钛酸铋钠介电调节

Journal of Physics and Chemistry of Solids67(2006)1803–1808Modification of dielectric relaxations in sodium bismuthtitanate with samarium dopingM.Raghavender,G.S.Kumar,G.PrasadÃDepartment of Physics,Osmania University,Hyderabad500007,IndiaReceived28December2005;received in revised form14March2006;accepted6April2006AbstractNa0.5Bi(0.5Àx)Sm x TiO3(NBST)ceramics with x¼0:05,0.1,and0.15are prepared through chemical route.The X-ray diffraction studies confirmed the formation of single phase.Dielectric measurements in the temperature region ranging from room temperature ($301C)to6001C at different frequencies(10kHz–1MHz)showed anomalies at130,306,and4741C(at10kHz frequency)for x¼0:05 sample.Other samples showed only two peaks.To establish the electrical nature of these relaxations,impedance measurements are done at different temperatures and frequencies.The relaxation time,obtained from both impedance and modulus data,is found to decrease with increase in temperature.The relaxations observed are of non-Debye type.Increase in samarium content increases the activation energy for relaxation.r2006Elsevier Ltd.All rights reserved.Keywords:A.Ceramic;B.Chemical synthesis;D.Dielectric properties;D.Ferroelectricity;D.Electrical properties1.IntroductionSodium bismuth titanate Na1=2Bi1=2TiO3(NBT)is arelaxor ferroelectric material with the general formulaA0xA001ÀxBO3.Smolensky et al.[1]havefirst reportedferroelectricity in sodium bismuth titanate.The ferroelec-tricity in NBT ceramic is attributed to(Bi1/2Na1/2)2+ions,especially Bi3+ions at the‘A’site of the perovskitestructure(ABO3)and due to rhombohedral symmetrybelow2001C.NBT is considered to be one of the excellentcandidates among lead-free piezoelectric materials and itsremanent polarization(P r)is38m C/cm2and coercivefield(E c)is73kV/cm at room temperature.It has high Curietemperature(T c¼3201C),and shows diffuse phasetransition(DPT)[1].In addition to the DPT,two morephase transitions were reported at2301C and betweentemperature range500–6001C.The peculiar phase transi-tions behavior of NBT has attracted considerable attentionof the investigators to examine the different dielectricanomalies and electrical state of each phase[2,3].Hong et al.[4]suggested that the intermediate phase is amixture of rhombohedral micropolar and tetragonalnonpolar matrix.Phase transition in NBT is an exceptionalcase of ferroelectric phase transition in that,the pointgroup of ferroelectric phase is not a subgroup of a highertemperature paraelectric phase.Tu et al.[5]described thepeculiarity of NBT phase transition between200and3001C in the context of incommensurate polar2commen-surate polar phase transition with increasing temperature.They suggested that the size of the incommensurate phaseto be of the order of nanometers,accounting for thestructural disparity between the incommensurate phasewith low symmetry and the ordered rhombohedral totetragonal matrix.Zvirgzds et al.[6]employedfixed frequency permittivitymeasurements to determine whether the tetragonal poly-morph is polar or non-polar.High temperature,fixed-frequency dielectric measurements reported on NBT‘revealed’a peak maximum in permittivity data at,450–5401C.This dielectric anomaly was related to atetragonal to cubic phase transition known to occur inNBT,from X-ray diffraction(XRD)and differentialscanning calorimetry(DSC)measurements at5401C.This/locate/jpcs0022-3697/$-see front matter r2006Elsevier Ltd.All rights reserved.doi:10.1016/j.jpcs.2006.04.004ÃCorresponding author.E-mail address:guduru_p@(G.Prasad).led Zvirgzds et al.[6]to postulate,on the basis offixed-frequency electrical data,that the tetragonal polymorph is polar.East and Sinclair[7]characterized the NBT using electric modulus spectroscopy.Their results demonstrate that the bulk permittivity of NBT decreases smoothly above3201C and does not show the presence of any anomaly in the region450–5401C as has been reported.Different dielectric anomaly studies reported different types of results.Most of the contradictions concern the number(two or three peaks in‘e’versus temperature plots)and location of different phases as well as the electrical state in each phase. Compositional modifications of NBT using dopant have attracted more interest,for only a small amount of dopant may modify the properties of NBT significantly.Recently lanthanum(La)was chosen to be a modifier in NBT system at A-site[8–10]and also at A and B sites simultaneously [11].Herabut et al.