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SynthesisandstructuralpropertiesofcubicG0-Rb2KMoO3F3oxyuorideV.
V.
Atuchina,*,T.
A.
Gavrilovab,L.
I.
Isaenkoc,V.
G.
Keslerd,M.
S.
Molokeeve,S.
A.
ZhurkovcaLaboratoryofOpticalMaterialsandStructures,InstituteofSemiconductorPhysics,SBRAS,Novosibirsk90,630090,RussiabLaboratoryofNanolithographyandNanodiagnostics,InstituteofSemiconductorPhysics,SBRAS,Novosibirsk90,630090,RussiacLaboratoryofCrystalGrowth,InstituteofGeologyandMineralogy,SBRAS,Novosibirsk90,530090,RussiadLaboratoryofPhysicalPrinciplesforIntegratedMicroelectronics,InstituteofSemiconductorPhysics,SBRAS,Novosibirsk90,630090,RussiaeLaboratoryofCrystalPhysics,InstituteofPhysics,SBRAS,Krasnoyarsk36,660036,RussiaReceived23September2011;receivedinrevisedform24October2011;accepted4November2011Availableonline10November2011AbstractHigh-temperatureG0polymorphofRb2KMoO3F3hasbeenpreparedbymeltsolidication.
Micromorphologyandchemicalpropertiesofthenalproductwereevaluatedbyscanningelectronmicroscopy(SEM)andX-rayphotoelectronspectroscopy(XPS).
Theelpasolite-relatedcrystalstructureofG0-Rb2KMoO3F3hasbeenrenedbyRietveldmethodatT=298K(spacegroupFm-3m,a=8.
92446(8)A,V=710.
76(1)A3;RB=3.
55%).
FerroelectricG1-Rb2KMoO3F3polymorph,earlierreportedatT0)manyoxyuoridesfromA2BMO3F3familyarefoundtobenoncentrosymmetricandpossesstheferroelectricproperties[2–5,7,9–12].
CompleteO/Fdisorderintheanionpositions,however,isfoundforhightemperaturecubicG0-phasewithspacegroupFm-3m[5,10,11,13–15].
Also,thestructuraltransitionsontemperaturevariationareknownformanyotheroxyuorideswithanioncontentO/F63andthisisanindicatorofthethingthatthecharacteristictemperatureofstructuraltransitionmaybestronglydependentontherealchemicalcompositionofoxyuoridecrystal,inparticulartheO/Fratio.
Asitwasobtained,cubicphaseG0-Rb2KMoO3F3existsabovetemperatureT>328K,andtwophaseswithferro-electricpropertieswereobservedatT<328K[3].
Thecrystallinematerialswithphasetransitionsfewaboveambienttemperatureareofspecialinterestforseveralapplicationsandthedetailedobservationofcrystalpropertiesisactualforsuchcompounds.
Earlier,theRb2KMoO3F3wassynthesizedbysolidstatereactionbetweenalkalimetaluoridesandMoO3atT=873Kinargonatmosphere[1,3].
AsinglecrystalgrowthofG0-Rb2KMoO3F3byBridgmantechniquewasalsoreported[13].
Inthesebothstudies,however,therewasnodetaileddescriptionofthetechnologicalconditionsusedforthecrystalsynthesis.
Commonly,thepolarityofMoO6xFxoctahedronisstronglydependentonO/FratiowithmaximumatO/F=3[16,17].
Respectively,tohavereproducibleresultsonphasecompositionandphysicalpropertiesofanoxyuoride,thetechnologyshouldbestabilizedwithrespecttouoridecomponentlossduringsynthesis.
ItshouldbealsoaccountedthattheappearanceofMo4+andMo5+ionsispossibleduetoincompleteoxidationofmolybdenumatomsinnonstoichio-metricoxyuoromolybdatespossessingnoticeableelectricalconductivity[18].
Thus,thepresentstudyisaimedatthedesignoftechnologyforthestablesynthesisofG0-Rb2KMoO3F3compoundandtheobservationofstructuralandmorphologicalpropertiesofthenalproduct.
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elsevier.
com/locate/ceramintAvailableonlineatwww.
sciencedirect.
comCeramicsInternational38(2012)2455–2459*Correspondingauthor.
