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Current Applied Physics 14 (2014) 407e414

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Current Applied Physics

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Influence of zirconium doping on structure, microstructure, dielectricand impedance properties of strontium bismuth niobate ceramics

Vinoth Shanmugama, R. Sridarane b, C. Deviannapoorani a, Raghavendra Kashyap a,Ramaswamy Murugan a,*

aDepartment of Physics, Pondicherry University, Puducherry 605 014, IndiabDepartment of Chemistry, Perunthalaivar Kamarajar Institute of Engineering and Technology, Karaikal, Puducherry, India

a r t i c l e i n f o

Article history:Received 4 September 2013Received in revised form14 December 2013Accepted 22 December 2013Available online 4 January 2014

Keywords:CeramicsAurivillius structureDielectricImpedance

* Corresponding author. Tel.: þ91 413 2654782; faxE-mail address: moranamurugan@pec.edu (R. Mur

1567-1739/$ e see front matter � 2014 Elsevier B.V.http://dx.doi.org/10.1016/j.cap.2013.12.020

a b s t r a c t

Efforts have been made in this work to enhance the dielectric properties of SrBi2Nb2O9 (SBN) by partialsubstitution of Zr4þ for Nb5þ. Systematic investigations on structure, microstructure, dielectric andimpedance properties of the SrBi2(Nb2�(4/5)xZrx)O9 [where, x ¼ 0, 0.1 and 0.2] ceramic samples werecarried out to understand the effect of substitution of Zr4þ for Nb5þ in SrBi2Nb2O9. The X-ray diffraction(XRD) investigations indicated that the lattice volume of SrBi2(Nb2� (4/5)xZrx)O9 with x ¼ 0.1 and 0.2decreases compared to SBN. The SEM investigations revealed an increase in the size of grains and thechange on shape of grains to elongated plate shaped structure with the increase of x (x ¼ 0.1 and 0.2) inSrBi2(Nb2�(4/5)xZrx)O9. Higher Curie temperature and enhanced peak dielectric constant at the Curietemperature were observed for both the SrBi2(Nb2�(4/5)xZrx)O9 with x ¼ 0.1 and 0.2 ceramic samplescompared to SBN. Among the investigated compositions the higher Curie temperature and enhancedpeak dielectric constant at the Curie temperature was observed for SrBi2(Nb2�(4/5)xZrx)O9 with x ¼ 0.1.

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1. Introduction

Bismuth layer structured ferroelectrics with Aurivillius typestructure received considerable attention in recent times becauseof their variety of device applications like non-volatile memory,electro-optical switches, and infrared detectors. Aurivillius familymaterials with general formula [Bi2O2]2þ [An�1BnO3nþ1]2� (wherethe sites A and B can be occupied by different elements) have acomplex structure given by [Bi2O2]2þ sheets intergrowths withperovskite blocks. Among the bismuth layer structured ferroelec-trics (BLSF’s), SrBi2Nb2O9 (SBN) and its solid solutions haveattracted much attention for the development of nonvolatilerandom access memories (NVRAMs) since they offer several ad-vantages like better fatigue properties, low operating voltages andlow leakage current.

The influence of substituting effect in BLSF’s was widely re-ported in literature aimed at improvement of the dielectric andferroelectric properties [1e7]. In particular, partial substitution ofV5þ with Nb5þ in SrBi2Nb2O9 found to have significant enhance-ment in ferroelectric properties [8e13]. Attempt has been alsomade to enhance the properties of SBN by partials substitution of

: þ91 413 2656988.ugan).

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Ta5þ and W6þ for Nb5þ in SrBi2Nb2O9 [6,14]. In the present worksystematic investigations on structure, microstructure, dielectricand impedance properties of the SrBi2(Nb2�(4/5)xZrx)O9 [where,x ¼ 0, 0.1 and 0.2] ceramic samples were carried out to understandthe effect of substitution of Zr4þ for Nb5þ in SrBi2Nb2O9.

