dependsjavmoo.com

ISSN0030-400X,OpticsandSpectroscopy,2009,Vol.
106,No.
4,pp.
556–563.
PleiadesPublishing,Ltd.
,2009.
OriginalRussianTextD.
A.
Spasskii,V.
N.
Kolobanov,V.
V.
Mikhalin,L.
Yu.
Berezovskaya,L.
I.
Ivleva,I.
S.
Voronina,2009,publishedinOptikaiSpektroskopiya,2009,Vol.
106,No.
4,pp.
625–632.
556INTRODUCTIONMolybdenum-containingcrystalsarepromisingforcryogenicscintillationbolometers.
The100Moisotopeisapotential(andmostpromising)sourceofdoublenon-neutrinoβ-decay0ν2β[1–4].
Thepossibilityofimplementinga0ν2βsourceanddetectingthisdecayinthesamematerialisveryattractivebecauseitallowsonetosignicantlyincreasethedetectionefciency.
Themainrequirementsforapotentialscintillatorareahighspeciclightyieldwithshortdecaytimes(lessthan10–4s),theabsenceofradioactiveisotopesfortheotherelementsinthecrystalcomposition,lowdensity,andsmalleffectiveatomicnumberZ(whichisneces-sarytoreducebackground).
Untilnow,themostatten-tionhasbeenpaidtothescintillationpropertiesofmolybdateswithascheelite-typestructure:AMoO4(A=Ca,Cd,Sr,Ba,Pb).
However,allofthecrystalsstudiedhaveanumberofdrawbacks,i.
e.
,lowlumines-cenceintensity,evenatlowtemperatures(BaMoO4);largeZ(PbMoO4);orthepresenceofradioactivecationisotopes(CdMoO4,PbMoO4).
Currently,CMoO4crystalsareconsideredasthemostappropriate.
How-ever,the48Caisotopeisasourceofthe2ν2βdecay,whichformsanunremovablebackgroundindetection.
Themagnesiummolybdate(MgMoO4)canbeanalter-nativetocrystalswithscheelitestructures.
Theabsenceoflong-livedradioactiveisotopesofmagnesium,aswellasthelowdensityofMgMoO4(4.
04g/cm3)areadvantagesincomparisonwithscheelitecrystals.
MgMoO4luminescencewasobservedforthefirsttimein[5];however,amoredetailedinvestigationoftheluminescenceandopticalpropertiesofthiscompoundhavebeenperformedonlyrecently[6–8].
Inthispaper,wereporttheresultsofstudyingtheluminescencecharacteristicsofMgMoO4crystalsgrownunderdifferentconditions.
Thereectionspec-traofMgMoO4weremeasuredforthersttimetakingintoaccountthecrystalstructureanisotropy.
Recently,muchattentionhasbeenfocusedoncharge-transferluminescence,whichwasobservedinanumberofoxy-gen-containingcompoundsdopedwithYb3+ions[9].
Therefore,wehavestudiedtheluminescencepropertiesofMgMoO4:Ybcrystals.
EXPERIMENTALTheluminescenceVUV-excitationspectra,trans-missionspectranearthefundamentalabsorptionedge,andluminescenceexcitationandreectionspectraintheenergyrange3.
7–25eVweremeasuredontheSuperlumisystem(DEZY,Hamburg)[10]attempera-turesof8–70Kandat300K.
TheluminescencespectraweremeasuredusinganARCSpectraProSP-308monochromator(inthespectrographmode)andnor-malizedtothespectralsensitivityfunctionofthedetectingcomplex.
Plane-parallelsurfacesoffreshcleavageswereanalyzed.
ThecrystalsunderstudyweregrownbytheCzo-chralskimethodinplatinumcruciblesinair.
SampleMgMoO4no.
1wasgrownfromastoichiometricmeltofMgOandMoO3ofhigh-puritygrade.
Thechargecom-ponentsweredriedandmixedinastoichiometricpro-portion,thenheatedinaplatinumcrucibleat700or930°Cfor6h.
X-raydiffractionanalysisshowedthatCONDENSED-MATTERSPECTROSCOPYLuminescencePeculiaritiesandOpticalPropertiesofMgMoO4andMgMoO4:YbCrystalsD.
A.
Spasskiia,V.
N.
Kolobanovb,V.
V.
Mikhalinb,L.
Yu.
Berezovskayac,L.
I.
Ivlevac,andI.
S.
