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Crystallization of amorphous bismuth titanate
Journal of Non-Crystalline Solids 293±295 (2001) 250±254
www.elsevier.com/locate/jnoncrysol
Crystallization of amorphous bismuth titanate Seiji Kojima a,*, Anwar Hushur a, Fuming Jiang a, Sinichi Hamazaki b, Masaaki Takashige b, Min-Su Jang c, Shiro Shimada d b
a Institute of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan College of Science and Engineering, Iwaki Meisei University, Iwaki, Fukushima 970-8551, Japan c RCDAMP, Pusan National University, Kumujeoung-Ku, Pusan 609-735, South Korea d Graduate School of Engineering, Hokkaido University, Supporo 060-8628, Japan
Abstract We studied the transformation of amorphous bismuth titanate by heat treatments. After an as-quenched amorphous sample was annealed at 500 °C, the lowest Raman peak became intense like a boson peak for glass. This fact indicates the formation of intermediate range order. The medium range correlation length of 7 nm calculated from the boson peak frequency is in agreement with the mean cluster size measured by atomic force microscopy (AFM). The dierential thermal analysis (DTA) shows the two-step crystallization at 608 and 830 °C on heating. It suggests the existence of a metastable state. The samples annealed at 770 and 1000 °C are identi®ed as the pyroclore and the layered perovskite structures, respectively. It strongly suggests that at ®rst a three-dimensional (3D) crystalline state appears by nucleation process from a 3D amorphous state and secondly it transforms into the pseudo-2D layered perovskite structure. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction Bismuth titanate Bi4 Ti3 O12 crystal is a wellknown ferroelectric material with the layered perovskite structure [1]. It undergoes a ferroelectric phase transition, and the displacive nature was con®rmed by the observation of the underdamped soft optic mode [2]. In 1990s it became a key material for ferroelectric random access memory [3]. In the fabrication of ferroelectric thin ®lms, the crystallization process from an amorphous state is very important [4]. Upon heating amorphous materials, they transform into a crystalline state at a certain temperature. This temperature is called * Corresponding author. Tel.: +81-298 53 5307; fax: +81-298 55 7440. E-mail address: [email protected] (S. Kojima).
the crystallization temperature Tcrys . However, in spite of the importance of the application, an understanding of such a crystallization process is still poor. Therefore, in this paper, we report the crystallization process of amorphous bismuth titanates studied by Raman and Brillouin scattering spectroscopy and dierential thermal analysis (DTA). The detailed results on structural changes studied by atomic force microscope (AFM) and Xray diraction will be published separately.
2. Experimental The amorphous samples were prepared by a twin-roller rapid quenching method as follows. For the preparation of Bi4 Ti3 O12 powder, Bi2 O3 and TiO2 were mixed in the ratio 2.4:3 (20% excess
0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 8 2 8 - 6
S. Kojima et al. / Journal of Non-Crystalline Solids 293±295 (2001) 250±254
Bi2 O3 ) and calcined ®rst at 600 °C, and second at 800 °C, both for 12 h. Amorphous samples were prepared using the twin-roller quenching system [4]. The mixed powder was melted at about 1200 °C and sprayed onto the twin-roller to quench rapidly into an amorphous ribbon 20±40 lm thick and 1±3 cm in length. For measurements both as-quenched and annealed samples were used. The DTA trace was obtained using a dierential thermo-analyzer (Rigaku, Thermo Plus) in the temperature range between room temperature and 950 °C. Raman scattering was measured by a triple-grating spectrometer (Jobin Yvon TR64000) with a photon counting system and an Ar ion laser (CR, 514.5 nm, 50 mW) [5]. Brillouin scattering was measured by the 3 3 Tandem Fabry±Perot Interferometer (Sandercock) with a photon counting system and an Ar ion laser (NEC, single frequency 514.5 nm, 50 mW) [6].