2. Experimental

Compounds of the nominal chemical formula SrBi2(Nb2�(4/5)

xZrx)O9 [where, x ¼ 0, 0.1 and 0.2] were prepared via conventionalsolid state reaction method using stoichiometric amount of highpurity chemicals SrCO3 (Merck, >99%), Bi2O3 (3.5 wt% excess Bi2O3to compensate the weight loss of the Bi2O3 during the sintering),Nb2O5 and ZrO2. The powders were ball milled using zirconia ballsin 2-propanol for about 6 h using a Pulverisette 7, Fritsch, Germany.After the evaporation of solvent, the mixtures were heated at700 �C in an open alumina crucible for 12 h and then cooled downto room temperature. The resultant powders were ground again foranother 6 h using zirconia balls in 2-propanol. After the evapora-tion of the solvent, the powders were ground and admixed with2 wt% polyvinyl alcohol as a binder and uniaxially pressed under250 Mpa into disks of approximately 10 mm in diameter and about1e2 mm in thickness. The pellets were sintered in air for 12 h at1150 �C for SrBi2(Nb2�(4/5)xZrx)O9 [where, x¼ 0.1 and 0.2] and 6 h at1200 �C for SrBi2(Nb2�(4/5)xZrx)O9 [where x ¼ 0] sample.

V. Shanmugam et al. / Current Applied Physics 14 (2014) 407e414408

The phase formation and crystallinity of the calcined sampleswere characterized by X-ray diffraction (X’Pert PRO PANalytical),using Cu-Ka radiation of wavelength 1.5418 �A in the range of20

� � 2q � 80�. Confocal micro-Raman spectra have been recorded

at room temperature in the range 50e1000 cm�1 using a Ramanmicroscope (Renishaw inVia Reflex) with a 50 mW internal Ar ionlaser source of wavelength 514 nm. The microstructure of the sin-tered sample surface was analyzed by scanning electron micro-scope (SEM) (Hitachi). Impedance measurements were performedon the prepared pellets using Au-electrodes (Au paste cured at600 �C for 1 h) over the temperature range from room temperature(30 �C) to 700 �C using a Wayne Kerr precision impedance analyzer6500B series in the frequency range 20 Hze15 MHz.

3. Results and discussion

3.1. XRD analysis

The measured powder X-ray diffraction (XRD) patterns ofSrBi2(Nb2�(4/5)xZrx)O9 [where, x ¼ 0, 0.1 and 0.2] along with theJCPDS pattern of SBN (JCPDS file no. 86-1190) are shown in Fig. 1.The X-ray diffraction peaks of SrBi2(Nb2�(4/5)xZrx)O9 with x ¼ 0, 0.1and 0.2 shown in Fig. 1(b), (c) and (d), matches closely in bothpositions and relative intensities of the diffraction peaks of thereported SBN (JCPDS (card no 86-1190) as shown in Fig. 1(a)). TheXRD pattern of SrBi2(Nb2�(4/5)xZrx)O9 with x¼ 0.1 shown as Fig. 1(c)revealed a single phase layered perovskite structure without anysecondary phase, indicating that Zr4þ is successfully incorporatedinto the crystal lattice of SBN. However the XRD pattern ofSrBi2(Nb2�(4/5)xZrx)O9 with x ¼ 0.2 shown as Fig. 1(d) indicated thepresence of Sr2Bi2O5 as secondary phase along with the majorlayered perovskite structure. The XRD pattern of SrBi2(Nb2�(4/5)

xZrx)O9 with x ¼ 0.2 shown as Fig. 1(d) also revealed the diffractionpeaks are relatively broadened compared to that of SrBi2(Nb2�(4/5)

xZrx)O9 with x¼ 0 and 0.1. The XRD pattern of SrBi2(Nb2�(4/5)xZrx)O9with x ¼ 0.1 and 0.2 could be indexed to an orthorhombic cell. Thestructural refinement with the dominant diffraction peaks indi-cated that the lattice parameters of SrBi2(Nb2�(4/5)xZrx)O9 with

Fig. 1. XRD patterns a) SBN (JCPDS; 86-1190) and SrBi2(Nb2�(4/5)xZrx)O9 with b) x ¼ 0,c) x ¼ 0.1 and d) x ¼ 0.2.

x ¼ 0.1 have slight changes compared to SrBi2Nb2O9 as shown inTable 1. However further substitution of Zr4þ for Nb5þ i.e. x ¼ 0.2 inSrBi2(Nb2�(4/5)xZrx)O9 leads to appreciable decrease of the latticevolume. The addition of zirconium improves the densificationprocess as can be seen in Table 1.

