a new polymorph of VO2 prepared by soft chemical methods

JOURNALOFSOLIDSTATECHEMISTRY138,178—182(1998)

ARTICLENO.SC977575

BRIEFCOMMUNICATION

ANewPolymorphofVO2PreparedbySoftChemicalMethods

DouglasHagrman,*JonZubieta,*ChristopherJ.Warren,-LindaM.Meyer,-MichaelM.J.Treacy,-

andRobertC.Haushalter-

*DepartmentofChemistry,SyracuseUniversity,Syracuse,NewYork13244;and-NECResearchInstitute,4IndependenceWay,Princeton,NewJersey08540

ReceivedOctober9,1996;inrevisedformJuly29,1997;acceptedJuly30,1997

AnovelpolymorphofVO2,designatedVO2(C),hasbeenpreparedbydehydrationofthenewlayeredvanadiumoxidehydrate,VO2·1/2H2O.ThehydrateispreparedinhighyieldasasinglephasefromthehydrothermalreactionofthehydrolyzateofVCl4andNaOHinamolarratioof1:4.5for96hoursat200°C.ThehydratecrystallizesinthetetragonalspacegroupI4/mmm,witha؍3.7211(6)A,c؍15.421(1)A,Z؍4,andR؍0.045asdeterminedbyX-raysinglecrystaldiffraction.ThestructureconsistsoflayersofVO5squarepyramids,eachofwhichsharesitsfourbasaledgeswithfourneighboringVO5squarepyramids.WatermoleculesthatresidebetweenthelayerscanberemovedreversiblytogivethenewlayeredoxideVO2(C).󰀁1998AcademicPress

Thecrystalchemistryofthebinaryvanadiumoxides,

VOV,variesinoxygencompositionsrangingfromx"1to

x"2.5anddisplaysawidevarietyofstructuretypes(1).

Severalofthesematerialsaretechnologicallyimportant

withusesinoxidationcatalysis(2)andbatteryapplications

(3).Atx"1,VOadoptsacubicrocksaltstructurewhich

containsahighpercentageofdefects.Thedefectswere

revealedwhenMathewsonetal.observedadifferenceinthe

X-raydensity6.50g/cm󰀳andthepycnometricdetermina-

tionof5.6g/cm󰀳(4).Atx"1.5V󰀲O󰀳adoptsastructure

similartocorundum(5).Asbrinketal.discoveredthemono-

cliniccrystallinephaseV󰀳O󰀵(6).Betweenthevalues

x"1.75and1.88thereexistthetriclinicMagneliphases

whichhavethegeneralformulaVLO󰀲L\󰀱(44n48)(7,8).

Atx"2,fivedifferentphaseshavebeenreported(8—13).

ThenextsetofimportantcrystallineVOVphasesexist

betweenx"2.17and2.33withthegeneralformula

VLO󰀲L>󰀱(n"3,4,and6)(14—17).Finally,themostoxygen-

richphase,V󰀲O󰀵,iswellknownandhasbeenstructurally

characterized(18).Physicalpropertiesofthevanadiumox-

idesvary,withelectricalconductivitydifferingfrommetallic

insomeVO󰀲polymorphsandV󰀲O󰀳tosemiconductingin

V󰀲O󰀵.SeveralpolymorphsofVO󰀲areknown.Ahightemper-

aturerutileVO󰀲phasewascrystallizedinthespacegroup

P4/mmm(8).AndersonrecognizedthesecondVO󰀲phase,

whichcrystallizesmonoclinicP2󰀱/c(9,10),whileatriclinic

VO󰀲phasefairlysimilartoAndersson’smonoclinicphase

hasalsobeenidentified(11).ThesethreeVO󰀲structuresare

similarandinterconverteduponheatingfrom325to340K.

TwoadditionalVO󰀲phasespreviouslyreportedaredesig-

natedVO󰀲(A)andVO󰀲(B).ThetetragonalVO󰀲(A)crystal-

lizesinthespacegroupP4󰀲/nmc(12)whilethemonoclinic

VO󰀲(B),firstcharacterizedbyTheobaldetal.,crystallizesin

spacegroupC2/m(13).Whenvieweddownthe(110)direc-

tion,VO󰀲(A)exhibitstwoedgesharingoctahedrawhich

edgesharewithtwoadditionaloctahedratoproducealayer

ofstairsteppingoctahedra.Theselayersarelinkedtoeach

otherattheprotrudingcornersofeachpairofoctahedrato

produceathree-dimensionalframework.Whenviewed

downthec-axis,VO󰀲(B)showsthesame+VO󰀶,sheetsas

VO󰀲(A);however,thesesheetsof+VO󰀶,octahedraproduce

twodistinctstructures,basedonthesuperpositionsofthe+VO󰀶,octrahedrallayers(12).VO󰀲(A)hasbeensynthesized

hydrothermallybyheatingasuspensionofVO(OH)󰀲to

250°Cfor48hrandbyheatingasuspensionofV󰀲O󰀳and

V󰀲O󰀵between220and330°C(12).Similarly,VO󰀲(B)is

preparedhydrothermallybyheatingasuspensionofV󰀲O󰀳andV󰀲O󰀵to220—150°C(19).

Wereportherethepreparationofanewpolymorphof

VO󰀲designatedVO󰀲(C),(1),whichisbuiltupfrom

O"VO󰀴󰁀󰀴layersandispreparedbydehydrating

VO󰀲·1/2H󰀲O,(2).Therewasarecentclaimofacompound

‘‘LiVV󰀲\BO󰀴\B(H󰀲O)’’,whosestructurewasdetermined

frompowderX-raydata,whichcrystallizesinthesame

spacegroupandhasessentiallyidenticalunitcellpara-

meterstothetitlecompound,VO󰀲·1/2H󰀲O,andanidenti-

callayerstructure.ThepositionsoftheLiatomswerenot

preciselydeterminedinthismaterial.Curiously,thepres-

enceofthelithiumcationsin‘‘LiVV󰀲\BO󰀴\B(H󰀲O)’’appar-

entlydoesnotdistortthestructure(20).

1780022-4596/98$25.00Copyright󰀁1998byAcademicPressAllrightsofreproductioninanyformreserved.Thehemihydrateispreparedbytreatingthehydrolyzate

ofVCl󰀴withaqueousNaOHhydrothermally.Aquated

‘‘VO󰀲>’’ispreparedbyverycarefullyaddingVCl󰀴dropwise

toice-coldwaterwithstirringtogiveasolution1.47molar

invanadium(Caution:Thisreactionisviolentlyexothermic

andevolveslargeamountsofHCl;thehydrolysisshouldbe

conductedinafumehoodwithpropershielding).To1.7mL

ofthissolutionisadded3mLofdistilledH󰀲Oand4.5mL

of2.5molarNaOH.Thismixtureissealedina23mL

polytetrafluoroethylene-linedstainlesssteelcontainerand

treatedfor96hat200°Cunderautogeneouspressure(40%

fillfactor).Theproduct,(2),isisolatedashighlycrystalline