[8]reported that the La-doped NBT results in enhancing its electrical properties due to super-paraelectric cluster polarization reorientation.Hence samarium(Sm)doping can influence the polar phases present in NBT at high temperatures.This paper presents the impedance spectroscopy studies of polycrystal-line Sm-doped NBT ceramics.The data is analysed in terms of complex impedance(Z*)or electrical modulus (M*)formalism,which help in eliminating stray frequency effects and determine the conductivity for comparative purposes.The electrical impedance formalism can be used to investigate the relaxations of defect species and the dielectric behaviour.This may provide an insight into polar phase transitions in the samples,especially the transitions present in the temperature region450–5501C.2.ExperimentalPolycrystalline samples of Na0.5Bi(0.5Àx)Sm x TiO3 (NBST)with x¼0:05,0.1,and0.15(NBST1,NBST2, and NBST3,respectively)were prepared through chemical route[12].The stoichiometric amounts of Na2CO3and Sm2O3were taken and converted to their respective nitrates by adding nitric acid solution.Bi(NO3)3Á5H2O was added along with TiO2to the above solutions.The entire mixture was slowly heated on a hot plate and citric acid was added. The pH was adjusted between6and7by adding dilute ammonium hydroxide solution.Ethylene glycol was added and heated over the temperature160–1801C to get the gel. The gel was heated to obtain the precursor.This precursor was heated at8001C for2h.The resultant powder was crushed and pressed in the form of pellets using polyvinyl alcohol as the binder.The pellets werefinally sintered at the temperature11501C for2h.Phase confirmation was done through XRD(Model Philips-PW3040/60,X’pert PRO Analytical).The silver paste was used as an electrode material on large area faces for electrical measurements. The samples were electrically poled by applying a constant DCfield of30kV/cm at1501C and allowed to cool rapidly to room temperature in the presence of thefield.Dielectric measurements were performed(using HP4192A Impe-dance Analyzer)in the frequency range of10kHz–1MHz from room temperature($301C)to6001C.The impedance properties were measured as a function of frequency ranging from500Hz to1MHz and at different tempera-tures from room temperature($301C)to6001C.The data acquisition is automated with a PC interfaced to the instrument.3.Results and discussionFig.1shows the powder X-ray diffraction(XRD) patterns of NBST samples.The powder XRD is indexed on the basis rhombohedral structure(JCPDS File:36-340). From the XRD pattern obtained the formation of single phase was confirmed.The lattice parameters were calcu-lated and tabulated in Table1.With the increase of samarium concentration the lattice parameters show a tendency to decrease.All the samples prepared have about 90%of X-ray(theoretical)density when compared with the experimental density.Fig.2shows the dielectric constant as a function of temperature at different frequencies.We can observe three peaks in dielectric constant plots of NBST1samples.In contrast to other studies[7],we observe dielectric anomaly between450and5501C.The peak temperatures are at130, 306and4741C at10kHz frequency for NBST1sample. The peak temperatures T1,T2and T3of all the samples are summarized in Table2and are found to be frequency dependent.With the increase of frequency,dielectricFig.1.X-ray diffractograms of(a)NBST1,(b)NBST2,and(c)NBST3. Table1Lattice parameters and percentage of density of NBST ceramics Sample Lattice parameters%Of densitya(A)a(deg)NBT a 3.98590.2491.9NBST1 3.88289.8989.5NBST2 3.88089.8091.2NBST3 3.86090.0192.3a After Aparna et al.[13].M.Raghavender et al./Journal of Physics and Chemistry of Solids67(2006)1803–1808 1804constant values have decreased and this decrease is more prominent for4741C peak for NBST1.The dielectric loss (Fig.3)also followed similar behavior as that of dielectric constant in general.However,at low frequencies and for temperature above4001C,the dielectric loss was found to increase with the increase of temperature.The low-temperature dielectric loss values are relatively small (approx10À2)and vary little with temperature.Increase of samarium concentration has resulted in decrease of dielectric constant and loss values in general.The peak at1301C observed for NBST1sample could not be recorded clearly for NBST2and