3.2. Raman scattering

The Raman spectra of SrBi2(Nb2�(4/5)xZrx)O9 with x ¼ 0, 0.1 and0.2 samples weremeasured at room temperature to understand theinfluence of Zr doping on the vibrational modes of SBN. The Ramanspectra measured at room temperature in the spectral range 50e1000 cm�1 shown in Fig. 2 revealed that the Raman profiles ofSrBi2(Nb2�(4/5)xZrx)O9 with x ¼ 0.1 and 0.2 are similar to that of theSBN sample. SBN samples exhibit intense Raman bands at around66, 202, 577 and 828 cm�1 and weak bands at around 170, 300, 446and 701 cm�1. For the modes above 300 cm�1, the strong bandobserved at 828 cm�1 corresponds to the stretching mode of NbO6octahedra and another band observed at 577 cm�1 corresponds to arigid sublattice mode [14e16].

The NbO6 octahedral stretching mode observed at 828 cm-1 forSBN shifted to 837 cm-1 for SrBi2(Nb2�(4/5)xZrx)O9 with x ¼ 0.1.However further substitution of Zr, i.e. for SrBi2(Nb2�(4/5)xZrx)O9with x ¼ 0.2, the NbO6 octahedral stretching mode exhibit shifttowards lower wave number to 834 cm�1. The NbO6 octahedralstretching mode appears to be relatively symmetric with Zr sub-stitution. The changes observed in the NbO6 octahedral stretchingmode might be due to the effect of partial substitution of Zr4þ forNb5þ. The band observed at 577 cm�1 for SBN corresponds to therigid sublattice mode exhibit red shift to 566 cm�1 for SrBi2(Nb2�(4/

5)xZrx)O9 with x ¼ 0.1 and 569 cm�1 for SrBi2(Nb2�(4/5)xZrx)O9 withx ¼ 0.2. The mode observed at 66 cm�1, assigned as a ‘rigid layer’mode reflecting the Bi3þ ions vibration in Bi2O2 layers do notexhibit any major shifts with increase in Zr substitution [16].Similarly the band at 202 cm�1 corresponds to the vibrations of Asite ions of the pseudo perovskite blocks also do not exhibit anymajor shift in its position with the Zr substitution in SBN.

3.3. Scanning electron microscope (SEM) analysis

The SEM images of SrBi2(Nb2�(4/5)xZrx)O9 where, x ¼ 0, 0.1 and0.2 are shown in Fig. 3(a)e(c), respectively. The SEM images of allthe samples show relatively dense structure, and although smallpores were found in the samples. An obvious difference among theimages is the grain size which depends on the amount of dopedzirconium. The micrograph for SBN shown as Fig. 3(a) indicatedthat the grains are not well defined. The increase of x inSrBi2(Nb2�(4/5)xZrx)O9 (Fig. 3(b) and (c)) indicated the growth ofgrain and the change on shape of grains to elongated plate shapedstructure. The plate like structure is typical of Aurivillius phase due

Table 1Lattice parameters, volume and relative density of SBN (SrBi2Nb2O9) sintered at1200 �C, SrBi2(Nb2�(4/5)xZrx)O9 with x ¼ 0.1 and x ¼ 0.2 samples sintered at 1150 �C.

Composition Lattice parameters(�A)

VolumeV ¼ abc (�A3)

Relativedensity (%)

SBN (SrBi2Nb2O9) a ¼ 5.518b ¼ 5.515c ¼ 25.112

764.202 94

SrBi2Nb1.92Zr0.1O9 a ¼ 5.500b ¼ 5.519c ¼ 25.110

762.201 95

SrBi2Nb1.84Zr0.2O9 a ¼ 5.465b ¼ 5.492c ¼ 25.046

751.725 98

Fig. 2. Raman spectra of SrBi2(Nb2�(4/5)xZrx)O9 with a) x ¼ 0, b) x ¼ 0.1 and c) x ¼ 0.2.

V. Shanmugam et al. / Current Applied Physics 14 (2014) 407e414 409

to the anisotropic nature of the crystal structure. The addition ofZrO2 might have promoted the formation of a melting phase at thegrain boundaries and facilitates the easy growth of grains and helpsin obtaining the dense ceramics by effectively increasing thediffusion distance between the grains.