251
an amorphous state was studied by heat treatments below the ®rst crystallization temperature Tcrys1 608 °C. As the annealing temperature increases, the lowest Raman band becomes sharp and the sample annealed at 500 °C clearly shows a boson peak like glass as shown in Fig. 2. This fact indicates that the medium range order appears by annealing. Such a low-frequency Raman scattering has been used to estimate the medium range order [7]. The structural correlation length or characteristic length L of the medium range order is given by the relation, xb SV =L, where xb , V , S is the boson peak frequency, sound velocity, and shape parameter, respectively [7]. The observed boson peak frequency xb is 45 cm 1 . The sound velocity is determined by backward Brillouin scattering geometry as shown in Fig. 3. From the frequency shift of 37.25 GHz and the obtained sound velocity is 4.79 km/s. Since the shape of the cluster is unknown, a shape parameter S is chosen to be 0.65. The characteristic length L is calculated to be 5.6
3. Medium range order of amorphous state At ®rst the crystallization process was investigated by DTA as shown in Fig. 1 from room temperature to 950 °C. The small endothermic peak around 540 °C is the glass transition. The two-step peaks at 608 and 830 °C indicate two-step crystallization on heating. Secondly, the change of
Fig. 1. DTA trace of amorphous bismuth titanate.
Fig. 2. Raman scattering spectrum of boson peak of amorphous bismuth titanate.
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S. Kojima et al. / Journal of Non-Crystalline Solids 293±295 (2001) 250±254
Fig. 3. Brillouin scattering spectrum of amorphous bismuth titanate.
nm. If the cluster shape is linear, then S 0:5 and L 7:2 nm. Next, we studied the medium range order by AFM. The surface roughness and elastic inhomogeneity were measured by AFM [8]. The tapping mode gives almost uniform ¯at surface. However, the force modulation mode of AFM shows nano-size heterogeneities. The obtained mean cluster size is about 7 nm, which is the same order as the characteristic length determined by Raman scattering. It may indicate that the shape of cluster is not sphere but linear shape. In other well-known ferroelectric BaTiO3 and PZT with the perovskite structure, the nano-size clusters were also observed in an amorphous state by high resolution transmission electron microscopy [9,10]. These facts indicate the existence of nano-size medium range order in amorphous states of ferroelectric materials. Such nano-size cluster may play a dominant role in the nucleation of crystallization process.
Fig. 4. Raman scattering spectra of amorphous bismuth titanate.
crystalline phase at Tcrys2 830 °C. Therefore, we studied at ®rst the metastable state which appears in the intermediate temperature range between Tcrys1 and Tcrys2 . The sample annealed at 770 °C (>Tcrys1 ) was studied by Raman scattering. It is found that the spectrum of this sample is nearly the same as that of a Bi2 Ti2 O7 single crystal of pyroclore structure grown by the ¯ux method as shown in Fig. 4. The X-ray powder diraction pattern was also compared, and it is concluded that the most parts of this metastable state are crystalline Bi2 Ti2 O7 with pyroclore structure. Very recently the existence of a metastable Bi2 Ti2 O7 state was reported in the crystallization process of Bi4 Ti3 O12 by Wen and Lu [11]. It indicates that amorphous samples cannot crystallize into the layered perovskite structure directly.
4. Metastable crystalline state As described in Section 3 there are two crystallization temperatures Tcrys1 608 °C and Tcrys2 830 °C as shown in Fig. 1. This fact indicates that an as-quenched amorphous sample ®rst crystallizes to a metastable state at Tcrys1 608 °C and then transforms irreversibly into a stable
5. Crystallization to layered perovskite structure We also studied the another crystalline state which appears above Tcrys2 830 °C. The sample annealed at 1000 °C (>Tcrys2 ) was studied by Raman scattering as shown in Fig. 5. The X-ray
S. Kojima et al. / Journal of Non-Crystalline Solids 293±295 (2001) 250±254
Fig. 5. Raman scattering spectra of amorphous bismuth titanate.
