Effect of La and Mn on the properties of alkalineniobate-based piezoelectric ceramics

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Effect of La and Mn on the properties of alkaline niobate-based piezoelectric ceramics

Henry E. Mgbemere & Gerold A. Schneider

To cite this article: Henry E. Mgbemere & Gerold A. Schneider (2016) Effect of La and Mn on the properties of alkaline niobate-based piezoelectric ceramics, Journal of Asian Ceramic Societies, 4:1, 97-101, DOI: 10.1016/j.jascer.2015.12.004

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Effect of La and Mn on the properties of alkaline niobate-based piezoelectric ceramics

Henry E. Mgbemere

, Gerold A. Schneider

aDepartmentofMetallurgical&MaterialsEngineering,UniversityofLagos,Nigeria

bInstituteofAdvancedCeramics,HamburgUniversityofTechnology,Denickestrasse15,21073,Hamburg,Germany

a r t i c l e i n f o

Articlehistory:

Received18September2015 Receivedinrevisedform 13November2015 Accepted20December2015 Availableonline7January2016

Keywords:

Ferroelectrics Lead-freeceramics (KxNa1−x)NbO3

Dopants

a b s t r a c t

Lead-freeferroelectric(K0.44Na0.52Li0.04)(Nb0.86Ta0.1Sb0.04)O3ceramicsco-dopedwithdifferentamounts ofbothLaandMnhavebeenproducedusingsolid-statesynthesismethod.Therelativedensityvalues oftheunmodifiedsamplearebetween92and96%anddecreasesto∼91%forthesamplewith1mol%

oftheco-doping.Bi-modalgraindistributionisobservedinthesampleswhiletheaveragegrainsize decreaseswithco-dopingduetograingrowthinhibitionbypinningofthegrainboundarymovement.

Thediffractionpatternsshowatransformationfromanorthorhombicphasetoapseudo-tetragonalphase withco-dopantsaddition.TheCurietemperatureandthetetragonal-orthorhombictransitiontempera- turesareloweredfrom∼9000at330^◦Cwithoutmodificationto∼4000attemperaturesbelow250^◦C withco-dopantaddition.Thedielectriclossvaluesofthesamplesalsodecreasefrom∼0.4to0.05for temperaturesupto250^◦Cwithco-doping.TheremnantpolarisationProfthesamplesdecreasesfrom

∼8.55kV/cmto∼6.57kV/cmwithco-dopantaddition.Thepiezoelectricchargecoefficient(d33),includ- ingthenormalisedstrainvalues,alsodecreasefrom∼400pm/Vand220pC/Nto157pm/Vand159pC/N, respectivelywithco-dopantsupto1mol%.

©2016TheCeramicSocietyofJapanandtheKoreanCeramicSociety.Productionandhostingby ElsevierB.V.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense(http://creativecommons.org/

licenses/by-nc-nd/4.0/).

1. Introduction

Lead zirconate titanate (Pb(ZrxTi1−x)O3 (PZT)) based piezo- electricceramicsfindsa lotof applicationinactuators,sensors, transducersandotherelectromechanicaldevicesbecauseoftheir excellentpiezoelectricpropertiesandreliabilityinservice.How- ever,lead(Pb),whichisoneofthemainconstituentelementsof theseceramics,isatoxicmaterial,andwhenreleasedtotheatmo- sphereduringceramicprocessing,it causesboth environmental andhealthproblems,especiallytochildren[1].Anotherconcern isthedisposalofthewasteleadinelectronicdevicesafterthey haveoutlivedtheirusefulness[2].Legislationshavebeenenacted bysomemulti-nationalgovernmentsliketheEuropeanCommis- sionbanningitsuseinalmostallproducts[3].Thereishoweverno

∗Correspondingauthor.

E-mailaddresses:h.mgbemere@unilag.edu.ng,henrymgbemere@yahoo.com (H.E.Mgbemere).

1 Theresearchworkleadingtothisarticlewasdonewhilethefirstauthorwas workingattheInstituteofAdvancedCeramics,HamburgUniversityofTechnology, HamburgGermany.