Fig. 3. SEM images of the SrBi2(Nb2�(4/5)xZrx)O9 with a) x ¼ 0, b) x ¼ 0.1 and c) x ¼ 0.2.

3.4. Dielectric studies

The complex permittivity (ε*), or dielectric constant, of a systemis defined by,

ε* ¼ ε

0 � jε00; (1)

ε* ¼ ε

0 � j�

s

uε0

�; (2)

where ε’ is real part of the dielectric constant, ε00 is the imaginarypart of the dielectric constant of the material, s is the conductivity,u is the angular frequency, and ε0 is the permittivity of the freespace.

Fig. 4 shows the temperature dependent dielectric constant ofSrBi2(Nb2�(4/5)xZrx)O9 [where, x¼ 0.1 and 0.2] samples measured at100 kHz with an oscillating amplitude of 50 mV. The SBN sampleshows a typical ferroelectric transition at around 435 �C (Curietemperature) with peak dielectric constant of 700 at the measuredfrequency of 100 kHz [8]. The transition temperature ofSrBi2(Nb2�(4/5)xZrx)O9 ceramics with x ¼ 0.1 is observed at 550 �Cwith the peak dielectric constant of 1872 at the measured fre-quency of 100 kHz. The temperature dependent dielectric constantmeasurement of SrBi2(Nb2�(4/5)xZrx)O9 ceramics with x¼ 0.2 showsrelatively broad peak at around 515 �Cwith peak dielectric constantof 866. Vanadium substituted SrBi2(V0.1Nb0.9)2O9 (SBVN) exhibitedCurie temperature at 477 �C with maximum dielectric constant of779 at the measured frequency of 100 kHz [17]. The temperaturedependent dielectric constant of SrBi2(Nb2�(4/5)xZrx)O9 ceramicswith x ¼ 0.1 and 0.2 measured at 100 kHz with an oscillatingamplitude of 50 mV indicated higher transition temperature andenhanced dielectric constant at the transition temperaturecompared to SBN, SBVN and other systems as shown in Table 2.

Fig. 5(a) and (b) shows the temperature dependent dielectricconstants of SrBi2(Nb2�(4/5)xZrx)O9 ceramic samples measured as afunction of temperature at different frequencies 10 kHz, 100 kHz,1 MHz, 15 MHz, with x ¼ 0.1 and 0.2 respectively. The dielectricconstants at low temperatures are the same regardless the

frequencies used for the measurements. The dielectric constantsmeasured at high temperatures vary markedly as shown in Fig. 5(a)and (b). At all measured frequencies SrBi2(Nb2�(4/5)xZrx)O9 exhibitssharp peak for x ¼ 0.1 and broad peak for x ¼ 0.2 at the transitiontemperature. The significant enhancement of the dielectric con-stant at high temperatures might be related to the oxygenvacancies.

The dielectric constant (εr) of a material has four polarizationcontributions: electronic polarization due to the displacement ofelectron shell relative to a nucleus; ionic polarization due to thedisplacement of charged ion or vacancies with respect to other

Fig. 4. Temperature-dependent dielectric constant of SrBi2(Nb2�(4/5)xZrx)O9 withx ¼ 0.1 and 0.2 measured at 100 kHz.

a

b

Fig. 5. Temperature-dependent dielectric constant of SrBi2(Nb2�(4/5)xZrx)O9 with a)x ¼ 0.1 and b) x ¼ 0.2 measured at various frequencies.

V. Shanmugam et al. / Current Applied Physics 14 (2014) 407e414410

ones; dipolar polarization due to the change of orientation of do-mains with permanent electric dipole moment in an applied elec-tric field and space charge polarization arises from electric chargeat structural interfaces. For the temperature-dependent dielectricconstant shown in Fig. 4 measured at a frequency of 100 kHz withan oscillating amplitude of 50 mV the contribution comes fromionic and atomic polarization only and space charge and dipolarpolarizations are saturated. Curie temperature, peak dielectricconstant, spontaneous polarization and coercive field of ferroelec-trics are largely dependent on the ionic displacements associatedwith the samples. Ionic polarization is strongly dependent on thecrystal structure, including density and lattice constant or unit cellvolume. The Zr doped SBN samples possesses lower values of latticeconstant a and c as compared to those of undoped SBN. However,the value of b is higher for the SrBi2(Nb2�(4/5)xZrx)O9 ceramics withx ¼ 0.1 compared to other samples. Doping at either A-site or at Bsite in perovskite structures influences orientation of lattice alongaeb and aec plane equally but in Aurivillius type this influence islimited to aec plane only. Therefore, in Aurivillius type structures

