Charge Transfer Luminescence of Yb3+

Journal of Luminescence 91 (2000) 177–193Charge transfer luminescence of Yb 3+L. van Pieterson*, M. Heeroma, E. de Heer, A. MeijerinkDebye Institute, Utrecht University, P. O. Box 80 000, 3508 TA Utrecht, The NetherlandsReceived 13 December 1999; received in revised form 21 February 2000; accepted 28 February 2000AbstractA systematic study of charge transfer (CT) luminescence from Yb 3+ is presented. CT luminescence was observed inLiYF 4 (7 eV), aluminates, phosphates, oxides (4 eV) and oxysulfides (3 eV). In all cases the CT emission bands arebroad and the Stokes shifts are large, ranging from 7000 cm 1 in oxysulfides and LiYF 4 and up to 17 000 cm 1 inaluminates. The influence of the size of the host lattice cation site on the charge transfer luminescence was investigated.A shift to longer wavelengths of the CT emission bands with increasing size of the cation site was observed. Typical(radiative) decay times for the CT luminescence are between 100 and 200 ns. The quenching of the Yb 3+ CTluminescence in the various host lattices was investigated and compared with the quenching of the Eu 3+ emission underCT excitation in the same lattices. A good correlation was found. The quenching temperatures of the Eu 3+ emission aremuch higher than that of the Yb 3+ CT luminescence. In lattices where Eu 3+ emission quenches at very high temperatures(>500 K) Yb 3+ CT luminescence could be observed. # 2000 Elsevier Science B.V. All rights reserved.PACS: 87.20e; 78.55m; 78.90+tKeywords: Yb 3+ ; Charge transfer1. IntroductionIn this century the luminescence of rare earthions has been well studied [1]. Especially since theapplication of luminescence from rare earth ionsin color television, fluorescent tubes and X-rayphosphors, numerous papers have appeared on4f n -4f n and 4f n1 5d–4f n emission. One kind of rareearth luminescence is still relatively unknown:charge transfer luminescence. This transition isthe reverse of the well-known charge transfer*Corresponding author. Tel.: +31-30-2533545; fax:+31-30-2532403.E-mail address: l.vanpieterson@phys.uu.nl (L. van Pieterson).absorption [2]. Up till now only three papers havereported charge transfer luminescence of a rareearth ion. In 1977, the first observation of thisphenomenom was made by Nakazawa. He reportedon charge transfer luminescence of theYb 3+ ion in phosphate and oxysulfide lattices[3,4]. Recently, Danielson et al., published oncharge transfer luminescence of the Ce 4+ ion inSr 2 CeO 4 [5].The charge transfer state is important inapplications. For example, in the red phosphorused in fluorescent tubes (Y 2 O 3 :Eu 3+ ), UVradiation is efficiently absorbed by a transition tothe charge transfer state (CTS) of the Eu 3+ -ion.After non-radiative decay to the lower 4f levels,luminescence occurs from the 5 D J states of Eu 3+ .0022-2313/00/$ - see front matter # 2000 Elsevier Science B.V. All rights reserved.PII: S 0022-2313(00)00214-3

178L. van Pieterson et al. / Journal of Luminescence 91 (2000) 177–193Eu 3+ -ion in the same host lattices under chargetransfer excitation.2. Experimental2.1. Sample preparationFor all systems, undoped host lattices wereprepared, as well as the Yb 3+ (1% and 3%) andEu 3+ (1%) doped lattices. The crystalline powdersamples were analyzed with X-ray diffractionusing a Philips PW1710 Diffractometer Controlusing Cu K a radiation (1.542 A˚ ).Fig. 1. Configurational-coordinate diagrams of Eu 3+ (4f 6 ) andYb 3+ (4f 13 ). In this diagram the potential energy is plotted as afunction of a configurational coordinate, which can be relatedto the metal-ligand distance in the vibrating complex [1].This