Impedance spectroscopy and piezoresponse force microscopy analysis of lead-free (1 − x) K0.5Na0.5NbO3 − xLiNbO3 ceramics.pdf

nNb

i b

Camndiamat

iNbe p

indicate that single phase was formed for pure KNN while a small amount of second phase (K Li Nb O ,

area of.5Na0.5

However, a few drawbacks still prevent wide scale industrial useof KNN-based materials owing to the volatility of the alkali speciesat high temperatures [2] and hygroscopic nature of the reactantpowders [3] leading to non-stoichiometry and inhomogeneouscompositions. All these problems require carefully controlled

2 3ceramics in order to improve themechanical quality factor Qm [10e12]. Also, numerous compositional engineering approaches havebeen explored to optimize the piezoelectric properties of KNNmaterials like adding BaTiO3 [13], SrTiO3 [14], LiNbO3 [15], LiTaO3[16], LiSbO3 [17], and ZnO [18] to form new solid solutions. Amongthese (K0.5Na0.5)NbO3 LiNbO3 (abbreviated as KNLN)-basedceramics was considered as an excellent candidate for lead-freepiezoelectric ceramics because of high piezoelectric propertiessuch as d33 210e300 pC/N and high Curie temperature (450 C).

* Corresponding author.

Contents lists available at

Current Appl

.e

Current Applied Physics 13 (2013) 430e440E-mail address: [email protected] (R. Rai).a solid solution of ferroelectric potassium niobate (KNbO3) andantiferroelectric sodium niobate (NaNbO3). KNN is the only knownlead-free system with a perovskite structure, which has a higherCurie temperature than the commonly used lead zirconium titanate(PZT). The high Curie temperature, relatively strong electrome-chanical coupling factor, presence of morphotropic phase boundary(MPB) and environmentally friendly constituents are severaladvantages of KNN system, thus making it a promising lead-freepiezoelectric material. Moreover, following the work of Saito et al.[1], KNN-based materials in fact outperformed other important Pb-free bismuth sodium titanate (BNT)-based family.

[4,5]. Various methods to overcome the drawbacks pertinent toKNN have been undertaken targeting mainly the sintering anddensication behavior of KNN-based ceramics. Densication ofKNN ceramics signicantly improved by hot-pressing, with sin-tered ceramics reaching w99% of the theoretical density and theresulting piezoelectric constant nearly twice the value of conven-tionally sintered KNN [6,7]. Dense ceramics with a high piezo-electric constant of 148 pC/N were achieved by Li et al. [8] usingadvanced processing methods compared to 90 pC/N for undopedKNN prepared using conventional methods [9]. Sintering aids likeCuO and Bi O have also been used to modify pure and doped KNNKeywords:CeramicsImpedance spectroscopyX-ray diffractionDielectric propertiesPhase transactions

1. Introduction

Recently, much attention in theceramics has been focused on (K01567-1739/$ e see front matter 2012 Elsevier B.V.http://dx.doi.org/10.1016/j.cap.2012.09.009analysis was performed using an equivalent circuit model. The impedance response in pure KNN andKNLN ceramics could be deconvoluted into two contributions, associated with the bulk (grains) and thegrain boundaries. Activation energies for conductivity were found to be strongly frequency dependent.The activation energy obtained from dielectric relaxation data was attributed to oxygen vacancies. FromPFM we found that the composition with 6.5 wt.% LN displays stronger piezocontrast as compared topure KNN implying an evidence of a pronounced piezoelectric coefcient.

2012 Elsevier B.V. All rights reserved.

lead-free piezoelectric)NbO3 (KNN) which is

manufacturing conditions and low reaction temperatures causingpoor densication behavior. The high dielectric losses related to theA-site vacancies and oxygen deciencies resulting from the evap-oration loss of alkali ions are also detrimental to the KNN ceramicsAccepted 12 September 2012Available online 18 September 2012 structure from orthorhombic to tetragonal with increase in LN content. The electrical behavior of the

ceramics was studied by impedance spectroscopy technique in the high temperature range. Impedance10 September 20126 4 10 30

w3%) was present in LN doped KNN ceramics. Phase analysis indicated the change in the crystalImpedance spectroscopy and piezorespoof lead-free (1 x) K0.5Na0.5NbO3 xLiRadheshyam Rai a,*, Indrani Coondoo a, Rashmi RanaDepartment of Materials and Ceramics Engineering and CICECO, University of Aveiro,b Ferroelectric Research Laboratory, Department of Physics, AN College, Patna 800013, IcDepartment of Mechanical Engineering and Centre for Mechanical Technology & Auto

a r t i c l e i n f o

Article history:Received 16 May 2012Received in revised form

a b s t r a c t

(1 x) K0.5Na0.5NbO3 xLferroelectric ceramics wer

journal homepage: wwwAll rights reserved.se force microscopy analysisO3 ceramics

, Igor Bdikin c, Seema Sharma b, Andrei L. Kholkin a

pus Universitario de Santiago, 3810-193 Aveiro, Portugal

ion, University of Aveiro, 3810-193 Aveiro, Portugal

O3 (where x 0.0, 5.0, 5.5, 6.0, and 6.5 wt.%) (KNLN) perovskite structuredrepared by the solid-state reaction method. X-ray diffraction patterns