Table 2Measured frequency, transition temperature (Tc) and dielectric constant (εr) of re-ported SBN and doped SBN samples.

S. no. SBN and doped SBN Measuredfrequency

Transitiontemp. (Tc)

Dielectricconstant(εr)

Ref.

1 SrBi2Nb2O9 100 kHz 440 1100 [5]2 Sr0.8Bi2.14(V0.2Nb0.8)2O9 100 kHz 493 2782 [12]3 SrBi2(Nb0.5Ta0.5)2O9 100 kHz w340 w385 [6]4 SrBi2Nb1.9Mo0.1O9 100 kHz 365 w610 [21]5 Sr1.1Bi1.933(W0.1Nb0.9)2O9 100 kHz w410 w950 [22]6 Ca0.1Sr0.9Bi2Nb2O9 100 kHz 475 w530 [23]7 Sr0.6Pb0.4Bi2Nb2O9 100 kHz 476 w1400 [24]8 Sr1�xLaxBi2Nb2O9 100 kHz w450 w1000 [25]9 Ba0.1Sr0.81La0.06Bi2Nb2O9 1 kHz 485 w650 [26]10 Ba0.1Sr0.81Gd0.06Bi2Nb2O6 1 kHz 470 788 [27]11 SrBi1.95Nd0.05Nb2O9 1 kHz 410 1252 [28]12 SrBi1.75Pb0.25Nb2O8.875 100 kHz 490 w1300 [29]13 SrBi3NbTi2O12 100 kHz 350 w130 [30]14 Sr0.6Bi2.266Nb2O9 100 kHz 525 1030 [31]15 SrBi2(Nb1.92Zr0.1)O9 100 kHz 550 1872 Present

work

tetragonal strain (c/a; strain along c-axis) is observed to be high.The investigation on the influence of Zr doping in SBN on thestructural properties like tetragonal strain (c/a; strain along c-axis)and orthorhombic distortion (b/a) revealed more tetragonal strainand less orthorhombicity. Higher Curie temperature and enhanceddielectric constant observed for SrBi2(Nb2�(4/5)xZrx)O9 ceramicswith x ¼ 0.1 is possibly due to its maximum permissible change inrattle space available between the octahedrons. The formation ofvacancies by the substitution of Zr4þ ions for Nb5þ in the SBNcannot be ruled out because of the constraint of maintaining overallcharge neutrality of the structure. When Zr4þ is incorporated forNb5þ into the SBN layered perovskite structure, oxygen vacancieswould be created. One oxygen vacancy will form with two tetra-valent zirconium ions entering the crystal structure, in order tokeep the electro neutrality and this reaction could be described by:

2ZrO2 ¼ 2Zr0Nb þ 4Ox

O þ V$$O ;

where Zr0Nb represents tetravalent Zr4þ occupied pentavalent Nb5þ

site with one effective negative charge and V$$O represents an oxy-

gen vacancy with two effective positive charges. Further detailed

V. Shanmugam et al. / Current Applied Physics 14 (2014) 407e414 411

studies of these ceramics are in progress to explore the origin of theenhanced dielectric properties of the Zr doped SBN ceramics.

An appreciable negative shift of Curie temperature of 20 �C inthe measured increasing frequency range 10 kHze15 MHz wasobserved for SrBi2(Nb2�(4/5)xZrx)O9 with x ¼ 0.1 as shown inFig. 5(a). However SrBi2(Nb2�(4/5)xZrx)O9 with x ¼ 0.2 sampleexhibit very small negative shift of 5 �C in the measured increasingfrequency range 10 kHze15 MHz (Fig. 5(b)). The well knowncharacteristic of relaxor ferroelectrics are the shift in Curie tem-perature towards higher temperature and strong frequencydispersion of dielectric constant with an increasing frequency.However, the negative shift of Curie temperature observed in thepresent work as a function of frequency might be a result of anoverlap of the true paraferroelectric transition peak with the defectinduced dielectric relaxation.

