CERAMICSINTERNATIONAL

Available online at www.sciencedirect.com

http://dx.doi.org/0272-8842/& 20

nCorrespondinE-mail addre

Ceramics International 40 (2014) 10641–10647www.elsevier.com/locate/ceramint

Synthesis of nano-crystalline potassium sodium niobate ceramic usingmechanochemical activation

Rajan Singha, Pankaj K. Patrob, A.R. Kulkarnic,n, C.S. Harendranatha

aCentre for Research in Nanotechnology and Science, Indian Institute of Technology-Bombay, Mumbai 400076, IndiabPowder Metallurgy Division, Materials Group, BARC, Vashi Complex, Navi Mumbai, India

cDepartment of Metallurgical Engineering and Materials Science, Indian Institute of Technology-Bombay, Mumbai 400076, India

Received 9 January 2014; received in revised form 17 February 2014; accepted 7 March 2014Available online 19 March 2014

Abstract

Single phase nano-crystalline K0.5Na0.5NbO3 (KNN) ceramic has been successfully synthesized by the mechanochemical activation method.High energy ball milling was carried out on a mixture of K2CO3, Na2CO3 and Nb2O5 powders for different times (6 h, 12 h, 24 h and 32 h).The 32 h mechanically activated sample was calcined at 550 1C for 2 h. This is about 300 1C lower as compared with conventional solid statereaction. X-ray diffraction pattern of calcined sample revealed single phase perovskites structure with orthorhombic phase. Particle size of thesepowders was measured using DLS. The Morphology of mixture powder, before and after calcination, was examined under FEG-SEM. Dielectric,ferroelectric and piezoelectric properties were evaluated and compared with reported results using other competing techniques.& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: C. Dielectric properties; C. Ferroelectric properties; KNN; Mechanochemical activation

1. Introduction

Perovskite Pb(Zr,Ti)O3 (PZT) and doped PZT-based ceramicsare widely used in many Industrial applications due to theirsuperior performance in ferroelectric, piezoelectric and pyro-electric properties [1,2]. However, PZT contains more than 60 wt% lead that raises serious environmental and safety concerns, dueto the toxicity of lead oxide and its high vapor pressure duringthe sintering process [3,4]. Therefore, there has been a growinginterest to develop lead-free ceramics for ferroelectric andpiezoelectric applications. Extensive research all around theworld in the last two decades has resulted in several lead freecandidate materials such as BaTiO3-based ceramics [5], Bilayered structures [4], alkaline niobate perovskites [4] and Bi-based perovskites [6], to name a few. Among them, muchattention has been paid to the alkaline niobate perovskites. Inparticular, potassium sodium niobate (K, Na)NbO3 (KNN) is

10.1016/j.ceramint.2014.03.04714 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

g author. Tel.: þ91 22 25767636; fax: þ91 22 25723480.ss: ajit.kulkarni@iitb.ac.in (A.R. Kulkarni).

considered to be an important lead free candidate material due toits high Curie temperature (420 1C) and promising ferroelectricand piezoelectric properties comparable with PZTs [7,8].The conventional synthesis of KNN ceramic requires high-

temperature and long time for calcination, which leads to thevolatilization of the alkali metal and consequent, non-stoichiometric and undesirable secondary phase formation.Furthermore, this method results in products with large andinhomogeneous particle sizes, due to particle coarsening thatdeteriorates the piezoelectric response [9]. In order to over-come these drawbacks, many wet chemical-based processingtechniques have been developed, such as, sol–gel [10], citrategel [11], co-precipitation [12], hydrothermal [13], to name afew. However, these techniques require high-purity inorganicmaterials that are typically much more expensive than thewidely available oxides and carbonates.Mechanochemical activation involves high energy ball milling

of a mixture of powders and is accompanied by chemicalreaction. Initially this technique was used for synthesizingmetallic alloys and inter-metallic compounds [14]. The main

R. Singh et al. / Ceramics International 40 (2014) 10641–1064710642

advantage of this method is that it permits the synthesis of nano-materials, in bulk, at an ambient temperature and in a very shortprocessing time [15]. In the past few years, several attempts havebeen made to synthesize various perovskites ceramic powdersusing mechanochemical activation starting from a powdermixture of carbonates and oxides, such as BaTiO3 [16], LiFeO2

[17], NaNbO3 [18], PZT [19] and KNN [9]. However, earlierreported literature on synthesis of KNN by the mechanochemicalactivation method did not result in single phase KNN duringmilling [9]. Even prolonged milling resulted in an amorphisationof the reactants.

