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Raman scattering study on ferroelectric (Ba0.32Sr0.68)2Nb2O7 ceramics
Materials Science and Engineering B 120 (2005) 45–49
Raman scattering study on ferroelectric (Ba0.32Sr0.68)2Nb2O7 ceramics Anwar Hushur a, ∗ , Yukikuni Akishige b , Seiji Kojima a a
Institute of Materials Science, Tsukuba University, Tsukuba, Ibaraki 305-8573, Japan b Faculty of Education, Shimane University, Matsue 690-8504, Japan
Abstract Raman scattering technique was applied to examine the Ba-doping effect to the two low temperature phase transitions of Sr2 Nb2 O7 (SN) in the temperature range from −190 to 600 ◦ C. The line shape of Raman spectra can be well fitted by multidamped harmonic oscillator model. We did not observe any soft mode related to the two low temperature phase transitions corresponding to those of the pure SN. It is correlated to the disappearance of the incommensurate phase in (Ba0.32 Sr0.68 )2 Nb2 O7 ceramics. However, the temperature dependence behavior of the three low frequency modes indicates another new structural phase transition around 270 ◦ C. It is considered that the reduction of the interlayer interaction caused by partial replacement of Sr-site by Ba-site, whose ionic radius is larger than that of Sr, may be the reason for the disappearance of the incommensurate phase transition in (Ba0.32 Sr0.68 )2 Nb2 O7 ceramics. © 2005 Elsevier B.V. All rights reserved. Keywords: Soft mode; Structural phase transition; Raman scattering; Sr2 Nb2 O7 ; Ferroelectric
1. Introduction Lead-based ferroelectric materials with perovskite structure, PZT and PMN-PT, have a good ferroelectric and piezoelectric properties and thus widely used as electromechanical devices. However, lead may affect natural environment. The development of lead-free ferroelectric materials required from the viewpoint of environmental problems. In recent years, a significant effort has been made to manufacture new lead-free ferroelectric or piezoelectric materials. SN is the highest Tc ferroelectric with the perovskite slab structure [1]. Low coercive field, low permittivity and high-heat resistance of this material enables its solid solutions Sr2 Tax Nb2−x O7 to be used as Pb-free non-volatile ferroelectric memory devices based on field effect transistors and capacitors, and optical waveguides [2–4]. Furthermore, its lead-free character has received much more attention as green materials in its application. The spontaneous polarization, Ps = 9 × 10−6 C/cm2 , is parallel to the c-axis and the coercive field is Ec = 6 kV/cm at room temperature [5]. The structure of SN have perovskite slabs parallel to (0 1 0), consists of an NbO6 octahedra and Sr atoms. It was known ∗
Corresponding author. Tel.: +81 29 853 5278; fax: +81 29 853 5262. E-mail address: [email protected] (A. Hushur).
0921-5107/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2005.02.001
that the octahedra distort and tilt on cooling and SN undergoes three phase transitions at 1342, 215 and −156 ◦ C, Cmcm → Cmc21 → Pbn21 → Pb11 [6,7]. The dynamics of phase transitions has been studied mainly by means of Raman scattering [8–11], ESR [12], and polarized far-infrared reflectivity and transmission measurements [13]. The ferroelectric soft mode was found in b(cc)b¯ scattering geometry below Tc = 1342 ◦ C. Below 215 ◦ C another soft mode appears showing the displacive character of this normal to incommensurate phase transition. Further decreasing the temperature, the soft mode coupled with another low-frequency mode, an intensity transfer occurs between the two modes. Raman, ESR and polarized far-infrared reflectivity and transmission results showed a non-classical behavior of the incommensurate order parameter with critical exponents β = 0.38, 0.34 and 0.3, respectively. As for Ba-doped SN, (Sr1−x Bax )2 Nb2 O7 , the Tc decreases from 1342 ◦ C with respect to x [6]. For x = 0.32 crystals, dielectric measurements show that the low temperature ferroelectric transition temperature disappears and a new structural phase transition occurs at 192 ◦ C [14]. Our high temperature Raman scattering study on (Ba0.32 Sr0.68 )2 Nb2 O7 (BSN) ceramics revealed that the soft mode frequency related to the order parameter of this Cmcm → Cmc21 high temperature ferroelectric phase transitions shows a non-classical behavior, and the critical exponent
46
A. Hushur et al. / Materials Science and Engineering B 120 (2005) 45–49
connected to the increase of the order parameters obtained as β = 0.377. From the relation of the soft mode frequency with temperature, it is also found that the ferroelectric phase transition temperature Tc decreases from 1342 to 1058 ◦ C with Ba concentration (x = 0.32) [15]. In this paper, we report our Raman scattering study of the Ba effect to another two low temperature phase transitions.
