Pressure-Induced Irreversible Phase Transition in the Energetic Material Urea Nitrate*

The behavior of energetic material Urea Nitrate ((NH$_{2})_{2}$COH$^{+}$ $\cdot$ NO$_{3}^{-}$, UN) has been investigated up to the pressure of $\sim$26 GPa. UN exhibits the typical supramolecular structure with uronium cation and nitrate anion held together by multiple hydrogen bonds in the layer. Both Raman and XRD data provide obvious evidence for the distorted phase transition in the pressure range $\sim$9--15 GPa. Further analysis indicates phase II has \textit{Pc} symmetry. The mechanism for the phase transition involves collapse of the initial 2D supramolecular structure to 3D hydrogen-bonded networks in phase \textit{Pc}. Importantly, the transition is irreversible and leads to a large reduction in volume on release of pressure. The density in phase \textit{Pc} has been increased by $\sim$11.8{\%} compared to the phase $P$2$_{1}$/$c$ under ambient conditions and therefore phase \textit{Pc} is expected to have much higher detonation power. This study opens new opportunities for preparing energetic materials with high density combining supramolecular chemistry with high-pressure techniques. Corresponding author. E-mail: zoubo@jlu.edu.cn Reference: JPCC. 2013, 117, 152.