Pressure-Induced Conformer Modifications and Electronic Structural

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Pressure-Induced Conformer Modifications and Electronic Structural Changes in 1, 3, 5-triamino-2, 4, 6-trinitrobenzene (TATB) Upto 20 GPa Xiaoyu Sun, Xiangqi Wang, Wentao Liang, Chan Gao, Zhilei Sui, Moxiao Liu, Rucheng Dai, Zhongping Wang, Xianxu Zheng, and Zengming Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03323 • Publication Date (Web): 03 Jul 2018 Downloaded from http://pubs.acs.org on July 4, 2018

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The Journal of Physical Chemistry

Pressure-Induced Conformer Modifications and Electronic Structural Changes in 1, 3, 5-triamino-2, 4, 6-trinitrobenzene (TATB) Upto 20 GPa Xiaoyu Sun1†, Xiangqi Wang1†, Wentao Liang1, Chan Gao1, Zhilei Sui1, Moxiao Liu2, Rucheng Dai3, Zhongping Wang3*, Xianxu Zheng4, and Zengming Zhang3,5* 1

Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, China

2

College of Computer Science and Technology, Chongqing University of Posts and Telecommunications, Chongqing 40065, China 3

The Centre for Physical Experiments, University of Science and Technology of China, Hefei, Anhui 230026, China

4

Institute of Fluid Physics, China Academy of Engineering Physics, Mianyang, Sichuan 621900, China

5

Key Laboratory of Strongly-Coupled Quantum Matter Physics, Chinese Academy of Sciences, School of Physical Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China †Co-authors:[email protected] and [email protected] *Corresponding Authors：(Z,Z) [email protected] and (Z,W) [email protected]

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Abstract: To probe the behavior of structural evolution and optical properties in solid energetic material TATB, X-ray diffraction, Raman and absorption spectroscopy were performed under high pressure up to 20 GPa. The absorption edge shifts to red and the color significantly varies with increasing pressure for TATB. The XRD patterns under high pressure indicate that TATB maintains the triclinic structure within this pressure range. An electronic structural change is observed at around 5 GPa, resulting from the modification of conformers of TATB, which is associated with the rotation of nitro and amino groups under high pressure. The current experimental results clarified the no phase transition appearance below 20 GPa and confirmed that the pressure-induced color change originates from the enhancing conjugation of π orbital due to the shorting C-NO2 bonds and the rotation of nitro groups with increasing pressure. The third-order Birch-Murnaghan equation of state is obtained up to 16.5 GPa, which is helpful for calculating researchers to verify correctness of their models.

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Introduction Energetic materials are widely applied in a lot of fields such as propellants, blasting agents, pyrotechnics and military. High pressure, high temperature and strong irradiation etc. are generated after igniting energetic materials. These extreme conditions can induce a series of phase transitions and complicate the process of detonation. The information during denotation is key to understand the properties of explosives and also difficult to be collected due to the ultrafast and damaging process 1-6

. The optional method is to observe some physical parameters under hydrostatic

pressure. 1, 3, 5-triamino-2, 4, 6-trinitrobenzene (TATB) is a class of layered aromatic explosive, which can release large amount of heat and energy at high rate detonation reactions 3. The high bonding rate of inter-molecular hydrogen bonding within the layer in TATB are responsible for its high energy density, good heat resistance and the exceptional insensitivity to external stimuli such as shock wave, friction and mechanical impact 4-6. There are some experimental and calculating works on the properties of TATB under high-pressure. Unlike RDX and HMX as energetic materials with saturation ring alkane, there exist a series of phase structures below 20 GPa

7,8

. Raman and

infrared of TATB under high pressure by Pravica et al. 9,10 and Satija et al. 11 revealed that TATB remains its structure stability below 20 GPa due to enhancing the hydrogen bonding of intra-molecular and inter-molecular within layer with increasing pressure. Davidson et al.

12

found that the first phase transition occurred around 28 GPa

according to their Raman spectra at higher pressure. On the other hand, Trott et al. 13 3 / 26



proposed a phase transition around 7 GPa from their observation of new vibration band using shock Raman. Stevens et al.

