Pressure dependence of several molecular vibration modes in Cs2TCNQ3 has been observed through the infrared absorption measurements. The behavior of the C-CN and C-H stretching modes suggests that phase transition takes place at around 3.6 GPa. The π* electrons, which are localized on TCNQ- molecules in the low-pressure phase, are delocalized significantly in the high-pressure phase. However, the radical-like and neutral-like molecules still coexist in the high-pressure phase indicating that the electrons are not completely delocalized. The electron-molecular-vibration (emv) coupled mode disappears in the high-pressure phase in coincident with the disappearance of the inter-radical charge transfer band S1, proving the strong coupling between them

Hasanudin

Kagayama, Tomoko

Kuroda, Noritaka

Sugimoto, Toyonari

カガヤマ, トモコ

クロダ, ノリタカ

スギモト, トヨナリ

ハサヌディン

加賀山, 朋子

杉本, 豊成

黒田, 規敬

Kumamoto University Repository System

熊本大学学術リポジトリ
Kumamoto University Repository System
Title Infrared study on pressure-induced charge
delocalization in Cs2TCNQ3
Author(s)Hasanudin; Kuroda, Noritaka; Kagayama, Tomoko;
Sugimoto, Toyonari
CitationJournal of Physics. Condensed Matter, 14(44):
10419-10422
Issue date2002-11-11
Type Journal Article
URL http://hdl.handle.net/2298/10528
Right ? 2002 Institute of Physics and IOP Publishing
Limited
Infrared study on pressure-induced charge delocalization in Cs2TCNQ3 
 
Hasanudin1, N. Kuroda1,2, T. Kagayama1, T. Sugimoto3 
1Department of Mechanical Engineering and Materials Science, Faculty of Engineering, 
Kumamoto University, Kumamoto 860-8555, Japan 
2CREST, Japan Science and Technology Corporation, Kawaguchi 332-0012, Japan 
3Research  Institute  for Advanced Science and Technology,  
Osaka Prefecture University, Sakai 599-8570, Japan 
 
Pressure dependence of several molecular vibration modes in Cs2TCNQ3 has been observed through the 
infrared absorption measurements. The behavior of the C-CN and C-H stretching modes suggests that phase 
transition takes place at around 3.6 GPa. The π* electrons, which are localized on TCNQ- molecules in the 
low-pressure phase, are delocalized significantly in the high-pressure phase. However, the radical-like and 
neutral-like molecules still coexist at high-pressure phase indicating that the electrons are not completely 
delocalized. The electron-molecular-vibration (emv) coupled mode disappears at high-pressure phase in 
coincident with the disappearance of the inter-radical charge transfer band S1, proving the strong coupling 
between them. 
Keywords: TCNQ compound, semiconductor-to-metal transition, molecular vibration. 
 
1. Introduction 
Cs2TCNQ3 crystallizes into a columnar structure 
consisting of a periodic stack of TCNQ0/TCNQ-/TCNQ- 
[1], resulting in a semiconductor with the lowest optical 
energy gap of 0.3 eV [2]. The radicals TCNQ- are 
dimerized, so that the uppermost valence band is formed 
by the π* orbit of spin singlet. Since the on-site 
repulsion energy U ≈ 1.2 eV [3] of the π* electrons is 
higher than the optical energy gap, Cs2TCNQ3 may be 
regarded as a charge-transfer semiconductor of the 
1/3-filled Hubbard system [4]. 
The high-pressure electric resistivity measurement 
has shown that this material undergoes the phase 
transition to a metallic state at around 3 GPa [5]. 
However, the optical energy gap remains unclosed [2] 
throughout the phase transition indicating that the 
metallic state in this material should be considered 
different from the ordinary metals. The results from the 
high-pressure Raman scattering experiment suggest that 
the electrons are completely delocalized from TCNQ- 
radical molecules at high pressure resulting in a 
homogeneous distribution of charge along the TCNQ 
molecular stack [6]. 
 The purpose of this work is to study the electronic 
mechanism underlying the above mentioned phase 
transition in Cs2TCNQ3. We measure the pressure 
dependence of infrared absorption due to molecular 
vibration. The infrared spectroscopy gives the 
complementary information on the charge distribution 
along the TCNQ stack to the Raman spectroscopy. In 
particular, the infrared spectroscopy allows us to 
observe the e-mv coupled modes, which are 
unobservable by the Raman spectroscopy. 
 