SciVerse ScienceDirect

ied Physics

lsevier .com/locate/cap

Until now, however, there is no report on detailed impedance andpiezoresponse force microscopy (PFM) analysis in KNLN ceramics.Consequently, the purpose of this paper is to investigate the effectof the amount of LiNbO3 onmicrostructure, dielectric, electrical andlocal piezoelectric properties of KNN ceramics.

2. Experimental procedure

Polycrystalline samples of (1 x) K0.5Na0.5NbO3 xLiNbO3(where x 0.0, 5.0, 5.5, 6.0, and 6.5 wt.%) (KNLN) ceramics wereprepared by the conventional mixed oxide route. The startingmaterials were high purity metal oxide or carbonate powders,K2CO3 (>99.8%), Na2CO3 (>99.8%), Li2CO3 (>99.99%), and Nb2O5(>99.9%). The powders were weighed according to the stoichio-metric proportion for the required compositions andmixed for 48 husing propan-2-ol and zirconia media. The powders were calcinedat 920 C for 4 h, after that the resulting powders were mixedthoroughly with a PVA binder solution. Cylindrical pellets wereprepared in a uniaxial press with a 10 mm diameter die at 100 MPaand then sintered at 1000 C for 4 h and nally cooled to roomtemperature at the rate of 180 C/h. Silver paste as electrodes wereapplied on the top and bottom surfaces on the samples for theelectrical measurements.

The formation and quality of compounds were veried with X-ray diffraction (XRD) technique. The XRD patterns of the

compounds were recorded at room temperature using X-raypowder diffractometer (Rigaku Miniex, Japan) with CuKa radia-tion (l 1.5405 A) in a wide range of Bragg angles 2q(20 2q 60) at a scanning rate of 2 min1. SEMmicrographs ofthe fractured surface of the sintered samples were obtained withthe help of JEM-2000FX (JEOL Ltd) scanning electron microscopeoperated at 20 keV. The dielectric properties of the samples weredetermined from RT (room temperature) to 450 C using animpedance analyzer (PSM Impedance Analyzer 1734) from 100 Hzto 1 MHz. A commercial Atomic Force Microscope (AFM, Multi-mode, Nanoscope IIIA) was used for the ferroelectric domainimaging. The microscope was equipped with an external lock-inamplier (SR-830, Stanford Research) and a function generator(FG120, Yokogawa). Stiff conducting Si cantilevers (42 Nm1, PPP-NCHR, Nanosensors) were used for the measurements performedin ambient environment. The tip had the shape of a polygon-basedpyramidwith the height of 10e15 mmand the radius rtipz 10 nm atthe apex. The PFM studies were carried out with Uac 5 V andf 50 kHz applied voltage on the tip.

3. Results and discussion

Fig. 1 shows XRD of the KNN LN ceramics for differentcompositions with x 0.0, 5.0, 5.5, 6.0, and 6.5 wt.%. The crystalstructure changes from orthorhombic to tetragonal with increasing

In

te

nsity ( a. u

. )

KNN

KNN - 5.0LN

KNN - 5.5LN

KNN - 6.0LN

KNN - 6.5LN

(1

01

)

(0

10

)

(0

02

)

(1

11

)

(2

02)

(0

20

)

(2

12

)

(1

21

)

(1

13

)

(3

11)

2)

In

te

nsity ( a. u

. )

KNN

KNN-5LN

KNN-5.5LN

KNN-6.0LN

KNN-6.5LN

Tetragonal phase appears

3. LN

a b

R. Rai et al. / Current Applied Physics 13 (2013) 430e440 43120 30 40 50 60

2 (degrees)

3.94

3.96

3.98

4.00

4.02

4.04

0 1 2

Latti

ce Pa

ram

eter

((

Conc

a b c

densitycFig. 1. (a) XRD of the (1 x) K0.5Na0.5NbO3 xLiNbO3 ceramics and (b) enlarged XRD pacompositions x 0.0, 5.0, 5.5, 6.0 and 6.5 wt.% respectively.44 45 46 47

