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Vacuum 99 (2014) 38e41

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Vacuum

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Double-hysteresis-like loops in Cr2Ti3O9-doped BaTiO3 ceramics

Yongping Pu, Dan Liu*, Xuan ShiSchool of Materials Science & Engineering, Shaanxi University of Science and Technology, Xi’an 710021, Shaanxi, China

a r t i c l e i n f o

Article history:Received 30 October 2012Received in revised form20 April 2013Accepted 23 April 2013

Keywords:BaTiO3

Double ferroelectric hysteresis loopPolarization

* Corresponding author. Tel.: þ86 29 86168803; faE-mail address: liudan062@yahoo.com.cn (D. Liu).

0042-207X/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.vacuum.2013.04.017

a b s t r a c t

Barium titanate (BaTiO3) ceramics doped with different content of Cr2Ti3O9 were obtained by sintering ina temperature rang from 1300 to 1350 �C in air for 2 h. The phase composition, perovskite structuralparameters and ferroelectric properties have been obtained by X-ray diffraction and ferroelectric mea-surements as a function of chemical composition and temperature. At low doping levels the peculiardouble-hysteresis-like loops were found to have striking similarities to aging tetragonal ferroelectrics,besides following random local strains mechanism driven by the displacement of oxygen octahedral inshrunk crystals caused by phase transition. The polar moments surrounding the shrunk crystal cells werepartially aligned and disrupted the long-range polar order due to the different final polarization directionby coupling between spontaneous polarization in original lattice. With temperature rising, the double-hysteresis-like loops can transform into a normal one because of the increase of long-range polar order.

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1. Introduction

Barium titanate is a ferroelectric compoundwith the perovskite-type structure and interesting electrical properties: ferroelectricity,high dielectric constant, piezoelectricity, and pyroelectricity. Ex-amples of practical applications are multilayer capacitors (MLC),sonar and ultrasonic transducers, positive temperature coefficient(PTC) heaters and sensors, and ferroelectric thin-film memories.BaTiO3 has four modifications of rhombohedral, orthorhombic,tetragonal and cubic phases, and the ferroelectric Curie tempera-ture (Tc) is 130 �C at which the tetragonal symmetry changes to thecubic one. One of the well-known properties of ferroelectrics is thepolarization versus electric field (PeE) hysteresis loop. The firstdiscovery of double-hysteresis-like loops in BaTiO3 was in 1953 byWalter J. Merz [1], and his work showed that the size and shape ofPeE curves change with the temperature from a line (higher tem-perature) to two overlapping loops (lower temperature). An agingeffect far below Curie temperature can induce the double PeE loopsobserved inmany kinds of different compositional ceramics such asacceptor-doped BaTiO3 ceramics, Mn-doped (Ba, Ca)TiO3 ceramics,KNbO3-based ceramics [2,3]. However, the double PeE loops of Mn-doped (Pb,La)(Zr,Ti)O3 ceramics were only observed in the virginstate according to Ziping Cao’s work [4]. Such double-hysteresis-like loops of Cr2Ti3O9-doped BaTiO3 ceramics were few reported.

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A symmetry conforming short-range ordering (SC-SRO) of pointdefects has been employed to explain these interesting double-likePeE loops behavior observed in acceptor doped BaTiO3 [5], Pb(Zr,Ti)O3 [6] and KNbO3 based ferroelectrics [7]. This type of loops wasnamed a constricted or clamped loop and this phenomenon usuallyappears in the poled and aged acceptor-doped ferroelectrics [8].However, the constricted loop of Cr2Ti3O9-doped BaTiO3 ceramicswas observed in the fresh state in this paper, which cannot bedirectly interpreted by this mode.

In the present paper, the crystal structure of Cr2Ti3O9-dopedBaTiO3 ceramics was analyzed. The mechanism of this abnormalhysteresis loop was also discussed by investigating Cr2Ti3O9 con-tent and temperature dependence of the PeE hysteresis loops andstrain versus electric field (SeE) curves.

2. Experimental

BaCO3 (d50 z 1.0 mm) and TiO2 (d50 z 0.6 mm) powders with apurity of 99.99%were thoroughlymixed according to the formula ofBaTiO3 and calcined at 1220 �C for 2 h so as to obtain a pure BaTiO3phase. In addition, Cr2O3 (d50 z 0.8 mm) and TiO2 powders with apurity of 99.99% at a molar ratio of Cr/Ti ¼ 2:3 were weighted andmixed well, then the mixture was calcined at 1270 �C for 6 h toobtain a pure Cr2Ti3O9 phase [9,10]. A series of samples with theformula (1 � x) BaTiO3 þ x Cr2Ti3O9 (x ¼ 0, 0.5, 1.0, 1.5 mol%) wereprepared, hereafter labeled as BTC0, BTC5, BTC10 and BTC15,respectively. These powders were weighted according to thenominal compositions, and mixed in distilled water by ball millingfor 4 h. After drying and granulating with polyvinyl alcohol (PVA,

Fig. 1. The XRD patterns of as synthesized BTC0, BTC5, BTC10 and BTC15 ceramics.