Tel.
:+73833308889;fax:+73833332771.
E-mailaddress:atuchin@thermo.
isp.
nsc.
ru(V.
V.
Atuchin).
0272-8842/$36.
00#2011ElsevierLtdandTechnaGroupS.
r.
l.
Allrightsreserved.
doi:10.
1016/j.
ceramint.
2011.
11.
0132.
ExperimentalTheRb2KMoO3F3compoundwasformedbysolidstatesynthesisinaccordancewiththeearlierproposedreaction[1,3]:2RbFKFMoO3Rb2KMoO3F3:HighpurityinitialreagentsKF2H2O,Rb2CO3,MoO3,NH4F(all99.
9%,CJSC''Plantofraremetals'',Russia)andaqueoushydrouoricacid(HF)(48%HFbyweight,CJSC''Plantofraremetals'',Russia)wereused.
ToavoidthedrasticcaptureofairagentsintoKFandRbFreagents,allthereactionsandheattreatmentswereproducedunderdriednitrogenatmosphereatincreasedpressure.
KFwassynthesizedbyvaporizationofamixtureofKF2H2Oandhydrouoricacid.
Then,theNH4FwasaddedtoKFpowderandthemixturewasheateduptoT=573KintotheclosedTeoncrucibletoremovetheresidualOH-groups(KOHspecies)inaccordancewiththereaction:KOHNH4FKFH2O"NH3":Then,tocreatethepuredryKFtheuorine-agentCF4wasaddedintothepowderandthemixturewasplacedintoaclosedplatinumcrucibleandheatedtoT=773K.
Then,thecooledKFpowderwasgrinded,placedintotheopenplatinumcrucibleandheatedtoT=1173K,thatisfaraboveKFmeltingtemperature,underdriednitrogenowforthetimet=3handcooledtoroomtemperatureattherate50K/h.
RbFwassynthesizedbythereactionofRb2CO3withexcesshydrouoricacidfollowedbyvaporizationuptosolidpowder.
Then,dryRbFwasfabricatedbythesameproceduresasthoseusedforKF,butthenalmelttreatmentwasproducedatlowertemperatureT=1073K.
Owingtotheirhygroscopicnature,thealkaliuorideproductswerekeptandmanipulatedunderdriednitrogenintoaglovebox.
ThestartingmixtureofKF,RbFandMoO3waspreparedinstoichiometriccompositionratiorelatedtoRb2KMoO3F3nominal.
Themixturewasgrinded,placedintotheopenplatinumcrucibleandheatedtoT=1073Kattherateof100K/h.
Then,themeltwasbeingcooledtoroomtemperaturetogetherwithfurnacefort=24h.
Thenalproductofthereactioninthemeltwasadenseuniformmilkcolordisk-likeagglomerateof$35mmindiameterand$10mminthickness.
Themicromorphologyoftheproductwasevaluatedwithscanningelectronmicroscopy(SEM)usingLEO1430device.
ChemicalcompositionandmolybdenumvalencestateweredenedbyX-rayphotoelectronspectroscopy(XPS)methodusingthesurfaceanalysiscenterSSC(Riber).
TopreparethesampleforXPSmeasurements,afragmentofRb2KMoO3F3wasgrindeduptopowderstateandpressedintoafreshlypreparedindiumfoil.
ThenonmonochromaticAlKaradiation(1486.
6eV)withthepowersourceof300Wwasusedfortheexcitationofphotoemission.
Themeasurementconditionsandenergyscalecalibrationmethodcanbefoundelsewhere[19,20].
Apronounceddriftofenergyscale,duetochargingeffects,wasdetectedbecauseofthedielectricnatureofRb2KMoO3F3surface.
ThisdriftwastakenintoaccountwithreferencetothepositionofC1s(284.
6eV)linegeneratedbyadventitiouscarbononthesurfaceofpowderas-insertedintovacuumchamber.
ThepowderX-raydiffractionpatternforRietveldanalysiswascollectedatroomtemperature(298K)withaBrukerD8ADVANCEdiffractometerintheBragg-BrentanogeometryandlinearVantecdetector.
Theoperatingparameterswere:CuKaradiation,tubevoltage40kV,tubecurrent40mA,stepsize0.