NBST3samples.However,the peak at3061C of NBST1sample is found to be less pronounced in NBST2and NBST3samples and the peak of4741C has been observed at high temperatures,i.e. 546and5011C for NBST2and NBST3,respectively.For NBST3sample,for high frequency,there is a shoulder in high-temperature region of dielectric loss curves.Curie–Weiss law could not befit to the high-temperature peak of the samples.However,these peaks follow modified Curie–Weiss law of diffuse phase transition(DPT).Similar behavior was reported for pure NBT samples earlier[5]. Fixed-frequency permittivity measurements at lower frequencies become increasingly dominated at elevatedFig.2.Variation of dielectric constant with temperature of(a)NBST1,(b)NBST2,and(c)NBST3samples.Table2Dielectric peak temperatures(in1C)of NBST ceramicsFrequency NBST1NBST2NBST3T1T2T3T1T2T3T1T2T3 10kHz130306474—305546—308501 50kHz130295457—302530—302476 100kHz131296446—300525—301457 500kHz136290——299525—300450 1MHz137290——297527—297451 M.Raghavender et al./Journal of Physics and Chemistry of Solids67(2006)1803–18081805temperatures by nonbulk,high polarization processes,such as sample/electrode contact effects.Measuring the a.c.response over a wide frequency range,e.g.10À2–107Hz,as opposed to a single frequency measurements can avoid such problems and ensure that reliable bulk characteristics,i.e.permittivity and conductivity values,can be obtained,especially at elevated temperatures.Also the modulus spectra along with impedance spectra can give additional information about the electrical nature of the phase transitions in the sample.The electric modulus,M üM 0þjM 00and complex permittivity formalisms,e *are related through e ü1=M Ã[14].The electric modulus formalism is dominated by small capacitance values and is therefore very sensitive to the intra-granular component of ceramic materials such as bulk capacitance (C bulk ),grain boundary capacitance (C gb ),and electrode capacitance (C el )[15].The intra-granular response of a dielectric material is normally modelled on an equivalent circuit consisting of an ideal resistor and capacitor connected in parallel.In general,a parallel resistor–capacitor (RC)element gives rise to a semicircle in the M *complex plane and a Debye peak in the spectroscopic plot of M 00against log f .At theM 00peak maximum,M 00max ¼1=2e o e00where e o is the permittivity of free space and o max RC ¼1.The heightand peak position of M 00spectra can therefore be used to calculate bulk permittivities and resistivities in poly-crystalline ceramics.The use of M 00spectroscopic plots to estimate bulk resistivities in ceramic materials,as opposed to the more conventional complex impedance plane plots (Z 0and Z 00),has been used for characterizing ceramic materials where the total resistivity is dominated by resistive grain boundaries [16].Since samarium doping can effect the grain boundary capacitance and conduc-tance.In the present study,these plots are used for electrical characterization of high temperature dielectric peaks.Fig.4(a)–(c)shows the variation of imaginary part of impedance (Z 00)and electric modulus (M 00)between 525and 6001C as a function of frequency for NBST1,NBST2,and NBST3samples,respectively.With the increase of frequency Z 00and M 00values initially increase then reach a maximum value,further increase of frequency decreases this value and the M 00values increases again.At the higher-frequency side all curves merge.With increasing tempera-ture the peak value shifts to higher frequency side and peak becomes broader.The peak frequency for M 00curves lie between 3Â104and 4Â105Hz.The M 00and Z 00peaks areFig.3.Variation of dielectric loss with temperature of (a)NBST1,(b)NBST2,and (c)NBST3samples.M.Raghavender et al./Journal of Physics and Chemistry of Solids 67(2006)1803–18081806not coincident and there is no evidence of any additional peaks at higher frequency side in the Z00spectrum.In the temperature between525and6001C the peak width at half maxima of Z00and M00curves is more than a decade indicating non-Debye relaxation behavior[16].This non-Debye relaxation behavior may be due to the distribution of relaxation times.Distribution of relaxation may be present in the sample due to the existence of the multiple polar phases in the sample.The peak frequency difference is found to decrease with the increase of samarium concentration.If a ceramic is well sintered,typically95%of