Irregularly mechanical and thermal response of the N-H modes in the FOX-7 energetic material

Abstract

With the aid of Raman spectroscopy and density functional theory (DFT) calculations, we investigated the bonding dynamics of FOX-7 under mechanical and thermal perturbations. Results revealed that mechanical compression shortens and stiffens the O···H nonbonds while simultaneously lengthening and softening the H-N covalent bonds, up to critical pressures PC of 4.5 GPa, being in line with the O···H-O of ice transiting from the VII/VIII to phase X at 60 GPa. Conversely, thermal heating impacts both O···H nonbonds and H-N covalent bonds at distinct rates. The stretching and wagging vibrational modes of the H-N bonds soften within a specific pressure range, attributable to O···H-N hydrogen bonds. Additionally, FOX-7 undergoes multiple phase transitions under high pressure, including a continuous α to α' phase transition (second-order or higher-order) and a discontinuous α' to ε transition (first-order). These findings provide new insight into the bonding dynamics of FOX-7, which is in line with O···H-O bond for water and ice, showing the essentiality of inter- and intramolecular coupling interaction.