Ferroelectrics Material Aspects Part 13 pptx

Ferroelectrics – Material Aspects 410 Kang, U.; Zhilin, A. A.; Logvinov, D. P.; Onushchenko, A. A.; Savost’yanov, V. A.; Chuvaeva, T. I. & Shashkin, A. V. (2001). Transparent Nd 3+ -activated glass-ceramics in the Li 2 O-Al 2 O 3 -SiO 2 system: physicochemical aspects of their preparation and optical characteristics. Glass Phys. Chem. 27, 4, 344-352, ISSN 1087-6596. Lin, H.; Liu, K.; Pun, E. Y. B.; Ma, T. C.; Peng, X.; An, Q. D.; Yu, J. Y. & Jiang, S. B. (2004). Infrared and visible fluorescence in Er 3+ -doped gallium tellurite glasses. Chem. Phys. Lett. 398, 1-3, 146-150, ISSN 0009-2614. Lynch, C. T. (1975). CRC Handbook of Materials Science, Vol. III, CRC Press, p. 170, Cleveland, Ohio. Mandal, D.; Banerjee, H. D.; Goswami, M. L. N. & Acharya, H. N. (2004). Synthesis of Er 3+ and Er 3+ :Yb 3+ doped sol-gel derived silica glass and studies on their optical properties. Bull. Mater. Sci. 27, 4, 367-372, ISSN 0250-4707. Mizuuchi, K. & Yamamoto, K. (1995). 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Introduction Many ferroelectric oxides possess the ABO 3 perovskite structure (Mitchell, 2002), in which the A-site cations are typically larger than the B-site cations and similar in size to the oxygen anion. Figure 1 shows a schematic drawing for this structure, where the A cations are surrounded by 12-anions in the cubo-octahedral coordination and the B cations are surrounded by 6-anions in the octahedral coordination. An ideal perovskite exhibits a cubic space group Pm3m. This structure is centrosymmetric and cannot allow the occurrence of ferroelectricity that is the presence of a switchable spontaneous electric polarization arising from the off-center atomic displacement in the crystal (Jaffe et al., 1971; Lines & Glass, 1977). The instability of ferroelectricity in the perovskite oxides is generally discussed with the Goldschimidt tolerance factor (t) (Goldschmidt, 1926 and Fig. 2), AO BO ()/2()trr rr=+ +, where r O , r A , and r B are the ionic radii of the O, A, and B ions. For a critical value t=1, the cubic paraelectric phase is stable. This unique case can be found in SrTiO 3 , which has an ideal cubic perovskite structure at room temperature and doesn’t show ferroelectricity down to the absolute 0 K (Müller & Burkard, 1979). However, ferroelectricity can be induced by the substitution of O 18 in this quantum paraelectric system at T<T c ~23 K (Itoh et al., 1999). When t>1, since the B-site ion is too small for its site, it can shift off-centeringly, leading to the occurrence of displacive-type ferroelectricity in the crystal. Examples of such materials are BaTiO 3 and KNbO 3 (Shiozaki et al., 2001). On the other hand, for t<1, the perovskite oxides are in general not ferroelectrics because of different tilts and rotations of BO 6 octahedra, which preserve the inversion symmetry. But exceptions may be found in the Bi- based materials, in which large A-site displacement is observed. This large A-site displacement is essentially attributed to the strong hybridization of Bi with oxygen (Baettig et al., 2005). Similar cases are observed in Pb-based materials, which commonly have large Pb displacement in the A-site (Egami et al., 1998) and strong covalent nature due to the unique stereochemistry of Pb (Cohen, 1992; Kuroiwa et al., 2001). Although BaTiO 3 - and PbTiO 3 -based ceramics materials have been widely used in electronic industry (Uchino, 1997; Scott, 2000), there remain some importance issues to be solved. One of such challenges is to seek novel compounds to replace the Pb-based materials, which have a large Pb-content and raises concerns about the environmental pollution (Saito et al. ,2004; Rodel et al.,2009). Ferroelectrics – Material Aspects 414 Fig. 1. The structure of an ABO 3 perovskite with the origin centered at (a) the B-site ion and (b) the A-site ion. 0.96 0.98 1.00 CaTiO 3 PbTiO 3 BaTiO 3 CdTiO 3 KTaO 3 KNbO 3 FerroelectricAntiferroelectric Quantum paraelectric (like) Tolerance fa ctor t （Sr,Ca)TiO 3 SrTiO 3 1.02 1.04 1.06 BaZrO 3 