(0

20)

(2

0

2 (degrees)

4 5 6

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

(wt%)

gm/c

m3

tterns of the ceramics for 2q 44e48 (c) lattice parameter and density of different

x. Since the crystal structure of KNN (perovskite structure) is verydifferent from LiNbO3 (ilmenite structure), the single phaseformation suggests that Li and Nb3 have diffused into the KNN

angles implying an increase in the lattice parameters withincreasing x. This may be attributed to the smaller ionic radii of Li

as compared to those of Na (1.16 A) and K (1.52 A). In the

Fig. 2. SEM micrograph illustrating typical microstructure for (1 x) K0.5Na0.5NbO3 xLiNbO3 ceramics of different compositions x 0.0, 5.0, 5.5, 6.0 and 6.5 wt.%.

R. Rai et al. / Current Applied Physics 13 (2013) 430e440432lattice, with Li entering the (Na0.5K0.5) site and Nb5 occupyingthe B site, to form a homogeneous solid solution. As x increases,a tetragonal phase appears and increases continuously. It is alsonoted that the diffraction peaks shift slightly toward low diffractionFig. 3. (a) Variation of dielectric constant, (b) loss (tan d) with temperature (c) plots of peak(1 x) K0.5Na0.5NbO3 xLiNbO3 ceramics of different compositions x 0.0, 5.0, 5.5, 6.0 anorthorhombic zone, the lattice constants c and a have very closevalues, which explains the two peak splitting of (200) reections ata 2q 45.5 for an orthorhombic structure, rather than three peaksplitting. Lattice constants and density of KNN-LN ceramicsr and Tc and (d) r and transition temperature vs. doping concentration at 100 kHz ford 6.5 wt.%.

depending on LiNbO3 concentration were calculated as shown inFig. 1(c). Lattice constants of KNN ceramics in orthorhombicsymmetry were calculated as a 3.9551 A, b 3.95765 A andc 4.0502 A, which are similar to the bulk KNN. As x changes thecrystal structures changed from orthorhombic to tetragonalsymmetry, which indicated that MPB coexisted orthorhombic andtetragonal phase. The small XRD peaks at 33 and 52 for KNNdoped with for x 5.5 and x 6.0% are attributed to an ortho-rhombic phase of Nb2O5 (O-Nb2O5) corresponding to JCPDS lenumber 527-1313. These peaks are probably due to the formation ofNb-rich second phase leading to the creation of A-site vacancies inthe ABO3 perovskite structure.

Fig. 2 presents the scanning electron micrographs for fracturedsurfaces of KNN xLN with 0.0, 5.0, 5.5, 6.0, and 6.5 wt.%. Thefracture surfaces show homogeneous grains with dense structure.The grain size of all samples is of the order of 2e4 mm.

The temperature dependence of the permittivity and loss shownin Fig. 3(a and b) conrms the presence of a phase transition fromorthorhombic to tetragonal at w200 C for pure KNN and tetrag-onal to cubic at w380 C. However, the composition containing6.5 wt.% LN, the peak related to phase transition temperature fromthe orthorhombic to the tetragonal is not observed. It is probablylocated below room temperature as has been observed in the caseof 7% lithium doped KNN ceramics [19]. It is also observed that with6.5 wt.% LN in KNN, the dielectric permittivity is reduced while theCurie temperature increased (Fig. 3c and d). The loss tangent (tan d)is also found to decrease with LN addition. A-site vacancy createdby the Nb rich phase (as is evident from XRD spectra), increases the

dielectric loss factor for x > 5.5% while the room temperaturedielectric constant value and the dielectric loss value are almostunaffected by the introduction of vacancies. The A- and B-siteelements have valence states of 1 and 5, respectively, whileoxygen has 2. In KNN ceramics, Nb is believed to be incorporatedat the B-site of the perovskite lattice and therefore acts as anacceptor. Oxygen vacancies are formed and when they combinewith Nb ions, defect dipoles are created. These defect dipoles movealong the direction of polarization and are based on the principle ofsymmetry conforming of point defects and lead to hard dopingeffects. It is well known that piezoelectrics with low permittivityare useful for energy harvesting because the gure of meritdescribes the operation of the energy harvesting system [20].According to this, a material with low permittivity will have highvoltage induced by strain. In this regard, the composition with6.5 wt.% LN is expected to exhibit a higher hij.

