Journalof Luminescence42 (1988) 173 180 173North-Holland, Amsterdam

IDENTIFICATION OF FLUORESCING PHASESIN Eu3~DOPED La

1~Sr015CuO4SUPERCONDUCTORS*

Brian M. TISSUEandJohnC. WRIGHTDepartmentof Chemistry,University of Wisconsin,Madison, Wisconsin53706, USA

Received14 July 1988Revised18 August 1988Accepted31 August 1988

Laser spectroscopyof a Eu3+ probe ion has beenusedto identify two impurity phasesin polycrystallinesamplesof

La~85Sr015CuO4. The impurity phasesare residualhexagonalLa203startingmaterial anda La silicate phasewhich forms

during sintenng from contaminationwith boat or furnace material. Laserspectroscopyof the Eu3~probe hasalsobeen

successfullyused to monitor the solid statereaction rateof the starting materials and to observeheterogeneitiesin thedistributionof theimpurity phasesin thefinal samples.

I. Introduction critical currentdensity~ Critical currentdensi-ties in polycrystalline La

1 85Sr015CuO4 are typi-Wepreviouslyreportedthe observationof fluo- cally 8 A/cm

2 [2] while densewires and filmsrescencein samplesof La

185Sr015CuO4doped have.~‘sof 20 50 A/cm2 [3 5]. Higher current

with Eu3~andPr3~[1]. Wereportherethe results densitieshavebeenachievedin films of the 90 Kof experimentsto determinethe specific site or superconductor,Y

1 Ba 2Cu307, which hasreceivedphase the fluorescing Eu

3~occupies in the much greaterattention[6]. Thesenumbers,as wellLa

1 85Sr015Cu04samples.The previouslyobserved as the following discussion,are meant to be ii-fluorescencehas beendeterminedto arise from lustrative rather than comprehensivesince pro-Eu

3± in a La silicate impurity phase. We have gress in high temperaturesuperconductivityhassubsequentlyobserveda newline in somesamples beenphenomenalsinceits discovery[7]. For rele-which correspondsto Eu3+ in unreactedLa203. vant newssunimariesseerefs. [8,9].This line is used to study the solid statereaction While the measuredcritical currentdensitiesinrate by monitoring the disappearanceof the theseceramicmaterialsare generallyquite low,Eu3~:La

203starting material with heating time. intragraincurrentdensitieshavebeenestimatedatWe also report on differencesin the distribution i0

5 to iO~A/cm2 [2,10]. The current limitingof the impurity phasesfrom sampleto sampleand factors are weak links in the currentpath duetofrom surfaceto bulk in a singlesample. the short coherencelengths, ~= 7—SO A [11 13],

The Eu3±ion servesas a very sensitiveprobeto of the superconductingelectrons.The presenceofthe presenceof insulating,impurity phasesin the weak links has been well established,althoughsuperconductorsamples.Thepresenceof impurity whether they are predominantly intra- or inter-phasesis veryimportantto the physicalproperties grain is still in question[14 16]. Regardless,theof the superconductorssuchas grain contactsand presenceof insulatingimpuritieswill decreasethe

critical current densities (e.g., 0.1 wt% Si02 in

* . Y1Ba Cu 07 reducedJ by a factor of almost2This work was supportedby theNational ScienceFounda- C

tion Division of Materials Researchunder Grant DMR [17]) and the overall superconductingproperties8513705. [18]. It thereforeis important to developsensitive

0022-2313/88/$03.50© ElsevierSciencePublishersB.V.(North-HollandPhysicsPublishingDivision)

174 B.M. Tissue,J.C. Wright / Identificationoffluorescingphasesin dopedsemiconductors

analyticaltechniquesto monitor insulatingimpur- agreeswith lanthanidedopingstudiesin the litera-ity phases.This work demonstratesthe usefulness ture [21 23].of rareearthprobeion spectroscopyfor analysing The spectroscopicapparatus for performingtheseimportantmaterials, site-selective laser spectroscopyhas been de-

scribed elsewhere[24]. Pellets were usedas is orgroundinto powderand pressedinto depressionson a coppersampleholder. The sampleholderwas

2. Expenmental mountedon the cold finger of a closed cycle He

refrigeratorcapableof maintaininga temperature

Sampleswerepreparedby solid statesintering variable from 300 K to approximately13 K. Thereaction of the appropriateamountsof La203, surfaceof the powderor pellet wasexcitedwith aEu203,SrCO3 and CuO. The La203 wasdried at N2 laser pumped dye laser (Rhodanune6G dye10000C for 4 h and for some experimentswas with a typical bandwidthof 0.7 cm 1 and pulsepresinteredwith Eu203to form Eu