X-ray (theoretical)density,and the bulk component is electrically homogeneous,then a single M00Debye peak width at half maxima approaching the theoretical value of1.14decades on a log frequency scale should be observed.Conversely,if a sample is poorly sintered and/or the bulk component is electrically inhomogeneous then M00spectra can contain two,overlapping M00Debye peaks or a single,asymmetric and significantly broadened peak with peak width at half maxima in excess of2decades on a log frequency scale[16]. The peak width at half maxima of the present system of samples varies from1.2to1.5.It is worth noting that M00spectra should only be employed to determine bulk dielectric characteristics if the response is Debye-like,i.e.the peak width at half maxima of M00is E1.14decades on a log frequency scale.When analyzing samples that show non-Debye-like responses,i.e. presence of significant M00peak broadening due to electrical inhomogeneities or the presence of a grain boundary component,then electric modulus complex plane plots should be employed[16,17]The plots of log t(‘t’is inverse of peak frequency)as a function of inverse of temperature show linear variation (figure not presented).The relaxation times,obtained from both impedance and modulus data,of dipoles is found to decrease with increase in temperature.The calculated activation energy for relaxation lies between 1.3and 1.6eV for all the samples.Increase in samarium content increases the activation energy for relaxation.M00spectra can be used to estimate bulk resistivities using the relationship o max RC¼1.log(s ac)versus103/T plot for NBST1sample is shown in Fig.5.This material is an electrical insulator,which obeys the Arrhenius law.The activation energy for conduction in high-temperature region lies between1.01and0.45eV.Conductivity plot clearly shows the kink around500–5101C for NBST1 sample.This kink correlates with dielectric data as an anomaly.4.ConclusionsIn conclusion,the present results indicate that there is an anomaly in dielectric properties of Sm-doped NBT in the temperature range of4501C–5501C.This peak corre-sponds to a broad shoulder in dielectric loss curves of NBST3samples.The impedance data indicates that the relaxations in the samples are due to the presence of polarizing species.The polarizing charges give non-Debye relaxations and contribute to grain boundary capacitance and conductance.The increase of samarium content results in disappearance of1301C peak of dielectric properties and brings out high-temperature polarization peak clearly.Fig.4.Variation of M00and Z00with frequency at different temperatures of(a)NBST1,(b)NBST2,and(c)NBST3samples(closed symbols are for M00).M.Raghavender et al./Journal of Physics and Chemistry of Solids67(2006)1803–18081807AcknowledgementsThe authors thank DRDO,India for financial support.One of the authors M.R.thanks to Prof S.V.Suryanarayanaand Prof.T.Bhimasankaram for their constant encourage-ment.The author M.R.also thanks Dr.M.Vithal,Department of Chemistry,Osmania University,Hydera-bad,India,for the facilities provided in the preparation of these samples through chemical route.References[1]G.A.Smolensky,V.A.Isupov,A.I.Agranovskaya,N.N.Krainik,Sov.Phys.Solid State 2(1961)2651.[2]J.Suchanicz,K.Poleder,A.Kania,J.Handerek,Ferroelectrics 77(1988)107.[3]T.V.Kruzina,V.V.Gene,V.A.Isupov,E.V.Sinyakov,Sov.Phys.Crystallogr.26(1981)482.[4]K.S.Hong,S.E.Park,J.Appl.Phys.79(1996)388.[5]C.-S.Tu,I.G.Siny,V.H.Schmidt,Phys.Rev.B 49(1994)11550.[6]J.V.Zvirgzds,P.P.Kapostins,J.V.Zvirgzde,Phys.Rev.B 40(1982)75.[7]J.East,D.C.Sinclair,J.Mater.Sci.Lett.16(1997)422.[8]A.Herabut,A.Safari,J.Am.Ceram.Soc.80(11)(1997)2954.[9]M.Aparna,T.Bhimasankaram,G.S.Kumar,G.Prasad,Mod.Phys.Lett.B 16(26)(2002)1007.[10]M.Aparna,G.S.Kumar,G.Prasad,Ferroelectrics 324(2005)63.[11]J.Yun Yi,J.-K.Lee,K.-S.Hong,J.Am.Ceram.Soc.85(12)(2002)3004.[12]H.Lee,M.Hong,S.Bae,H.Lee,E.Park,K.Kim,J.Mater.Chem.13(2003)2626.[13]M.Aparna,T.Bhimasankaram,G.S.Kumar,G.Prasad,Mod.Phys.Lett.B 16(26)(2002)1007.[14]J.Ross Macdonald,Impedance Spectroscopy Emphasizing SolidMaterials and Systems,Wiley,New York,1987(Chapter 1).[15]J.T.S.Irvine,D.C.Sinclair,A.R.West,Adv.Mater.2(1990)132.[16]D.C.Sinclair,A.R.West,J.Appl.Phys.66(1989)3850.[17]N.Hirose,A.R.West,J.Am.Ceram.Soc.79(1996)1633.Fig.5.Arrhenius plot for NBST1sample.M.Raghavender et al./Journal of Physics and Chemistry of Solids 67(2006)1803–18081808。