Fig. 4(a and c) shows variation of the real part of impedance(Z0) as a function of frequency (103e106 Hz) for differenttemperatures and compositions. The pattern shows a sigmoidalvariation as a function of frequency in the low frequency regionfollowed by a saturation region in the high frequency region. Thissuggests of the manifestation of the mixed nature of polarizationbehavior in the material. The merger of the real part of impedance(Z0) in the frequency domain for all temperatures indicatesa possibility of the release of the space charge as a result oflowering in the barrier properties of the material. These resultsindicate that the electrical conduction will increase with rise intemperature and the phenomenon is dependent on the release of

60

80 5LN 5.5LN 6LN 6.5LN

RT

300

400 5LN 5.5LN 6LN 6.5LN

RT

0

a b

R. Rai et al. / Current Applied Physics 13 (2013) 430e440 43310 100 1000

0

20

40

Frequency (kHz)

Z'(k

)

10 100 1000

5

10

15

20

Frequency (kHz)

Z'(k

)

5LN 5.5LN 6LN 6.5LN

400 0Cc

0Fig. 4. (a, c) Variation of real part impedance (Z ) and (b, d) imaginary part of the impedanceramics with frequency at different temperatures.10 100 1000

0

100

200

Frequency (kHz)

z"(k

)

10 100 10000

2

4

6

8

Frequency (kHz)

z"(k

)

5LN 5.5LN 6LN 6.5LN

At 400 0Cd

00ce (Z ) of (1 x) K0.5Na0.5NbO3 xLiNbO3 (where x 0.0, 5.0, 5.5, 6.0, and 6.5 wt.%)

the space charge. The extent of steepness in the low frequencyregion is observed to have a very strong dependence of Z0 on thecomposition irrespectively of temperature. Further, the imped-ance value is observed to increase rst in the low frequency regionwith rise in temperature followed by a relative decrease atsubsequently higher temperatures. The result may be related toa change in the charge-ordering pattern with temperature; theeffect showing clearly perceptible change in its behavior in thelow frequency region. A decreasing trend of Z0 with temperaturerise suggests the presence of negative temperature coefcient ofresistance (NTCR) in the material in the low frequency region buttends to merge in the high frequency region at almost alltemperatures. These results indicate a possibility of increase in acconductivity with increasing temperature in the high frequencyregion, possibly due to the release of space charge. Fig. 4(b and d)exhibits frequency and temperature dependence of imaginarypart of impedance (Z00) for KNLN solid-solutions. The trend of thevariation of Z00 with frequency is the typical of the presence of theelectrical relaxation in the materials which is temperature-dependent. The asymmetric broadening of the peaks suggeststhe presence of the electrical process in the material with a spreadof relaxation time. The shifting of the peaks indicates that the netrelaxation time is decreasing with the increase in temperature. Inlow temperature/frequency range, Z00 has very high value for allthe compositions which decreases with increasing frequency andattains low values. This is an interesting (unusual) trend in thevariation of Z00 with frequency/temperature for KNLN ceramicsamples. The appearance and nature of peaks at a characteristicangular frequency umax ( 2pfmax) provides information on thetype and strength of the dielectric relaxation phenomenon

occurring in the material. The nature of this pattern suggests thepresence of a weak dielectrical relaxation above 400 C in KNLNsystem. At low temperatures (400 C), the spectrum showsa monotonous decrease having dispersive nature in the lowfrequency region followed by a plateau at higher frequencies. Thisobservation clearly testies the presence of electronic, dipolar andspace charge polarizations in the material, and the appearance ofthe peak(s) being a result of dipolar contribution.

Fig. 5(a and c) shows the variation of real part of modulus asa function of frequency at different temperatures for all composi-tions of KNLN. The variation of M0 with frequency for all thesamples shows a dispersion tending toward MN (the asymptoticvalue of M0 at higher frequencies) and dispersion shifts towardhigher frequency side with increasing temperature. Monotonousdispersionwith increasing frequency at lower temperaturesmay bedue to the short range mobility of charge carriers. It comprises ofthe features such as a very low value (approaching to zero) ofM0 inthe low frequency region and a continuous dispersion withfrequency. Such results may possibly be related to a lack ofrestoring force governing the mobility of the charge carriers underthe action of an induced electric eld. This behavior supports thatas the frequency increases; each ion moves a shorter and shorterpath of electric eld, until the electric eld changes so rapidly thatthe ions only rattle within the connement of their potentialenergy wells. The value of M0 decreases with increasing tempera-ture in the studied frequency range. Fig. 5(b and d) depicts variationof imaginary part of modulus (M00) with frequency. High frequencyrange side represents that ions are separately conned to theirpotential wells, and the ion can make only the localized motionwithin the wells [21,22].