In this paper, we report synthesis of single phase nanocrystal-line KNN ceramic, using mechanochemical activation, in aneffort to decrease the calcination temperature as compared toconventional solid state reaction. The morphology, structure andelectrical properties of the ceramics so obtained are presented.

Fig. 1. TGA and derivative of TGA curve for 32 h mechanically activatedmixture powders.

2. Materials and methods

The starting materials used in this study were K2CO3,Na2CO3 (all 99% pure, Merck India) and Nb2O5 powder(99.9%, Aldrich, USA). The alkaline carbonate powders werefirst dried at 200 1C for 1 h prior to use due to theirhygroscopic nature. High energy ball-milling was carried outin a high energy mill, SPEX8000D, for mechanochemicalactivation of the samples. The milling speed was 875 cycles(back and forth) per minute. The milling was interrupted atregular intervals (6 h, 12 h, 24 h and 32 h) to collect samplesfor analysis. The 32 h milled sample was calcined and sinteredat 550 1C and 1100 1C for 2 h, respectively, in air. The densityof the sintered samples was determined using the Archimedesmethod of fluid displacement (ASTM #C 373-88).

Thermogravimetric analysis (TGA) was performed, inflowing air, at a heating rate of 10 1C/min using PerkinElmerPyris 1 Thermo-gravimetric analyzer. To determine the phasepurity and lattice parameters of calcined powders, X-raydiffraction data was recorded in the 2θ scan range of 10–901using X-ray diffractometer (PANalytical X-ray diffractometer)and Rietveld refined using Fullprof program. X-ray sourcewas Cu-Kα (λ¼1.541 Å) radiation. The morphology ofmechanically activated powders was examined using a JEOL(JSM-7600F) Field Emission Scanning Electron Microscope(FEG-SEM). For electrical measurements, silver paste wasapplied on both sides of the sample and dried at 100 1C for 1 hto remove organic solvent. Dielectric measurements ofthe samples were carried out over a temperature range of50–500 1C using a computer interfaced NovoControl dielectricAlfa analyzer in the frequency range from 1 Hz to 1 MHz.Variation of electrical polarization as a function of electric field(P–E loop) at room temperature and at 2 Hz was observedusing ferroelectric test system (aixACT TF analyzer, Aachen,Germany). Mechanical displacement was measured usingmechanical displacement sensor (SIOS). During the polariza-tion measurement, an electric field of 10–50 kV/cm wasapplied. To prevent the breakdown from the edges, sampleswere immersed in silicone oil during the measurement.

3. Results and discussion

3.1. Thermal analysis

Fig. 1 shows results of Thermo-gravimetric analysis (TGA),and its corresponding derivative, for the mixture powdersmechanochemically activated for 32 h. The experiment wascarried out from room temperature to 1000 1C. From the figureit can be seen that the powder mixture goes through two mainstages of thermal decomposition. In the initial stage, fromroom temperature to about 150 1C, the weight loss (7.61%) ismainly due to moisture and water with maximum weight lossrate at 135 1C. In the second stage, the main part of the curvefrom 400�550 1C, the weight loss (19.38%) is due to thedecomposition of carbonates. The peak at 450 1C in thederivative curve indicates the starting of carbonate decomposi-tion. The major peak at 510 1C corresponds to the highest rateof decomposition. Absence of a peak after 550 1C suggeststhat the reaction is completed at 550 1C.

3.2. Phase analysis (XRD)

The XRD pattern of mixture of powders, mechanicallyactivated for different times, is shown in Fig. 2. It can be seenthat before activation both the alkaline carbonates, [K2CO3

(JCPDS #87-0730) and Na2CO3 (JCPDS #19-1130)], andNb2O5 (JCPDS #27-1313) phase is present [20–22]. However,after 32 h mechanochemical activation only Nb2O5 phase(JCPDS #27-1313) was detected and no peak belonging toalkaline carbonates has been observed. This suggests the lossof the long-rang periodicity in the carbonates crystal structureto the point at which it is undetectable using the XRD [23].The loss of long-range periodicity in the carbonates whenmilled together with Nb2O5 is an evidence of an interactionbetween the reactants and confirms that the chemical reactionoccurs during milling [23]. Further, no new peak has beendetected after mechanical activation. However, the peakintensity decreases as milling time increases. The decrease inintensity and increase in background is related to the formationof an amorphous phase. This amorphous phase has formed dueto the reconstruction of the CO3

2� ions into a carbonate

Fig. 2. X-ray diffraction patterns of mixture powders mechanically activatedfor different times.

