In 1951, the notion of anti-ferroelectricity based on the phenomenological theory was firstly proposed by C. Kittel (Kittel, 1951), and who predicted exist of the anti-ferroelectric materials and its some intrinsic characteristics. Subsequently, a double hysteresis (P–E) loop and a ferroelectric-antiferroelectric (FE-AFE) phase transition were observed in PbZrO3 (Shirane et al., 1951, 1952) and Pb(Zr, Ti)O3 (Sawaguchi, 1953) ceramics with a perovskite structure. Since then, the double hysteresis loop, one important macroscopic effect, has been regarded as a typical characteristic of antiferroelectric materials. This kinds of antiferroelectric behavior is also observed in Pb(Zr, Sn, Ti)O3 and Pb(La, Zr, Sn, Ti)O3 ceramics (Berlincourt, 1963, 1964; Biggers & Schulze, 1974; Gttrttritja et al., 1980; Shebanov et al., 1994).
Interestingly, the double hysteresis loops have been observed in BaTiO3 (Merz, 1953), BaTiO3-based (Ren, 2004; Zhang & Ren, 2005, 2006; Liu et al., 2006), (Na0.5Bi0.5)TiO3-based (Takenaka, 1991; Sakata & Masuda, 1974; Tu et al., 1994; Sakata et al., 1992), (Ba, Sr)TiO3 (Zhang et al., 2004), KNbO3 (Feng & Ren, 2007, 2008), BiFeO3 (Yuen et al., 2007) and other lead-based perovskite ceramics such as Pb(Yb0.5Ta0.5)O3 (Yasuda & Konda, 1993), Pb(Fe2/3W1/3)O3-Pb(Co1/2W1/2)O3 (Uchino & Nomura, 1978), Pb(Sc0.5Ta0.5)O3 (Chu et al., 1993) and Pb(Co1/2W1/2)O3 (Hachiga et al., 1985) over the past decades. However, the observed double hysteresis loops have different physical origins.
For the physical origins of the double P-E loops observation in the perovskite structure materials, they can be generally classified into several groups below: (1) the double P-E loops result from antiferroelectric components. i.e., the observed double P-E loops in PbZrO3 and Pb(Zr, Ti)O3 ceramics. (2) the double P-E loops result from an electric field-induced antiferroelectric-to-ferroelectric phase transition as shown observed in Pb(La, Zr, Sn, Ti)O3 ceramics. (3) two successive paraelectric-antiferroelectric-ferroelectric (PE-AFE-FE) phase transitions is often observed in highly ordered perovskite lead-based and lead-free complex compounds, and the double P-E loops are expected to accompany this phase transition. Such case often occur in Pb(Yb0.5Ta0.5)O3, Pb(Fe2/3W1/3)O3-Pb(Co1/2W1/2)O3, Pb(Sc0.5Ta0.5)O3 and Pb(Co1/2W1/2)O3 and (Na0.5Bi0.5)TiO3-based ceramics. (4) an aging effect far below Curie temperature ( Tc ) can induce the double P-E loops observation in many kind different compositional ceramics such as acctor-doped BaTiO3 ceramics, Mn-doped (Ba, Sr)TiO3 ceramics, KNbO3-based ceramics and BiFeO3 ceramics. The aging-induced double P-E loops has been the highlights based on the high-quality publications and their cited times. (5) for the observed double P-E loops in BaTiO3 crystals at the Curie point, its origin can not attributed to any one group mentioned above. The BaTiO3 crystals go from a paraelectric to a ferroelectric state, while (Pb, Ba)ZrO3 ceramics go from an antiferroelectric to a ferroelectric state when an electric field is applied. It is generally believed that the Curie point of the BaTiO3 crystal shifts to higher temperature when a dc bias field is applied. The more detailed discussion about the origin of the double P-E loops in the BaTiO3 crystal can consult the previous literatures titled “A theory of double hysteresis for ferroelectric crystals” (Srivastava, 2006).
After a brief review of the physical origin of double hysteresis loops in different perovskite structure (A+B5+O3, A2+B4+O3, AA’BO3, ABB’O3, etc) ceramic materials, this chapter begins with aging effect, namely a gradual change in physical properties with time. This is followed by discussions of the aging-induced double hysteresis loops in Bi doped (Ba, Ca)TiO3 and Bi doped (Ba, Sr, Ca)TiO3 ferroelectric ceramics. Some emphasis will be on the roles of acceptor-doping and donor-doping in understanding the physics of these materials.
