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Raman scattering spectra of internal modes in triglycine sulphate
ELSEVIER
Physica B 219&220 (1996) 529 531
Raman scattering spectra of internal modes in triglycine sulphate A. Sakai *, A. Araya Department q["Electrical and Electronic Engineerinq, Muroran Institute ol Technology, Mizumoto-eho 2 7-1, Muroran 050, Japan
Abstract Raman scattering spectra of triglycine sulphate have been observed as a function of temperature with a special attention to internal modes. The CH2 symmetric stretching vibration splits into two modes in the ferroelectric phase. The temperature dependence of the frequency difference between these split modes is in good agreement with that of the spontaneous polarization. In addition to the CH2 symmetric stretching vibration, the NH3 rocking vibration and the CH2 wagging vibration show similar temperature dependence. This result indicates that the change of the environment of the glycine molecule is strongly related to the order parameter in the phase transition.
1. Introduction
2. Experimental procedure
Triglycine sulphate (NH2CH2COOH)3H2SO4 (TGS) has been considered one of the most important ferroelectric crystals since its ferroelectricity was discovered. The phase transition at T c - 49 °C has been well described by an order-disorder type mechanism. In order to study the phonon contribution to the phase transition, optic-phonon modes have been investigated by Raman scattering [1-3]. Though the soft optic phonon does not appear near Tc, the phonon modes show anomalies at Tc in both the externaland internal-mode regions. In the external region, for example, the relation between phonon modes and the dielectric constant has been discussed [2]. On the other hand, in the internal region, the quantitative study has not been carried out so far. The purpose of the present paper is to report the temperature dependence of internal modes. Internal modes of the molecule at a site in a crystal reflect both the environment and the symmetry of that site. Thus, the study of internal modes is useful as a probe to detect a change of the environment of the molecule [4, 5]. In terms of the microscopic viewpoint, the temperature dependence of the internal modes is important for the discussion of the phase transition mechanism.
Spectra were observed by using a micro-probed Raman scattering system with a backscattering configuration. The light source was an argon-ion laser operated at 514.5 nm line. The laser beam was focused to 10 tam in spot size on a sample surface. The clean, flat and homogeneous surface has been carefully selected as an observed region. The scattered light was analyzed by a triple grating monochromator (JASCO, NR-1800) and detected by a CCD (charge coupled device) system. The instrumental resolution was about 2 cm -~ . The temperature of the sample was controlled within 0.1 °C in the temperature range from -193 °C to 70 °C. The spectra were analyzed by least-squares method using a Lorentz-type function.
* Corresponding author. 0921-4526/96/$15.00 @ 1996 Elsevier Science B.V. All rights reserved SSD1 0921-4526{95)00801-2
3. Results and discussion Raman scattering spectra at room temperature show a fairly good agreement with those reported previously [ 1-3]. Since intramolecular forces are generally expected to be stronger than intermolecular forces, the frequencies of internal modes in the crystal are found at the same frequency region in the free molecule. Thus, the observed phonon peaks are successfully assigned, according to the infrared and Raman investigation [1].
530
A. SakaL A. Araya/ Physica B 219&220 (1996) 529 531 (x I0 "z)
As the temperature decreases to the ferroelectric phase, several internal modes show considerable changes near T¢. The typical z(yy)2 spectra are shown in Fig. 1 at various temperatures. The internal mode at about 2990 cm-] in Fig. 1 is assigned to a CH2 symmetric stretching vibration in the glycine molecule. In the ferroelectric phase, this stretching mode splits into two modes which have the same scattering intensities and line widths. The degree of splitting increases with decreasing temperature. The peak
12.5 )0
10
O
G~
°%~Oo
°\o
7.5
2.5
O°C
0
....
0
--
Frequency
•
--
Spontaneous polarization
I ....
i ....
shift
1
s ....
=
i . . i ~
-200 -150 -100 -50 0 Temperature (°C)
50
Fig. 3. Temperature dependence of anomalous part of the frequency shift (open circles) of the CH2 symmetric stretching vibration. Filled circles correspond to the spontaneous polarization. (xlO-2) 20
. . . .
2900
a
. . . .
n .
.
n ,
I
,
,
2950 3000 3 0 5 0 Frequency shift (cm d)
,
12 DO
,
o Oo.o
3100
I0 000
15
oo 6) O0
Fig. 1. Temperature dependence of Raman spectra of TGS in the z(yy)~ geometry. The mode at about 2990 cm - l is assigned to a CH2 symmetric stretching vibration in the glycine molecule.
*•o 000
t,i
0o 0
s ~
10
3000
4
2995 )
0
0
0
0
0
0
0
0
0
0
O --
Frequencyshift
• -
Spontaneous polarization
J
•
0 0
-200 -150 -100
2990
-50
0
50
Temperature (°C) OlIZDO
Fig. 4. Temperature dependence of anomalous part of the frequency shift (open circles) of the NH3 rocking vibration. Filled circles correspond to the spontaneous polarization.