PeerreviewunderresponsibilityofTheCeramicSocietyofJapanandtheKorean CeramicSociety.

overallsuitablereplacementyetforitsuseinpiezoelectricceram- ics, and so, it is still being allowed pending until a suitable replacementcanbefound.

Thesearchforpossiblelead-freereplacementcompositionsfor PZTceramicshasmoreorlessbeenfocussedon(Bi_0.5Na_0.5)TiO₃ [4–6]and(KxNa1−x)NbO3[7–9]basedceramicsystemsandreports in the literature so far show that the later exhibits slightly higherpiezoelectricpropertiesbutwithlowertemperaturestabil- ity[2,10,11].TheresearchonKNNceramicsbeganinthelate1940s butwasovertakenbyPZTbecauseofitsbetterproperties.Alotof researchhasbeendoneon(KxNa_1−x)NbO₃(KNN)basedpiezoelec- tricceramicsinthelast10yearsaccountingformorethan85%ofall publishedworksinthefield[2,12].Poorsinteringandlowpiezo- electricpropertiesarethemainproblemswithpureKNNceramics [13]. To overcome these problems, their ease of sintering and piezoelectricpropertieshavebeenimprovedbysubstitutingthe mainelementswithdopants.Someofthesecombinationsinclude KNN–Ba[14],KNN–SrTiO₃[15,16],KNN–LiNbO₃[17],KNN–LiTaO₃ [18],KNN–LiSbO3 [19],(K,Na,Li)(Nb,Ta,Sb)O3 [20]andpureKNN withsinteringaidslikeCuO[21],ZnO[22],MnO₂[23]andBi₂O₃ [24].

The KNN-based ceramics modified with Li, Ta and Sb first reported by Saito et al. [20] remains one of the most studied http://dx.doi.org/10.1016/j.jascer.2015.12.004

2187-0764©2016TheCeramicSocietyofJapanandtheKoreanCeramicSociety.ProductionandhostingbyElsevierB.V.ThisisanopenaccessarticleundertheCCBY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

98 H.E.Mgbemere,G.A.Schneider/JournalofAsianCeramicSocieties4(2016)97–101 compositions in terms of piezoelectric charge coefficient (d₃₃)

values and efforts are still being made by various researchers toimproveontheexistingvalues[25–27].Manganesehasbeen reportedtoimprovethedensificationofKNNceramics,suppress graingrowthandhelpstoincreasetheelectricalresistivityofthe piezoceramic material,that is to improve resistivity[23,28,29].

Lanthanumhasalsobeenreportedtoimprovethepropertiesof PZT ceramics [30] and of BNT ceramics, provided it is <2at.%

[31,32].SincethepropertiesofBNTwereimprovedonaddition ofLa,itisalsobelievedthatitmayhavethesameeffectonKNN ceramics.Gaoetal.studiedtheeffectofCeandLaonKNNceram- ics and reported that provided the doping amount is <1mol%, thedielectric andferroelectricpropertiesvaluesaremaintained [33]. The effectof La₂O₃ on (K_0.5Na_0.5)(Nb_0.96Sb_0.04)O₃ showed that the grain size reduced, while the density, dielectric con- stantandpiezoelectriccoefficientvaluesincreasedupto0.6mol%

dopantaddition[34].BaTiO₃ hasbeenco-dopedwithMnandLa in order tostudy theireffects on theproperties of theceram- ics[35].Theauthorswanttodeterminethecombinedeffectsof twointerestingdopants(LaandMn)onthepropertiesofthewell established(K0.44Na0.52Li0.04)(Nb0.86Ta0.1Sb0.04)O3 ceramics. The objectivethereforeistoinvestigatetheeffectsofco-dopingonthe structure,dielectricandpiezoelectricpropertiesoftheseceramics.

