Order-Disorder Phase Transitions in Quasicrystals

Hiroshi Abe ^a,., Yukihiro Yoshimura^b, Yusuke Imai ^a, Takefumi Goto ^b, Hitoshi Matsumoto ^a
^a Department of Materials Science and Engineering, National Defense Academy, Yokosuka 239-8686, Japan
^b Department of Applied Chemistry, National Defense Academy, Yokosuka 239-8686, Japan

Journal of Molecular Liquids 150 (2009) 16.

Abstract

Phase behavior of roomtemperature ionic liquid (RTIL)-H₂Omixtures (x=0.0-98.5 mol% H₂O) was investigated by in-situ observations of simultaneous X-ray diffraction and differential scanning calorimetry method, and Raman spectroscopy: The RTIL is N, N-diethyl-N-methyl-N-2-methoxyethyl ammonium tetrafluoroborate, [DEME][BF₄]. We found that the crystal structure at low temperature is almost the same as that of pure [DEME][BF₄] over the whole range of water concentration. Above 60 mol%, H₂O crystal appears inside the RTIL crystals. The difference in the enthalpy between the liquid and the solid state was estimated as a function of H₂O concentration. In Raman spectra, a nearly-free hydrogen bonded band of H₂O appears clearly below 90 mol%.We correlate the existence of nearly-free bonds in the [DEME][BF₄]-H₂O mixtures with the abrupt change of the enthalpy at around 90 mol%.

	Fig. 1. X-ray diffraction pattern of [DEME][BF₄]-H₂O mixture at 60.5 mol% H₂O both on cooling and heating. On cooling, amorphous solid of [DEME][BF₄] is formed at low temperature and a small amount of H₂O ice crystal appears below -60 °C. Closed triangles and open circle reveal the calculated 2θ values of Bragg reflections of H₂O ice crystal and the Al sample holder, respectively. Upon heating, “cold crystallization” is observed at around -60 °C and the solid melts above -10 °C.
	Fig. 2. DSC thermogram of the [DEME][BF₄]-H₂O mixture at 60.5 mol% H₂O by the simultaneous X-ray diffraction and DSC measurements. A weak exothermal peak is observed on cooling. Considering the X-ray diffraction pattern in Fig. 1, glass transition of [DEME][BF₄] (T_g) and crystallization of a small amount of H₂O ice crystal (T_I) occurs simultaneously. Upon heating, “cold crystallization” of [DEME][BF₄] is observed at T_cc accompanying with the exotherm. Additionally, two kinds of melting points, T_m1 and T_m2, are observed.
	Fig. 3. “Kinetic phase diagrams” of [DEME][BF₄]-H₂O mixtures on (a) cooling and (b) heating. Closed circles represent observed data, which are determined by the simultaneous X-ray diffraction and DSC measurements with a cooling rate of 8 °C/min and a heating rate of 3 °C/min. Open squares are data points obtained by conventional DSC measurements. x_cc and x_I are defined to be a critical concentration of the “cold crystallization” and the formation of H₂O ice crystal, respectively. Tcc is the “cold crystallization” temperature with an exothermal peak on heating. T_m1 and T_m2 are the melting temperatures, respectively. L, C, A, I and U mean the liquid, [DEME][BF₄] crystal, amorphous, H₂O ice and undefined phase. The U is classified further into (I+A)-, (I+C)- and (I+A+C)-phases.
	Fig. 4. Cooling rate dependence of the glass transition temperature T_g and heating rate dependence of the “cold crystallization” temperature T_cc, and two kinds of melting points T_m1 and T_m2 (at 6.7 mol% H₂O). Tg depends on the cooling rate, although T_cc shows little heating rate dependence.
	Fig. 5. H₂O concentration dependence of the X-ray crystallinity, η. Very small crystallinity is seen around 6 mol%H₂O (A-phase). This is concordantwith the “kinetic phase diagram” on cooling.
	Fig. 6. (a) Change in the enthalpy difference between the liquid and solid states, ΔH, as a function of H₂O concentration, x. As indicating by arrows in the figure, anomalous increases of ΔH are observed below x_cc. (b) Change in the ΔH/η normalized by the crystallinity, η, as a function of x.
	Fig. 7. Raman spectra for thewater-[DEME][BF₄] solutions in the regionof 2700-3800 cm^-1 at x=95mol % (upper) and x=45mol %measured at roomtemperature. The solid circles are the experimental data. The dashed and solid lines represent the fitting curves and sum of the fitting curves, respectively.
	Fig. 8. Spectral changes in the nearly-free hydrogen bonded band of water-[DEME][BF₄] solution at (a) x=2.9 mol%, (b) x=6.7 mol% and (c) x=45 mol % as a function of temperature. The cooling rate was 5 °C/min.
	Fig. 9. Dependence of the water concentration on the relative ratio of integrated intensities of I_(ν(NFHB) and I_(ν(OH).

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ab@nda.ac.jp

Department of Materials Science and Engineering
National Defense Academy

Last Modified: April 1, 2009