2. Experimental procedures
2.1. Ceramics synthesis
A conventional solid reaction route was employed to synthesize ceramics samples. Reagent grade BaCO3 (99.8%), Bi2O3 (99.8%), SrCO3 (99.8%), CaCO3 (99.8%) and TiO2 (98%) as the raw materials were weighed according to the compositions (Ba1-xCax)1-1.5yBiyTiO3 (Bi-BCT, x =0.10, 0.20 and 0.30, y=0.05) and (Ba1-xCax/2Srx/2)1-1.5yBiyTiO3 (Bi-BCST, x=0.10, 0.20 and 0.40, y=0 and 0.05). (Ba0.925Bi0.05)(Ti0.90Ca0.10)O3 (designated hereafter Bi-BTC) was prepared to compare the effect of Ca substitution at the Ti sites of Bi-BCT. After ball-milled in alcohol for 6 h using agate balls in a planetary mill, the slurry was dried, and then calcined at 1100 o C for 4-5h. The calcined powder was ball milled and dried again to obtain homogeneous powder. Pellets of 10 mm in diameter and ~1 mm thick were pressed using 5 % PVA binder. Slow heating at 500 °C for 3-4h burned out the binder. The samples were sintered at 1240 oC -1300°C in air for 3 h, with heating rates of 200°C/h. The samples were cooled with the furnace.
The phase structures of the ceramics at different temperature were checked by the X-ray powder diffraction (XRD, D/Max2200 RZGAKV: Rigaku Inc.D, Japan) on an automated Rigaku D/max 2400 X-ray diffractometer with rotating anode using CuK radiation. The microstructures were examined by a scanning electron microscopy (SEM, Quanta 200 FEG System: FEI Co., USA) with X-ray energy dispersive spectroscopy (EDS) for chemical analysis. Raman scattering investigation was performed at room temperature by using an ALMEGA dispersive Raman spectrometer (ALMEGA, Therm Nicolet, Madison,WI).
2.3. Property measurements
After polishing, the dimensions were measured before silver electrodes were deposited on the pellets, then the specimens were fired at 810 °C for 10 mins. Dielectric properties at frequencies ranging from 0.1kHz to 100 kHz were measured with an Agilent 4284A LCR meter, as samples were heated at a rate of 2 °C/min from negative 80 to positive 200 °C. Hysteresis loops were measured in a wide temperature range using a computer-controlled, modified Sawyer-Tower circuit at frequency of 1 Hz. Current-field relation was measured on an automatic ferroelectric test system of aixACT TF-ANALY2ER2000. Applied electric field signal is triangular, and a period time is a second.
3. Results and discussion
3.1. Structural analysis
It has been reported that the solubility limit of Bi is around 5 at. % in BaTiO3 (Zhou et al., 1999), and around 10 at. % in (Ba0.2Sr0.8)TiO3 (Zhou et al., 2000), respectively. Moreover, 5 at. % of Bi doping can be fully incorporated into the perovskite lattice of Ba1-xSrxTiO3 (x < 0.80) (Zhou et al., 2001). It is then possible to assume that 5 at. % of Bi doping can be fully incorporated into the perovskite lattice of (Ba1-xCax)TiO3 (Bi-BCT, x=0.10, 0.20 and 0.30) and (Ba1-xCax/2Srx/2)TiO3 (Bi-BSCT, x=0.10, 0.20 and 0.40). XRD analysis confirmed this assumption. Fig. 1 show the XRD patterns of 5 at. % of Bi doped (Ba1-xCax)TiO3 ceramics. The XRD patterns of all the compositions showed mainly a single perovskite phase.