2. Experimentalprocedure

The sampleswere synthesised using themixed-oxide route fromthefollowingrawpowders:K₂CO₃,Na₂CO₃,Li₂CO₃(99+%), Sb₂O₃, Nb₂O₅, Ta₂O₅ (99.5%) (Chempur Feinchemikalien und ForschungsGmbH,Karlsruhe,Germany)andMnO2,La2O3(99+%) (AlfaAesarGmbH,Karlsruhe,Germany).Therawpowderswere firstdriedinanovenfor4hatatemperatureof220^◦Ctoensure thatlittleornomoistureispresent.Stoichiometriccompositions ofthepowderswerefirstweighed,mixedandattritionmilledfor 4husingethanolassolventand3mmdiameterZrO₂ballsasthe millingmedia.Theethanolwasseparatedfromthemilledpow- derusingasolventextractor.Calcinationofthemilledpowderwas carriedoutat750^◦Cfor4htoensurethatthevolatilecomponents oftheraw powdersareremovedsothattherequired composi- tion(K0.44Na0.52Li0.04)(Nb0.86Ta0.1Sb0.04)O3isformed.Itisbelieved thatSb₂O₃beingveryunstable,readilyoxidisesfromthe+3tothe +5valencestateinthepresenceofairatelevatedtemperatureto ensurestoichiometry.0,0.25,0.5and1mol%eachofbothLa2O3

andMnO2 rawpowderswereaddedtothesampleandthenthe milling,solventextractionandcalcinationstepswererepeatedto ensurethatthepowdersarehomogenous.

Thepowderswerepressedintodiscsof12.5mmdiameterand 4.5mminitiallywithauniaxialpressoperatingat40MPafor30s andlaterwithacoldisostaticpressat500MPafor2min.Thepellets weresinteredinachamberfurnaceat1075^◦Cfor1hwithaheating andcoolingrateof3^◦C/minand10^◦C/min,respectively.Theden- sityofthesampleswasdeterminedusingtheArchimedesmethod whilethecrystalstructurewasexaminedusingX-raydiffraction analysiswithCuK_␣radiation(D8Discover,BrukerAXSKarlsruhe, Germany).Samplesformicrostructuralexaminationwerepolished andthermallyetchedat925^◦Cfor30min.Themicrostructurewas

observedusinga scanningelectronmicroscope(LEO1530SEM, Gemini/Zeiss,Oberkochen,Germany)whilethegrainsizemeasure- mentswerecarriedoutusingthemeaninterceptlengthmethod fromatleastsixdifferentareasoftheimage.Aminimumof100 grainswascountedintheanalysisoftheaveragegrainsize.

Silver paints actingas electrodeswere appliedonboth sur- facesofthesamplestobeusedforelectricalmeasurements.The temperaturedependenceofthedielectricpropertiesoftheceram- icswasmeasuredfrom20Hzto1MHzwithanLCR meter(HP 4284A,Agilent Technologies,Inc.,Palo Alto,USA) attachedtoa heatingfurnace.Thepolarisationhysteresiscurveswereobtained usingastandardSawyer-Towercircuitwhilethestrainhystere- siscurveswereobtainedusinganinductivetransducerdevice.A completehysteresisloopmeasurementwasperformedin200s.

Thepiezoelectriccoefficientd33wasmeasuredusingalowsignal displacementtransducer(HottingerBaldwinMesstechnikGmbH, Darmstadt,Germany)connectedtoalock-inamplifierwhilethe slopeofthestrainhysteresisloopwasusedtoobtainthehighsignal piezoelectricchargecoefficient.

3. Resultsanddiscussion

The density values for the La and Mn co-doped (K0.44Na0.52Li0.04)(Nb0.86Ta0.1Sb0.04)O3 ceramics are as shown inTable1.Aminimumof12sampleswasusedinthecalculationof thedensityvalues.Thetheoreticaldensityvaluesforthesamples arealsocalculatedusingtheX-raydiffractionpatternsanditwas assumedthattheorthorhombicphaseistheonlyphasepresent.

