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Development of Novel Ferroelectric Materials
A P P L I C A T I O N
47 Development of Novel Ferroelectric Materials Yuji Noguchi, Masaru Miyayama Nonvolatile memories using ferroelectric function have attracted a great deal of worldwide interest for next-generation memory devices because of the lowenergy consumption, high-speed read/write, and potentially high density. The widespread applications of the ferroelectric memories require ferroelectric thin films with a larger remanent polarization (Pr), high endurance against repeated switching of bistable polarization states, and low-temperature synthesis as low as 650 C [1e4]. Up to now, two kinds of ferroelectric oxides have been recognized as candidates for the memory materials: one is lead zirconate titanate (PZT) with perovskite structure and the other is bismuth layerestructured ferroelectrics (BLSFs, see Fig. 47.1) with layered perovskite structure such as SrBi2Ta2O9 (SBT) and Bi4Ti3O12 (BiT) [3,4]. Ferroelectric PZT shows a superior polarization property such as a relatively larger Pr of 20e40 mC/cm2, whereas PZT films with Pt electrodes suffer from fatigue problems and then expensive Ir or IrOx electrodes are required to be used in memory devices [1,3]. Furthermore, PZT-based materials contain a large amount of toxic lead, and the use of lead element in these electronic devices would be restricted in the near future by the RoHS directives. Thus, lead-free materials with sufficient ferroelectric properties are required to protect the environment and the ecosystem. Layered ferroelectric SBT and BiT have been widely studied as candidate materials for ferroelectric memories because of their high fatigue endurance [4]. SBT, however, shows a relatively low Pr of 7e10 mC/cm2, leading to a difficulty in establishing high-density ferroelectric memories using SBT. The substitution of La into BiT enables us to obtain ferroelectric films at a relatively low deposition temperature of 650 C with a larger Pr (20 mC/cm2), but this Pr value is still lower than that of PZT films. Thus, novel Pb-free ferroelectrics with a larger Pr are expected for next-generation high-density ferroelectric memories. In this chapter, it is described that novel ferroelectrics composed of two kinds of BLSFs, so-called
Nanoparticle Technology Handbook, Third Edition https://doi.org/10.1016/B978-0-444-64110-6.00054-8
“superlattice-structured ferroelectrics,” show superior polarization properties [5e10]. The novel ferroelectrics with a large Pr of 50 mC/cm2 compared with PZT are promising not only for memory materials but also for Pb-free piezoelectric ones.
1. CRYSTAL STRUCTURE OF BISMUTH LAYEReSTRUCTURED FERROELECTRICS In the crystal structure of BLSFs, perovskite blocks (Am1BmO3mþ1) are sandwiched between Bi2O2 layers, and the perovskite blocks are composed of m layers of BO6 octahedra with A-site cations (see Fig. 47.1). The Bi2O2 layers play a significant role not only in the polarization properties but also in the high durability of ferroelectric capacitor. Furthermore, spontaneous polarization (Ps) and insulating properties strongly depend on m of the perovskite blocks [11e13]. The Bi2O2 layers act as insulating paraelectric layers and control the electronic response (electrical conductivity, band gap, etc.), while the ferroelectricity arises mainly in the perovskite blocks. The crystal structure of BLSFs is briefly characterized by m, and the dielectric and ferroelectric anisotropy strongly depends on the value of m. Recently, superlattice-structured BLSFs, discovered by Kikuchi et al. [5,6], have received a renewed interest as a promising candidate for ferroelectric materials with a relatively large Pr [7e10]. For superlattice-structured BiTeSBTi, two kinds of perovskite blocks of BiT (m ¼ 3) and SrBi4Ti4O15 (SBTi: m ¼ 4) are in turn sandwiched between Bi2O2 layers. A lattice mismatch between the two perovskite blocks and their different chemical characters induce the large lattice distortion in the Bi2O2 layers, leading to a quite distinct type of the ferroelectric displacement of the Bi in the Bi2O2 layers along the a-axis [7]. This Bi displacement is suggested to contribute partly to a larger Pr observed for the BiTeSBTi ceramics than the constituent BiT and SBTi [7]. While most investigations on the superlatticestructured BLSFs to data have been performed on bulk
651
© 2018, 2012, 2007 Elsevier B.V. All rights reserved.