2. Experimental Raman scattering are excited using a diode-pumped solidstate laser at a wavelength of 532 nm and a power of about 100 mW. The light beam, passing through a narrow band pass filter, a half wavelength plate, and a polarizer, is focused onto a sample by lens. Scattered light from a sample is then collected by another lens using the backward scattering geometry, and analyzed by a triple-grating spectrometer of additive dispersion (Jobin Yvon T64000). The spectral resolution was about 2 cm−1 . The room temperature spectra were measured in the range of 7–1000 cm−1 . Low frequency spectra from −80 to 200 cm−1 were obtained at the wide temperature range from −190 to 600 ◦ C. The sample was inserted into a cryostat cell (THMS600, Linkham), whose temperature stability was ±0.1 ◦ C from −190 to 600 ◦ C in temperature-dependent Raman measurements.
3. Results and discussion Whole Raman spectra measured from 7 to 1000 cm−1 at room temperature are shown in Fig. 1. At room temperature 22 distinct bands have been observed. Factor group analysis
of the high temperature ferroelectric phase of SN have been done by Buixaderas et al. [13]. However, they did not show the contribution of different ions to the lattice modes. In order to clarify this, we also did the factor group analysis using the structural data from the 3D X-ray diffraction [1], it gives 66 lattice modes at the centre of the Brillouin zone (Γ ), with the atoms in the 4a and 8b Wickoff-sites, 19 of A1 symmetry, 14 of A2 symmetry, 14 of B1 symmetry, and 19 of B2 symmetry. The contribution of Sr and Nb ions to the lattice modes is Γ Sr = Γ Nb = 4A1 + 2A2 + 2B1 + 4B2 , and from oxygen ions is Γ O = 11A1 + 10A2 + 10B1 + 11B2 . The total lattice modes are ΓSr2 Nb2 O7 = 19A1 + 14A2 + 14B1 + 19B2 . The acoustic modes are A1 + B1 + B2 . The A1 , B1 and B2 modes are both Raman and infrared active, while A2 mode is only Raman active. In a low temperature phase, the decrease in structure symmetry produces more Raman active modes. Although the number of the Raman modes observed at room temperature in present study is less than the theoretical mode number, mode counting in the Raman spectra of ceramic samples is very difficult due to the possible overlapping and broadening of the bands. Bands formally allowed but too weak to observe. The lowest mode was observed at 24.8 cm−1 . The strongest and sharpest mode was observed at 60.7 cm−1 . Two shoulder peaks were observed at 43.1 and 52.4 cm−1 . As the temperature increases, the Raman modes become broad and intense up to measured temperature as shown in Fig. 2. However, some modes disappear completely at certain temperature, which corresponds to the new structural phase transition. For example, the lowest peak at 24.8 cm−1 at room temperature disappears above Tr ∼ 250 ◦ C. The mode at 52.4 cm−1 approaches to the mode at the 43.1 cm−1 in the spectra, an intensity transfer between the two modes occur. Then the peak at 43.1 cm−1 shows softening for increasing temperature. It is
Fig. 1. Raman spectra of (Ba0.32 Sr0.68 )2 Nb2 O7 at room temperature from 7 to 1000 cm−1 . Inset: for 7–200 cm−1 .
A. Hushur et al. / Materials Science and Engineering B 120 (2005) 45–49
Fig. 2. Low frequency Raman spectra in (Ba0.32 Sr0.68 )2 Nb2 O7 obtained between −80 and 200 cm−1 at temperatures 600, 500, 340, 280, 250, 220, 200, 100, 26, −60 and −190 ◦ C, from top to bottom. The arrows indicate the lowest mode observed at 24.8 cm−1 at room temperature.
