Effect of Sn content on dielectric, piezoelectric and ferroelectric properties for Pb(Zr0.35Ti0.65)1−xSnxO3 ceramics near morphotropic phase boundary

Journal of Alloys and Compounds 627 (2015) 238–243

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Effect of Sn content on dielectric, piezoelectric and ferroelectric properties for Pb(Zr0.35Ti0.65)1xSnxO3 ceramics near morphotropic phase boundary Zhuo Xing, Yujun Feng, Xiaoyong Wei ⇑ Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an 710049, China

a r t i c l e

i n f o

Article history: Received 1 October 2014 Received in revised form 20 November 2014 Accepted 8 December 2014 Available online 15 December 2014 Keywords: Pb(ZrSnTi)O3 Piezoelectric ceramics Rayleigh law Domains

a b s t r a c t ABO3 perovskite structure Pb(Zr0.35Ti0.65)1xSnxO3 (PZST) piezoelectric ceramics were synthesized by conventional solid state reaction method with the compositions near tetragonal to rhombohedral morphotropic phase of boundary (MPB). The influences of Sn content on dielectric, piezoelectric and ferroelectric properties of PZST piezoceramics were investigated while Zr/Ti ratio was fixed. With increasing Sn content, the c/a ratios and Curie temperatures TC decrease monotonically. Rayleigh law was explored to evaluate the piezoelectric behaviors as a function of loading electric field strength. Both Ray0 leigh coefficients ad and dinit are increased with increasing Sn content, indicating that Sn content considerably increase the mobility of domain walls and also the piezoelectric nonlinear responses. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Ferroelectric materials, which possess a spontaneous electric polarization and the polarization can be reversed by the application of an external electric field, have been widely used in many fields, such as sensors, actuators, non-volatile memories, and energy harvesting devices [1–6]. Among these ferroelectric materials, lead-based solid solution ceramics, represented by Pb(Zr1xTix) O3 (lead zirconate titanate, PZT) ceramics, have attracted many attentions for last few decades, because of their superior piezoelectric properties [1,5–7]. The phase structure of PZT is highly composition-dependent. For the composition with Zr/Ti  52/48, a morphotropic phase boundary (MPB), in which both tetragonal and rhombohedral phase coexist, was identified. Interestingly, both dielectric and piezoelectric properties show maximum values at MPB [8–11]. It shown that enhanced polarizability arises from the coupling between the two equivalent energy states, allowing optimum domain reorientation during poling [12,13]. Furthermore, Noheda et al. attributed the enhanced properties to a third monoclinic phase, which assisted the polarization reversal between the tetragonal phase and rhombohedral phase readily [9,10]. Recently, Li et al. emphasized that the enhancement of piezoelectric coefficient d can be partly attributed to the increase of dielectric permittivity e, since the coefficient d of perovskite ⇑ Corresponding author. Tel./fax: +86 29 82668679. E-mail address: [email protected] (X. Wei). http://dx.doi.org/10.1016/j.jallcom.2014.12.032 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.

ferroelectrics is proportional to PQe (P, spontaneous polarization, Q is electrostrictive coefficient) [14]. The solid solution of Pb(Sn1xTix)O3 (PST) system has already been reported to exhibit some interesting properties which are different from PZT [15–21]. It has been found that when Sn content is in the range of 60–80 mol%, PST crystallizes into ferroelectric rhombohedral phase. However it cannot form perovskite phase at atmospheric pressure, when Sn content is higher than 80% [17]. Furthermore, for PST ceramics, similar to that of the PZT ceramics, their MPB separating the rhombohedral and tetragonal phases was observed with a Sn/Ti mole ratio of 50/50. Actually, the Pb(ZrSnTi)O3 (PZST) solid solutions have received more general interest from the point view of fundamental research and application. The phase diagram of PZST is shown in Fig. 1 [1]. Adjusting the Zr/Ti/Sn mole ratio, different phase structures were obtained. From the point view of application, the compositions of PZST with high Zr concentration (Zr mol% > 60%) have been intensively investigated near antiferroelectric (AT and AO) to ferroelectric (FR(LT)) phase transition region. However, another region between FR(LT) and FT with high Ti concentration is not well understood. Although very little works have been conducted in this region, the piezoelectric and dielectric properties as a function of composition are not systematically studied. To ferroelectric materials, properties come from both intrinsic and extrinsic contributions [22,23]. It is well accepted that the intrinsic contributions to dielectric and piezoelectric properties of a ferroelectric originate from field-induced changes of polarization

