Synthesis, Crystal Structure and Magnetic Properties of the Linear-Chain Cobalt Oxide Sr5Pb

The novel spin-chain cobalt oxide Sr5Pb3CoO12 [P-62m, a = 10.1093(2) A and c = 3.562 51(9) A at 295 K] is reported. Polycrystalline sample of the compound was studied by neutron diffraction (at 6 and 295 K) and magnetic susceptibility measurements (5 to 39

Synthesis,CrystalStructureandMagneticPropertiesoftheLinear-ChainCobalt

OxideSr5Pb3CoO12

K.Yamaura,1,2, Q.Huang,3,4andE.Takayama-Muromachi1

1

arXiv:cond-mat/0110292v1 [cond-mat.str-el] 15 Oct 2001

AdvancedMaterialsLaboratory,NationalInstituteforMaterialsScience,1-1Namiki,Tsukuba,Ibaraki305-0044,Japan

2

JapanScienceandTechnologyCorporation,Kawaguchi,Saitama332-0012,Japan

3

NISTCenterforNeutronResearch,NationalInstituteofStandardsandTechnology,Gaithersburg,Maryland20899

4

DepartmentofMaterialsandNuclearEngineering,UniversityofMaryland,CollegePark,Maryland20742

(Dated:Oct5th,2001)

Thenovelspin-chaincobaltoxideSr5Pb3CoO12[P¯62m,a=10.1093(2) Aandc=3.56251(9) Aat295K]isreported.Polycrystallinesampleofthecompoundwasstudiedbyneutrondi raction(at6and295K)andmagneticsusceptibilitymeasurements(5to390K).ThecobaltoxidewasfoundtobeanalogoustothecopperoxideSr5Pb3CuO12,whichiscomprisedofmagnetic-linear

A.Althoughthecobaltoxidechains(µe of3.64µBperCo)arechainsatinter-chaindistanceof10

substantiallyantiferromagnetic(θW=-38.8K),neitherlow-dimensionalmagnetismnorlong-rangeorderinghasbeenfound;alocal-structuredisorderinthechainsmightimpactonthemagnetism.Thiscompoundishighlyelectricallyinsulating.

I.

INTRODUCTION

Inordertorevealnatureofcorrelatedelectronsincon-densedmattersandtoconstructadvancedmodelsforthose,onedimensional(1D)electronicsystemasaba-sisofthemodelshasbeensubjectedforbothexper-imentalandtheoreticalinvestigations.Intheexperi-mentalpart,avarietyofquantumphenomenahasbeenindeedfoundforthequasi-1Dcompoundsinpastfewdecades;Tomonaga-Luttinger-typeelectricconductivityforBaVS3[1],PeierlsinstabilityforCuGeO3[2],charge-andspin-densitywavesfor(TMTSF)2AsF6andTTF-TCNQ[3,4],spinon-holonseparationforSrCuO2[5],HaldanegapforNi(C2H8N2)2NO2(ClO4)[6],andsuper-conductivityfor(TMTSF)2PF6[7].Thesecharacteris-ticphenomenamightre ectthenatureofthecorrelatedelectrons,andintensiveresearchonthematerialsplayedanimportantroletosteadilyadvanceunderstandingofthenature.Wehaverecentlybeenexploringnovelquasi-1Dcompoundsinorderto ndadditionalsystemsshow-ingcorrelationsamongtheirmagnetic,electronictrans-portproperties,andcrystalstructure.

Thelinear-chaincobaltoxideSr5Pb3CoO12wasre-centlydiscoveredinacourseofstudiesofquasi-1Dmagneticmaterialsandelectrical-carrier-dopedthose[8,9,10,11,12].Thepolycrystallinesampleofthecompoundwasobtainedbyhigh-temperaturesolid-statereaction,andthensubsequentlyinvestigatedbyx-raydi raction,magneticsusceptibility,andthermogravimet-ricanalysis(TGA)studies.Crystalstructurewasinves-tigatedindetailbypowder-neutron-di ractionat6and295K.Thecompoundwasfoundisostructuraltothe1DantiferromagneticcopperoxideSr5Pb3CuO12[P¯62m,

a=10.1089(6)Aandc=3.5585(2)A],inwhich

dis-

The novel spin-chain cobalt oxide Sr5Pb3CoO12 [P-62m, a = 10.1093(2) A and c = 3.562 51(9) A at 295 K] is reported. Polycrystalline sample of the compound was studied by neutron diffraction (at 6 and 295 K) and magnetic susceptibility measurements (5 to 39

matoronthecountersideandanauto-divergence-slitsys-tem.TheoxygencontentofthesampleswasmeasuredinacommercialTGAapparatus(PYRIS1,developedbyPerkinElmer,Inc)byheatingsmallamountofeachsample(powder,～20mg)in3%hydrogeninargonataheatingrateof5 Cperminuteto700 Candholdingfor6hours.