0168,countingtime1sperstep.
Thedatawerecollectedover2urange5–1108.
ThepeakpositionsweredeterminedwiththeprogramEVAavailableinthePCsoftwarepackageDIFFRAC-PLUSsuppliedfromBruker.
3.
ResultsanddiscussionSEMimagesofRb2KMoO3F3surfaceareshowninFig.
1.
Toseetheinternalmorphology,thebulkagglomeratewascrackedmechanicallyandthefragmentsurfaceswereevaluatedbySEM.
AsitisevidentfromtheimageshowninFig.
1,thesurfaceishillyandwithoutnoticeablecrystalfacets.
TheuniformgraycontrastintheseSEMimagesindicatesthechemicaluniformityofthesample.
DarksportsvisibleinFig.
1aaretheporesappeared,asitseems,duetobulkmaterialFig.
1.
SEMimagesof(a)crackedsurfaceand(b)aporeofG0-Rb2KMoO3F3fragment.
V.
V.
Atuchinetal.
/CeramicsInternational38(2012)2455–24592456shrinkageoncoolingaftersolidication.
OnesuchporeisshowninFig.
1b.
Interestingformationswerefoundinthecentralpartofexternalsurfaceofthediskformedaftermeltsolidication.
AnexampleofsuchformationsisshowninFig.
S1.
Thispartofthediskagglomeratewasnotcontactingthewallsoftheplatinumcrucible.
Theregularrectangularspotsweredetectedatthesurfaceoftheformations.
Asitseems,thesesquare-likespotsappearedduetocubicRb2KMoO3F3crystalsgrowinginthediskbulk.
ThesurveyphotoemissionspectrumisshowninFig.
2.
BesidesspectralfeaturesrelatedtoconstituentelementcorelevelsandAugerlines,theweaksignalofC1scorelevelwasfoundinthespectrum.
TheC1slineisapparentlyduetothepresenceofhydrocarbonscapturedatthesamplesurfacefromtheair.
InFig.
3theMo3ddoubletrecordedbyXPSisshown.
Itiswellknownthatthebindingenergy(BE)ofMo3d5/2andMo3d3/2componentsisstronglydependentonthemolybdenumionvalencestateinoxides[18,21].
InRb2KMoO3F3theMo3ddoubletispartlycrossingtheRb3pdoubletandanaccuratepeakttingprocedurewasimplementedtorevealtheshapeandBEpositionofMo3d5/2andMo3d3/2components.
TheMo3d5/2andMo3d3/2componentsarenarrowandsymmetrical.
TheBEvaluesdenedfortheMo3d5/2andMo3d3/2componentsbyttinganalysisareshowninFig.
3.
ThesevaluesarewellrelatedtotheBEvaluestypicalofMo6+ionsinoxides[21,22].
ThechemicalcompositionofRb2KMoO3F3samplewasestimatedwithXPSusingtheRb3d,K3p3/2,Mo3d5/2,O1sandF1slinesandtabulateddataontheatomicsensitivityfactors(ASF)[21].
Theresultsofcalcula-tionsareshowninTable1.
TheconstituentelementratioestimatedbyXPSiswellrelatedtothatofstoichiometricRb2KMoO3F3.
Fig.
4showstheXRDcurverecordedforthepowdersampleofRb2KMoO3F3.
OnlyfewpeaksofveryweakintensityandrelatedtoforeignphaseswererevealedintheXRDpattern.
ThestructureofG0-phaseofRb2KMoO3F3isisostructuraltoelpasolite(sp.
gr.
Fm-3m),soweusethesecoordinatesofatomstorenethestructureofRb2KMoO3F3.
AllrenementsanddataprocessingwereperformedbyDDMprogram[23].
ThePearsonVIIfunctionwasusedtomodelthepeakproles.
TherenementofthestructurewithFm-3mspacegroupwasstableanditledtominimalR-factor(Table2).
Thecoordinatesofatoms,isotropicthermalparametersandoccupationsofatompositionsaregiveninTable3.
Experimental(dots)andtheoretical(lines)X-raydiffractionpatternsofG0-Rb2KMoO3F3areshowninFig.
4.
ThemainbondlengthsarereportedinTable4.
