High-Pressure Experimental and DFT-D Structural Studies of the Energetic Material FOX-7

Abstract

This work reports the hydrostatic compression of the perdeuterated α-form of FOX-7 using neutron powder diffraction to follow the structural changes up to 4.58 GPa at room temperature. The equation of state for the hydrostatic compression of the α-form over the range 0–4.14 GPa has been determined, and a phase transition was observed over the pressure range 3.63–4.24 GPa. On the basis of dispersion-corrected density functional theory (DFT-D) calculations performed on the γ-form over a range of pressures, the high-pressure form observed in the neutron diffraction experiments can unambiguously be identified as being different from the γ-form and should therefore be denoted as the ε-form. Based on similarities between the simulated and experimental powder diffraction patterns of the γ- and ε-forms, it is suggested that the ε-form adopts a planar, layered structure. The structural responses to pressure of the α-form observed experimentally are reproduced by DFT-D calculations, but in-depth analysis of the bond lengths, angles, dihedrals, and vibrational frequencies calculated in the DFT-D simulations identified a very subtle second-order phase transition at 1.9 GPa. This corroborates results obtained from previous far- and mid-IR vibrational spectroscopic studies. These very small changes in molecular geometry do not manifest themselves in either the measured or calculated lattice parameters or unit-cell volumes and are much smaller than can be detected by diffraction experiments. The results of phonon calculations were compared with experimental inelastic neutron scattering measurements and were used to investigate the effect of pressure on the heat capacities of α-FOX-7. The simulations predict very weak pressure dependencies (approximately −1 J K–1 mol–1 GPa–1), in accordance with the conclusions reached in our previous studies of the energetic material RDX.

Predicting the reactivity of energetic materials: an ab initio multi-phonon approach

“…The PBE-D2 scheme has been previously demonstrated to work well for molecular energetic materials. [66][67][68] It was therefore chosen as a starting point for the calculations presented here. All input structures were taken from the Cambridge Crystallographic Database ( , and TATB (TATNBZ 73 ) or were recently determined (AGTZ, GTZ, DGTZ).…”

Theoretical insights on a series of difluoramino group-based energetic molecules

“…Becke, 3‐parameter, Lee‐Yang‐Parr (B3LYP) method of density functional theory has been demonstrated as a reliable way and economical tool in predicting the accurate energies, molecular structures, and vibrational frequencies of unknown energetic materials because it cannot only provide accurate geometries and energies, but also require less computer resources than other computational methods . Therefore, computations in this paper were conducted at B3LYP/6‐31G ( d , p ) level with Gaussian 03 software .…”

Structural and Vibrational Properties of Crystalline β‐Octahydro‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocine at High Temperatures: Ab Initio Molecular Dynamics Studies

“…Raman spectroscopy can provide powerful insights into the fundamental vibrational characteristics of energetic materials. Therefore, much more attention has been paid to investigate their vibrational properties under extreme conditions …”

“…The calculated pressure‐induced frequency shifts indicate that the pressure produces a more significant influence on the ring deformation and stretching vibrations than on other modes. Hunter et al . researched the structural responses of α ‐FOX‐7 ( α ‐1,1‐diamino‐2,2‐dinitroethene) to pressure by periodic density functional theory with dispersion correction (DFT−D) calculations.…”