The structural evolution of Bi-doped BCST ceramics samples from x=0.10 to 0.40 is shown in Fig. 2. The XRD results of 5 at. % Bi-doped BCST are consistent with those of (Ba1-xSrx)1-1.5yBiyTiO3 (Zhou et al., 2001) and Bi-BCT, showing that they are a single phase. The crystalline symmetry of Bi-doped BCST ceramics is a rhombohedral for x=0.40. However, it tends appreciably towards the tetragonal for x=0.20. The rhombohedral phase is illustrated by the increased splitting between the (021) and (003) peaks in two-theta near 39° while the tetragonal phase is illustrated by the increased splitting between the (002) and (200) peaks in two-theta near 45°. There are splits in the peaks observed at two-theta of about 39° and 45°, respectively, as shown in Fig.3, indicating the coexistence of the tetragonal and rhombohedral phases of Bi-BCST for x=0.10 and that of Bi-BCT for x=0.10 and 0.20.
To check the crystal symmetry, X-ray powder diffraction (XRD) at different temperatures was performed for Bi-BCT and Bi-BSCT (x=0.10, y=0.05). Fig. 4 shows the form of one of the structure sensitive maxima in the XRD patterns. It is found from the changes in the (002)/(200) refection with temperature that Bi-BCT ceramics have a tetragonal structure throughout the whole temperature range from 280 K up to 320 K while Bi-BSCT ceramics have a tetragonal structure throughout the whole temperature range from 280 K up to 300 K.
3.2. Ferroelectric properties
Fig. 5 and Fig. 6 show the plots of polarization P vs electrical field E (P-E) at elevate temperature for Bi-BCT and Bi-BCST (x=0.10 and 0.20, y=0.05), respectively. A well-behaved hysteresis loop can be observed at 280 K for Bi-BCT (x=0.10 and 0.20). When the temperature was increased, the double hysteresis loop, typical of antiferroelectric materials, was observed at 300 K for Bi-BCT and at 280 K for Bi-BSCT, with a nearly linear P-E relationship at the mid-part of the hysteresis loop. With further increased temperature, hysteresis could not be detected, the P-E loops were slim and showed dielectric quasilinearity over a wide electric field range.
Note that a remarkable double-like P-E loop is not observed for Bi-BCT (x=0.30). However, the E dependence of P and current density (J) (J-E) relationships have shown that four remarkable J peaks are observed for Bi-BCT (x=0.30), indecating of the existence of double-like P-E loop for x=0.30 in Bi-BCT. Talking about that the present chapter foucses on the double P-E loops in Bi-BCT and Bi-BSCT ceramics, the related data for J-E relationship are not shown here.
The P-E loops transform from the normal hysteresis loop to an interesting double-like hysteresis loop, and then to the nearly linear one with increasing temperature. These characteristics obtained from the loops at elevated temperature suggest that different ferroelectric behaviors have occurred in Bi-BCT. It seems that two successive paraelectric-antiferroelectric-ferroelectric (PE-AFE-FE) phase transitions exist in Bi-BCT ceramics. By contrast, this similar transform from the normal to the double to the quasilinear can not be detected in Bi-BSCT ceramics. Polarization data at a lower temperature could not be obtained for Bi-BSCT ceramics due to the limitation of the present measuring equipment. However, one can assume that an ferroelectric–antiferroelectric transformation occurs at a lower temperature (temperature is less than 280 K) for Bi-doped BCST ceramics. This assumption needs to be confirmed experimentally. The appropriate research is now being carried out.
A linear dielectric response is observed for Bi-BCT and Bi-BSCT, that is, the P-E relationship is close to linear, unlike that for normal ferroelectrics and lead-based relaxors. The quasi-linear P-E relationship looks somewhat similar to that taken on the antiferroelectric sample. In this case, the linear P-E dependence is typical for antiferroelectric materials. In order to demonstrate that dielectric quasi-linearity in a certain electric field range is a typical antiferroelectric behavior or not, a complete investigation including the electric field-induced and temperature-induced structure phase transition has been performed in our previous literatures.
3.3. Dielectric properites
The temperature dependence of the dielectric constant and the dielectric loss of Bi-BCT and Bi-BSCT ceramics for different frequencies are shown in Fig. 7(A)-(D). The dielectric constant has a broad maximum at a temperature of the peak dielectric constant ( Tm ). Tm increases with increasing frequency. For example, Tm is equal to 341 K at 1 kHz and 347 K at 1 MHz for Bi-BCT (x=0.10), respectively. With decreasing temperature, the value of dielectric loss increases rapidly around the temperature of the dielectric loss. With increasing frequencies, the peak dielectric constant decreases and Tm shifts to high temperature. Such change trends of the dielectric constant and the dielectric loss with the frequencies and the temperatures is a type of dielectric relaxation behavior, which has been reported in detail in the solid state physics text book.
