Combined Raman Scattering and X-ray Diffraction Study of Phase Transition of the Ionic Liquid [BMIM][TFSI] Under High Pressure

In situ solidification of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM][TFSI]) from melt has been investigated by using a diamond anvil cell under pressures up to about 6.0 GPa at room temperature. The results indicate that non-monotonic pressure induced Raman shifts and variation of the full width at half-maximum are suggestive of a phase transition at about 1.8 GPa upon compression, which was further confirmed to be a phase transition from liquid to a superpressurized glass by line broadening measurements of the sharp R 1 ruby fluorescence line and synchrotron X-ray diffraction measurements. The conformational changes of C 1 and C 2 conformers of the [TFSI] anion under high pressure are discussed; the C 2 conformers are predominant under high pressures. In addition, the Raman spectra of [BMIM][TFSI] under different conditions are compared in detail. The results show that the structure of the glassy state at low temperature is different from that of the high pressure state, while the glassy state is mainly composed of C 2 anion conformers, which is similar to the high pressure state.