652
47. DEVELOPMENT OF NOVEL FERROELECTRIC MATERIALS
c
Bi2O2 layer
Perovskite layer (m :the number of BO6octahedra
b a
BBTi layer (m=4)
Ti
Ta Sr
(A)
Bi
(B)
BiT layer (m=3)
Ti Bi/Ba
(C)
(D)
FIGURE 47.1
Crystal structure of bismuth layerestructured ferroelectrics (BLSFs). BLSFs are composed of the alternate stacking of Bi2O2 layers and perovskite layers along the c-axis. The number of BO6 octahedra in one perovskite layer is defined as m. (A) SrBi2 Ta2O9 (SBT: m ¼ 2). (B) B4Ti3O12 (BiT: m ¼ 3). (C) BaBi4Ti4O15 (BBTi: m ¼ 4). (D) Bi4Ti3O12eBaBi4Ti4O15 (BiTeBBTi: m ¼ 3e4).
ceramics to elucidate fundamental properties, their dielectric and ferroelectric tensor is not understood, mainly because of the lack of the single crystals. Here, the properties of the single crystals of superlattice-structured BiTeBaBi4TiO15 (m ¼ 3e4) are reviewed. BiTeBBTi has an ideal chemical formula of BaBi8Ti7O27, and the m ¼ 3 (BiT) and m ¼ 4 (BaBi4TiO15: BBTi) layers are alternately stacked in the structure along the c-axis. The measurements of the single crystals have demonstrated that the superlattice-structured BiTeBBTi exhibited a Ps of 52 mC/cm2 [9,10], which was larger than that of BiT (46 mC/cm2) and BBTi (16 mC/cm2).
2. CRYSTAL GROWTH AND EXPERIMENTAL PROCEDURE First, BiT, BBTi, and BiTeBBTi powders were prepared by a solid-state reaction. The mixture of BaCO3 (99.99%), Bi2O3 (99.99%), and TiO2 (99.99%) with its stoichiometric composition was calcined at 1050 C for 5 h several times with intermediate grinding. The X-ray powder diffraction data agreed well with the pattern calculated by the Rietveld analysis. Time-of-flight neutron powder diffraction was collected using the Vega diffractometer at KENS, and structural parameters were refined using the program RIETAN-TN. Single crystals of BiTeBBTi were grown by a self-flux method using the BiTeBBTi powder. The BiTeBBTi powder was mixed with Bi2O3 as a flux and put into a Pt crucible. The material was heated at 1200 C for 5 h
and then cooled to room temperature at 200 C/h. The single crystals obtained were plate-like thin sheets, with 0.1e0.4 mm thickness and 3 mm2 3 mm2 planar dimensions. After heat treatment at 950 C for 24 h in air, annealing under high-pressure oxygen at 35 MPa was performed at 750 C for 12 h to reduce oxygen vacancies in the crystals. The composition of the crystals determined by inductively coupled plasma emission spectrometry was Ba0.75Bi8.32Ti7O27, which is a Ba-deficient and Bi-excess composition compared with the ideal one (BaBi8Ti7O27). For electrical measurements, the crystals were cut so as to apply electric field along the a(b) directions according to the crystallographic axis determined by the 90 degrees domain structure. The small crystals of 1 mm3 0.2 mm3 0.25 mm3 (Au electrodes were sputtered on 1 mm2 0.2 mm2 faces) were used for polarization measurements. Single crystals of BiT and BBTi were also grown separately, and their properties were compared with those of the BiTeBBTi crystals.
3. LAYERED STRUCTURE, DIELECTRIC, AND LEAKAGE CURRENT PROPERTIES OF BiTeBBTi CRYSTALS Fig. 47.2A shows the q 2q X-ray diffraction (XRD) pattern of BiTeBBTi single crystals. Apparent 00l reflections originating from the superlattice structure were observed. Fig. 47.2B shows the transmission electron micrograph of a BiTeBBTi crystal. Because the crystals
653
4. GIANT POLARIZATION IN BiTeBBTi CRYSTALS
0016
0012 0013
20
log (current density/A · cm-2)
10
15 0011
12
0014
-6
009
9
007
6
005
004
Intensity (a. u.)