process is shown in Fig. 1(a). In Eu 3+ ,luminescence from the charge transfer state cannotbe observed, because there will always be fastrelaxation to the 5 D J states.Yb 3+ (4f 13 ) is an ion for which charge transferluminescence can be expected [3,4]. In this ion,the only excited 4f state, 2 F 5/2 , is located 10 000 cm 1above the ground state 2 F 7/2 . Fig. 1(b) shows theconfiguration-coordinate model of Yb 3+ [1].Because of the large energy difference betweenthe charge transfer state and the highest excited4f state, charge transfer luminescence can beobserved.Here we report a systematic study of chargetransfer luminescence from Yb 3+ . We studiedYb 3+ charge transfer luminescence in the phosphates,borates, aluminates, oxysulfides, oxyhalogenides,oxides and fluorides. The influence of thehost lattice and the size of the cation site for whichYb 3+ substitutes is investigated. Temperaturedependentmeasurements were performed to getan insight in the influence of the host lattice onthe quenching temperature of the Yb 3+ chargetransfer luminescence. The results are comparedwith the temperature quenching measured for the2.1.1. REPO 4Powder samples of REPO 4 (RE=Sc, Lu, Y, La)were prepared by firing RE 2 O 3 (4 N) and(NH 4 ) 2 HPO 4 in air at 13508C for 3 h. ScPO 4 ,LuPO 4 and YPO 4 all have a tetragonal crystalstructure (zircon-type), LaPO 4 has a monoclinicstructure (monazite type) [6].2.1.2. REBO 3Powder samples of REBO 3 (RE=Sc, Y, La)were prepared by firing stoichiometric amounts ofRE 2 O 3 (4 N) and boric acid in air at 5008C for 5 h.Then, after adding 20% excess boric acid, thesamples were heated for 8 h at 9508C. ScBO 3 hasthe rhombohedral calcite structure, YBO 3 has thehexagonal pseudovaterite structure, and LaBO 3had the orthorhombic aragonite structure [7].2.1.3. RE 2 O 2 SY 2 O 2 S and La 2 O 2 S were prepared by the sulfidefusionmethod [8] which involves a solid–meltreaction between RE 2 O 3 (4 N) and Na 2 S x .RE 2 O 3 ,Na 2 CO 3 , and S (ratio 1 : 1 : 2.97) were mixed andpreheated at 2008C, then fired at 11008C innitrogen atmosphere for 4 h. It is then washedseveral times with hot water and dilute HCl (2%).Y 2 O 2 S and La 2 O 2 S both have a hexagonal crystalstructure [9].2.1.4. Na/LiREO 2NaREO 2 and LiREO 2 powders (RE=Sc, Y,La) were prepared by firing RE 2 O 3 (4 N) and

L. van Pieterson et al. / Journal of Luminescence 91 (2000) 177–193 179Na 2 CO 3 resp. Li 2 CO 3 at 9008C in a dry nitrogenatmosphere for 3 h. NaScO 2 has the a-NaFeO 2 structure, NaLaO 2 and LiScO 2 have thea-LiFeO 2 structure [10], LiYO 2 has a monoclinicstructure [11].2.1.5. LiYF 4LiYF 4 was prepared by the Bridgman method ina vitreous carbon crucible using a Philips PH 1006/13 high-frequency furnace. The crystal growthmelt contained 5 mol% of Yb 3+ .YF 3 , YbF 3 and15% excess of LiF were mixed and preheated overnight at 3008C in a nitrogen atmosphere. Then thesample was heated to 5508C andthecrystal-growthchamber was flushed with SF 6 to remove traces ofoxygen. At 7008C, again under nitrogen, the samplewas heated to the melting point (9008C). Then thesample was slowly cooled to room temperature.LiYF 4 has the inverse scheelite structure [12].2.1.6. RE 2 O 3 , REAlO 3 and YAGSc 2 O 3 and Y 2 O 3 crystals doped with 3% Yb 3+and YAlO 3 and YAG crystals doped with 2%Yb 3+ were prepared by Mr. Mix of the ‘Institutfu¨ r Laser-physik’ of the University of Hamburg[13,14]. YAlO 3 and YAG crystals doped with 2%Yb 3+ were prepared by the Czochralski method,YAG in an oxidizing atmosphere (N 2 +1% O 2 ),YAlO 3 in a reducing atmosphere (N 2 +1% H 2 ).Polycrystalline samples of YAG host lattice andYAG doped with 1% Eu 