Oxidation state of uranium in metamict and annealed zircon

26 Pages·2003·0.8 MB·English

INSTITUTEOFPHYSICSPUBLISHING JOURNALOFPHYSICS:CONDENSEDMATTER
J.Phys.:Condens. Matter15(2003)3445–3470 PII:S0953-8984(03)60426-1
Oxidation state of uranium in metamict and
annealed zircon: near-infrared spectroscopic
quantitative analysis
MingZhang1,EkhardKHSalje1andRodneyCEwing2
1DepartmentofEarthSciences,UniversityofCambridge,DowningStreet,
CambridgeCB23EQ,UK
2DepartmentofNuclearEngineeringandRadiologicalSciences,DepartmentofGeological
Sciences,UniversityofMichigan,AnnArbor,MI48109-2104,USA
E-mail: [email protected]
Received6March2003
Published12May2003
Onlineatstacks.iop.org/JPhysCM/15/3445
Abstract
Radiationandthermallyinducedchangesintheoxidationstateofuraniumin
metamict zircon have been systematically analysed, for the first time, using
polarized near-infrared spectroscopy. The results showed that in damaged
zircon U ions in crystalline domains exhibited relatively sharp, anisotropic
signalsfromtetravalentandpentavalentUionsincrystallinedomains(U4+
crystal
and U5+ ). The linewidths and peak positions of the 4834 cm−1 band
crystal
(U4+ ,E (cid:1) c) and the 6668 cm−1 band (U5+ ,E ⊥ c) are a non-
crystal crystal
linear function of the self-radiation dose. They reach nearly constant values
at doses greater than ∼3.5 × 1018 α-eventsg−1. Quantitative analysis of
U4+ and U5+ signals revealed that the intensity ratio (U4+ /U5+ )
crystal crystal crystal crystal
exhibited a nearly linear increase as a function of dose and decreased on
heating. This suggests that radiation leads to an alteration of the oxidation
states of U and the tetravalent state is more preferable in radiation-damaged
zircons. U ions associated with amorphous materials (U ) gave rise
amorphous
to broad and isotropic signals and they were mainly in the tetravalent state
(U4+ ), although small amounts of pentavalent U ions (U5+ ) may
amorphous amorphous
exist. The dose dependenceof the intensity ratio of U /U did not
amorphous total
followthatofthereportedfractionoftheamorphousdomain,butgaveclearly
lower values. Thisimplies the potentialpreferentialoccurrenceof U ions or
possible U enrichment in the crystalline regions. Annealing intermediately
andhighlydamagedzirconhaddifferentimpactsontheoxidationstate ofU.
Anintermediatelydamagedzircon(withadoseof∼5.2×1018α-eventsg−1),
annealedathightemperaturesbetween500and1800K,showedasystematic
increaseinthesignalsofU4+ andU5+ above700KwhileU4+ bands
crystal crystal crystal
disappearedattemperaturesabove1600K.Incontrast,ahighlydamagedzircon
0953-8984/03/203445+26$30.00 ©2003IOPPublishingLtd PrintedintheUK 3445
3446 MZhangetal
(witha doseof∼15.9×1018 α-eventsg−1)exhibitedthepresenceofU4+
crystal
andU5+ above1000K,withadramaticincreaseinsignalsnear1400K.
crystal
1. Introduction
Zircon(ZrSiO ;I4 /amd)hasastructurethatisbuiltofchainsofalternatingSiO tetrahedra
4 1 4
andZrO triangulardodecahedraparalleltothecaxis(Speer1980). UandTharecommonly
8
foundinnaturalzirconatconcentrationsthattypicallyarenogreaterthan5000ppm. Many
zirconcrystalsarepartiallymetamict(anaperiodicoramorphousstate; Ewing1994,Weber
etal1988,Saljeetal1999)asaresultofself-radiationdamageassociatedwiththeα-decay
eventsoftheincorporatedU,Thandtheirdaughterproducts.
Priortothisstudytherehavebeenonlylimitedstudiesoftheeffectofradiationdamageon
theoxidationstateofuranium(Ellsworth1925,Tomkeleff1946,Hutton1950,Frondel1958).
TogainanunderstandingoftheoxidationstateofU,whichmayoccurinthetetra-,penta-and
hexa-valentstates,isofimportancebecausethecrystalchemistryandmobilityofUvarieswith
oxidationstate,andtheseprocessescanhaveaprofoundeffectontheresultsofU/Pbdating.