Intheunmodifiedsample,valuesrangingfrom92%to96%ofthe theoreticaldensityareobtained.WhenLaandMnco-dopingare introduced,therelative densityvalueslightlydecreasesbutthe deviation fromthe mean density value also decreases.For the co-dopedsamples,thehighestrelativedensityvalue(94.9±0.9%) isobtainedwiththesamplemodifiedwith0.5mol%anddecreases to90.96±0.5% with1mol%. Mn and Lahave been reportedto improvethedensificationofpiezoelectricceramicsthroughpin- ningofthegrainboundarymovement [28,34].In this case,the co-dopants ensure that while the density value decreases,the deviationfromthemeanvalueofthedensityalsodecreases.The theoreticaldensityvaluesforeachcompositionwascalculatedand itis4.79g/cm³fortheunmodifiedsampleandgraduallyincreases to4.84g/cm³forthesamplewith1mol%co-doping.

Thescanningelectronmicroscope(SEM)imagesofthepolished surfaceofthesampleswithdifferentco-dopingamountsareshown inFig.1.Allthemicrographshavegrainswithquasi-cubicmorphol- ogy,whichhasbeenreportedforKNNceramics[36].Grainsthat arerelativelynon-uniforminsizeandcontainingfeweramounts ofporesareformed.

FortheunmodifiedsampleinFig.1a,aninhomogeneousgrain size distribution can be observed. Some grains are very large relative to others and this abnormal grain growth leads to a bimodalgrainsizedistributionwithlargegrainsbeingsurrounded by smallergrains. Some of the smallergrains have sizesof as small as ∼600nm. The average grain size for the large grains is ∼4.9±1.0␮m, and for the small grains, it is ∼1.7±0.8␮m.

Unevenlydistributedandsizedporescanalsobeseenatthegrain Table1

Datashowingthedensity,dielectricandpiezoelectricpropertiesof(K0.44Na0.52Li0.04)(Nb0.86Ta0.1Sb0.04)O3ceramicsco-dopedwithdifferentamountsofLaandMnatroom temperature.

Dopingamount (mol%)

εr@1kHz tan␦(1kHz) d33(pC/N) Normalisedstrain, d^∗₃₃(pm/V)

Theoretical density(g/cm³)

Relative density(%)

Remnantpolarisation, Pr(␮C/cm²)

Coercivefield, Ec(kV/cm)

0 ∼1146 0.1515 220 400±10 4.79 94.25±2.3 ∼18.9 8.55

0.25 ∼1397 0.0812 196 259±5 4.81 93.87±0.6 ∼10.0 6.57

0.5 ∼1311 0.0283 167 283±5 4.82 94.87±0.9 ∼10.0 6.57

1.0 ∼1357 0.0293 159 157±4 4.84 90.96±0.5 ∼7.5 6.57

Fig.1.Scanningelectronmicroscopeimagesofthethermallyetchedsurfacesof(K0.44Na0.52Li0.04)(Nb0.86Ta0.1Sb0.04)O3ceramicssinteredat1075^◦Cfor1hinairatmosphere showing(a)theundopedceramic,(b)ceramicwith0.25mol%ofLa,Mnand(c)with0.5mol%ofLa,Mn.

boundariesandtherearealsosignsofliquid phaseatthegrain boundaries.Fig.1bshowsthemicrostructureofthesamplewith 0.25mol%oftheco-dopants.Thesurfacesofthesampleshaveboth smoothandroughgrains,whichmaybeattributedtothedifferent crystallographicplanes,whichbehavedifferentlyduringetching.

Thehigherenergeticplanestrytoreverttothelowerenergetic planesandtheresultisaroughsurface.Itstillhasabimodalgrain sizedistributionbutwithfewerpores.Thelargegrainshaveamean grainsizeof2.6±0.5␮mwhilethesmallgrainshaveameangrain sizeof1.1±0.5␮m.Themicrostructuresofthesamplesco-doped with0.5mol%and1mol%,respectivelyarenotsignificantlydiffer- entandsoonlythesamplewith0.5mol%ispresented.Theamount ofgrainswithlargesizescontinuestodecreasesuchthatthevol- umeofgrainswithsimilarsizesincreases.Thelargegrainshavean averageparticlesizeof1.9±0.4␮mwhilethesmallgrainshavean averagesizeof0.8±0.4␮m(Fig.1c).Mnisknowntocreateoxy- genvacanciesinKNNceramics,whichhindersthemovementofthe grainboundariesandinhibitsgraingrowthleadingtolesservolume ofporesinthemicrostructure[28,37].InLa-doped(Bi0.5Na0.5)TiO3

ceramicsat1at.%,thegrainsizehasbeenreportedtoincrease[31].

