unusually soft ferroelectricity in (ba1−xsrx)tio3 ceramics

Phys. Status Solidi A 208, No. 8, 1960–1963 (2011) / DOI 10.1002/pssa.201127086 p s sa

statu

s

soli

di

www.pss-a.comph

ysi

ca

applications and materials science

Unusually soft ferroelectricity in(Ba1�xSrx)TiO3 ceramics

Le Wang, Xiaoli Wang*, Jing Shi, Ruiting Sun, and Bo Liy

MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, School of Science,

Xi’an Jiaotong University, Xi’an 710049, P.R. China

Received 16 February 2011, revised 14 March 2011, accepted 25 March 2011

Published online 14 April 2011

Keywords ceramics, ferroelectricity, phase transitions, polarization

* Corresponding author: e-mail [email protected], Phone: þ86-29-82663747, Fax: þ86-29-82667872yPresent address: Henan Institute of Metrology, Zhengzhou, P.R. China

Temperature evolution of polarization versus electric field of

(Ba0.4Sr0.6)TiO3 and (Ba0.9Sr0.1)TiO3 ceramics was investi-

gated. Polarization characteristics of (Ba0.9Sr0.1)TiO3 belong to

ferroelectrics with diffuse phase transition (DPT), while the

ones of (Ba0.4Sr0.6)TiO3 are rather different. (Ba0.4Sr0.6)TiO3

ceramics display very soft ferroelectricity and extremely high

dielectric tunability. The long-range ferroelectric ordering in

(Ba0.4Sr0.6)TiO3 ceramics disappears completely below tem-

perature Tm of maximum dielectric permittivity em, which is

different from normal DPT ferroelectrics. The special ferro-

electric behavior is hypothesized to be attributed to random

arrangements of the distorted TiO6 octahedra.

� 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction BaTiO3 is a typical ferroelectricperovskite compound with three structural phase transform-ations from ferroelectric rhombohedral phase to orthorhom-bic phase (�190 K), to tetragonal phase (�275 K), andto paraelectric cubic phase (�400 K). SrTiO3 is an incipientferroelectric perovskite compound with a nonferroelectricantiferrodistortive phase transition at 105 K [1].(Ba1�xSrx)TiO3 solid solution ceramics exhibit a sequenceof ferroelectric transitions similar to that of BaTiO3 withSr2þ concentrations ranging from 0 to 80 at.%. On theSrTiO3 side, the polarization behavior is complicated by thecompetition between ferroelectric and antiferrodistortiveinstabilities [2, 3]. The diffuse ferroelectric phase transitionof (Ba1�xSrx)TiO3 ceramics with x< 0.8 has been deducedfrom the studies of their dielectric properties, crystalstructures, lattice dynamics, and thermodynamic calcu-lations [2–10]. Direct ferroelectric experiments wereinsufficient and fragmentary in the reported works. Due torestrictions of measurement technology, low temperatureferroelectric data were even rare.

In this article, we reported a ferroelectric investigationover a wide temperature range for (Ba0.4Sr0.6)TiO3 and(Ba0.9Sr0.1)TiO3 ceramics. (Ba0.4Sr0.6)TiO3 ferroelectricceramic with diffuse phase transition (DPT) presents verysoft ferroelectricity, and its temperature evolution of long-

range ferroelectric ordering is rather different from those ofnormal DPT ferroelectrics.

