Pressure-Induced Irreversible Phase Transition in the Energetic Material Urea Nitrate: Combined Raman Scattering and X‑ray Diffraction Study

In situ high-pressure Raman spectroscopy and synchrotron X-ray diffraction (XRD) have been employed to investigate the behavior of the energetic material urea nitrate ((NH₂)₂COH⁺·NO₃^–, UN) up to the pressure of ∼26 GPa. UN exhibits the typical supramolecular structure with the uronium cation and nitrate anion held together by multiple hydrogen bonds in the layer. The irreversible phase transition in the range ∼9–15 GPa has been corroborated by experimental results and is proposed to stem from rearrangements of hydrogen bonds. Further analysis of XRD patterns indicates the new phase (phase II) has Pc symmetry. The retrieved sample is ∼10.6% smaller than the ambient phase (phase I) in volume owing to the transformation from two-dimensional (2D) hydrogen-bonded networks to three-dimensional (3D) ones. The mechanism for the phase transition involves the cooperativity of noncovalent interactions under high pressure and distortions of the layered structure. This work suggests high pressure is an efficient technique to explore the performance of energetic materials, and to synthesize new phases with high density.