Raman Spectroscopic Studies of Pressure-induced
Glassy Phase Transitions on cyclohexene*
ZHANGYI HU and HAIFEI ZHENG **
(School of Earth and Space Sciences, Peking University, Beijing 100871, China)
ABSTRACT: Raman spectroscopic study of cyclohexene was conducted in a moissanite anvil cell up
to 1.5 GPa at 26℃. Two new solid phases were observed. The phase formed at around 0.9 GPa is
confirmed as a glassy phase (termed as Phase A). The other solid phase (termed as Phase B) is formed
at 1.5GPa, with a first-order transition from Phase A to Phase B. Raman shift behaviors of 21
vibrational modes at the two transition points are summarized, providing primary data for the
mechanism of phase transition.
Key Words: cyclohexene, high pressure, glassy phase transition, Raman spectroscopy
INTRODUCTION
Cyclohexene, C6H10, is one of the organic molecular crystals with
pseudospherical cage or monocyclic ring structures that typically exhibit
order-disorder phase transitions in the solid state [1, 2, 3]. Phase behavior at low
temperatures has been studied extensively, which shows that the behavior involves
metastable and “glassy crystalline” states, depending on the rate of cooling and
thermal history of the sample [2, 3]. Recently, the crystal structures of three known
ambient-pressure phases have been determined [3]. However, the phase behavior and
vibrational modes of cyclohexene at high pressure is still not studied.
Raman shift as a function of pressure provides a valuable tool to indicate phase
transition and analyze the behavior of different vibrational modes at the transition
points. Such behaviors may serve as a path to look into the mechanism of the phase
transition. In this study, properties of cyclohexene at high pressure are investigated by
in situ Raman spectroscopic measurements and the techniques of DAC [4], which
provides some primary data for further theoretical and experimental work.
EXPERIMENTAL METHODS
The equipment used to generate high pressure is similar to the Mao-Bell diamond
anvil cell (DAC) [4], with the diamond anvils replaced by moissanite anvils. The
transparency of moissanite allows in situ optical observation and Raman spectra
measurements. A small quartz chip, as an internal pressure gauge, was loaded with the
sample into a hole in a stainless steel gasket (1.0 mm thick, 0.25 mm in diameter).
Pressure in the sample chamber was gradually increased by compressing the two
anvils. Raman spectra were recorded simultaneously using a Renishaw 1000
spectrometer. The 514.5 nm argon-ion laser beam, as the excitation light source, was
focused on the sample to a spot size of 1 μm, with an output of 25 mW. All spectra
*
This research was supported by grants from National Nature Science Foundation of China
(40173019, 10299040) and Jun Zheng funding.
**
Corresponding author, E-mail: [email protected]
were taken at room temperature.
Pressure in the chamber is calculated according to the following formula given by
Schmidt et al. [5]:
P(MPa) = 0.36079·(vp－464)2 +110.86·(vp－464),
where vp is the Raman wave number of quartz measured under the applied pressure
and 464 is the Raman wave number of quartz at ambient conditions.
Cyclohexene is a clear colorless liquid with a melting point of 170 K and a
boiling point of 345 K. The sample (95%) is obtained from Beijing chemical agent
company with no further purification before use. All the observed mode frequencies in
this work could be assigned to vibrational modes of cyclohexene [6, 8] or moissanite
[16], indicating that the influence of the 5% impurity on the Raman signal is
negligible.
The process of the experiment is shown on Fig. 1. The sample was compressed
very slowly: 96 steps of compression or decompression were carried out, taking about
15 hours. The spectra were collected with an equilibrium time of 2 or 3 minutes after
every step of compression or decompression. Several measurements with an
equilibrium time of about 10 minutes and one 17 minutes were obtained, in order to
evaluate the equilibrium state. These data points well fit the trend defined by the rest
data points (Fig. 1, 2). It is evident that a waiting time of 2 or 3 minutes was enough
for full equilibrium.
Fig. 1 Experimental process.
RESULTS AND DISCUSSION
The glassy phase (phase A)
Microscopic observation showed that the sample gradually solidified (Phase A) at
around 0.9 GPa. The quartz chip didn’t move even after the anvil cell was shaken
severely. In a following duplicate test, after every step of compression, we put the
anvil cell upside down and looked down into the chamber with a microscope. In the
first few steps, the quartz chip was always found at the bottom, because it sunk
quickly enough; as the pressure increased, it could be observed that the quartz chip
sunk gradually to the bottom; when it was around 1.0 GPa, the quartz sunk very
slowly, and finally couldn’t move, which was confirmed by the fact that it “hanged”
in the chamber for the following 6 days, motionless. The observation showed that the
liquid became more and more viscous and finally turned into a solid.
