Pressure-induced amorphization of ionic liquid [HMIM][PF6]

•Amorphization and conformational equilibrium of [HMIM][PF6] have been investigated.•A phase transition of liquid to glass occurs at about 3.4 GPa upon compression.•The fraction of AAAA conformer increases with increasing pressure.•Nanoscale structural heterogeneities play an important role in the process.•It gives a deeper insight into pressure-induced amorphization of ionic liquids.

Phase behavior of ionic liquid [HMIM][PF6] has been investigated under high pressure up to 5.6 GPa at room temperature. The results indicated that [HMIM][PF6] might experience a phase transition at about 3.4 GPa upon compression, which could be identified as solidification to superpressurized glass by ruby R1 line broadening measurement and synchrotron X-ray diffraction (XRD) patterns. For conformational equilibrium, the fraction of all-anti (AAAA) conformer increased upon compression, while the conformational change was independent of temperature. These facts indicated that there were large differences of the structure in response to the extreme conditions, especially in the structure of the cation.

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