Structural, vibrational, and quasiparticle band structure of 1,1-diamino-2,2-dinitroethelene from ab initio calculations

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, β, 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 → β → γ) and the high-pressure phase transform path (α → 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.

Low-frequency lattice vibrations from atomic displacement parameters of α-FOX-7, a high energy density material

Highly anharmonic thermal vibrations may serve as a source of structural instabilities resulting in phase transitions, chemical reactions and even the mechanical disintegration of a material. Ab initio calculations model thermal motion within a harmonic or sometimes quasi-harmonic approximation and must be complimented by experimental data on temperature-dependent vibrational frequencies. Here multi-temperature atomic displacement parameters (ADPs), derived from a single-crystal synchrotron diffraction experiment, are used to characterize low-frequency lattice vibrations in the α-FOX-7 layered structure. It is shown that despite the limited quality of the data, the extracted frequencies are reasonably close to those derived from inelastic scattering, Raman measurements and density functional theory (DFT) calculations. Vibrational anharmonicity is parameterized by the Grüneisen parameters, which are found to be very different for in-layer and out-of-layer vibrations.

Pressure-induced phase transitions and mechanical properties of insensitive high explosive 1,1-diamino-2,2-dinitroethylene

The structural and mechanical properties of an insensitive high-explosive 1,1-diamino-2,2-dinitroethylene (FOX-7) polymorphs were studied using dispersion-corrected density functional theory calculations. The predicted lattice parameters of FOX-7 polymorphs agree well with the available single-crystal X-ray diffraction data. From our elastic modulus calculations, we found that the ε phase has the highest shear modulus G, Young’s modulus E, longitudinal speed C L, and shear speed C S, respectively. Moreover, both α and α′ phase are brittle, ε phase is ductile nature. The results of Hirshfeld surfaces and fingerprint plots indicate that the α and α′ phase possess similar molecular packing modes. Meanwhile, the ε phase is found to have the strongest π…π interactions because of the nearly planer molecules formed a planar layer in the crystal. The pressure effects on the α and α′ phase presented an obvious anisotropy, a pressure-induced phase transition from phase α′ (P21/n) to ε phase (P1) was studied. And we also analyze the influence of pressure on the electronic structure.

Study of the Elastic Properties of the Energetic Molecular Crystals Using Density Functionals with van der Waals Corrections

We studied the elastic properties of crystalline energetic materials within the framework of density functional theory with van der Waals interactions (DFT-D3(BJ) and rev-vdW-DF2). The full sets of elastic constants were computed. The computed parameters are in good agreement with the experimental data. Among the crystals studied in this work, FOX7 had the lowest compressibility value of 0.0034 GPa^-1 and had the highest anisotropy. Crystalline pentaerythritol tetranitrate had almost isotropic mechanical properties.

Possible pre-phase transition of the α-HMX crystal observed by the variation of hydrogen-bonding network under high pressures

The variation of non-covalent interactions in the HMX crystal under high pressures was investigated through disassembling the hydrogen-bonding network.

Interaction Mechanisms of Insensitive Explosive FOX-7 and Graphene Oxides from Ab Initio Calculations

Energetic material–graphene oxide (EM–GO) composites exhibit excellent thermal stability and insensitivity to mechanical stimuli. The interfacial interactions play an important role in affecting the structural and electrical properties of EM–GO composites. FOX-7 crystal with a wave-shaped layer structure is an ideal prototype system for matching with oxygen-rich GO monolayers to form FOX-7–GO composites. Here, we conducted a systematic investigation on FOX-7–GO composites by dispersion-corrected density functional approach. Our results revealed that there exists relatively strong interaction in the FOX-7–GO interface, which stems from the synergistic effect of interfacial charge transfer and hydrogen bonds. The electronic structure analyses demonstrated that GO can hybridize with FOX-7 to reduce charge accumulation on the FOX-7 surface. These theoretical results are useful for clarifying the interfacial effects on the sensitivity of FOX-7–GO composites.