VoroninacaSkobeltsynInstituteofNuclearPhysics,MoscowStateUniversity,Moscow,119992RussiabFacultyofPhysics,MoscowStateUniversity,Moscow,119991RussiacProkhorovGeneralPhysicsInstitute,RussianAcademyofSciences,Moscow,119991Russiae-mail:deris2002@mail.
ruReceivedOctober20,2008Abstract—Magnesiummolybdateisconsideredasapromisingmaterialforcryogenicscintillationbolometers.
TheluminescencepropertiesofMgMoO4havebeeninvestigatedonsinglecrystalsgrownfrommeltsofsto-ichiometricandnonstoichiometriccompositionsandoncrystalsdopedwithYb3+ions.
TheiropticalpropertiesareinterpretedtakingintoaccounttheanisotropyoftheMgMoO4crystalstructure.
PACSnumbers:78.
55.
-m,78.
40.
-qDOI:10.
1134/S0030400X09040171OPTICSANDSPECTROSCOPYVol.
106No.
42009LUMINESCENCEPECULIARITIESANDOPTICALPROPERTIES557synthesisat700°CleadstotheformationofbothMgMoO4andMg2Mo3O11phases.
At930°C,onlytheβ-MgMoO4phaseissynthesized.
Themeltingtemper-atureofMgMoO4is1320°C.
Thepullingraterangedfrom1to3mm/h.
SampleMgMoO4no.
2wasgrownfromanonstoichiometricmeltwithadecitofMoO3(3wt%).
SampleMgMoO4no.
3wasobtainedfromastoichiometricmeltwithadditionof5wt%Yb2O3.
β-MgMoO4isabiaxialanisotropiccrystalwiththeunit-cellparametersa=10.
273,b=9.
288,c=7.
025,andβ=106.
96°;itbelongstothespacegroupC2/m[11].
ItisknownthatthereflectionofanisotropicmolybdatecrystalsdependsontherelativeorientationoftheircrystallographicaxesandtheelectricvectorEoftheincidentpolarizedsynchrotronradiation.
There-fore,thesamplesunderstudywereorientedalongthecrystallographicaxesbyelectronbackscattereddiffrac-tion(EBSD)usingaCRYSTALattachment(OxfordInstrumentsINCASystem)toaJSM5910-10LV(Jeol)scanningelectronmicroscope.
ItwasshownthattheMgMoO4cleavageplanebelongsto{9110}crystallo-graphicplanes.
Thus,thecrystallographicaxiscliesintheMgMoO4cleavageplane.
Duringmeasurements,thesampleswereorientedsoastomakethecaxisbeeitherparallel(c||E)orperpendicular(c⊥E)totheprojectionoftheincidentradiationvectorEontothesampleplane.
RESULTSANDDISCUSSIONAbsorptionSpectraofMgMoO4TheabsorptionspectraatT=10and300KareshowninFig.
1.
Atatemperatureof10K,theabsorp-tioncoefcientbeginstosignicantlyincreaseat3.
9eV.
Withanincreaseintemperatureto300K,theabsorptionedgeundergoesaredshiftandbecomesat-ter.
SuchbehaviorischaracteristicofthefundamentalabsorptionedgeofdielectricsandcanbedescribedbytheUrbachformula[12]whereσistheslope,Tistemperature,kBistheBoltz-mannconstant,andα0andE0arethecoordinatesoftheintersectionpointsoftheUrbachedgesatdifferenttem-peratures,extrapolatedtolargeabsorptioncoefcients.
ApproximationoftheabsorptionspectrabytheUrbachformulagavethefollowingvaluesofσandE0forMgMoO4:σ=0.
32andE0=4.
49eVatT=300Kandσ=0.
02andE0=4.
57eVatT=10K.
NotethatE0isclosetotheexcitonpeakpositionintheabsorptionspectrumandthattheslopeparameterσdependsontemperatureandcangiveinformationaboutthepossi-bilityofexcitonself-trappinginthesecompounds[12].
ThereectionspectraofMgMoO4samplesmea-suredinthefundamentalabsorptionrangewererecal-culatedintoabsorptionspectra,becausethelatteryieldmoreexactenergiesofinterbandelectronictransitions.
Wealsocalculatedtheenergydependencesoftheimag-inaryandrealpartsofthepermittivityandenergy-lossfunction.
TherecalculationwasperformedaccordingtotheKramers–KronigrelationsusingfastFouriertrans-form[13].
Thesecalculationsrequireinformationaboutthebehaviorofthereectioncoefcientintheentirefrequencyrange.
Theexperimentalreectionspectraweremeasuredatenergiesfrom4to25eV.
Thereec-αE()α0σE0E–kBT-–,exp=0.
0203.
8Intensity,arb.
unitsPhotonenergy,eV4.
04.
20.
041234110α,cm–1Fig.
1.
