Enhanced the tunability properties of pure (Ba,Sr)TiO3 lead free

ferroelectric by polar nano-region contributions

Abd El-razek Mahmoud1*, Samar Moen1, M.K.Gerges1

1Physics Department, Faculty of Science, South Valley University, Qena 83523, Egypt

A B S T R A C T

In the present work, a series compositions of (Ba1-xSrx)TiO3 (0≤x≤0.4) lead-free ferroelectric

(BST) were successfully prepared by sol-gel method. The effects of Sr2+ on the optical

properties, crystal structure and morphology of BaTiO3 were systematically investigated.

Compositions with x<0.4 exhibit single phase tetragonal perovskite at room temperature, while

a cubic structure of BST has been detected in the ceramic with x=0.4. Phase transition

temperature was shifted toward lower temperature by increasing Sr-doping. Moreover, detailed

tunability analysis for the present ceramics confirmed an enhancement by Sr-addition and the

maximum values were observed at Sr=0.2. The enhancement back to present multiple

components contribute to the dc-field induced dielectric constant, where at low electric fields

the Langevin theory indicate present polar nano-region (PNR) contributions into tunability

data, however domain switching contributions at high electric field was described by Johnson

equation. Ferroelectric properties appraised by P-E loop and positive up negative down

(PUND) methods have been introduced. The results shown the remnant polarization appraised

by PUND method is much lower than polarization estimated by P-E loop indicate the

difference values of Pr is mostly owing to leakage current contribution in P-E method.

Key words: (Ba,Sr)TiO3; Tunability; Sol gel; PNR; PUND

Corresponding author: abdelrazek.mahmoud@sci.svu.edu.eg

Introduction

Such kind of non-linearity dielectric materials in ferroelectric phase such as BaTiO3 (below

phase transition) possess an interesting functional properties in different area of electronic

applications such as tunable microwave, piezoelectricity, resonators, capacitors, oscillators,

actuators and so on[1]. However, mostly of tunable applications required a materials with low

relative permittivity, low dielectric loss and high response of permittivity with external applied

field[2,3] at room temperature. Pure BT prepared by solid state reaction technique have large

domain size in micro-meter with longe range order which imply it can possess high relative

value of dielectric constant even at room temperature and this is not suitable for tunable

applications[4,5]. Such kind of ferromagnetic materials have shown great performance when

used as phase shifting in phased antennas applications, due to controlled the change of the

magnetic permeability of the material under the application of a dc-magnetic field[6]. However,

these materials are very costly and have to be usually implemented in large size devices, posing

serious limitations in practical applications. Moreover, there are different models have been

derived to understand the mechanism or the relation between permittivity and dc-electric field

for different systems. For instance, permittivity dependent high electric field-normally refer to

domain switching-, had been derived by Johnson from the Helmholts free energy when high

electric field was applied into ferroelectric material[7]. Another model for ferroelectric domain

switching of PZT was proposed by Uchida et al [8], and they suggested it is 90o domain

reorientation by applying high electric field. Diamond [9] proposed that the nonlinearity of

dielectric constant of BST is attributed to field induced phase transition from non-ferroelectric

to ferroelectric. For ferroelectric system with a single polarization mechanism, the dc-tunability

data can be descriped by Landau–Ginzburg–Devonshire (LGD) model [10]. Another model has

been derived by Langevin [11] for interpretation the nonlinearity of permittivity dependent low

electric field (below domain switching) based on polar nanoregion (PNR), where PNR can

define as nano-scale areas with spontaneous polarization and the dipoles are easily re-orientable

under low external electric fields [12]. Recently, many researchers have been interested in

(Ba,Sr)TiO3 (BST) ferroelectric materials, which have shown superior tunable dielectric

properties for potential applications in microwave devices, such as antennas, resonators,

tunable filters and phase shifters in the microwave frequency range[13,14]. Pure

(Ba0.6Sr0.4)TiO3 ceramics prepared by solid state reaction technique and sintered at 1350°C for

2h showed high permittivity (~ 4400) and large dielectric loss (~ 0.1), which precluded their

use in most tunable applications[15]. Adding non-ferroelectric materials with low permittivity

values such as (MgO, Al2O3, BaWO4) into BST ferroelectric compound shown effective

enhancement the functional properties of BST including reducing the permittivity which can

make the material a more suitable in tunable applications[16,17]. Addition of MgO, was

reported suppress the dielectric constant of BST and as consequently enhanced their tunable

properties. Low percentage of Al2O3 – not excess 1%- shown a great effect on reducing the

dielectric properties of BST and improved the tunable properties at room temperature. One of

the appropriate method can pinched the dielectric properties of BST is reducing the domain size

from Polar micro-region to polar nano-region. This can achieve by prepare the material in nano

range of particles by physical method such as High-activation energy ball milling or chemical

method such as Sol gel method[18,19].