Structural, mechanical properties, and vibrational spectra of LLM-105 under high pressures from a first-principles study

In this work, we report the structure, mechanical properties, and vibrational spectra of 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105), an energetic molecular crystal, with a first-principles method based on density functional theory (DFT) using the recentely developped HASEM package. The elastic constants, acoustic velocity, and parameters of equations of state were calculated, and the predicted ordering of stiffness constants is C ₃₃ (38.5 GPa) > C ₁₁ (24.0 GPa) > C ₂₂ (17.7 GPa). We also investigated the structure and equation of state of LLM-105 under hydrostatic pressure up to 100 GPa. The predicted structures are in good agreement with experimental results available from ambient pressure to 20 GPa. Under compressions, the LLM-105 crystal exhibits anisotropic compressibility, with a highly incompressible response along the a-axis and c-axis. It is worth noting that there is a sudden change in the lattice parameters and change rate of volume at ~30 GPa. Based on the intermolecular interaction analysis and vibrational spectra, a phase transition at the hydrostatic pressure of ~30 GPa is predicted.

Hydrostatic Pressure Effects on Structural and Electronic Properties of ETN and PETN from First-Principles Calculations

We studied the structural and electronic properties of pentaerythritol tetranitrate (PETN) and erythritol tetranitrate (ETN) crystals within the framework of density functional theory with van der Waals interactions. The computed lattice parameters have good agreement with experimental data. Electronic and structural properties of the crystals under 0-20 GPa hydrostatic pressure were studied. The parameters of equations of state calculated from the theoretical data show good agreement with experiment within the studied pressure intervals. We have also calculated the detonation velocity and pressure.

DFT-D studies on structural variation and absorption properties of crystalline benzotrifuroxan under high pressure

The structural, electronic, and absorption properties of crystalline benzotrifuroxan (BTF) under hydrostatic compression of 0–150 GPa have been studied by using density functional theory with dispersion correction. The crystal structure of BTF was relaxed using three types of van der Waals corrections such as the PBE-G06, PBE-TS, and PW91-OBS functionals at ambient conditions. The results indicate that PBE-G06 is the best functional for studying BTF. The a and c directions may be more sensitive to van der Waals interactions than the b direction. The structure is the stiffest in the b direction in the whole pressure range. At 107 GPa, BTF decomposes by the breaking of the N2−O2 and O1−N6 bonds. An analysis of the band gap and density of states indicates that BTF becomes more and more sensitive under compression. The absorption spectra show that BTF has relatively high optical activity with increasing pressure; moreover, the absorption region is broadened in the energy range of below 1.5 and above 16.5 eV, respectively. This work may provide useful information in understanding how BTF behaves under high pressure.

Polymorphism and thermodynamic ground state of silver fulminate studied from van der Waals density functional calculations

Silver fulminate (AgCNO) is a primary explosive, which exists in two polymorphic phases, namely, orthorhombic (Cmcm) and trigonal (R3) forms at ambient conditions. In the present study, we have investigated the effect of pressure and temperature on relative phase stability of the polymorphs using planewave pseudopotential approaches based on Density Functional Theory (DFT). van der Waals interactions play a significant role in predicting the phase stability and they can be effectively captured by semi-empirical dispersion correction methods in contrast to standard DFT functionals. Based on our total energy calculations using DFT-D2 method, the Cmcm structure is found to be the preferred thermodynamic equilibrium phase under studied pressure and temperature range. Hitherto Cmcm and R3 phases denoted as α- and β-forms of AgCNO, respectively. Also a pressure induced polymorphic phase transition is seen using DFT functionals and the same was not observed with DFT-D2 method. The equation of state and compressibility of both polymorphic phases were investigated. Electronic structure and optical properties were calculated using full potential linearized augmented plane wave method within the Tran-Blaha modified Becke-Johnson potential. The calculated electronic structure shows that α, β phases are indirect bandgap insulators with a bandgap values of 3.51 and 4.43 eV, respectively. The nature of chemical bonding is analyzed through the charge density plots and partial density of states. Optical anisotropy, electric-dipole transitions, and photo sensitivity to light of the polymorphs are analyzed from the calculated optical spectra. Overall, the present study provides an early indication to experimentalists to avoid the formation of unstable β-form of AgCNO.