Toinvestigatethecrystalstructurefurther,thesam-pleannealedinthecompressedgaswasagainstudiedbyneutrondi raction.Theneutrondataat6and295KwereobtainedbytheBT-1high-resolutionpowderdi ractometerattheNISTCenterforNeutronResearch.ACu(311)monochromatorwasemployedtoproduce′acoherentneutronbeam(λ=1.5401 A)with15,20′,and7′collimatorsbeforeandafterthemonochromator,andafterthesample,respectively.Theneutrondi rac-tionpatternsweremeasuredbetween8and160degreesindi ractionangleat0.05degreesstep.Withtheneu-tronpro les,crystalstructureparameterswerere nedtoahighdegreeofagreementbyRietveldcalculationsontheprogramGSAS[13].Thetotalnumbersofre ectionanddatapointwere172and3039,respectively.Neutronscatteringamplitudeinthecalculationswereset0.702,0.940,0.253,and0.581(×10 12)cmforSr,Pb,Co,andO,respectively[13].

Themagneticpropertiesofthesampleswerestudiedbyacommercialapparatus(MPMSsystem)between5and390K,developedbyQuantumDesign,Inc.Themagneticsusceptibilitydatawerecollectedat50kOeoncooling,andmagnetizationcurveswererecordedbetween-55and55kOeaftercoolingtheeachsampleat5and150K.Allthepelletsthusobtainedweretooelectricallyinsulatingtobesubjectedforfurtherelectronictransportmeasurements;beyondthe10M limitofaconversionaltwo-terminaltesteratroomtemperature.

III.RESULTSANDDISCUSSIONS

Thepowderx-raypatternsoftheannealedsampleand

theas-madesamplearepresentedinFigs.1aand1b,re-spectively.Recalledthehexagonalunitcellofthestruc-tureoftheanalogouscopperoxideSr5Pb3CuO12(P¯a=10.1089(6) Aandc=3.5585(2) 62m,

A)[8],ahexago-nalunitcellwas,at rst,testedtoqualitativelyanalyze

thebothpatterns.Almostpeakswereclearlyfoundtobeattheexpectedpositionsfromthehexagonalsymme-tryandlatticeparameters,asmarkedbyeachhklno-tation(Fig.1a),exceptseveralsmallpeaksindicatedbysolidstars. Thelatticeparameters werere nedtoa=10.11(1)Aandc=3.567(1)Abyaleastsquares t-tingfortheannealedsamplepattern(Fig.1a),anda=10.12(1) Aandc=3.558(1) Afortheother(Fig.1b).Thebothpatternsindicatequalityofthesamples.Althoughthehexagonalmodelwaseventuallyfoundreasonabletoexplainthealmostx-raypeaksforthebothcobaltox-idesaswellasthecopperoxide,thestar-markedminorpeaks,however,remainedtobeuncharacterized,indicat-

2

ingpossiblepresenceofeithersmallamountofimpuritiesotherwisesomewhatstructuralmodulation.Anattempttoprepareamuchpuresample,whichmaynotshowtheextrapeaks,bymeansofoptimizingtheheatingcondi-tionsandstartingcompositionshasbeenmade,however,thatisunsuccessfulthusfar.

Inordertomeasuretheoxygencontentofthecobaltoxide,theTGAstudywasmadeontheselectedsam-ples;thedataareshowninFig.2,wheredottedandsolidcurvesindicateweightchangefortheas-madeandan-nealedsamples,respectively.Aclearweightlosswasfoundaboveapproximately400 Cinthebothmeasure-ments.Basedonthehypothesisthattheobservedweightlossistotallyduetooxygenreduction,theoxygenquan-titywasthencalculatedtobe12.19and12.32molespertheformulaunitfortheas-madeandannealedsamples,respectively.Thehigh-oxygenpressureannealingdoesnotappeartoproduceasigni cantincrementofoxy-genquantityintheas-madecobaltoxide.Thisfactissupportedbythex-raydata;rathersmallchanges(lessthan0.25%)inlatticeparameterswasfoundaftertheannealing.Asthesamplescontinuetolousethoseweightaftertheoxygenreductioncompleted(nulldatasecuredtheaccuracyofthemeasurements),probablyduetoavolatilityoflead,asmallamountofextraloosingshouldbesuperimposedonthemajorsteps,whichmayleadanoverestimationsomewhatintotheoxygen-quantitycal-culations.