R. Rai et al. / Current Applied Physics 13 (2013) 430e4404340 00Fig. 5. (a, c) Plot of real part of modulus (M ) and (b, d) imaginary part of modulus (M ) withceramic.frequency of (1 x) K0.5Na0.5NbO3 xLiNbO3 (where x 0.0, 5.0, 5.5, 6.0, and 6.5 wt.%)

Fig. 6(aee) shows complex impedance spectrum (Nyquist plot)of samples of different compositions at 300, 350, 400, 425 and450 C, respectively. We have tted the experimental data (Fig. 6f)for all the compositions with the equivalent circuit usingcommercially available software Z Simp Win Version 2. There isa close agreement observed between experimental and tted data.It is observed that at lower temperatures a single semi-circular arcappears. This single semicircular arc suggests the presence of graininterior (bulk) property of the material [23]. However, at highertemperatures, another arc is present, and the spectrum comprises

two semicircular arcswith their centers lying below the real axis forall compositions (temperatures above 400 C). The high frequencysemi-circle (rst arc) can be attributed to the bulk (grain interior)properties of the material arising due to a parallel combination ofbulk resistance (Rb) and bulk capacitance (Cb). The low frequency(second) semicircular arc of the impedance spectrum at elevatedtemperatures (pattern at 425 and 450 C, respectively) has beenattributed to the presence of grain boundary arising due toa parallel connection of grain boundary resistance (Rgb) andcapacitance (Cgb). Two semi-circular arcs of the impedance

0 200 400 6000

100

200

300

400

500

600

z'(k)

z"(k

)

5LN 5.5LN 6LN 6.5LN

300 0C

0 50 100 150 200 2500

50

100

150

200

250

z'(k)

z"(k

)

5LN 5.5LN 6LN 6.5LN

350 0C

0 5 10 15 20 250

5

10

15

20

25

z'(k)

z"(k

)

5LN 5.5LN 6LN 6.5LN

400 0C

0 2 4 60

2

4

6

z'(k)

z"(k

)

5LN 5.5LN 6LN 6.5LN

425 0C

4

2

3

4

"(k

)

5LN 5.5LN 6LN 6.5LN

450 0C

15

20Z measured- 5LN Z calculated- 5LN Z measured- 5.5LN Z calculated- 5.5LN Z measured- 6LN Z calculated- 6LN Z measured- 6.5LN Z calculated- 6.5LN

)

(Q)

(R)

(Q)

(R)

a

c

e f

d

b

R. Rai et al. / Current Applied Physics 13 (2013) 430e440 4350 1 2 30

1

z'(k)

zFig. 6. (aef). Variation of real and imaginary part of impedance of different (1 x) K0.5Na0.5Nand 450 C respectively.0 5 10 15 200

5

10

' ()

'' (bO3 xLiNbO3 (where x 0.0, 5.0, 5.5, 6.0, and 6.5 wt.%) ceramics at 300, 350, 400, 425

spectrum may be due to grain bulk and grain boundary contribu-tions, which is consistent with the brick-layer model for a poly-crystalline material [24]. These semicircular arcs can be expressedin terms of an equivalent circuit (Fig. 6f) built up by cascading oftwo parallel resistance and capacitance connected in series (RCcircuits), each being responsible for a semi-circle. As a result, boththe grain bulk and the grain boundary phenomena occur simulta-neously with increasing temperature. This effect may be related tothe change in microstructure governed predominantly by thethermal state of the material, which throws a signicant light onthe relationship of temperature-dependent microstructure andelectrical properties of the materials. The intercept of the semi-circular arc on the real axis gives the dc resistance of the material. Itis seen from Fig. 6 that the dc resistance decreases with the increasein temperature as well as the LN content in the KNN ceramics. The

Table 1The value of bulk resistance (Rb), grain boundary resistance (Rgb), bulk capacitance(Cg) and grain boundary capacitance (Cgb) of LN doped KNN ceramics of differentcompositions x 0.0, 5.0, 5.5, 6.0 and 6.5 wt.%.

x Temp (C) Rg (kU) Rgb (kU) Cg (nF) Cgb (nF)

5 400 4.668 7.087 3.683 2.426425 2.174 2.978 5.765 3.732450 1.539 2.279 2.346 1.921

5.5 400 13.133 22.459 2.859 1.672425 2.029 2.978 4.536 3.089450 1.468 1.879 2.102 1.642

6 400 3.680 4.885 4.006 3.081425 2.249 3.228 3.504 2.441450 1.942 3.667 2.530 1.334

6.5 400 9.348 14.596 1.839 1.178425 3.845 6.438 4.471 2.670450 1.970 3.139 2.144 1.345

