1 NaMnO2

LETTERSPUBLISHEDONLINE:18MAY2014|DOI:10.1038/NMAT3964DirectvisualizationoftheJahn–TellereectcoupledtoNaorderinginNa5/8MnO2XinLi1,XiaohuaMa1,DongSu2,LeiLiu1,RobinChisnell3,ShyuePingOng1†,HailongChen1†,AlexandraToumar1,Juan-CarlosIdrobo4,YuechuanLei1,JianmingBai5,FengWang6,JereyW.Lynn7,YoungS.Lee3andGerbrandCeder1*ThecooperativeJahn–Tellereect(CJTE)referstothecorre-lationofdistortionsarisingfromindividualJahn–Tellercentresincomplexcompounds1,2.Theeectusuallyinducesstrongcouplingbetweenthestaticordynamiccharge,orbitalandmagneticordering,whichhasbeenrelatedtomanyimportantphenomenasuchascolossalmagnetoresistance1,3andsuper-conductivity1,4.HerewereportaNa5/8MnO2superstructurewithapronouncedstaticCJTEthatiscoupledtoanunusualNavacancyordering.WevisualizethiscoupleddistortionandNaorderingdowntotheatomicscale.TheMnplanesareperiodicallydistortedbyachargemodulationontheMnstripes,whichinturndrivesanunusuallylargedisplacementofsomeNaionsthroughlong-rangedNa–O–Mn3+–O–Nainteractionsintoahighlydistortedoctahedralsite.Atlowertemperatures,magneticorderappears,inwhichMnatomicstripeswithdierentmagneticcouplingsareinterwovenwitheachother.Ourworkdemonstratesthestronginteractionbetweenalkaliordering,displacement,andelectronicandmagneticstructure,andunderlinestheimportantrolethatstructuraldetailsplayindeterminingelectronicbehaviour.NaTMO2(TMD3dtransitionmetalions)compoundswithalternatingNaandTMlayershavebeenstudiedextensivelyfortheirpotentialapplicationinrechargeablebatteries57orastheparentmaterialsofthesuperconductivecobaltate5,810.NacanbeelectrochemicallyandreversiblyremovedfromthesematerialscreatingNaxTMO2(0x1)compoundsinaprocesscalledde-intercalation.SuperstructuresduetoNa-vacancy(VNa)orderinghavebeenobservedandidentifiedtobedominatedbytheelectrostaticinteractionsinNaxVO2(ref.6)andNaxCoO2(refs5,8).However,NaxMnO2isexpectedtobemorecomplicatedasitmixesMn3Cions,whichexhibitoneofthelargestJahnTellerdistortionsintransitionmetalcompounds,andformsantiferromagnetic(AF)Mn3Catomicstripes1113,withMn4Cions,whicharenotJahnTelleractiveandcanformferromagneticorAFnearest-neighbourcouplings,dependingonthecompetitionbetweendierentdirectandindirectexchangemechanisms14.Assuch,NaxMnO2iswellsuitedtostudytheinterplaybetweentheVNaordering,CJTEandthemagneticproperties.TherehavebeencontinuouseortstodirectlyvisualizetheCJTEusingscanning/transmissionelectronmicroscopy3,1517(S/TEM).Here,electrondiraction,synchrotronX-raydiraction(XRD),densityfunctionaltheory(DFT)andaberration-correctedatomic-resolutionSTEMimagingareusedtodeterminethesuperstructureofelectrochemicallyformedNa5=8MnO2,andtovisualizeinsodiumintercalationcompoundsthecooperativedistortionoftheMnJahnTellercentresandtheircouplingtoNaordering.Ratherthanbeingdominatedbyelectrostaticinteractions,weshowheredirectexperimentalevidencefromSTEMimagingthatthesuperstructureinNa5=8MnO2ismainlycontrolledbyJahnTellerdistortionswhichinducespecificlong-rangedNaVNainteractionsthroughMnchargeandd-orbitalorderings.Weuseneutronpowderdiraction,magneticsusceptibilitymeasurementsandDFTcomputationstodemonstratethata`magneticstripesandwich'structureisformedatlowtemperatures,whichcausesapronouncedchangeofthemagneticproperties.ElectrochemicalNaremovalfromNaMnO2initiallyisknowntooccurthroughatwo-phasereaction,forminganewphaseNaxMnO2(ref.18).Figure1a,bshowsthestructureofconventionalmonoclinicNaMnO2,whichisusedtoindextheelectrondiractionpatternsofNaxMnO2showninFig.1ce.Theformationofasuperstructureisclearfromthe(200),(122),(122)diractionspots.IntheZ-contrastimageshowninFig.2b,eachdotcorrespondstoeitheraNaorMnatomicstripeprojectedalongthe[010]orbdirection.TheperiodicintensitymodulationofonebrightandthreedarkdotsintheNaplaneisproportionaltotheNaconcentrationinthesestripes.ThesuperstructurehklpeaksandSTEMZ-contrastinformationecientlylimitthepossibleNaorderingsinthiscompound.Todetermineauniqueorder,weperformedanexhaustivesearchofthepossiblesuperstructuresinthesupercellsupto32formulaunitsforseveralxvaluesinNaxMnO2.TheonlysuperstructurethatmatchesalloftheelectrondiractionsandSTEMimagesoccursatxD5=8.ThesynchrotronXRDrefinement(SupplementaryFig.1andTables2and3)wasperformedonthisparticularsuperstructuremodelstartingwithDFTrelaxedioncoordinatesandshowsagoodfit.Furthermore,DFTcalculationsshowthatthissuperstructurehasthelowestenergyamongthe300dierentNaarrangementsthatwecalculatedforxD5=8,withitsenergybelowthetielineconnectingthetwolowestenergystructuresatneighbouringNacompositions,indicatingthatitisthermodynamicallystable(SupplementaryFig.2).Onthebasisof1DepartmentofMaterialsScienceandEngineering,MassachusettsInstituteofTechnology,Cambridge,Massachusetts02139,USA,2CenterforFunctionalNanomaterials,BrookhavenNationalLaboratory,Upton,NewYork11973,USA,3DepartmentofPhysics,MassachusettsInstituteofTechnology,Cambridge,Massachusetts02139,USA,4CenterforNanophaseMaterialsSciences,OakRidgeNationalLaboratory,OakRidge,Tennessee37831,USA,5NationalSynchrotronLightSource,BrookhavenNationalLaboratory,Upton,NewYork11973,USA,6Su