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Pb(Zr0.35Ti0.65)1xSnxO3 piezoelectric ceramics were fabricated using conventional mixed oxide approach. The compositions studied in present work are given in Fig. 1, near the MPB region. High purity oxide powders: PbO (99%), ZrO2 (99%), SnO2 (99.5%), TiO2 (98%) (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China)

Room temperature XRD patterns of Pb(Zr0.35Ti0.65)1xSnxO3 ceramic powders with x = 0.22, 0.24, 0.26, 0.28 and 0.3 are shown in Fig. 2(a). The h–2h scanning angle is from 20° to 70°. It was found that only the peaks corresponding to a pure perovskite phase could be identified for the patterns collected from the calcined powders of all studied compositions. No pyrochlore or other secondary phase can be detected. As shown in Fig. 2(b), it was observed that with increasing Sn content, the (0 0 2) and (2 0 0) peaks moved closer gradually, indicating a decrease trend of tetragonality. According to Fig. 1, there is a rhombohedral–tetragonal phase boundary located around the composition with x = 0.3. Note that the (1 1 1) peak becomes broad as x increase. This phenomenon can be attributed to the existence of rhombohedral phase, since only the decrease of the tetragonality cannot result in such a broadening. Noheda et al. has found a monoclinic phase at MPB region in PZT ceramics, which display a similar diffraction feature as we observed in PZST ceramics [10]. However, as argued by Jin et al., a nanoscale coherent mixture of microdomains might be interpreted as an adaptive ferroelectric phase, while microdomainaveraged crystal lattice is monoclinic [37,38]. Therefore, whether there exist a monoclinic phase in PZST ceramics, further investigation is highly expected. As increasing the Sn content from 0.22 to 0.3, the composition approached the MPB region gradually. It can be seen in Fig. 2(c) that the c/a ratio decreased from 1.023 to 1.014, when the Sn content was increased from 0.22 to 0.30. The

1)

(11 1)

(10

Sn increases

(00 (10 1) 0)

Intensity (a.u.)

(a)

(20 (222) 0) (21 2)

2. Experimental procedures

3. Results and discussion

(11 (212) 1)

where E0 is the amplitude of the driving field E = E0sin(xt), is the initial piezoelectric coefficient, including the contributions from intrinsic piezoelectric effect of the lattice and the reversible domain walls vibration. The irreversible Rayleigh coefficient ad represents irreversible displacement of domain walls. Although the linear description of piezoelectric coefficients is valid approximation at lower field and stress levels, normally the field strength is limited much lower than the coercive field (Ec); it becomes more and more inaccurate when higher external fields are applied [31]. The existence of threshold field Et indicates that above which the piezoelectric coefficient and dielectric permittivity are expected to be constant with field [32,33]. The Rayleigh region is between the Et and Ec. In the Rayleigh region, the piezoelectric coefficient and dielectric permittivity show linear increase with increasing the electric field strength [23,34]. In this work, the Sn content on phase structure and related piezoelectric and dielectric properties for the composition of Pb(Zr0.35Ti0.65)1xSnxO3 was systematically investigated. The Rayleigh law was applied to study the electric field dependence of the piezoelectric coefficients. It was found that both ad and 0 dinit are increased with increasing Sn content, indicating that Sn content considerably increase the mobility of domain walls and also the piezoelectric nonlinear responses.

(10 (2 2) (2010) 1)

ð1Þ 0 dinit

(00 (202) 0)

0

0)

