In-situ Raman Scattering And First-principles Calculations Study Of Structural Stability Of Typical Energetical Materials Under High Pressures

Understanding of the relationship between the properties and structures of materials is an important topic in the fields of physics, material science and chemistry. Study substance microstructure is helpful to promote performance and synthesize the new materials.Although the energetic material is widely used for many years in the areas of military and civil engineering, the study on the microstructral stability and energy liberation mechanism is relatively limited. In particular, building a detailed molecular mechanism of the detonation is a common concern in the fields of modern detonation physics, weapon science and high pressure condensed matter physics. The detonation involves complex pressure and temperature condition, and experiences a complicated process of chemical and physical change. In another words, the nature of detonation mechanism is to reveal the structural transform or decomposition reaction of the explosives under extreme conditions. Thus,studies of the pressure effects on the structure response are important for understanding and modeling energetic materials. Moreover, a good knowledge of pressure-induced structural change is useful to understand the earlier stage reaction of the explosive. Therefore,investigations of the structural stability of the energetic materials under high pressure are important to establish a satisfied molecular mechanism of detonation. Based on the above considerations, the Diamond Anvil Cell (DAC), two-stage light -gas gun, Raman spectral measurement and DFT calculation were employed to study the structural stability of the typical energetic materials, including nitromthane (NM), nitrobenzene (NB),1,3-Diamino-2,4,6-trinitrobenzene (DATB) and nitrogen-rich energetic compound 3,4-diamino-1,2,4-triazoliuml-aminotetrazol-5-oneate (ATO�DATr). Specific contents are as follows:Firstly, we conduct the structural stability of nitrobenzene under high pressure. As a simplest structure of the aromatic nitro compounds, nitrobenzene is investigated as a model for understanding structural properties in nitro derivatives of benzene and anilines. Using the Raman spectroscopic technique, the high-pressure structure and vibrational modes of solid NB are examined under hydrostatic compression up to 10 GPa. The Raman spectra suggest that a subtle structural alteration occurred around 5 GPa. Also, to get further insight into the high-pressure Raman spectral changes, the dispersion corrected density functional theory(DFT-D) calculations are performed to exam pressure effects on the molecular geometry. The calculated data show a distinct change in the bond lengths, bond angles and dihedral angles around 7 GPa, indicating a structural change is occuring. Combining experimental and calculated results, suggest that NB molecules are distorted, and molecular structure is readjusted when the conformational change take place under high-pressure.Secondly, we study the structural stability of nitromethane under shock-compression conditions. So far,understanding the reaction and detonation mechanism of energetic materials remain a difficult problem, and people put a great deal of time and effort into that.Nitromethane, the simplest nitro compound, is employed to exam the structural response to shock-compression of the energetic materials. Combine two-stage light-gas gun with Raman spectroscopy and thermal radiation measurements, we obtained the Raman spectrum and ignition delay time of liquid nitromethane under shocked-compression conditions. The experimental results indicate that the sample nitromethane is still transparent before the detonation occuring. In addition, Raman characteristic peaks of possible reaction products are observed for the first time. It is suggested that the C-H bond is early destructed under shock compression process, according to the assigned vibrational modes of the products.These interesting results also provide new insight into study of the initiation reaction and detonation mechanism of other energetic materials.Thirdly, we determine the structural change and stability of DATB. DATB is a well-known explosive of the nitro derivatives of benzene and anilines, which molecular structure was similar to the famous insensitive explosive TATB. However, the structural information is poorly understood under high-pressure, especially the pressure-induced structural transformation remains an opening problem. Dispersion corrected density functional theory (DFT-D) calculation is performed to examine the structural response of DATB in the pressure range of 0-15 GPa. The calculated results of the crystal structure,molecular geometry and intermolecular close contacts are in good agreement with the experimental data at ambient pressure. To get further insight into the structural response to pressure of DATB, the crystal structure, elastic constants and mechanical stability of DATB are also studied under high pressure. The calculation results show that the pressure depenced of the lattice constants, molecular geometry and elastic constants are remarkably changed around 7.5 GPa. In addition, the elastic constants of DATB are not satisfied the mechanical stability criteria at 7.3 GPa, indicating that the DATB crystal structure is unstable.Finally, we investigate the pressure effect on the crystalline structural stability of ATO � DATr. The nitrogen-rich energetic compound ATO � DATr is considered as an insensitive potential explosive due to its good detonation velocity and excellent detonation pressure. The kind of nitrogen-rich energetic compounds have been has attracted greater attention from scholars both at home and abroad because of their high heat of formation and nitrogen content as well as detonation products are environment friendly. To get further insight into the high-pressure behavior of these nitrogen-rich energetic compounds, the dispersion corrected density functional theory calculations are employed to study the crystal structure and molecular geometry. In addition, the intermolecular interactions including hydrogen bonds are examined by the Hirshfeld surfaces and two-dimensional fingerprint plots. These calculated results are in good agreement with the available experimental values.To gain insight into the pressure effect on crystal ATO � DATr, we further study the lattice parameters,equations of state and electronic band gap under hydrostatic pressure from 0 to 50 GPa. Moreover, the variations of Hirshfeld surfaces and corresponding fingerprint plots with pressure are determined. Subsequently, the effects of hydrostatic compression on ATO�DATr are dertemined. It is found that ATO �DATr exhibits anisotropic compressibility.The calculated bulk modulus of ATO � DATr is higher than other known energetic materials.Moreover, the number of closer contacts increased under pressure, while the overall shortening of these longer contacts is related to decrease of the maximum value de. The results shown above suggest that the decreased compressibility is related to the increased intermolecular interactions of ATO � DATr under high pressure.