Available online at ScienceDirect

ScienceDirectJ. Mater. Sci. Technol., 2013, -(-), 1e7

Mo6D Modified (K0.5Na0.5)NbO3 Lead Free Ceramics: Structural, Electrical

and Optical Properties

Jyoti Rani1), Piyush Kumar Patel1), Nidhi Adhlakha1), Hemant Singh1), K.L. Yadav1)*,Satya Prakash2)

1) Smart Materials Research Laboratory, Department of Physics, Indian Institute of Technology Roorkee, Roorkee 247667, India2) Metallurgical and Materials Engineering Department, Indian Institute of Technology Roorkee, Roorkee 247667, India

[Manuscript received May 12, 2013, in revised form July 16, 2013, Available online xxx]

* Corresp273560;1005-03JournalLimited.http://dx

Please

Lead free polycrystalline ceramics (K0.5Na0.5)Nb(1�x)MoxO3 (x ¼ 0, 0.02, 0.04, 0.06 and 0.08) have beensynthesized via solid state reaction method. The formation of single phase perovskite structure up to 6 mol%of Mo6þ has been confirmed by X-ray diffraction pattern. Impedance spectroscopy reveals that bulkresistance decreases with increasing temperature, which indicates negative temperature coefficient ofresistance (NTCR) behaviour of the compounds. The diffuse reflectance spectroscopy results indicate a redshift of the band gap energy of K0.5Na0.5NbO3 (KNN, from 4.28 to 3.61 eV) with increasing Mo6þ

concentration due to structural modification. The photoluminescence spectra of doped samples are composedof two emission bands at room temperature. One emission band is near band edge ultraviolet (UV) emission(w354 nm) and other is visible emission band (w397 nm) which may explore the possibility of theseceramics to be used in optical device applications.

KEY WORDS: Ceramics; Characterization; Dielectric constant; Impedance spectroscopy; Optical band gap

1. Introduction

Lead zirconium titanate (PZT)-based materials are of sub-stantial interest due to their potential applications such as highdielectric capacitors, random access memories, transducers andelectro-optic devices etc[1]. Lead oxide is the main constituent ofPZT-based material which is detrimental for environment as wellas for living beings. Therefore, a great attention has been givento the lead free ceramics, such as Bi0.5Na0.5TiO3 (BNT)-basedceramics, BaTiO3-based ceramics, BiFeO3-based ceramics,(K,Na)NbO3-based ceramics[2], bismuth-layered structure ce-ramics[3,4] and tungsten bronze-type ceramics[5] etc. Amongthem, potassium sodium niobate (K0.5Na0.5NbO3) (KNN) hasbeen proposed as a promising candidate because of its high Curietemperature, good ferroelectric and piezoelectric properties[6e8].KNN belongs to the perovskite orthorhombic system with aspace group Amm2. It is a solid solution of ferroelectric potas-sium niobate (KNbO3) having Curie temperature of w435 �Cand antiferroelectric sodium niobate (NaNbO3) with Curie

onding author. Ph.D.; Tel.: þ91 1332 285744; Fax: þ91 1332E-mail address: klyadav35@yahoo.com (K.L. Yadav).

02/$e see front matter Copyright� 2013, The editorial office ofof Materials Science & Technology. Published by ElsevierAll rights reserved..doi.org/10.1016/j.jmst.2013.10.022

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temperature of w355 �C[9]. The composition K/Na ¼ 50/50which is close to the morphotrophic phase boundary, has beenreported to exhibit a moderate dielectric constant and an opti-mum piezoelectric response[10].Different techniques like spark plasma and high energy mill-

ing[11,12] etc. are being used to overcome the alkali evaporationduring high temperature synthesis of KNN. Chemical modifi-cation by using various dopants such as Liþ, Mg2þ, Ca2þ, Sr2þ,Ba2þ, La3þ, Ta5þ, Ga3þ and Fe3þ etc.[10,13e17] at A or B site orby making its solid solution with LiNbO3, SrTiO3 and BaTiO3

