title: In situ observation of multiple phase transitions in low-melting ionic liquid [BMIM][BF4] under high pressure up to 30 GPa.
abstract: In situ characterization of phase transitions and direct microscopic observations of a low-melting ionic liquid, 1-butyl-3-methyl imidazolium tetrafluoroborate ([BMIM][BF(4)]), has been performed in detail by Raman spectroscopy. Compression of [BMIM][BF(4)] was measured under hydrostatic pressure up to ~30.0 GPa at room temperature by using a high-pressure diamond anvil cell. With pressure increasing, the characteristic bands of [BMIM][BF(4)] displayed nonmonotonic pressure-induced frequency shifts, and it is found to undergo four successive phase transitions at around 2.25, 6.10, 14.00, and 21.26 GPa. Especially, above a pressure of 21.26 GPa, luminescence of the sample occurs, which is connected with the most significant phase transition at around this pressure. It was indicated that the structure change under high pressure might be associated with a conformational change in the butyl chain. Upon releasing pressure, the spectrum was not recovered under a pressure up to 1.16 GPa, thereby indicating that this high-pressure phase remains stable over a large pressure range between 30 and 1.16 GPa in low-melting ionic liquid [BMIM][BF(4)]. Although the sample was kept under the normal pressure for 24 h, the spectrum was recovered, and it showed that the phase transition of [BMIM][BF(4)] was reversible. In other words, such a low-melting ionic liquid [BMIM][BF(4)] remains stable even after being treated under so a high pressure of up to 30 GPa.
authors.affiliation: The High Pressure Research Center of Science and Technology, Zhengzhou University of Light Industry, Zhengzhou 450002, China. zz_sulei@zzuli.edu.cn
authors.forename: Lei
authors.initials: L
authors.lastname: Su
authors.forename: Xiang
authors.initials: X
authors.lastname: Zhu
authors.forename: Zheng
authors.initials: Z
authors.lastname: Wang
authors.forename: Xuerui
authors.initials: X
authors.lastname: Cheng
authors.forename: Yongqiang
authors.initials: Y
authors.lastname: Wang
authors.forename: Chaosheng
authors.initials: C
authors.lastname: Yuan
authors.forename: Zhenping
authors.initials: Z
authors.lastname: Chen
authors.forename: Chunli
authors.initials: C
authors.lastname: Ma
authors.forename: Fangfei
authors.initials: F
authors.lastname: Li
authors.forename: Qiang
authors.initials: Q
authors.lastname: Zhou
authors.forename: Qiliang
authors.initials: Q
authors.lastname: Cui