AgNbO 3 NaNbO 3 PbZrO 3 BO 6 tilting(zone boundary) BiFeO 3 B-site off-center(zone center) BiMnO 3 P s Fig. 2. Tolerance factor of typical dielectric oxides. The discovery of extremely large polarization (52 µC/cm 2 ) under high electric field in the AgNbO 3 ceramics (Fu et al., 2007) indicates that Ag may be a key element in the designs of lead-free ferroelectric perovskite oxides (Fu et al., 2011a). With the advance of first-principles calculations (Cohen, 1992) and modern techniques of synchrotron radiation (Kuroiwa et al., 2001), we now know that the chemical bonding in the perovskite oxides is not purely ionic as we have though, but also possesses covalent character that plays a crucial role in the occurrence of ferroelectricity in the perovskite oxides (Cohen, 1992; Kuroiwa et al., 2001 ). It is now accepted that it is the strong covalency of Pb with O that allows its large off-center in the A-site. Although Ag doesn’t have lone-pair electrons like Pb, theoretical investigations suggest that there is hybridization between Ag and O in AgNbO 3 (Kato et al., 2002; Grinberg & Rappe, 2003,2004), resulting in a large off-center of Ag in the A-site of perovskite AgNbO 3 (Grinberg & Rappe, 2003,2004). This prediction is supported by the results from X-ray photoelectron spectroscopy, which suggest some covalent characters of the chemical bonds between Ag and O as well as the bonds between Nb and O (Kruczek et al., 2006). Moreover, bond-length analysis also supports such a theoretical prediction. Some of the bond-lengths (~2.43 Å) in the structure (Sciau et al., 2004; Yashima et al. 2011) are significantly less than the sum of Ag + (1.28 Å) and O 2- (1.40 Å) ionic radii (Shannon, 1967). All these facts make us believe that AgNbO 3 may be used as a base compound to develop novel ferroelectric materials. Along such a direction, some interesting results have been obtained. It was found that ferroelectricity can be Ferroelectricity in Silver Perovskite Oxides 415 induced through the chemical modification of the AgNbO 3 structure by substitution of Li (Fu et al., 2008, 2011a), Na(Arioka, 2009; Fu et al., 2011b), and K (Fu et al., 2009a) for Ag. Large spontaneous polarization and high temperature of para-ferroelectric phase transition were observed in these solid solutions. In the following sections, we review the synthesis, structure, and dielectric, piezoelectric and ferroelectric properties of these solid solutions together with another silver perovskite AgTaO 3 (Soon et al.,2009, 2010), whose solid solutions with AgNbO 3 are promising for the applications in microwaves devices due to high dielectric constant and low loss (Volkov et al. 1995; Fortin et al., 1996; Petzelt et al., 1999; Valant et al., 2007a). 2. AgNbO 3 2.1 Synthesis Both single crystal and ceramics of AgNbO 3 are available. Single crystal can be grown by a molten salt method using NaCl or V 2 O 5 as a flux (Łukaszewski et al., 1980; Kania, 1989). Ceramics samples can be prepared through a solid state reaction between Nb 2 O 5 and silver source (Francombe & Lewis, 1958; Reisman & Holtzberg, 1958). Among the silver sources of metallic silver, Ag 2 O and Ag 2 CO 3 , Ag 2 O is mostly proper to obtain single phase of AgNbO 3 (Valant et al., 2007b). For silver source of Ag 2 O, thermogravimetric analysis indicates that phase formation can be reach at a firing temperature range of 1073-1397 K (Fig. 3). The issue frequently encountered in the synthesis of AgNbO 3 is the decomposition of metallic silver, which can be easily justified from the color of the formed compounds. Pure powder is yellowish, while grey color of the powder generally indicates the presence of some metallic silver. It has been shown that the most important parameter that influences the phase formation is oxygen diffusion (Valant et al., 2007b). In our experiments to prepare the AgNbO 3 ceramics, we first calcined the mixture of Ag 2 O and Nb 2 O 5 at 1253 K for 6 hours in O 2 atmosphere and then sintered the pellet samples for electric measurements at 1323 K for 6 hours in O 2 atmosphere (Fu et al., 2007). Insulation of these samples is very excellent, which allows us to apply extremely high electric field to the sample (Breakdown field >220 kV/cm. For comparison, BaTiO 3 ceramics has a value of ~50 kV/cm.) 2.2 Electric-field induced ferroelectric phase Previous measurements on D-E hysteresis loop by Kania et al. (Kania et al., 1984) indicate that there is small spontaneous polarization P s in the ceramics sample of AgNbO 3 . P s was estimated to be ~0.04 µC/cm 2 for an electric field with an amplitude of E=17 kV/cm and a frequency of 60 Hz. Our results obtained at weak field have confirmed Kania’s reports (Fig. 4 and Fu et al., 2007). The presence of spontaneous polarization indicates that AgNbO 3 must be ferroelectric at room temperature. The good insulation of our samples allows us to reveal a novel ferroelectric state at higher electric field. As shown in Fig.4, double hysteresis loop is distinguishable under the application of E~120 kV/cm, indicating the appearance of new ferroelectric phase. When E>150 kV/cm, phase transformation is nearly completed and very large polarization was observed. At an electric field of E=220 kV/cm, we obtained a value of 52 µC/cm 2 for the ceramics sample. Associating with such structural change, there is very large electromechanical coupling in the crystal. The induced strain was estimated to be 0.16% for the ceramics sample (Fig.5). The D-E loop results unambiguously indicate that the atomic displacements are ordered in a ferri-electric way rather than an anti-ferroelectric way in the crystal at room temperature. Ferroelectrics – Material Aspects 416 400 600 800 1000 1200 1400 96 97 98 99 100 101 1333 K 1397 K TG (%) T (K) 1073 K 730 K Ag 2 O Ag In air Fig. 3. Thermogravimetric curves of the AgNbO 3 formation in air using Ag 2 O or metallic Ag powder as the silver source (Valant et al., 2007b). The curves are normalized to a weight of single-phase AgNbO 3 . Temperatures of phase formation completed and decomposition are also indicated. For case of Ag 2 O, decomposition of Ag 2 O into Ag and oxygen occurs at temperature of ~730 K. -60 -30 0 30 60 -4 -2 0 2 4 -200 -100 0 100 200 -10 0 10 -150 -100 -50 0 50 100 150 -10 -5 0 5 10 -100 0 100 -10 -5 0 5 10 -200 -100 0 100 200 -40 0 40 -200 -100 0 100 200 -50 0 50 D (μC/cm 2 ) E (kV/cm) Fig. 4. D-E hysteresis loops for poly-crystalline AgNbO 3 at room temperature. -200 -100 0 100 200 0.00 0.05 0.10 0.15 Strain (%) E (kV/cm) Fig. 5. Strain vs electric field for poly-crystalline AgNbO 3 at room temperature. Ferroelectricity in Silver Perovskite Oxides 417 2.3 Room-temperature structure There are many works attempting to determine the room-temperature structure of AgNbO 3 (Francombe & Lewis, 1958; Verwerft et al., 1989; Fabry et al., 2000; Sciau et al., 2004; Levin et al., 2009). However, none of these previous works can provide a non-centrosymmetric structure to reasonably explain the observed spontaneous polarization. Very recently, this longstanding issue has been addressed by R. Sano et al. (Sano et al., 2010). The space group of AgNbO 3 has been unambiguously determined to be Pmc2 1 (No. 26) by the convergent- beam electron diffraction (CBED) technique, which is non-centrosymmetric and allows the appearance of ferroelectricity in the crystal (Fig.6). Pmc2 1 ( T =298K) Site x y z U ( Å 2 ) A g 1 4c 0.7499(3) 0.7468(3) 0.2601(5) 0.0114(2) A g 2 2b 1/2 0.7466(6) 0.2379(5) 0.0114(2) A g 3 2a 0 0.7424(4) 0.2759(6) 0.0114(2) Nb1 4 c 0.6252(2) 0.7525(5) 0.7332(2) 0.00389(18) Nb2 4 c 0.1253(2) 0.24159 0.27981 0.00389(18) O1 4 c 0.7521(9) 0.7035(12) 0.7832(24) 0.0057(5) O2 2b 1/2 0.804(3) 0.796(3) 0.0057(5) O3 4 c 0.6057(7) 0.5191(18) 0.4943(18) 0.0057(5) O4 4 c 0.6423(7) 0.0164(18) 0.539(2) 0.0057(5) O5 2a 0 0.191(3) 0.256(3) 0.0057(5) O6 4 c 0.1339(9) 0.0410(17) 0.980(2) 0.0057(5) O7 4 c 0.1154(8) 0.4573(17) 0.5514(19) 0.0057(5) Table 1. Structural parameters of AgNbO 3 at T=298K (Yashima et al., 2011). Number of formula units of AgNbO 3 in a unit cell: Z=8. Unit-cell parameters: a = 15.64773(3) Å, b = 5.55199(1) Å, c = 5.60908(1) Å, α=β=γ= 90 deg., Unit-cell volume: V = 487.2940(17) Å 3 . U (Å 2 )=Isotropic atomic displacement parameter. Fig. 6. Convergent-beam electron diffraction (CBED) pattern of AgNbO 3 taken at the [100] incidence. In contrast to a mirror symmetry perpendicular to the b*-axis, breaking of mirror symmetry perpendicular to the c*-axis is seen, indicating that spontaneous polarization is along the c-direction (Taken by R. Sano & K. Tsuda (Sano et al., 2010)). Ferroelectrics – Material Aspects 418 On the basis of this space group, M. Yashima (Yashima et al., 2011) exactly determined the atom positions (Table 1) in the structure using the neutron and synchrotron powder diffraction techniques. The atomic displacements are schematically shown in Fig.7. In contrast to the reported centrosymmetric Pbcm (Fabry et al. 2000; Sciau et al. 2004; Levin et al. 2009), in which the Ag and Nb atoms exhibit antiparallel displacements along the b-axis, the Pmc2 1 structure shows a ferri-electric ordering of Ag and Nb displacements (Yashima et al., 2011) along the c-axis of Pmc2 1 orthorhombic structure. This polar structure provides a reasonable interpretation for the observed polarization in AgNbO 3 . Fig. 7. (a) Ferrielectric crystal structure of AgNbO 3 (Pmc2 1 ) at room temperature. The atomic displacements along the c-axis lead to the spontaneous polarization in the crystal. (b) For comparison, the patterns for the previously reported Pbcm (Sciau et al., 2004) are also given. A cross (+) stands for the center of symmetry in the Pbcm structure. (by M. Yashima (Yashima et al., 2011) ). 2.4 Dielectric behaviours and phase transitions Initial works on the phase transitions of AgNbO 3 and their influence on the dielectric behaviors were reported by Francombe and Lewis (Francombe & Lewis, 1958) in the late 1950s, which trigger latter intensive interests in this system (Łukaszewski et al., 1983; Kania, 1983, 1998; Kania et al., 1984, 1986; Pisarski & Dmytrow, 1987; Paweczyk, 1987; Hafid et al., 1992; Petzelt et al., 1999; Ratuszna et al., 2003; Sciau et al., 2004). The phase transitions of AgNbO 3 were associated with two mechanisms of displacive phase transition: tilting of oxygen octahedra and displacements of particular ions (Sciau et al., 2004). Due to these two mechanisms, a series of structural phase transitions are observed in AgNbO 3 . The results on dielectric behaviors together with the reported phase transitions are summarized in Fig.8. Briefly speaking, the structures of the room-temperature (Yashima et al., 2011) and the high temperature phases (T> T O1-O2 =634 K ) (Sciau et al., 2004) are exactly determined, in contrast, those of low-temperature (T<room temperature) and intermediate phases ( T C FE =345 K <T< T O1-O2 ) remain to be clarified. In the dielectric curve, we can see a shoulder around 40 K. It is unknown whether this anomaly is related to a phase transition or not. It should be noticed that Shigemi et al. predicted a ground state of R3c rhombohedra phase similar to that of NaNbO 3 for AgNbO 3 (Shigemi & Wada, 2008). Upon heating, there is an anomaly at T C FE =345 K, above which spontaneous polarization was reported to disappear (Fig.8 (c) and Kania et al., 1984). At the same temperature, anomaly of lattice distortion was observed (Fig.8 (c) and Levin et al., 2009). The dielectric anomaly at T C FE =345 K was attributed to be a ferroelectric phase transition. Upon further heating, there is a small peak at T=453 K, which [...]