Fig.2showstheX-raydiffraction(XRD)patternsforsamples modifiedwithdifferentamounts ofthedopants.Thepeak pos- itionsdidnotchangesignificantlywithdopingbutthepeakshapes changed.Intheunmodifiedstate,theorthorhombicphaseisthe dominantstructureatroomtemperature.Thereisareportinthe literaturethat states thata two-phaseorthorhombic tetragonal coexistenceis observed[38].When0.25mol% ofthe co-dopant is addedto theceramic, thepeak splittingbetweenthe ortho- rhombicandtetragonalphasesattheBraggangleof∼47^◦isroughly equal and indicatesthat thestructure is close tothepolymor- phicphaseboundaryposition.Thevolumeofthetetragonalphase

graduallyincreaseswhilethatoftheorthorhombicphasedecreases withmoredopantaddition.Apseudocubicphasehasbeenreported whenthereisLasubstitutionontheA-siteofthelattice[33].Some extrapeaks(markedwithcirclesin Fig.2)canbeobservedbut couldnotbeidentifiedbecauseitsvolumeisverylittleintheextra peaksthatwerefound.Asecondphase relatedtoNa2Ti4O9 has beenreportedwhentheA-andB-siteofKNNceramicshasbeen simultaneouslydopedwithLaandTi,respectively[39].

Fig.3ashowsthetemperaturedependenceofthedielectriccon- stantvaluesmeasuredonheatingat1kHzforthesampleco-doped

Fig.2. X-raydiffractionpatternsof(K0.44Na0.52Li0.04)(Nb0.86Ta0.1Sb0.04)O3ceramics dopedwithdifferentamountsofdopants.Tracesofanextraphasecanbeobserved onthepatterncontaining1mol%ofLaandMn.

100 H.E.Mgbemere,G.A.Schneider/JournalofAsianCeramicSocieties4(2016)97–101

500 400

300 200

100 0

2000 4000 6000 8000 10000

140 120 100 80 60 40 120020 1600 2000 2400

Dielectricconstant (εr)

Temperature (°C)

0 mol%

0.25 mol%

0.5 mol%

1 mol%

(a)

Temp. (°C)

500 400

300 200

100 0

0 5 10 15

300 250 200 150 100 0,0 50 0,1 0,2 0,3 0,4 0,5

tanδ

Temperature (°C) 0 mol%

0.25 mol% 0.5 mol% 1 mol% (b)

tanδ

Temperature (°C)

Fig. 3. Temperature dependence of (a) the dielectric constant values for (K0.44Na0.52Li0.04)(Nb0.86Ta0.1Sb0.04)O3 ceramics measured at 1kHz and (b) the dielectricloss(tanı)valuesfor(K0.44Na0.52Li0.04)(Nb0.86Ta0.1Sb0.04)O3ceramicsmea- suredat1kHz.Theinsetinthegraphmagnifiestheregionofthegraphfrom20^◦C to250^◦Candfrom20^◦Cto300^◦Cfor(a)and(b),respectively.

withdifferentamountof Laand Mn. Thereis a slight increase inthedielectricconstantvaluesatlowertemperatureswhenthe samples are modified with the dopants. Both phase transition temperatures(Tc and T_T-O)decrease withincreasing amountof thedopants.TheTcdecreasesfrom∼330^◦Cto∼240^◦Cwhenthe dopantsareaddedbutdo notdecreasesubstantiallywithmore additions.Additionofthedopantsalsoledtoa decreaseinthe dielectric constant values atthe Tc. While a dielectric constant valueof∼9000wasobtainedinthesamplewithoutthedopants, 5000andbelowisobtainedinthedopedsamplesattheTc.Sim- ilar resultshave been reported for La2O3 and MnO2 co-doped 0.02Pb(Y_2/3W_1/3)O3–0.98Pb(Zr0.52Ti0.48)O3[40].Broadeningatthe dielectricconstantpeakisalsoobservedforthedopedsamples.The highertheamountofdopingused,thebroaderisthepeakvalueat theTc,whichisanindicationofpossiblerelaxorbehaviour.