Theannealedsamplewassubjectedtoneutrondi rac-tionstudyat6and295Ktoobtaindetailsofthelocal-chainstructureanddegreeofoxygennonstoichiometry.TheanalysisoftheneutrondatausingtheRietveldtechniquefundamentallyfollowedthewaydevelopedinthestructuralstudiesontheanalogouscopperoxideSr5Pb3CuO12[8,11,12].Asaresult,agoodachievementwassuccessfullyobtainedbytheRietveldcalculationsontheneutronpro les(Fig.3),suggeststheaveragecrystalstructuralmodel(P¯62m)isreliablewiththecobaltox-ideaswellasthecopperoxide[8,11,12].InthemainpanelofFig.3,theobservedandcalculatedpro lesarepresentedatthebestquality(5–7%levelsinagreementfactors).Shownbelowthepro lesasadeferenceplotbetweenthose,qualityofthere nementisindeedonaconvincinglevel.ThestructureparametersatthebestqualityarelistedinTableI,andselectedinteratomicdistancesandbondangleswerecalculatedfromthepa-rameters(TableII).Latticeconstantsat295Kofthehexagonal unitcellarea=10.1093(2) Aandc=3.56251(9)A,whichessentiallymeetthex-rayparameters.Duringthere nements,temporary tsusingun- xedisotropic-atomic-displacementparametersandoc-cupancyfactorsforO(4)andO(5)hadleadonlyincred-ibleresults,probablyduetotoolowlevelsofthoseoc-cupancyfactorslessthan0.1.Thesituationwasnotim-provedevenforthelowtemperaturedata.TheisotropicatomicdisplacementparametersattheoxygensitesO(4)and O(5)were,therefore, xedataconceivablelevel,2.0A2(1.2 A2at6K);calculationsthereafterappearedto

The novel spin-chain cobalt oxide Sr5Pb3CoO12 [P-62m, a = 10.1093(2) A and c = 3.562 51(9) A at 295 K] is reported. Polycrystalline sample of the compound was studied by neutron diffraction (at 6 and 295 K) and magnetic susceptibility measurements (5 to 39

improvethesituation.Duetothesamereasons,theoc-cupancyfactorofO(5)wasconstrainedtobeequaltothatofO(4).TheoccupancyfactorsofO(2)andO(3),thenormallyoccupiedsites,were xedtobefullyoccu-piedinthe nalcalculationbecauseoxygenvacancywasfoundwithinonestandarddeviationof1.00.ThepossiblemixingbetweenSrandPbwastestedinpreliminaryman-ner,howeversubstantialdegreeofthemixingwasnotde-tected.ThedisplacementparameterofCoisunusuallylarge,probablyre ectingthelocalstructuredisorderasinthecopperoxide[8,11,12].Theoccupancyfactorsinthepartiallyoccupiedsites,O(1),O(4),andO(5),areslightlyhigherthantheexpectedvalues2/3,1/12,and1/12,respectively,fromtheoxygen-stoichiometriccomposition12molespertheformulaunit.Theoxygenquantitycalculatedfromthepresentneutronparametersis12.42molespertheformulaunit.Althoughtheesti-mationfromtheneutrondatamatchesthatoftheTGAdata(12.32moles)within1%level,webelievetheoxygenquantitymaybeslightlyoverestimatedbecausetheoccu-pancyfactorsofO(4)andO(5)aretoolowtobeaccurate.Astheneutrondi ractionstudyonthecopperoxidedidnothelpto gureouttheprobablelocal-structuredis-ordersandoxygennonstoichiometry[8,11,12],furtherattentionsonthepresentanalysis,includingcalculationswithanisotropicallythermalparametersofmetals,didnotshedlightontheproblems,either.Theoxygencom-positionofthecobaltoxidemightbeslightlysuperstoi-chiometric( 12.4),butnotfarfromthestoichiometricSr5Pb3CoO12.Todeterminetheoxygenquantitycon-clusively,furtherinvestigationswouldberequiredaftermuchhigh-puresamplebecomesavailable.