Inaddition,aspointedoutinourrecentstudy(Zhangetal2002),Uionabsorptionspectraare
very sensitive to local symmetry; thus, by monitoringthe spectralvariationsof U ions, one
obtains an insight into local coordinationenvironmentsas a functionof increasing α-decay
dose. Chemical analyses by electron microprobe, mass spectroscopy and ion microprobe
(e.g. recent studies by Chapman and Roddick 1994, Davis and Krogh 2000, Carrigan et al
2002,Geisleretal2002)havebeenusedtostudyUconcentrationsinzircon,particularlythe
lossofUbydiffusionanditseffectonagedetermination. Thebulkchemicalanalysisprovides
almostnostructuraloroxidationstateinformationonuranium.
Althoughvibrationalspectroscopyissensitivetovibrationalmovementsofmoleculesand
atomsincrystals,thetypicalconcentrationsofUinnaturalzircon(e.g.thousandsofpartsper
million)arenothighenoughtocausesignificantchangesinvibrationalspectra(inthefrequency
regionbelow1500cm−1). However,spectrarelatedto theelectronicstate ofU ionscanbe
obtainedinthenear-infrared(NIR)region. Innaturalzircon,UatomsreplaceZratomsand
occupytheZr4+site. TheZr4+siteisnoncentrosymmetric,D . Thuselectronictransitionsfor
2d
paramagneticimpurities(e.g.Uions)arepossible. AndersonandPayne(1939)andAnderson
(1962, 1963) reported characteristic optical absorption spectra of U ions in natural zircon.
Experimentalandtheoreticalworkhasbeenperformedtogainanunderstandingoftheenergy
levelsofUions;U4+andU5+spectraofU-dopedsyntheticandheatednaturalzirconhavebeen
obtained(JuddandRunciman1976,Kisliuketal1967,Richmanetal1967,Mackeyetal1975,
VanceandMackey1974,1975,1978). TheU energylevelscanbelabelledintermsoffive
irreduciblerepresentationsoftheD pointgroup:foursinglets(cid:2) ,(cid:2) ,(cid:2) ,(cid:2) andonedoublet
2d 1 2 3 4
(cid:2) . Thedegenerate5f2(SL)J termssplitinto70levels: 16(cid:2) +9(cid:2) +12(cid:2) +12(cid:2) +21(cid:2) ,
5 1 2 3 4 5
i.e.49singlets(cid:2) ,(cid:2) ,(cid:2) ,(cid:2) and21doublets(cid:2) .
1 2 3 4 5
In a recent NIR study on metamictization and recrystallization of zircon (Zhang et al
2002),weinvestigatedtheimpactofradiationondefects(e.g.UandOHions)innaturalzircon
andshowedtheevolutionofU4+ andU5+ spectra,aswellasmodificationoftheOHsignals
withincreasingdegreeofdamage. Theresultsshowedthatzirconsampleswithintermediate
degreesofdamageexhibitedcharacteristicfeaturesthataredifferentfromthoseofcrystalline
orheavilydamagedzircon. Thechanges,‘seen’byphonons(withacorrelationlengthofafew
unitcells)anddefectssuchasUandOHions,wereexplainedbyaradiation-inducedchange
oflocalsymmetryoramodificationofthelocalconfigurationandachangeoftheoxidation
stateofuranium.
Oxidationstateofuraniuminmetamictandannealedzircon 3447
As a continuation of this previous work (Zhang et al 2002), we report, in this paper,
newlyrecordeddataandfurtheranalysisoftheUionspectra. Wehaveobtainedquantitative
information on the spectral profiles of U ions as a function of increasing damage and have
extracted the different contributionsrelated to U ions in aperiodic and crystalline domains.
In this application of infrared spectroscopy to the study of radiation damage in zircon, we
demonstratethatitispossibletoseparatetheUsignalsrelatedtorecrystallizedandamorphous
regions and to obtain the ratio of the two types of signals as a function of the degree of
radiationdamage. Toourknowledge,theseresultsarethefirstoftheirkind. Wehaveobtained
aquantitativeanalysisofUionsignalsformetamictzirconand,byouranalysis,haveextracted
thedistributionofthedifferentoxidationstatesofUincrystallineandamorphousregionsin
metamictandrecrystallizedzircon.