Thetemperaturedependenceofthedielectricloss(tanı)forthe samplesmeasuredat1kHzisshowninFig.3b.Theinsetshows thatattemperaturesbelow250^◦C, theco-dopants areeffective atreducingthelossbehaviouroftheceramic.Intheunmodified sample, thedielectric loss increasesfrom 0.15 to ∼0.45 as the measurementtemperatureincreases.Additionof0.25mol%ofthe co-dopantsledtoadecreaseinthelossvaluesfrom∼0.4to0.05.

Co-dopingwith0.5mol%givesthebestresultwithvaluesbelow 0.03evenatveryhightemperatures.Furtheradditionupto1mol%

increasesthedielectriclossofthesample.

Thepolarisationhysteresiscurvesforthesamplesareshown inFig.4.Allthemeasuredsamplesattainedsaturationpolarisation

Fig. 4. A plot of the polarisation–electric field hysteresis curves for (K0.44Na0.52Li0.04)(Nb0.86Ta0.1Sb0.04)O3ceramicsco-dopedwithdifferentamounts ofLaandMn.

whenanelectricfieldof20kV/cmisapplied.Theadditionoftheco- dopantstotheceramicsledtoareductioninboththecoercivefield (E_c)andtheremnantpolarisation(P_r)values.Inthesamplewithno modification,thePrandEcvaluesare∼18␮C/cm²and∼9kV/cm, respectively.Additionof0.25mol%eachoftheco-dopantsledtoa reductionintheP_randE_cvaluesto∼10␮C/cm²and∼6.25kV/cm, respectively.Withincreasingdopantamounts,slightlyreducedPr

valuesareobtainedbutthereisnosignificantdifferenceintheEc

values.ThedecreaseinPrvalueswithincreasingdopantamountis probablyduetothepinningeffectofthedomainwalls,whichisa resultoftheincreaseinthenumberofdefectsinthelattice.

Fig.5showsthestrainhysteresiscurvesfortheceramicsmod- ified with different amounts of the co-dopants. Samples with dopantsupto0.5mol%couldbemeasuredandhavethetypical butterflyshape,which indicatesthepresenceofferroelectricity.

Duetolargeleakage current,agood hysteresiscurvecouldnot beobtainedforthesamplemodifiedwith1mol%ofthedopants.

Withincreasingamountofthedopants,theareaofthehysteresis loopdecreases,whichalsoagreeswiththeirdecreasingpiezoelec- tricactivity.Thepiezoelectricchargecoefficient(d₃₃)valuesforthe samplesareshowninTable1.Thehighestvalueofthenormalised strain(400±10pm/V)isobtainedintheunmodifiedsample.Asthe amountofco-dopantsincreases,thed₃₃valuegenerallydecreases

Fig. 5. A plot of the strain–electric field hysteresis curves for (K0.44Na0.52Li0.04)(Nb0.86Ta0.1Sb0.04)O3ceramicsco-dopedwithdifferentamounts ofLaandMn.

butthereductiondoesnotdrasticallychangewiththeamountsof thedopants.