etc.[18e20] have been used to improve its dielectric and electricalproperties. Method of synthesis has a marked influence ondensity and electric properties of KNN. Liu et al.[21] reportedhigher density and dielectric constant for KNN, when its pelletswere buried in calcined KNN powder during sintering, ascompared to bare KNN pellet. These changes arise due to dif-ference in concentration and type of oxygen vacancies that in-crease with increasing Na/K evaporation. Since grain, grainboundary and electrodeespecimen interface are contributed tothe electrical properties of polycrystalline specimens, impedancespectroscopy may be used as a tool to distinguish the contribu-tions of these effects. The output response of the impedancemeasurement when plotted in a complex plane appears in theform of successive semicircles representing electrical phenom-enon due to grain, grain boundary effect and interfacial polari-zation[22,23]. Using elemental substitution, one can tailor the

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Fig. 1 (a) XRD patterns of the (K0.5Na0.5)Nb(1�x)MoxO3 (x ¼ 0, 0.02, 0.04 and 0.06), (b) XRD pattern for x ¼ 0.08 with FE-SEM micrograph in inset,(c) merging of peaks (202) and (020).

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properties of the ceramics for any specific application. Till now,there are few studies on optical behaviour of pure and dopedbulk KNN ceramics. Molybdenum oxides (MoO3) are one of themost attractive metal oxides due to their optical characteristics.Many reports are available regarding the optical properties ofMoO3

[24e27]. Application of Mo6þ as a dopant for KNN ce-ramics has not yet been reported. So an attempt has been madefor Mo6þ doping in KNN at Nb5þ site, with a hypothesis thatMo6þ may improve the optical properties and give rise to asystem with soft characteristics as already established in the caseof donor doped PZT ceramics[28,29]. The influence of doping onthe structural, dielectric, complex impedance and optical prop-erties of KNN ceramic has been discussed in this paper.

2. Experimental

Polycrystalline ceramics of (K0.5Na0.5)Nb(1�x)MoxO3 (x ¼ 0,0.02, 0.04, 0.06, and 0.08) were synthesized by solid state re-action method. The chemicals Na2CO3 (Qualigns, 99.9%),K2CO3 (Himedia, 99%), Nb2O5 (Himedia, 99.9%), MoO3

(Himedia, 99.5%) were used as received. For the synthesis of(K0.5Na0.5)Nb(1�x)MoxO3 (x ¼ 0, 0.02, 0.04, 0.06, and 0.08),analytical grade of above mentioned chemicals was taken in

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appropriate proportion and mixed thoroughly in acetone mediafor better mixing. Approximately 5 wt% of K2CO3 and Na2CO3

were initially added in excess to compensate for K and Na lossduring calcination and sintering process. These mixed powderswere calcined in air at 825 �C for 4 h. The heating rate of thefurnace was set at 5 �C/min. The calcined powders were finallygrounded and characterized by using an X-ray powder diffrac-tometer (Brueker D8 Advance) with CuKa radiation in the anglerange of 20� � 2q � 60� at a scanning rate of 1� min�1. Thecalcined powders were pressed using a hydraulic press intopellets of diameter 9e10 mm and thickness 0.9e1.2 mm byapplying pressure of w6 � 107 kg/m2. The pellets of x ¼ 0 andx ¼ 0.02 compositions were sintered at 1115 �C for 2 h but forother concentrations of Mo6þ, the sintering was done at lowertemperature, i.e. 1080 �C for 2 h in an air atmosphere to preventmelting of the samples. The melting temperature of MoO3 is low(w795 �C) and for higher concentration of Mo6þ this causesmelting of samples. These sintered pellets were coated with highpurity silver paint on two parallel surfaces and then dried at250 �C for 30 min before performing electrical measurements.The capacitance and impedance measurement of the samples atdifferent frequencies and temperatures were carried out usingHIOKI 3532-50 LCR meter. Morphological analyses of the

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Fig. 2 FE-SEM photographs of (K0.5Na0.5)Nb(1�x)MoxO3: (a) x ¼ 0, (b) x ¼ 0.02, (c) x ¼ 0.04, (d) x ¼ 0.06.

