Transformation of hydrazinium azide to molecular N₈ at 40 GPa

Hydrazinium azide (HA) has been investigated at high pressures to 68 GPa using confocal micro-Raman spectroscopy and synchrotron powder x-ray diffraction. The results show that HA undergoes structural phase transitions from solid HA-I to HA-II at 13 GPa, associated with the strengthening of hydrogen bonding, and then to N₈ at 40 GPa. The transformation of HA to recently predicted N₈ (N≡N⁺-N^--N=N-^-N-⁺N≡N) is evident by the emergence of new peaks at 2384 cm^-1, 1665 cm^-1, and 1165 cm^-1, arising from the terminal N≡N stretching, the central N=N stretching, and the N-N stretching, respectively. However, upon decompression, N₈ decomposes to ε-N₂ below 25 GPa, but the remnant can be seen as low as 3 GPa.