2. Experimentalmethods
2.1. Samples
The samples used in this study were mainly gemstone zircons from Sri Lanka, previously
investigated by a number of different analytical methods (e.g. x-ray diffraction, FT-Raman
andmicro-Ramanspectroscopy,IRandmicro-IRspectroscopy,electronmicroscopy,electron
microprobe, nuclear magnetic resonance) (Ellsworth et al 1994, Farnan and Salje 2001,
Murakami et al 1991, R´ıos and Salje 1999, R´ıos et al 2000, Salje et al 1999, Zhang and
Salje 2001, 2003, Zhang et al 2000a, 2000b, 2000c, 2002). Detailed informationon these
samples can be found in these references. These samples were chosen because they are
well-characterizedcrystals of gemstone quality, have low levels of chemicalimpurities and
lackofgrowthzoning. Moreimportantly,SriLankanzirconscanbefoundwhichareeither
crystallineormetamictduetothewiderangeofUandThconcentrationsfoundinthesesamples
(HollandandGottfried1955). ThecrystalsusedinthisstudywerefurtherinvestigatedusingIR
microscopy(beamsizesof80–150µm)andtheresultsshowedthatthespectralvariationsfrom
pointtopointwereverysmall. ApartfromtheseSriLankancrystals,gem-qualitycrystalline
zircon crystals from other localities studied previously (crystal Mud Tank, from Australia,
studied by Murakamiet al 1991, and crystalMoroto, studied by Zhang et al 2000a, 2000b
andFarnanandSalje2001)aswellasagemcrystalwithunknownlocality(originallylabelled
Vigene) were used for the low-temperature measurement. More detailed information on
crystalsMudTankandMorotocanbefoundinthereferencesgivenabove.Electronmicroprobe
analysisshowedthatthecrystallabelledVigeneconsistedofSiO 33.39,ZrO 66.59,HfO
2 2 2
0.86, Y O 0.03, ThO 0.01, UO 0.01 wt%, andFe, Ca andAl with concentrationsbelow
2 3 2 2
ourdetectionlevel. Itslatticeparameters(a =6.6053Å,c =5.9802ÅandV =260.96Å3)
weredeterminedbyx-rayGuinierpowderdiffractionwithSiasinternalstandard. Thissample
showedpolarizedIRreflectionfeaturesinthemid-IRregion,similartothoseofacrystalline
zirconasreportedbyZhangetal (2000c)anditsreflectivityat850cm−1(E ⊥ c)is0.97,a
value close to thatof undamagedzircon (Zhangand Salje 2001). FT-Raman analysis(with
an instrumentalresolution of 2 cm−1) showed that its ν Raman mode (B , antisymmetric
3 1g
stretching)islocatednear∼1007cm−1withameasuredfullwidthathalfmaximum(FWHM)
of∼4cm−1. Theradiationdosewasestimatedtobelessthan1.0×1018α-eventsg−1based
onacomparisonwithpublisheddata(e.g.HollandandGottfried(1955)andMurakamietal
(1991))forx-raydata,Zhangetal(2000b)forpolarizedIRdataandZhangetal(2000c)for
Ramandata). Naturalzirconmaybesubjectedtothermalannealingduringgeologicalperiods,
thusaffectingtheamountofradiationdamagethatisretainedinthesample. Inthisstudy,the
self-radiationdose(basedontheageofthezirconanditsUandThcontents)forthenatural
3448 MZhangetal
zirconcrystalshasbeenusedtocharacterizetheamountofdamage. Atpresentthisisthebest
parametertouseindeterminingtheextentofdamageandwillallowustoassesstherelative
changesinthecrystallineandmetamictdomainsofthezirconcrystals.
The crystallographic orientations of the crystals were determined from the external
morphologyor by using x-ray precession techniques and an optical polarizing microscope.
Itwasimpossibletoorientsampleswithdoseshigherthan∼10×1018α-eventsg−1,asthere
werenoeasilyobtainedBraggreflectionsandthesehigh-dosecrystalsallhadirregularexternal
shapes. Crystalzirconplateswerecutparalleltothecaxisandthenpolished.Thermallytreated
sampleswerepreparedbyannealingthesamplebetween500and1800Kinairinavertical
furnace. The annealing temperature was measured with a Pt–PtRh thermocouple that was
placedjustabovethesamples. Thezirconwasannealedatahightemperaturefor1handthen
quenched in air. After measurementsat room temperature, the samples were annealed at a
highertemperature.
