High pressure spectroscopic studies of hydrazine (N2H4)

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.