14

also supported this proposal based on the

subtle cusp of P-V curve at 8 GPa. Recently, Ojeda reported pressure-induced hydrogen bonding formation and molecular rearrangement in TATB by using atomistic and electronic level computations 15. An isosymmetric phase transition was predicted at 1.5 GPa, resulting in new hydrogen bonds formation along c-axis. Kohno also supported spatial structure changes of TATB in the pressure range 2-4 GPa using molecular dynamics and first-principles calculations 4. These results from experiments and calculations were contradiction each other, so it needs to clarify the phase structure below 20 GPa for TATB. TATB can undergo a process of greening after UV irradiation. We also found the phenomenon after irradiating TATB by divergent 325nm laser. Comparison of Raman spectra for sample before and after irradiation indicates that UV laser modified the structure of original molecular. Manaa et al.

16

revealed that the greening resulted

from the formation of the mono nitroso derivative. Both two groups from Davidson and Stevens found the pressure-induced discoloration from yellow to orange and further to dark red with increasing pressure 12,14. Color change is relative to electronic structure of material. As for the color change of TATB, the several hypothetical originations such as energy band distortion, dimerization and chemical reaction under high pressure are proposed. But as far as, no experimental data explain the phenomenon. Kakar et al. provided an estimate of 6.6 eV for the band gap of TATB combining the valence-band photoemission data and the X-ray absorption 4 / 26


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spectroscopy (XAS) data

17

. Wu et al. gave a band-gap range of 4.6 eV to 5.7 eV

based on the complete active space multiconfiguration self-consistent field (CASSCF) calculations 18. Several calculating works also provided the band gap values, but there exist significant differences 19-21. As mentioned above, it is significance to clarify the phase structure and origination of color change for TATB under high pressure below 20 GPa. In this work, XRD, absorption and Raman under high pressure are employed to investigate the structure stability and electronic structure change for TATB. The calculation based on density function theory further explain the mechanism of pressure-induced discoloration for TATB.

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Experimental Section and Calculations. Ⅰ Experimental section. The TATB samples were provided by Chinese academy of engineering physics. The crystal structure of the samples was characterized by using an x-ray diffractometer (Rigaku, Smartlab 9) under Cu Kα radiation (λ=1.542 Å). The particle size was estimated by a scanning electron microscope (Hitachi, SU8010). Absorption spectra were measured by a home-built confocal microscope system with an ultraviolet-visible (UV-VIS) spectrometer (Zolix-Omni-λ750), a standard light

source

(EQ-99CAL

LDLS)

and

a

photomultiplier

tube

detector

(Zolix-HVC1800). Angle-dispersive x-ray diffraction experiments were performed at the 15U1 beam line of the Shanghai Synchrotron Radiation Facility (SSRF, Shanghai) with synchrotron radiation at λ=0.6199 Å

22

. The x-ray beam was focused into a size of

approximately 6×4 µm2. The diffraction data were recorded by using a two-dimensional (2D) imaging plate detector. The orientation and distance of the detector are calibrated through the use of a CeO2 standard. The diffraction patterns were integrated and collected with the program Fit2D. Raman spectra were recorded by an integrated laser Raman system (LABRAM HR, Jobin Yvon) with a confocal microscope, a stigmatic spectrometer, and a multichannel air-cooled CCD detector. The 632.8 nm line from a He-Ne laser was used as the excitation source at a power level of 7 mW to avoid any laser-induced 6 / 26


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damage. All spectra were recorded in the backscattering geometry at room temperature. High-pressure was generated by a diamond anvil cell (DAC) with 400 µm diameter culets. A stainless steel gasket was preindented to a thickness of 60 µm and then a hole with a diameter of 150 µm was drilled to be used as sample chamber. Helium was used as pressure-transmitting medium in high-pressure experiments. The pressure was measured by the standard ruby ﬂuorescence technique 23. Ⅱ Calculations details. The density function theory method implemented in the VASP package

24

was

used to relax the structure under hydrostatic-pressure up to 14 GPa. The exchange-correlation

functional

is

treated

with

the

Generalized

Gradient

Approximation (GGA) following the Perdew-Burke-Ernzerhof formulation (PBE) 25-27

. The self-consistent convergence criteria of energy were set at 10−6 and 10−5 eV

for electronic and ionic relaxations, respectively. These calculations were performed with an 8 × 8 × 8 Monkhorst−Pack k-point mesh and plane-wave cut-off energy of 600 eV was set to produce a well-converged structure. And the DTF-D2 approach of Grimme 28,29 was employed to address the long-range dispersion correction.