SchematicstructuralviewsofSr5Pb3CoO12weredrawnfromtheneutrondata(295K)inFigs.4aand4b.Itisclearthatthecobaltcompoundhasalinear-chainstructurebasis,inwhichcobalt–oxygenpolyhedraformchainsatinter-chaindistanceof10.1 A[8].Apartofthechainalongc axisispresentedinFig.4b.Allofcobaltandoxygensitesareshownbythedottedcirclessmallerandlarger,respectively,andaprobablearrangementofthoseatomsisindicatedbysolidand lledcircles.In-tolerablyshortbonddistances(TableII)betweencobaltandoxygenatomswereprecludedfrommakingthear-rangement.Theirregulararrangementofcobaltatoms,asshowninFig.4b,andprobablepresenceoflocaldis-placementsofatomsmayaccountfortheobservedunusu-allylargeatomicdisplacementparameters2intheaverage-structuredatasuchas3.8 A2and2.9 AforCoandO(1),respectively.Thec axisconstant(3.56251(9) A)re ectstheaverageofCotoCodistancesinthechains;almostarecloseto～3.56 A,butfewpartsat～2.61 Amaybesomewhatinvolved.Sincetheoxygen-coordinationen-vironmentofallcobaltatomsisnotunique,adegreeofmagneticuniformityofthepresentchaincompoundmightbeaslowasthatofthecopperoxideSr5Pb3CuO12,farratherthanotherwell-studiedspin-chaincompounds[14,15,16,17].

Thelow-temperaturestructurewasinvestigatedat6

3

K(insetofFig.3)inthesamemanner,andtheobtainedstructureparameters,calculatedinteratomicdistancesandbondanglesaresummarizedinTablesIandIIaswell.Thehexagonalunitcellslightlyshrinks(0.21%alonga axisand0.17%alongc axis)bythecooling,whiletheCositesplittingismuchconspicuousthanthatat295K(+11.5%insplittingdistance)asshowninFigs.5aand5b.Anytracesofmagneticorderingwerenotdetectedinthedi ractionpro les.

Themagnetic-susceptibilitydataofthesamplesan-nealed(opencircles)andas-made(closedcircles)areshowninFig.6;Tvs1/χandTvsχareplottedinthemainpanelandtheinset,respectively.Thedataofmagnetic elddependenceofthemagnetizationat5and150KareshowninFig.7.SeeninthemainpanelofFig.6,theinversemagneticsusceptibilityislinearlyde-pendentontemperatureaboveapproximately130Kforbothsamples.TheCWlawwas,then,appliedtoanalyzethetwosetsofthehigh-temperatureportionofthedata.Theformulato tthedatabyaleast-squaresmethod,asindicatedbythesolidlines(mainpanelofFig.6),was

2χ(T)=

Nµe

The novel spin-chain cobalt oxide Sr5Pb3CoO12 [P-62m, a = 10.1093(2) A and c = 3.562 51(9) A at 295 K] is reported. Polycrystalline sample of the compound was studied by neutron diffraction (at 6 and 295 K) and magnetic susceptibility measurements (5 to 39

spins(S)was

MS(α)=NgSµB

2S+1×

2S

α

1

2S

α,(2)

whereα=gSµBH/kBT.Thee ectiveSwasesti-matedtobe～1.39fortheannealedand～1.48fortheas-madesamplesfromcalculationsusingtheformulaµe =2

The novel spin-chain cobalt oxide Sr5Pb3CoO12 [P-62m, a = 10.1093(2) A and c = 3.562 51(9) A at 295 K] is reported. Polycrystalline sample of the compound was studied by neutron diffraction (at 6 and 295 K) and magnetic susceptibility measurements (5 to 39

¯m.ThelatticeTABLEI:StructureparametersofSr5Pb3CoO12at295K( rstline)and6K(secondline).Spacegroup:P62

parametersarea=10.1093(2) A,c=3.56251(9) Aat295K,anda=10.0877(2) A,c=3.55587(9) Aat6K.Thevolume

33

ofthehexagonalunitcellis315.31(2) Aat295Kand313.38(2) Aat6K.Thecalculateddensityis6.89g/cm3at295Kand6.93g/cm3at6K.Atom

Site

x

y

z

n

B( A2)

Rp=

5.12%

6.79%

Rwp=

6.66%8.60%

χ2=

1.9701.428

FIG.1:Plotsofx-raypro les(CuKα)ofpowdersamplesofthecobaltoxide(a)annealedinthecompressedoxygen-argongasand(b)as-madeinair.Thedatawereobtainedatroomtemperature.Smallpeaksmarkedbystarshavenotbeenindexedbythehexagonal-unit-cellmodel.

