Molecular Structures, Dipole Moments, and Electronic Properties of ß-HMX under External Electric Field from First-Principles Calculations.

In order to investigate the impact of an external electric field on the sensitivity of ß-HMX explosives, we employ first-principles calculations to determine the molecular structure, dipole moment, and electronic properties of both ß-HMX crystals and individual ß-HMX molecules under varying electric fields. When the external electric field is increasing along the [100], [010], and [001] crystallographic directions of ß-HMX, the calculation results indicate that an increase in the bond length (N1-N3/N1'-N3') of the triggering bond, an increase in the main Qnitro (N3, N3') value, an increase in the minimum surface electrostatic potential, and a decrease in band gap all contribute to a reduction in its stability. Among these directions, the [010] direction exhibits the highest sensitivity, which can be attributed to the significantly smaller effective mass along the [010] direction compared with the [001] and [100] directions. Moreover, the application of an external electric field along the Y direction of the coordinate system on individual ß-HMX molecules reveals that the strong polarization effect induced by the electric field enhances the decomposition of the N1-N3 bonds. In addition, due to the periodic potential energy of ß-HXM crystal, the polarization effect of ß-HMX crystal caused by an external electric field is much smaller than that of a single ß-HXM molecule.

First-principle calculations of the electronic, vibrational, and thermodynamic properties of nitrogen-rich salt of 3,6-dinitramino-1,2,4,5-tetrazine [(NH4)2(DNAT)].

CONTEXT: Energy-containing materials such as explosives have attracted considerable interest recently. In the field of high-energy materials, tetrazine and its derivatives can largely meet the requirements of high nitrogen content and oxygen balance. Nitrogen-rich energetic salts are important research subjects. Nitrogen-rich salt of 3,6-dinitramino-1,2,4,5-tetrazine is a high-energy nitrogen-rich material, but there are few related studies. This paper systematically studies the crystal structure and electronic, vibrational, and thermodynamic properties of (NH4)2(DNAT). The lattice parameters of (NH4)2(DNAT) are observed to align well with the experimental values. The properties of electrons are analyzed by band structure and density of states (DOS). The phonon dispersion curves indicate that the compound is dynamically stable. The vibrational modes of bonds and chemical groups are described in detail, and the peaks in the Raman and infrared spectra are assigned to different vibration modes. Based on the vibration characteristics, thermodynamic properties such as enthalpy (H), Helmholtz free energy (F), entropy (S), Gibbs free energy (G), constant volume heat capacity (CV), and Debye temperature (Θ) are analyzed. This article can pave the way for subsequent work or provide data support to other researchers, promoting further research. METHODS: In this study, we utilized the density functional theory (DFT) for our calculations. The exchange-correlation potential and van der Waals interactions were characterized based on the GGA-PBE + G function calculation. We obtained Brillouin zone integrals using Monkhorst-Pack k-point grids, with the k-point of the Brillouin zone set to a 2 × 2 × 2 grid. During the self-consistent field operation, we set the total energy convergence tolerance to 5 × 10-6 eV per atom. The cut-off energy for the calculation was established at 830 eV. Additionally, the states of H (1s1), C (2s2 2p2), N (2s2 2p3), and O (2s2 2p4) were treated as valence electrons in our study.

Structural, mechanical, electronic, vibrational properties and hydrogen bonding of a novel energetic ionic 5, 5'-dinitroamino-3, 3'-azo-oxadiazole 4, 7-diaminopyridazino [4, 5-c] furoxan salt.

CONTEXT AND RESULTS: The structure, mechanical, electronic, vibration, and hydrogen bonding properties of a novel high-energy and low-sensitivity 5, 5'-dinitroamino-3, 3'-azo-oxadiazole 4, 7-diaminopyridazino [4, 5-c] furoxan salt have been studied by density functional theory. The calculated vibrational properties show that the low-frequency mode is mainly contributed by the vibration of the -NO2 group, and the high-frequency mode is mainly contributed by the vibration of the -NH2 group and the N7-H3 bond which protonates the cation. In addition, it is analyzed that the first bond to break may be the N-NO2 bond. The calculated hydrogen bond properties indicate that the hydrogen bond between water molecules and cations is N7-H3… O5 (1.563 Å), which is the shortest hydrogen bond among all hydrogen bonds. The presence of this exceptionally short hydrogen bond renders the N7-H3 and H6-O5 bonds resistant to disruption at high frequencies, underscoring the pivotal role of hydrogen bonding in stabilizing the structure of energetic materials. Given the absence of experimental and theoretical data on the electronic, mechanical, and vibrational properties of the material thus far, our calculations offer valuable theoretical insights into the ionic salts of high energy and low sensitivity. COMPUTATIONAL METHODS: All calculations have been carried out based on density functional theory (DFT) and implemented in the CASTEP code. The mode-conserving pseudopotential is utilized to describe the plane wave expansion function, while the PBE functional within the generalized gradient approximation (GGA) is employed to characterize the exchange-correlation interaction. Additionally, dispersion correction is applied using Grimme's DFT-D method.