In this study, we aim to enhancement the tunable properties of pure BST by reducing the

domain size to be in nano range and as consequently pinched the dielectric properties by using

Sol-gel method. Furthermore to understand the mechanism of the electric field dependence of

the dielectric constant in (Ba1-xSrx)TiO3 (0≤x≤0.4) with particular attention to the polar nano-

regions contribution of polar nano-regions on the tunability behaviour. The interpretation is

supported by the study of the P-E loops and the PUND curves.

Experimental

Perovskite (Ba1-xSrx)TiO3 ceramics (0.0≤x≤ 0.4) were synthesized using the sol-gel

technique. High purity of barium acetate Ba(CH3COO)2 (Alphchem 99.99% assay), strontium

acetate Sr(CH3COO)2 (Alphachem 99.99%) and titanium isopropoxide Ti[OCH(CH3)2]4

(Alphachem 99.99%) were weighed in stoichiometric and have been used as a raw materials for

fabricate BST powder. Glacial acetic acid (CH3COOH) (Technochem 98%) was used as a

solvent for both barium acetate and strontium acetate, while 2-propanol [(CH3)2CHOH]

(Technochem 98%) was used as a solvent for titanium isopropoxide. The procedure for the

preparation of BST ceramics is reported in the flowchart shown in Fig.1 and described in

details in our previous work [20,21]. The calcined powders at 1000oC/3h were mixed with a

binder polyvinyl alcohol and pressed into discs of 10mm diameter and 1.5mm thickness. The

discs were sintered at 1300oC for 2h in air. Small amount of calcined powder was mixed with

potassium bromide (KBr) and Fourier transform infrared-spectrometer (FT/IR-4100) was used

to identify the infrared-active functional groups. The phase identification was carried out by

analysing the X-ray diffraction (XRD) patterns obtained with PANalytical X'Pert PRO

diffractometer. The morphology of BCT ceramics was examined by scanning electron

microscopy (SEM) (JEOL JSM-840A). The sintered samples were electroded with fired silver

paste for dielectric, tunability, piezoelectric and ferroelectric measurements. The dielectric

properties were measured on unpoled ceramics using an LCR meter (HIOKI 3532-50 LCR

HITESTER). The electric-field dependence of the dielectric response (tunability) and

ferroelectric properties were measured at room temperature using a specific tester (RADIANT

Precision II Multiferroic Ferroelectric Test System 10kV HVI-SC Model 609B).

Results and discussion

3.1. Structural and dielectric properties

Fig.2 shows the Fourier transform infrared spectra of (Ba1-xSrx)TiO3 (0 ≤ x ≤ 0.4) calcined

powders. The spectrum of pure BaTiO3 calcined at 900°C for 3h shows a band around 1430cm-

1 and a band around 856cm-1, which correspond to vibrational modes characteristic of the

𝐶𝑂32−group in the BaCO3 [22]. The bands close to 567cm-1 (broad) and 406cm-1 (sharp) have

been assigned to υ(Ti-O) stretching vibration and δ(Ti-O) torsional bending vibration,

respectively [23]. The spectrum of BT at high calcination temperature (950oC) shows similar

features, but with significantly lower intensities of the CO3 bands. High calcination temperature

(1000oC) leads to the disappearance of the bands belonging to CO3 vibrations and the only

presence of Ti-O vibrational modes can be observed. The X-ray diffraction (XRD) patterns of