3. Infraredtechnique
InfraredspectrawererecordedusingtwoIRspectrometers. ABrukerFT-IR66vspectrometer
attachedtoaninfraredmicroscopewasusedtorecordtheinfraredspectrabetween500and
13000cm−1atroomtemperature.Aliquid-nitrogen-cooledMCTdetector,coupledwithaKBr
beamsplitterandaGlobarsource,wereusedtorecordthespectrabetween500and5000cm−1.
Forthefrequencyregionof3000–11000cm−1,atungstensource,anMCTdetectorandaCaF
2
beamsplitterwereusedwithspectralresolutionsof2and4cm−1. AKRS5wire-gridpolarizer
wasemployed. Allthespectrawereaveragedover512scans. ABruker113vIFSspectrometer
wasusedtorecordpolarizedandunpolarizedinfraredspectrabetween4000and8000cm−1
atlow temperatures. Aninstrumentalresolutionof1cm−1 and350scanswereadopted. A
Leyboldtwo-stageclosed-cycleheliumcryostatwithaworkingtemperaturerangeof13–310K
wasusedforthelow-temperaturemeasurements(Zhangetal1996). Thecryostatwascoupled
with a Leybold TLC60 temperature controller and a Leybold cryo-compresser. A sample
holderwasmadefromhigh-thermal-conductivityoxygen-freecopper. A gold-coatedlattice
made from oxygen-freecopper was installed at the sample position to improvethe thermal
contactbetweenthesampleandthesampleholder. Onetemperaturesensor,positionednear
the heatingunit, was used to controlthe temperatureof the cryostatwhile anotherSi-diode
temperature sensor (LakeShore, DT-470-DI-13, calibrated by the manufacturer) was glued
ontothecentreofthesampleholderformeasuringthesampletemperature. Thesamplewas
placed in a vacuumchamber equipped with KRS5 windows. Sample and reference single-
beamspectrameasuredatthesametemperaturesandinthesamepolarizationconditionswere
usedforcalculatingtheabsorptionspectra. Allabsorptionspectrawererecordedasabsorbance
α,withα =−log (I /I ),where I isthesingle-beamtransmissionintensity. The
10 sample reference
softwareOPUS-IRwasusedfordataanalysis. InordertoobtainpeakpositionsandFWHMs,
themeasureddatawerefittedtotheLorentzianfunctionswithlinearbaselines.
4. Analysisandresults
4.1. Radiation-inducedspectralchanges
PolarizedUionspectra(between4000and11000cm−1)ofcrystallineandmetamictzircon
areshowninfigure1(a). Forcrystallinezircon,ourdataareconsistentwithpublishedresults
(Richman et al 1967, Vance and Mackey 1978), although some bands are too weak to be
resolved at room temperature. We recorded absorption bands at 4838 and 10922 cm−1 in
Oxidationstateofuraniuminmetamictandannealedzircon 3449
weaklydamagedsamplesandtheywereassignedtoU4+ (Richmanetal1967,Vanceand
crystal
Mackey 1978). We use the subscript ‘crystal’ to indicate that the signal is associated with
crystalline ZrSiO . The sharp bands near 6668 and 9030 cm−1 (E ⊥ c) are assigned as
4
signalsdueto U5+ inopticalandZeemanstudies(VanceandMackey1974,1975). Judd
crystal
and Runciman (1976) proposed that they were due to (cid:2) → (cid:2) and (cid:2) → (cid:2) transitions.
6 6 7 7
Experimentalresults(VanceandMackey1978)confirmedtheirpredictionandrevealedthat
therewasnoaccidentaldegeneracyinthegroundstateofUions. Theseauthorsfoundthatit
wascausedbyimpurity-inducednewtransitions. Inarecentclassicalatomicsimulationstudy
of zircon using the energy minimization approach, Akhtar and Waseem (2001) calculated
defect energies to predict the most favourable site for an impurity ion in the host lattice of
zirconandfoundthatpentavalentionscouldexistinzirconbymetalvacancycompensation
orelectroncompensationmechanisms.
The impact of radiation on the U ion spectra is characterizedby three types of change
describedbelow.