diraction of the same sample was also measured. These data are compared with those of a ceramic Bi4 Ti3 O12 sample of layered perovskite structure fabricated by the standard method, and good agreement is obtained. Consequently, it is concluded that the ferroelectric phase with layered perovskite structure is obtained after the sample is annealed at 1000 °C. In fact, on the cooling run of DTA trace shows the endothermic anomaly obviously at about 660 °C as shown in Fig. 1. It indicates the ferroelectric phase transition of Bi4 Ti3 O12 with layered perovskite structure. It indicates that a 3D pyroclore structure transforms into the pseudo two-dimensional (2D) layered perovskite structure above Tcrys2 .
6. Summary Crystallization process of as-quenched amorphous bismuth titanate Bi4 Ti3 O12 was studied by using Raman, Brillouin, DTA, X-ray diraction and AFM. The sample annealed at 500 °C shows a boson peak like glass. It is found that the intermediate structural correlation length of 7.2 nm estimated from a boson peak is in agreement with
253
the mean cluster size of 7 nm determined by AFM force modulation mode in the same sample within experimental uncertainty. These results indicate the formation of nano-size medium range order before crystallization. Such nano-size order may play a dominant role in nucleation of crystallization process. The origin of high dielectric constants in amorphous ®lms can be attributed to nano-size clusters like polar micro-regions in relaxor ferroelectrics. The DTA results show two-step crystallization on heating. The sample annealed at 770 °C (>Tcrys1 608 °C) was crystalline Bi2 Ti2 O7 with the pyroclore structure. Whereas the sample annealed at 1000 °C (>Tcrys2 830 °C was the crystalline Bi4 Ti3 O12 with the layered perovskite structure. Therefore, at ®rst a 3D crystalline state appears by nucleation process from a 3D amorphous state. Secondly the pseudo-2D layered perovskite structure is formed from a 3D crystalline state. Since it is not certain about the general crystallization mechanism of a pseudo-2D crystalline state from an amorphous state, theoretical study is necessary to make clear the mechanism of the successive crystallization from a 3D amorphous state to a 2D crystalline state through a 3D crystalline state. Acknowledgements This work is partially supported by the Grantin-Aid from the JSPS (No. 10440115) and the Japan±Korea Joint-Research program from JSPS and KOSEF. One of the authors (M.-S.J.) is thankful to University of Tsukuba for the hospitality in Institute of Materials Science by the international exchange program. References [1] E.C. Subarao, J. Phys. Chem. Solids 23 (1962) 665. [2] S. Kojima, S. Shimada, Physica. B 219&220 (1996) 617. [3] C.A. Arajo, J.D. Cuchiaro, M.C. Scottand, L.D. MaMillan, Nature 374 (1995) 627. [4] M.S. Jang, S.H. Kim, H.J. Kim, Y.S. Yang, S.Y. Jeong, J. Kor. Phys. Soc. 28 (1995) S605. [5] S. Kojima, Phys. Rev. B 47 (1993) 2924. [6] F.M. Jiang, S. Kojima, Appl. Phys. Lett. 77 (2000) 1271.
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S. Kojima et al. / Journal of Non-Crystalline Solids 293±295 (2001) 250±254
[7] T. Achibat, A. Boukenter, E. Duval, J. Chem. Phys. 99 (1993) 2046. [8] M. Takashige, S. Hamazaki, Y. Takahashi, F. Shimizu, T. Yamaguchi, M.S. Jiang, S. Kojima, Jpn. J. Appl. Phys. 31 (2000) 3716.
