Dynamic pressure-induced amorphous transition and crystallization behavior of 4-methylpyridine

The phase transitions of 4-methylpyridine (4MP) were studied under static pressure and dynamic pressure by in situ Raman spectroscopy. Under static compression, 4-methylpyridine underwent three phase transitions: a liquid to crystal I transition at 0.44 GPa; followed by transformation into crystal II at 2.17 GPa and crystal III at 10.36 GPa. Under dynamic compression, 4-methylpyridine was compressed into an amorphous phase as the pressure increased from 0.36, 0.38, and 0.33 GPa to 4.42, 6.80, and 11.53 GPa within 5 ms, respectively. Through the fitting of partial Raman peak linewidth with pressure, the compression rate has a significant effect on the disorder degree of crystal structure. During the pressure relief process, the amorphous 4-methylpyridine transformed into a new crystal Ⅰ’ at 2.40 GPa, and then returned to the initial liquid state at roughly 0.58 GPa. The phase transitions of 4-methylpyridine dynamic pressure can be explained by the faster compression rate that drives systems to break chemical equilibrium and generate a new phase.