(Ba1-xSrx)TiO3 (0 ≤ x ≤ 0.4) calcined powders (1000oC/2h) are displayed in Fig.(3). The

diffractograms of the calcined powder with 0 ≤ x ≤ 0.2 can be indexed with the tetragonal

structure of BT with P4/mm space group (PDF 01-075-0473 card in JCPDS database).The

diffraction peaks of the composition x=0.4 can be indexed with the cubic symmetry of BT

perovskite structure (Pm-3m space group) (PDF 01-074-1967 card in JCPDS database). This

confirms that Sr2+ ions have successfully diffused into the BT lattice to form BST homogenous

single phase perovskite structure. Additional peaks belonging to BaCO3, TiO2 and BaO were

observed in all patterns except in the x=0.2 compound. Fig.(4) shows the Rietveld refinements

of (Ba1-xSrx)TiO3 sintered ceramics (1300oC/2h) which was carried out with Fullprof software

to estimate the accurate values of the lattice parameters. The figure displays also the

diffractograms of the ceramics x=0 and x=0.2 zoomed in the interval 2θ = 44° - 46°. It can be

noticed that pure BT exhibits a split peak containing (002)/(200) reflections, while x=0.4 shows

only the (200) single peak, evidencing the effect of Sr addition on the structural properties of

BT. Furthermore, Scherer equation was applied to estimate the crystallite size of the sintered

ceramics and the results were found as (D=110nm for BT), (D=90nm for Sr=0.05), (D=75 for

Sr=0.1), (D=60nm for Sr= 0.2) and (D=50nm for Sr=0.4). Fig.5 shows the surface

morphologies of (Ba1-xSrx)TiO3 (0 ≤ x ≤ 0.2) sintered ceramics. As can be observed, the grain

size decreases with increasing the Sr- content (as shown by the grain size distributions in the

insets). The EDS analysis confirmed the absence of contaminations or undesired elements.

Fig.6 shows the temperature dependence of the dielectric permittivity and loss of BST

ceramics at different frequencies (1, 10, 100 and 500 kHz) during heating from room

temperature up to approximately 190°C. Pure BT shows the presence of two phase transitions:

(i) orthorhombic-tetragonal (TO-T) at about 35°C and (ii) tetragonal-cubic (TT-C) at 135°C. The

other compositions show only the tetragonal-cubic phase transition, whose corresponding

temperature (TT-C) decreases with increasing Sr content. The phase transition temperature TO-T

shifted below room temperature by Sr addition. The maximum value of permittivity was

observed in pure BT in the entire temperature range studied. The permittivity’s decrease upon

Sr substitution is due to a smaller contribution of domain walls to the permittivity, caused by

the Sr addition which leads to a less distorted, more centro-symmetric and less polar structure.

The relative permittivity slightly decreases with increasing frequency at low temperature, while

it becomes nearly frequency-independent at high temperature (from ~110oC up to Curie

temperature). The dielectric loss (tan δ) increase with increasing frequency in the whole

temperature range and the maximum value of loss was detected at near the phase transition

temperature.

3.2. Tunability Properties

The dc-electric field dependence of the permittivity at room temperature of (Ba1-xSrx)TiO3

(0≤x≤0.4) ceramics is shown in Fig.(7a). It can be observed that all the compositions display

similar behavior, characterized by a decrease of the dielectric permittivity with increasing

applied field. The variation of the dielectric constant with an applied DC electric field is termed

tunability, which can be defined by the following relationship:

𝑇𝑢𝑛𝑎𝑏𝑖𝑙𝑖𝑡𝑦 (%) = 𝜀𝑟(0) − 𝜀𝑟(𝐸)

𝜀𝑟(0) (1)

where 𝜀𝑟(0) is the dielectric constant in absence of the DC electric field and 𝜀𝑟(𝐸) is the

dielectric constant in presence of an applied DC electric field of magnitude E. The tunability of

the BST ceramics at different dc electric fields is displayed in Fig. (7b). It can be seen that in

compositions with 0 ≤ x ≤ 0.2, the tunability increases with increasing Sr-addition in the entire

dc field range considered. In particular, under a dc field of 3kV/cm, the tunability increased

from 18.4% at x=0.0, to 19.9% for x=0.05, 23.34% for x=0.1 and 25.43% for x=0.2, while at

x=0.4, the tunability decreased to 13.14%.

