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Effect of an electric field on the low frequency Raman spectrum of KNbO3
Volume 98A, number 1,2
PHYSICS LETTERS
3 October 1983
EFFECT OFAN ELECTRIC FIELD ON THE LOW FREQUENCY RAMAN SPECTRUM OF KNbO3 1 Takeshi SHIGENARI Department of Engineering Physics and Institute of Laser Sciences, The University of Electro-Communications, 1-5-1, Chofugaoka, Chofu-shi, Tokyo 182, Japan Received 28 June 1983
The effect of an electric field on the low-frequency B
1 in a KNbO 2 mode at 56 cm 3 crystal has been observed as a small shift of the frequency in Raman spectrum. It means that the mode is not due to the disorder-induced TA modes with a flat dispersion but is a Brillouin zone center optical mode. However, the mode does not show clear ferroelectric soft-mode behavior as usualiy observed in other ferroelectrics.
KNbO3 (potassium niobate) is orthorhombic Bmm2 at room temperature and undergoes a phase transition to tetragonal at 225°Cand trigonal at —50°C.Since the early diffuse X-ray scattering data by Comes et al. [1], a wide variety of experimental results have been essentially well explained by a CLG (Comès—Lambert —Guinier) model [1]. The existence of a chain-like disorder along the orthorhombic b-axis results in an anomalous dispersion curve which is flat over an extensive range of the Brillouin zone in a plane perpendicular to the b-axis. However, the assignment of the low-frequency modes, one of which might be a ferroelectric soft mode, seems to be stifi controversial. Neutron scattering data by Currat et al. [2] have shown that the transverse acoustic mode is flat over 80% of the Brillouin zone and reaches about 50 cm~at the zone boundary. Similarly the lowest transverse optical mode starts at 23 cm~from the F point and approaches a constant energy of about 150 cm’. On the other hand, from Raman scatteringdata, Bozinis et al. [3] assigned the strong underdamped mode at 56 cm—’ to be a zone center lowest B2 optical mode dosely related to the orthorhombic—tetragonal transition at 225°C.More recent Raman data by Quittet et al. [4] have ascribed the mode around 150 cm—’ to the one-phonon density 1 Present address: Department of Physics, City College of
New York, New York, NY 10031, USA.
0.031-9163/83/0000—0000/$ 03.00 © 1983 North-Holland
of states of the TO branch which become Raman active due to the existing disorder. Since a ferroelectric mode is sensitive to external electric fields [5], as has been suggested by Scott [6], a study ofthe electric field induced Raman effect would be useful to see whether the 56 cm~mode is a zone-center ferroelectric mode or a disorder-induced mode reflecting a one-phonon density of states of the TA branch. The purpose of the present paper is to report that there is a small but defmite effect of an extemal electric field on the 56 cm’ mode. Single ciystals of KNbO3 made by Toshiba Electric Comp. Ltd. were cut to thin plates with thickness of about 1 mm. Silver electrodes were evaporated to the surfaces of 8 X 5 mm and the incident beam is either parallel or perpendicular to the electrode. In the latter case, light goes through a small transparent part of the electrodes. Prior to the evaporation, the samples were poled in an oil bath at 70°with an electric field of maximally 20 V/mm to make them single domain crystals. An Ar laser at 514.5 nm and a conventional Raman spectrometer were used in a right-angle scattering geometry. We used a dc electric field, rather than ac or pulsed field, followed by an appropriate data processing, which has been proven to be equally effective [7]. Spectra obtained in various scattering geometries were similar to those reported previously [4]. First we tried to use a Y-cut plate (X, Y,Z are 63