(A)
40
30
2 (deg, Cuk ) m a(b) axis
c axis
4
BiT layer (m = 3)
3 4
3 3
1200
a axis 6000
900
4000
600
2000
300
Dielectric permittivity (-)
Dielectric permittivity (-)
1500 1 MHz
c axis 300
400
500
600
BBTi crystals
700
20
40
60
80
FIGURE 47.4 Leakage current density as a function of electric field applied along the a-axis (25 C).
(C)
were easily damaged by electron beam irradiation, this photograph was taken within a few seconds. Fig. 47.2C shows a micrograph of the region enclosed by the dashed line in Fig. 47.2B. The thickness of the constituent layers was approximately 1.5 and 2.0 nm, and these values are in good agreement with half the lattice parameters c of BiT and BBTi, respectively. These results clearly indicate that the alternative superlattice stacking of m ¼ 3 and m ¼ 4 layers is maintained not only in nanometer scale but also in a wider-ranging area. In addition to the alternate stacking, a stacking fault along the c-axis was observed (m ¼ -3-4-3-4-3-3-4-). Fig. 47.3 shows the temperature dependence of dielectric permittivity of BiTeBBTi crystals along the a(b) and c axes at a frequency of 1 MHz. The dielectric permittivity along the a-axis was approximately 1000 at 25 C, while that along the c-axis was 195. The Curie
0
-8
E // a axis (kV/cm)
FIGURE 47.2 Results of the structural analysis of BiTeBBTi single crystals: (A) q 2q X-ray diffraction pattern showing 00l reflections from the crystals surface; (B) transmission electron micrograph in the a(b)ec plane; and (C) the schematic crystal structure.
8000
BiT-BBTi crystals
0 BBTi layer (m = 4)
4
(B) 2 nm
-7
-9
3
2 nm
BiT crystals
0
Temperature (°C)
FIGURE 47.3 Temperature dependence of dielectric permittivity of BiTeBBTi single crystals (1 MHz).
temperature (TC) estimated from the permittivity peak was 540 C, and this TC was intermediate between those of BiT (TC ¼ 675 C) and BBTi (TC ¼ 410 C) [7e10]. Compared with BiTeBBTi (BaBi8Ti7O27) ceramics with TC of 500 C, the BiTeBBTi crystals showed a 40 C higher TC. The higher TC observed for Ba-deficient BiTe BBTi crystals is suggested to originate from the Bi substitution [14] at the perovskite A site. The leakage currents along the a(b)-axis as a function of electric field at 25 C (Fig. 47.4) show that the BiT crystals exhibited a poor value of w107 A/cm2 above 25 kV/cm, while the BBTi crystals showed a lower current of w109 A/cm2. Note that the leakage current of the BiTeBBTi crystals remained a sufficiently low value, which was one-fifth that of the BiT crystals. A simple equivalent circuit of parallel conduction of individual BiT and BBTi layers cannot explain this behavior.
4. GIANT POLARIZATION IN BiTeBBTi CRYSTALS Fig. 47.5 indicates the polarization hysteresis loops of single crystals along the a(b)-axis at 25 C. The Ps and the coercive field (Ec) of BiTeBBTi were 52 mC/cm2 and 120 kV/cm, respectively. Note that this Ps value is larger than those of BiT (46 mC/cm2) and BBTi (16 mC/cm2) crystals. It has been widely known that BiT has the largest Ps among the BLSFs and that the Ps of BBTi is much smaller [13]. If the intrinsic ferroelectric distortions in BiT and BBTi were maintained in the superlattice structure, the Ps of BiTeBBTi would be the average of those for BiT and BBTi (approximately 30 mC/cm2). However, the Ps observed for the BiTeBBTi crystals is much larger than the average Ps. The enhanced Ps of the BiTeBBTi crystals implies that the lattice strain
654
47. DEVELOPMENT OF NOVEL FERROELECTRIC MATERIALS
BiT-BBTi crystals
Polarization (µ C/cm2)
75 50
BiT crystals
25 0 -25
BBTi crystals
-50 -75 -3 00
1 Hz -200
-100
0
100
200
300
E // a axis (kV/cm)
FIGURE 47.5
Polarization hysteresis loops along the a-axis (25 C).