(1) Isotropic, broad features appear in metamict zircon and their intensity increases with
increasing degree of damage. As shown in figure 1(b), a broad feature occurs in the
polarizedspectra(E (cid:1) candE ⊥ c)between4000and5600cm−1 inheavilydamaged
samples. ThisfeatureisassignedasU4+ (thesubscript‘amorphous’indicatesthat
amorphous
the signal is associated with the radiation-induced amorphous, or aperiodic, domains
of ZrSiO ) absorption because they are absent in the crystalline sample and disappear
4
whenannealedathightemperatures. AlthoughtheU4+ signalswerethedominant
amorphous
componentsinspectraofhigh-dosesamples(dosegreaterthan∼8×1018α-eventsg−1),a
broad,isotropicfeaturealsoappearednear6700cm−1,butwithaveryweakintensity. The
presenceofthissignalindicatesthatasmallnumberofUionsareinthepentavalentstate
(U5+ )inmetamictnaturalZrSiO . Inhighlydamagedzircons,theratiobetweenthe
amorphous 4
U4+ and U5+ absorptionareas (integratedbetween 4500and 5600 cm−1 for
amorphous amorphous
theU4+ signal,andbetween6200and7150cm−1fortheU5+ signal)gavean
amorphous amorphous
averagedvalueof∼10%.
(2) The characteristic bands of crystalline materials show a systematic broadening and
frequencyshift(figures1(a)and2).
(3) TheU5+ bandnear6668cm−1showsarelativelystrongresponsetoradiationdamagein
crystal
comparisonwithU4+ asitbecamealmostunresolvableabove7.5×1018α-eventsg−1,
crytstal
whileweakU4+ signals(e.g.near4830cm−1)arestillpreserved.
crystal
4.2. QuantitativeanalysisofU4+ andU5+ signals
Spectral subtractions were conducted to separate the signals associated with crystalline
materialsfromthoseofamorphousphases. Inthisapproach,thedamagedzirconwastreatedas
atwo-phasesystem(althoughtheeffectivemediumapproach(ZhangandSalje2001)isusually
a better method, the differentU concentrationsin zirconswith differentdegreesof damage
madetheapplicationdifficult). Thesubtractionwascompletedbetweentwopolarizedspectra
withdifferentorientationsrecordedfromthesamecrystal. Theideaisbasedonthefactthat,
foradamagedsample,thesignalsrelatedtocrystallineregionsremainessentiallyanisotropic
even for damaged samples, whereas, signals from amorphous domains are independent of
crystallographic orientations (i.e. the amorphization process destroys the long-range order
of the crystalstructure). Radiation-inducedloss of anisotropyby metamictizationof zircon
hasbeenshownpreviouslybydifferentstudies, e.g.refractiveindex(HollandandGottfried
3450 MZhangetal
(a)
(b)
Figure1.PolarizedUionspectraofmetamictzircon(a)between4000and11000cm−1(theNIR
dataarefromZhangetal2002). Theindicatedradiationdoserepresentstheself-radiationdoseof
thenaturalzirconcrystalsaccumulatedsincetheirgrowth. (b)Detailedfeaturebetween4000and
5800cm−1. ThebroadandisotropicU4+featurein4000–5600cm−1(E⊥c)wasproducedby
metamictization.
Oxidationstateofuraniuminmetamictandannealedzircon 3451
Figure2. Differencespectrainthefrequencyregionof6200–7100cm−1(AE⊥c−AE(cid:1)c,where
Arepresentsabsorptionspectra).
1955),electronmicroscopy(Murakamietal 1991,Meldrumet al 1998),polarizedinfrared
spectroscopyinmid-IR(Zhangetal2000c),far-IR(ZhangandSalje2001)andnear-IR(Zhang
et al 2002). Similar methodswere successfully used to separate the OH signals associated
with crystalline phases from those of amorphous domains in metamict titanite (Salje et al
2000, Zhang et al 2001). The advantage of using difference spectra from the same crystal
is that this method can reduce the possible errors related to the measurements of different
samples and also avoids the potential uncertainty caused by spectral variations associated
with the amorphouszirconunderfurtherbombardments. To acquirebetter dataforspectral
subtractionandintegration,particularlyforhighdosecasesinwhichverylimitedcrystalline
regions remain, special measures were taken. Polarized reference single-beam spectra for
bothE (cid:1)candE ⊥cwererecordedfirstinidenticalexperimentalconditionsexceptrotating
thepolarizerremotelyormanually;then,aftersampleloading,polarizedsamplesingle-beam
spectrawererecordedinthesameexperimentalconditionsasusedinthereferencespectrum
measurements. Thesemeasuresresultedinessentiallyconsistentbaselinesandsignal–noise
ratios. In particular, this method ensured that the data measured with E (cid:1) c and E ⊥ c
arefromthesamesampleareas, with identicalopticalpathsandsamplethicknesses. These
factorsmightaffectthe outcomeof the subtractionsand peak profile analysis(curvefit and
integration). Typicaldifferencespectraobtainedthroughthespectrumsubtractionareplotted
in a stack in figures 2 and 3(a) (the inset part). The reasonable baselines and bandshapes
suggestthatthemeasurementsandsubtractionswerewellperformed.