J. Rani et al.: J. Mater. Sci. Technol., 2013, -(-), 1e7 3

ceramic pellets were done by field emission scanning electronmicroscopy (FE-SEM, FEI quanta 200F) operating at 20 kV. Theestimated optical band gap energy of the compounds wascalculated with the help of UVeVis diffuse reflectance spec-trophotometer (Shimadzu UV-2450) in the wavelength range of200e800 nm using BaSO4 as the reference. The photo-luminescence analysis was conducted by using a fluorescencespectrophotometer (Hitachi F-4600) at room temperature. Thepowder samples were dispersed in Millipore� water to measurethe photoluminescence (PL) spectra.

3. Results and Discussion

3.1. Structural and microstructural properties

Fig. 1(a) shows the X-ray diffraction (XRD) patterns of(K0.5Na0.5)Nb(1�x)MoxO3 (x ¼ 0, 0.02, 0.04 and 0.06) ceramics.The observed peaks indicate the formation of pure perovskitephase for pure and Mo6þ doped KNN up to x ¼ 0.06. This is theclear indication for the formation of a homogenous solid solution,in which Mo6þ ion has diffused into KNN lattice. Furtherincreasing the Mo6þ to 8 mol% in KNN, impurity phases appearas seen in Fig. 1(b). The impurity phases were identified asK2Nb8O21 and K2Nb6O16 (* and # marked, respectively) whichwere further matched with international center for diffraction data(JCPDS) with file Nos. 31-1060 and 28-0788, respectively. Theformation of secondary phases indicates that the solubility limit ofMo6þ in the lattice of KNN has reached 6 mol% of Mo6þ.

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XRD pattern (Fig. 1(b)) and FE-SEM micrographs (inset ofFig. 1(b)) of x ¼ 0.08 composition show a large amount ofimpurity phases. Due to appearance of impurity phase, furtherstudy was not carried out for this composition.The enlarged view of XRD pattern for (K0.5Na0.5)

Nb(1�x)MoxO3 (x ¼ 0, 0.02, 0.04 and 0.06) in the angle rangefrom 44� to 47� is shown in Fig. 1(c). It depicts that the twodiffraction peaks (202) and (020) start to merge into a singlepeak with increasing concentration of Mo6þ. This indicates thatthe ceramics have a tendency to transform into another phase,e.g. pseudocubic, as the concentration of Mo6þ increases.Fig. 2 shows the FE-SEM micrographs of (K0.5Na0.5)

Nb(1�x)MoxO3 (x ¼ 0, 0.02, 0.04 and 0.06) sintered ceramics.All the micrographs show cuboidal shaped grains. There is nomuch change in grain size or shape on Mo6þ doping up tox ¼ 0.06. The average grain sizes of the compounds werecalculated by linear intercept method and found to be in therange of w5e7 mm.

3.2. Dielectric properties

The variation of dielectric constant ( 3) with temperature for(K0.5Na0.5)Nb(1�x)MoxO3 (x ¼ 0, 0.02, 0.04 and 0.06) ceramicsat frequency 1 kHz is shown in Fig. 3(a). This depicts that pureKNN undergoes two types of phase transitions: (i) orthorhombicto tetragonal transition (TOeT) at 200 �C and (ii) tetragonal tocubic phase transition (TC) at 395 �C. Mo6þ doped samples alsoexhibit similar phase transitions with different TOeT and TC.

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Fig. 3 Variation of: (a) dielectric constant ( 3), (b) dielectric loss (tand)of (K0.5Na0.5)Nb(1�x)MoxO3 (x ¼ 0, 0.02, 0.04 and 0.06) withtemperature at frequency 1 kHz.

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There is no systematic change in TOeT but TC shifted slightlytowards low temperature side from 395 to 380 �C withincreasing concentration of Mo6þ. The dielectric constant de-creases with broad dielectric peak at TC as the concentration ofMo6þ increases. This broadening may be due to compositionalfluctuation. Fig. 3(b) shows the dielectric loss of (K0.5Na0.5)Nb(1�x)MoxO3 (x ¼ 0, 0.02, 0.04 and 0.06) as a function oftemperature at frequency 1 kHz. The dielectric loss at roomtemperature varies in the range of w0.035e0.055 for x ¼ 0e0.06 Mo6þ concentration.