induced by the alternate stacking of m ¼ 3 and m ¼ 4 layers with different cell size promotes ferroelectric distortion. One of the origins of the larger Ps of the BiTeBBTi crystals is the Bi substitution at the Ba site. For strontium bismuth tantalate with Sr-deficient and Bi-excess composition, the Bi substitution at the Sr site occurs, and the charge difference between Bi3þ and Sr2þ is compensated through the formation of Sr vacancies [14]. This enhances Ps from 19 mC/cm2 (SrBi2Ta2O9) to 29 mC/cm2 (Sr0.81Bi2.13Ta2O9) and raises TC from 295 to 410 C [14]. It has been reported for BBTi ceramics with random orientation that the Bi substitution at the Ba site enhances Pr from 14 mC/cm2 (BaBi4Ti4O15) to 18 mC/cm2 (Ba0.9Bi4.07Ti4O15) and leads to an increase in TC from 415 to 440 C. The compositional analysis of the BiTeBBTi crystals shows the Ba-deficient and Bi-excess composition (Ba0.75Bi8.32Ti7O27), indicating that excess Bi is substituted at the Ba site. Indeed, higher TC was observed for the BiTeBBTi crystals (540 C) than for the stoichiometric ceramics (490 C). This is direct evidence of the Bi substitution at the Ba site. However, the effects of the Bi substitution alone cannot explain the large Ps of the BiTeBBTi crystals. It is considered that the lattice distortion induced by the alternate stacking of m ¼ 3 and m ¼ 4 layers plays a dominant role in enhancing Ps of BiTeBBTi. Because BBTi contains Ba ions with large ionic radius at the A site, parameters a and b are larger than those of BiT. The lattice mismatch between the BiT and BBTi reaches 0.42% along the a-axis and 0.88% along the b-axis. The parameters a and b of BiTeBBTi were the averages of those of values for the BiT and BBTi. These results imply that tensile and compressive stresses built up in the m ¼ 3 and m ¼ 4 layers of the superlattice structure, respectively. These stresses are suggested to be concentrated on the Bi2O2 layers that interleave the two kinds
of perovskite block. This crystallographic environment induces a local symmetry breaking of the Bi2O2 layers. All Bi ions of the Bi2O2 layers in BiT and BBTi are identical from the crystallographic point of view. In contrast, the Bi ions of the Bi2O2 layers in the BiTeBBTi are divided into two cations: one is connected to the perovskite blocks of the m ¼ 3 layer and the other is adjacent to that of the m ¼ 4 layer. The symmetry breaking leads to an unusual ferroelectric displacement of the Bi ions of the Bi2O2 layers in the BiTeBBTi. It has been reported that the Bi ions of the Bi2O2 layers in BiTeSrBi4Ti4O15 are displaced along the a-axis (the polar direction) by 2% of the parameter a from the corresponding positions of the high-temperature tetragonal structure. Similar displacements of the Bi ions are expected to the BiTeBBTi, which enhance Ps. In addition to the lattice distortions of the Bi2O2 layers, the alternate stacking of the m ¼ 3 and m ¼ 4 layers is considered to promote the ferroelectric distortions in the perovskite blocks in the BiTeBBTi. Superlattice-structured BiTeBBTi single crystals were grown by a self-flux method, and the structure was confirmed by XRD and transmission electron microscope analyses. Dielectric measurements showed that TC of the BiTeBBTi was 540 C, which is 40 C higher than that of stoichiometric BiTeBBTi ceramics. The Ps of the BiTeBBTi crystals along the a(b)-axis was 52 mC/cm2, which is larger than those of BiT and BBTi crystals. It is suggested that the Bi substitution at the Ba site and the local symmetry breaking of the Bi2O2 layers are partially responsible for the larger Ps of BiTeBBTi.