Volume 98A, number 1,2
PHYSICS LETTERS
taken parallel to the orthorhombic a, b, c axes), since the ferroelectric polarization in the trigonal phase would occur in this direction. At room temperature a maximum of 600 V/mm was applied but no change in the spectrum due to the field was observed. At 700 V /mm the crystal become opaque due to heating and/or electrical breakdown. The 56 cm~mode was completely missing at low temperatures below 50°C.At temperatures between 0 and —50°C,a field of 200 V /mm made cracks in the sample and no electric field effect could be measured. Next, we applied the field along the orthorhombic c-axis using Z-cut plates. In this case a maximum of 1000 V/mm could be applied at room temperature before the sample became opaque. Fig. 1 shows the spectrum measured in the polarization configuration ofZ(YZ)X without electric field (b) and the field-in-
KNbO3 Z(YZ)x 300K
(a)
-100
~
RT (a)
I000V/mm_OV i~V/mm
~Th0c)s
0
100
200 crn~
Cc)
(b)
•
~
_____
0
1 0 50 100 150 200 250cm • . . . Fig. 1. Raman spectrum m the configuration of Z(YZ)X at room termperature. X, Y and Z stand for a, b and c axes in the orthorhombic phase. (a) shows the difference in scattering intensity between two spectra, that is, the spectrum without the field [shown in (b)] is subtracted from that with the field of 1000 V/mm along the Z-axis. Result of the fitting by a damped harmonic oscillator is given by dots in (c) with the observed spectrum given by a solid line. The solid line in (a) simply shows the average of data points.
64
KNbO3 XZ(Y,XZ)XZ
~
~
~
I X(w)
duced part (a). The small (approximately 2% of the intensity) increase and decrease are clearly seen around 70 cm~and 20 cm’, respectively. Such changes can be attributed to a result of the shift of the strong 56 cm~mode to the high-frequency side by 5±3cm~. When the field is 500 V/mm, no electric field effect was observed. It should be noted that the lineshape of the underdamped 56 cm—1 mode can be fitted very well to a simple damped harmonic oscillator even in the presence of the field with a characteristic frequency w 0 = 56 cm’ and a damping constant y = 56 cm~(fig. lc). In the configuration XZ(Y,XZ)XZ, using another sample but cut from the same crystal, no field effect was observed at room temperature (fig. 2a). However, by cooling to —40.7°C,stifi in the orthorhombic phase,
.
:N~/~J:1o~2oocrn1
(b)
3 October 1983
~ ~
200Crn~ .‘ ••..~,, ~ .•.-:• ~
260 V/mm Fig. 2. Spectrum in a configuration of XZ(Y, YZ)X2 at room temperature (a), and at —40.7°C(b). The lower part of (a) and (b) are the difference of the spectrum with and without the field indicated. Note the field effect can be seen in (b) but not in (a).
Volume 98A, number 1,2
PHYSICS LETTERS
JcNbO
XZ( Y,XZ
3
)x~
~
296 K
/
3 October 1983
is much weaker in KNbO3 than in SrTiO3 This is partly .
because in KNbO3, being Raman active without extemal field, the effect is proportional to the field intensity, while in SrTiO3, the effect on the Raman inactive mode is proportional to the square of the intensity. Futhermore, in contrast to the fact that soft modes for various ferroelectrics have been observed
I
~
to be always near the transition ature, the 56 overdamped cm~mode in KNbO3 remains temperunder-
3
________
mode disappears damped above T~. byAthe lthough, ordering as shown of the inpolarization fig. 3, the
‘~ ________
/\
~26
_______________________
0
100
200
300
cm~
Fig. 3. Temperature dependence of the imaginary part of the susceptibility obtained from the spectrum in the absence of field. Note that the 56 cm1 mode is missing in the trigonal phase as 224.5 K.
