ARTICLE IN PRESS

Physica B 404 (2009) 3457–3461

Contents lists available at ScienceDirect

Physica B

0921-45

doi:10.1

� Corr

E-m

journal homepage: www.elsevier.com/locate/physb

Shape memory effect in PZST system at exact morphotropic phase boundary

Abhishek Pathak a, C. Prakash b, Ratnamala Chatterjee a,�

a Advanced Ceramic Laboratory, Department of Physics, IIT Delhi 110016, Indiab Directorate of ER and IPR, DRDO, DRDO Bhawan, New Delhi 110105, India

a r t i c l e i n f o

Article history:

Received 16 February 2009

Accepted 22 May 2009

Keywords:

Shape memory effect

Morphotropic phase boundary

Antiferroelectric

PZST

26/$ - see front matter & 2009 Elsevier B.V. A

016/j.physb.2009.05.044

esponding author.

ail address: rmala@physics.iitd.ac.in (R. Chatt

a b s t r a c t

Unmodified lead zirconate titanate stannate (PZST) system with compositional formula Pb

[(Zr0.7Sn0.3)xTi(1�x)]O3, where 0.92rxr0.94 has been studied at Morphotropic phase boundary (MPB)

for dielectric, ferroelectric and shape memory effect. Field-induced strain measurements are presented

to show that �0.08% remnant strain pertaining to shape memory can be observed in PZST ceramics at

near-exact MPB.

& 2009 Elsevier B.V. All rights reserved.

1. Introduction

Stress, temperature or electric field-induced antiferroelectric(AFE) to ferroelectric (FE) phase transition is responsible for shapememory effect (SME) in many lead based systems [1–8]. PragyaPandit et al. [4] reported that the composition in the vicinity ofMPB is favorable for such type of phase transition. The free energydifference between the AFE and FE states at MPB is very small, sothat parent AFE phase can be driven to FE phase by application ofelectric field. If this induced metastable FE phase remains after theremoval of electric field than it produces remnant strain in thesample. If this remnant strain can be removed by applying electricfield in reverse direction, it is called shape memory effect [9–14].

Jaffe [15] has studied and given a phase-diagram of niobiummodified (2% Nb) lead zirconate stannate titanate system (PNZST)at room temperature (RT) (see Fig. 1). In this phase diagram ofPNZST system, there are two MPBs between AFE and FE phases.MPB between orthorhombic AFE (Ao) and rhombohedral FE (FR) isnot suitable for shape memory application due to large electricfield requirement for phase switching. Generally dielectricbreakdown takes place before the phase switching [16].Berlincort [17] found that substitution of Sn4+ for Zr–Ti ratioextends the tetragonal AFE phase. We studied this system byvarying Ti4+ content and keeping Zr–Sn ratio constant.

Till date, research papers have shown field induced strainproperties in various modified PZST systems viz., PNbZST [18],PLaZST [19], PYZST [12] and PNdZST [20]. We present here theresults of our studies on field-induced strain and polarization

ll rights reserved.

erjee).

measurements on unmodified PZST system with variation in Ti4+

content. Taking clue from this Fig. 1, we are working close to MPBof tetragonal AFE (AT) and rhombohedral FE (FR) phases, shown bya circle in this figure.

The aim of this work is to approach the MPB as close aspossible, so as to obtain a combination of FE and AFE phase in thesame composition. The series of ceramic samples with chemicalformula Pb [(Zr0.7Sn0.3)xTi(1�x)]O3, where x was initially varied asx ¼ 0.92, 0.93, 0.94, were prepared. Ti4+ substitution favorsferroelectricity due to the smaller radii. Compositions x ¼ 0.92

Fig. 1. Phase diagram of PNZST system at room temperature.

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A. Pathak et al. / Physica B 404 (2009) 3457–34613458

and 0.93 were found to be ferroelectric at room temperaturewhereas the composition with x ¼ 0.94 was observed to beantiferroelectric. Value of x was then varied upto third decimalpoint between x ¼ 0.93 and 0.94 (0.933, 0.935 and 0.938), toapproach the exact MPB.

Fig. 2. XRD patterns at room temperature.