References [1] Y. Arimori, T. Eshita, Oyo Butsuri 69 (2000) 1080e1084 (in Japanese). [2] H. Ishiwara, FED. J. 11 (2000) 52e66 (in Japanese). [3] M. Okuyama, Denki Gakkai Gakujyutsu Ronbunshi E 121 (2003) 537e541 (in Japanese). [4] Y. Fujisaki, Kino Zairyo 23 (2003) 22e30 (in Japanese). [5] T. Kikuchi, J. Less Common Met. 48 (1976) 319e323. [6] T. Kikuchi, A. Watanabe, K. Uchida, Mater. Res. Bull. 12 (1977) 299e304. [7] Y. Noguchi, M. Miyayama, T. Kudo, App. Phys. Lett. 77 (2000) 3639e3641. [8] Y. Goshima, Y. Noguchi, M. Miyayama, App. Phys. Lett. 81 (2002) 2226e2228. [9] T. Kobayashi, Y. Noguchi, M. Miyayama, Jpn. J. Appl. Phys. 43 (2004) 6653e6657. [10] T. Kobayashi, Y. Noguchi, M. Miyayama, App. Phys. Lett. 86 (2005) 012907. [11] T. Takenaka, Choonpa Techno 13 (8) (2001) 2e12 (in Japanese). [12] Y. Noguchi, M. Miyayama, Kino Zairyo 21 (9) (2001) 31e36 (in Japanese). [13] H. Irie, M. Miyayama, T. Kudo, Jpn. J. Appl. Phys. 40 (2001) 239e243. [14] Y. Noguchi, M. Miyayama, T. Kudo, Phys. Rev. B 63 (2001) 214102.
47 Development of Novel Ferroelectric Materials Yuji Noguchi, Masaru Miyayama Nonvolatile memories using ferroelectric function have attracted a great deal of worldwide interest for next-generation memory devices because of the lowenergy consumption, high-speed read/write, and potentially high density. The widespread applications of the ferroelectric memories require ferroelectric thin films with a larger remanent polarization (Pr), high endurance against repeated switching of bistable polarization states, and low-temperature synthesis as low as 650 C [1e4]. Up to now, two kinds of ferroelectric oxides have been recognized as candidates for the memory materials: one is lead zirconate titanate (PZT) with perovskite structure and the other is bismuth layerestructured ferroelectrics (BLSFs, see Fig. 47.1) with layered perovskite structure such as SrBi2Ta2O9 (SBT) and Bi4Ti3O12 (BiT) [3,4]. Ferroelectric PZT shows a superior polarization property such as a relatively larger Pr of 20e40 mC/cm2, whereas PZT films with Pt electrodes suffer from fatigue problems and then expensive Ir or IrOx electrodes are required to be used in memory devices [1,3]. Furthermore, PZT-based materials contain a large amount of toxic lead, and the use of lead element in these electronic devices would be restricted in the near future by the RoHS directives. Thus, lead-free materials with sufficient ferroelectric properties are required to protect the environment and the ecosystem. Layered ferroelectric SBT and BiT have been widely studied as candidate materials for ferroelectric memories because of their high fatigue endurance [4]. SBT, however, shows a relatively low Pr of 7e10 mC/cm2, leading to a difficulty in establishing high-density ferroelectric memories using SBT. The substitution of La into BiT enables us to obtain ferroelectric films at a relatively low deposition temperature of 650 C with a larger Pr (20 mC/cm2), but this Pr value is still lower than that of PZT films. Thus, novel Pb-free ferroelectrics with a larger Pr are expected for next-generation high-density ferroelectric memories. In this chapter, it is described that novel ferroelectrics composed of two kinds of BLSFs, so-called
Nanoparticle Technology Handbook, Third Edition https://doi.org/10.1016/B978-0-444-64110-6.00054-8
“superlattice-structured ferroelectrics,” show superior polarization properties [5e10]. The novel ferroelectrics with a large Pr of 50 mC/cm2 compared with PZT are promising not only for memory materials but also for Pb-free piezoelectric ones.
1. CRYSTAL STRUCTURE OF BISMUTH LAYEReSTRUCTURED FERROELECTRICS In the crystal structure of BLSFs, perovskite blocks (Am1BmO3mþ1) are sandwiched between Bi2O2 layers, and the perovskite blocks are composed of m layers of BO6 octahedra with A-site cations (see Fig. 47.1). The Bi2O2 layers play a significant role not only in the polarization properties but also in the high durability of ferroelectric capacitor. Furthermore, spontaneous polarization (Ps) and insulating properties strongly depend on m of the perovskite blocks [11e13]. The Bi2O2 layers act as insulating paraelectric layers and control the electronic response (electrical conductivity, band gap, etc.), while the ferroelectricity arises mainly in the perovskite blocks. The crystal structure of BLSFs is briefly characterized by m, and the dielectric and ferroelectric anisotropy strongly depends on the value of m. Recently, superlattice-structured BLSFs, discovered by Kikuchi et al. [5,6], have received a renewed interest as a promising candidate for ferroelectric materials with a relatively large Pr [7e10]. For superlattice-structured BiTeSBTi, two kinds of perovskite blocks of BiT (m ¼ 3) and SrBi4Ti4O15 (SBTi: m ¼ 4) are in turn sandwiched between Bi2O2 layers. A lattice mismatch between the two perovskite blocks and their different chemical characters induce the large lattice distortion in the Bi2O2 layers, leading to a quite distinct type of the ferroelectric displacement of the Bi in the Bi2O2 layers along the a-axis [7]. This Bi displacement is suggested to contribute partly to a larger Pr observed for the BiTeSBTi ceramics than the constituent BiT and SBTi [7]. While most investigations on the superlatticestructured BLSFs to data have been performed on bulk
651
© 2018, 2012, 2007 Elsevier B.V. All rights reserved.