(1,2)AbsorptionspectraofMgMoO4no.
1atT=(1)300and(2)10KandtheluminescenceexcitationspectrameasuredatT=10Kforsamples(3)MgMoO4no.
1(Elum=2.
3eV)and(4)MgMoO4no.
2(Elum=2.
0eV).
558OPTICSANDSPECTROSCOPYVol.
106No.
42009SPASSKIIetal.
tioncoefcientRwasextrapolatedintheenergyrangeE>25eVaccordingtothelawR~E–4.
Inthelow-energyregion,weappliedtheself-consistentprocedureforextrapolatingthereectioncoefcient,whichusestherefractiveindexasaparameter.
ThecriterionforrecalculationcorrectnesswasthevalidityoftheKram-ers–Kronigrelationsforthecalculatedpermittivity,aswellastheconvergenceofthecalculatedfourtypesofthesumrule,obtainedfromtheimaginarypartofthepermittivityandtheenergy-lossfunction.
TheobtainedabsorptionandintrinsicluminescenceexcitationspectraareshowninFig.
2.
Dependingonthesampleorientation,uptoeightabsorptionpeaksareobservedintheenergyrange4–25eV;theirenergypositionsatT=10KarelistedinTable1.
TheenergiesoftheabsorptionpeaksexceedsthevalueE0givenbytheUrbachformula.
Indeed,theabsorptionspectrumlacksanypronouncedstructureintheenergyrange4.
5–4.
6eV,whichcouldbeassignedtotheformationofanexcitonatthefundamentalabsorptionedge.
Theabsenceofexcitonischaracteris-ticofmolybdateswithscheelitestructure,forexample,CaMoO4andSrMoO4,forwhichthelow-energyreflec-tionpeakisdeterminedbyinterbandtransitions[14,15].
However,onecannotexcludethattheexcitonpeakishiddenduetothepresenceofstructuraldefectsintheMgMoO4samplesunderstudy.
Forexample,for0.
506Intensity,arb.
unitsPhotonenergy,eV481510202512*6(b)0.
5*10601.
0*1060.
5012*6(a)0.
5*10601.
0*1061.
01.
5*106α,cm–1Fig.
2.
(1)AbsorptionspectracalculatedaccordingtotheKramers–Kronigrelationsfromtheexperimentalreectionspectraand(2)intrinsicluminescenceexcitationspectra(Elum=2.
3eV)foraMgMoO4crystalwiththeorientations(a)c||Eand(b)c⊥E.
Thearrowsindicatethemostpronouncedantibaticpeculiaritiesintheabsorptionandluminescenceexcitationspectra.
Table1.
AbsorptionpeakenergiesforMgMoO4atdifferentorientationsofthecrystallographicaxiscwithrespecttotheprojectionoftheelectricvectorEofsynchrotronradiationincidentonthesamplesurface(T=10K)CrystalorientationAbsorptionpeakenergies,eV123456789c||E5.
15.
86.
256.
99.
110.
614.
117.
0~21c⊥E5.
25.
75–6.
89.
111.
213.
916.
5~21OPTICSANDSPECTROSCOPYVol.
106No.
42009LUMINESCENCEPECULIARITIESANDOPTICALPROPERTIES559LuAlO3,reliableinformationabouttheexistenceofanexcitonpeakatthefundamentalabsorptionedgecouldbeobtainedonlybymeasuringthereectionfromthesurfaceofsingle-crystallmshavinglessstructuraldefectsincomparisonwithbulksinglecrystals[16].
Apparently,thelow-energyabsorptionpeakintheMgMoO4samplesunderstudyisduetotheinterbandelectronictransitions.
Thepositionoftherst-peakmaximumintheabsorptionspectrumyieldsthefollow-ingestimationfromabovefortheMgMoO4bandgap:Eg30K,theexperimentaltemperaturedepen-denceoftheluminescenceintensityforMgMoO4no.
1differsfromthatcalculatedfromtheMottformula.
Thereasonforthisisthatanadditionalbandarisesintheluminescencespectrumasaresultofthesignicantdecreaseintheintrinsicluminescenceintensity(approximatelybytwoordersofmagnitude).
Indeed,anincreaseintemperatureto30Kshiftsthebandpeakfrom2.
3to2.
0eV;thebandbecomesnonelementaryandcanberepresentedasasuperpositionoftwoele-mentaryluminescencebandspeakingat2.
3and1.
9eV.
Forsampleno.
2(grownfromachargewithadecitofMoO3),thenonelementaryluminescenceband,whichcanalsobedecomposedintoGaussiancomponents,isobservedevenatT=10K.
Theresultsofdecomposi-tionintoGaussiancomponentsforsamplesno.
