Pressure-Induced Frustration–Frustration Process in 1-Butyl-3-methylimidazolium Hexafluorophosphate, a Room-Temperature Ionic Liquid

Abstract

We have found that the room-temperature ionic liquid (RTIL) reveals outstanding pressure-induced phase changes from a liquid state to a crystal polymorph and finally to a glass form upon compression by up to 8 GPa. The RTIL is 1-butyl-3-methylimidazolium hexafluorophosphate, [C4mim][PF6], which offers the opportunity to investigate a variety of fluctuations in one system and can be completely recovered without dissociation or polymerization, even after decompression. Similar to charge frustration, spin ice-like frustration, and geometric frustration in high potential spintronics/multiferroic materials, the RTIL frustrations are classified into charge (scalar), orientation (vector), and coordination number (topology). Degrees of freedom at each state of [C4mim][PF6] are described by charge balancing, molecular orientational order/disorder, molecular conformations of the C4mim(+) cation, and the coordination number. Here, we show a novel "conformation glass" induced by high pressure.

Stability of the Liquid State of Imidazolium-Based Ionic Liquids under High Pressure at Room Temperature

“…These p g values are lower than those of commonly used pressure media, such as a 4:1 mixture of methanol and ethanol (9.8 GPa) and 2-propanol (4.2 GPa), but are slightly higher than or comparable to those of glycerol (1.4 GPa) and silicone oil (0.9 GPa). 50 If we consider the result of [C 4 mim][PF 6 ], which crystallizes upon compression, 51 the results of [C n mim]-based ILs can be separated into, at least, three distinct groups on the basis of their behavior: (1) pressure-induced crystallization upon compression due to the reduction of lattice energies through disruption of packing efficiencies, 51 (2) superpressurization upon compression, and (3) decompression-induced crystallization from the superpressurized glass. More advanced experiments, however, are required to present a thorough discussion.…”

High-pressure phase behavior of the room temperature ionic liquid 1-ethyl-3-methylimidazolium nitrate

“…However, this trend remained true for [emim][BF 4 ] [9] and/or 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF 6 ]) [26] under high pressure conditions (GPa range) in the densely packed state, regardless of the crystallization or amorphization of the sample.…”

“…Because of the direct correlation between the specific phase behavior of RTILs and the isomerism of the imidazolium cation [9,24,26,27], the conformational changes of this cation were investigated according to the pressure-induced phase changes. Planar (P)-nonplanar (Np) Pressure /GPa CH stretching frequency /cm -1 equilibrium shifts for the C2N1C7C8 angle of the ethyl chain are reflected in the Raman spectrum detected from 400 to 460 cm − 1 .…”

Long Periodic Structure of a Room‐Temperature Ionic Liquid by High‐Pressure Small‐Angle X‐Ray Scattering and Wide‐Angle X‐Ray Scattering: 1‐Decyl‐3‐Methylimidazolium Chloride

“…An HP‐crystal polymorph of [C 10 mim][Cl] was observed, although an LT‐crystal polymorph did not appear. In contrast, in case of [C 4 mim][PF 6 ], both the LT‐ and HP‐crystal polymorphs were detected.…”

“…With respect to the cation's molecular conformation, the gauche portion increased under HP. Holding of the alkyl side‐chain was also observed in the pressure‐induced amorphization of [C 4 mim][PF 6 ] . The gauche spatial constraint in the amorphous phase was distinguishable using the Raman band.…”