652
47. DEVELOPMENT OF NOVEL FERROELECTRIC MATERIALS
c
Bi2O2 layer
Perovskite layer (m :the number of BO6octahedra
b a
BBTi layer (m=4)
Ti
Ta Sr
(A)
Bi
(B)
BiT layer (m=3)
Ti Bi/Ba
(C)
(D)
FIGURE 47.1
Crystal structure of bismuth layerestructured ferroelectrics (BLSFs). BLSFs are composed of the alternate stacking of Bi2O2 layers and perovskite layers along the c-axis. The number of BO6 octahedra in one perovskite layer is defined as m. (A) SrBi2 Ta2O9 (SBT: m ¼ 2). (B) B4Ti3O12 (BiT: m ¼ 3). (C) BaBi4Ti4O15 (BBTi: m ¼ 4). (D) Bi4Ti3O12eBaBi4Ti4O15 (BiTeBBTi: m ¼ 3e4).
ceramics to elucidate fundamental properties, their dielectric and ferroelectric tensor is not understood, mainly because of the lack of the single crystals. Here, the properties of the single crystals of superlattice-structured BiTeBaBi4TiO15 (m ¼ 3e4) are reviewed. BiTeBBTi has an ideal chemical formula of BaBi8Ti7O27, and the m ¼ 3 (BiT) and m ¼ 4 (BaBi4TiO15: BBTi) layers are alternately stacked in the structure along the c-axis. The measurements of the single crystals have demonstrated that the superlattice-structured BiTeBBTi exhibited a Ps of 52 mC/cm2 [9,10], which was larger than that of BiT (46 mC/cm2) and BBTi (16 mC/cm2).
2. CRYSTAL GROWTH AND EXPERIMENTAL PROCEDURE First, BiT, BBTi, and BiTeBBTi powders were prepared by a solid-state reaction. The mixture of BaCO3 (99.99%), Bi2O3 (99.99%), and TiO2 (99.99%) with its stoichiometric composition was calcined at 1050 C for 5 h several times with intermediate grinding. The X-ray powder diffraction data agreed well with the pattern calculated by the Rietveld analysis. Time-of-flight neutron powder diffraction was collected using the Vega diffractometer at KENS, and structural parameters were refined using the program RIETAN-TN. Single crystals of BiTeBBTi were grown by a self-flux method using the BiTeBBTi powder. The BiTeBBTi powder was mixed with Bi2O3 as a flux and put into a Pt crucible. The material was heated at 1200 C for 5 h
and then cooled to room temperature at 200 C/h. The single crystals obtained were plate-like thin sheets, with 0.1e0.4 mm thickness and 3 mm2 3 mm2 planar dimensions. After heat treatment at 950 C for 24 h in air, annealing under high-pressure oxygen at 35 MPa was performed at 750 C for 12 h to reduce oxygen vacancies in the crystals. The composition of the crystals determined by inductively coupled plasma emission spectrometry was Ba0.75Bi8.32Ti7O27, which is a Ba-deficient and Bi-excess composition compared with the ideal one (BaBi8Ti7O27). For electrical measurements, the crystals were cut so as to apply electric field along the a(b) directions according to the crystallographic axis determined by the 90 degrees domain structure. The small crystals of 1 mm3 0.2 mm3 0.25 mm3 (Au electrodes were sputtered on 1 mm2 0.2 mm2 faces) were used for polarization measurements. Single crystals of BiT and BBTi were also grown separately, and their properties were compared with those of the BiTeBBTi crystals.