2. Experimental 
The Cs2TCNQ3 salt is synthesized from the reaction 
of TCNQ with an excess of CsI, and the single crystals 
of this material were obtained by recrystallization of the 
salt from acetonitrile-ether solution. The as grown single 
crystals of Cs2TCNQ3, about 10 µm thick, are used as 
the samples. The pressure is generated using DAC. The 
daphne oil and flourinert (FC-40) are used as the 
pressure-transmitting medium for the measurement in 
the C-CN and C-H stretching region, respectively. The 
unpolarized infrared absorption is measured using a 
spectrometer system (Jasco FT/IR-410) equipped with a 
microscope. The ruby florescence method is used for 
calibrating the pressure.  
 
3. Results  
Figure 1 shows the absorption spectra in the 
C-CN stretching region at several pressures. The 
features at 1205 and 1211 cm-1 at the ambient pressure 
arise from neutral and radical molecules, respectively. 
These features show a blue-shift with increasing 
pressure, while increasing their energy spacing. At 
around 3.6 GPa they abruptly displace toward each 
other about half the way of the energy spacing. It is 
noteworthy that in the high-pressure phase radical-like 
and neutral-like molecules still coexist.  
 
Fig. 1. Absorption spectra in the C-CN 
stretching region at several pressures. 
 
The feature at 1183 cm-1 is the symmetric ag mode. 
The ag modes are originally infrared inactive in an 
isolated TCNQ molecule. They become infrared active 
as the result of the interaction of the molecular vibration 
with charge transfer, which is known as the electron 
molecular vibration (e-mv) coupling [7]. The frequency 
of the e-mv mode remains almost unchanged at low 
pressures. The e-mv mode disappears above 3.6 GPa, in 
coincidence with the disappearance of the electronic 
band S1, which is the counterpart of the e-mv mode [1,8]. 
Pressure dependencies of those modes are shown in Fig. 
2. 
 
Fig. 2. Pressure dependencies of the frequency 
of molecular vibration modes in the C-CN  
stretching region 
 
Figure 3 shows the absorption spectra in the C-H 
stretching region at several pressures. The b2uν36  mode 
belonging to radical and neutral molecules are observed 
at 3038 and 3054 cm-1, respectively, at ambient pressure. 
They are blue-shifted with increasing pressure while 
maintaining their energy spacing. When the pressure 
exceeds 3 GPa, the mode belonging to the neutral 
molecules abruptly displaced toward low energy side so 
that the energy spacing abruptly decreases. At around 
0 2 4 6
1180
1190
1200
1210
1220
1230
1240
 
Pressure (GPa)
Fr
eq
ue
nc
y 
(c
m
-1
)
 emv
 Radical
 Neutral
1160 1180 1200 1220 1240 1260
0.2
0.4
0.6
0.8
 0 GPa
 3.1
 4.0
 5.2
A
bs
or
ba
nc
e
Wavenumber (cm-1)
3.6 GPa they recover blue-shifting while maintaining 
their spacing. The pressure dependencies of the 
frequency of those modes are shown in Fig. 4. 
We have intended to observed other modes such as 
C=C and C=N vibrations, but failed because of the 
strong absorption of the diamond and the pressure 
transmitting medium. 
Fig. 3. The C-H stretching modes at  
several pressures. 
Fig. 4. The pressure dependencies of the C-H 
stretching modes. 
 
 
4. Discussion 
The results explained above clearly show that a 
pressure-induced electronic phase transition occurs at 
around 3.6 GPa. The energy spacing between the neutral 
and radical modes of both vibrations that abruptly 
decreases at high pressure indicates that the π* electrons 
are significantly delocalized from radical to neutral 
molecules. However, in the high-pressure phase 
radical-like and neutral-like molecules still coexist 
indicating that the π* electrons are not completely 
delocalized. This makes significant difference from 
results the Raman scattering measurement, which 
suggested that at high-pressure phase TCNQ molecules 
have a homogeneous ionicity [6]. 
The disappearance of the e-mv coupled mode 
above 3.6 GPa in coincidence the disappearance of the 
S1 band are reminiscent of the results from temperature 
dependent optical absorption measurement in 
(NMe4)2TCNQ3 [7]. The S1 band is assigned to the 
electron transfer between the adjacent radical molecules. 
The close resemblance between the properties of S1 and 
the e-mv coupled modes suggests the strong coupling 
between them.  
 