The tangent loss can be expressed as,

tan d ¼ ε00

ε0 ; (3)

where ε0 is real part of the dielectric permittivity, ε00 is the imaginarypart of the dielectric permittivity of the material. Fig 6(a) and (b)shows temperature dependent tangent loss measured at different

a

b

Fig. 6. Temperature-dependent tangent loss of SrBi2(Nb2�(4/5)xZrx)O9 with a) x ¼ 0.1and b) x ¼ 0.2 measured at various frequencies.

frequencies 10 kHz, 100 kHz, 1 MHz, 15 MHz, for zirconium-dopedSrBi2(Nb2�(4/5)xZrx)O9 with x ¼ 0.1 and 0.2, respectively. In generalthe tangent loss for this composition decreases with increase infrequency and shows no significant anomaly near the Curie tem-perature. The formation of higher concentration of charge carriersat higher temperature might be the reason for the increase intangent loss at higher temperature.

3.5. AC conductivity behavior

The frequency dependent conductivity is often used to deriveinformation regarding the dynamic behavior of ion conductingmaterials. The ac conductivity behavior is analyzed for many ionicsolids, glasses, polymers, and semiconductors, by using Jonscher’suniversal power law (UPL) [18],

s0ðuÞ ¼ sdc þ Aun: (4)

A is pre-factor that depends on temperature and compositionand it is defined as A ¼ (sdc/up

n). The above equation is rewritten as,

s0ðuÞ ¼ sdc

�1þ

�u

up

�n�: (5)

The above equation is called framework of AlmondeWest con-ductivity formalism [19,20] where sdc is the frequency independentdc conductivity, u is angular frequency, up is the hopping frequencyof the charge carriers and n is the dimensionless frequency expo-nent which lies in the range 0 < n < 1. Frequency dependentconductivity s0 of SrBi2(Nb2�(4/5)xZrx)O9 ceramics with x ¼ 0.1 and0.2 at various temperatures are shown as Fig. 7(a) and (b),respectively. Two clear dispersion regions were found in the fre-quency dependent conductivity s0 spectra for the investigatedsamples as shown in Fig. 7(a) and (b). The lower frequency partcorresponds to the relaxation processes in grain boundaries, andthe ion blocking character of the electrode. At high frequency theconductivity displays a dispersive behavior attributed to therelaxation processes in grains as generally observed in ionicconductors.

The conductivity spectra for different temperatures are fitted toEq. (5) independently for both the investigated samples using non-linear least square (NLLS) fitting of LevenbergeMarquard methodand the parameters sdc, up and n are extracted from the analysis.The parameters obtained by fitting are tabulated in Tables 3 and 4for SrBi2(Nb2�(4/5)xZrx)O9 with x ¼ 0.1 and 0.2 compounds,respectively.

3.6. Electric modulus analysis

Complex modulus formalism is a very important tool to deriveinformation related to charge transport processes in ionic solids.The complex electric modulus (M*) has been calculated from thecomplex impedance (Z*) data using the relation,

M*ðuÞ ¼ 1=ε*ðuÞ ¼ iuC0Z* ¼ M0ðuÞ þ iM}ðuÞ

¼ MN

241�

ZN

0

expð � iutÞ�dfðtÞdt

�dt

35; (6)

where ε*(u) is the complex permittivity, C0 is the geometricalcapacitance ¼ ε0A/t (ε0 ¼ permittivity of free space, A ¼ area of theelectrode and t ¼ thickness),M0 ¼ real part of the electric modulus,M00 ¼ imaginary part of the electric modulus, MN ¼ 1/εN (εN is thehigh frequency asymptotic value of the real part of the dielectric

a

b

Fig. 7. Frequency dependent conductivity measured in the temperature interval from400 to 700 �C of SrBi2(Nb2�(4/5)xZrx)O9 with a) x ¼ 0.1 and b) x ¼ 0.2. The solid lines arebest fit to the AlmondeWest conductivity formalism [19].

