Irregular mechanical and thermal responses of the N-H modes in the FOX-7 energetic material

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 H⋯O nonbonds while simultaneously lengthening and softening the N-H covalent bonds, up to a critical pressure P_C of 4.5 GPa, being in line with the O-H⋯Obond of ice transiting from the VII/VIII to phase X at 60 GPa. Conversely, thermal heating impacts both H⋯O nonbonds and N-H covalent bonds at distinct rates. The stretching and wagging vibrational modes of the N-H bonds soften within a specific pressure range, attributable to N-H⋯O hydrogen bonds. The rocking vibration of the N-H bond in-plane possesses the positive compressibility and positive thermal expansivity. 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 dynamics for water and ice, showing the essentiality of inter- and intramolecular coupling interactions.