0

d33 ðE0 Þ ¼ dinit þ ad  ðE0 Þ;

were used as starting materials. The samples were sintered in sealed crucibles at temperature of 1220 °C for 2 h. The details of the ceramic processing can be found in Refs. [35,36]. The crystalline phases of studied samples were identified by X-ray diffraction (XRD, D/Max-IIIC, Rigaku, Japan) analysis using Cu Ka radiation on sintered powders operating at 40 kV and 100 mA. Scanning electronic microscopy (SEM, FEI Quanta 250 FEG, Hillsboro, Oregon, USA) was used to investigate the microstructures. The grain size was roughly estimated by the SEM photos using the interpolating method. A multi-frequency LCR meters (E4980A, Agilent, Palo Alto, CA, U.S.) was used to measure the dielectric permittivity as functions of temperature and frequency. Polarization–electric field (P–E) hysteresis loops and strain–electric field (S–E) curves were measured based on a Sawyer–Tower circuit (TF analyzer 2000, aixACCT, Aachen, Germany) combine with a photonic displacement sensor (MTI2000, MTI Instruments, Washington, U.S.) at room temperature. Small signal piezoelectric coefficients were measured on disk samples using a Berlincourt d33 meter (ZJ-3A, Institute of Acoustics, CAS, Beijing, China).

(11

and deformation within a single domain [24,25]. In contrast, extrinsic contributions to dielectric and piezoelectric properties include displacement of domain walls and other factors, such as, interphase boundary movement [26,27]. The nonlinear contributions from extrinsic mechanisms can be calculated from the field-dependent piezoelectric coefficients using Rayleigh law. Analysis of the dielectric and piezoelectric response with the framework of Rayleigh law enables us to describe quantitatively the contribution of irreversible movement of domain walls [28–30]. The Rayleigh law [28] for piezoelectric response can be expressed as:

20

30

40

50

60

70

Fig. 1. Phase diagram of PbSnO3–PbZrO3–PbTiO3 (PZST) ternary system [1] and the compositions studied in present work (red dot). AT – tetragonal antiferroelectric phase; AO – orthorhombic antiferroelectric phase; FR(HT) – high temperature rhombohedral ferroelectric phase; FR(LT) – low temperature rhombohedral ferroelectric phase; FT – tetragonal ferroelectric phase. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

0)

(c)

1.035

c/a

Sn increases

(20

(00

1) (11

Intensity (a.u.)

(b)

2)

2θ (degrees)

1.020

1.005 40

2θ (degrees)

45

0.21

0.27

0.24

0.30

x

Fig. 2. Room temperature XRD patterns of Pb(Zr0.35Ti0.65)1xSnxO3 ceramic powders with different Sn content from x = 0.22 to x = 0.30 (a) 2h = 20–70°, (b) 2h = 37–48°, (c) and c/a ratio as a function of Sn concentration.

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(a)

(b)

(c)

20 μm

20 µm

20 µm

(d)

(e) Grain size (μm)

14

(f)

12 10 8 0.22 0.24 0.26 0.28 0.30

20 µm

20 µm

x

Fig. 3. SEM micrographs of Pb(Zr0.35Ti0.65)1xSnxO3 ceramic samples on broken surfaces. (a) x = 0.22; (b) x = 0.24; (c) x = 0.26; (d) x = 0.28; (e) x = 0.30, and (f) grain size as a function of x.

1k Hz 10k Hz 100k Hz 1M Hz

2x104

(a)

1x104

Only the dielectric constant measured at 1 kHz shows a strong increase above the Curie temperature TC, which was mainly due to the increase of conductivity, especially at high temperature. For dielectric constant measured at other frequencies, very tiny frequency dispersion can be observed. Fig. 4(f) shown the dielectric properties of various compositions measured at 1 MHz as a function of temperature. It can be seen that TC decreases monotonically with increasing Sn concentration, from 320 °C to 273 °C, also accompanying with a broadening of the dielectric peak. P–E hysteresis loops and current–electric field (I–E) curves of studied samples with different Sn content are shown in Fig. 5. Among all samples, the composition with x = 0.26 possesses a maximum polarization of 24 lC/cm2, with a remnant polarization Pr  12.4 cm2, and Ec  8.9 kV/cm. Within a maximum electric field

1k Hz 10k Hz 100k Hz 1M Hz

2x104

(b)

1x104

Dielectric constant

Dielectric constant

3x104

Dielectric constant

c/a ratio for composition of x = 0.28 is bigger than its neighboring components, which may be caused by the fluctuations of experimental data. SEM microscopic photographs of the fractured surfaces of Pb(Zr0.35Ti0.65)1xSnxO3 ceramics are shown in Fig. 3. The porosity level of all studied compositions is similar, and the fracture propagation of all samples is intergranular. For the composition of x = 0.28 (on tetragonal side near the MPB) has the minimum of grain size for about 8.5 lm, and the maximum was MPB rhombohedral side for 13.5 lm. Temperature dependence of dielectric constant for compositions with x = 0.22–0.30 were measured at 1 kHz, 10 kHz, 100 kHz and 1 MHz, and shown in Fig. 4(a)–(e). For each composition, only a single dielectric peak can be observed, which corresponds to the ferroelectric to paraelectric phase transition.