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Results and discussion XRD pattern of TATB is well refined by Rietveld analysis as shown in Fig.1. The result demonstrates that the sample crystallizes in triclinic structure with space group of P1 and contains no any impurity. The SEM image of TATB sample reveals its multi-layered morphology and grain size of 10-20 µm as seen in the inset of Fig.1.

Fig. 1. XRD pattern of TATB powder crystals, and SEM image of TATB powder crystals in the inset. Figure 2 shows a significant change in color with the increase of the pressure. Below 4 GPa, the sample remains original light yellow. From 4 to 8 GPa, the color deepens gradually. Until about 12 GPa, the color becomes orange. The color continues to deepen and becomes dark red with the further loading. During the process of decompression back to ambient condition, the sample returns to light yellow once again, and the color is gradually restored. In general, the color variation would accompany a shifting of the absorption edge in the visible region.

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Fig. 2. Discoloration of TATB under high pressure. Figure 3(a) is the absorption spectra of TATB sample under high pressure up to 21 GPa. It is known that the relationship between the absorption coefficient (α) and the optical bandgap (Eg) for direct-gap semiconductor obeys the following formula (1) 30

:

(1)

Where A is the parameter that relates to the effective masses associated with the valence and conduction bands, and hν is the photon energy. Therefore, the absorption edge is given by extrapolating the linear portion of the plot (αhν)2 versus hν to α=0 as seen in the inset of Fig. 3(b). Eg decreases with the increasing pressure as shown in Fig. 3(b). The experimental value of Eg is 2.50 eV, which is very similar to the calculation result of 2.51 eV by the method of Generalized Gradient Approximation (GGA) in the ambient condition. The calculating energy-band diagram of TATB crystal structure is shown in the Fig. S1 of Supporting Information (SI). Figure 3(b) shows a clear red-shifted for absorption edge with the increase of pressure, which is consistent with the color changes in Fig. 2. This phenomenon indicates a decrease in 9 / 26



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the energy gap due to the variation of the lowest unoccupied molecular orbitals (LUMO) and highest occupied molecular orbitals (HOMO)

31

. The absorption edge

stays around 5 GPa and then rapidly decreases with further compression as shown in Fig. 3(b). What reason results in the inflection point of Eg at 5 GPa? Is this a signal of the structural phase transition? Further work will prove and improve our conjecture.

Fig. 3. (a) Absorption spectra of TATB under high pressure; (b) The absorption edge of TATB under high pressure. The absorption edge is given by the tangent method in the inset. The synchrotron XRD patterns of TATB are shown in Fig. 4(a) under high pressure up to 16.5 GPa. All diffraction peaks can be indexed with the triclinic phase structure. With the increasing pressure, all of peaks shift to the larger angles with different shift rate due to the anisotropy of the TATB as shown in Fig. 4(b). The peak for (002) fastly shifts below 4 GPa and then slows its rate with the further compression. This indicates that the larger space between adjacent layers is facilely

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compressed along c-axis at the initail loading stage. The diffraction peaks broaden with the increasing pressure due to the microscopic stress. The lattice of TATB up to 16.5 GPa is much more compressible along the c direction as seen in Fig. 5(a), which is consistent with the above different shift rate of the diffraction peaks. The constant ratio of a and b verified Olinger and Cady’s assumption 32. Figure 5(b) is the unit cell volume at different pressure. The third-order Birch-Murnaghan equation of state (EOS) 33

in Eq. (2) agrees well with experimental data for TATB.