The tunability phenomenon is not exclusively observed in ferroelectric materials, but it has also

been reported in relaxors such as (Ba,Sn)TiO3[24], non-ferroelectric dielectric materials like

BaWO4 [25], and ferroelectric materials in their paraelectric phase such as SrTiO3[26]. It is

worth to recall that the macroscopic tunability can reflect a number of bias field-induced effects

occurring at different material length scales. These include: i) intrinsic mechanisms active at

the unit cell scale, such as lattice phonons; ii) the response of polar nano-regions (PNR) at the

nano-scale; iii) ferroelectric domain switching and domain wall movement at the submicron

scale, and iv) interfacial grain-grain boundary effects at the micron scale [11].

The dc-tunability data can be described with appropriate models that are able to account the

different underlying mechanisms active in specific ranges of temperature and DC electric field.

In polar dielectrics, the tunability is usually described by the multi-polarization mechanisms

model proposed by Ang and Yu [27]

𝜀𝑟(𝐸) =𝜀𝑟(0)

[1 + 𝛼(𝜀0𝜀𝑟(0))3

𝐸2]1/3+ ∑

𝑃0

𝜀0[cosh(𝐸𝑥)]−2 (2)

where Ʃ indicates the sum over different cluster-polarization, α is the temperature-independent

coefficient, 𝑃0 is the effective polarization of one cluster dependence on temperatures, and x is

defined as 𝑥 =𝑃0𝑉

𝐾𝐵𝑇, where V is the volume or size of the cluster and KB is Boltzmann’s

constant.

Also, by assume a small polarization and high dc-electric field able for domain switching was

applied into ferroelectric material or in the state close to ferro-paraelectric phase transition, the

tunability data can be described by Johnson scenario [7] model by the following equation (3)

𝜀𝑟(𝐸)

𝜀𝑟(0)=

1

{1 + 𝛼[𝜀𝑜𝜀𝑟(0)]3𝐸2}1/3 (3)

However, Johnson model was modified by Langevin model to consider the non-domain

switching extrinsic polarization contributions at low DC-electric field, where Johnson equation

is usually completed with one (L1) or more (L2) of Langevin-type terms dependent on the

applied field associated with different polarization contributions such as domain switching and

polar nanoregion contributions [11]. The Langevin model can express by the following

equation;

𝜀𝑟(𝐸) =𝜀𝑟(0)

[1 + 𝛼(𝜀0𝜀𝑟(0))3

(𝐸 𝐸𝑐−+ )2]1/3

+ ∑𝑃0

𝜀0[cosh(𝐸𝑥)]−2 (4)

The dc- electric field dependence permittivity at room temperature of (Ba1-xSrx)TiO3

(0.0<x<0.4) sintered ceramics shown in Fig.(6.a). As observed, all the compositions have

shown the same behavior, where dielectric permittivity decreases with increasing the applied

field which is attributed to the lattice deformation as an intrinsic contributions and as

consequently variation the domain wall structure due to applying high external electric field

[28]. Tunability of the present ceramics versus dc-electric field displayed in Fig. (6.b), The

figure shown the tunability increased by Sr-addition, where it found 18.4% for x=0.0, 19.9%

for x=0.05, 23.34% for x=0.1 and 25.43% for x=0.2, then it was observed decreased to 13.14%

for x=0.4 at (3kV/cm). Permittivity versus dc-field for all compositions can be described by

Johnson equation (no.3) for understanding the mechanism of tunability behavior of the present

ceramics. Fig. (8) shows the fitting of the experimental data of (Ba1-xSrx)TiO3 (0.0<x<0.4)

sintered ceramics, arranged in the plot of [εr(0)/εr(E)]3 against (E)2. The fitting was carried out

using Johnson’s model.

Pure BT ceramic show only one linear region over the entire electric field range, indicating

that the permittivity dependence on the external dc field can be approximately described solely

by the Johnson model.

however, the other compositions (0.05≤x≤0.4) showed two different regions of linear fitting . In

these compositions, the linear fitting with large slope at low electric field indicate the dc-

tunability data can describe by Johnson equation combined with one Langevin-type term (L1),

while the linear fitting at high electric field indicate that the tunability date can explain by

Johnson equation only. Present two linear fitting indicated that an additional extrinsic effect

into the tunability such as polar nano-regions contributions [10]. As observed by SEM (Fig 4),

the particles size were observed decreased by Sr-addition which lead to decrease the domain

size, so that increasing the tunability by Sr-addition could be owing to increase the effect of