1.80x104

(c)

1k Hz 10k Hz 100k Hz 1M Hz

1.35x104 9.00x103 4.50x103

0 200

100

300

1.5x104 1.0x10

(d)

4

5.0x103

1.25x104

Dielectric constant

Dielectric constant

1k Hz 10k Hz 100k Hz 1M Hz

2.0x104

200

300

100 150 200 250 300

Temperature (oC)

Temperature (oC)

Temperature (oC)

1k Hz 10k Hz 100k Hz 1M Hz

1.00x104 7.50x103

(e)

5.00x103 2.50x10

3

Dielectric constant

100

(f)

Sn Tc (ć

1.8x104

0.22 0.24 0.26 0.28 0.30

1.2x104 6.0x103

320 309 302 287 273

0.0 100

200

300

Temperature (oC)

100

200

Temperature

300

(oC)

100

200

300

Temperature (oC)

Fig. 4. Temperature dependence of dielectric constant of Pb(Zr0.35Ti0.65)1xSnxO3 ceramics, (a) x = 0.22, (b) x = 0.24, (c) x = 0.26, (d) x = 0.28, (e) x = 0.3 measured at 1 kHz, 10 kHz, 100 kHz and 1 MHz. (f) Dielectric constant of all studied samples as a function of temperature measured at 1 MHz.

241

-12 -1.0x10-4

(a) -20

0

20

-40

Polarization (µC/cm2)

-20

0

20

0.0

0 -12

-2.0x10-4

(c)

-24 -40

40

Electric Field (kV/cm)

Electric Field (kV/cm) P I

24

-1.0x10-4

(b)

-20

40

2.0x10-4

12 0.0

0

I (A)

-40

-10

-12

(d)

-24 -40

-20

0

20

-2.0x10-4

Polarization (µC/cm2)

-24

0.0

0

2.0x10-4

12

0

20

40

-4.0x10-4

4.0x10-4 2.0x10-4

12

0.0

0

-2.0x10-4

-12

(e)

-24

40

-20

Electric Field (kV/cm) P I

24

I (A)

0.0

0

10

4.0x10-4

P I

24

I (A)

12

1.0x10-4

I

Polarization (µC/cm2)

1.0x10-4

P

20

I (A)

Polarization (µC/cm2)

P I

24

I (A)

Polarization (µC/cm2)

Z. Xing et al. / Journal of Alloys and Compounds 627 (2015) 238–243

-40

Electric Field (kV/cm)

-20

0

20

-4.0x10-4

40

Electric Field (kV/cm)

Fig. 5. Ferroelectric P–E hysteresis loops and I–E curves for Pb(Zr0.35Ti0.65)1xSnxO3 ceramics with different Sn content. (a) x = 0.22⁄, (b) x = 0.24⁄, (c) x = 0.26, (d) x = 0.28 and (e and d) x = 0.3. ⁄Suggests that the compositions possessing a pinning effect.

of 40 kV/cm at 1 Hz, apparent pinched hysteresis loops were observed in the samples with lower Sn content (x = 0.22, 0.24), while for other samples normal hysteresis loops were observed. However, it is not always accurate to detect the pinning effect through P–E loops, especially to some samples with weak pinning effect. From I–E curves, it is observed that compositions with pinched P–E hysteresis loops (x = 0.22, 0.24) exhibit double I–E peaks with respect to increasing or decreasing the electric field. The appearance of the double current peaks can be seen as the evidence of the existence of the pinning effect. While as shown in Fig. 5(d), although the composition x = 0.28 shows normal

ferroelectric loop without pinched shape, the I–E curve shows weak double peaks. It is probably due to the internal stress induced by the large c/a ratio, which makes domain walls switch difficult [23]. As have been reported in PZT ceramics, Ec in tetragonal phase is higher than that in rhombohedral phase, and a high remnant polarization is obtained also in rhombohedral phase than that in tetragonal phase [29]. Therefore, the missing of pinning effect in sample of x = 0.28 might be related to the low distortion of the crystal lattice because of the rhombohedral phase. As suggested by XRD patterns in Fig. 2, the broadening of (1 1 1) peak is owing to the appearance of the rhombohedral phase.