Fig. 3. X-ray diffraction pattern of 32 h mechanically activated samplecalcined at 550 1C for 2 h.

Table 1Comparison between calculated and observed (JCPDS Card no. 71-0946)lattice parameters for KNN ceramic synthesized by mechanochemicalactivation.

Calculated Observed Change (%)

a¼3.9641 Å a¼3.971 Å 0.1737b¼5.6650 Å b¼5.692 Å 0.4743c¼5.6976 Å c¼5.719 Å 0.3741V¼127.95 Å3 V¼129.27 Å3 1.02

Fig. 4. Particle size distribution of KNN powders produced by 32 h of highenergy milling and calcined at 550 1C for 2 h.

R. Singh et al. / Ceramics International 40 (2014) 10641–10647 10643

complex [24]. Further, the difference in diffraction peakintensity between non-activated and activated sample indicatesthe loss of crystal structure produced by the distortion andaccumulation of defects in the structure. This suggests thatmechanochemical activation induces several chemical andstructural changes in the mixture of powders [25].

Fig. 3 shows the XRD pattern of sample achieved for 32 hmechanical activation, followed by calcination at 550 1C for2 h. The calcination temperature was decided with the help ofTGA result. It may be noted that the XRD pattern reveals asingle phase perovskite structure with an orthorhombic phase.The XRD pattern was indexed with JCPDS Card no. #71-0946[26]. The calcination temperature for conventional synthesis ofKNN powders is 850 1C [7]. Thus, the mechanochemicaltreatment of powder mixtures resulted in single phase formationat a lower temperature, lower by 300 1C, as compared to theconventional synthesis method. The XRD pattern was Rietveldrefined using Fullprof program. The lattice parameters at roomtemperature for KNN after refinement are; a¼3.9641 Å,b¼5.6650 Å, c¼5.6976 Å and α¼β¼γ¼901. The latticeparameters calculated by refinement vary slightly with theobserved data (JCPDS #71-0946), a comparison of which isshown in Table 1.

3.3. Particle size measurement (DLS)

Particle size distribution of KNN powders, mechanicallyactivated for 32 h and calcined at 550 1C for 2 h, is shown inFig. 4. It can be seen that the distribution is nearly homo-geneous. The measurement was repeated for three times and ineach experiment we found the similar distribution of particles.The mean particle size (d50) of the powders was �100 nm.

3.4. Morphology analysis

FEG-SEM micrograph of samples, mixture of K2CO3,Na2CO3 and Nb2O5, prepared before and after mechanicalactivation is shown in Fig. 5. The particle sizes and morphol-ogies, before and after mechanical activation, appear different.Before activation, the sample appears only as a simple mixtureof carbonates and Nb2O5, as shown in Fig. 5(a). However,after 32 h mechanochemical activation, the sample appears as acomposite of particulate material, as shown in Fig. 5(d). This isdue to the solid state reaction between carbonates and Nb2O5

particles, stimulated by mechanical stress via the grinding

Fig. 5. FEG-SEM morphology of mixture powders mechanically activated for (a) 0 h, (b) 12 h, (c) 24 h, (d) 32 h, (e) 32 h milled and calcined at 550 1C for 2 h and(f) microstructure of sintered product.

R. Singh et al. / Ceramics International 40 (2014) 10641–1064710644

beads in the mill. It may also be noted that as the milling timeincreases the particle size decreases and, after 32 h milling, ahomogeneous mixture with spherical particles having meansize of about �105 nm (Fig. 5(d)) was obtained. Further, someparticle agglomerates were found which is due to the largespecific surface area of the particles. Fig. 5(e) shows the

morphology of KNN particles obtained after 32 h mechanicalactivation and calcined at 550 1C for 2 h. After calcination theagglomerates are broken and particles become more homo-geneous. Fig. 5(e) shows FEG-SEM microstructure of frac-tured surface of KNN ceramic sintered at 1100 1C for 2 h. Thesample shows well developed grains having very low porosity.