Raman spectra and vibrational properties of FOX-7 under pressure and temperature: First-principles calculations.

FOX-7 (1,1-diamino-2,2-dinitroethene) as one of the widely studied insensitive high explosives exists five polymorphs (α, ß, γ, α', ε) whose crystal structures have been determined by XRD (X-rays Diffraction) and which are investigated by a density functional theory (DFT) approach in this work. The calculation results show that the GGA PBE-D2 method can reproduce the experimental crystal structure of FOX-7 polymorphs better. The calculated Raman spectra of FOX-7 polymorphs were compared in detail and fully with the experimental Raman spectra data and it was found that the calculated Raman spectra frequencies have an overall red-shift in middle band (800-1700 cm-1), and that the maximum deviation does not exceed 4 % (The maximum point is the mode of CC in plane bending). The high-temperature phase transform path (α â†’ ß â†’ Î³) and the high-pressure phase transform path (α â†’ α'→ε) can be well represented in the computational Raman spectra. In addition, crystal structure of ε-FOX-7 was performed up to 70 GPa to probe Raman spectra and vibrational properties. The results showed that the NH2 Raman shift is jittering with pressure (not smooth compared to other vibrational modes) and NH2 anti-symmetry-stretching appears red-shifted. The vibration of hydrogen mixes in all of other vibrational modes. This work shows that the dispersion-corrected GGA PBE method can reproduce the experimental structure, vibrational properties and Raman spectra very well.

Pressure and temperature effects on the Raman spectra of LLM-105.

Currently, only one crystal structure of LLM-105 (2,6-diamino-3,5-dinitropyrazine-1-oxide) (P21/n) has been discovered, and there are still debates on its phase transition point and phase diagram. Based on previous work, we performed crystal structure, Raman spectra, and vibrational properties calculations on LLM-105 crystal. Our results indicate that the crystal structure of LLM-105 remains stable until compressed to 49 GPa, beyond which it may undergo two phase transitions at pressure intervals of 49.0-49.1 GPa and 51.4-51.5 GPa, respectively. Analysis of Raman shift results suggests that these two phase transitions may be reversible, with an intermediate phase possibly acting as a transition phase. Additionally, based on the quasi-harmonic approximation, we fitted the experimental data of LLM-105 lattice expansion state, obtaining the volume at zero pressure and using it for Raman spectra calculations. The results demonstrated the accuracy of this quasi-harmonic approximation method in describing the redshift of Raman peaks during the heating process and the excitation ratio of Raman peaks in different wavenumber ranges.

The comparative study of structural, electronic, and optical properties of hydrogen peroxide and its dihydrate under pressures: first-principle calculations.

Hydrogen peroxide (H2O2) is used as a fuel and propellant in fuel cells and rockets due to its prominent oxidizing and combustion properties. In addition, hydrogen peroxide, as the energetic material with the simplest molecular structure, exhibits general detonation performance under external stimulation. Based on the first-principle method, we calculated the two crystal structure, electronic properties related to sensitivity closely, optical properties of pure hydrogen peroxide, and 48wt.% hydrogen peroxide (H6O4) under pressure. We found that the band gaps of H2O2 and H6O4 become larger under pressure and the former is larger than the latter; neither has the tendency of metallization phase change. The added peak II of TDOS from H6O4 compared with H2O2 come from molecular H2O in crystal structure. The pressure-induced peak (peak 2 and peak II of TDOS from H2O2 and H6O4) splitting is caused by changes (stronger) in the intermolecular hydrogen bond environment in the crystal under pressure. The specific macroscopic optical properties have the characteristics of overall blue-shift under pressure, which is due to the blueshift of the conduction band and the increase of the band gap. We hope to provide some reference and guidance for deeper future research.