3

2

2

Strain (‰)

Strain (‰)

0

1

0

(a) -20

0

20

(b) -1 -40

40

-20

Electric field (kV/cm)

0

0 -1 -40

40

(c) -20

0

20

Electric field (kV/cm)

2

2

1

0

-1 -40

20

1

Electric field (kV/cm)

Strain (‰)

-1 -40

Strain (‰)

Strain (‰)

2 1

1

0

(d) -20

0

20

Electric field (kV/cm)

40

(e) -1 -40

-20

0

20

40

Electric field (kV/cm)

Fig. 6. Strain–electric field (S–E) curves for Pb(Zr0.35Ti0.65)1xSnxO3 ceramics with different Sn content.

40

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Z. Xing et al. / Journal of Alloys and Compounds 627 (2015) 238–243

0.22

0.24

0.26

0.28

and rhombohedral phases near MPB. But the temperature stability of composition x = 0.3 is lower than that of x = 0.26, considering the lowest TC (273 °C). With increasing of Sn content, the mechanical quality factor Qm decreased substantially. Due to the decrease of the c/a ratio, the compositions exhibit some ‘‘soft’’ features as frequently reported PZT ceramics, for instance, higher d33, lower Qm and disappearance of the pinning effect [39–41]. It is worth noting that traditional softening effect mainly resulted by the donor dopant doping, while in this work, the soft features is due to the crystal lattice distortion. 0 In the Rayleigh region, the real piezoelectric coefficient d33 was obtained from the peak-to-peak longitudinal strain Sp–p, measured for each excitation electric field (E0). A typical hysteresis strain– 0 field loop is shown in Fig. 8(a). The d33 was calculated as followed:

0.30

252

Qm

210 168 126

Qm kp

0.33

k31

0.4

0.22

0.3

k31

kp

0.5

0.11 d33

d33

390

0

d33 ¼

260

Sp—p : 2E0

ð2Þ 0

130 0.22

0.24

0.26

0.28

0.30

x Fig. 7. Qm, kp, k31, and d33 as a function of x for Pb(Zr0.35Ti0.65)1xSnxO3 ceramics measured at room temperature.

Strain–electric field curves for Pb(Zr0.35Ti0.65)1xSnxO3 ceramics with different Sn content were shown in Fig. 6. It can be seen, for the composition with x = 0.26 and 0.3, their maximum strain is larger than 3‰. In contrast, for other three compositions, their maximum strain is less than 1.3‰. In consistent with the P–E loops, samples without pinning effect show larger strain response with respect to the same driving electric field. These data suggests that the domain wall motion, especially the domain wall switching, is depressed to a large extent, by the internal stress owing to the high crystal lattice distortion. Longitudinal piezoelectric constant d33 was measured in wellaged samples after poling (24 h after poling). d33 as a function of Sn content was shown in Fig. 7. The d33 of the samples were found to be around 180–450 pC/N, among which the highest value was obtained in the sample x = 0.30. Two compositions (x = 0.26, 0.30) exhibit high values due to the coexistence between the tetragonal

-0.04

600

600

500

500

400 300 200 100

-10

-5

-E0

0

5

d33' (pm/V)

P-P Strain

0.00

d33' (pm/V)

Strain (%)

0.04 (a)