Fig. 6. HR-TEM micrograph of mixture powders mechanically activated for32 h and calcined at 550 1C for 2 h. Inset shows selected area electrondiffraction (SAED) pattern of KNN powder.

Fig. 7. Temperature dependence of (a) dielectric constant and (b) tangent loss of KNN ceramic measured at different frequencies. In both the figures(a) and (b), inset shows the magnified values between temperature range50 1C and 150 1C.

R. Singh et al. / Ceramics International 40 (2014) 10641–10647 10645

The mean grain size was below �0.5 mm and density wasfound to be 4.20 g/cm3.

The HR-TEM micrograph of KNN powders, mechanicallyactivated for 32 h, followed by calcinations, is shown in Fig. 6.The average particle size, revealed by image analysis, wasabout 70 nm. Selected area electron diffraction (SAED) patternof KNN powders, shown in the inset of Fig. 6, exhibits spottyrings revealing the polycrystalline nature of the sample.

Fig. 8. P–E loop and bipolar strain of KNN ceramic measured at roomtemperature.

3.5. Dielectric properties

The temperature dependence of dielectric constant (ε0) andloss (tanδ) at different frequencies are plotted for mechano-chemically synthesized KNN ceramic in Fig. 7. The sampleshowed two peaks in ε0 values (Fig. 7(a)); one near 190 1C andanother near 365 1C corresponding to the orthorhombic–tetra-gonal phase transition (TT�O) and tetragonal cubic phasetransition (Curie temperature – Tc), respectively [7,8]. Thereported Tc for pure KNN ceramic is 420 1C [7]. The lower Tc(365 1C) obtained for mechanochemically activated KNN sam-ple in this study can be attributed to the refinement in grain size[27]. The room temperature dielectric constant was found to be680 at 1 kHz, which is higher than those obtained for conven-tional solid state reaction [7], partial co-precipitation and evenhot pressed synthesized samples [12,8]. The enhanced dielectricproperties are due to the small and fine-grained microstructure ofthe as prepared KNN ceramics [28]. It can also be noted that, thedielectric constant increases with increase in temperature andreaches a maximum at Tc and then starts decreasing. In thehigher temperature regions, the higher dielectric constant is dueto the contributions from space charge polarization, which arisesfrom mobility of ions and imperfections in the material [12]. The

room temperature tangent loss is 0.035 (Fig. 7(b)) at 1 kHz andis around 3–10% for mechanochemically activated KNN samplein the temperature range of 50–200 1C and becomes more andmore appreciable at higher temperatures. Further, strong fre-quency dispersion can be seen (1 kHz, 10 kHz and 100 Hz) intanδ values above 200 1C for the KNN ceramics, as shown inFig. 7(b). The tanδ value increases with increase in temperatureand, after reaching peaks at Tc, they increase rapidly owing toconductivity losses [29]. A comparison of εr, tanδ, Pr, Ec and

Table 2Comparison of dielectric, ferroelectric and piezoelectric properties for reported KNN ceramics synthesized by different techniques.

Property KNN by conventional synthesis [7] Hot pressedKNN [8]

KNN by partialco-precipitation [12]

This study (mechanochemical activation)

% TD 94.24 98.89 99.5 93.1εr (1 kHz) 290 420 648 680tanδ (1 kHz) 0.04 0.035 0.05 0.035d33 (pC/N) 80 160 71 95Pr (μC/cm2) 12.6 33 9.84 12.75Ec (kV/cm) 7.28 17 10.93 12.50

R. Singh et al. / Ceramics International 40 (2014) 10641–1064710646

d33 values obtained at 1 kHz for the KNN ceramics, synthesizedby different methods is tabulated in Table 2.

3.6. P–E Loop

Fig. 8 shows polarization versus electric field hysteresis(P-E) loop and bipolar strain of mechanochemically synthe-sized KNN ceramic. The sample shows well saturated P–E loopwith a relatively large remnant polarization Pr (12.75 μC/cm2)and low coercive field Ec (12.5 kV/cm). The observed Pr of theKNN ceramic prepared by the mechanochemical activation methodis smaller than those obtained from hot-pressed (33 μC/cm2)ceramics [8]. However, it is higher than those obtained frompartial co-precipitation (9.84 μC/cm2) and conventional solid statereaction synthesized samples [7,12]. The obtained Ec is lower thanthat of the hot-pressed (17 kV/cm) samples [8] and higher thanthose provided by partial co-precipitation (10.93 kV/cm) andconventional solid state reaction (7.28 kV/cm) synthesized sam-ples [7,12]. The sample shows a butterfly-shaped S–E curve that istypically seen in ferroelectric materials [30]. The maximumbipolar strain (Smax) obtained was 0.0934%.