3.3. Complex impedance analysis

The analysis of complex impedance (Z*) can give informationof the frequency dependent properties of materials. The value ofZ* can be evaluated as follows:

Z� ¼ Z 0 � jZ 00 ¼ Rs � juCs

(1)

where Z 0 and Z 00 are the real and the imaginary part of Z*, Rs andCs is the resistance and capacitance in series, respectively,u ¼ 2pf is the angular frequency and j ¼ ffiffiffiffiffiffiffi�1

pis the

imaginary factor. The complex impedance spectra (Nyquistplot) of (K0.5Na0.5)Nb(1�x)MoxO3 (x ¼ 0, 0.02, 0.04 and 0.06)at different temperatures are shown in Fig. 4. The real andimaginary parts of the impedance are multiplied by the areaand divided by the thickness of the sample to eliminate theeffect of size of the sample. The value of impedance increaseswith increasing concentration of Mo6þ. Mo6þ may act as adonor dopant at Nb5þ site in KNN and may induce thedecrease of the conductivity as in the case of donor dopedPZT[30] and Nb doped BFO ceramic[31]. This decrease in

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conductivity may be due to the decrease in oxygen vacanciesin KNN which are created during sintering. Moreover, Mosubstitution for Nb5þ may impart a charge imbalance, due towhich Mo may undergo valency fluctuation in order tomaintain the charge balance in the system[32]. These may bethe possible reason for the increase in impedance. The plotsfor pure as well as Mo6þ doped KNN up to x ¼ 0.04 consistof single semicircular arc at higher temperatures, suggestingthat the electrical properties occurring in these compositionshave single relaxation which may be possibly due to thecontribution from the bulk material (grain interior properties).But there are two semicircular arcs for x ¼ 0.06 composition(Fig. 4(d)) which indicates that the grain boundary phase isalso activated at this composition. The semicircular arc athigher frequencies for x ¼ 0.06 at temperature 400, 450,500 �C is shown in the inset of Fig. 4(d) for a clear view. Thefirst semicircular arc is due to bulk contribution, while thesecond one is due to grain boundary contribution[33]. Theequivalent circuits corresponding to the semicircular arcs areshown in the insets of Fig. 4. In series combination of parallelRC circuit, one RC circuit represents bulk resistance (Rb) andbulk capacitance (Cb) of the samples while another representsgrain boundary resistance (Rgb) and grain boundarycapacitance (Cgb). It is observed that all the plots showdepressed semicircular arcs which represent non-Debye type ofrelaxation phenomenon in the material. As the temperatureincreases, the semicircular arcs become smaller and the angleof depression decreases. This shows the negative temperaturecoefficient of resistance (NTCR) behaviour similar to that ofsemiconductors[23].

3.4. Optical band gap calculation

Fig. 5 shows the diffuse reflectance spectra of (K0.5Na0.5)Nb(1�x)MoxO3 (x ¼ 0, 0.02, 0.04 and 0.06) ceramics for thedetermination of optical band gap of the compounds. The bandgap energy is determined by the use of KubelkaeMonk (KM)formalism[34,35]:

FðRÞ ¼ ð1� RÞ22R

(2)

where R is experimental reflectance. Within the energy rangecontaining the absorption edge features, F(R) can be assumedto be proportional to the absorption coefficient (a)[34,36]. Theoptical band gap energy (Eg) value can be evaluated with thehelp of fundamental absorption, which corresponds to electronexcitation from the valence band to conduction band. Therelation between the absorption coefficient (a) and the incidentphoton energy (hn) for the direct band gap is as follows:

hna ¼ A�Eg � hn

�1=2(3)

where h is the Planck constant, n is the frequency, a is theabsorbance coefficient, A is a constant, Eg is the optical bandgap energy. The extrapolation of linear part of (ahn)2 vs (hn)curves to the x-axis provides the band gap energy. In KNN,the band gap corresponds to the transition from the top of thevalence band occupied by O2p electron state to the bottom ofthe conduction bands dominated by the empty Nb4d electronstates[37]. The value of band gap energy of KNN was found todecrease with increasing concentration of Mo6þ. The band gap

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Fig. 4 Complex impedance spectra of (K0.5Na0.5)Nb(1�x)MoxO3 at temperatures of 350, 400, 450 and 500 �C for: (a) x ¼ 0, (b) x ¼ 0.02, (c) x ¼ 0.04,(d) x ¼ 0.06.