Table 1Lattice parameters, remnant polarization and coercive field for BTC0, BTC5, BTC10and BTC15 ceramics.

a (�A) c (�A) a2c (�A3) Pr (mC/cm2) Ec (kV/cm)

BTC0 3.994 4.022 64.154 9.64 3.03BTC5 3.995 4.020 64.142 1.61 2.28BTC10 4.001 4.006 64.133 1.45 1.81BTC15 4.007 4.045 64.935 2.55 3.75

Y. Pu et al. / Vacuum 99 (2014) 38e41 39

5 wt%), the well-mixed powders were pressed into a disk (F12 � 1.6 mm) at 120 MPa and sintered in a temperature rang from1300 to 1350 �C in air for 2 h.

The crystal structure of the samples was identified using an X-ray diffractometer (XRD; Model D-MAX 2200 pc, Rigaku Co., Tokyo,

Fig. 2. PeE curves of BTC0, BTC5, BTC10 and

Japan), in conjunction with Cu Ka radiation, operated at 50 kV,100 mA, and a scanning rate of 3�/min within the range of 2q from10� to 70�. For dielectric measurements, Ag electrodes were screenprinted with Ag paste on both sides of the pellets and heat treatedat 600 �C for 20 min. The PeE and SeE curves were measured usinga modified Sawyer-Tower circuit. The electric field signal wastriangular wave.

3. Results and discussion

Fig. 1 shows the XRD patterns of as synthesized BTC0, BTC5,BTC10 and BTC15 ceramics. The crystalline phase of pure BaTiO3 atroom temperature was tetragonal. When the content of Cr2Ti3O9was low, the XRD patterns showed a single tetragonal phase.However, a secondary phase, identified as the hexagonal phase ofBaCrO3, was detected with increasing of doping content. In order todemonstrate the distortion of the BaTiO3 unit cell, the lattice pa-rameters of ceramic samples were calculated and listed in Table 1.In Table 1, a of BaTiO3 decreases monotonically with increase ofCr2Ti3O9 amount; 3.994 �A (0 mol%), 3.995 �A (5 mol%), 4.001 �A(10 mol%) and 4.007 �A (15 mol%). On the contrary, c of BaTiO3 de-creases and jumps to high value with Cr2Ti3O9 content; 4.022 �A(0 mol%), 4.020�A (5 mol%), 4.006�A (10 mol%) and 4.045�A (15 mol%). This parameter change indicated that BaTiO3 changed fromtetragonal to cubic with increase of Cr2Ti3O9 amount until its10 mol% concentration and then jumped to tetragonal again. Itmeans that ferroelectric properties could be degraded due to theloss of the anti-symmetry, and the shrinkage of the lattice volumewas found because of the phase transition, as shown in Table 1.

The PeE hysteresis loops of BTC0, BTC5, BTC10 and BTC15 ce-ramics at room temperature are shown in Fig. 2. In contrast withthe normal PeE loops for the BTC0 samples, the doped samplespossessed an interesting double PeE loop, which very similar tothat of the aged acceptor-doped tetragonal ferroelectrics such as

BTC15 ceramics at room temperature.

Fig. 3. SeE curves of BTC0, BTC5, BTC10 and BTC15 ceramics at room temperature.

Y. Pu et al. / Vacuum 99 (2014) 38e4140

the A2þB4þO3 system [11]. The remnant polarization and coercivefield decreased with increasing of Cr2Ti3O9 content, when x below1.5 mol% (Table 1). The remnant polarization and coercive field ofBTC15 then increased because of its tetragonal phase. Moreover, arecoverable electrostrain of 0.03% at 35 kV/cm, accompanying thedouble PeE loop, was achieved in BTC samples (Fig. 3). Thisrecoverable electrostrain curve was indeed different from thebutterfly irrecoverable electrostrain curve as obtained in BaTiO3samples due to the existence of a recoverable domain switching.