3+ were made by firingstoichiometric amounts of RE 2 O 3 and calcinedAl 2 O 3 at 13508C for 15 h. LaAlO 3 was made byfiring La 2 O 3 and calcined Al 2 O 3 at 12758C for 8 h.The oxides are of the cubic bixbyit type,YAlO 3 has the orthorhombic perovskite structure,LaAlO 3 has the rhombohedral perovskite structureand YAG has the cubic garnet structure [9].2.1.7. REOXREOX crystalline powders (RE=Y, La; X=Cl,Br) were prepared by dissolving the rare earthoxides (4 N) in concentrated hydrochloric acid andhydrobromic acid, respectively, evaporating untildryness, and firing in air at 7008C for 5 h. LaOFcrystalline powder was prepared by reaction ofLa 2 O 3 with NH 4 F at 10508C. The REOX powders(RE=Y, La; X=Cl, Br) all have the tetragonalPbFCl structure, LaOF has a tetragonal crystalstructure [9].2.2. Optical spectroscopyLuminescence and reflection measurements onundoped samples and Yb 3+ -doped samples wereperformed at the HIGITI experimental station [15]of the Synchrotronstrahlungslabor HASYLAB atDESY, Hamburg (Germany). Diffuse reflectionspectra in the vacuum ultraviolet (VUV) rangewere measured by scanning the excitation andemission monochromators synchronously, withsmall excitation and large emission slit. In excitationand reflection the spectral resolution wasbetter than 0.5 nm using a modified WadsworthMounting 1 m monochromator with a 1200 l/mmgrating blazed at 150 nm. In the ultraviolet (UV)and vacuum ultraviolet (VUV) emission measurementsthe resolution was ca. 2 nm using a SEYA0.2 m monochromator with a 1200 l/mm gratingblazed at 150 nm and a Hamamatsu MCP 1645U-09 channel plate detector. The UV and visibleemission measurements were recorded with a 0.22 mSPEX monochromator with a 100 l/mm gratingblazed at 450 nm and detected with a Tektronicsccd-array or a Hamamatsu R943-02 photomultiplier.The resolution was ca. 1 nm. The temperatureof the sample could be varied between 10 K androom temperature using a cold finger cryostat.Excitation and reflection spectra were corrected forthe spectral distribution of the synchrotron radiationintensity and the sensitivity of monochromators anddetectors using reference spectra recorded for MgF 2(total reflection). Since there were no correctionfactors available for the spectral response of theemission monochromators and detectors, the emissionmeasurements shown here are not corrected.The SEYA monochromator and detector have a lowsensitivity for wavelengths longer than 300 nm. TheSPEX monochromator and ccd/PM detector have alow sensitivity for wavelengths shorter than 300 nm.This may influence the reported positions for theemission maxima.The luminescence properties of the Eu 3+ -dopedsamples were measured on a SPEX 1680 spectrofluorometer,equipped with 0.22 m double monochromators.This apparatus was adapted for VUV

180L. van Pieterson et al. / Journal of Luminescence 91 (2000) 177–193excitation measurements [16]. The spectral resolutionof this setup is about 0.5 nm. Excitationspectra measured on this apparatus were correctedfor the lamp intensity using excitation spectra ofsodium salysilate emission as a reference. Emissionspectra were corrected for the spectral responseof the monochromator and the detector usingcorrection spectra provided by the manufacturer.Temperature-dependent measurements could beperformed between 5 and 850 K. In the lowtemperatureregion a cold finger cryostat was used.For measurements at temperatures higher thanroom temperature a home-made high-temperaturesample holder was used.3. ResultsTo distinguish between Yb 3+ charge transferluminescence and luminescence of the host lattice,it is necessary to have a good knowledge abouthost lattice luminescence. Luminescence bandsobserved in the spectra of Yb 3+ -doped lattices,which are not observed in the spectra of theundoped samples, can be assigned to Yb 3+ . Forthis reason, both undoped samples and Yb 3+ -doped samples were prepared and measured. Acareful analysis of the reflection and luminescencespectra of the doped and undoped samplesprovides information on the CT luminescence ofYb 3+ in the different host lattices.3.1. PhosphatesFig. 