0 O0
2985
0 o O O
OO
,0 0 0
2980
2975
-200
.......................... -150 -100 -50 0 Temperature (°C)
50
Fig. 2. Temperature dependence of the peak frequency of the CH2 symmetric stretching vibration.
frequencies of these modes are shown in Fig. 2 as a function of temperature. The crystal symmetry changes from paraelectric C~hP21/m to ferroelectric C~-P21 at Tc. Below To, the inversion symmetry of the TGS crystal disappears so that a spontaneous polarization appears. The change of the symmetry at the molecular site produces the inequivalent molecules in the unit cell. The splitting of the internal modes is caused by the inequivalence of the molecular sites.
A. Sakai, A. Araya/ Physica B 219&220 (1996)529-531 In the framework of the perturbation approach, it is expected that the degree of splitting corresponds to the degree of inequivalence. To discuss the splitting of internal mode more quantitatively, the temperature dependence of the frequency difference Av between two modes is compared with that of the spontaneous polarization Ps [6]. The result is indicated in Fig. 3. Both temperature dependences are in good agreement with each other. This me,ms that Av is well expressed by a relation Av oc P~. It is concluded that the change of the environment is proportional to P~ which acts as an order parameter, since the change of the environment around the molecule produces a shift of the peak frequency of the internal mode. In addition to the CH2 symmetric stretching vibration, other internal modes also show the same temperature dependence. The NH3 rocking-vibration mode splits into two modes. Fig. 4 shows the temperature dependence of the frequency difference. The scattering intensity of the CH2
531
wagging vibration mode increases with decreasing temperature. The temperature dependence of these internal modes is in good agreement with that of the spontaneous polarization. Therefore, our result indicates that the change of the environment of the glycine molecule is strongly related to the order parameter in the phase transition.
References
[1] V. Winterfeldt and G. Schaack, Ferroelectrics 15 (1977) 21. [2] P.O. Cervenka, A.D.P. Rao and S.P.S. Porto, Ferroelectrics I 1 (1976) 511. [3] E. Silberman, S.H. Morgan and J.M. Springer, J. Raman Spectrosc. 10 (1981) 248. [4] A. Sakai and T, Yagi, J. Phys. Soc. Japan 56 (1987) 2637. [5] Y. Yagi and A. Sakai, Ferroelectrics 89 (1989) 61. [6] J.A, Gonzalo and J.R. Lopez-Alonso, J. Phys. Chem. Solids 25 (1964) 303.
Physica B 219&220 (1996) 529 531
Raman scattering spectra of internal modes in triglycine sulphate A. Sakai *, A. Araya Department q["Electrical and Electronic Engineerinq, Muroran Institute ol Technology, Mizumoto-eho 2 7-1, Muroran 050, Japan
Abstract Raman scattering spectra of triglycine sulphate have been observed as a function of temperature with a special attention to internal modes. The CH2 symmetric stretching vibration splits into two modes in the ferroelectric phase. The temperature dependence of the frequency difference between these split modes is in good agreement with that of the spontaneous polarization. In addition to the CH2 symmetric stretching vibration, the NH3 rocking vibration and the CH2 wagging vibration show similar temperature dependence. This result indicates that the change of the environment of the glycine molecule is strongly related to the order parameter in the phase transition.
1. Introduction
2. Experimental procedure
Triglycine sulphate (NH2CH2COOH)3H2SO4 (TGS) has been considered one of the most important ferroelectric crystals since its ferroelectricity was discovered. The phase transition at T c - 49 °C has been well described by an order-disorder type mechanism. In order to study the phonon contribution to the phase transition, optic-phonon modes have been investigated by Raman scattering [1-3]. Though the soft optic phonon does not appear near Tc, the phonon modes show anomalies at Tc in both the externaland internal-mode regions. In the external region, for example, the relation between phonon modes and the dielectric constant has been discussed [2]. On the other hand, in the internal region, the quantitative study has not been carried out so far. The purpose of the present paper is to report the temperature dependence of internal modes. Internal modes of the molecule at a site in a crystal reflect both the environment and the symmetry of that site. Thus, the study of internal modes is useful as a probe to detect a change of the environment of the molecule [4, 5]. In terms of the microscopic viewpoint, the temperature dependence of the internal modes is important for the discussion of the phase transition mechanism.
Spectra were observed by using a micro-probed Raman scattering system with a backscattering configuration. The light source was an argon-ion laser operated at 514.5 nm line. The laser beam was focused to 10 tam in spot size on a sample surface. The clean, flat and homogeneous surface has been carefully selected as an observed region. The scattered light was analyzed by a triple grating monochromator (JASCO, NR-1800) and detected by a CCD (charge coupled device) system. The instrumental resolution was about 2 cm -~ . The temperature of the sample was controlled within 0.1 °C in the temperature range from -193 °C to 70 °C. The spectra were analyzed by least-squares method using a Lorentz-type function.