5. References 
1. C.J. Fritchie, Jr., P. Arthur, Jr., Acta Cryst. 21 (1966) 
139. 
2. Hasanudin, T. Kagayama, N. Kuroda, T. Sugimoto, 
Phys. Stat. Sol. (b) 223 (2001). 
3. K. D. Cummings, D. B. Tanner, J. S. Miller, Phys. Rev. 
B 24 (1981) 24. 
4. Z.G. Soos and D.J. Klein, J. Chem. Phys. 55 (1971) 
3284. 
5. S. Matsuzaki, Synth. Met. 61 (1993) 207. 
6. S. Matsuzaki, Y. Matsushita, M. Sano, Solid State 
Commun. 74 (1990) 1265. 
7. M.J. Rice, Solid State Commun. 31, (1979) 93. 
8. Hasanudin, N. Kuroda, T. Sugimoto, Synth. Met. 120 
(2001) 1045. 
3000 3050 3100 3150
0.2
0.4
0.6
5.7 GPa
3
2 
0 
Ab
so
rb
an
ce
Wavenumber (cm-1)
0 1 2 3 4 5 6
3040
3050
3060
3070
 Neutral
 Radical
Fr
eq
ue
nc
y 
(c
m
-1
)
Pressure (GPa)


Infrared study on pressure-induced charge delocalization in Cs2TCNQ3

Kumamoto University Repository

熊本大学学術リポジトリInfrared study on pressure-induced chargedelocalization in Cs2TCNQ3journal orpublication titleJournal of Physics. Condensed Mattervolume 14number 44page range 10419-10422year 2002-11-11URL http://hdl.handle.net/2298/10528doi: 10.1088/0953-8984/14/44/304Infrared study on pressure-induced charge delocalization in Cs2TCNQ3  Hasanudin1, N. Kuroda1,2, T. Kagayama1, T. Sugimoto3 1Department of Mechanical Engineering and Materials Science, Faculty of Engineering, Kumamoto University, Kumamoto 860-8555, Japan 2CREST, Japan Science and Technology Corporation, Kawaguchi 332-0012, Japan 3Research  Institute  for Advanced Science and Technology,  Osaka Prefecture University, Sakai 599-8570, Japan  Pressure dependence of several molecular vibration modes in Cs2TCNQ3 has been observed through the infrared absorption measurements. The behavior of the C-CN and C-H stretching modes suggests that phase transition takes place at around 3.6 GPa. The π* electrons, which are localized on TCNQ- molecules in the low-pressure phase, are delocalized significantly in the high-pressure phase. However, the radical-like and neutral-like molecules still coexist at high-pressure phase indicating that the electrons are not completely delocalized. The electron-molecular-vibration (emv) coupled mode disappears at high-pressure phase in coincident with the disappearance of the inter-radical charge transfer band S1, proving the strong coupling between them. Keywords: TCNQ compound, semiconductor-to-metal transition, molecular vibration.  1. Introduction Cs2TCNQ3 crystallizes into a columnar structure consisting of a periodic stack of TCNQ0/TCNQ-/TCNQ- [1], resulting in a semiconductor with the lowest optical energy gap of 0.3 eV [2]. The radicals TCNQ- are dimerized, so that the uppermost valence band is formed by the π* orbit of spin singlet. Since the on-site repulsion energy U ≈ 1.2 eV [3] of the π* electrons is higher than the optical energy gap, Cs2TCNQ3 may be regarded as a charge-transfer semiconductor of the 1/3-filled Hubbard system [4]. The high-pressure electric resistivity measurement has shown that this material undergoes the phase transition to a metallic state at around 3 GPa [5]. However, the optical energy gap remains unclosed [2] throughout the phase transition indicating that the metallic state in this material should be considered different from the ordinary metals. The results from the high-pressure Raman scattering experiment suggest that the electrons are completely delocalized from TCNQ- radical molecules at high pressure resulting in a homogeneous distribution of charge along the TCNQ molecular stack [6].  The purpose of this work is to study the electronic mechanism underlying the above mentioned phase transition in Cs2TCNQ3. We measure the pressure dependence of infrared absorption due to molecular vibration. The infrared spectroscopy gives the complementary information on the charge distribution along the TCNQ stack to the Raman spectroscopy. In particular, the infrared spectroscopy allows us to observe the e-mv coupled modes, which are unobservable by the Raman spectroscopy.  2. Experimental The Cs2TCNQ3 salt is synthesized from the reaction of TCNQ with an excess of CsI, and the single crystals of this material were obtained by recrystallization of the salt from acetonitrile-ether solution. The as grown single crystals of Cs2TCNQ3, about 10 µm thick, are used as the samples. The pressure is generated using DAC. The daphne oil and flourinert (FC-40) are used as the pressure-transmitting medium for the measurement in the C-CN and C-H stretching region, respectively. The unpolarized infrared absorption is measured using a spectrometer system (Jasco FT/IR-410) equipped with a microscope. The ruby florescence method is used for calibrating the pressure.   