In this work, we also measured the piezoelectric coefficientof KNN ceramic prepared by the mechanochemical activationmethod. The piezoelectric constant (d33) was found to be95 pC/N. This is superior to d33 obtained from conventionalsolid state reaction (80 pC/N) and partial co-precipitation(71 pC/N) synthesized KNN ceramics but lower than the hotpressed (160 pC/N) KNN ceramic [7,8,12].

4. Conclusions

Single phase nano-crystalline KNN ceramic powders havebeen successfully synthesized by the mechanochemical activa-tion method. This method is attractive for commercial applica-tions as huge amount of nano-material can be produced, in arelatively short time, as compared to chemical routes. Singlephase KNN ceramic has been obtained by high energy ballmilling for 32 h followed by calcination at 550 1C for 2 h.Thus, the mechanochemical treatment of powder mixturesresulted in single phase formation at a lower temperature,lower by 300 1C, as compared to conventional solid statereaction synthesis. The as prepared ceramic powder showedparticle size of about �78 nm. The mechanochemical activa-tion method has resulted in the highest room temperaturedielectric constant (680), so far reported, as compared to

conventional solid state reaction, partial co-precipitation andhot pressed synthesized samples. While Pr (12.75 μC/cm2) andd33 (95 pC/N) obtained by this method is lower than thereported values for the hot pressed method, it is superior toreported results for conventional solid state reaction and partialco-precipitation techniques.

Acknowledgments

We express our thanks to SAIF, IIT-Bombay and Departmentof Science and Technology, Govt. of India, for FEG-SEM andHR-TEM studies.

References

[1] N. Setter, D. Damjanovic, L. Eng, G. Fox, S. Gevorgian, S. Hong,A. Kingon, H. Kohlstedt, N.Y. Park, G.B. Stephenson, I. Stolitchnov, A.K. Taganstev, D.V. Taylor, T. Yamada, S. Streiffer, Ferroelectric thinfilms: review of materials, properties, and applications, J. Appl. Phys. 100(2006) 051606-1–051606-46.

[2] K. Uchino, Ferroelectric Devices, Marcel Dekker Inc., New York, 2000.[3] E. Mensur, M. Papila, Microstructural features and electrical properties of

copper oxide added potassium sodium niobate ceramics, Ceram. Int. 36(2010) 1921–1927.

[4] M.D. Maeder, D. Damjanovic, N. Setter, Lead free piezoelectricmaterials, J. Electroceram. 13 (2004) 385–392.

[5] Z. Yu, C. Ang, R. Guo, A.S. Bhalla, Piezoelectric and strain properties ofBa(Ti1�xZrx)O3 ceramics, J. Appl. Phys. 92 (2002) 1489–1493.

[6] Y. Hiruma, R. Aoyagi, H. Nagata, T. Takenaka, Ferroelectric andpiezoelectric properties of (Bi1/2K1/2)TiO3 ceramics, Jpn. J. Appl. Phys.44 (2005) 5040–5044.

[7] L. Egerton, D.M. Dillom, Piezoelectric and dielectric properties ofceramics in the system potassium-sodium niobate, J. Am. Ceram. Soc.42 (9) (1959) 438–442.

[8] R.E. Jaeger, L. Egerton, Hot pressing of potassium-sodium niobates,J. Am. Ceram. Soc. 45 (5) (1962) 209–213.

[9] T. Rojac, M. Kosec, B. Malic, J. Holc, Mechanochemical synthesis ofNaNbO3, KNbO3 and K0.5Na0.5NbO3, Sci. Sinter. 37 (2005) 61–67.

[10] J. Fang, X. Wang, L. Li, Li, Sb- and Ta-modified (K, Na)NbO3

nanopowder prepared via a water-based sol–gel method, Phys. StatusSolidi: Rapid Res. Lett. 6 (3) (2012) 132–134.