Fig. 7. (aee) Variation of real and imaginary part of modulus with temperature of (1

R. Rai et al. / Current Applied Physics 13 (2013) 430e440436x) K0.5Na0.5NbO3 xLiNbO3 (where x 0.0, 5.0, 5.5, 6.0, and 6.5 wt.%) ceramics.

values of the electrical parameters by tting the measured datawith the equivalent circuit at 400 C are given in Table 1. Theobserved depressed semicircular arcs having centers lying belowthe real impedance (Z0) axis is due to the presence of distributedphase elements. The relaxation process associated with thisobservation is non-ideal in nature. The non-ideal behavior origi-nated from several factors grain orientations, like grain sizedistributions, grain boundaries, atomic defect distribution andstressestrain phenomena.

A relative comparison of the complex impedance spectrum andmodulus spectrum for all the concentrations of LN in KNN showsone arc in the modulus pattern with different radii (Fig. 7) whereastwo arcs in the impedance spectrum. This result may be explainedin term of the capacitance effect by the fact that in an equivalentcircuit comprising of two parallel RC circuit elements (connected inseries), the resistance of one may be much lower than that of theother, but the two capacitance values may not be too differentleading to a well resolved arc pattern in the modulus spectrumunlike that in the impedance pattern [23, 24]. Furthermore, themodulus spectrum shows a notable change in the shape withincreasing temperature suggesting a probable change in thecapacitance value of the KNLN material as a function of tempera-ture as well as concentration of LiNbO3. This suggests that thedielectric behavior of the solid-solution is controlled by the LiNbO3addition.

The variation of normalized complex electric impedance withnormalized frequency is shown in Fig. 8. This is known as a mastercurve, which enables us to shed light on the dielectric behavior ofthe material as a function of temperature. The overlap of the curves

at different temperatures into a single master curve indicates thatthe relaxation describes the same mechanism at differenttemperatures. This behavior is represented by the non-exponentialtype of conductivity relaxation [25]. A non-exponential typeconductivity relaxation suggests the possibility that the ionmigration takes place via hopping mechanism accompanied bya consequential time dependent mobility of other charge carriers ofthe same type in the vicinity. The master modulus shows (1) theslight shift in the impedance peak pattern, (2) the similar shape andpattern with slight variation in full width at half maximum(FWHM) with increasing temperature, and (3) the asymmetricnature of the curves which is a characteristic of the non-exponential behavior of conductivity. The FWHM of the peaks iswider than that of the Debye peak, which also suggests the pres-ence of non-Debye-type relaxation phenomena.

Fig. 9 shows the variation of AC conductivity (sac) withtemperature at 10 kHz frequencies for all samples. The nanoappreciable variation of sac with temperature indicates themultiple relaxations. The ac conductivity is dependent on thecapacitance of the sample given by the expression.

sac 6000

where 0 is the vacuum dielectric constant, 6 is the angularfrequency, 0 and 00 are the real and imaginary part (the dielectricloss) of the dielectric permittivity. The AC conductivity increaseswith increase in temperature showing a negative temperaturecoefcient of resistance (NTCR) behavior. Arrhenius equation wasused for the calculation of activation energy for the ceramics,

R. Rai et al. / Current Applied Physics 13 (2013) 430e440 437Fig. 8. (aed) Impedance scaling behavior of (1 x) K0.5Na0.5NbO3 xLiNbO3 (where x 0.0, 5.0, 5.5, 6.0, and 6.5 wt.%) ceramics in the master curves.

bO3

R. Rai et al. / Current Applied Physics 13 (2013) 430e440438s s0Ea=kBT

sac is the AC conductivity, Ea is the activation energy, kB is theBoltzmann constant, T is the temperature in Kelvin scale and s0 isthe dc conductivity.

The activation energy calculated at 10 kHz in the temperaturerange (390e330 C) and (200e150 C) is between 1.30 eV and0.55 eV respectively. The low value of activation energy obtainedcould be attributed to the inuence of electronic contribution to theconductivity; The increase in conductivity with temperature may

Fig. 9. Temperature dependence of the sac of (1 x) K0.5Na0.5NbO3 xLiNbe attributed to the oxygen vacancies due to the loss of oxygenduring the sintering process and leave behind free electronsmaking them n type semiconductor [26e28]. In oxide ferroelec-trics, doubly charged oxygen vacancies are the most mobile chargesand play an important role in the conduction process. The volatil-ization of A-site elements (Na and K) will result in the deciency inA-site cations, and thereafter the generation of oxygen vacanciesdue to the valence balance, which may alter the lattice distortionand thus inuence the dielectric behavior of these ceramics.