also observed that a significant drop in intensity of the mode at 52.4 cm−1 on heating. Light scattering intensity of Stoke shifts is given by Si Im IS (ω) = [n(ω) + 1] (1) ωi2 − ω2 + iγi ω where ωi and γ i are the phonon frequency and the damping constant, respectively, and Si the constant representing the oscillator strength. The term, n(ω) =
1 exp(¯hω/kB T ) − 1
is so-called Bose–Einstein factor. The first term on the righthand side of Eq. (1) represents the population density of phonons, whereas the second term is the susceptibility function for multidamped harmonic oscillators. We fit the Raman scattering intensity using Eq. (1). As the spectra were recorded in the same spectroscopic conditions for all temperatures, we can thus obtain the thermal behavior of all characteristics of each phonon (frequency, damping, and strength). Figs. 3 and 4 display the temperature-dependent frequency and damping. On heating, the mode at 52.4 cm−1 softens while the mode at 43.1 cm−1 shows slight hardening with the temperature approaching 270 ◦ C and then pushes it down to lower frequencies. This implies that the ionic motions associated with the two-phonon modes are transferred to each other. This makes much more complicate the mechanism of these two low frequency modes around 270 ◦ C. The lowest mode observed at 24.8 cm−1 at room temperature shows hardening and damping of this mode decrease on heating. Finally, it interacts with the second lowest mode and disappeared above 250 ◦ C. As mentioned earlier, the octahedra distort and tilt on cooling and pure SN undergoes three phase transitions at 1342, 215 and 156 ◦ C, Cmcm → Cmc21 → Pbn21 → Pb11. There are three optical soft modes corresponding to above
47
Fig. 3. Temperature dependence of the phonon frequency, ω, for 24.8, 43.1, 52.4 and 60.7 cm−1 modes at room temperature, as deduced by the fit.
phase transitions. Our previous result shows the low frequency mode observed at 43.1 cm−1 at room temperature in BSN ceramics exhibits a large softening with increasing temperature from the value 43.1 cm−1 at room temperature down to 20 cm−1 at 970 ◦ C as shown in Fig. 5. Its damping increases so large that the soft mode becomes overdamped (γ/ω > 1). This soft mode is A1 (z) symmetry in a high temperature ferroelectric phase with space group Cmc21 and responsible for the Cmcm → Cmc21 ferroelectric phase transition [15]. The mode at 52.4 cm−1 shows slight softening on approaching 270 ◦ C from both low and high temperature sides. We did not observe any soft mode related to the two low temperature phase transitions corresponding to those of pure SN, indicating the disappearance of the incommensurate phase. It is a good accordance with the dielectric measurement [14]. However, an unusual effect is the behavior of the lowest mode at 24.8 cm−1 which interacts with the soft mode and then disappears around 270 ◦ C. It is interesting to no-
Fig. 4. Temperature dependence of the phonon damping, γ, for 24.8, 43.1, 52.4 and 60.7 cm−1 modes at room temperature, as deduced by the fit.
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in SrTiO3 . Ca2 Nb2 O7 has a perovskite slab structure analog to the pure SN. Ca ions have smaller ionic radii than Sr ions so that the lattice parameters of Ca2 Nb2 O7 are smaller than pure SN [19]. The lattice has been shrunk by the changes of the Ca ions to the Sr ions. It is reasonable to assume that Ba doping to the SN would relax the lattice and reduce the interlayer interaction and also reduce the interaction of the neighboring octahedra. As a consequence, the tendency of the tilting of octahedra decreases, stability of octahedra increases. This may be the reason for the disappearance of the incommensurate phase transition in BSN ceramics. Considering the new phase below Tr = 270 ◦ C, it seems unite cell doubling occurs below Tr and the softening of the lowest mode at 24.8 cm−1 on cooling may be related to the unite cell doubling. Fig. 5. The temperature dependence of the soft optical phonon frequency (dark square). The solid line denotes the fit ωs = A(Tc − T)β [15].
