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
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-
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-
The structural evolution of Bi-doped BCST ceramics samples from
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
Note that a remarkable double-like
A linear dielectric response is observed for Bi-BCT and Bi-BSCT, that is, the
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-
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
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
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
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-
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
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
When the measurement temperature is reduced to 280 K from 300 K, the double
The above results describing the natural aging effect in (Ba1-
It seems from the change of shape of hysteresis loops that these two ceramic samples undergo a ferroelectric-antiferroelectric-paraelectric (
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
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-
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 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).
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