For all 21 vibrational modes studied here, plots of Raman shift versus pressure
show no abrupt change at 0.9GPa, which is inconsistent with the previous Raman
studies of first-order phase transitions (e. g. liquid-crystal transition) [10, 11]. For 17
of the 21 modes, there is a discernable change in slope (Table 2, Fig. 2) at 0.9 GPa,
which is consistent with Raman studies of a second-order phase transition of sodium
nitrite at 0.9+0.1GPa [14, 15]. Phase A is determined as an amorphous phase, because
the transition between liquid and glass is a typical second-order transition [1, 12].
Fig. 2. Raman shifts of three vibrational modes plotted against pressure: (a): ν5 (CH2 str.); (b): ν6 (
C=C str.); (c): ν18 (C-C str.). solid circles, liquid; hollow triangle, phase A; solid square, phase B
Fig. 1 shows a linear dependence between sample pressures and run times in
liquid and Phase A, with changed slopes but no break at the transition point. The
second-order transition between the two phases can provide a possible explanation.
The chamber was compressed evenly, and therefore the decrease of slope at 0.9 GPa
indicates a sudden drop of the sample’s bulk modules, which resulted from a sudden
increase of ( ∂ V/ ∂ P)T. The discontinuity in ( ∂ V/ ∂ P)T represents a second-order phase
transition[1, Ehrenfest’s theory].
The other solid phase (Phase B)
For 17 of the 21 modes, plots of Raman shift versus pressure show abrupt changes
at around 1.5 GPa (Fig. 2), indicating a first-order phase transition [10, 11].The
pressure dropped by about 80 MPa at this point. The abrupt changes in Raman shifts
showed that the transition was so fast that the coexisting of the two phases was not
observed.
Spectroscopic studies
The spectrum on cyclohexene has been studied in details, and the vibrational
assignment has been done thoroughly [2, 6]. The point group of cyclohexene
molecule is C2. There are 42 kinds of vibrational modes, 22 of which belong to the A
species and 20 to the B species [6, 7]. 30 of the modes give observable Raman bands
[6, 8, 13]. Together with some overtone bands，they construct the Raman spectrum of
cylcohexene. In this work the Raman spectra in the frequency range from 100cm-1 to
4000cm-1 (Fig. 3) is studied at the pressure up to 1.5 GPa. 21 vibrational modes are
analyzed. The rest bands are unable to yield valid results because of weak signal or
sluggishness to pressure change. The overtone bands are also neglected.
During the whole experiment, no peak became sharper or broader, no peak
disappears and no new peak was observed. The spectra are similar with those on the
four known lower-temperature solid phases in Ref. 2. However, a difference was
observed in the frequency range from 1000 cm-1 to 1100cm-1, which is summarized in
Table 1.
Fig. 3. Raman spectra of cyclohexene at different pressures up to 1.43 GPa.
Table 1. Vibrational data (cm-1) for cyclohexene in the frequency range of 1000-1100
In this work
Liquid, Phase A, B
1068m
From Ref. 2
Phase gI
Phase II
Phase III
Phase I
Assignment a
1075mw
1077mw
1076mw
1075mw
ν14
1070m
1068s
1066m
1066m
}
ν15
1040m
}
ν35
1067m
1048mw
1042m
1064m
1041m
1043m
1039m
a
1040m
Following Ref. 6
Part of the spectra is shown in Fig. 4, where the two peaks are definite in this
range. However, three main peaks are clearly discernible in the same range of the
spectra on the four low-temperature phases (see in Fig.1 of Ref. 2). This spectroscopic
difference shows that the two high-pressure solid phases are different from the known
low-temperature phases.
Raman shifts are plotted against pressure for 21 vibrational modes in this study,
and we find that different modes behave differently in the two transition points. At the
transition point around 0.9 GPa, the slope ( dω / dP ) may become steeper, gentler or
stay unchanged, but at the transition point around 1.5GPa, the Raman shift abruptly
jump, drop or move continuously. Typical peak behaviors are illustrated in Fig. 2, and
peak behaviors of 21 vibrational modes are summarized in Table 2.
Fig. 4. Part of the spectra on the three phases in this work. Two peaks in the range of 1000-1100,
where there will be three major peaks in the low-temperature phases [2].