tice that the disappearance of the lowest mode and energy transfer between another two low frequency modes occurred at same temperature region. The mode frequency of the lowest mode shows hardening on heating and the damping, γ, decreases. It may indicate a new structural phase transition around 270 ◦ C (Tr ). At 192 ◦ C (Tr ), a new structural phase transition observed by dielectric measurement on the b-plate BSN single crystals [14]. The reason for the differences of Tr by two method is not clear. Because of the difficulties to grow a larger size single crystal for BSN (x = 0.32), the present Raman study carried out on BSN ceramics with traditional backward scattering geometry. In near future, we will carry out micro-Raman scattering on BSN single crystals in order to clarify such differences. As we shown earlier, observed soft mode frequency, ωs , at room temperature for BSN ceramics is much smaller than pure SN single crystals. Since partial replacement of Sr-site by Ba ions, whose ionic radius is larger than that of Sr ions, should lead to the decrease of interlayer interactions, as a consequence the frequency of this soft mode can be expected to decrease with increasing Ba doping. The soft mode is probably connected with NbO6 octahedra tiltings. Below Tc the ordered octahedra rotate within the layers, in the bc plane. The eigen vector of the ferroelectric soft mode is connected with these tiltings. In incommensurate phase, the neighboring octahedra of pure SN had opposite tilts, incommensurately modulated with the wave vector k = ±(1 − δ)(1/2)a* , a* is reciprocal lattice vector [16].The value of δ was very small (0.02 at 215 ◦ C) and decrease slowly down to 0.01 at 20 ◦ C. Usually A-site doping has a big effect on the shape of the octahedra in perovskite structure. When A-site doped with ions, which has smaller ionic radii than A-site ions, firstly, the lattice would shrink and the octahedra will tilt upon further increasing the doping ratio [17,18]. It is a common concept that the changes of Ba ions in BaTiO3 to the Sr ions cause octahedra tilting in the ferroelectric phase. The tilting of octahedra is the order parameter of ferroelectric phase transition
4. Summary Raman scattering was investigated in Ba-doped (Sr1−x Bax )2 Nb2 O7 (x = 0.32) ceramics. Low frequency spectra from −80 to 200 cm−1 was obtained at the wide temperature range from −190 to 600 ◦ C. The line shape of Raman spectra can be well fitted by multidamped harmonic oscillator model. We did not observe any soft mode related to the two low temperature phase transition corresponding to those of pure SN. On heating, the mode at 52.4 cm−1 softens while the mode at 43.1 cm−1 shows slight hardening with the temperature approaching 270 ◦ C and then pushes it down to lower frequency, indicating that the ionic motions associated with the two phonon modes are transferred to each other. An unusual effect is the behavior of the lowest mode at 24.8 cm−1 which interacts with the soft mode and then disappears around 270 ◦ C. It is interesting to notice that the disappearance of the lowest mode and energy transfer between another two low frequency modes occurred in the same temperature region. The mode frequency of the lowest mode shows hardening on heating and the damping, γ, decreases. It is correlated to a new structural phase transition around 270 ◦ C (Tr ). Partial replacement of Sr-site by Ba ions, whose ionic radius are larger than that of Sr ions, should induce the decrease of interlayer interactions and also reduces the interaction of the neighboring octahedra. As a consequence, the tendency of the tilting of octahedra decreases, and the stability of octahedra increases. This may be the reason for the disappearance of the incommensurate phase transition in BSN ceramics. Considering the new structural phase transitions, it seems unite cell doubling occurs below Tr and the softening of the lowest mode at 24.8 cm−1 on cooling may be related to the unite cell doubling. Acknowledgement This work was supported in part by the 21st century COE program under the Japanese Ministry of Education, Culture, Sports, Science and Technology.
A. Hushur et al. / Materials Science and Engineering B 120 (2005) 45–49
References [1] N. Ishizawa, F. Marumo, T. Kawamura, M. Kimura, Acta Cryst. B31 (1975) 1912. [2] Y. Fujimori, N. Izumi, T. Nakamura, Jpn. J. Appl. Phys. 37 (1998) 5207. [3] Y. Fujimori, T. Nakamura, A. Kamisawa, Jpn. J. Appl. Phys. 38 (1999) 2285. [4] A. Ishitani, M. Kimura, Appl. Phys. Lett. 29 (1976) 289. [5] S. Nanamatsu, M. Kimura, K. Doi, M. Takahashi, J. Phys. Soc. Jpn. 30 (1971) 300. [6] S. Nanamatsu, M. Kimura, T. Kawamura, J. Phys. Soc. Jpn. 38 (1975) 817. [7] K. Ohi, M. Kimura, H. Ishida, H. Kakinuma, J. Phys. Soc. Jpn. 46 (1979) 1387. [8] S. Kojima, M.T. Nakamura, K. Ohi, H. Kakinuma, Solid State Commun. 31 (1979) 755. [9] S. Kojima, M.-S. Jang, K. Ohi, T. Nakamura, J. Phys. Soc. Jpn. 50 (1981) 2787.