As we know, one of the most valuable features of ferroelectricbehavior is that ferroelectric ceramics can be transformed intopolar materials by applying appropriate electric fields or mechan-ical stresses. A random local strain mode caused by oxygen octa-hedral rotations in the crystal cells was proposed to explain thedouble-hysteresis-like loop in un-doped Pb(Zr,Ti)O3 [12]. Theypostulated the existence of localized regions of stress inside thegrains where the values of the dielectric polarization were lowerthan its average. On the basis of this model, we can assume that

Fig. 4. The projection of BT (a) and BTC (d) structure on (010) face (Ps stand for the spontaoxygen octahedral), and schematic diagram of the polar micro-region for BT and BTC ceram

random local strains result from the displacement of oxygen octa-hedral, which induce the constricted loop. For tetragonal BaTiO3(BTC0 and BTC15), the central Ti4þ ion was closer to one of the O2�

ions and it will be energetically favorable for the Ti4þ ion locatedmore distantly from that O2� ion on the opposite side, thusengendering a similar displacement of all the Ti4þ ions in aparticular column in the same direction. Coupling betweenneighboring columns occurs in BaTiO3 so that all the Ti4þ ions weredisplaced in the same direction and formed the spontaneous po-larization (Ps) (Fig. 4(a)). With applied electric field, 180� domainsswitched producing net overall polarity but no dimensional change,and 90� domains switched accompanied by small elongation withan increase in electric field (Fig. 4(b)). Once the field was removed,the domains were partial deviation from the direction of the elec-tric field in order to buffer the stresses form ceramics. This part ofelongation manifested remnant polarization (Fig. 4(c)). As for BTC5and BTC10, the phase transition may cause a displacement of ox-ygen octahedral perpendicular to the direction of spontaneouspolarization, resulting in the random local strains. The final polar-ization direction was different from Ps by coupling between Ps inoriginal lattice (Fig. 4(d)). Again, the polar moments surroundingshrunk crystal cells were partially aligned and disrupted the long-range polar order (Fig. 4(e)). The polarization direction was morerandom than BaTiO3’s because the random local strains were notswitched or ordered by the electric field, resulting in a decreasingremnant polarization (Fig. 4(f)). This was also approved by the dateof Table 1.

Fig. 5 plots the PeE curves for BTC5 ceramics at different tem-peratures of room temperature, 40, 60, 80 and 100 �C in order toinvestigate the temperature dependence of long-range polar order.On heating to 60 �C, remnant polarization (Pr) and coercive field (Ec)were 1.18 mC/cm2 and 1.74 kV/cm, respectively. The constrictedmagnitude of the PeE curve weakened greatly. Upon heating to80 �C, a normal hysteresis loop with Pr of 1.21 mC/cm2 and Ec of2.30 kV/cmwas observed. While further heating to 100 �C, Pr and Eccontinued to increase to 1.57 mC/cm2 and 3.14 kV/cm, respectively.As shown in Fig. 6, the temperature dependence ofmaximum strainSmax of BTC5 is also noted that the long-range polar order increasedwith increasing temperature. These results indicate the switching

neous polarization of BT; Pd stand for the polarization caused by the displacement ofics.

Fig. 5. PeE curves of BTC5 ceramics at room temperature, 40 �C, 60 �C, 80 �C and 100 �C.

Fig. 6. The temperature dependence of maximum strain Smax of BTC5.

Y. Pu et al. / Vacuum 99 (2014) 38e41 41

of polar moments around the shrunk crystal cells can increasegradually because of the heat activation.

4. Conclusion

In conclusion, the structural and ferroelectric properties of(1 � x) BaTiO3 þ x Cr2Ti3O9 (x ¼ 0, 0.5, 1.0, 1.5 mol%) ceramicsprepared using a conventional solid-state method were investi-gated. The introduction of Cr2Ti3O9 induces the phase transitionand the shrinkage of some local crystal cells in the lattice. Theremnant polarization and coercive field decreased with theincreasing of Cr2Ti3O9 content (x < 1.5 mol%). And then an inter-esting double PeE loop and recoverable electrostrain curveappeared which can be explained by the random local strainmechanism driven by the displacement of oxygen octahedral inshrunk crystal cells. The double-hysteresis-like loops can transforminto a normal one because of the increasing of long-range polarorder by heat activation.

Acknowledgments

This research was supported by the National Natural ScienceFoundation of China (nos. 51072106 and 51102159), the New Cen-tury Excellent Talents Program of Chinese Education Ministry (no.NCET-11-1042), the Foundation of Shaanxi Educational Committee(no. 12JK0447), the International Science and Technology Cooper-ation Project Funding of Shaanxi Province (no. 2012KW-06), andthe Academic Leaders Cultivation Program and Graduate Innova-tion Fund of Shaanxi University of Science and Technology.

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