a slight shift due to a field of 260 V/mm has been detected as shown in fig. 2b. The temperature dependence of the x”(w) obtained from the spectrum without the field is given in fig. 3. The cause of the difference between the effects in the two configurations is not clear but may be due to the difference in the direction of the propagation of the phonon involved. In the XZ(Y, XZ)XZ configuration, related phonons propagate along the orthohombic (001) axis, while in the Z(YZ)X configuration, they propagate in the direction of (101) along which the sample has been poled in advance. Further cooling down to the trigonal phase caused cracks in the sample. In any case the field dependence of the change in the spectrum could not be measured. The observed small but definite effect of an electric effect on the 56 cm~mode indicates that it is a Brillouin zone center mode. A few wavenumber shift towards the high-frequency side at 1000 V/mm is similar to the case of the ferroelectric mode in SrTiO 3 [5—7].The field effect on Raman intensity, however,
is the ferroelectric soft mode triggering the ortho1 mode in a plane perpendicular tonot thechange b-axis in56thecm trigonal phase, temperature. its frequency So, it isdoes unlikely that theappreciable with rhombic—trigonal The possibility that transition. the mode is the lowest zonecenter mode closely related to the orthorhombic—tetragonal transition is not excluded. However, the present result does not agree with the neutron data which have suggested the lowest zone-center optical mode to be located at 23 cm~.In addition, the reason why the disorder-induced spectrum at about 50 cm~ is not observed in the Raman spectrum is still unclear at the present stage. Further investigation of neutron diffraction would be interesting in this context. The author thanks Drs. N. Ichinose and T. Fukuda of Toshiba Electric Company, Ltd. for providing excellent crystals. He also thanks Dr. Y Takagi for his collaboration in the experiment. References [1] R. Comes, H. Lambert and A. Guinier, C.R. Acad. Sci. Paris 266 (1968) 957. [2] R. Currat, R. Comes, B. Dorner and E. Wiesendanger, J. Phys. C7 (1974) 252. 13] D•G. Bozinis and J.P. Hurrell, Phys. Rev. B13 (1976) 3109. [4] A.M• Quittet, H.I. Bell, H. Krauzman and P.M. Raccah, Phys. Rev. B14 (1976) 5068. [5] P.A. 613. Fleury and J.M. Worlock, Phys. Rev. 174 (1968) [6] J.F. Scott, Phys. Rev. B15 (1977) 2826. [7] T. Shigenari and H. Ebashi, J. Phys. C14 (1981) 969.
65
PHYSICS LETTERS
3 October 1983
EFFECT OFAN ELECTRIC FIELD ON THE LOW FREQUENCY RAMAN SPECTRUM OF KNbO3 1 Takeshi SHIGENARI Department of Engineering Physics and Institute of Laser Sciences, The University of Electro-Communications, 1-5-1, Chofugaoka, Chofu-shi, Tokyo 182, Japan Received 28 June 1983
The effect of an electric field on the low-frequency B
1 in a KNbO 2 mode at 56 cm 3 crystal has been observed as a small shift of the frequency in Raman spectrum. It means that the mode is not due to the disorder-induced TA modes with a flat dispersion but is a Brillouin zone center optical mode. However, the mode does not show clear ferroelectric soft-mode behavior as usualiy observed in other ferroelectrics.
KNbO3 (potassium niobate) is orthorhombic Bmm2 at room temperature and undergoes a phase transition to tetragonal at 225°Cand trigonal at —50°C.Since the early diffuse X-ray scattering data by Comes et al. [1], a wide variety of experimental results have been essentially well explained by a CLG (Comès—Lambert —Guinier) model [1]. The existence of a chain-like disorder along the orthorhombic b-axis results in an anomalous dispersion curve which is flat over an extensive range of the Brillouin zone in a plane perpendicular to the b-axis. However, the assignment of the low-frequency modes, one of which might be a ferroelectric soft mode, seems to be stifi controversial. Neutron scattering data by Currat et al. [2] have shown that the transverse acoustic mode is flat over 80% of the Brillouin zone and reaches about 50 cm~at the zone boundary. Similarly the lowest transverse optical mode starts at 23 cm~from the F point and approaches a constant energy of about 150 cm’. On the other hand, from Raman scatteringdata, Bozinis et al. [3] assigned the strong underdamped mode at 56 cm—’ to be a zone center lowest B2 optical mode dosely related to the orthorhombic—tetragonal transition at 225°C.More recent Raman data by Quittet et al. [4] have ascribed the mode around 150 cm—’ to the one-phonon density 1 Present address: Department of Physics, City College of
New York, New York, NY 10031, USA.