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[10] K. Ohi, H. Omura, T. Kazumi, M. Takashige, M.-S. Jang, J. Phys. Soc. Jpn. 51 (1982) 3745. [11] K. Ohi, S. Kojima, Jpn. J. Appl. Phys. 24 (1985) 817. [12] Y. Akishige, T. Kubota, K. Ohi, J. Phys. Soc. Jpn. 52 (1983) 1290. [13] E. Buixaderas, S. Kamba, J. Petzelt, J. Phys: Condens. Matt. 13 (2001) 2823. [14] Y. Akishige, M. Kamata, K. Fukano, J. Kor. Phys. Soc. 42 (2003) S1187. [15] A. Hushur, Y. Akishige, S. Kojima, Ceram. Int. 30 (2004) 2023–2027. [16] N. Yamoto, K. Yagi, G. Honjo, M. Kimura, T. Kawamura, J. Phys. Soc. Jpn. 48 (1980) 185. [17] X.H. Dai, Z. Xu, J.F. Li, D. Viehland, J. Appl. Phys. 77 (1995) 3354. [18] X.H. Dai, D. Viehland, J. Appl. Phys. 80 (1996) 1919. [19] V.A. Isupov, Ferroelectrics 220 (1999) 79.
Raman scattering study on ferroelectric (Ba0.32Sr0.68)2Nb2O7 ceramics Anwar Hushur a, ∗ , Yukikuni Akishige b , Seiji Kojima a a
Institute of Materials Science, Tsukuba University, Tsukuba, Ibaraki 305-8573, Japan b Faculty of Education, Shimane University, Matsue 690-8504, Japan
Abstract Raman scattering technique was applied to examine the Ba-doping effect to the two low temperature phase transitions of Sr2 Nb2 O7 (SN) in the temperature range from −190 to 600 ◦ C. The line shape of Raman spectra can be well fitted by multidamped harmonic oscillator model. We did not observe any soft mode related to the two low temperature phase transitions corresponding to those of the pure SN. It is correlated to the disappearance of the incommensurate phase in (Ba0.32 Sr0.68 )2 Nb2 O7 ceramics. However, the temperature dependence behavior of the three low frequency modes indicates another new structural phase transition around 270 ◦ C. It is considered that the reduction of the interlayer interaction caused by partial replacement of Sr-site by Ba-site, whose ionic radius is larger than that of Sr, may be the reason for the disappearance of the incommensurate phase transition in (Ba0.32 Sr0.68 )2 Nb2 O7 ceramics. © 2005 Elsevier B.V. All rights reserved. Keywords: Soft mode; Structural phase transition; Raman scattering; Sr2 Nb2 O7 ; Ferroelectric
1. Introduction Lead-based ferroelectric materials with perovskite structure, PZT and PMN-PT, have a good ferroelectric and piezoelectric properties and thus widely used as electromechanical devices. However, lead may affect natural environment. The development of lead-free ferroelectric materials required from the viewpoint of environmental problems. In recent years, a significant effort has been made to manufacture new lead-free ferroelectric or piezoelectric materials. SN is the highest Tc ferroelectric with the perovskite slab structure [1]. Low coercive field, low permittivity and high-heat resistance of this material enables its solid solutions Sr2 Tax Nb2−x O7 to be used as Pb-free non-volatile ferroelectric memory devices based on field effect transistors and capacitors, and optical waveguides [2–4]. Furthermore, its lead-free character has received much more attention as green materials in its application. The spontaneous polarization, Ps = 9 × 10−6 C/cm2 , is parallel to the c-axis and the coercive field is Ec = 6 kV/cm at room temperature [5]. The structure of SN have perovskite slabs parallel to (0 1 0), consists of an NbO6 octahedra and Sr atoms. It was known ∗
Corresponding author. Tel.: +81 29 853 5278; fax: +81 29 853 5262. E-mail address: [email protected] (A. Hushur).