The possible mechanism for the relaxor behavior observation in Bi-doped SrTiO3 (Ang et al., 1998), Bi-doped Ba1-xSrxTiO3 (Zhou et al., 2001), Ca-doped SrTiO3 (Bednorz & Müller, 1984) and Li-doped KTaO3 (Toulouse et al., 1994) has been discussed in detail in the previous publications. A widely accepted viewpoint is that the dielectric relaxation behavior in these systems was due to a random electric field induced ferroelectric domain state. According to their viewpoint, the Bi3+ ions which substitute for A-site ions in BCT and BCST ceramics can also be located at off-center positions and A-site vacancies may also appear to compensate for the charge misfit arising from the A-site ions substituted by Bi3+ ions. A random electric field formed by off-center Bi3+ ions and Bi3+–VA dipoles would then suppress the ferroelectricity of BCT and BCST and result in the relaxor behavior observed for Bi-doped BCT and BCST. If Ca2+ ions can locate at B-sites like Ti4+ ions, to balance the charge misfit, a next-neighbor oxygen can be vacant, and form a Ca2+–VO neutral center. Such Ca2+–VO centers form dipoles and thus set up local electric fields, which suppress the ferroelectricity of BCT and BCST and result in the relaxor behavior observed in BCT and BCST.
In most cases of ferroelectric phase transition, where the new ordered phase originates from structural changes, there will be a peak in the dielectric spectrum but not all peculiarities or peaks correspond to a structural phase transition. For example, All classical relaxors, such as Pb(Mg1/3Nb2/3)O3 (PMN) and low x (1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (Bokov & Ye, 2006), show the dielectric peaks but do not undergo the ferroelectric (or antiferroelectric) phase transition. Therefore, the dielectric peak may only indicate the possible phase transition. If the FE-AFE-PE transition has occurred in Bi-BCT, there will be two corresponding dielectric peak in the dielectric spectrum. However, the dielectric spectrum of Bi-BCT and Bi-BSCT show only one peak at temperature ranging from 190 K to 428 K as seen in Fig.7.
On the other hand, there is no direct indication of the appearance of antiferroelectric components in (Ba,Ca)TiO3 (Han et al., 1987; Zhuang, et al., 1987; Mitsui & Westphal, 1961; Baskara & Chang, 2003). Therefore, the aging-induced effect should be responsible for the double ferroelectric hysteresis observation in Bi-BCT and Bi-BSCT ceramics.
3.4. Ferroelectric aging effect
The interesting aging-induced double P-E hysteresis loop was also reported in various ferroelectric systems. Many mechanisms such as the grain boundary effect (Karl & Hardtl, 1978), the domain-wall pinning effect (Postnikov et al, 1970), the volume effect (Lambeck & Jonker, 1986) and symmetry-conforming short range ordering (SC-SRO) mechanism of point defects (Ren, 2004; Zhang & Ren, 2005, 2006; Liu et al., 2006) have been proposed to explain this phenomenon. Although all these models seem to be able to provide a self-consistent explanation of the double hysteresis loop in aged ferroelectric crystals, the grain boundary effect cannot explain the perfect double P-E loop exists in the aged single-crystal sample, the domain-wall pinning effect cannot explain the restoration of the initial multidomain state from a single-domain state because there would be no domain wall to be dragged back, and the volume effect is based on a key assumption that there exist dipolar defects and they follow spontaneous polarization after aging (Zhang & Ren, 2005, 2006). Compared all these models, SC-SRO mechanism, which also is a volume effect, provides a microscopic explanation for the origin of aging, and it involves no assumption. The conformation of the defect dipole with spontaneous polarization naturally comes from the symmetry-conforming property of the defects. Generally, all these models agree that defects play a decisive role in the aging induced phenomena. However, they differ much in the driving force for defect migration.
