High pressure stability of hydrazine (H$_{2}$N-NH$_{2})$: Implications for energetic hydronitrogen compounds

Hydrazine (H$_{2}$N-NH$_{2})$ is a metastable, high energy density molecule that is relevant to planetary physics and plays an important role in industrial synthesis and propellant applications. Theoretical calculations have predicted the existence of ``hydronitrogen'' extended solids that hold great potential as a high energy density material (HEDM). Exploring the high pressure-temperature ($P-T)$ stability of hydrazine will provide crucial insights into hydrogen bonded -N-H networks under these conditions. Further, related simple molecules such as CH$_{4}$, NH$_{3}$, CO, and CO$_{2}$ have been shown to have rich high $P-T$ phase diagrams, often forming extended amorphous solids. Here, we report the first comprehensive study of hydrazine to 50 GPa at ambient temperature, using both \textit{in situ} vibrational spectroscopy and synchrotron x-ray diffraction to elucidate structural changes driven by compression. Liquid hydrazine solidifies into a monoclinic structure at 0.5 GPa that is isomorphous with the low-$T$ solid phase. Further compression drives structural re-ordering and at least 2 phase transformations to 20 GPa, with complex anisotropic hydrogen bonding interactions. Surprisingly, no evidence for the formation of extended amorphous solids was observed to the highest pressure studied.