0921-5107/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2005.02.001
that the octahedra distort and tilt on cooling and SN undergoes three phase transitions at 1342, 215 and −156 ◦ C, Cmcm → Cmc21 → Pbn21 → Pb11 [6,7]. The dynamics of phase transitions has been studied mainly by means of Raman scattering [8–11], ESR [12], and polarized far-infrared reflectivity and transmission measurements [13]. The ferroelectric soft mode was found in b(cc)b¯ scattering geometry below Tc = 1342 ◦ C. Below 215 ◦ C another soft mode appears showing the displacive character of this normal to incommensurate phase transition. Further decreasing the temperature, the soft mode coupled with another low-frequency mode, an intensity transfer occurs between the two modes. Raman, ESR and polarized far-infrared reflectivity and transmission results showed a non-classical behavior of the incommensurate order parameter with critical exponents β = 0.38, 0.34 and 0.3, respectively. As for Ba-doped SN, (Sr1−x Bax )2 Nb2 O7 , the Tc decreases from 1342 ◦ C with respect to x [6]. For x = 0.32 crystals, dielectric measurements show that the low temperature ferroelectric transition temperature disappears and a new structural phase transition occurs at 192 ◦ C [14]. Our high temperature Raman scattering study on (Ba0.32 Sr0.68 )2 Nb2 O7 (BSN) ceramics revealed that the soft mode frequency related to the order parameter of this Cmcm → Cmc21 high temperature ferroelectric phase transitions shows a non-classical behavior, and the critical exponent
46
A. Hushur et al. / Materials Science and Engineering B 120 (2005) 45–49
connected to the increase of the order parameters obtained as β = 0.377. From the relation of the soft mode frequency with temperature, it is also found that the ferroelectric phase transition temperature Tc decreases from 1342 to 1058 ◦ C with Ba concentration (x = 0.32) [15]. In this paper, we report our Raman scattering study of the Ba effect to another two low temperature phase transitions.
2. Experimental Raman scattering are excited using a diode-pumped solidstate laser at a wavelength of 532 nm and a power of about 100 mW. The light beam, passing through a narrow band pass filter, a half wavelength plate, and a polarizer, is focused onto a sample by lens. Scattered light from a sample is then collected by another lens using the backward scattering geometry, and analyzed by a triple-grating spectrometer of additive dispersion (Jobin Yvon T64000). The spectral resolution was about 2 cm−1 . The room temperature spectra were measured in the range of 7–1000 cm−1 . Low frequency spectra from −80 to 200 cm−1 were obtained at the wide temperature range from −190 to 600 ◦ C. The sample was inserted into a cryostat cell (THMS600, Linkham), whose temperature stability was ±0.1 ◦ C from −190 to 600 ◦ C in temperature-dependent Raman measurements.
3. Results and discussion Whole Raman spectra measured from 7 to 1000 cm−1 at room temperature are shown in Fig. 1. At room temperature 22 distinct bands have been observed. Factor group analysis
of the high temperature ferroelectric phase of SN have been done by Buixaderas et al. [13]. However, they did not show the contribution of different ions to the lattice modes. In order to clarify this, we also did the factor group analysis using the structural data from the 3D X-ray diffraction [1], it gives 66 lattice modes at the centre of the Brillouin zone (Γ ), with the atoms in the 4a and 8b Wickoff-sites, 19 of A1 symmetry, 14 of A2 symmetry, 14 of B1 symmetry, and 19 of B2 symmetry. The contribution of Sr and Nb ions to the lattice modes is Γ Sr = Γ Nb = 4A1 + 2A2 + 2B1 + 4B2 , and from oxygen ions is Γ O = 11A1 + 10A2 + 10B1 + 11B2 . The total lattice modes are ΓSr2 Nb2 O7 = 19A1 + 14A2 + 14B1 + 19B2 . The acoustic modes are A1 + B1 + B2 . The A1 , B1 and B2 modes are both Raman and infrared active, while A2 mode is only Raman active. In a low temperature phase, the decrease in structure symmetry produces more Raman active modes. Although the number of the Raman modes observed at room temperature in present study is less than the theoretical mode number, mode counting in the Raman spectra of ceramic samples is very difficult due to the possible overlapping and broadening of the bands. Bands formally allowed but too weak to observe. The lowest mode was observed at 24.8 cm−1 . The strongest and sharpest mode was observed at 60.7 cm−1 . Two shoulder peaks were observed at 43.1 and 52.4 cm−1 . As the temperature increases, the Raman modes become broad and intense up to measured temperature as shown in Fig. 2. However, some modes disappear completely at certain temperature, which corresponds to the new structural phase transition. For example, the lowest peak at 24.8 cm−1 at room temperature disappears above Tr ∼ 250 ◦ C. The mode at 52.4 cm−1 approaches to the mode at the 43.1 cm−1 in the spectra, an intensity transfer between the two modes occur. Then the peak at 43.1 cm−1 shows softening for increasing temperature. It is
Fig. 1. Raman spectra of (Ba0.32 Sr0.68 )2 Nb2 O7 at room temperature from 7 to 1000 cm−1 . Inset: for 7–200 cm−1 .