[9] Y. Xu, J.D. Mackenzie, J. Non-Cryst. Solids 176 (1994) 1. [10] Y. Xu, J.D. Mackenzie, J. Non-Cryst. Solids 246 (1999) 136. [11] C.Y. Wen, C.H. Lu, Ferroelectrics Lett. 26 (1999) 125.
www.elsevier.com/locate/jnoncrysol
Crystallization of amorphous bismuth titanate Seiji Kojima a,*, Anwar Hushur a, Fuming Jiang a, Sinichi Hamazaki b, Masaaki Takashige b, Min-Su Jang c, Shiro Shimada d b
a Institute of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan College of Science and Engineering, Iwaki Meisei University, Iwaki, Fukushima 970-8551, Japan c RCDAMP, Pusan National University, Kumujeoung-Ku, Pusan 609-735, South Korea d Graduate School of Engineering, Hokkaido University, Supporo 060-8628, Japan
Abstract We studied the transformation of amorphous bismuth titanate by heat treatments. After an as-quenched amorphous sample was annealed at 500 °C, the lowest Raman peak became intense like a boson peak for glass. This fact indicates the formation of intermediate range order. The medium range correlation length of 7 nm calculated from the boson peak frequency is in agreement with the mean cluster size measured by atomic force microscopy (AFM). The dierential thermal analysis (DTA) shows the two-step crystallization at 608 and 830 °C on heating. It suggests the existence of a metastable state. The samples annealed at 770 and 1000 °C are identi®ed as the pyroclore and the layered perovskite structures, respectively. It strongly suggests that at ®rst a three-dimensional (3D) crystalline state appears by nucleation process from a 3D amorphous state and secondly it transforms into the pseudo-2D layered perovskite structure. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction Bismuth titanate Bi4 Ti3 O12 crystal is a wellknown ferroelectric material with the layered perovskite structure [1]. It undergoes a ferroelectric phase transition, and the displacive nature was con®rmed by the observation of the underdamped soft optic mode [2]. In 1990s it became a key material for ferroelectric random access memory [3]. In the fabrication of ferroelectric thin ®lms, the crystallization process from an amorphous state is very important [4]. Upon heating amorphous materials, they transform into a crystalline state at a certain temperature. This temperature is called * Corresponding author. Tel.: +81-298 53 5307; fax: +81-298 55 7440. E-mail address: [email protected] (S. Kojima).
the crystallization temperature Tcrys . However, in spite of the importance of the application, an understanding of such a crystallization process is still poor. Therefore, in this paper, we report the crystallization process of amorphous bismuth titanates studied by Raman and Brillouin scattering spectroscopy and dierential thermal analysis (DTA). The detailed results on structural changes studied by atomic force microscope (AFM) and Xray diraction will be published separately.
2. Experimental The amorphous samples were prepared by a twin-roller rapid quenching method as follows. For the preparation of Bi4 Ti3 O12 powder, Bi2 O3 and TiO2 were mixed in the ratio 2.4:3 (20% excess
0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 8 2 8 - 6
S. Kojima et al. / Journal of Non-Crystalline Solids 293±295 (2001) 250±254
Bi2 O3 ) and calcined ®rst at 600 °C, and second at 800 °C, both for 12 h. Amorphous samples were prepared using the twin-roller quenching system [4]. The mixed powder was melted at about 1200 °C and sprayed onto the twin-roller to quench rapidly into an amorphous ribbon 20±40 lm thick and 1±3 cm in length. For measurements both as-quenched and annealed samples were used. The DTA trace was obtained using a dierential thermo-analyzer (Rigaku, Thermo Plus) in the temperature range between room temperature and 950 °C. Raman scattering was measured by a triple-grating spectrometer (Jobin Yvon TR64000) with a photon counting system and an Ar ion laser (CR, 514.5 nm, 50 mW) [5]. Brillouin scattering was measured by the 3 3 Tandem Fabry±Perot Interferometer (Sandercock) with a photon counting system and an Ar ion laser (NEC, single frequency 514.5 nm, 50 mW) [6].