The novel spin-chain cobalt oxide Sr5Pb3CoO12 [P-62m, a = 10.1093(2) A and c = 3.562 51(9) A at 295 K] is reported. Polycrystalline sample of the compound was studied by neutron diffraction (at 6 and 295 K) and magnetic susceptibility measurements (5 to 39

TABLEII:SelectedinteratomicdistancesandanglesofSr5Pb3CoO12at295K( rstline)and6K(secondline).Atoms

Distances( A)

Atoms

Distances( A)

O(1)–Co–O(1)O(1)–Co–O(4)O(1)–Co–O(5)O(1)–Co–O(5)O(1)–Co–O(5)O(1)–Co–O(5)

113.5(6)112.2(7)121.96(2)121.18(32)71.2(8)69.1(8)82.7(12)82.3(12)100.7(10)101.0(9)120.4(20)126.0(20)

O(4)–Co–O(5)O(4)–Co–O(5)O(4)–Co–O(5)O(4)–Co–O(5)Co–O(5)–CoCo–O(5)–Co

44.5(11)45.1(11)55.3(21)58.1(14)121.0(10)124.9(11)147.5(21)150.2(21)155.9(26)149.9(23)163.6(18)160.3(15)

The novel spin-chain cobalt oxide Sr5Pb3CoO12 [P-62m, a = 10.1093(2) A and c = 3.562 51(9) A at 295 K] is reported. Polycrystalline sample of the compound was studied by neutron diffraction (at 6 and 295 K) and magnetic susceptibility measurements (5 to 39

FIG.2:Thermogravimetricanalysisdataforthepowdersamplesas-made(brokencurve)andannealedinthecompressedoxygen-argongas(solidcurve).Thesampleshavebeenstudiedinamixedgas,3%hydrogeninargon,atheatingratioof5

Cperminute.Theweightlossatthemajorstepswere8.903%(solidcurve)and8.758%(brokencurve),whichsuggestingoxygenquantityofthesamples12.32and12.19molespertheformulaunit,

respectively.

FIG.3:Plotsoftheobserved(crosses)andcalculated(solidcurve)neutrondi ractionpro les(295K,λ=1.5401 A)ofthepowdersampleofSr5Pb3CoO12,annealedinthecompressedgas.TheverticalbarsindicatecalculatedpositionsforthenuclearBraggre ections.Thelowerpartshowsdi erencebetweenthepro les.Thedataat6Kandthesubsequentanalysisofthosearepresentedintheinset.

The novel spin-chain cobalt oxide Sr5Pb3CoO12 [P-62m, a = 10.1093(2) A and c = 3.562 51(9) A at 295 K] is reported. Polycrystalline sample of the compound was studied by neutron diffraction (at 6 and 295 K) and magnetic susceptibility measurements (5 to 39

FIG.4:(a)SchematiccrystalstructureviewofSr5Pb3CoO12drawnfromthe295K-neutrondata.Thehexagonalunitcellisindicatedbythebrokenlines.(b)Schematicviewofthecobalt–oxygenchainalongc–axis,showingtherandomlyoccupiedoxygenpositions[O(1),O(4),andO(5)],andaprobablearrangementofcobaltandoxygenatomswithintheaveragestructural

model.

FIG.5:Comparisonofthechainstructuresat(a)295Kand(b)6KofSr5Pb3CoO12alongc–axis.Theisotropicatomicdisplacementsareshown.

The novel spin-chain cobalt oxide Sr5Pb3CoO12 [P-62m, a = 10.1093(2) A and c = 3.562 51(9) A at 295 K] is reported. Polycrystalline sample of the compound was studied by neutron diffraction (at 6 and 295 K) and magnetic susceptibility measurements (5 to 39

FIG.6:Temperaturedependenceoftheinversemagneticsusceptibilityandthemagneticsusceptibilityat50kOeofthecobaltoxidesas-made(closedcircles)andannealedinthecompressedoxygen-argongas(opencircles).Thesolidlineindicates tstotheCurie-Weiss

law.

FIG.7:Appliedmagnetic elddependenceofthemagnetizationoftheannealedsampleofSr5Pb3CoO12at5and150K.Thesolidcurvesarethedataat5Kobtainedpriortotheannealing.Estimatedspinnumbers(S=1.48and1.39)bytheCurie-Weiss tstothehightemperaturedatawereemployedtocomputethebrokencurvesforfreespinsat5KusingtheBrillouinfunction.

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