"

{1 + 4[ 1]}

(2)

where V0 and V are the unit cell volume at ambient and pressure P, respectively. B0 is the bulk modulus at ambient condition, and is its pressure derivative. The fitted results is shown in Fig. 5(b), B0=18.727 GPa and =5.186. The current values of B0 and are similar to the results from Olinger’s work below 8 GPa experiment below 13.22 GPa

32

and Stevens’s

14

. The unit cell volume collapses to 30% of ambient

condition at 16.5 GPa. The result confirmed the prediction by Wu et al. based the LDA calculation 18. XRD patterns indicate that there is no new diffraction peak in the entire loading pressure process and no discontinuous phenomenon in P-V curve proposed by Stevens et al.

14

. The current experimental result confirmed that no

first-order structural phase transition was detected up to 16.5 GPa.

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Fig. 4. (a) XRD patterns of TATB under high pressure; (b) The shifts of (002) and (1-31) peaks of TATB under high pressure.

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Fig. 5. (a) The lattice parameters of TATB under high pressure; (b) The pressure-volume curve of TATB, the solid line represents the fitted result using 3rd order Birch-Murnaghan equation of state. In order to explore further the real reason of color and electronic structure change under compression, ab initio energy calculations were also performed for TATB. The comparison between experimental and theoretical lattice parameters at 13 / 26



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ambient conditions in this work is shown in the Table I. Our GGA calculations overestimate a, b and c by 1.8%, 1.3% and 7.8%, respectively. And the comparison between experimental and theoretical lattice parameters and equation of state under compression in this work is shown in the Fig. S2 and Fig. S3 of SI. For our calculation results, the pressure dependence of the a and b lattice parameters show an almost identical decrease with increasing pressure from around 9.2 to 8.7 Å, about 5.4% reduction at 14 GPa. And the pressure dependence of the c lattice parameter exhibits a dramatic decrease with increasing pressure from around 7.4 to 5.7 Å, about 23.0% reduction at 14 GPa. Compared with the previous reference

20

, we found that our

calculation results shows better agreements with our experimental results. Table I. Comparison of experimental and calculating cell parameters of TATB crystal at ambient conditions α°

β°

γ°

parameter

a (Å)

b (Å)

c (Å)

Exp.

9.0280

9.0871

6.8434

108.2547 91.6245 120.1643

Cal.

9.1963

9.2094

7.4281

106.0677 92.2081 120.0885

The rotation of nitro and amino groups are found from the evolution of molecular structure in the unit cell. The dihedral angles between planes of NO2 or NH2 and aromatic ring are defined as important parameters determining the extent of rotating. The dihedral angles of the amino groups and the nitro groups as a function of external pressure are calculated using Vienna Ab initio Simulation Package (VASP) simulations

24

, and the results are presented in Fig. 6. The torsion angles define by

C6-C1-N1-O1, C2-C3-N3-O3, C4-C5-N5-O5, C3-C2-N2-H2, C5-C4-N4-H4 and 14 / 26


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C1-C6-N6-H6 (γ1, γ2, γ3, γ4, γ5 and γ6). For the nitro groups, the dihedral angles (γ1, γ2 and γ3) increase monotonously by about 6° with the increase of pressure up to 14 GPa. However, the dihedral angles (γ4, γ5 and γ6) of amino groups are different from nitro groups. γ4 increases from 0.5° to 1.5° and the trends of γ5 and γ6 are similar, firstly increase and then decrease and finally become steady. The slight rotations of the nitro groups result in that the oxygen atoms are more prone to warping out-of-plane as shown in Fig. 7. With increasing pressure, the length decreases for all C-N bonds and the dihedral angle increases for C-C-N-O as seen in Fig. 8. On the one side the increasing dihedral of C-C-N-O decreases the overlapping volume between p-orbital of lone pair electrons of N atom and larger π orbital of aromatic ring, on the other side the shortening C-N bond enhances the overlapping of P-orbital and big π orbital. The same competition also exists during the rotation of amino group. Combining the contribution from the rotations of amino groups and nitro groups, the conjugation of big π orbital enhances due to the joint P-orbital of N atom with the increasing pressure. The increasing conjugation of big π orbital lowers the excited energy from HOMO electrons and further induces the redshift of absorption edge 18,34. Due to the existence of the competition for shortening C-N bonds and rotating nitro and amino groups, after the exciting energy reduces at the initial stage below 4 GPa, the absorption edge keeps stable from 4 to 6 GPa, then the further loading compression results in the rapid decreasing band gap. It is concluded that there is an electronic transition resulting from the conformer modifications near 5 GPa.