3~:La203.The energywas 3 X 10 ~ J) and fluorescencewas col-

starting materialswere mixed by grinding in an lectedat 900 to the excitation beam.7F

0 —* D0agatemortarandpestle,pressedinto a pellet and fluorescenceexcitation spectrawere obtainedbyheatedin air at 1000°Cfor 12 14 h. On cooling directly monitoring the fluorescenceusinga mech-the black pellet wasground,repelleted,sinteredat anical chopper to block the laser light. Fluo-1000°Cin air for an additional 12 14 h, and rescencespectrawereobtainedwith a I m mono-cooled slowly (<20°C/mm) to room temper- chromatorand dry iced cooledEMI 9658RPMT.ature.For heating,the pelletsrestedin porcelain, All spectrawere recordedwith a gatedintegratoralumina or platinum containersand were placed andstrip chart recorder.in a Lindberg box furnace with a Moldathermheatingelement,which consistsof approximately50% alumina and50% silica. 3. Results

The phase purity of the samples was de-termined by powder X-ray diffraction using a Figures 1(a,b) show typical

7F0 —

5D0 excita-

Nicolet 12/V diffractometerandCuKt radiation. tion spectraof (Eu004La18Sr0i6)~Cu04for xSingle phasesamplescould be preparedby care-fully weighing the starting materials (to within0.3%) and using dried La203. Undried La203 Energy (cmi)consistsof a large proportionof La(OH)3 which, 17400 17200whenusedas is, resultsin a deficiencyof La and I I I

therefore multiphase samples containing La1 ~Sr0~5Cu04and La4SrCu5O134[19,20]. Eu

3~was (c~solublein La

1 85Sr015Cu04up to 10% Eu3~,but

new X-ray diffraction lines appearedat 20% indi-cating a new phasewas forming (all percentagesare mol% unlessindicatedotherwise).The super- J J

conductingpropertiesof samplespreparedby theaboveprocedurewere checkedby measuringthe (b)AC susceptibility.UndopedLa

185Sr015Cu04 hada superconductingonsettemperature(7~)of 38.5K and showedcomplete flux shielding at 10 K 574 ‘ 578 ‘ 582while a 1% Eu

3~dopedsamplehad a 7~of 37 K Wavelength(nm)and a broad, steppedtransition with a reduced Fig. 1. 7F

0 —p

5D0 excitation spectraat 13 K monitoring all

percentageof flux shieldingat 10 K. The Eu3± fluorescencein (a) 004 La

1 8Sr016)096CuO4,and(b) 004~

solubility and suppressionof 7~in our samples La1 5Sr016)10CuO4.

B.M. Tissue,J.C. Wright / Identificationoffluorescingphasesin dopedsemiconductors 175

Energy (cm—i) 900 1000°C. The sample was predominantly16600 16000 15400 single-phaseSrO with some extra broad lines of

I about10% of the major SrO line. SinteringEu203

andEu2CuO4.It couldnot be determinedif Eu3~

~ and CuO at 1000°Cproduceda mixture of CuOwassoluble in CuO, andno fluorescencecould be

I I I I I observedin this material. The 7F0 —* ~D0 excita-

tion spectra of Eu203, 0.1% Eu3~:La

203,and0.5% Eu

3~:Sr0are shown in fig. 3. Eu203 shows

(b~two linesat 580.8 and582.1 nm, correspondingtothe two cationsitesin the cubic C phase[25], and

I I I I I I

or secondaryphases.Figure 3(b) shows a single(I) small lines at higher energieswhich are vibronicsline as expected for the hexagonalA phaseof

Eu3~:La

203[26]. The5D

0 —~ ~F2 fluorescence

I spectraof Eu203 and0.1%Eu3tLa

2O3are shown600 620 640Wavelength (nm) in figs. 4(a,b),respectively.