A. Hushur et al. / Materials Science and Engineering B 120 (2005) 45–49
Fig. 2. Low frequency Raman spectra in (Ba0.32 Sr0.68 )2 Nb2 O7 obtained between −80 and 200 cm−1 at temperatures 600, 500, 340, 280, 250, 220, 200, 100, 26, −60 and −190 ◦ C, from top to bottom. The arrows indicate the lowest mode observed at 24.8 cm−1 at room temperature.
also observed that a significant drop in intensity of the mode at 52.4 cm−1 on heating. Light scattering intensity of Stoke shifts is given by Si Im IS (ω) = [n(ω) + 1] (1) ωi2 − ω2 + iγi ω where ωi and γ i are the phonon frequency and the damping constant, respectively, and Si the constant representing the oscillator strength. The term, n(ω) =
1 exp(¯hω/kB T ) − 1
is so-called Bose–Einstein factor. The first term on the righthand side of Eq. (1) represents the population density of phonons, whereas the second term is the susceptibility function for multidamped harmonic oscillators. We fit the Raman scattering intensity using Eq. (1). As the spectra were recorded in the same spectroscopic conditions for all temperatures, we can thus obtain the thermal behavior of all characteristics of each phonon (frequency, damping, and strength). Figs. 3 and 4 display the temperature-dependent frequency and damping. On heating, the mode at 52.4 cm−1 softens while the mode at 43.1 cm−1 shows slight hardening with the temperature approaching 270 ◦ C and then pushes it down to lower frequencies. This implies that the ionic motions associated with the two-phonon modes are transferred to each other. This makes much more complicate the mechanism of these two low frequency modes around 270 ◦ C. The lowest mode observed at 24.8 cm−1 at room temperature shows hardening and damping of this mode decrease on heating. Finally, it interacts with the second lowest mode and disappeared above 250 ◦ C. As mentioned earlier, the octahedra distort and tilt on cooling and pure SN undergoes three phase transitions at 1342, 215 and 156 ◦ C, Cmcm → Cmc21 → Pbn21 → Pb11. There are three optical soft modes corresponding to above
47
Fig. 3. Temperature dependence of the phonon frequency, ω, for 24.8, 43.1, 52.4 and 60.7 cm−1 modes at room temperature, as deduced by the fit.
phase transitions. Our previous result shows the low frequency mode observed at 43.1 cm−1 at room temperature in BSN ceramics exhibits a large softening with increasing temperature from the value 43.1 cm−1 at room temperature down to 20 cm−1 at 970 ◦ C as shown in Fig. 5. Its damping increases so large that the soft mode becomes overdamped (γ/ω > 1). This soft mode is A1 (z) symmetry in a high temperature ferroelectric phase with space group Cmc21 and responsible for the Cmcm → Cmc21 ferroelectric phase transition [15]. The mode at 52.4 cm−1 shows slight softening on approaching 270 ◦ C from both low and high temperature sides. We did not observe any soft mode related to the two low temperature phase transitions corresponding to those of pure SN, indicating the disappearance of the incommensurate phase. It is a good accordance with the dielectric measurement [14]. However, an unusual effect is the behavior of the lowest mode at 24.8 cm−1 which interacts with the soft mode and then disappears around 270 ◦ C. It is interesting to no-
Fig. 4. Temperature dependence of the phonon damping, γ, for 24.8, 43.1, 52.4 and 60.7 cm−1 modes at room temperature, as deduced by the fit.