3. LAYERED STRUCTURE, DIELECTRIC, AND LEAKAGE CURRENT PROPERTIES OF BiTeBBTi CRYSTALS Fig. 47.2A shows the q 2q X-ray diffraction (XRD) pattern of BiTeBBTi single crystals. Apparent 00l reflections originating from the superlattice structure were observed. Fig. 47.2B shows the transmission electron micrograph of a BiTeBBTi crystal. Because the crystals
653
4. GIANT POLARIZATION IN BiTeBBTi CRYSTALS
0016
0012 0013
20
log (current density/A · cm-2)
10
15 0011
12
0014
-6
009
9
007
6
005
004
Intensity (a. u.)
(A)
40
30
2 (deg, Cuk ) m a(b) axis
c axis
4
BiT layer (m = 3)
3 4
3 3
1200
a axis 6000
900
4000
600
2000
300
Dielectric permittivity (-)
Dielectric permittivity (-)
1500 1 MHz
c axis 300
400
500
600
BBTi crystals
700
20
40
60
80
FIGURE 47.4 Leakage current density as a function of electric field applied along the a-axis (25 C).
(C)
were easily damaged by electron beam irradiation, this photograph was taken within a few seconds. Fig. 47.2C shows a micrograph of the region enclosed by the dashed line in Fig. 47.2B. The thickness of the constituent layers was approximately 1.5 and 2.0 nm, and these values are in good agreement with half the lattice parameters c of BiT and BBTi, respectively. These results clearly indicate that the alternative superlattice stacking of m ¼ 3 and m ¼ 4 layers is maintained not only in nanometer scale but also in a wider-ranging area. In addition to the alternate stacking, a stacking fault along the c-axis was observed (m ¼ -3-4-3-4-3-3-4-). Fig. 47.3 shows the temperature dependence of dielectric permittivity of BiTeBBTi crystals along the a(b) and c axes at a frequency of 1 MHz. The dielectric permittivity along the a-axis was approximately 1000 at 25 C, while that along the c-axis was 195. The Curie
0
-8
E // a axis (kV/cm)
FIGURE 47.2 Results of the structural analysis of BiTeBBTi single crystals: (A) q 2q X-ray diffraction pattern showing 00l reflections from the crystals surface; (B) transmission electron micrograph in the a(b)ec plane; and (C) the schematic crystal structure.
8000
BiT-BBTi crystals
0 BBTi layer (m = 4)
4
(B) 2 nm
-7
-9
3
2 nm
BiT crystals
0
Temperature (°C)
FIGURE 47.3 Temperature dependence of dielectric permittivity of BiTeBBTi single crystals (1 MHz).
temperature (TC) estimated from the permittivity peak was 540 C, and this TC was intermediate between those of BiT (TC ¼ 675 C) and BBTi (TC ¼ 410 C) [7e10]. Compared with BiTeBBTi (BaBi8Ti7O27) ceramics with TC of 500 C, the BiTeBBTi crystals showed a 40 C higher TC. The higher TC observed for Ba-deficient BiTe BBTi crystals is suggested to originate from the Bi substitution [14] at the perovskite A site. The leakage currents along the a(b)-axis as a function of electric field at 25 C (Fig. 47.4) show that the BiT crystals exhibited a poor value of w107 A/cm2 above 25 kV/cm, while the BBTi crystals showed a lower current of w109 A/cm2. Note that the leakage current of the BiTeBBTi crystals remained a sufficiently low value, which was one-fifth that of the BiT crystals. A simple equivalent circuit of parallel conduction of individual BiT and BBTi layers cannot explain this behavior.
4. GIANT POLARIZATION IN BiTeBBTi CRYSTALS Fig. 47.5 indicates the polarization hysteresis loops of single crystals along the a(b)-axis at 25 C. The Ps and the coercive field (Ec) of BiTeBBTi were 52 mC/cm2 and 120 kV/cm, respectively. Note that this Ps value is larger than those of BiT (46 mC/cm2) and BBTi (16 mC/cm2) crystals. It has been widely known that BiT has the largest Ps among the BLSFs and that the Ps of BBTi is much smaller [13]. If the intrinsic ferroelectric distortions in BiT and BBTi were maintained in the superlattice structure, the Ps of BiTeBBTi would be the average of those for BiT and BBTi (approximately 30 mC/cm2). However, the Ps observed for the BiTeBBTi crystals is much larger than the average Ps. The enhanced Ps of the BiTeBBTi crystals implies that the lattice strain
654
47. DEVELOPMENT OF NOVEL FERROELECTRIC MATERIALS
BiT-BBTi crystals
Polarization (µ C/cm2)
75 50
BiT crystals
25 0 -25
BBTi crystals
-50 -75 -3 00
1 Hz -200
-100
0
100
200
300
E // a axis (kV/cm)
FIGURE 47.5
Polarization hysteresis loops along the a-axis (25 C).