Fig. 2.5D

0 —.7F

2 fluorescence spectra at 13 K of (a) The7F

0 —* ~D~ excitation spectrum of(Eu002La183Sr015)094CuO4exciting at 578.2 nm, (b) (Eu0~ 0.5%Eu

3~:SrOin fig. 3(c) shows many linesmdi-La

1 8Sr0 16)10CuO4 exciting at 581.0 nm, and (c) Eu002La1 98 cating that multiple sites or phasesare present.Cu04excitingat 581.0 nm. Multiple sites are expectedsince the Eu

3~sub-

stitutes for divalent Sr and therefore requirescharge compensation.The chargecompensation

0.96 and 1.0. The x = 0.96 samplewas a mixture can occur by variousmeanscreatingdistinct crys-of La

1 85Sr015Cu04and La45rCu50134while the tallographic environments,including clusters ofx — 1.0 samplewas single-phaseLa1 85Sr015Cu04. two or moreEu

3~ions. From the 5D0 —~

7F2 fluo-

Figures 2(a,b) show the5D

0 —*

7F2 fluorescence

spectraof the 578.2 and 581.0 nm lines of figs.1(a,b), respectively.The fluorescenceis very weak Energy (cm~)

but strongenough to obtain spectra.Figure 2(c) 17400 17200shows the

5D0 —~

7F2 fluorescencespectrum of

1%Eu3~:La

2CuO4,which also had a7F

0 —~ ~ D0excitation line at 581.0nm, andwasmoreintenseand therefore better resolved,The 581.0 nm linewas only observedwhen sampleswere prepared I I I I I I

exactly on stoichiometry or when excessLa203 (b) Eu3~La20~,L

was present.The 578.2 nm line was observedinstoichiometricand La-deficientsamples. _______________ _______

Eu203, Eu

3~:La2O3,Eu

3~:Sr0and Eu3~:Cu0 I I I I I I

were investigatedto determineif any of the fluo-rescencein the superconductorsarisesfrom Eu3+

T3+~Sr0in unreactedstarting materials. Eu

203 and 0.1%Eu

3~:La203were dried at 1000°Cfor 4 h. The

Eu203 formedpredominantlycubic C phase,with I574 578 582some additional small broad lines in the diffrac-

Wavelength (nm)tion pattern,while theLa203 formedpurehexag- Fig. 3.

7F0-.

5D0 excitation spectraat 13 K monitonng all

onal A phase[25]. 0.5% Eu3~:SrOwas prepared fluorescence of (a) Eu

203, (b) 0.1%Eu3~:La

2O3,and (c)by sintering Eu203 and SrCO3 for 6 h at 0.5%Eu

3~:SrO.

176 B.M. Tissue,J.C. Wright / Identificationoffluorescingphasesin dopedsemiconductors

Energy (cm 1) Comparing spectra1(b) to 3(b) and 2(b) to16600 15800 4(b), the line at 581.0 nm in the Eu004La18Sr016

I I I I I I Cu04sampleis dueto Eu3~in unreactedhexago-

nal La 203. By comparingthe signal intensityob-servedin fig. 1(b) to the intensityof a samplewith1% excess La (not shown), this particular

I I I I I Eu004La18Sr016Cu04sampleis estimatedto con-

tain 1 x 10 2% excessLa. Assumingthat asignal-(b) Eu

3~La203 to-noise ratio of 2 is sufficient for detection,the

detection limit of residual La 201 inLa1 85Sr015Cu04is estimatedtobe I x 10 ~mol%.

The broad line at 578.2 nm, which previouslyI I I I I was attributedto the superconductor[1], hasbeen

600 620 640Wavelength (nm) moredifficult to identify andmuch of the rest of

this discussionfocuseson this line. In order toFig. 4.

5D0 —.

7F2 fluorescencespectraat 13 K of (a) Eu203

exciting at 580.7 nm, and (b) 0 1%Eu3~:La

2O3exciting at characterizethe 578.2 nm line, further spectral581.0nm. studies were performed using fluorescence line

narrowing (FLN) and examiningthe temperaturedependenceandthe effect of La deficiencyon the

rescencespectraof Eu3~:Sr0shownin fig. 5, the spectra.The FLN was performed by recording

excitation lines can be groupedinto three setsof 5D0 —~ ~F2 fluorescencespectraas the laserwas

two lines each.Efficient energy transfer occurs steppedacross the7F

0 —*

5D0 excitation line in

betweenions resulting in the samefluorescence incrementsof 0,22 nm. The broadwidth of thespectrumregardlessof which line of a set is ex-

7F0 —~ ~D~transitionindicatesthat thereis a great

cited. The Eu3~ions giving rise to thesepairs of deal of disorderin the Eu3~lattice positions.The

linesare probablyassociatedas dimersor clusters. 5D0 —+ ~F2 fluorescencespectraare much sharper

becauseof FLN. The line positions shift as thelaserexcitesdifferent siteswithin the

