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

Jushan Wang, Zhaoyang Zheng, Yangyang Zeng, Zanhao Wang, Guoyang Yu, Qi Long Yan, Huashan Li, Yanqiang Yang, Biao Wang

Research output: Contribution to journalArticlepeer-review

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 H⋯O nonbonds while simultaneously lengthening and softening the N-H covalent bonds, up to a critical pressure PC 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.

Original languageEnglish
JournalPhysical Chemistry Chemical Physics
DOIs
StateAccepted/In press - 2025

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