... dielectric response of the disordered antiferroelectric Ag(Ta,Nb)O3 system, Ferroelectrics 223, pp 235-246 Pirc, R & Blinc, R (1999) Spherical random-bond–random-field model of relaxor ferroelectrics, Phys Rev B 60, pp 134 70 -134 78 Pisarski, M & Dmytrow, D (1987) Phase transitions in ceramic AgNbO3 investigated at high hydrostatic pressure, Ferroelectrics 74, pp 87-93 Ratuszna, A.; Pawluk, J &Kania, A (2003)... ferrielectric within this composition range This is also supported by the temperature dependence of dielectric constant (Fig.16) On the other hand, for the Na-rich composition, particularly, x>0.8, we 426 Ferroelectrics – Material Aspects observed large remanent polarization with value close to the saturation polarization at high field This result indicates that a normal ferroelectric phase is stable in... into the sealed zirconia tube after the evacuation Upon heating, the pressure of sealed oxygen gas increased and reached ~13 atm when the powder compact was sintered at 1573K for 2 hours This eventually led to formation of dense polycrystalline AgTaO3 432 Ferroelectrics – Material Aspects Fig 22 Schematic diagram of the self-customized furnace employed for the sintering at high oxygen pressure 6.2 Phase... considering the practical applications 9 Acknowledgment Part of this work was supported by the Collaborative Research Project of Materials and Structures Laboratory of Tokyo Institute of Technology, and Grant-in-Aid for Scientific Research, MEXT, Japan 436 Ferroelectrics – Material Aspects 400 300 200 800 x=0 600 1MHz 100kHz 10kHz 1kHz x=0.035 1MHz 100kHz 10kHz 1kHz 400 200 100 600 ε ε 300 x=0.05 800 x=0.002... the p-cut Ag1-xLixNbO3 single crystals 3.4 Dielectric behaviours and proposed phase diagram Figure 13 shows the dielectric behaviours of the ferroelectric Ag1-xLixNbO3 solid solutions For comparison, the temperature dependence of dielectric constant of AgNbO3 is also 424 Ferroelectrics – Material Aspects shown It can be seen that solid solution with x > xc shows different temperature evolutions of... Simulation 34, pp 1105-1114 Shiozaki, Y.; Nakamura, E & Mitsui, T (2001) Ferroelectrics and Related Substances, Vol 36, Pt A1, Springer, Berlin Soon, H P.; Taniguchi, H & Itoh, M (2009) Ferroelectricity triggered in the quantum paraelectric AgTaO3 by Li-substitution, Appl Phys Lett 95, pp 242904 442 Ferroelectrics – Material Aspects Soon, H P.; Taniguchi, H &Itoh, M (2010) Dielectric and soft-mode... also associated with an anomaly of 220o d-spacing However, current structural investigations using x-ray and neutron diffraction do not find any symmetric changes associated with the 420 Ferroelectrics – Material Aspects dielectric anomalies at 456 K and 540 K, and the structure within this intermediate temperature range was assumed to be orthorhombic Pbcm (Sciau et al., 2004) At T=TCAFE=631 K, there... 50 0 20 300 K PDF No 01-072 -138 3 30 40 50 2θ (Deg) 60 70 80 Fig 23 XRD traces for AgTaO3 obtained at 300 K and 68.4 K together with the standard pattern given by the powder diffraction file (PDF) No 01-072 -138 3 433 Ferroelectricity in Silver Perovskite Oxides (a) ε 360 AgTaO3 270 Fitting with Barrett's relation ε=C/((T1/2)coth(T1/2T)-T0)+ε0 4 C=4.80*10 K T1=43.4K 180 T0= -137 K ε0=41.5 90 1 10 (b) 0.010... the Barrett’s relation (Barrett, 1952) that is characteristic for the quantum paraelectric system (Abel, 1971; Höchli & Boatner,1979; Itoh et al., 1999), suggesting that AgTaO3 may be a 434 Ferroelectrics – Material Aspects quantum paraelectric On the other hand, two step-like dielectric anomalies corresponding to the phase transitions from monoclinic to tetragonal and tetragonal to cubic were observed... to improve the material performance, to understand the basic physics of the ferroelectricity/piezoelectricity of the materials, and to seek novel promising compounds among the discovered solutions or alloys with other ferroelectric systems Moreover, integration techniques of thin films are also a direction for the future works when considering the practical applications 9 Acknowledgment Part of this . anti-ferroelectric way in the crystal at room temperature. Ferroelectrics – Material Aspects 416 400 600 800 1000 1200 1400 96 97 98 99 100 101 133 3 K 139 7 K TG (%) T (K) 1073 K 730 K Ag 2 O Ag In. the Pb-based materials, which have a large Pb-content and raises concerns about the environmental pollution (Saito et al. ,2004; Rodel et al.,2009). Ferroelectrics – Material Aspects 414. constant (Fig.16). On the other hand, for the Na-rich composition, particularly, x>0.8, we Ferroelectrics – Material Aspects 426 observed large remanent polarization with value close