 0

2

4 6 8 10 12 14 Pressure (GPa)

60 Interlayer angle (deg.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(b)

40

20

0 0

2

4

6

8 10 12 14

Pressure (GPa)


Page 34 of 37

Page 35 of 37


(a)

C C2

A

C1

C2 C1 C2 C1

B C (b)

c

A

a C c

C (c) 7.0 Intermolecular distance (Å)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


b

B

C1(A) - C1(C) or C2(A) - C2(C)

6.9

C1(A) - C1(B) or C2(A) - C2(B)

6.8 6.7 6.6

C1(A) - C1(B)

6.5

C2(A) - C2(B)

6.4 0

2

4

6

8

10

12

Pressure (GPa)


14


Figure 9. Dreger et al. (a)

< 4.5 GPa 6

3 5 4

2 1 7 8

> 4.5 GPa

2

3 5 4 6

(b)

1

a c

7

8

b

c

5

2.4 1

Hydrogen-bond length (Å)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

3

2.2 6 7

2.0

2 4

1.8

8

1.6 0

2

4

6

8

10 12 14

Pressure (GPa)


Page 36 of 37

Page 37 of 37

TOC

Volum e/Z (Å3 ) Param e ters (Å)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


12

c

11 a

7 6

b~c

b P21/n (Z=4)

125 110

P1 (Z=2)

95 0

2

4

6

8

10

12

14

Pres sure (GPa)


High-Pressure Structural Response of an Insensitive Energetic Crystal

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