Two distinct pressure-induced phase transitions in 1-butyl-2,3-dimethylimidazolium tetrafluoroborate

The pressure-induced phase transition behavior of the ionic liquid (IL) 1-butyl-2,3-dimethylimidazolium tetrafluoroborate ([C4dmim][BF4]) in the crystalline state was investigated by Raman spectroscopy at 298 K. The observed spectral changes indicate that four phase transitions may occur up to 8 GPa. The obtained results were compared with those reported in a previous study on supercooled [C4dmim][BF4], where crystallization occurred at ∼1.3 GPa and unexpectedly, only the gauche conformer was present in the high-pressure crystalline state. Typically, the trans conformer dominates the crystalline structures of most imidazolium-based ILs at high-pressures. This study reports two unique pressure-induced phase transition patterns in [C4dmim][BF4], highlighting the different conformational behavior under applied pressure.