251
an amorphous state was studied by heat treatments below the ®rst crystallization temperature Tcrys1 608 °C. As the annealing temperature increases, the lowest Raman band becomes sharp and the sample annealed at 500 °C clearly shows a boson peak like glass as shown in Fig. 2. This fact indicates that the medium range order appears by annealing. Such a low-frequency Raman scattering has been used to estimate the medium range order [7]. The structural correlation length or characteristic length L of the medium range order is given by the relation, xb SV =L, where xb , V , S is the boson peak frequency, sound velocity, and shape parameter, respectively [7]. The observed boson peak frequency xb is 45 cm 1 . The sound velocity is determined by backward Brillouin scattering geometry as shown in Fig. 3. From the frequency shift of 37.25 GHz and the obtained sound velocity is 4.79 km/s. Since the shape of the cluster is unknown, a shape parameter S is chosen to be 0.65. The characteristic length L is calculated to be 5.6
3. Medium range order of amorphous state At ®rst the crystallization process was investigated by DTA as shown in Fig. 1 from room temperature to 950 °C. The small endothermic peak around 540 °C is the glass transition. The two-step peaks at 608 and 830 °C indicate two-step crystallization on heating. Secondly, the change of
Fig. 1. DTA trace of amorphous bismuth titanate.
Fig. 2. Raman scattering spectrum of boson peak of amorphous bismuth titanate.
252
S. Kojima et al. / Journal of Non-Crystalline Solids 293±295 (2001) 250±254
Fig. 3. Brillouin scattering spectrum of amorphous bismuth titanate.
nm. If the cluster shape is linear, then S 0:5 and L 7:2 nm. Next, we studied the medium range order by AFM. The surface roughness and elastic inhomogeneity were measured by AFM [8]. The tapping mode gives almost uniform ¯at surface. However, the force modulation mode of AFM shows nano-size heterogeneities. The obtained mean cluster size is about 7 nm, which is the same order as the characteristic length determined by Raman scattering. It may indicate that the shape of cluster is not sphere but linear shape. In other well-known ferroelectric BaTiO3 and PZT with the perovskite structure, the nano-size clusters were also observed in an amorphous state by high resolution transmission electron microscopy [9,10]. These facts indicate the existence of nano-size medium range order in amorphous states of ferroelectric materials. Such nano-size cluster may play a dominant role in the nucleation of crystallization process.
Fig. 4. Raman scattering spectra of amorphous bismuth titanate.
crystalline phase at Tcrys2 830 °C. Therefore, we studied at ®rst the metastable state which appears in the intermediate temperature range between Tcrys1 and Tcrys2 . The sample annealed at 770 °C (>Tcrys1 ) was studied by Raman scattering. It is found that the spectrum of this sample is nearly the same as that of a Bi2 Ti2 O7 single crystal of pyroclore structure grown by the ¯ux method as shown in Fig. 4. The X-ray powder diraction pattern was also compared, and it is concluded that the most parts of this metastable state are crystalline Bi2 Ti2 O7 with pyroclore structure. Very recently the existence of a metastable Bi2 Ti2 O7 state was reported in the crystallization process of Bi4 Ti3 O12 by Wen and Lu [11]. It indicates that amorphous samples cannot crystallize into the layered perovskite structure directly.
4. Metastable crystalline state As described in Section 3 there are two crystallization temperatures Tcrys1 608 °C and Tcrys2 830 °C as shown in Fig. 1. This fact indicates that an as-quenched amorphous sample ®rst crystallizes to a metastable state at Tcrys1 608 °C and then transforms irreversibly into a stable
5. Crystallization to layered perovskite structure We also studied the another crystalline state which appears above Tcrys2 830 °C. The sample annealed at 1000 °C (>Tcrys2 ) was studied by Raman scattering as shown in Fig. 5. The X-ray
S. Kojima et al. / Journal of Non-Crystalline Solids 293±295 (2001) 250±254
Fig. 5. Raman scattering spectra of amorphous bismuth titanate.
