Compression Rate-Driven Amorphization in Pyrrole: Static vs. Dynamic Pressure Conditions

The phase transition of pyrrole was studied under static pressure and dynamic pressure by in situ Raman spectroscopy. Under static compression, pyrrole underwent three phase transitions: a liquid-to-crystal (phase Ⅰ) transition at 1.15 GPa, followed by transformation into phase II at 7.45 GPa and the start of amorphous phase transition at 16 GPa. Under dynamic compression, pyrrole was compressed into an amorphous phase as the pressure increased from 0.71 GPa to 5.02 GPa, 8.07 GPa, 10.30 GPa, 11.30 GPa and 12.84 GPa within 5 ms, respectively. Rapid compression rate and high pressure provide a greater degree of disorder in amorphous pyrrole by fitting the partial Raman peak linewidth with pressure. During the pressure relief process, the amorphous pyrrole crystallized and transformed into phase II at 3.20 GPa and then reverted to its initial liquid state at around 0.55 GPa. The phase behaviors can be attributed to different solidification mechanisms of liquid pyrrole induced by different compression rates under the two compression conditions.