Fig. 5 Plot of (ahn)2 vs hn for the (K0.5Na0.5)Nb(1�x)MoxO3 at room temperature: (a) x ¼ 0, (b) x ¼ 0.02, (c) x ¼ 0.04, (d) x ¼ 0.06.

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Fig. 6 Room temperature photoluminescence spectra of (K0.5Na0.5)Nb(1�x)MoxO3 for x ¼ 0, 0.02, 0.04 and 0.06.

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energy for (K0.5Na0.5)Nb(1�x)MoxO3 (x ¼ 0, 0.02, 0.04 and 0.06)were found to be 4.28, 4.11, 3.82 and 3.61 eV, respectively. Oneof the reasons for this decrease in band gap energy is that theband gap energy of the bulk MoO3 is low (w2.9 eV)[24],therefore with increasing concentration of Mo6þ the band gapenergy of KNN decreases. Another reason of this red shift inband gap energy of KNN may be attributed to structuralmodification, which is also confirmed by XRD pattern.Generally in ABO3-type perovskite compounds, the opticalproperties are mainly determined by the oxygen-octahedralstructure[38]. In Mo doped KNN, the oxygen-octahedral mayshrink due to smaller radius of Moþ6 than that of Nbþ5 whichmay be responsible for red shift in band gap energy. Thereforeband gap of KNN can be tuned with Moþ6 doping.

3.5. Photoluminescence property

For further study of optical property of pure as well as Moþ6

doped KNN ceramics, photoluminescence spectroscopy wasused. Room temperature photoluminescence (PL) spectra of(K0.5Na0.5)Nb(1�x)MoxO3 for x ¼ 0, 0.02, 0.04 and 0.06 in thewavelength range of 335e450 nm are shown in Fig. 6. Thesuspension of powder samples was excited by photons ofwavelength 300 nm. One emission band w397 nm in visibleregion (violet emission) was observed for pure KNN. For Moþ6

doped (K0.5Na0.5)Nb(1�x)MoxO3 (x ¼ 0.02, 0.04, and 0.06),another emission band was also observed in UV region(w354 nm) along with visible band around w397 nm. Theappearance of emission band w354 nm in Moþ6 doped KNNmay be attributed to the presence of Moþ6 ions because MoO3

shows PL at room temperature[24,25]. Similar type of emissionhas already been reported by Navas et al.[24] in MoO3 film. Thereare many reports on the appearance of PL spectra due to dopingof various elements[39,40]. The presence of near band UV emis-sion in doped sample (w354 nm) may be ascribed to excitonrecombination that is the recombination of electrons in con-duction band and holes in valence band. The electron holerecombination occurs during inelastic excitoneexciton colli-sion[41]. Presence of visible emission band may be ascribed tothe transitions of excited optical centres in the deep levels. Thedeep level emission is usually attributed to the presence of im-purities and defects[24]. This photoluminescence behaviour of(K0.5Na0.5)Nb(1�x)MoxO3 ceramics may lead to its application inoptical devices.

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4. Conclusion

Polycrystalline (K0.5Na0.5)Nb(1�x)MoxO3 have been synthe-sized by solid state reaction method. There is a change in phaseinto another phase, i.e. pseudocubic, as the concentration ofMo6þ increases in KNN. Mo6þ ions are found to have limitedsolubility upto 6 mol% in the host compound KNN. Furtherincreasing the Mo6þ to 8 mol% leads to the formation of im-purity phases. The impedance analysis reveals mainly graincontribution upto 4 mol% whereas for 6 mol% of Mo6þ the grainboundary contribution is also observed. The red shift in the bandgap energy is observed with increasing concentration of Mo6þ inKNN. Interestingly, near band edge UV emission has been seenin Mo6þ doped KNN along with visible emission band of pureKNN in photoluminescence spectra at room temperature.

AcknowledgementAuthors acknowledge the financial support from Council of

Scientific and Industrial Research, New Delhi India under theresearch Grant No. 03(1156)/10/EMR II.

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