diraction of the same sample was also measured. These data are compared with those of a ceramic Bi4 Ti3 O12 sample of layered perovskite structure fabricated by the standard method, and good agreement is obtained. Consequently, it is concluded that the ferroelectric phase with layered perovskite structure is obtained after the sample is annealed at 1000 °C. In fact, on the cooling run of DTA trace shows the endothermic anomaly obviously at about 660 °C as shown in Fig. 1. It indicates the ferroelectric phase transition of Bi4 Ti3 O12 with layered perovskite structure. It indicates that a 3D pyroclore structure transforms into the pseudo two-dimensional (2D) layered perovskite structure above Tcrys2 .
6. Summary Crystallization process of as-quenched amorphous bismuth titanate Bi4 Ti3 O12 was studied by using Raman, Brillouin, DTA, X-ray diraction and AFM. The sample annealed at 500 °C shows a boson peak like glass. It is found that the intermediate structural correlation length of 7.2 nm estimated from a boson peak is in agreement with
253
the mean cluster size of 7 nm determined by AFM force modulation mode in the same sample within experimental uncertainty. These results indicate the formation of nano-size medium range order before crystallization. Such nano-size order may play a dominant role in nucleation of crystallization process. The origin of high dielectric constants in amorphous ®lms can be attributed to nano-size clusters like polar micro-regions in relaxor ferroelectrics. The DTA results show two-step crystallization on heating. The sample annealed at 770 °C (>Tcrys1 608 °C) was crystalline Bi2 Ti2 O7 with the pyroclore structure. Whereas the sample annealed at 1000 °C (>Tcrys2 830 °C was the crystalline Bi4 Ti3 O12 with the layered perovskite structure. Therefore, at ®rst a 3D crystalline state appears by nucleation process from a 3D amorphous state. Secondly the pseudo-2D layered perovskite structure is formed from a 3D crystalline state. Since it is not certain about the general crystallization mechanism of a pseudo-2D crystalline state from an amorphous state, theoretical study is necessary to make clear the mechanism of the successive crystallization from a 3D amorphous state to a 2D crystalline state through a 3D crystalline state. Acknowledgements This work is partially supported by the Grantin-Aid from the JSPS (No. 10440115) and the Japan±Korea Joint-Research program from JSPS and KOSEF. One of the authors (M.-S.J.) is thankful to University of Tsukuba for the hospitality in Institute of Materials Science by the international exchange program. References [1] E.C. Subarao, J. Phys. Chem. Solids 23 (1962) 665. [2] S. Kojima, S. Shimada, Physica. B 219&220 (1996) 617. [3] C.A. Arajo, J.D. Cuchiaro, M.C. Scottand, L.D. MaMillan, Nature 374 (1995) 627. [4] M.S. Jang, S.H. Kim, H.J. Kim, Y.S. Yang, S.Y. Jeong, J. Kor. Phys. Soc. 28 (1995) S605. [5] S. Kojima, Phys. Rev. B 47 (1993) 2924. [6] F.M. Jiang, S. Kojima, Appl. Phys. Lett. 77 (2000) 1271.
254
S. Kojima et al. / Journal of Non-Crystalline Solids 293±295 (2001) 250±254
[7] T. Achibat, A. Boukenter, E. Duval, J. Chem. Phys. 99 (1993) 2046. [8] M. Takashige, S. Hamazaki, Y. Takahashi, F. Shimizu, T. Yamaguchi, M.S. Jiang, S. Kojima, Jpn. J. Appl. Phys. 31 (2000) 3716.
[9] Y. Xu, J.D. Mackenzie, J. Non-Cryst. Solids 176 (1994) 1. [10] Y. Xu, J.D. Mackenzie, J. Non-Cryst. Solids 246 (1999) 136. [11] C.Y. Wen, C.H. Lu, Ferroelectrics Lett. 26 (1999) 125.