High pressure spectroscopic studies of hydrazine (N₂H ₄)

Pressure-induced changes (up to 20 GPa) to the structure and bonding in hydrazine (N₂H₄) have been investigated using diamond anvil cell (DAC) with in situ Raman spectroscopy. Liquid hydrazine solidifies at 0.3 GPa into a crystalline phase and its structure was established using synchrotron x-ray diffraction (XRD) measurements. The high pressure phase is monoclinic (P2₁) with a remarkable correspondence to the low-T monoclinic structure that is also seen in the similar Raman spectral features. With increasing pressure, the modifications to N-H...N hydrogen bonding is observed with emergence of new contacts beyond 5 GPa as well as appearance of new lattice modes. This is accompanied by small changes in the pressure dependency (dv/dP) of frequencies. Based on these observations, we conclude a sluggish phase transition in the 5-7 GPa range accompanied by selective strengthening and restructuring of hydrogen bonding network. Inelastic Neutron Scattering (INS) measurements performed in the 10-250 K range indicates that the order-disorder phase transition (observed in thin films at 175-80 K) driven by conformational changes is not observed in a bulk sample.