Table 4Parameters derived from the fits of ac conductivity measurements for SrBi2(Nb2-(4/5)xZrx)O9 with x ¼ 0.2.

Temp (�C) sdc (S cm�1) A (S cm�1 rad�n) n up (rad S�1)

400 1.90 � 10�7 1.95 � 10�11 0.72 3.46 � 105

450 4.20 � 10�7 6.20 � 10�11 0.67 5.22 � 105

500 1.00 � 10�6 9.90 � 10�11 0.69 6.35 � 105

510 1.40 � 10�6 6.60 � 10�10 0.55 1.11 � 106

520 1.80 � 10�6 6.70 � 10�10 0.56 1.32 � 106

550 2.80 � 10�6 7.10 � 10�10 0.58 1.58 � 106

600 6.00 � 10�6 7.30 � 10�10 0.60 3.34 � 106

650 1.40 � 10�5 8.30 � 10�10 0.63 5.12 � 106

700 2.40 � 10�5 8.70 � 10�10 0.64 8.68 � 106

a

b

V. Shanmugam et al. / Current Applied Physics 14 (2014) 407e414412

permittivity) and the relaxation function 4(t) gives the time evo-lution of the electric field within the material.

Fig. 8(a) and (b) shows the variation of real part of the electricmodulus (M0) with log (f) of SrBi2(Nb2�(4/5)xZrx)O9 with x ¼ 0.1 and0.2, measured in the temperature range from 400 to 700 �C. It ischaracterized by the low value of M0 in the low frequency regionand a continuous dispersion with frequency to saturate at a

Table 3Parameters derived from the fits of ac conductivitymeasurements for SrBi2(Nb2�(4/5)

xZrx)O9 with x ¼ 0.1.

Temp (�C) sdc (S cm�1) A (S cm�1 rad�n) n up (rad S�1)

400 4.90 � 10�7 9.00 � 10�11 0.65 6.87 � 105

450 1.30 � 10�6 6.20 � 10�10 0.55 8.89 � 105

500 3.25 � 10�6 9.20 � 10�10 0.58 1.18 � 106

530 6.40 � 10�6 1.50 � 10�9 0.57 2.47 � 106

540 7.60 � 10�6 4.50 � 10�9 0.47 2.61 � 106

550 9.80 � 10�6 7.90 � 10�9 0.45 5.74 � 106

600 2.40 � 10�5 9.30 � 10�9 0.49 3.27 � 107

650 4.00 � 10�5 1.00 � 10�8 0.51 7.23 � 107

700 7.20 � 10�5 2.00 � 10�8 0.5 8.80 � 107

maximum value designated as MN, attributed to the conductionphenomenon arises from the short range mobility of charge car-riers. Decrease in the value of MN and shift of dispersion regiontowards higher frequency side was observed with the rise in tem-perature up to 540 �C for SrBi2(Nb2�(4/5)xZrx)O9 with x ¼ 0.1 and up

Fig. 8. Plot of real part of modulus (M0) of SrBi2(Nb2�(4/5)xZrx)O9 with a) x ¼ 0.1 and b)x ¼ 0.2 compounds with frequency.

V. Shanmugam et al. / Current Applied Physics 14 (2014) 407e414 413

to 510 �C for SrBi2(Nb2�(4/5)xZrx)O9 with x ¼ 0.2. However withfurther increase of temperature from 550 �C for SrBi2(Nb2�(4/5)xZrx)O9 with x¼ 0.1 and 520 �C for SrBi2(Nb2�(4/5)xZrx)O9 with x¼ 0.2 anincrease in the value ofMN was noticed corresponding to the Curietemperature.

Fig. 9(a) and (b), respectively, shows the variation of imaginarypart of the electric modulus (M00) with log (f) of SrBi2(Nb2�(4/5)xZrx)O9 with x ¼ 0.1 and 0.2, measured at different temperatures in thetemperature range from 400 to 700 �C. The low-frequency side ofthe M00 peak represents the range of frequencies in which chargecarriers can move over a long distance. The high frequency side ofthe M00 peak represents the localized motion within the well. Themaxima of the imaginary component of modulus (M00

max) shifttowards higher relaxation frequency with the rise in temperatureas shown in Fig. 9(a) and (b). The peak frequency fm represents themost probable conductivity relaxation frequency from the relationfmsm ¼ 1, where sm is the characteristic relaxation time.