7F0—~ ~

Energy (cm~) inhomogeneousdistribution of the line. Figure 616600 15800 summarizesthe observedshifts as a function of

I I I I I I the laserposition. The5D

0 energyis equalto the

differencebetween D0 energyand the energy ofdye laserenergywhile the

7F2 energylevels are the

the5D

0 —s7F

2 transitions.The datapoints on the

I I I I I extremitiesare lessintensebecauseof the inhomo-geneousdistribution of the line. The lowest

7F2

level is much less intensethan the threehigher7F

2levels as canbe seenin the fluorescencespectrumin fig. 2(a). The nearlylinear shift with excitation

I I I I I energyindicates that the line is predominantlyasingle inhomogeneousprofile rather than severaloverlappinglines as previously thought [1]. Thespectraare qualitatively different on the wings

I I I I indicating that the Eu3± ionsare in fairly different

600 620 640 environments.Wavelength (nm) The position of the 5D0 level was recordedat

Fig. 5.5D

0—s

7F2 fluorescencespectraat 13 K of 0.5%Eu

3~SrOexciting at(a) 575.3 or 575.5 nm, (b) 578.4 or 578.7 nm, and(c) temperaturesfrom 13 to 300 K sinceseveralgroups

580.4or 582.3nm. have reported orthorhombic distortions in

B.M. Tissue,J.C. Wright / Identificationoffluorescingphasesin dopedsemiconductors 177

17400phasedueto contamination.In oneset of samplestermineif the fluorescencearisesfrom an impurity

0 Al203 was carefullydopedin to replaceup to 2%of the Cu siteswith Al. No changeswere observed

17200in theX-ray diffraction patternsand therewasno

1500 - enhancementof the 578.2nm peak.Sinteredmix-S.

tures of 0.05 0.6 A1203:1.0 Eu004La18Sr016CuO4

resulted in multiphasesamplescontainingLa185: Sr015Cu04, La4SrCu5O134,and LaA1O3. Againtherewas no enhancementof the 578.2nm peak.Doping Si02 into Eu004La18Sr016CuO4 re-

S1200

1100 ~ ~ • sulted in a large enhancementof the 578.2 nmpeak. Sintered mixtures of 0.01 0.2 Si02:1,0Eu004La18Sr016Cu04resultedin multiphasesam-

ples containingLa1 85Sr015Cu04,CuO, and a newphase. The intensity of the 578.2 nm line was- ~ tion patternas shownin fig. 7. The new linesaredirectly correlatedto the new lines in the diffrac-

900 -

I left unmarked in the powder X-ray diffractionI I

576 578 580 pattern of the 0.2SiO2 : 1.0 Eu004La18Sr016CuO4Excitation wavelength (nm) sample shown in fig. 8. A Hanawalt search of

Fig. 6. Positionsof the7F

2 energylevels,determinedfrom the the JCPDS powder X-ray diffraction data-~‘D0—s

7F2 fluorescencespectra,as a functionof the

7F0 —s

5D0

base matched these new lines to several Laexcitationenergyin (Euo02La1 83Sr0is )o 94CuO4.

silicate phases such as LiLa9(Si04)602 andNaLa9(Si04)602[31]. Wethink that the 578.2nm

La185Sr015Cu04at 220 K to 130 K [27—30]. line arises from Eu3~in a La silicate phaseal-

Structuralphasetransitionsshouldalter the local thoughthe exactstoichiometryof the phasecouldenvironmentof the Eu3+ probe ion and causea not be determined.From fig. 7 and the observedshift in line position. No sharp break was ob- signal levels in the superconductorsamples,theserved,suggestingthat eitherthe phasetransition detection limit of silica is estimatedto be 0.03is too subtleto affect the rareearthenvironment, mol%.or that the Eu3~is segregatedin a different phase Attemptsto makeEu3~dopedLa—silicates[32]from the superconductor. by sinteringat 1000°Cin air for longerthan 24 h

We have observed that the intensity of the578.2 nm line is irreproducibleand is absentin

8) . •somestoichiometricsamples,while it is enhanced

0x 12~when samplesare preparedLa-deficient, A sys- —.

tematicstudyof theeffect of La deficiencyshowed -~‘ - : ~that the intensity of the 578.2 nm line wasmaxi- ~ 4o 2.