Fig. 10(a) shows a representative AFM image of the ceramicsample. Following the topography acquisition, the AFM wasswitched to the piezoresponse force microscopy (PFM) regime inwhich the conducting tip is scanned in contact mode while an acvoltage (Vac) is applied between the tip and Ag electrode (Fig. 10b).

Fig. 10. AFM images of KN 6.5 LN ceramic samples: (a) topogrIn these conditions, we could measure both out-of-plane (OOP) andin-plane (IP) polarization components [29]. After the scan we wereable to detect IP signal due to a shear component of the piezotensor(d15), corresponding to the polarization parallel to the surfacesample, and OOP that reects polarization perpendicular to thesurface [normal component of the piezotensor (d33)]. Comparisonof the topography (Fig. 10a) and PFM (Fig. 10b and c) images atteststhat the polishing scratches and other corrugations of the samplesurface are not reected on the corresponding PFM images thusgiving an evidence of the piezoelectric nature of the contrast. ForOPP component (Fig. 10b), the contrast is roughly proportional to

(where x 0.0, 5.0, 5.5, 6.0, and 6.5 wt.%) ceramics at 10 kHz frequencies.the effective d33 coefcient and determined by the projection of thepolarization vector P on the normal N to the ceramic surface. Thedark and bright contrasts correspond to the polarization headdirected to the sample bulk or to the surface, respectively. IP pie-zoresponse image (Fig. 10c) is complementary to the OPP pictureand reects the effective shear d15 piezoelectric coefcientproportional to the corresponding IP component of thepolarization.

Ferroelectric domain distributions distributions of the pure and6.5 wt.% LN concentration samples are shown in Fig. 11(a and b).The different grains of the respective materials are represented bythe colored regions in the gure. The histograms of the OPP PFMsignal from images are represented in Fig. 11c. It has a clearasymmetric shape with a maximum slightly shifted toward the

aphy, (b) OPP PFM, and (c) IP PFM. Uac 5 V, f 50 kHz.

ed PR. Rai et al. / Current Applinegative or positive values. It means that the number of grains withthe polarization head terminated at the free surface exceeds thenumber of oppositely oriented dipoles. Under large magnication,a shoulder on the positive slope was observed that may representa maximum in the number of grains having the same effectivepiezoelectric coefcient. Since the sign of the piezosignal is referredto the direction of the polarization projection onto the samplesnormal, it can be inferred that all investigated samples haveaverage non-zero piezoresponse in the unpoled state. In our de-nition, it corresponds to the polarization direction directed fromthe free surface into sample (self-polarization effect). This corre-sponds to the peak shift as observed in Fig.11c. The shifts to positiveand negative values is related to different signs of self-polarizationsand related to the preferred orientation of surface domains. Theeffective piezoelectric coefcient, d33eff was determined from thepeak half width (Fig. 11d). It is observed that the composition with6.5 wt.% LN displays stronger piezocontrast as compared to pureKNN implying an evidence of a pronounced piezoelectric coef-cient. A broad response in Fig. 11(c) for 6.5 wt.% LN corresponds tothe trend shown in Fig. 11(d).

4. Conclusions

(1 x) K0.5Na0.5NbO3 xLiNbO3 (x 0e6.5 wt.%) lead freepiezoelectric ceramic has been synthesized by conventional solidstate reaction technique. The effect of different amounts of Li andNb content on the phase structure and electric properties of KNLNceramics were systematically investigated. XRD studies showed thetransition from orthorhombic to tetragonal above x 5.5 wt.% ofLN. Detailed studies of dielectric and electrical properties indicatethat the material exhibits: (i) shift of Curie temperature toward

Fig. 11. OPP PFM images of pure (a) and 6.5 wt.% LN (b), (c) and (d) represent the LN conrepresenting piezoelectric activity.hysics 13 (2013) 430e440 439higher temperature side on increasing LN concentration, (ii)decrease in dielectric constant and loss on increase in LN content,(iii) follows Arrhenius relation in higher temperature region, (iv)bulk or grain conduction up to 400 C, (v) grain boundaryconduction up to 450 C, (vi) temperature dependent relaxationphenomena. The activation energy values obtained for different LNcontent suggest that the electrical conduction in KNLN is mainlydue to the mobility of the ionized oxygen defects. We observed thatthe compositionwith 6.5 wt.% LN displays stronger piezocontrast ascompared to pure KNN implying an evidence of a pronouncedpiezoelectric coefcient.