As shown in Fig. 9(a) and (b) the magnitude of the peak de-creases with increase in temperature as one approach the Curie

a

b

Fig. 9. Plot of imaginary part of modulus (M00) of SrBi2(Nb2�(4/5)xZrx)O9 with a) x ¼ 0.1and b) x ¼ 0.2 compounds with frequency.

temperature, and again increases with increase in temperature.Decrease in the value of themaxima of the imaginary component ofmodulus (M00

max) was observed with the rise in temperature up to540 �C for SrBi2(Nb2�(4/5)xZrx)O9 with x ¼ 0.1 and up to 510 �C forSrBi2(Nb2�(4/5)xZrx)O9 with x ¼ 0.2. However with further increaseof temperature from 550 �C for SrBi2(Nb2�(4/5)xZrx)O9 with x ¼ 0.1and 520 �C for SrBi2(Nb2�(4/5)xZrx)O9 with x¼ 0.2 an increase in thevalue ofM00

max was noticed corresponding to their respective phasetransition temperature.

The scaling behavior of SrBi2(Nb2�(4/5)xZrx)O9 with x ¼ 0.1 and0.2 was studied by plotting normalized parameters (i.e. M00/M00

maxvs. log (f/fmax); fmax is frequency corresponding to M00

max) atdifferentmeasuring temperatures and is shown in Fig.10(a) and (b).The overlapping of all the peaks at different temperatures into asingle master curve reveals that the relaxation describes the samemechanism at various temperatures.

The full width at half maximum (FWHM) value derived from themodulus curve of SrBi2(Nb2�(4/5)xZrx)O9 with x ¼ 0.1 and 0.2 werefound to be 1.40 and 1.51 decade, respectively. The derived fullwidth at half maximum (FWHM) values for the compositionSrBi2(Nb2�(4/5)xZrx)O9 with x¼ 0.1 and 0.2 are wider than the widthof a typical Debye peak (1.14 decade), suggesting the presence ofnon-Debye type conductivity relaxation phenomena in the inves-tigated samples.

a

b

Fig. 10. Modulus scaling behavior of SrBi2(Nb2�(4/5)xZrx)O9 with a) x ¼ 0.1 and b)x ¼ 0.2.

V. Shanmugam et al. / Current Applied Physics 14 (2014) 407e414414

4. Conclusion

A systematic investigation on structure, microstructure, dielec-tric and impedance properties of SrBi2(Nb2�(4/5)xZrx)O9 with x¼ 0.1and 0.2 ceramics were carried out using XRD, SEM and impedancespectroscopic techniques. XRD investigations on SrBi2(Nb2�(4/5)

xZrx)O9 with x ¼ 0.1 revealed slight changes in the lattice constantscompared to SBN. In addition to the shift of dominant diffractionpeaks the SrBi2(Nb2�(4/5)xZrx)O9 with x ¼ 0.2 sample shows thepresence of minor secondary phase Sr2Bi2O5. The microstructuralinvestigation by SEM revealed that the increase of x in SrBi2(Nb2�(4/

5)xZrx)O9 increases the growth of grain and the change on shape ofgrains to elongated plate shaped structure. The increase of Zr4þ inSrBi2(Nb2�(4/5)xZrx)O9 might promotes the formation of a lowmelting phase at the grain boundaries and facilitates the easygrowth of grains and helps in obtaining the dense ceramics. HigherCurie temperature and enhanced peak dielectric constant at theCurie temperature were observed for the SrBi2(Nb2�(4/5)xZrx)O9ceramic samples investigated in this work compared to SBN.Among the investigated compositions higher Curie temperatureand the maximum peak dielectric constant is observed forSrBi2(Nb2�(4/5)xZrx)O9 ceramic with x ¼ 0.1. The negative shift ofCurie temperature observed with increasing measured frequencyfor SrBi2(Nb2�(4/5)xZrx)O9 [x¼ 0.1 and 0.2] samplesmight be a resultof an overlap of the true paraferroelectric transition peak with thedefect induced dielectric relaxation. From the impedance andmodulus spectroscopic studies the investigated materials showedrelaxation effects which are non-Debye type.

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