ThecrystalstructureofG0-Rb2KMoO3F3isshowninFig.
5anditcanbeconsideredasasequenceofcorner-linkedMo(O,F)6andK(O,F)6octahedra.
ItisinterestingtocomparethecellparametervalueobtainedforG0-Rb2KMoO3F3inRef.
[13]tothatdenedinthepresentstudy.
Thevaluea=8.
945(5)AspeciedforsinglecrystalsampleofG0-Rb2KMoO3F3atT=343Kisnoticeablyhigherthanthevaluea=8.
92446(8)AfoundinthepresentstudyatFig.
2.
SurveyphotoemissionspectrumfromG0-Rb2KMoO3F3.
Fig.
3.
DetailedspectrumofMo3dandRb3pdoubletsfromG0-Rb2KMoO3F3.
Table1Constituentelementratiointhesynthesizedsample.
Element(corelevel)Rb(Rb3d)K(K3p3/2)Mo(Mo3d5/2)O(O1s)F(F1s)Sample0.
210.
100.
090.
350.
25Rb2KMoO3F3,nominal0.
200.
100.
100.
300.
30Table2Themainparametersofcrystalstructureprocessingandrenement.
SpacegroupFm-3ma(A)8.
92446(8)V(A3)710.
76(1)2u-intervalrange(8)5–110Numberofreexions39Numberofrenementparameters8RB(%)3.
55RDDM(%)10.
97V.
V.
Atuchinetal.
/CeramicsInternational38(2012)2455–24592457T=298K.
Thisdrasticdisparity,however,seemsberelatedtoadifferenceinthetemperatureofmeasurements.
Earlier,astrongdependenceofthecellparameterontemperatureincubicG0-Rb2KMoO3F3wasspeciedovertherangeT=343–473K[13].
Iftoextrapolatethea(T)curvedowntoT=298K,onecancalculatethevaluea$8.
928Aaccountingforapossibleerrorina(T)curvedetermination.
Thisvalueisinareasonablerelationwiththatobtainedinourexperiment.
So,itmaybeconcludedthatthetemperaturerangeofexistenceofcubicmodicationG0-Rb2KMoO3F3canbeexpandeduptoT=298K.
Fig.
4.
Experimental(dots)andtheoretical(lines)X-raydiffractionpatternsofG0-Rb2KMoO3F3atT=298K.
Peaksofalienphasesaremarkedbyarrows.
Table3Coordinatesofatoms,isotropicthermalparameters(Biso)andoccupationofatompositions(p)ofstructureRb2KMoO3F3atroomtemperature.
AtompXYZBiso.
(A2);Uij(A2)Rb1.
01/41/41/44.
04(5)Mo1.
00003.
72(5)K1.
01/21/21/21.
9(1)F0.
50.
2119(5)00U11=U22=0.
0144(5);U33=0.
0085(9)O0.
50.
2119(5)00U11=U22=0.
0144(5);U33=0.
0085(9)Table4ThemaininteratomicdistancesinstructureRb2KMoO3F3atroomtemperature.
BondLength(A)Mo–O(F)1.
891(5)Rb–O(F)3.
174(5)K–O(F)2.
571(5)Fig.
5.
CrystalstructureofG0-Rb2KMoO3F3atT=298K.
V.
V.
Atuchinetal.
/CeramicsInternational38(2012)2455–245924584.
ConclusionsTheRb2KMoO3F3oxyuoromolybdateispreparedbyaccuratesolidstatesynthesis.
However,ferroelectricG1-Rb2KMoO3F3phasewhichexistencewasearlierreportedovertherangeT<328Kisnotfoundbyourmeasurements.
AtT=298KthecubicG0-Rb2KMoO3F3wasfoundbycrystalstructureanalysisofourpolycrystallinesample.
Respectively,thetemperatureofthephasetransitionG0$G1shouldbeshiftedatleasttotherangeT<298K,ifthetransitioneveroccurs.
AcknowledgmentsThisstudywaspartlysupportedbyRFBR(Grant09-02-00062)andSBRAS(Grant34).
AppendixA.
SupplementarydataSupplementarydataassociatedwiththisarticlecanbefound,intheonlineversion,atdoi:10.
1016/j.
ceramint.
2011.
11.
013.
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