The 4834 cm−1 band (U4+ , E (cid:1) c) and the 6668cm−1 band (U5+ , E ⊥ c) were
crystal crystal
usedmainlyforspectralanalysis,astheyarerelativelyintenseandbetterseparatedfromthe
3452 MZhangetal
(a)
(b)
Figure3. DosedependenceofpeakpositionsandFWHMobtainedfromSriLankazircon(full
squaresrepresentdataobtainedatroomtemperature,andopencirclesat20K):(a)FWHMofthe
4834cm−1band(U4+ );(b)peakpositionofthe4834cm−1band;and(c)peakpositionofthe
crystal
U5+ bandnear6668cm−1. Theinsetinfigure3(a)showsthedifferencespectrabetween4200
crystal
and5500cm−1(AE(cid:1)c−AE⊥c,),whicharenormalizedtoshowdetailedchangesoftheband.The
curvesarevisualguides.
other bands. Peak profile analysis was performedby fitting the U4+ and U5+ bandsin
crystal crystal
thedifferencespectratoLorentzianfunctionswithlinearbaselinessoastogainquantitative
results from the measured data. The FWHMs and peak positions of the U4+ and U5+
crystal crystal
Oxidationstateofuraniuminmetamictandannealedzircon 3453
(c)
Figure3.(Continued.)
are plotted as a function of radiation dose in figures 3(a)–(c). As the data were obtained
from the differencespectra and the contributionsof the amorphousdomains were removed
byspectralsubtraction,thechangesshowninfigures3(a)–(c)reflectmainlythebehaviourof
U ions in the crystalline regions. The peak profiles of the U4+ near 4834 cm−1 shows a
crystal
non-linearchangewithradiationdose(figure3(a)). Withincreasingdose,itsFWHMexhibits
a dramatic increase in the dose region of ∼0 and ∼3.5 (1018 α-eventsg−1), as shown in
figure 3(a). It becomesalmostsaturated with furtherincreasingdose. We considerthatthe
dramaticfrequencychangeand line-broadeningin these U ion bandshasthe same physical
nature as those frequency changes in Raman bands (e.g. the ν mode near 1007 cm−1 as
3
reported by Zhang et al (2000c)) that occur in the same dose region. They are associated
with radiation-induceddefectivelattices. But the room-temperaturedata with doses of less
than1.0(1018 α-eventsg−1)(figures3(a)–(c))areofamorecomplexnature. Wenotedwith
surprise that, for crystalline zircon, the bands show FWHM values of 50–60 cm−1 at room
temperature. Thisobservationledustosuspectthatthesecrystallinezirconscouldbemore
damagedthanwehadinitiallyestimated. Ourlow-temperatureresultsat20K(opencircles)
showthattheplateaushowninfigures3(a)and(b)inthislow-doseregionismainlycaused
bytemperature-inducedlinebroadeningorfrequencyshiftratherthan,orweaklyrelatedto,
radiationdamageorstructuraldisordering.