To check further whether there is a diffusional aging effect, the samples were “de-aged” by holding them at 470 K for 1 h, followed by a quick cooling to room temperature above Curie temperature. The room temperature hysteresis loops were measured simultaneously. Fig. 8 shows the experimental results of the hysteresis loops for the Bi-BCT in de-aged (fresh) and aged state, respectively. The de-aged sample shows a normal hysteresis loop, but all the aged samples show interesting double hysteresis loops. The change from the single P-E loops in the de-aged sample to the double P-E loops in the aged sample should exclude antiferroelectric components and any electric field-induced PE-FE phase transition near Curie temperature. It indicates that the aging effect must be responsible for the double P-E loops observed in Bi-BCT.
3.5. Raman analysis
The aging effect in acceptor-doped ferroelectrics is generally considered to be due to the migration of oxygen vacancies (which are highly mobile) during aging (Ren, 2004; Zhang & Ren, 2005, 2006; Liu et al., 2006; Zhang et al., 2004; Feng & Ren, 2007, 2008). However, the O2- vacancies in our Bi-BCT and Bi-BSCT samples were not formed artificially by substitution of lower-valence ions for Ti ions on the B-sites. To understand the aging effect in Bi-BCT, we need to analyze its defect structure. For Bi doped BCT and BSCT with the ABO3 pervoskite structure [(Ba0.90Ca0.10)0.925Bi0.05TiO3 and (Ba0.90Sr0.05Ca0.05)0.925Bi0.05TiO3], there are two possible vacancies: first, the Bi3+ ions substituted for A-site divalent ions (Ba2+, Sr2+ or/and Ca2+) in BCT and BSCT can be located at off-center positions of the A-site, so that A-site vacancies are formed to compensate the charge imbalance arising from the substitution, and second, that Ca2+ ions substitute for Ti4+ ions in BCT, and cause the formation of O2– vacancies to balance the charge misfit. The previous experimental results from equilibrium electric conductivity (Han et al., 1987), scanning electric microscopy (Zhuang, et al., 1987), neutron diffraction (Krishna et al, 1993) and Raman and dielectric spectroscopies (Zhuang, et al., 1987; Chang & Yu, 2000; Park et al., 1992), have given evidence that a small amount of Ca2+ ions can substitute for Ti4+ ion causing the formation of O2– vacancies to balance the charge misfit, although the ionic radius and chemical valence of the Ca2+ ions is very different from those of the Ti4+ ions. 4 mol% Ca2+ ions were found to have substituted for the Ti4+ ions even when the molar ratio of (Ba+Ca)/Ti was 1 for the starting materials used by Krishna et al. in their studies of Ba0.88Ca0.12TiO3 samples prepared by the solid-state reaction technique. Following the above-mentioned suggestion, it seems that substitution of the Ca2+ ions for the Ti4+ ions had occurred in the Bi-BCT and Bi-BSCT ceramics prepared by the solid-state reaction technique.
Since aging is controlled by the migration of mobile oxygen vacancies, an experimental study of the formation of O2– vacancies in Bi-BCT by Raman scattering at room temperature was performed with the results shown in Fig. 9. (Ba0.925Bi0.05)(Ti0.90Ca0.10)O2.90 (Bi-BTC) ceramics were prepared in order to compare the effect of Ca substitution at the Ti sites of Bi-BCT. In single crystal and ceramic samples of BaTiO3, almost the same Raman bands, such as those at 165 cm-1 [A(TO)], 173 cm-1 (mixed modes), 266 cm-1 [A(TO)], 306 cm-1 [E(TO)], 470 cm-1 [E(T)+A(L)], 516 cm-1 [A(T)] and 712 cm-1 [A(LO)+E(LO)], were observed (Burns, 1974; Begg et al., 1996). Very similar results were also observed in (Ba1-xCax)TiO3 and BaTi1-yCayO3 ceramics (Chang & Yu, 2000). For A-site substitution, with increasing x, the Raman bands related to the phonon vibration of the Ba-O bonds shift to higher frequency (512 and 719 cm-1 for x = 0.005, 521 and 730 cm–1 for x = 0.20) while the Raman bands caused by the phonon vibration of the Ti-O bonds shift to lower frequency (259 and 306 cm -1 for x = 0.005, 248 and 298 cm–1 for x = 0.20). For B-site substitution, Ba-O bonds are closely related to the formation of the 517 and 718 cm-1 bands, and Ti-O bonds are closely related to the formation of the 257 and 307 cm-1 bands in BaTi1-yCayO3. Fig.9 shows the bands 299, 520 and 723 cm -1 were independent of the formation of CaBa and CaTi in Bi-BCT and Bi-BTC. From these results we can conclude that Ba-O bonds are closely related to the formation of the 520 and 723 cm-1 bands, and Ti-O bonds to the formation of the 262 and 299 cm-1 bands. We also find the development of a weak new Raman band at 827cm-1 for Bi-BTC and Bi-BCT (see Fig. 9). Almost the same Raman bands (832 cm-1) have been observed in Ba(Ti0.985Ca0.005Nb0.01)O3 and BaTi1-yCayO3 (y=0.005 and 0.015) ceramics (Chang & Yu, 2000). This higher frequency Raman band has resulted from the formation of CaTi defects. That is, Ca2+ ion substitution for B-site Ti4+ ions has occurred in Bi-BCT, and O2- vacancies have formed to compensate for the charge imbalance. The present results give critical evidence for the formation of O2- vacancies.
