Structures, Mechanical Properties, Equations of State, and Electronic Properties of β-HMX under Hydrostatic Pressures: A DFT-D2 study (original) (raw)
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We investigate the equation of states of the beta\betabeta-polymorph of cyclotetramethylene tetranitramine (HMX) energetic molecular crystal using DFT-D2, a first-principles calculation based on density functional theory (DFT) with van der Waals (vdW) corrections. The atomic structures and equation of states under hydrostatic compressions are studied for pressures up to 100 GPa. We found that the N-N bonds along the minor axis of the ring are more sensitive to the variation of pressure, which indicates that they are potential "weak spots" in atomic level within a single molecule of beta\betabeta-HMX. Our study suggested that the van der Waals interactions are critically important in modeling this molecular crystal.
Predicting Elastic Properties of β-HMX from First-principles calculations
The Journal of Physical Chemistry B, 2015
We investigate the performance of van der Waals (vdW) functions in predicting the elastic constants of β cyclotetramethylene tetranitramine (HMX) energetic molec- ular crystals using density functional theory (DFT) calculations. We confirm that the accuracy of the elastic constants is significantly improved using the vdW correc- tions with environment dependent C 6 together with PBE and revised PBE exchange- correlation functionals. The elastic constants obtained using PBE-D3(0) calculations yield the most accurate mechanical response of β-HMX when compared with experi- mental stress-strain data. Our results suggest that PBE-D3 calculations are reliable in predicting the elastic constants of this material.
Elastic Coefficients of β-HMX as Functions of Pressure and Temperature from Molecular Dynamics
Crystals, 2020
The isothermal second-order elastic stiffness tensor and isotropic moduli of β-1,3,5,7- tetranitro-1,3,5,7-tetrazoctane (β-HMX) were calculated, using the P21/n space group convention, from molecular dynamics for hydrostatic pressures ranging from 10−4 to 30 GPa and temperatures ranging from 300 to 1100 K using a validated all-atom flexible-molecule force field. The elastic stiffness tensor components were calculated as derivatives of the Cauchy stress tensor components with respect to linear strain components. These derivatives were evaluated numerically by imposing small, prescribed finite strains on the equilibrated β-HMX crystal at a given pressure and temperature and using the equilibrium stress tensors of the strained cells to obtain the derivatives of stress with respect to strain. For a fixed temperature, the elastic coefficients increase substantially with increasing pressure, whereas, for a fixed pressure, the elastic coefficients decrease as temperature increases, in acco...
2006
We report the results of a comparative study of pentaerythritol tetranitrate (PETN) at high compression using classical reactive interatomic potential ReaxFF and first-principles density functional theory (DFT). Lattice parameters of PETN I, the ground state structure at ambient conditions, is obtained by ReaxFF and two different density functional methods (plane wave and LCAO pseudopotential methods) and compared with experiment. Calculated energetics and isothermal equation of state (EOS) upon hydrostatic compression obtained by DFT and ReaxFF are both in good agreement with available experimental data. Our calculations of the hydrostatic EOS at zero temperature are extended to high pressures up to 50 GPa. The anisotropic characteristics of PETN upon uniaxial compression were also calculated by both ReaxFF and DFT.
The Journal of Physical Chemistry C, 2017
The structural response of a novel, insensitive energetic crystaldihydroxylammonium 5,5′-bistetrazole-1,1′-diolate (TKX-50) was examined under high pressure. Using synchrotron single-crystal X-ray diffraction measurements, details of molecular, intermolecular, and crystal changes were determined to ∼10 GPa to understand its structural stability. The experimental results showed that TKX-50 exhibits highly anisotropic compression and significantly lower volume compressibility than currently known energetic crystals. These results are found to be in general agreement with our previous predictions from the DFT calculations. Additionally, the experimental data revealed anomalous compressionan expansion of the unit cell along the a axis (negative linear compressibility, NLC) upon compression to ∼3 GPa. The structural analyses demonstrated that this unusual effect, the first such observation in an energetic crystal, is a consequence of the highly anisotropic response of 3D motifs, comprised of two parallel anions [(C 2 N 8 O 2) 2− ] linked with two cations [(NH 3 OH) + ] through four strong hydrogen bonds. The present results demonstrate that the structural stability of TKX-50 is controlled by the strong and highly anisotropic intermolecular interactions, and these may contribute to its shock insensitivity.