* Corresponding author. 0921-4526/96/$15.00 @ 1996 Elsevier Science B.V. All rights reserved SSD1 0921-4526{95)00801-2
3. Results and discussion Raman scattering spectra at room temperature show a fairly good agreement with those reported previously [ 1-3]. Since intramolecular forces are generally expected to be stronger than intermolecular forces, the frequencies of internal modes in the crystal are found at the same frequency region in the free molecule. Thus, the observed phonon peaks are successfully assigned, according to the infrared and Raman investigation [1].
530
A. SakaL A. Araya/ Physica B 219&220 (1996) 529 531 (x I0 "z)
As the temperature decreases to the ferroelectric phase, several internal modes show considerable changes near T¢. The typical z(yy)2 spectra are shown in Fig. 1 at various temperatures. The internal mode at about 2990 cm-] in Fig. 1 is assigned to a CH2 symmetric stretching vibration in the glycine molecule. In the ferroelectric phase, this stretching mode splits into two modes which have the same scattering intensities and line widths. The degree of splitting increases with decreasing temperature. The peak
12.5 )0
10
O
G~
°%~Oo
°\o
7.5
2.5
O°C
0
....
0
--
Frequency
•
--
Spontaneous polarization
I ....
i ....
shift
1
s ....
=
i . . i ~
-200 -150 -100 -50 0 Temperature (°C)
50
Fig. 3. Temperature dependence of anomalous part of the frequency shift (open circles) of the CH2 symmetric stretching vibration. Filled circles correspond to the spontaneous polarization. (xlO-2) 20
. . . .
2900
a
. . . .
n .
.
n ,
I
,
,
2950 3000 3 0 5 0 Frequency shift (cm d)
,
12 DO
,
o Oo.o
3100
I0 000
15
oo 6) O0
Fig. 1. Temperature dependence of Raman spectra of TGS in the z(yy)~ geometry. The mode at about 2990 cm - l is assigned to a CH2 symmetric stretching vibration in the glycine molecule.
*•o 000
t,i
0o 0
s ~
10
3000
4
2995 )
0
0
0
0
0
0
0
0
0
0
O --
Frequencyshift
• -
Spontaneous polarization
J
•
0 0
-200 -150 -100
2990
-50
0
50
Temperature (°C) OlIZDO
Fig. 4. Temperature dependence of anomalous part of the frequency shift (open circles) of the NH3 rocking vibration. Filled circles correspond to the spontaneous polarization.
0 O0
2985
0 o O O
OO
,0 0 0
2980
2975
-200
.......................... -150 -100 -50 0 Temperature (°C)
50
Fig. 2. Temperature dependence of the peak frequency of the CH2 symmetric stretching vibration.
frequencies of these modes are shown in Fig. 2 as a function of temperature. The crystal symmetry changes from paraelectric C~hP21/m to ferroelectric C~-P21 at Tc. Below To, the inversion symmetry of the TGS crystal disappears so that a spontaneous polarization appears. The change of the symmetry at the molecular site produces the inequivalent molecules in the unit cell. The splitting of the internal modes is caused by the inequivalence of the molecular sites.
A. Sakai, A. Araya/ Physica B 219&220 (1996)529-531 In the framework of the perturbation approach, it is expected that the degree of splitting corresponds to the degree of inequivalence. To discuss the splitting of internal mode more quantitatively, the temperature dependence of the frequency difference Av between two modes is compared with that of the spontaneous polarization Ps [6]. The result is indicated in Fig. 3. Both temperature dependences are in good agreement with each other. This me,ms that Av is well expressed by a relation Av oc P~. It is concluded that the change of the environment is proportional to P~ which acts as an order parameter, since the change of the environment around the molecule produces a shift of the peak frequency of the internal mode. In addition to the CH2 symmetric stretching vibration, other internal modes also show the same temperature dependence. The NH3 rocking-vibration mode splits into two modes. Fig. 4 shows the temperature dependence of the frequency difference. The scattering intensity of the CH2
531
wagging vibration mode increases with decreasing temperature. The temperature dependence of these internal modes is in good agreement with that of the spontaneous polarization. Therefore, our result indicates that the change of the environment of the glycine molecule is strongly related to the order parameter in the phase transition.
References
[1] V. Winterfeldt and G. Schaack, Ferroelectrics 15 (1977) 21. [2] P.O. Cervenka, A.D.P. Rao and S.P.S. Porto, Ferroelectrics I 1 (1976) 511. [3] E. Silberman, S.H. Morgan and J.M. Springer, J. Raman Spectrosc. 10 (1981) 248. [4] A. Sakai and T, Yagi, J. Phys. Soc. Japan 56 (1987) 2637. [5] Y. Yagi and A. Sakai, Ferroelectrics 89 (1989) 61. [6] J.A, Gonzalo and J.R. Lopez-Alonso, J. Phys. Chem. Solids 25 (1964) 303.