3.6. Origin of double-like P-E loops in Bi-BCT and Bi-BSCT
As mentioned above, there are two kinds of vacancy, A-site vacancies and oxygen vacancies, around an acceptor Ca2+ ion. Considering the mobility of oxygen vacancies and the immobility of cation vacancies at ordinary temperatures, the observation of double P-E loops in Fig. 8 can be explained by a special aging effect related to the defect symmetry defined as the conditional probability of finding O2- vacancies at site i (i=1–6) of an ABO3 single cell (Ren, 2004; Zhang & Ren, 2005, 2006; Liu et al., 2006; Zhang et al., 2004; Feng & Ren, 2007, 2008). Fig. 10 depicts how the double P-E loops are produced in a single-crystal grain of the aged Bi-BCT sample. The high-temperature sintering gives the oxygen and cation vacancies sufficient mobility. Thus, in the paraelectric phase, the probability of finding an O2- vacancy and a A-site vacancy around an acceptor Ca2+ ion will adopt a cubic symmetry, according to the SC-SRO mechanism of point defects [Fig. 10(d)] (Ren, 2004).
For the de-aged tetragonal samples, which are formed by immediately cooling from the paraelectric state at 470 K down to 300 K, the SRO distribution of point defects retains the same cubic symmetry as that in the cubic paraelectric phase because the diffusionless paraeletric-ferroelectric transition cannot alter the original cubic SRO symmetry of point defects (Ren, 2004). As a result, the de-aged ferroelectric state has tetragonal crystal symmetry, but cubic defect symmetry; thus the two symmetries do not match [see Fig. 10(a)]. According to the SC-SRO mechanism (Ren, 2004; Zhang & Ren, 2005, 2006; Liu et al., 2006; Zhang et al., 2004; Feng & Ren, 2007, 2008), such a state [Fig. 10(a)] is unstable due to the mismatch between the defect symmetry and the crystal symmetry. After aging for a long time, the defect symmetry in each domain follows the polar tetragonal crystal symmetry and exhibits a defect dipole moment following the polarization direction of the residing domain. Every domain is in its stable state, as shown in Fig. 10(b). The SRO symmetry of O2− vacancies around the Ca2+ ion can be gradually changed into a polar tetragonal symmetry (which produces a defect dipole PD) [see Fig. 10(b) and 10(e)] by the migration of mobile O2− vacancies, which is the same process as for the acceptor-doped case (Ren, 2004; Zhang & Ren, 2005, 2006; Liu et al., 2006; Zhang et al., 2004; Feng & Ren, 2007, 2008). However, the SRO symmetry of A-site vacancies around Ca2+ the ion still remains cubic because the cation vacancies are immobile at such temperatures (Tan et al., 1999) [Fig. 10(b) and 10(f)]. When an electric field is applied to the naturally aged tetragonal Bi-BCT sample, PS is switched to the field E direction while PD keeps its original direction during such a sudden process [Fig. 10(b) to 10(c)]. Therefore, after removing the electric field [Fig. 10(c) to 10(b)], the unchanged defect symmetry and the associated PD cause reversible domain switching. As a consequence, the original domain pattern is restored [Fig. 10(b)] so that the defect symmetry and dipole moment follow the crystal symmetry in every domain. An interesting double hysteresis loop in the P-E relation for Bi-BCT is expected to accompany this reversible domain switching, as observed in Fig. 8. Clearly, the explanation is the same as that for acceptor-doped ABO3 ferroelectrics (Ren, 2004; Zhang & Ren, 2005, 2006), since aging originates from the mismatch between the defect symmetry and the crystal symmetry after a structural transition. Comparing the double P-E loop “2“ with “3“ in Fig. 8, it can be seen that the naturally aging-induced double loop is obvious if the Bi-BCT samples are given a longer period of aging (33 months), which indicates that a longer time was required to establish an equilibrium defect state at room temperature (300 K).