Fig. 8(b)–(f) shows the ac electric field-dependent d33 for Pb(Zr0.35Ti0.65)1xSnxO3 ceramics measured at 0.05 Hz with 0 x = 0.22–0.30. The calculated d33 for each composition under various electric fields are fitted linearly. Within a suitable field amplitude, the piezoelectric coefficients exhibited good linear behavior as a function of driving electric field. It can be observed is that the piezoelectric response could be well described using Rayleigh law [28–30]. According to the different Ec of each composition, the PZST compositions exhibited linear Rayleigh behavior over the range of 1.5 < E0 < 13 kV/cm. The tetragonal compositions x = 0.22 and 0.24 have higher threshold field (7 kV/cm) than other compositions (2–5 kV/cm). After determining the ac electric field dependence of piezoelec0 tric coefficient d33 (E0), the Rayleigh coefficient ad was obtained from a linear fitting based on Eq. (1), and was shown in Fig. 9. The errors were determined from a least squares analysis of the Rayleigh plots. It is observed that dinit increased with increasing Sn content, and reached a maximum (250) at x = 0.28. The dinit value is reversible contributions, including the intrinsic contributions from lattice and reversible domain walls motion. The reversible piezoelectric response was found to reach the maximum values when composition approached the MPB region, being on the order of 114.7, 172.4, 244.4, 257.6 and 257.8 for compositions x = 0.22, 0.24, 0.26, 0.28 and 0.3, respectively, being attributed to

+E0 10

200

(b)

100

750 900 1050 1200 1350

800

900

700

800

(d)

360

d33' (pm/V)

d33' (pm/V)

d33' (pm/V)

480

600 500

(e) 400

(c) 440 660 880 1100 1320

Electric field (V/mm)

840

600

300

Electric field (V/mm)

Electric Field (kV/cm)

720

400

700 600 500

(f)

210 420 630 840 1050

189 252 315 378 441

237 316 395 474 553

Electric field (V/mm)

Electric field (V/mm)

Electric field (V/mm)

0

Fig. 8. (a) A typical S–E curve, and the d33 was calculated from the measured peak-to-peak strain, based on Eq. (2). The ac electric field–dependent converse piezoelectric 0 coefficients d33 for Pb(Zr0.35Ti0.65)1xSnxO3 ceramics measured at 0.05 Hz. The fits to Eq. (1) are shown with red lines. (b) x = 0.22, (c) x = 0.24, (d) x = 0.26, (e) x = 0.28, (f) x = 030.

400

2.0

300

1.5

200

1.0

100 0.5 0 0.0 0.22

0.24

0.26

0.28

243

References

α'd (10-15m2/V2)

d'init (pm/V)

Z. Xing et al. / Journal of Alloys and Compounds 627 (2015) 238–243

0.30

Sn content 0 dinit

Fig. 9. Calculated and ad for Pb(Zr0.35Ti0.65)1xSnxO3 ceramics. The errors are from the least squares analysis. Red lines are guide for eye. The maximum ad of 0.89 (pm/V)(V/mm)1 is observed for the composition with x = 0.3. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

the flattening of the free energy profile [42]. The extrinsic contribution also increased as the compositions approaching MPB, suggesting that the irreversible domain wall motion was enhanced. It was observed in PZT ceramics that the extrinsic domain wall motion in rhombohedral phase was larger than that in tetragonal phase [43]. 0 The dinit and ad are both low in tetragonal side. This result indicates that the displacement of the domain wall is difficult, because of the strong local internal stress, which was created by the high distortion of tetragonal cell [30]. This observation is consistence with other ferroelectric system, for example, BiFeO3–(K0.5Bi0.5)TiO3– PbTiO3 [44], PZT films [45], doped PZT ceramics [46]. It is well known that the spontaneous strain of the tetragonal elemental cell is greater than 2%, while the strain is less than 0.5% in the rhombohedral cell [1]. 4. Conclusions To summarize, Pb(Zr0.35Ti0.65)1xSnxO3 ferroelectric ceramics near the MPB region were prepared using a routine solid state reaction method. The c/a ratio and Tm, decrease with increasing of Sn content. Pinning effect was observed in compositions x = 0.22, 0.24 and 0.28, which might be related to their high c/a ratio. The Rayleigh law was used to evaluate the contribution of the irreversible domain wall motion to the piezoelectric response of PZST 0 ceramics. Both ad and dinit are increased by increasing Sn content, indicating that Sn content substantially increase the mobility of domain walls and also the piezoelectric nonlinear response. It was suggested that irreversible domain wall motion is increased as the composition approached to MPB region. Acknowledgments This work is supported by the International Science & Technology Cooperation Program of China (2013DFR50470) and ‘‘111’’ project (B14040). The SEM work was done at International Center for Dielectric Research (ICDR), Xi’an Jiaotong University, Xi’an, China. The authors also thank Mr. Yongyong Zhuang for his help in using SEM.

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