1andno.
2arelistedinTable3.
Theexcitationspectrumoftheluminescencebandat1.
9eVnearthefundamentalabsorptionedge(Fig.
1)ischaracterizedbytheformationofanadditionalexci-tationbandat4.
0eV.
At10K,thisbandisinthetrans-parencyrangeofMgMoO4.
Therelativeweightofthelow-energyluminescencebandincreasesuponlumi-nescenceexcitationat4.
0eV.
Wecanconcludethattheluminescencebandat1.
9eVisduetothedefectscausedbythecrystaldeviationfromstoichiometry.
Twooverlappingluminescencebandsofcompara-bleintensitywereobservedfornominallypureMgMoO4atT=9K[6].
Thebandpeaksobtainedinthisstudyarered-shiftedby~0.
2eVwithrespecttothedataof[6].
Ourconclusionaboutthedefectnatureofthe1.
9-eVluminescencebandisinagreementwithoneofthesuggestionsmadein[6]aboutthenatureofthelow-energybandat~2.
1eV.
Notethattheabsenceofasignicantcontributionofthelow-energydefectbandatT=10KforsampleMgMoO4no.
1indicatesitshighstructuralquality.
Theluminescencespectrumofmagnesiummolyb-datedopedwithytterbiumions(sampleno.
3)contains,alongwiththeintrinsicluminescenceband,additionalnarrowluminescencebandsat1.
21,1.
24,and1.
27eV,whichcorrespondstothef–ftransitionsinYb3+ions[9].
Thecharge-transferluminescence,characteristicofsomecomplexoxides(garnets,perovskites,borates)dopedwithYb3+,wasnotobservedinMgMoO4:Yb.
Notethattheintrinsicluminescenceintensityforsamplesno.
2andno.
3ismuchlowerthanforsampleno.
1.
Apparently,thisisduetotheformationofcom-petingluminescencecenters,whicharerelatedtothepresenceofbothstructuraldefectsandimpuritycentersinthenonstoichiometric(no.
2)anddoped(no.
3)sam-ples.
Theluminescenceintensityofmagnesiummolyb-dateandsomeothermolybdatesandtungstatesatlowtemperatures(T=10K)wascomparedunderthesameexperimentalconditions.
Toexcludetheenergylossduetothethermalizationofchargecarriers,theexcita-tionenergyforeachcrystalwaschosentocorrespondtothefundamentalabsorptionedge.
Thisapproachmakesitpossibletoshowthepotentialspeciclightyieldofacrystalwhenthecompetingchannelsofnon-radiativeenergyrelaxationaresuppressed.
Compari-sonwasperformedforMgMoO4no.
1,ZnMoO4,BaWO4,andCdWO4samples.
MgMoO4,ZnMoO4,andBaWO4singlecrystalsweregrownbytheCzo-chralskimethodattheGeneralPhysicsInstitute,Rus-sianAcademyofSciences.
TheCdWO4samplewaspreparedfroma1-cm3crystalwithaspeciclightyieldTable3.
EnergiesandFWHMsoftheintrinsicluminescenceband(Emax1)andtheluminescencebandduetostructuralde-fects(Emax2)forMgMoO4no.
1andno.
2samplesatdifferenttemperaturesandexcitationenergiesCrystalT,KEex,eVEmax1,eVFWHM1,eVEmax2,eVFWHM2,eVMgMoO4no.
1105.
42.
310.
67––MgMoO4no.
1705.
42.
300.
691.
90(1.
21)0.
57MgMoO4no.
2105.
42.
310.
641.
92(0.
43)0.
53MgMoO4no.
2104.
02.
310.
651.
89(1.
28)0.
57Note:Theweightofalow-energyluminescencebandwithrespecttothehigh-energybandisgiveninparenthesesafterthemaximumvalue.
562OPTICSANDSPECTROSCOPYVol.
106No.
42009SPASSKIIetal.
of~20000photons/meVatT=300K;thecrystalwasgrownbytheCzochralskimethodattheScienticandTechnicalComplex"InstituteforSingleCrystals"(Kharkov,Ukraine).
ItisshownthattheluminescenceintensityofMgMoO4no.
1isanorderofmagnitudehigherthanthatofBaWO4butmuchlowerthantheluminescenceintensityofCdWO4andZnMoO4(byfactorsofabout40and10,respectively),whichcanbeaseriousproblemforpracticalapplicationofMgMoO4asascintillator.
LuminescenceExcitationSpectraTheintrinsicluminescenceexcitationspectraofMgMoO4no.
1areshowninFig.
2.