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Fig. 6. Dihedral angles of the amino groups and nitro groups of TATB under high pressure.

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Fig. 7. Structural schematic diagram of TATB molecule at (a) 1 atm and (b) 5 GPa.

Fig. 8. Lengths of the C-N bonds of TATB under high pressure. The shortening C-N bond adds the force constant and results in the bule shift of vibration frequency for C-N stretching mode. The prediction is verified by Raman spectra of TATB under high pressure as shown in Fig. 9. The bond length of C-NH2 always is less than that of C-NO2 with increasing pressure as displayed in Fig. 8(a). The shorter bond length implies the stronger force constant and higher vibration frequency. So Raman peak at 1168 cm-1 is response to C-NH2 stretching vibration and peak at 1142 cm-1 for C-NO2 stretching mode in Fig. 9(a). Pressure dependence of Raman shift for the two main modes is shown in Fig. 9(b). The frequency shift of C-NH2 stretching mode exhibits a jumping down around 5 GPa due to the maximum rotating angle of amino groups at this pressure as seen in Fig. 6. The relative

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intensities for the two peaks reverse with increasing pressure. Figure 9(c) is the intensity ratio versus pressure. The ratio inflects around 5 GPa due to the combining action from rotation of nitro and amino groups, shortening C-N bonds and distortion of aromatic ring with increasing pressure. The inflection point demonstrates that there exists an electron structure transition around 5 GPa. The shift of major Raman modes as a function of pressure is shown in Fig. 10. It is clear that some vibrational modes show clear discontinuity near 5 GPa. Such as lattice vibration, ring distorsion & NH2 wagging, NH2 rocking & C-NO2 stretching, NH2 out-of-plane twisting, C-NO2 stretching, ring stretching & C-NH2 stretching, NH2 stretching vibration modes, etc. The fact further supports that an electron structural transition occurs around 5 GPa.

Fig. 9. (a) Raman spectra of TATB in the range of 1120-1280 cm-1 below 10 GPa; (b) Raman shifts of C-NO2 stretch and C-NH2 & ring stretch vibration modes under high pressure; (c) The intensity ratio-pressure curve (C-NO2 stretch and C-NH2 & ring 18 / 26


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stretch vibration modes of TATB).

Fig. 10. Frequency shifts of major Raman vibration modes under high pressure. Conclusions In summary, the structural stability and electronic structure change of energetic materials TATB was studied under high pressure below 20 GPa. High pressure XRD patterns reveal that TATB maintains the triclinic structure. The third-order Birch-Murnaghan EOS for TATB was obtained up to 16.5 GPa, which is useful to check the corretness of the calculating model. The dependence of absorption edge on pressure indicates that there is a significant change in electronic structure at around 5 GPa. The essential reason is the modification of conformers of TATB, which is related to the rotation of nitro and amino groups under high pressure. First principle calculations are consistent with the above experimental results. The present work 19 / 26



gives a more comprehensive understanding of the energetic material TATB in extreme conditions, which is of great significance for blasting and detonation applications of explosives. Supporting Information The calculating energy-band diagram of TATB crystal structure at ambient condition is supplied as Supporting Information (SI). Acknowledgment This work was supported by the Science Challenge Project (No. TZ2016001), beamline 15U1 of the Shanghai Synchrotron Radiation Facility (SSRF) and Supercomputing Center of University of Science and Technology of China.

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Fig. 3. (a) Absorption spectra of TATB under high pressure; (b) The absorption edge of TATB under high pressure. The absorption edge is given by the tangent method in the inset. 88x44mm (300 x 300 DPI)



Figure S2. The comparison between experimental and theoretical lattice parameters under compression in this work 82x82mm (300 x 300 DPI)


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Figure S3. The comparison between theoretical and experimental equation of state in this work 82x82mm (300 x 300 DPI)


Pressure-Induced Conformer Modifications and Electronic Structural

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