48
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in SrTiO3 . Ca2 Nb2 O7 has a perovskite slab structure analog to the pure SN. Ca ions have smaller ionic radii than Sr ions so that the lattice parameters of Ca2 Nb2 O7 are smaller than pure SN [19]. The lattice has been shrunk by the changes of the Ca ions to the Sr ions. It is reasonable to assume that Ba doping to the SN would relax the lattice and reduce the interlayer interaction and also reduce the interaction of the neighboring octahedra. As a consequence, the tendency of the tilting of octahedra decreases, stability of octahedra increases. This may be the reason for the disappearance of the incommensurate phase transition in BSN ceramics. Considering the new phase below Tr = 270 ◦ C, it seems unite cell doubling occurs below Tr and the softening of the lowest mode at 24.8 cm−1 on cooling may be related to the unite cell doubling. Fig. 5. The temperature dependence of the soft optical phonon frequency (dark square). The solid line denotes the fit ωs = A(Tc − T)β [15].
tice that the disappearance of the lowest mode and energy transfer between another two low frequency modes occurred at same temperature region. The mode frequency of the lowest mode shows hardening on heating and the damping, γ, decreases. It may indicate a new structural phase transition around 270 ◦ C (Tr ). At 192 ◦ C (Tr ), a new structural phase transition observed by dielectric measurement on the b-plate BSN single crystals [14]. The reason for the differences of Tr by two method is not clear. Because of the difficulties to grow a larger size single crystal for BSN (x = 0.32), the present Raman study carried out on BSN ceramics with traditional backward scattering geometry. In near future, we will carry out micro-Raman scattering on BSN single crystals in order to clarify such differences. As we shown earlier, observed soft mode frequency, ωs , at room temperature for BSN ceramics is much smaller than pure SN single crystals. Since partial replacement of Sr-site by Ba ions, whose ionic radius is larger than that of Sr ions, should lead to the decrease of interlayer interactions, as a consequence the frequency of this soft mode can be expected to decrease with increasing Ba doping. The soft mode is probably connected with NbO6 octahedra tiltings. Below Tc the ordered octahedra rotate within the layers, in the bc plane. The eigen vector of the ferroelectric soft mode is connected with these tiltings. In incommensurate phase, the neighboring octahedra of pure SN had opposite tilts, incommensurately modulated with the wave vector k = ±(1 − δ)(1/2)a* , a* is reciprocal lattice vector [16].The value of δ was very small (0.02 at 215 ◦ C) and decrease slowly down to 0.01 at 20 ◦ C. Usually A-site doping has a big effect on the shape of the octahedra in perovskite structure. When A-site doped with ions, which has smaller ionic radii than A-site ions, firstly, the lattice would shrink and the octahedra will tilt upon further increasing the doping ratio [17,18]. It is a common concept that the changes of Ba ions in BaTiO3 to the Sr ions cause octahedra tilting in the ferroelectric phase. The tilting of octahedra is the order parameter of ferroelectric phase transition
4. Summary Raman scattering was investigated in Ba-doped (Sr1−x Bax )2 Nb2 O7 (x = 0.32) ceramics. Low frequency spectra from −80 to 200 cm−1 was obtained at the wide temperature range from −190 to 600 ◦ C. The line shape of Raman spectra can be well fitted by multidamped harmonic oscillator model. We did not observe any soft mode related to the two low temperature phase transition corresponding to those of pure SN. On heating, the mode at 52.4 cm−1 softens while the mode at 43.1 cm−1 shows slight hardening with the temperature approaching 270 ◦ C and then pushes it down to lower frequency, indicating that the ionic motions associated with the two phonon modes are transferred to each other. An unusual effect is the behavior of the lowest mode at 24.8 cm−1 which interacts with the soft mode and then disappears around 270 ◦ C. It is interesting to notice that the disappearance of the lowest mode and energy transfer between another two low frequency modes occurred in the same temperature region. The mode frequency of the lowest mode shows hardening on heating and the damping, γ, decreases. It is correlated to a new structural phase transition around 270 ◦ C (Tr ). Partial replacement of Sr-site by Ba ions, whose ionic radius are larger than that of Sr ions, should induce the decrease of interlayer interactions and also reduces the interaction of the neighboring octahedra. As a consequence, the tendency of the tilting of octahedra decreases, and the stability of octahedra increases. This may be the reason for the disappearance of the incommensurate phase transition in BSN ceramics. Considering the new structural phase transitions, it seems unite cell doubling occurs below Tr and the softening of the lowest mode at 24.8 cm−1 on cooling may be related to the unite cell doubling. Acknowledgement This work was supported in part by the 21st century COE program under the Japanese Ministry of Education, Culture, Sports, Science and Technology.
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