Table 2. Vibrational data (cm-1) for cyclohexene and the peak behavior of 21 modes
Raman
Peak
Shift
behavior
Assignment
c
dω / dP (cm-1/GPa)
d
L to
A
a
A to
B
b
L
A
B
3206
-
-
ν1
0.99Kl
11.8+0.2
11.3+0.2
9+1
2940
+
-
ν2
0.99Kd
8.5+0.2
9.4+0.4
6+1
2914
+
-
ν3
1.0Kd
8.5+0.3
10.2+0.6
-
2882
+
+
ν26
0.99Kd
7.5+0.3
12.2+0.4
7+3
2863
+
0
ν4
0.99Kd
6.5+0.2
2838
+
+
ν5
0.99Kd
2.8+0.1
1654
0
-
ν6
0.79KD+0.12KR
4.12+0.03
3.4+0.5
1266
0
+
ν32
0.11KR+0.11Hφ+0.1Hψ+0.13Hθ+0.56Hγ
3.46+0.04
2.9+0.4
1241
0
+
ν11
0.21Hθ+0.65Hγ
3.01+0.07
2.3+0.6
1223
+
0
ν12
0.16KR+0.21Hφ+0.21Hψ+0.33Hγ
2.8+0.2
3.3+0.2
2.3+0.6
1065
-
+
ν15
0.38Hθ+0.30Hγ+0.13HΓ+0.31τD
6.1+0.1
4.9+0.1
4.7+0.6
1038
-
0
ν35
0.20KD+0.13Hφ+0.26Hψ+0.1Hθ+0.56Hγ
5.8+0.2
5.4+0.1
5.5+0.7
905
0
0
ν17
0.69KR+0.12Hγ+0.10KD
875
-
0
ν38
0.81KR
7.2+0.3
825
-
0
ν18
0.83KR
7.9+0.1
725
+
-
ν39
0.14KR+0.18Hε+0.37Hγ+0.14HΓ
645
-
-
ν40
495
+?
-
456
0
395
282
9.2+0.3
3.9+0.1
5.78+0.06
4.1+0.5
4+1
5.3+0.2
2.9+0.5
6.51+0.09
2+1
4.7+0.7
5+2
0.66HΓ
6.1+0.5
4.5+0.4
3+2
ν20
0.11Hε+0.58Hω+0.11Hγ
3.2+0.5
4.0+0.5
-
-
ν41
0.57Hω+0.34Hγ
-
-
ν21
0.31HΓ+0.34τD+0.1τT+0.1τR
6.0+0.2
5.1+0.2
4.2+0.9
-
-
ν22
0.49 Hω+0.1τD+0.27τR
12.4+0.6
9.0+0.5
≈0
4.4+0.1
-
a
Peak behavior in phase transition point from liquid to phase A: “0”=consistent slope;
“+”=steeper slope; “-”=gentler slope. e. g. Fig. 2: (a): +; (b): 0; (c): -.
b
Peak behavior in phase transition point from phase A to phase B: “0”=continuous Raman shift;
“+”=jump; “-”=drop. e.g. Fig.2: (a): +; (b): 0; (c): -.
c
K=stretching; H=bending; τ =Torsion. For more symbols see Ref. 6, 9
d
Pressure derivative of Raman shifts. “L”=Liquid; “A”=Phase A; “B”= Phase B; “-”= unable to
yield valid result.
There seems to be some regularity at the liquid-glass transition point, e. g. all the
modes with bond torsion (τD, τT, τR) shows a decreased slope. We presume that the
peak behaviors of the modes are decided by simple superposition of the behaviors of
its component bond movements. As a result, the behaviors of different bond
movements of cyclohexene at the liquid-glass transition point can be revealed. A
rudimentary analysis of bond movements controlled by pressure is given in Table 3.
Table 3. Features of bond movements at liquid-glass transition point
+
0
-
Kd Hδ
KD Hθ Hγ
Kl τD τT
Hε
Hω Hφ Hψ
τR
“+” or “-”: bond movements which become more sensitive, or less sensitive to pressure
change after vitrified from liquid, respectively.
CONCLUSIONS
At 26 ℃, Cyclohexne undergoes a liquid-glass phase transition around 0.9 GPa,
and a solid-solid phase transition around 1.5 GPa. Different vibrational modes behave
differently in the two transition points. Spectroscopic characters indicate that these
two high pressure phases are different from the known solid phases of cyclohexene at
room pressure.
ACKNOWLEDGEMENTS
We thank Jinqiu Ren of Department of Geology, Peking University for assisting
with the measurement of Raman spectra and Xi Liu of Department of Geology,
Peking University for reading the manuscript.
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