6000

4000

2000

0

Die

lect

ric C

onst

ant

6000

4000

2000

0

Die

lect

ric C

onst

ant

x = 0.92

1 kHz10 kHz100 kHz200 kHz500 kHz

x = 0.933At 1 Hz

x = 0.935x = 0.938

40 80 120 160 200Temperature (°C)

40 80 120 160 200Temperature (°C)

Fig. 3. Dielectric constant of PZST com

2. Experimental procedure

All samples used in this study were prepared by solid-stateroute. Purity of the starting raw materials was 99.9% (AldrichChem. Ltd.). The mixed powders were calcined at 950 1C for 4 hand pellets were sintered at �1250 1C for 2 h. Relative density ofall the sintered pellets was more than 95%. XRD of the sinteredpellets showed pervoskite structure.

For electrical characterizations, the sintered samples werepolished to obtain parallel and smooth faces. Gold electrodes weremade by thermal evaporation on the faces of the pellets. Afterelectroding, the samples were heat-treated at 450 1C for 30 min toensure the contact between the electrodes and the ceramicsurfaces. The dielectric properties of the sintered samples werestudied as functions of both temperature and frequency with theimpedance analyzer (HP 9142A). The capacitance and the di-electric loss tangent were determined in the temperature range30 1CrTr220 1C with the frequency ranging from 1 kHzrfr500kHz. Measurements were carried out at the heating rate of0.5 1C/min. The ferroelectric hysteresis (P–E) loops were measuredusing work station (Radiant Technologies, USA) at room tempera-ture and 0.1 Hz frequency. Field induced strain measurementswere carried out on a LVDT based strain meter (SS 50 strainmeasurement system, Sensor Tech. Ltd., Canada).

3. Results and discussion

Fig. 2 show room temperature X-ray diffraction patterns of allcompositions. It is clear that all compositions have pervoskite

6000

4000

2000

0

Die

lect

ric C

onst

ant

4000

2000

0

Die

lect

ric C

onst

ant

x = 0.93

x = 0.94

1 kHz10 kHz100 kHz200 kHz500 kHz

1 kHz10 kHz100 kHz200 kHz

40 80 120 160 200Temperature (°C)

40 80 120 160 200Temperature (°C)

positions at varying temperature.

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40

30

20

10

0

-10

-20

-30

-40

Pol

ariz

atio

n (µ

c/cm

2 )

40

30

20

10

0

-10

-20

-30

-40

Pol

ariz

atio

n (µ

c/cm

2 )

40

30

20

10

0

-10

-20

-30

-40

Pol

ariz

atio

n (µ

c/cm

2 )

40

30

20

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0

-10

-20

-30

-40

Pol

ariz

atio

n (µ

c/cm

2 )

40

30

20

10

0

-10

-20

-30

-40

Pol

ariz

atio

n (µ

c/cm

2 )

40

30

20

10

0

-10

-20

-30

-40P

olar

izat

ion

(µc/

cm2 )

x = 0.92 x = 0.93 x = 0.933

x = 0.94x = 0.938x = 0.935

-40 -30 -20 -10 0 10 20 30 40Electric Field (Kv/cm)

-40-60 -20 0 20 6040Electric Field (Kv/cm) Electric Field (Kv/cm) Electric Field (Kv/cm)

-40 -30 -20 -10 0 10 20 30 40Electric Field (Kv/cm)Electric Field (Kv/cm)

-40 -20 0 20 40

-80 -60 -40 -20 0 20 40 60 80 -60 -40 -20 0 20 40 60

Fig. 4. P–E hysteresis loops of PZST compositions.

Table 1

Compositions Remnant

polarization

(PR) (mc/cm2)

Spontaneous

polarization

(Ps) (mc/cm2)

Coercive field

at 0.1 Hz (EC)

(kV/cm)

x ¼ 0.92 28.21 30.62 9.576

x ¼ 0.93 27.44 30.06 9.332

x ¼ 0.933 37.01 41.26 9.82

x ¼ 0.935 32.22 35.21 12.21

x ¼ 0.938 16.56 29.91 17.20

x ¼ 0.94 02.11 33.72 5.781

A. Pathak et al. / Physica B 404 (2009) 3457–3461 3459

structure. Singlet of 111 peak and doublet of 200 peak confirm thetetragonal structure of x ¼ 0.95 composition (Fig. 2b). Thetetragonality decreases with increase in titanium content andx ¼ 0.92 composition shows rhombohedral structure.

Figs. 3(a–d) show the dielectric behavior of all the sixcompositions (x ¼ 0.92, 0.93, 0.933, 0.935, 0.938 and 0.94) withtemperature and frequencies. Value of transition temperatures(161 1CrTCr167 1C) and dielectric constants (4400rerr6270)are observed to decrease with decreasing Ti4+ content. Theferroelectric transition temperatures for all compositions werefound to be almost same (TC) for all frequencies (non-relaxorbehavior).