0.031-9163/83/0000—0000/$ 03.00 © 1983 North-Holland
of states of the TO branch which become Raman active due to the existing disorder. Since a ferroelectric mode is sensitive to external electric fields [5], as has been suggested by Scott [6], a study ofthe electric field induced Raman effect would be useful to see whether the 56 cm~mode is a zone-center ferroelectric mode or a disorder-induced mode reflecting a one-phonon density of states of the TA branch. The purpose of the present paper is to report that there is a small but defmite effect of an extemal electric field on the 56 cm’ mode. Single ciystals of KNbO3 made by Toshiba Electric Comp. Ltd. were cut to thin plates with thickness of about 1 mm. Silver electrodes were evaporated to the surfaces of 8 X 5 mm and the incident beam is either parallel or perpendicular to the electrode. In the latter case, light goes through a small transparent part of the electrodes. Prior to the evaporation, the samples were poled in an oil bath at 70°with an electric field of maximally 20 V/mm to make them single domain crystals. An Ar laser at 514.5 nm and a conventional Raman spectrometer were used in a right-angle scattering geometry. We used a dc electric field, rather than ac or pulsed field, followed by an appropriate data processing, which has been proven to be equally effective [7]. Spectra obtained in various scattering geometries were similar to those reported previously [4]. First we tried to use a Y-cut plate (X, Y,Z are 63
Volume 98A, number 1,2
PHYSICS LETTERS
taken parallel to the orthorhombic a, b, c axes), since the ferroelectric polarization in the trigonal phase would occur in this direction. At room temperature a maximum of 600 V/mm was applied but no change in the spectrum due to the field was observed. At 700 V /mm the crystal become opaque due to heating and/or electrical breakdown. The 56 cm~mode was completely missing at low temperatures below 50°C.At temperatures between 0 and —50°C,a field of 200 V /mm made cracks in the sample and no electric field effect could be measured. Next, we applied the field along the orthorhombic c-axis using Z-cut plates. In this case a maximum of 1000 V/mm could be applied at room temperature before the sample became opaque. Fig. 1 shows the spectrum measured in the polarization configuration ofZ(YZ)X without electric field (b) and the field-in-
KNbO3 Z(YZ)x 300K
(a)
-100
~
RT (a)
I000V/mm_OV i~V/mm
~Th0c)s
0
100
200 crn~
Cc)
(b)
•
~
_____
0
1 0 50 100 150 200 250cm • . . . Fig. 1. Raman spectrum m the configuration of Z(YZ)X at room termperature. X, Y and Z stand for a, b and c axes in the orthorhombic phase. (a) shows the difference in scattering intensity between two spectra, that is, the spectrum without the field [shown in (b)] is subtracted from that with the field of 1000 V/mm along the Z-axis. Result of the fitting by a damped harmonic oscillator is given by dots in (c) with the observed spectrum given by a solid line. The solid line in (a) simply shows the average of data points.
64
KNbO3 XZ(Y,XZ)XZ
~
~
~
I X(w)
duced part (a). The small (approximately 2% of the intensity) increase and decrease are clearly seen around 70 cm~and 20 cm’, respectively. Such changes can be attributed to a result of the shift of the strong 56 cm~mode to the high-frequency side by 5±3cm~. When the field is 500 V/mm, no electric field effect was observed. It should be noted that the lineshape of the underdamped 56 cm—1 mode can be fitted very well to a simple damped harmonic oscillator even in the presence of the field with a characteristic frequency w 0 = 56 cm’ and a damping constant y = 56 cm~(fig. lc). In the configuration XZ(Y,XZ)XZ, using another sample but cut from the same crystal, no field effect was observed at room temperature (fig. 2a). However, by cooling to —40.7°C,stifi in the orthorhombic phase,
.