[11] Y. Cao, K. Zhu, H. Zheng, J. Qiu, H. Gu, Synthesis of potassium sodiumniobate powders using an EDTA/citrate complexing sol–gel method,Particuology 10 (2012) 777–782.

[12] P. Kumar, M. Pattanaik, Sonia, Synthesis and characterizations of KNNferroelectric ceramics near 50/50 MPB, Ceram. Int. 39 (2013) 65–69.

[13] F. Zhang, Lu. Han, S. Bai, T. Sun, T. Karaki, M. Adachi, Hydrothermalsynthesis of (K,Na)NbO3 particles, Jpn. J. Appl. Phys. 47 (9) (2008)7685–7688.

R. Singh et al. / Ceramics International 40 (2014) 10641–10647 10647

[14] T. Sritharan, F.Y.C. Boey, A. Srinivas, Synthesis of complex ceramics bymechanochemical activation, J. Mater. Process. Technol. 192–193 (2007)255–258.

[15] P. Balaz, P. Pourghahramani, E. Dutkova, E. Turianicova, J. Kova,A. Satka, Mechanochemistry in preparation of nanocrystalline semicon-ductors, Phys. Status Solidi C 5 (12) (2008) 3756–3758.

[16] E. Brzozowski, M.S. Castro, Synthesis of barium titanate improved bymodifications in the kinetics of the solid state reaction, J. Eur. Ceram.Soc. 20 (2000) 2347–2351.

[17] V. Berbenni, A. Marini, P. Matteazzi, R. Ricceri, N.J. Welhamc, Solid-state formation of lithium ferrites from mechanically activated Li2CO3–

Fe2O3 mixtures, J. Eur. Ceram. Soc. 23 (2003) 527–536.[18] A. Castro, B. Jimenez, T. Hungrıa, A. Moure, L. Pardo, Sodium niobate

ceramics prepared by mechanical activation assisted methods, J. Eur.Ceram. Soc. 24 (2004) 941–945.

[19] S.E. Lee, J.M. Xue, D.M. Wan, J. Wang, Effects of mechanical activationon the sintering and dielectric properties of oxide-derived PZT, ActaMater. 47 (9) (1999) 2633–2639.

[20] K2CO3, Powder Diffraction File (PDF-2 Database) No. 87-0730.[21] Na2CO3, Powder Diffraction File (PDF-2 Database) No. 19-1130.[22] Nb2O5, Powder Diffraction File (PDF-2 Database) No. 27-1313.

[23] T. Rojac, M. Kosec, P. Segedin, B. Malic, J. Holc, The formation of acarbonate complex during the mechanochemical treatment of aNa2CO3�Nb2O5, Solid State Ionics 177 (2006) 2987–2995.

[24] T. Rojac, A. Bencan, H. Ursic, B. Malic, M. Kosec, Synthesis of aLi- and Ta-modified (K,Na)NbO3 solid solution by mechanochemicalactivation, J. Am. Ceram. Soc. 91 (11) (2008) 3789–3791.

[25] J. Luo, Synthesis of NiCuZn-ferrite powders by means of mechanochem-ical treatment, Mater. Res. Bull. 48 (2013) 3527–3529.

[26] KNbO3, Powder Diffraction File (PDF-2 Database) No. 71-0946.[27] Y. Zou, Y. Wu, X. Guo, S. Tong, Z. Wang, L. Zhang, Effect of particle size

on the densification and dielectric properties of BaTiO3 ceramics prepared byliquid phase sintering, Phys. Status Solidi A 209 (2) (2012) 243–247.

[28] M.M.V. Petrovic, J.D. Bobic, A.M. Radojkovic, J. Banys, B.D. Stojanovic,Improvement of barium titanate properties induced by attrition milling,Ceram. Int. 38 (2012) 5347–5354.

[29] H. Yang, Y. Lin, F. Wang, H. Luo, Chemical synthesis of K0.5Na0.5NbO3

ceramics and their electrical properties, Mater. Manuf. Process. 23 (2008)489–493.

[30] S. Zhang, L. Wang, Y. Chen, Phase characteristics and piezoelectricproperties in the Bi0.5Na0.5TiO3–BaTiO3–K0.5Na0.5NbO3 system, J. Am.Ceram. Soc. 93 (6) (2010) 1561–1564.