induced by the alternate stacking of m ¼ 3 and m ¼ 4 layers with different cell size promotes ferroelectric distortion. One of the origins of the larger Ps of the BiTeBBTi crystals is the Bi substitution at the Ba site. For strontium bismuth tantalate with Sr-deficient and Bi-excess composition, the Bi substitution at the Sr site occurs, and the charge difference between Bi3þ and Sr2þ is compensated through the formation of Sr vacancies [14]. This enhances Ps from 19 mC/cm2 (SrBi2Ta2O9) to 29 mC/cm2 (Sr0.81Bi2.13Ta2O9) and raises TC from 295 to 410 C [14]. It has been reported for BBTi ceramics with random orientation that the Bi substitution at the Ba site enhances Pr from 14 mC/cm2 (BaBi4Ti4O15) to 18 mC/cm2 (Ba0.9Bi4.07Ti4O15) and leads to an increase in TC from 415 to 440 C. The compositional analysis of the BiTeBBTi crystals shows the Ba-deficient and Bi-excess composition (Ba0.75Bi8.32Ti7O27), indicating that excess Bi is substituted at the Ba site. Indeed, higher TC was observed for the BiTeBBTi crystals (540 C) than for the stoichiometric ceramics (490 C). This is direct evidence of the Bi substitution at the Ba site. However, the effects of the Bi substitution alone cannot explain the large Ps of the BiTeBBTi crystals. It is considered that the lattice distortion induced by the alternate stacking of m ¼ 3 and m ¼ 4 layers plays a dominant role in enhancing Ps of BiTeBBTi. Because BBTi contains Ba ions with large ionic radius at the A site, parameters a and b are larger than those of BiT. The lattice mismatch between the BiT and BBTi reaches 0.42% along the a-axis and 0.88% along the b-axis. The parameters a and b of BiTeBBTi were the averages of those of values for the BiT and BBTi. These results imply that tensile and compressive stresses built up in the m ¼ 3 and m ¼ 4 layers of the superlattice structure, respectively. These stresses are suggested to be concentrated on the Bi2O2 layers that interleave the two kinds
of perovskite block. This crystallographic environment induces a local symmetry breaking of the Bi2O2 layers. All Bi ions of the Bi2O2 layers in BiT and BBTi are identical from the crystallographic point of view. In contrast, the Bi ions of the Bi2O2 layers in the BiTeBBTi are divided into two cations: one is connected to the perovskite blocks of the m ¼ 3 layer and the other is adjacent to that of the m ¼ 4 layer. The symmetry breaking leads to an unusual ferroelectric displacement of the Bi ions of the Bi2O2 layers in the BiTeBBTi. It has been reported that the Bi ions of the Bi2O2 layers in BiTeSrBi4Ti4O15 are displaced along the a-axis (the polar direction) by 2% of the parameter a from the corresponding positions of the high-temperature tetragonal structure. Similar displacements of the Bi ions are expected to the BiTeBBTi, which enhance Ps. In addition to the lattice distortions of the Bi2O2 layers, the alternate stacking of the m ¼ 3 and m ¼ 4 layers is considered to promote the ferroelectric distortions in the perovskite blocks in the BiTeBBTi. Superlattice-structured BiTeBBTi single crystals were grown by a self-flux method, and the structure was confirmed by XRD and transmission electron microscope analyses. Dielectric measurements showed that TC of the BiTeBBTi was 540 C, which is 40 C higher than that of stoichiometric BiTeBBTi ceramics. The Ps of the BiTeBBTi crystals along the a(b)-axis was 52 mC/cm2, which is larger than those of BiT and BBTi crystals. It is suggested that the Bi substitution at the Ba site and the local symmetry breaking of the Bi2O2 layers are partially responsible for the larger Ps of BiTeBBTi.
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