Figs. 4(a–f) show the P–E hysteresis loops for all sixcompositions at room temperature. The P–E loops are wellsaturated and smooth. The samples were dense enough tosustain large electric fields without dielectric breakdown.

Compositions x ¼ 0.92 and 0.93 show clear ferroelectric loops,whereas the x ¼ 0.94 sample clearly is antiferroelectric in nature.The antiparallel arrangement of dipoles is responsible for thedouble hysteresis loop. In between, x ¼ 0.935 and 0.938 samplesshow pinched loops that appear to be in intermediate statebetween a typical ferroelectric and antiferroelectric. Values ofremnant polarizations (PR), spontaneous polarizations (Ps) andcoercive fields (EC) for all compositions are tabulated in Table 1.Drastic fall in value of PR for x ¼ 0.938 indicated the possiblepresence of AFE phase along with FE in this composition. Becausethe value of remnant polarization is very low for AFEcompositions. The composition with x ¼ 0.94, clearly shows AFEbehavior. It is clear from these measurements that a small changein composition at MPB can produce large change in the properties.

Figs. 5(a–f) show electric field-induced strain butterfly loopsfor all six compositions. The measurements were made at roomtemperature and 50 Hz frequency (unlike P–E measurements thatwere made at 0.1 Hz). Values of maximum strain, remnant strainand coercive field (at 50 Hz) are given in Table 2. For0.92rxr0.935, typical ferroelectric butterfly loops wereobserved. However, for compositions with x 4 0.935, as thefield increases, a large and abrupt strain change (�0.35% forx ¼ 0.94 and �0.22% for x ¼ 0.938) was observed above criticalfield E ¼ 32 kV/cm due to forced AFE to FE phase transition. Thislarge jump in strain occurs because of the different cell volume ofboth phases and it is clear that double loops in P–E hysteresismeasurements is truly due to the AFE property of thecomposition. The ferroelectric phase change was sustained even

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Fig. 5. Field-induced strain loops of PZST compositions.

Table 2

Composition Maximum

strain (%)

Remnant

strain (%)

Coercive field

at 50 Hz

(EC) (kV/cm)

x ¼ 0.92 0.115 0.042 6.52

x ¼ 0.93 0.108 0.0341 7.21

x ¼ 0.933 0.0766 0.0251 7.38

x ¼ 0.935 0.135 0.0287 8.4

x ¼ 0.938 0.219 0.06 13.01

x ¼ 0.94 0.336 0.0347 4.55

Fig. 6. Remnant strain with varying Ti4+ content.

A. Pathak et al. / Physica B 404 (2009) 3457–34613460

after removal of electric field, giving rise to remnant strain �0.06%in the x ¼ 0.938 sample. Therefore this sample at near-exact MPBis defined as a shape memory ceramic. Initial state of sample (AFEphase) was observed to be obtained by application of a negativeelectric field of �13 kV/cm. At MPB, values of free energies of AFEand FE phases are comparable. So, when AFE phase is driven to FE,it remains in this phase even after field is reduced to zero, causingremnant strain at E ¼ 0, which can be obtained back either byreversing the direction of electric field or by proper heattreatment.

4. Conclusion

Fig. 6 conclusively demonstrates that the remnant strain peaksat MPB and remains almost same for all other compositionswhether they are in AFE or FE phase. Large remnant strain ispossible only when both phases coexist in the same composition,as in case of Pb [(Zr0.7Sn0.3)0.938Ti0.062]O3. An important point to

note is that the values of strains shown in this report are allmeasured at 50 Hz frequency (due to experimental limitations).Corresponding values at lower frequencies would becomparatively larger.

A comparison of relevant properties of Pb-based systems[12,16,18,19,21–24] with those obtained from present work isshown in Table 3.

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Table 3

Materials Dielectric

constant

References Maximum

strain (%)

Remnant

strain (%)

PZST 6000 Present

work

0.35 0.08

PNbZST – 16 0.31 0.22

PLaZST – 18 0.2–0.9 –

PYZST 3500 12 0.18 0.06

PZST–PMN 14,000 24 0.11 0.04

Mn-doped PST 47,000 19 0.1 –

Nd-doped

PZST

2400 21 0.45 –

PZSrT – 22 0.49 0.063

PZN–PT 32,000 23 0.5 0.075

A. Pathak et al. / Physica B 404 (2009) 3457–3461 3461

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