PNR contributions, where PNR can define as nano-scale areas with spontaneous polarization

and the dipoles are easily re-orientable under small or low external electric fields [19] which

can give a contributions into permittivity and tunability as well at these range of fields. Decline

slopes at high electric field can explain as the large electric fields can convert the PNR into

micro or macro-scale of domains and consequently their contribution cannot detected at high

fields. Decreasing of tunability data at x=0.4 can confirm by decline the slope of linear fitting

at low electric field and this can interpret due to decay of PNR contribution due to form

paraelectric phase at room temperature. It is worth to notice that, the tunability increase rapidly

with electric field however at higher bias field it tend to be saturated. This could be owing to at

high dc field, polar nano-domains would grow up accompanied by congelation of polar nano-

domains which leads to a reduction of dielectric permittivity [17]. A similar behavior of

tunability versus dc-field has been reported[19]. So, we can conclude that pure nano-BST with

Sr=0.2 ferroelectric material has an acceptable and appropriate tunability properties which

makes the material a promising candidate for tunable devices applications.

3.3. Ferroelectric properties

The polarization-electric field (P-E) hysteresis loops of BST ceramics are presented in Fig.

(9). The measurements were performed at room temperature by applying triangular waveforms

of 30 kV/cm amplitude and 1Hz frequency. The remnant polarization (Pr) decreases with

increasing Sr addition due to a suppression of the structure polarity in absence of an applied

electric field, as also indicated by the decrease of the Curie point observed in the permittivity

measurements. All P-E loops show polarization gap in correspondence of the negative remnant

polarization. The polarization gap can arise when the polarization relaxation takes place

between electric field cycles, due to the presence of an internal field that tends to reduce the

remnant polarization in absence of an applied field.

Fig. (10) shows a ferroelectric properties of three different compositions (Ba1-xSrx)TiO3 (x=0.0,

0.1 and 0.4 mol%) ceramics appraised by positive up negative down pulses method (PUND)

with pulse delay (1000ms) and pulse width (10ms). PUND method initially reported by Scott

et-al is being used to measure the switching charge density by applying four voltage pulses into

the sample. The first pulse is used to measure the total amount of polarization (P*), while the

second pulse (P^) was applied to measure the imaginary part of polarization. So that the

switching charge density (QSW) can define as the following equation;

𝑄𝑠𝑤 = ( 𝑃∗ − 𝑃^) (4)

Third (-P*) and Fourth (-P^) pulses were used for the same purpose of first and second pulses

but in negative bias of electric field (pulses in down direction).

Pure BT shown high switching charged density value (Qsw = 1.239µC/cm2) compared to the BT

doped Sr. In both of Sr=0.1 and 0.385, both of P* and P^ are close to each other’s, which

confirm the switching charge density is close to zero value (Qsw ~ 0). These results confirm

that, the remnant polarization estimated by P-E loop is due to the contribution of leakage

current polarization with absent any contribution of switching charged density. Similar results

for different composition have been reported[29].

Fig.(11) shows strain induced dc electric field of (Ba1-xSrx)TiO3 bipolar ceramics, (0.0<x<0.4)

at room temperature. The figure shown the piezoelectric properties decreased with increasing

Sr content and completely suppressed at 0.4 of Sr addition due to form pure BST cubic phase.

A significantly high converse piezoelectric response with d33=336pm/v has been observed in

pure BT which considered a promising material for piezoelectric applications.

Conclusion

Lead-free ferroelectric (Ba1-xSrx)TiO3 (0≤x≤0.4) perovskite structure were synthesized by

sol gel method. All compositions belong x<0.4 exhibit tetragonal phase however x=0.4 a cubic

phase structure at room temperature has been detected. Tetragonal-cubic phase transition were

shifted toward lower temperature by increasing content of Sr2+. Tunability properties of the

present compositions can be enhanced by Sr addition, where the highest tunability data were

observed at x=0.2 due to present the polar nano-regions contributions at low electric fields.

While at high electric field, the domain switching effect only can contribute into the tunability

data. Ferroelectric properties measured by P-E loop and PUND methods shown a different

values of remnant polarization and the difference can represent the switching charged density.