Acknowledgments

The authors are grateful to the Foundation for Science andTechnology of Portugal (FCT) for nancial support within theprojects PTDC/CTM-CER/115085/2009 and PTDC/FIS/108025/2008.Thework is partly supported by the EU network Enermat.aa. SeemaSharma wish to acknowledge Department of Science and Tech-nology, Govt of India (SR/S2/CMP-39/2006), India for the nancialsupport under a research project scheme.

References

[1] Y. Saito, H. Takao, T. Tani, T. Nonoyama, K. Takatori, T. Homma, T. Nagaya,M. Nakamura, Nature 432 (2004) 84e87.

[2] E. Hollenstein, M. Davis, D. Damjanovic, N. .Setter, Appl. Phys. Lett. 87 (2005)182905.

[3] H.L. Du, Z.M. Li, F.S. Tang, S.B. Qu, Z.B. Pei, W.C. Zhou, Mater. Sci. Eng. B 131(2006) 83.

[4] P.C. Goh, K. Yao, Z. Chen, J. Am. Ceram. Soc. 92 (2009) 1322.[5] N. Hagh, A. Safari, J. Electroceram. 18 (2007) 339.[6] R.E. Jaeger, L. Egerton, J. Am. Ceram. Soc. 45 (1962) 209.

centration dependence of the piezoresponse histograms of out-of-plane (OPP) signal

[7] R.P. Wang, R.J. Xie, T. Sekiya, Y. Shimojo, Mater. Res. Bull. 39 (2004) 1709.[8] J.F. Li, K. Wang, B.P. Zhang, L.M. Zhang, J. Am. Ceram. Soc. 89 (2006) 706.[9] J.G. Hao, Z.J. Xu, R.Q. Chu, Mater. Chem. Phys. 118 (2009) 229.[10] M. Matsubara, T. Yamaguchi, W. Sakamoto, J. Am. Ceram. Soc. 88 (2005) 1190.[11] J.G. Hao, Z.J. Xu, R.Q. Chu, J. Mater. Sci. 44 (2009) 6162.[12] J.G. Hao, Z.J. Xu, R.Q. Chu, J. Electron. Mater. 39 (2010) 347.[13] Y. Guo, K. Kakimoto, H. Ohsato, Jpn. J. Appl. Phys. 43 (2004) 6662e6666.[14] Y. Guo, K. Kakimoto, H. Ohsato, Solid State Commun. 129 (2004) 279e284.[15] Y. Guo, K. Kakimoto, H. Ohsato, Appl. Phys. Lett. 85 (2004) 4121e4123.[16] Y. Guo, K. Kakimoto, H. Ohsato, Mater. Lett. 59 (2005) 241e244.[17] S.J. Zhang, R. Xia, H. Hao, H.X. Liu, T.R. Shrout, Appl. Phys. Lett. 92 (2008)

152904.[18] S.H. Park, C.W. Ahn, S. Nahm, Jpn. J. Appl. Phys. 43 (2004) 1072e1074.[19] E. Hollenstein, D. Damjanovic, N. Setter, J. Eur. Ceram. Soc. 27 (2007) 4093.

[20] R. Elfrink, J. Micromech. Microeng. 19 (2009) 094005.[21] R.N.P. Choudhary, Dillip K. Pradhan, C.M. Tirado, G.E. Bonilla, R.S. Katiyar,

Phys. Status Solidi (b) 244 (2007) 2254e2266.[22] A. Shukla, R.N.P. Choudhary, Physica B Condens. Matter 406 (2011) 2492e

2500.[23] D.C. Sinclair, A.R. West, J. Appl. Phys. 66 (1989) 3850.[24] A. Shukla, R.N.P. Choudhary, A.K. Thakur, J. Phys. Chem. Solids 70 (2009) 1401.[25] A. Shukla, R.N.P. Choudhary, Curr. Appl. Phys. 11 (2011) 414.[26] J.R. Macdonald, Impedance Spectroscopy, Emphasizing Solid Materials and

Systems, Wiley Interscience Publ., New York, 1987.[27] S. Poykko, D.J. Chadi, Appl. Phys. Lett. 76 (2000) 499e501.[28] S. Steinsvik, R. Bugge, J. Gjonnes, J. Tafto, T. Norby, J. Phys. Chem. Solids 58

(1997) 969e976.[29] N. Balke, I. Bdikin, S.V. Kalinin, A.L. Kholkin, J. Am. Ceram. Soc. 92 (2009) 1629.

R. Rai et al. / Current Applied Physics 13 (2013) 430e440440

Impedance spectroscopy and piezoresponse force microscopy analysis of lead-free (1 x) K0.5Na0.5NbO3 xLiNbO3 ceramics1. Introduction2. Experimental procedure3. Results and discussion4. ConclusionsAcknowledgmentsReferences