2 Experimental procedures Ceramics of (Ba0.4Sr0.6)-TiO3 and (Ba0.9Sr0.1)TiO3 were prepared by a conventionalmixed-oxide route [11]. Stoichiometric amounts of reagentsBaCO3, SrCO3, and TiO2 powders were wet mixed by ballmilling, and then presintered between temperatures 1373 and1423 K for 2 h. The presintered powder was ball milled anddried. Pellets 12 mm in diameter and �1 mm thick werepressed using 10% polyvinyl alcohol binder. The pelletswere fired between temperatures 1573 and 1653 K for 2–3 h.The dielectric permittivity was measured on an automatedsystem, within a temperature control sample chamber and anAgilent 4284A LCR meter were controlled by a personalcomputer. For ferroelectric hysteresis loop measurements, asinusoidal signal of 1 Hz, generated by a personal computerwith a PCI6221 Data Acquisition (DAQ) card, was amplifiedthrough a Trek 610E high-voltage supply/amplifier/controller and applied to the sample. Current throughthe sample was collected by the DAQ card, and convertedto a digital signal within. The hysteresis loop wasobtained through charge integration. For obtaining initialpolarization curves, the ceramic samples were heated to473 K to recover a history-independent state, and then were


Phys. Status Solidi A 208, No. 8 (2011) 1961

Original

Paper

Figure 1 (online color at: www.pss-a.com) Temperature depend-ences ofdielectricpermittivity eatdifferent frequencies (a) and1/eat1 kHz (b) for (Ba0.4Sr0.6)TiO3 and (Ba0.9Sr0.1)TiO3 ceramics.

Figure 2 Hysteresis loops for (Ba0.9Sr0.1)TiO3 ceramic at 378 K(a), and (Ba0.4Sr0.6)TiO3 ceramic at 213 K (b) and 203 K (c).

cooled to the desired temperature for each polarizationmeasurement.

3 Results and discussion Figure 1a shows dielectricpermittivity e from 0.1 kHz to 100 kHz as a function oftemperature for ceramics of (Ba0.4Sr0.6)TiO3 and(Ba0.9Sr0.1)TiO3. The temperatureTm of maximum dielectricpermittivity em for the two compositions is 211 and373 K, respectively. Compared with (Ba0.9Sr0.1)TiO3,(Ba0.4Sr0.6)TiO3 has a much higher em. There is anotherobvious anomaly on the rising side of e(T) peak of(Ba0.4Sr0.6)TiO3 around 173 K, which should be correspond-ing to phase transition between ferroelectric orthorhombicand tetragonal symmetries. Although the e(T) peak is gettingsmeared, 1/e of (Ba0.4Sr0.6)TiO3 above Tm displays a similardeparture from Curie–Weiss law to that of (Ba0.9Sr0.1)TiO3

(see Fig. 1b).Figure 2 depicts loops of polarization P versus electric

field E for (Ba0.9Sr0.1)TiO3 and (Ba0.4Sr0.6)TiO3 ceramics attemperatures around their respective Tm. The field-inducedtransition between paraelectric state and ferroelectricdomains does not occur in the two compositions, whichcan be observed in the parent BaTiO3 ceramic at tempera-tures just above Tc and in some BaTiO3-based relaxorsaround Tm [12, 13]. (Ba0.9Sr0.1)TiO3 shows a tiny hysteresisloop with a weak remnant polarization Pr of 0.0026 C/m2 at

www.pss-a.com

378 K (5 K higher than Tm), which might be caused bycritical phase fluctuations from paraelectric to microdomains[6]. The P(E) loop of (Ba0.4Sr0.6)TiO3 at 213 K (2 K higherthan Tm) presents reversible polarization with strongnonlinearity. On cooling, hysteresis loop cannot bedetected in (Ba0.4Sr0.6)TiO3 ceramics until 203 K (8 K lowerthan Tm). The weak remnant polarization Pr of 0.0031 C/m2

at 203 K suggests there are a few long polar order regions.It indicates that the long-range ferroelectric ordering in(Ba0.4Sr0.6)TiO3 takes place at temperatures below Tm. Thepolarization evolution is contrary to known DPT ferro-electrics [12].

Figure 3 plots temperature dependences of remnantpolarization Pr for the two compositions. (Ba0.4Sr0.6)TiO3

displays a much slower evolution of long-range ferroelectric


1962 L. Wang et al.: Unusually soft ferroelectricity in (Ba1�xSrx)TiO3 ceramicsp

hys

ica ssp st

atu

s

solid

i a

Figure 3 (online color at: www.pss-a.com) Temperature depend-encesofremnantpolarizationPr forceramicsof(Ba0.4Sr0.6)TiO3 and(Ba0.9Sr0.1)TiO3.

