Vibrational Dynamics and Phase Transitions of Hydrazine to 50 GPa

The high pressure behavior of hydrazine, N₂H₄, has been investigated to 50 GPa at room temperature using infrared and Raman spectroscopy to explore pressure induced phase transitions and changes in hydrogen bonding. Three solid-solid phase transitions were detected at 11, 21, and 32 GPa on room temperature compression through dramatic changes in the lattice vibration and N-H stretching regions with increasing pressure in both measurement techniques. The transition to phase IV, which appears at 32 GPa, exhibits increased hydrogen bonding with significant hysteresis, persisting to 9 GPa on decompression. This work presents a detailed analysis of the pressure dependence of mode shifts and calculations of mode Grüneisen parameters as well as a determination of an approximate thermodynamic Grüneisen parameter. We compare these results to the behavior of other small molecular materials such as ammonia and water and explore the evolution of hydrogen bonding in hydrazine toward the symmetrically hydrogen bonded state, which has previously been suggested by theoretical computations.