:N~/~J:1o~2oocrn1
(b)
3 October 1983
~ ~
200Crn~ .‘ ••..~,, ~ .•.-:• ~
260 V/mm Fig. 2. Spectrum in a configuration of XZ(Y, YZ)X2 at room temperature (a), and at —40.7°C(b). The lower part of (a) and (b) are the difference of the spectrum with and without the field indicated. Note the field effect can be seen in (b) but not in (a).
Volume 98A, number 1,2
PHYSICS LETTERS
JcNbO
XZ( Y,XZ
3
)x~
~
296 K
/
3 October 1983
is much weaker in KNbO3 than in SrTiO3 This is partly .
because in KNbO3, being Raman active without extemal field, the effect is proportional to the field intensity, while in SrTiO3, the effect on the Raman inactive mode is proportional to the square of the intensity. Futhermore, in contrast to the fact that soft modes for various ferroelectrics have been observed
I
~
to be always near the transition ature, the 56 overdamped cm~mode in KNbO3 remains temperunder-
3
________
mode disappears damped above T~. byAthe lthough, ordering as shown of the inpolarization fig. 3, the
‘~ ________
/\
~26
_______________________
0
100
200
300
cm~
Fig. 3. Temperature dependence of the imaginary part of the susceptibility obtained from the spectrum in the absence of field. Note that the 56 cm1 mode is missing in the trigonal phase as 224.5 K.
a slight shift due to a field of 260 V/mm has been detected as shown in fig. 2b. The temperature dependence of the x”(w) obtained from the spectrum without the field is given in fig. 3. The cause of the difference between the effects in the two configurations is not clear but may be due to the difference in the direction of the propagation of the phonon involved. In the XZ(Y, XZ)XZ configuration, related phonons propagate along the orthohombic (001) axis, while in the Z(YZ)X configuration, they propagate in the direction of (101) along which the sample has been poled in advance. Further cooling down to the trigonal phase caused cracks in the sample. In any case the field dependence of the change in the spectrum could not be measured. The observed small but definite effect of an electric effect on the 56 cm~mode indicates that it is a Brillouin zone center mode. A few wavenumber shift towards the high-frequency side at 1000 V/mm is similar to the case of the ferroelectric mode in SrTiO 3 [5—7].The field effect on Raman intensity, however,
is the ferroelectric soft mode triggering the ortho1 mode in a plane perpendicular tonot thechange b-axis in56thecm trigonal phase, temperature. its frequency So, it isdoes unlikely that theappreciable with rhombic—trigonal The possibility that transition. the mode is the lowest zonecenter mode closely related to the orthorhombic—tetragonal transition is not excluded. However, the present result does not agree with the neutron data which have suggested the lowest zone-center optical mode to be located at 23 cm~.In addition, the reason why the disorder-induced spectrum at about 50 cm~ is not observed in the Raman spectrum is still unclear at the present stage. Further investigation of neutron diffraction would be interesting in this context. The author thanks Drs. N. Ichinose and T. Fukuda of Toshiba Electric Company, Ltd. for providing excellent crystals. He also thanks Dr. Y Takagi for his collaboration in the experiment. References [1] R. Comes, H. Lambert and A. Guinier, C.R. Acad. Sci. Paris 266 (1968) 957. [2] R. Currat, R. Comes, B. Dorner and E. Wiesendanger, J. Phys. C7 (1974) 252. 13] D•G. Bozinis and J.P. Hurrell, Phys. Rev. B13 (1976) 3109. [4] A.M• Quittet, H.I. Bell, H. Krauzman and P.M. Raccah, Phys. Rev. B14 (1976) 5068. [5] P.A. 613. Fleury and J.M. Worlock, Phys. Rev. 174 (1968) [6] J.F. Scott, Phys. Rev. B15 (1977) 2826. [7] T. Shigenari and H. Ebashi, J. Phys. C14 (1981) 969.
65