Pure BT shown appropriate value of switching charged density (Qsw= 1.06µC/cm2), however

x=0.4 shown (Qsw ~ 0) due to form cubic phase at RT.

Acknowledgment

This Project was supported financially by the academy of Scientific Research and

Technology (ASRT), Egypt, Grant No 6700.

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Figures caption

Fig.1. Flowchart of synthesis BST by sol–gel method[15].

Fig.2. FTIR spectra of (Ba1-xSrx)TiO3 (0 ≤ x ≤ 0.4) calcined powder at (900-1000oC).

4000 3000 2000 1000 0

C

O3

T

i-O

T

i-O

Sr=0.4,Tc=1000oC

Sr=0.2,Tc=1000oC

Sr=0.1,Tc=1000oC

Sr=0.05,Tc=1000oC

Sr=0,Tc=1000oC

Sr=0,Tc=950oC

Sr=0,Tc=900oC

Wavenumber(Cm-1)

Tra

nsm

itta

nce

(%)

C

O3

Fig. 3. X-ray diffraction patterns of (Ba1−xSrx)TiO3 (0.0 ≤ x ≤ 0.4) calcined powder.

Fig. 4. Rietveld refined of (Ba1-xSrx)TiO3 (0<x<0.4) ceramics sintered at 1300oC/2h.

Fig. 5. SEM micrographs of the (Ba1-xSrx)TiO3 ceramics sintered at 1300oC and EDS of Sr=0.1.

1 2 3 4 5 60

2

4

6

8

10

Co

un

t

Particle size(m)

Sr=0.025

1 2 3 4 5 6 7 80

1

2

3

4

5

6

Co

un

t

Particle size(m)

Sr=0.0

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.00

4

8

12

16

20

Co

un

t

Particle size(m)

0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.40

1

2

3

4

5

6

7

8

Co

un

t

Particle Size (m)

1.0 1.2 1.4 1.6 1.8 2.0 2.20

1

2

3

4

5

6

Co

un

t

Particle size (m)

Fig. 6.Temperature dependence of dielectric permittivity and loss at different frequencies (a-e); frequency

dependence of dielectric loss at room temperature (f); permittivity at room temperature and 1kHz frequency for

different Sr content (inset f).

20 40 60 80 100 120 140 160 180 200

1000

2000

3000

4000

5000

Temperature(oC)

Pe

rmit

tiv

ity

()

Sr=0.0

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14 1KHz

10KHz

100KHz

500KHz

Die

lectr

ic L

oss (

tan

)

TO-T

TT-C

20 40 60 80 100 120 140 160 1800

1000

2000

3000

4000

5000

Temperature(oC)

Pe

rmit

tiv

ity

()

1KHz

10KHz

100KHz

500KHz

Sr=0.05

0.0

0.2

0.4

0.6

0.8

1.0

Die

lec

tric

Lo

ss

(ta

n

)

20 40 60 80 100 120 140 160 1800

500

1000

1500

2000

2500

3000

Temperature(oC)

Pe

rm

itti

vit

y ()

1KHz

10KHz

100KHz

500KHz

Sr=0.1

0.0

0.1

0.2

0.3

0.4

0.5

Die

lec

tric

Lo

ss

(ta

n

)

20 40 60 80 100 120 140 160 1800

500

1000

1500

2000

2500

Temperature(oC)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Perm

itti

vit

y ()

Temperature(oC)

1KHz

10KHz

100KHz

500KHz

Sr=0.2

Die

lec

tric

Lo

ss

(ta

n)

20 40 60 80 100 120 140 160 180100

200

300

400

500

600

700

800

900

Temperature(oC)

Pe

rmit

tiv

ity

()

Sr=0.4 1KHz

10KHz

100KHz

500KHz

0.0

0.2

0.4

0.6

0.8

1.0

Die

lectr

ic L

oss (

tan

)

1 10 100 1000

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Die

lec

tric

lo

ss

(ta

n)

Log Frequency (KHz)

Sr=0.0

Sr=0.05

Sr=0.1

Sr=0.2

Sr=0.4

0 5 10 15 20 25 300.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7x=0.2

x=0.1

x=0.05

x=0.0

DC bias field(kV/cm)

Tu

nab

ilit

y(%

)

0.4

b

0 5 10 15 20 25 30

400

800

1200

1600

2000

2400

Pe

rmit

tiv

ity

()

DC bias field(kV/cm)

x=0.0

x=0.05

x=0.1

x=0.2

x=0.4

a

Fig. 7. DC bias field dependences of dielectric permittivity (a) and tunability (b) of the

(Ba1-xSrx)TiO3 (0<x<0.4) ceramics measured at room temperature.