2 shows the reflection spectra of ScPO 4 andScPO 4 doped with 3% Yb 3+ . The host latticeabsorption edge is observed at 168 nm. In thispaper the absorption edge is defined as the lowestenergy at which the absorption reaches its maximumvalue (see arrow in Fig. 2). The reflectionspectrum of ScPO 4 doped with 3 mol% of Yb 3+shows the same absorption bands as the undopedsample and in addition a strong absorption bandwith a maximum at 195 nm. This extra absorptioncan be assigned to the 2 F 7/2 ! CT transition of theYb 3+ ion in ScPO 4 . At wavelengths longer than200 nm the signal decreases slowly. The slowFig. 2. Reflection spectrum of the ScPO 4 host lattice (brokenline) and ScPO 4 doped with 3% Yb 3+ (solid line), measuredat 10 K.decrease may be related to the very low intensityof the synchrotron in this wavelength region whichintroduces relatively large errors in the correctedspectrum.Fig. 3 shows the emission and excitation spectraof ScPO 4 and ScPO 4 doped with 1% Yb 3+ . Theemission spectrum of the ScPO 4 host lattice showsseveral broad bands with maxima at 211, 350 and470 nm. The emission at 211 nm can only beexcited with radiation of energies higher than theband gap (l5168 nm). The other emission bandscan also be excited at wavelengths longer than168 nm and are attributed to defect centers in theScPO 4 lattice.If we compare the emission spectrum of ScPO 4doped with Yb 3+ with the spectrum of the hostlattice, two strong extra bands are observed withmaxima at 270 and 370 nm. These bands areassigned to transitions from the charge transferstate to the 2 F 7/2 and 2 F 5/2 state of Yb 3+ . Theenergy separation between the two emission bandsis 10 000 cm 1 , in agreement with the separationbetween the 2 F 5/2 and 2 F 7/2 states of Yb 3+ . Thefull-width at half-maximum (FWHM) of thebands is 6 000 cm 1 . The Stokes shift of the CT! 2 F 7/2 emission is 14 500 cm 1 . The Stokes shiftis the energy difference between the maximum ofthe CT absorption band and the CT ! 2 F 7/2emission band. The CT ! 2 F 7/2 emission band isoften located in a spectral region (250–400 nm)where both SEYA monochromator with detectorand SPEX monochromator with ccd-array have alow sensitivity. For this reason, in this paper theStokes shift is calculated from the position of theCT ! 2 F 5/2 emission minus 10 000 cm 1 , this

L. van Pieterson et al. / Journal of Luminescence 91 (2000) 177–193 181emission is in most cases observed in a spectralregion where the SPEX/ccd sensitivity curve isrelatively flat. Emission due to the transitionbetween the two 4f states can be observed assharp peaks in the infrared at 979 nm.In the excitation spectrum of the Yb 3+ emissionat 270 nm, the host lattice absorption edge isobserved at 168 nm. At 195 nm the 2 F 7/2 ! CTexcitation band is observed. The position of thisexcitation band is in good agreement with thecharge transfer absorption band in the reflectionspectrum.At low temperature, a decay time of 163 25 nshas been measured for the charge transfer luminescence,similar to the decay times measured forYb 3+ charge transfer luminescence in LuPO 4 andYPO 4 [3,4]. To study the temperature quenchingof the charge transfer