When the measurement temperature is reduced to 280 K from 300 K, the double P-E loop becomes a normal one. The change of shape of the P-E loops in Fig. 5 and Fig. 6 can be explained as follows: normally, the coercive field increases with decreasing temperature, especially in some lead-based ceramic samples (Chu et al., 1993; Sakata, et al., 1992). When the samples were measured at low temperature, the coercive field may become higher than the driving force for reversible domain switching. As a result, the PD creating the driving force is not enough to switch a reversible domain and thus result in a single P-E loop observation at low temperature for Bi-BCT. A similar change from double to single P-E loops with temperature has been observed in specially aged KNbO3-based ceramics (Feng & Ren, 2007).
The above results describing the natural aging effect in (Ba1-xCax)0.925Bi0.05TiO3 ferroelectrics are very similar to the special aging effect in acceptor-doped BaTiO3 materials (Ren, 2004; Liu et al., 2006; Zhang et al., 2004; Feng & Ren, 2007; Yuan et al., 2007). The interesting double P-E loop behavior between the natural aging effect of ABO3 compound with A-site substitution and the special aging effect of ABO3 compound with B-site substitution can be explained by the SC-SRO mechanism of point defects. As discussed above, such a mechanism relies essentially on symmetry, not on the crystal structure or ionic species. The key idea of the SC-SRO mechanism is that an equilibrium defect state tends to adopt a match between the defect symmetry and the crystal symmetry in every domain. This tendency does not depend on the crystal structure or ionic species (Ren, 2004; Liu et al., 2006; Zhang et al., 2004; Feng & Ren, 2007, 2008; Yuen et al., 2007).
It seems from the change of shape of hysteresis loops that these two ceramic samples undergo a ferroelectric-antiferroelectric-paraelectric (FE-AFE-PE) transition, similar to that observed in (Na0.5Bi0.5)TiO3-based ceramics (Takenaka, 1991; Sakata & Masuda, 1974; Tu et al., 1994; Sakata et al., 1992). Some researchers thought that an intermediate phase with double hysteresis loops is as proof of an additional antiferroelectric transition (Sakata & Masuda, 1974; Tu et al., 1994; Balashova & Tagantsev, 1993). Indeed, heterophase fluctuations proposed between the paraelectric and the ferroelectric phase are unreasonable because of energy considerations, but it is possible that at a higher temperature there occurs an extra local phase transition to a nonpolar state. The energy gap between this phase and the ferroelectric phase is relatively small, and therefore heterophase fluctuations between the two phases can take place (Uchino & Nomura, 1978). Inhomogeneities due to chemical disorder cause nonuniformity in the free energy of the two states throughout the material, increasing the probability of heterophase fluctuations. For the selected samples Bi-BSCT, disordering in the Ba, Sr, Ca and Bi cations distributed in A-sites will be favorable for increasing the fluctuation probability, and resulting in an intermediate phase with double hysteresis loops between the paraelectric and the ferroelectric phase. However, it has not been confirmed by other experimental measurements whether the intermediate phase exists in Bi-BSCT. Further experiments on Bi-BCT and Bi-BSCT with TEM for observing the intermediate phase transition need to be carried out.
