Photon Factory Activity Report 2014 #32 (2015) B

BL-18C/2013G017

Polymorphs in Room Temperature Ionic Liquids under High Pressure
Hiroshi Abe1,*, Hiroaki Kishimura1, Yukihiro Yoshimura2, Mayumi Takaku3 and Nozomu Hamaya3
1
Dept. of Mater. Sci. & Eng., National Defense Academy, Yokosuka 239-8686, Japan
2
Dept. of Appl. Chem., National Defense Academy, Yokosuka 239-8686, Japan
3
Graduate School of Hum. & Sci., Ochanomizu University, Tokyo 112-8610, Japan
1 Introduction
Room temperature ionic liquids (RTILs) are recently
known as novel solvent in green chemistry. We havealreadyreported
outstanding
liquidhierarchy,
nanoheterogeneity of liquid structures, crystal
polymorphs, and multiple glass transition, and so on. Thetopics are summarized in the recently published paper [1].
Focusing on pressure-induced phase changes, we areinvestigating high pressure (HP) phases in Table 1. For acomparison, low temperature (LT) phases are alsodenoted. Red colored phases were determined by ourgroup. In Table 1, cc and dc represent cold crystallizationand decompression crystallization, respectively. A varietyof solid phases are results in the inherent properties ofcation and anion.
To fill the solid phases in Table 1 is necessary todistinguish the pressure effect from thermal one relatingwith characteristic cation and anion.
2 ExperimentHigh-pressure X-ray diffraction experiments werecarried out by using a Mao-Bell type diamond anvil cell(DAC) in the BL-18C of the Photon Factory at the HighEnergy Accelerator Research Organization in Japan. Data
were collected at room temperature. Two dimensional
diffraction patterns were obtained using an Imaging-Platesystem (FLA 7000,GE Healthcare Life Sciences Co.).
[DEME][BF4](N,N-diethyl-N-methyl-N-(2methoxyethyl) ammonium tetrafluoroborate) [2, 3],[C2mim][BF4](1-ethyl-3-methylimidazoliumtetrafluoroborate)[4],[C4mim][PF6](1-butyl-3methylimidazolium
hexafluorophosphate)[5],[C2mim][NO3] (1-ethyl-3-methylimidazolium nitrate) [6],anda series of [Cnmim][BF4] (1-alkyl-3-methylimidazolium tetrafluoroborate) [7] are selected in thisstudy.
3 Results and Discussion
The intrinsic features of anions in Table 1 are describedas follows; (i) NO3- anion possesses the property of
proton capturing, (ii) BF4- anion is identified bygeometrical freezing factor, (iii) PF6- anion causes
rotational disorder, (iv) TFMS- anion is chemically stableacid, and (v) TFSI- has two stable conformers.
PF6- anion is a clue to interpret the HP effect insolidifications. For instance, HP crystals of [C4mim][PF6]below 1 GPa are the same as LT ones [5]. Above 1 GPa,
entirely different crystal structures appeared. Thus, LT
crystal polymorphs are not equal to HP ones.
Orientational disorder of PF6- anion plays an important

Table 1. Low-temperature (LT) and high pressure (HP)phases of RTILs.
role with molecular packing efficiency under highpressure. Furthermore, in [DEME][PF6], different kind ofHP crystal polymorph was observed on decompression
process. Crystal structure analysis is now in progress. The
result will provide cation effect in HP crystal polymorphs
in the RTILs.
Acknowledgement
Authors appreciate Dr. S. Nakano of NIMS and Dr.
Kikegawa of KEK for experimental supports.
References[1] H. Abe, T. Takekiyo, M. Aono, H. Kishimura, Y.
Yoshimura, N. Hamaya, J. Mol. Liq. (2015), in press.
[2] Y. Yoshimura, H. Abe, Y. Imai, T. Takekiyo, N.
Hamaya, J. Phys. Chem. B 117 (2013) 3264.
[3] H. Abe, Y. Imai, T. Takekiyo, Y. Yoshimura, N.
Hamaya, IOP Conf. Series: Mater. Sci. Eng. 54 (2014)
012003.
[4] Y. Yoshimura, H. Abe, T. Takekiyo, M. Shigemi, N.
Hamaya, R. Wada, M. Kato, J. Phys. Chem. B 117(2013) 12296.
[5] H. Abe, T. Takekiyo, N. Hatano, M. Shigemi, N.
Hamaya, Y. Yoshimura, J. Phys. Chem. B 118 (2014)1138.
[6] Y. Yoshimura, T. Takekiyo, H. Abe, N. Hamaya, J.
Mol. Liq. 206 (2015) 89.
[7] Y. Yoshimura, M. Shigemi, M. Takaku, M. Yamamura,T. Takekiyo, H. Abe, N. Hamaya, D. Wakabayashi,
K. Nishida, N. Funamori, T. Sato, T. Kikegawa,J. Phys. Chem. B 119 (2015) 8146.
* ab@nda.ac.jp