Figure 4 (online color at: www.pss-a.com) Hysteresis loop andcorresponding e(E) loop of BaTiO3 ceramic at 298 K (a) and (b),(Ba0.9Sr0.1)TiO3 ceramic at 243 K (c) and (d), and (Ba0.4Sr0.9)TiO3

ceramic at 163 K (e) and (f). A red line indicates the initial state.Polarization versus external field at the initial state for the threeceramics (g).

ordering than (Ba0.9Sr0.1)TiO3 around their respective Tm. Therapid increase of remnant polarization in (Ba0.4Sr0.6)TiO3 oncooling below Tm demonstrates an accelerated diffuseparaelectric–ferroelectric phase transition and formation ofdomains [2].


Figure 4c–f shows field dependences of polarization Pand derived dielectric permittivity (¼@D/@ET) for(Ba0.9Sr0.1)TiO3 at 243 K, and for (Ba0.4Sr0.6)TiO3 at163 K, respectively. The selected temperatures are 10 Klower than their respective ferroelectric orthorhombic–tetragonal phase transition temperature. The hysteresis loopand corresponding permittivity of BaTiO3 at 298 K are alsoplotted for comparison (Fig. 4a and b). (Ba0.4Sr0.6)TiO3 has ahigher saturation polarization (0.197 C/m2) and much lowercoercive field (0.05 MV/m) than those of BaTiO3 (0.190 C/m2 and 0.18 MV/m) and (Ba0.9Sr0.1)TiO3 (0.138 C/m2 and0.21 MV/m). That is, (Ba0.4Sr0.6)TiO3 is a very soft ferro-electric. The variations of initial P(E) and e(E) curves of(Ba0.9Sr0.1)TiO3 and BaTiO3 are similar, but those of(Ba0.4Sr0.6)TiO3 are evidently different. In addition, thedielectric permittivity is nonlinear for (Ba1�xSrx)TiO3

ferroelectrics even at low field of 100 kV/m (100 V/mm).One interesting thing is that linear dielectric behavior isdiscovered in BaTiO3 between 0.24 and 0.33 MV/m and in(Ba0.9Sr0.1)TiO3 between 0.16 and 0.23 MV/m. Thephenomenon might be helpful both in physical study andnew device design. The derived permittivity maximumfor BaTiO3 and (Ba0.9Sr0.1)TiO3 occurs around externalfield of 0.486 and 0.439 MV/m, respectively, which is aboutthree times as their respective Ec. The behavior has beenattributed to the domain reorientation [14, 15]. For(Ba0.4Sr0.6)TiO3, however, there is not linear dielectricbehavior and the derived permittivity maximum occurs at0.07 MV/m, a little above Ec of 0.05 MV/m. Electricfield dependences of initial polarization of the threecompositions are plotted in Fig. 4g. Induced polarization of(Ba0.4Sr0.6)TiO3 presents a step-like rising when externalfield varies from 0 to 0.02 MV/m (¼20 V/mm). Thecorresponding dielectric permittivity is extremely high andvery sensitive with field.

Average cell volume and cage size of O6-octahedra in(Ba0.9Sr0.1)TiO3 should be smaller than those of the parentBaTiO3, because radius of Sr2þ (0.154 nm) is smaller thanthat of Ba2þ (0.174 nm). Therefore, (Ba0.9Sr0.1)TiO3 showssmaller spontaneous polarization comparing with the parentBaTiO3. In (Ba0.4Sr0.6)TiO3 composition, with close con-tents of Ba2þ and Sr2þ ions and their random A-siteoccupations, the O6-octahedra might twist. A Ti4þ ion ineach twisting O6-octahedron will have its preferred ferro-electric-active shifting site. The distortion ways of adjoiningO6-octahedra are disorder since the arrangements of Ba2þ