Apossibleexplanationfortheobservedreductioninpiezoelec- tric properties and increasingconductivity leadingto dielectric breakdown in the samples with increasing amount of the co- dopants is given below. The ionic radius, valence state and coordinationnumberofanionistheoreticallyusedtopredictits positionontheperovskitelattice.La³⁺(1.06 ˚A)isbelievedtoenter theA-siteofthelatticesinceitssizeisclosertothoseofNa⁺(1.02 ˚A) andK⁺(1.39 ˚A)Mn,whichismulti-valentintheory,andcanenter boththeAandBsitesoftheperovskitelattice.OntheA-siteofthe lattice,thecoordinationnumberofK⁺,Na⁺andLi⁺is12.Lawith avalenceof+3,acoordinationnumberof12andionicradiusof 1.06 ˚A,therefore,fitswellintotheA-site.OntheB-site,Nb⁵⁺,Ta⁵⁺

andSb⁵⁺havecoordinationnumbersof6.Mnhasdifferentoxida- tionstatesandsoitspositionintheperovskitelatticedependson itsoxidationstate.Acurrentreportintheliteratureonthestateof Mnindicatesthatthepossiblestateforitispredominantly+4and sometimes+2[29].OntheassumptionthatLawilloccupytheA-site positionwhileMnwilloccupytheB-sitepositionoftheperovskite lattice,La³⁺ontheA-sitewillcreateA-sitevacancieswhereasMn willcreateoxygenvacanciesasshownusingtheKröger–Vinknota- tion,asshowninEqs.(1)–(3).WithLa³⁺ontheA-siteofthelattice (Eq.(2)),itisalsopossiblethatoxygengaswillbeliberatedleading totheformationofelectrons.Mnwithavalencestateof+4will leadtothecreationofoxygenvacanciesintheperovskitelattice.

La₂O₃^3ABO−→³2La^••_A +3O^×_O+4V_A (1) La₂O₃^2ABO−→³2La^••_A +1

2O₂(g)↑+4e+2O^×_O (2) 2MnO₂^2ABO−→³2Mn_B+3O^×_O+V^••_O (3) SimultaneousdopingoftheAandtheBsitesoftheperovskitelat- ticecancreateinternalbiasfield,whichleadstotheformationof defectdipoles.InastudyofBi_0.5Na_0.5TiO₃–Bi_0.5Li_0.5TiO₃–BaTiO₃ ceramicsco-dopedwithLaandFeontheAandBsitesrespectively, itwasreportedthatinternalbiasfieldwascreated,whichledtothe formationofdefectdipolesofthetype(Fe

Ti–V^••_O)[32].Thissitua- tionresultsinanasymmetricstrainbehaviour,whichisobserved inourwork.

4. Conclusion

(K_0.44Na_0.52Li_0.04)(Nb_0.86Ta_0.1Sb_0.04)O₃ceramicsmodifiedwith differentamounts ofbothLaand Mnhavebeenpreparedusing theconventionalmixed-oxidemethod.Therelative densityval- uesslightlydecreasedfrom94.25±2.3%intheunmodifiedsample to 91±0.5% in the 1mol% La and Mn sample. The degree of scatterin thedopedsamplesis howeverlower. Bi-modal grain size distribution is observed in all the samples and the aver- agegrainsizedecreaseswithincreasingco-dopantamounts.The crystalstructure of theceramicschangedwithco-dopantaddi- tionsfromadominantorthorhombicphasethroughatwo-phase orthorhombic-tetragonalcoexistencetoapseudo-cubicphasewith 1mol%.The dielectricconstant and dielectric lossvalues inthe ceramicsatlowertemperaturesimprovedwhenthedopantsare added. The phase transition temperatures (Tc and T_T-O) in the ceramicsareloweredwithco-dopantadditionsbutthereisverylit- tlechangeinthetransitiontemperatureswithincreasingamount ofthedopants.Thepiezoelectricandferroelectricpropertiesofthe ceramicsdecreasewithdopantadditionbutnosignificantdiffer- enceinthePrandEcvaluesareobservedwithincreasingamount

ofthedopants.Itisbelievedthatthedopantsbeingaliovalentto theunmodifiedcomposition,introducedcation,oxygenvacancies anddefectdipolesintotheperovskitelattice.

Acknowledgements

Theresearchleadingtotheseresultshasreceivedfinancialsup- portfromDeutscheForschungsgemeinschaftundergrantno.SCHN 372/16-1.

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