Here, we need to mention that BaTiO3 ferroelectric systems with dielectric relaxation behavior can usually be formed by doping point defects (impurity or doping) into a normal ferroelectric system. Bi-doped BCST is in such case. These random point defects distort the surrounding crystal lattice and thus generate random local fields. The random distribution of local fields brings about significant effects: the long-range ordering of electric dipoles is prohibited or destroyed while the local short-range ordering is retained. In simpler language, the random point defects will crush the macro-size domains into nano-size domains. Thus, only the local ordered polar nano-domain exists in Bi-doped BCT and BCST. The nearly linear P-E response (see also Fig. 5 and Fig. 6) is due to the random local field, stemming from the lattice defects by dopants, which has a strong frustration to the nano-domains. This has contributed significantly to our understanding of the contradictions between high dielectric constant and dielectric linearity in the most relaxors. In addition, the linear dielectric response in solid solutions with the dielectric relaxation behavior may provide possibilities for applications.
It is clear that an aging effect exists in Bi-BCT ceramics in the present report, so does it in the more complicated compositions Bi-BSCT ceramics. Based on the SC-SRO mechanism, the diffusional age-induced effect is the main reason causing the double hysteresis loops observed in Bi-BCT. Similarly, the double hysteresis loops observed between the paraelectric and the ferroelectric states for Bi-BSCT ceramics can be also explained by a point-defect-mediated reversible domain switching mechanism. However, talking about the complexity of ceramic compositions, it is unclear whether the aging effect is only reason resulting in the observed double hysteresis loops for Bi-BCT and Bi-BSCT ceramics according to the suggestion mentioned above, which needs to be confirmed by further experimental evidence.
(Ba1-xCax)1-1.5yBiyTiO3 (Bi-BCT, x =0.10, 0.20 and 0.30, y=0.05), (Ba1-xCax/2Srx/2)1-1.5yBiyTiO3 (Bi-BCST, x=0.10, 0.20 and 0.40, y=0 and 0.05) and (Ba0.925Bi0.05)(Ti0.90Ca0.10)O3 (Bi-BTC) perovskite ceramics were prepared through solid-state reaction technique. The structural, dielectric and ferroelectric properties and ferroelectric aging effect of Bi-BCT and Bi-BSCT ceramics were investigated.
5 at. % of Bi doping can be fully incorporated into the perovskite lattice of Bi-BCT and Bi-BSCT ceramics. The crystalline symmetry of Bi-BCT and Bi-BCST ceramics is a rhombohedral phase for x=0.40 while it is the tetragonal phase for x=0.30, and the crystalline symmetry of ceramics samples tends appreciably towards the coexistence of the tetragonal and rhombohedral phases for Bi-BCT (Bi-BCST) with the compositions of x=0.10 and x=0.20. XRD patterns at elevated temperature has demonstrated that the (002)/(200) refection of Bi-BCT can keep spliting throughout the whole temperature range from 280 K up to 320 K while that of Bi-BSCT can keep spliting throughout the whole temperature range from 280 K up to 300 K.
A typical relaxor behavior, a similar behavior to lead-based relaxor ferroelectrics, has been observed in Bi-BCT and Bi-BSCT ceramics. A random electric field is suggested to be responsible for the relaxor behavior observations. The dielectric peak corresponding to the ferroelectric-antiferroelectric transition cannot be found in the dielectric spectrum within the temperature range 198 K to 430 K for Bi-BCT and Bi-BSCT.
The P-E relationship at elevated temperature shows that an abnormal double-like P-E loop was observed for Bi-BCT and Bi-BSCT. The de-aging experiment has shown that there is a diffusional aging effect in Bi-BCT ceramics. The change from the single P-E loops in the de-aged sample to the double P-E loops in the aged sample excludes the existence of ferroelectric-antiferroelectric transition in Bi-BCT. Raman scattering gives critical evidence for the formation of O2- vacancies in Bi-BCT. A point-defect-mediated reversible domain switching mechanism is suggested to be responsible for the double hysteresis loops observed between the paraelectric and the ferroelectric states for Bi-BCT ceramics. It is expected that the abnormal ferroelectric behavior of Bi-BSCT ceramics can be explained using a point-defect-mediated reversible domain switching mechanism. The striking similarity of the aging effect between nonequivalence substitution for A-site ions and B-site ions of ABO3 systems indicates a common physical origin of aging.
The author acknowledges the financial support of the Scientific Research Program Funded by Shaanxi Provincial Education Department (Program No.2010JK655). This work was also supported by the PUNAI Education Scholarship through PuYang Refractory Group Co., Ltd. (PRCO).