Thestructuralanisotropymanifestsitselfinthesespectra,whicharedifferentfordifferentorientationswithrespecttoE.
Apossiblereasonforthedifferenceintheexcitationspectraisthesurfacelossfactor.
Generally,theexcita-tionspectraaremodulatedbytheabsorptionspectra,duetowhichantibaticspecicfeaturesariseinthestructureoftheexcitationandabsorptionspectra[13].
Theantibaticspecicfeaturesthataremostpro-nouncedintheabsorptionandexcitationspectraareindicatedbyarrowsinFig.
2.
SincethepresenceandrelativeintensityoftheabsorptionpeaksinMgMoO4dependonthecrystalorientation,themodulationoftheexcitationspectraobtainedintheparallelandperpen-dicularorientationsofcandEwillbedifferent.
Notethatinthecaseofthec||Eorientationtheexci-tationspectrumcontainstwoadditionalpeaksat4.
05and4.
4eV.
Theyareabsentforthec⊥Eorientation,andtheirappearancecannotbeexplainedbythesurfacelossfactor,becausepronouncedabsorptionpeaksareabsentinthisenergyrange.
Atthesametime,theinten-sityoftheexcitationspectrumincreasessimultaneouslywiththeabsorptioncoefcient(Fig.
1,curves2,3).
Apparently,thepresenceofthesepeaksonlyatthesampleorientationc||Echaracterizestheanisotropyoftheenergybandlevelsnearthefundamentalabsorptionedge.
Thesignicant(byapproximatelyanorderofmag-nitude)decreaseintheluminescenceintensitywithanincreaseintheexcitationenergyfrom4.
5–5.
5to11–12eVindicatestheexcitontypeofenergytransfertointrinsicluminescencecentersinthepresenceofeffec-tivecompetingchannelofenergyrelaxation.
Anincreaseintheexcitationenergyincreasestheaveragedistancebetweenthefreeelectronandhole.
Intheabsenceofeffectivecompetingrelaxationchannel,theprobabilityforafreeelectronandholetoformanexci-tondependsweaklyontheexcitationenergyintherangefromthefundamentalabsorptionedgetothebeginningofphotonmultiplication.
Thisbehaviorwasobserved,forexample,fortheluminescenceexcitationspectrumofself-trappedexcitonsinundopedCdWO4[26].
Inthepresenceofacompetingrelaxationchannel,theprobabilityofexcitonformationdecreaseswithanincreaseintheaveragedistancebetweenanelectronandhole.
SinceonlyoneluminescencebandisobservedintheMgMoO4no.
1crystalatT=10K(Fig.
4,curve1),thecompetingchannelisnonradiative.
Atphotonenergiesabove12eVtheintensityofexcitationspectraincreasesduetotheformationoflow-energyelectron–holepairsasaresultofmultipli-cationofelectronicexcitations.
CONCLUSIONSTheluminescenceandopticalpropertiesofMgMoO4crystalsgrownfromstoichiometricandnon-stoichiometricmeltsandcrystalsdopedwithYb3+ionswereinvestigated.
Considerationofthestructuralanisotropymadeitpossibletoexplainthemainpecu-liaritiesoftheabsorptionspectraintheenergyrange4–25eV.
Thebandgapisestimatedtobeintherange4.
52eV.
Itisshownthattheintrinsiclumines-cenceat2.
3eViscausedbytheemissionofexcitonsself-trappedatMoO4complexes.
Theexistenceofaneffectivenonradiativechannelofenergyrelaxation,whichcompeteswithintrinsicluminescencecenters,isestablished.
Itisconcludedthattheluminescencebandat1.
9eVisduetothedefectsofthesamplecrystalstructure.
DopingofMgMoO4withytterbiumionsleadstotheoccurrenceofimpurityluminescencebandsintheIRregion;charge-transferluminescenceisnotobservedinthiscase.
Theintrinsicluminescenceinten-sityofMgMoO4at10KismuchlowerthanthatofCdWO4andZnMoO4;hence,applicationofthiscrystalasancryogenicscintillatorislimited.
ACKNOWLEDGMENTSThisstudywassupportedbygrantDFG436RUS113/437/0-3andtheRussianFoundationforBasicResearch,projectno.
06-02-16339.
WearegratefultoG.
ZimmererforpermittingexperimentsonSUPER-LUMIandtoG.
Striganyukforhishelpinmeasure-ments.
WearealsogratefultoD.
ShaturaforsupplyingtheX-raydiffractiondata,L.
Iskhakovaforthesampleorientation,andI.
TupitsinaandL.
Nagornayaforsup-plyingtheCdWO4sample.
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TranslatedbyYu.
Sin'kov