The Journal of Physical Chemistry A, 2014
Pressure effects on the Raman vibrations of an energetic crystal FOX-7 (1, 1diamino-2, 2-dinitroethene) were examined using density functional theory (DFT) calculations. High accuracy calculations were performed with a periodic plane-wave DFT method using normconserving pseudopotentials. Different exchange−correlation functionals were examined for their applicability in describing the structural and vibrational experimental data. It is shown that the PBE functional with an empirical dispersion correction by Grimme, PBE-D method, reproduces best the molecular geometry, unit cell parameters, and vibrational frequencies. Assignments of intramolecular Raman active vibrations are provided. The calculated pressure dependence of Raman shifts for the intramolecular and lattice modes were found to be in good agreement with the experimental data; in particular, the calculations predicted correctly a decrease of frequencies for the NH 2 stretching modes with pressure. Also, in accord with experiments, the calculations indicated some instances of modes mixing/coupling with increasing pressure. This work demonstrates that the dispersion-corrected PBE functional can account for the structural and vibrational properties of FOX-7 crystal at ambient and high pressures.
Journal of Applied Physics, 2010
Hydrostatic and uniaxial compressions of 1,3,5-triamino-2,4,6-trinitrobenzene were investigated using first-principles density functional theory with an empirical van der Waals correction. The equilibrium structural and elastic properties and the hydrostatic equation of state are in good agreement with available experimental data. Physical properties such as the principal stresses, shear stresses, band gap, and the change in energy per atom as a function of compression ratio V / V 0 in the directions normal to the ͑100͒, ͑010͒, ͑001͒,͑110͒, ͑101͒, ͑011͒, and ͑111͒ crystallographic planes were calculated, showing highly anisotropic behavior under uniaxial compressions.
Electronic structure of solid nitromethane: Effects of high pressure and molecular vacancies
The combined effect of pressure and molecular vacancies on the atomic structure and electronic properties of solid nitromethane, a prototypical energetic material, is studied at zero temperature. The self-consistent charge density-functional tight-binding method is applied in order to investigate changes induced in the band gap of this system by uniform and uniaxial strain of up to 70%, corresponding to static pressure in the range of up to 200 GPa. The effects of molecular vacancies with densities ranging from 3% to 25% have also been considered. A surprising finding is that uniaxial compression of about 25-40 GPa along the b lattice vector causes the C-H bond to be highly stretched and leads to proton dissociation. This event also occurs under isotropic compression but at much higher pressure, being indicative of a detonation chemistry which is preferential to the pressure anisotropy. We also find that the band gap, although evidently dependent on the applied strain, crystal anisotropy and vacancy density, is not reduced considerably for electronic excitations to be dominant, in agreement with other recent first-principles studies.
Elastic Properties of Molecular Crystals Using Density Functional Calculations
AIP Conference Proceedings, 2004
The elastic properties of several molecular crystals (PE, PETN, urea) have been investigated using plane-wave pseudopotential methods based on density functional theory (DFT). The lattice constants, elastic constants and bulk modulus of the molecular crystals were calculated and compared with experiments. Two prevalent density functional methods, LDA and GGA-PBE were tested and compared with each other. We find that LDA typically overestimates the stiffness of these crystals at least by factor of two, while the GGA calculations with the PBE exchange-correlation functional are more reasonable. A large cutoff of the plane-wave basis and a modest sampling of k space are required to describe the molecular crystals. The elastic behavior of urea and PE crystals under uniaxial compressions show interesting anisotropic effects.
High-Pressure Crystal Structures of an Insensitive Energetic Crystal: 1,1-Diamino-2,2-dinitroethene
The Journal of Physical Chemistry C, 2016
Understanding the insensitivity/stability of insensitive high explosive crystals requires detailed structural information at high pressures and high temperatures of interest. Synchrotron single-crystal X-ray diffraction experiments were used to determine the high-pressure structures of 1,1-diamino-2,2dinitroethene (FOX-7), a prototypical insensitive high explosive. The phase transition around 4.5 GPa was investigated and the structures were determined at 4.27 GPa (α′ phase) and 5.9 GPa (ε phase). The α′ phase (monoclinic, P2 1 /n), structurally indistinguishable from the ambient α phase, transforms to the new ε phase (triclinic, P1). The most notable features of the ε phase, compared to those of the α′ phase, are formation of planar layers and flattening of molecules. Density functional theory (DFT-D2) calculations complemented the experimental results. The results presented here are important for understanding the molecular and crystalline attributes governing the high-pressure insensitivity/stability of insensitive high explosive crystals.