3. Results  Figure 1 shows the absorption spectra in the C-CN stretching region at several pressures. The features at 1205 and 1211 cm-1 at the ambient pressure arise from neutral and radical molecules, respectively. These features show a blue-shift with increasing pressure, while increasing their energy spacing. At around 3.6 GPa they abruptly displace toward each other about half the way of the energy spacing. It is noteworthy that in the high-pressure phase radical-like and neutral-like molecules still coexist.   Fig. 1. Absorption spectra in the C-CN stretching region at several pressures.  The feature at 1183 cm-1 is the symmetric ag mode. The ag modes are originally infrared inactive in an isolated TCNQ molecule. They become infrared active as the result of the interaction of the molecular vibration with charge transfer, which is known as the electron molecular vibration (e-mv) coupling [7]. The frequency of the e-mv mode remains almost unchanged at low pressures. The e-mv mode disappears above 3.6 GPa, in coincidence with the disappearance of the electronic band S1, which is the counterpart of the e-mv mode [1,8]. Pressure dependencies of those modes are shown in Fig. 2.  Fig. 2. Pressure dependencies of the frequency of molecular vibration modes in the C-CN  stretching region  Figure 3 shows the absorption spectra in the C-H stretching region at several pressures. The b2uν36  mode belonging to radical and neutral molecules are observed at 3038 and 3054 cm-1, respectively, at ambient pressure. They are blue-shifted with increasing pressure while maintaining their energy spacing. When the pressure exceeds 3 GPa, the mode belonging to the neutral molecules abruptly displaced toward low energy side so that the energy spacing abruptly decreases. At around 0 2 4 61180119012001210122012301240 Pressure (GPa)Frequency (cm-1) emv Radical Neutral1160 1180 1200 1220 1240 12600.20.40.60.8 0 GPa 3.1 4.0 5.2AbsorbanceWavenumber (cm-1)3.6 GPa they recover blue-shifting while maintaining their spacing. The pressure dependencies of the frequency of those modes are shown in Fig. 4. We have intended to observed other modes such as C=C and C=N vibrations, but failed because of the strong absorption of the diamond and the pressure transmitting medium. Fig. 3. The C-H stretching modes at  several pressures. Fig. 4. The pressure dependencies of the C-H stretching modes.   4. Discussion The results explained above clearly show that a pressure-induced electronic phase transition occurs at around 3.6 GPa. The energy spacing between the neutral and radical modes of both vibrations that abruptly decreases at high pressure indicates that the π* electrons are significantly delocalized from radical to neutral molecules. However, in the high-pressure phase radical-like and neutral-like molecules still coexist indicating that the π* electrons are not completely delocalized. This makes significant difference from results the Raman scattering measurement, which suggested that at high-pressure phase TCNQ molecules have a homogeneous ionicity [6]. The disappearance of the e-mv coupled mode above 3.6 GPa in coincidence the disappearance of the S1 band are reminiscent of the results from temperature dependent optical absorption measurement in (NMe4)2TCNQ3 [7]. The S1 band is assigned to the electron transfer between the adjacent radical molecules. The close resemblance between the properties of S1 and the e-mv coupled modes suggests the strong coupling between them.   5. References 1. C.J. Fritchie, Jr., P. Arthur, Jr., Acta Cryst. 21 (1966) 139. 2. Hasanudin, T. Kagayama, N. Kuroda, T. Sugimoto, Phys. Stat. Sol. (b) 223 (2001). 3. K. D. Cummings, D. B. Tanner, J. S. Miller, Phys. Rev. B 24 (1981) 24. 4. Z.G. Soos and D.J. Klein, J. Chem. Phys. 55 (1971) 3284. 5. S. Matsuzaki, Synth. Met. 61 (1993) 207. 6. S. Matsuzaki, Y. Matsushita, M. Sano, Solid State Commun. 74 (1990) 1265. 7. M.J. Rice, Solid State Commun. 31, (1979) 93. 8. Hasanudin, N. Kuroda, T. Sugimoto, Synth. Met. 120 (2001) 1045. 3000 3050 3100 31500.20.40.65.7 GPa32 0 AbsorbanceWavenumber (cm-1)0 1 2 3 4 5 63040305030603070 Neutral RadicalFrequency (cm-1)Pressure (GPa)

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Infrared study on pressure-induced charge delocalization in Cs2TCNQ3

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