luminescence, temperaturedependentlifetime measurements and intensitymeasurements were performed. In Fig. 4, theintensity of the 270 nm emission band and thedecay time of this emission are given as a functionof temperature. There is a good agreementbetween the decrease in intensity and decay time.If we define the quenching temperature as thetemperature at which the luminescence intensity orthe decay time have decreased to half of its initialvalue, the quenching temperature of the Yb 3+charge transfer luminescence is estimated to beabout 225 K.Also for Yb 3+ in other orthophosphates luminescence,reflection and lifetime measurementswere performed and compared with the luminescenceof the undoped host lattices. In Table 1, theFig. 3. Emission and excitation spectra of the ScPO 4 hostlattice (a, b) and of ScPO 4 doped with 1% Yb 3+ (c, d) measuredat 10 K. The emission spectra for the ScPO 4 host lattice andYb 3+ -doped sample were both recorded for excitation at140 nm, for the excitation spectrum of the host lattice the emissionwavelength was at 210 nm, for the Yb 3+ -doped sample itwas at 270 nm. In the emission spectra the broken linerepresents measurements performed with a SEYA monochromator,the solid line represents measurements performed with aSPEX monochromator equipped with a Tektronics ccd-array.Fig. 4. Decay time (*), on the left axis, and luminescenceintensity (&), on the right axis, of the charge transferluminescence at 270 nm as a function of temperature forScPO 4 :Yb 3+ (l exc =140 nm).

182L. van Pieterson et al. / Journal of Luminescence 91 (2000) 177–193Table 1Positions of absorption edges and emission bands in the orthophosphatehost lattices REPO 4 (RE=Sc, Lu, Y, La)data obtained for the orthophosphate host latticesare summarized and Table 2 summarizes theresults of the orthophosphates doped with Yb 3+ .In the case of LaPO 4 :Yb 3+ no evidence for Yb 3+charge transfer luminescence could be obtained,indicating that the charge transfer luminescence isquenched already at the lowest temperatures inthis host lattice.3.2. OxidesAbsorption edge (nm)Emission (nm)ScPO 4 168 211, 350, 470LuPO 4 140 Not measuredYPO 4 144 233, 440LaPO 4 153 262, 328Fig. 5 shows the reflection spectrum of LiScO 2and LiScO 2 doped with 3% Yb 3+ . The host latticeabsorption edge is observed at 187 nm. In theYb 3+ -doped sample, an extra absorption isobserved with a maximum at 209 nm. Thisabsorption band is assigned to the charge transferband of Yb 3+ .Fig. 6 shows the emission and excitation spectraof the LiScO 2 host lattice and LiScO 2 doped with3% Yb 3+ . The emission spectrum of the hostlattice shows one broad band at 250 nm whenexciting at 180 nm. In the excitation spectrum theabsorption edge is observed at 186 nm. Whenexciting at longer wavelengths, defect emission isobserved between 350 and 450 nm.In the emission spectrum of LiScO 2 :Yb 3+ twobroad bands are observed at 312 and 448 nmwhich are not present in the spectrum of theundoped lattice. The energy separation betweenthese bands is about 10 000 cm 1 . The FWHM ofthe bands is 6000 cm 1 and the Stokes shift is16 000 cm 1 . At 972 nm the transition between the4f states of Yb 3+ is observed. In the excitationspectrum of LiScO 2 :Yb 3+ the host lattice absorptionedge is observed at 188 nm. At longerwavelengths an extra band is observed with theonset at 225 nm and a maximum at 208 nm, whichcan be assigned to the charge transfer band ofYb 3+ .The decay time of the charge transfer luminescenceis 190 25 ns and the quenching temperatureis about 180 K in LiScO 2 . Table 3 summarizesthe results for the NaREO 2 and LiREO 2 hostlattices. Data on NaYO 2 and LiYO 2 host latticesare lacking due to problems with synthesis. Table 4shows charge transfer luminescence data obtainedfor NaREO 2 and LiREO 2 lattices doped withYb 3+ . In NaLaO 2 and LiLaO 2 no Yb 3+ chargetransfer luminescence was observed, indicating thatFig. 