0 200 400 600 800 1000

0

5

10

15

20

25

30

35

40

E2(kV/cm)

[(0

)/(

E)]

3 J

Y=0.2481+0.038X

Sr=0.0

0 200 400 600 800 1000

0

5

10

15

20

25

30

35

[(0

)/(E

)]3

Y=0.51+0.041x

J+L1

J Sr=0.05

0 200 400 600 800 1000

0

5

10

15

20

25

30

35

[(0

)/(

E)]

3

Y=0.53+0.045x

J+L1

J

S=0.1

0 200 400 600 800 1000

0

5

10

15

20

25

30

35

40

J

Y=0.5+0.02x[(0

)/(

E)]

3

Sr=0.4

J+L1

0 200 400 600 800 1000

0

5

10

15

20

25

30

35

[(0

)/(

E)]

3

Y=0.54+0.056x

J+L1

J

Sr=0.2

E2(kV/cm)

Fig. 8. Electric field dependences of dielectric permittivity of (Ba1-xSrx)TiO3 (0<x<0.4)

estimated by Johnson and Langevin models at room temperature.

-30 -20 -10 0 10 20 30

-18

-15

-12

-9

-6

-3

0

3

6

9

12

15

E(kV/cm)

P(

C/c

m2)(Sr=0.0)

-30 -20 -10 0 10 20 30

-10

-8

-6

-4

-2

0

2

4

6

8

E(kV/cm)

P(

C/c

m2)(Sr=0.05)

-30 -20 -10 0 10 20 30

-8

-6

-4

-2

0

2

4

6

8

P(

C/c

m2)

E(kV/cm)

(Sr=0.1)

-30 -20 -10 0 10 20 30

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

E(kV/cm)

P(

C/c

m2)(Sr=0.2)

-30 -20 -10 0 10 20 30

-4

-3

-2

-1

0

1

2

3

P(

C/c

m2)

E(kV/cm)

(Sr=0.4)

0.0 0.1 0.2 0.3 0.4

0

1

2

3

4

5

6

Pr(

C/c

m2)

Sr-content (mol %)

Fig. (9). P-E hysteresis loops of the(Ba1-xSrx)TiO3 bipolar ceramics (0<x<0.4) at 1Hz at room

temperature.

Fig. 10. PUND sequence of (Ba1-xSrx)TiO3 ceramics (x=0, 0.1 and 0.4) with inset of switched

charge ferroelectric density (QSW) value.

Po

lari

zati

on

(µ

C/c

m2)

Pulse Top/Down Sequence

P^=5.715

-P*=-6.958

-P^=-5.718

Pulse delay

Sr=0.0, Qsw=1.239 µc/cm2P*=6.954

P*=3.842P^=3.457

-P*=-3.845-P^=-3.461

P*=1.305 P^=1.301

-P*=-1.298 -P^=-1.304

Sr=0.1, Qsw=0.385 µc/cm2

Sr=0.4, Qsw=0.004 µc/cm2

PU

N D

Fig. 11. Strain induced electric field of (Ba1-xSrx)TiO3 ceramics (0<x<0.4) and piezoelectric

coefficient variation Sr2+ doping.

Table.1. Lattice parameters estimated by reitveld refinement, max permittivity (εmax), Curie

temperature (Tc) and piezoelectric coefficient (d33) of (Ba1-xSrx)TiO3 ceramics (0<x<0.4).

Sample

a(Å) b(Å) c(Å) εmax Tc(oC)

Sr=0.0

4.0732 4.0732 4.1210 5030 127.5

Sr=0.05

3.9925 3.9925 4.0541 4510 122.5

Sr=0.1

3.9815 3.9815 3.9978 2230 102

Sr=0.2

3.9702 3.9702 3.9872 2200 73

Sr=0.4

3.9652 3.9652 3.9652