8)mum in samplespreparedwith a4 5% deficiency 6 ~of La. The maximum in the intensity in non- o20 -

0stoichiometricsamplessuggeststhat the578.2 nm LU -

1•line is dependenton some phaseequilibria in the I I I I

(5sample. N- 0 5 10 15 20

Sincethe aboveresultsindicate that the 578.2 % Si02

nm line originates from a separatephaseother Fig. 7. Plot of the intensity of the 578.2 nm excitation line(dots)and the heightof the 29 — 28.2°X-ray diffraction line

than the superconductor,sampleswere prepared (x) of multiphasesamplesof x Si02:1.0. Eu0~La18Sr016Cu-intentionally doped with A1203 and Si02 to de- O4wherex = 0.02 to 0.2 (2 to 20% Si02).

178 B.M. Tissue,J.C. Wright / Identificationoffluorescingphasesin dopedsemiconductors

100 0 1.0 Eu3~:La

203:SrC03:Cu0

~‘80 .. 1000°C/ air

1)0.6

~ 60 > 0.4

40~O.20

r~20 j~ ?~0%~ 000~

__________________________________________ Heating time (h)0 I I I I Fig. 10. Kinetics of thesolid statereactionof La1 85Sr0 15CuO420 30 40 50 60 70 monitoring the disappearanceof Eu

3~:La2O3.The crosses

2e (degrees) were sintered pellets of 1.85(0.1%Eu3~:La

2O~):O.15

Fig. 8. Powder X-ray diffraction pattern of the multiphase SrCO3:1.OCuO and the dots were powders ofsample obtained on sintering 0.2 Si02:1.0 (Eu004La18 1.85(0.5%Eu

3tLa2O3):O.15SrCO3:1.OCuO.Both setsof sam-

Sro.16)10CuO4. CuO lines are markedwith x andLa 85Sr0 ~ pies were heatedat 1000°Cin air and were regroundtoCu04lines aremarkedwith ~ powder to make themeasurements.The dotshave been nor

mahzedto comparewith the crosses.The line is drawn asaguidefor theeye.

resultedin mixtures of the unidentifiedphaseandLa203. Figure 9 shows the

7F0 —s

5D0 excitation in fig. 2(a), although it is broaderand slightly

and5D

0 —s7F

2 fluorescencespectra of a 0.1% shifted. The shift can be explainedby differencesEu

3~•1 La:2 Si02 sample. The excitation spec- in the FLN sincethe excitationwavelengthsof the

trum showsthat two sitesor phasesare present(as two spectrawere slightly different. It is possiblewell as excessLa 203) and that the local environ- that the presenceof Cu and Sr in the phasement is verydisordered.Comparingfig. 9(a) to the equilibrium in the superconductorhelpscrystallizesuperconductorin fig. 1(a), the excitationlines are the impurity phaseandthis could accountfor thein the sameregion but the superconductoris not differencesin the spectra.as broad and has only a shoulderon the low In the courseof this work the solid statereac-energyside of the main line ratherthana distinct tion rate of the starting materialswas studiedbypeak. The fluorescencespectrum in fig. 9(b) is monitoring the disappearanceof Eu

3~ La 203 withvery similar to the spectrumof the superconductor heating time. This hasprovento be a very sensi-

tive and convenientmethod of monitoring theprogress of the solid state reaction. The pre-sintered Eu3tLa

2O3, SrCO3, and CuO starting~0203

materialswere ground until homogeneous.Theywerethenpelleted,andfired for varyinglengthsof

I I I time,with samplesfired for 2 h or longerundergo-574 578 I 582

Excitation Wavelength (nm) ing an intermediategrinding. The intensityof the7F0 —s

5D0 transitionat 581.0 nm was then mea-

suredto determinethe amountof unreactedLa203in the samples.Typical dataare shownin fig. 10for samplesheatedin air at 1000°C.The time= 0

_____________________________________ samplewasmixed with undopedLa1 85Sr015Cu04600 620 640 to make the optical penetrationdepth similar to

Fluorescence Wavelength (nm) sinteredsamplessince the pellets are light greyFig. 9. Spectraat 13 K of sintered0.1% Eu

3~:1La203:2Si02, before firing and black afterward.This method

(a)7F

0—.