and Sr2þ ions at A-sites are irregular. Therefore, theferroelectric-active shifting directions of Ti4þ ions inadjoining O6-octahedra are random, and their interactionsare frustrated to some extent. The situations make longferroelectric order regions form at lower temperatures. Thereshould exist very strong ionic polarization in the state [10], sothat the e(T) peak of (Ba0.4Sr0.6)TiO3 appears at temperaturehigher than where domains form. The derived permittivitymaximum of (Ba0.4Sr0.6)TiO3 ceramics emerges around Ec,which is apparently different from those of BaTiO3 and(Ba0.9Sr0.1)TiO3 (Fig. 4). The extremely soft ferroelectricity

www.pss-a.com

Phys. Status Solidi A 208, No. 8 (2011) 1963

Original

Paper

at temperatures far below Tm suggests that the grains of(Ba0.4Sr0.6)TiO3 ceramics should be full of very tinydomains (microdomains). It coincides with our presumption,that is, the random array of twisting TiO6 octahedra is hard toform ferroelectric domains with large scales.

Based on the analysis above, we can further hypothesizethat the analogous soft ferroelectric characteristics and giantdielectric tunability should also exist in (Ba1�xSrx)TiO3

(0.3< x< 0.7) solid solutions.

4 Conclusions The long-range ferroelectric orderingin (Ba0.4Sr0.6)TiO3 ceramics disappears completelybelow Tm, which is different from (Ba0.9Sr0.1)TiO3 andother known DPT ferroelectrics. More importantly,(Ba0.4Sr0.6)TiO3 ceramics display very soft ferroelectricityand extremely high dielectric tunability.

Acknowledgements This work was supported by the Fundsof National Natural Science Foundation of China (project no.50772087).

References

[1] K. A. Muller and H. Burkard, Phys. Rev. B 19, 3593 (1979).

www.pss-a.com

[2] V. V. Lemanov, E. P. Smirnova, P. P. Syrnikov, and E. A.Tarakanov, Phys. Rev. B 54, 3151 (1996).

[3] C. Menoret, J. M. Kiat, B. Dkhil, M. Dunlop, H. Dammak,and O. Hernandez, Phys. Rev. B 65, 224104 (2002).

[4] L. Benguigui and K. Bethe, J. Appl. Phys. 47, 2787(1976).

[5] V. S. Tiwari, N. Singh, and D. Pandey, J. Phys.: Condens.Matter 7, 1441 (1995).

[6] N. Singh, A. P. Singh, C. D. Prasad, and D. Pandey, J. Phys.:Condens. Matter 8, 7813 (1996).

[7] D. A. Tenne, A. Soukiassian, X. X. Xi, H. Choosuwan,R. Guo, and A. S. Bhalla, Phys. Rev. B 70, 174302(2004).

[8] D. Fuks, S. Dorfman, S. Piskunov, and E. A. Kotomin, Phys.Rev. B 71, 014111 (2005).

[9] V. B. Shirokov, V. I. Torgashev, and A. A. Bakirov, Phys.Rev. B 73, 104116 (2006).

[10] T. Teranishi, T. Hoshina, H. Takeda, and T. Tsurumi, J. Appl.Phys. 105, 054111 (2009).

[11] X. Wang and B. Li, Solid State Commun. 149, 537 (2009).[12] L. Wang, X. Wang, and B. Li, Solid State Commun. 149,

1877 (2009).[13] X. Wang and W. Cao, Appl. Phys. Lett. 90, 042913 (2007).[14] W. J. Merz, Phys. Rev. 95, 690 (1954).[15] D. Fu, H. Taniguchi, M. Itoh, S. Koshihara, N. Yamamoto,

and S. Mori, Phys. Rev. Lett. 103, 207601 (2009).


unusually soft ferroelectricity in (ba1−xsrx)tio3 ceramics

Documents