Oneof theimportantobservationsfromthisstudyis thatin metamictizationthe U5+
crystal
band near 6668 cm−1 (E ⊥ c) showed a much stronger decrease in intensity than that of
the U4+ band near 4834 cm−1 (E (cid:1) c) and the former became almost undetectable near
crystal
adoseof7.5(1018 α-eventsg−1)whileforU4+ thesignalremainedrelativelyintense. In
crystal
ordertocomparethetwotypesofsignalsquantitatively,thebandareasofbothsignalswere
obtainedbyintegratingthedatashowninthedifferencespectra(figures1(b)and2). Theratio
betweenthesetwobandareasisplottedtorevealtheirrelativechange(figure4(a)). Thedose
dependenceoftheratioshowsasystematicincreasewithincreasingdegreeofdamage. Asthe
3454 MZhangetal
datawereextractedfromthesignalsmainlyrelatedtocrystallineregions,theresultsshownin
figure4(a)representchangesoccurringintheremainingcrystallinedomains. Wewereunable
toobtaindatapointsatdoseshigherthan∼7.5(1018α-eventsg−1),becausetherelativelysharp
U5+ signal becomesundetectable in the differencespectra (figure 2) as describedearlier.
crystal
Withincreasingradiationdose,thesystematicchangeoftheratiobetweenU4+ andU5+
crystal crystal
ionsimpliesradiation-inducedvariationsoflocalchargestate. Weshallshowinalatersection
that,duringhigh-temperatureannealing,theintensityratiobetweenU4+ andU5+ exhibits
crystal crystal
anoppositechange: asystematicincrease,indicatingthatthechangeshowninfigure4(a)is
causedbyradiationdamage.
The ratio of the U ion signals contributed by those U ions in amorphous volumes
(U =U4+ +U5+ )andthetotalUions(U =U +U4+ +U5+ )
amorphous amorphous amorphous total amorphous crystal crystal
wasestimated(figure4(b)). Wefoundthatitwasverydifficultorimpossibletotraceorinte-
gratetheU5+ componentinzirconwithintermediatedegreesofdamage,becauseofthe
amorphous
lowintensityofthecomponentandthepresenceofweaksharpbandsintheregion,evenfor
E (cid:1)c. InallourdeterminationsoftheU5+ ,thecontributionofU5+ wasconsidered
amorphous amorphous
as10%oftheU4+ (seeabove)valueobtainedfromhighlydamagedsamples(i.e.itwas
amorphous
assumedthattheratioofU4+ toU5+ wasaconstantvalueinamorphousdomains).
amorphous amorphous
Thisassumptionisunlikelytoresultinanysignificanterrorsorinfluenceourdetermination
oftheratio,becausetheU5+ signalisveryweakandplaysamuchlessimportantrolein
amorphous
theratio.
4.3. Uionspectrumofdecomposedzircon
Tocomparethespectraldifferencebetweenmetamictanddecomposedzircon,wemeasured
a decomposed zircon crystal from an unknown locality. The crystal (sample Cam1) was
previouslystudiedusingRaman,x-rayandinfrared(Zhangetal2000c,2002). Thesampleisa
greengemstone,likemostoftheheavilydamagedsamplesanalysed.However,itshowsRaman
signals of ZrSiO and additional sharp Raman bands at 146, 260, 312, 460 and 642 cm−1,
4
characteristicoftetragonalZrO . X-raydiffractiondataofZhangetal(2000c)revealedthatthis
2
zirconcrystalisonlypartiallymetamict(withcellparametersa = 6.6070Å,c = 6.0221Å)
andconfirmedthepresenceofsignificantamountsofcrystallineZrO ,consistentwithinfrared
2
andRamandata. Thisdecomposedzirconexhibitsmulti-phonon(overtoneandcombination)
bandsverydifferentfromthemetamictzirconduetothepresenceofglassySiO (Zhangetal
2
2002). The U ionspectra ofthis crystalexhibitwell-resolvedspectralfeaturesand sharpU
ionsbandsincontrasttootherpartiallydamagedsamples(figure5). TheU5+ signalnear
crystal
6650cm−1isalmostundetectableinthissample. Basedonthehigh-temperaturedata,which
shallbedescribedinalatersection,weconsiderthatthisdecomposedsamplewassubjected
tohightemperatureannealing.
4.4. Low-temperaturespectra
Low-temperaturemeasurementsinthenear-IRwerecompletedwithtwoobjectives:
(1) to check whether radiation damage or structural disorder was responsible for the large
room-temperaturevalues(∼50–60cm−1)oftheFWHMsrecordedincrystallinesamples
(e.g.figure3(a)),and
(2) tocompareUionspectraofcrystallineandmetamictzirconatlowtemperatures.
Thelow-temperaturenear-IRspectrafromtwosampleswithdifferentdegreesofdamageare
shown in figures 6(a) and (b) (with E (cid:1) c), and 6(c) and (d) (with E ⊥ c). Because U