5D0 excitation spectramonitoring all fluorescence,

and(b)5D

0 —o

7F2 fluorescencespectraexcitingat 577.8 nm. provides limited compensation for penetration

B.M. Tissue,J.C. Wright / Identificationoffluorescingphasesin dopedsemiconductors 179

depth differencesbecauseit approximatesa ho- batchto batchandfrom surfaceto bulk in a singlemogeneousdistribution of stronglyabsorbingpar- sample.tides with a heterogeneousdistribution of quitedifferently absorbingparticles.As canbe seeninfig. 10 the La203 in thesetwo samplesis almostcompletely gone in 12 18 h. However, varying Referencesamountsof unreactedEu

3~:La 203 havebeenob-served in other samples preparedin a similar [1] B.M. TissueandJ.C. Wright, J. Lumin. 37 (1987)117.

manner,evenafter 24—40 h of heating.This ob- [2] S. Senoussi,M. Oussenaand M. Ribault,Phys.Rev. B 36

servationindicates that the superconductorscan (1987)4003.

bedifficult to preparereproduciblyand that sam- [3] K. Matsuzaki,A. Inoue, H.M. Kimura, K. Moroishi andT. Masumoto,J. Mater. Sci. Lett. 6 (1987)990.piespreparedidenticallycanhavevarying amounts [4] H. Adachi, K. Setsuneand K. Wasa, Phys. Rev. B 35

of residualstartingmaterial.We think that minor (1987)8824.

differences in the starting stoichiometryare re- [5] 0. Kohno, Y. Ikeno, N. Sadakata,M. Sugimoto and M.

sponsiblefor the differencesobservedin different Nakagawa,Japan.J. AppI. Phys.26 (1987)L759.

setsof samples. [6] P. Chaudhari,R.H. Koch, RB. Laibowitz, T.R. McGuireand R.J. Gambino,Phys. Rev. Lett. 58 (1987) 2684.Heterogeneitieswerealso observedwithin single [7] J.G.BednorzandK.A. Muller, Z. Phys.B 64 (1986) 189.

samples.The 578.2 nm line was generally more [8] L. Garwin andP. Campbell,Nature330 (1987) 611.intense when using pellets rather than ground [9] T. Forgan,Nature329 (1987)483.

samples.In one sampleof Eu004La18Sr016Cu04, [10] D.P. Hampshire, X. Cai, J. Seuntjens and D.C.

Larbalestier,Supercond.Sci. Technol. 1 (1988) 12.only the La silicate line at 578.2nm wasobserved [11] K.W. Kwok, G.W. Crabtree,D.G. Hinks, D.W. Capone,when spectroscopywas performedon the surface J D Jorgensennnd K Zhang, Phys Rev. B 35 (1987)

of the pellet while only the Eu3~:La

203line at 5343.

581.0 nm was observedwhen the groundpowder [12] 5. Uchida, H. Takagi, K. Kishio, K. Kitazawa, K. Fueki

wasused. Another sample,which had beenfired and S. Tanaka,Japan.J. Appi. Phys.26 (1987)L443.[13] M. Suzuki, Y. Enomoto,K. Moriwaki and T. Murakanii,

in an aluminaboat,hadabroad7F

0 —* ~D0 excita- Japan.J. Appi. Phys.26 (1987)L1921.

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using the pellet but no signalwasobservedfrom Physica148B (1987) 149.

the ground powder.We attribute this to Eu3~in [15] B. Renker, I. Apfelstedt, H. Kupfer, C. Pohtis, H.

A1203 on the surfaceof the pellet. Theseresults Rietschel,W. Schauer,H. WUhl, U. Gottwick, H. Kneis-

sd, U. Rauchschwaibe,H. Spille andF. Steglich,Z. Phys.indicate that the surface of the pellet can be B 67 (1987) 1.

different from the bulk and is consistentwith the [16] Y.-M. Chiang, D.A. Rudman,D.K. Leung, J.A.S. Ikeda,

578.2 nm line arising from reactionof the pellets A. RoshkoandB.D. Fabes,PhysicaC 152 (1988)77.with outsidecontaminants. [17] R.A. Camps, J.E. Evetts, B.A. Giowacki, S.B. Newcomb,

RE. SomekhandW.M. Stobbs,Nature329 (1987) 229.[18] C. Uher and A.B. Kaiser,Phys.Rev. B 37 (1988) 127.[19] J.G. Bednorz,M. Takashigeand K.A. Muller, Mat. Res.

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lattice, While laserspectroscopycannotbeusedto (1987) 1003.characterizethe new superconductorsdirectly, it [23] I. Felner, So!. St. Commun.62 (1987)791.

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It has also shown that heterogeneitiesexist from NewYork, 1964) p. 1.

180 B.M. Tissue,J.C. Wright / Identificationoffluorescingphasesin dopedsemiconductors

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