5. Reflection spectrum of LiScO 2 (broken line) and LiScO 2doped with 3% Yb 3+ (solid line) at 10 K.Table 2Positions of the charge transfer emission and absorption bands for Yb 3+ in orthophosphate host lattices. For decay time ðtÞ measurements(at 10 K) and determination of the quenching temperature ðT q Þ, the CT ! 2 F 7/2 emission is monitoredAbsorption max. (nm) Emission (nm) Stokes shift (cm 1 ) tðnsÞ T q ðKÞScPO 4 195 270, 370 14 500 163 225LuPO 4 210 290, 440 15 000 175 250YPO 4 210 304, 445 15 000 120 290LaPO 4 228 } } } }

L. van Pieterson et al. / Journal of Luminescence 91 (2000) 177–193 183Table 3Positions of absorption edges and emission bands in NaREO 2(RE=Sc, La) and LiREO 2 (RE=Sc, La) host latticesAbsorption edge (nm)Emission (nm)NaScO 2 189 400 (very broad)NaLaO 2 242 Not measuredLiScO 2 188 250, 350–450LiLaO 2 229 400Fig. 6. Emission and excitation spectra of the LiScO 2 host lattice(a, b) and LiScO 2 doped with 3% Yb 3+ (c, d), T ¼ 10 K. Thehost lattice emission spectrum was recorded for excitation at180 nm, the excitation spectrum was measured for the 272 nmemission. The emission spectrum of the Yb 3+ -doped sample wasrecorded for excitation at 200 nm, while the excitation spectrumwas measured for the 300 nm emission. In Fig. 6 (c), the brokenline represents measurements performed with a SEYA monochromator,the solid line represents measurements performed witha SPEX monochromator equipped with a Tektronics ccd-array.the CT luminescence is quenched in these hostlattices.The reflection spectra of Y 2 O 3 and Y 2 O 3 dopedwith 3% Yb 3+ are shown in Fig. 7. The hostlattice absorption edge is observed at 208 nm, inagreement with results by Tomiki et al., whoobserved the exciton creation at 5.88 eV (210 nm)[17]. In the Yb 3+ -doped Y 2 O 3 the charge transferabsorption band is observed at 227 nm.Fig. 8 shows the emission and excitation spectraof the Y 2 O 3 host lattice and Y 2 O 3 :Yb 3+ . Hostlattice emission is observed at 370 nm. In theexcitation spectrum, the host lattice excitationedge is observed at 206 nm.The emission spectrum of Y 2 O 3 doped with 3%Yb 3+ shows two broad bands with maxima at 368and 533 nm. The energy separation between thesebands is 8500 cm 1 , which is less than the energyseparation of 10 000 cm 1 between the 2 F 5/2 and2 F 7/2 states of Yb 3+ . This may be caused byunderlying host lattice defect emission and by thefact that the emission spectra are not corrected.The FWHM of the bands is 7000 cm 1 . At 995 nm,the transition between the 2 F 5/2 and 2 F 7/2 state canbe observed. In the excitation spectrum, an extraband is observed with a maximum at 228 nm, ingood agreement with the position of the chargetransfer band in the reflection spectrum. TheStokes shift is 15 000 cm 1 . In Table 5, data onluminescence properties of Yb 3+ -doped and undopedoxide lattices are summarized.In addition to the oxides discussed above, welooked for Yb 3+ CT luminescence in LaYO 3 .Inthis lattice the host lattice absorption edge is observedat 206 nm. No indication for CT luminescencefrom Yb 3+ was found, not even at thelowest temperatures.

184L. van Pieterson et al. / Journal of Luminescence 91 (2000) 177–193Table 4Positions of the charge transfer emission and absorption bands for Yb 3+ in NaREO 2 and LiREO 2 host lattices. For decay time (t)measurements (at 10 K) and determination of the quenching temperature ðT q Þ, the CT ! 2 F 7/2 emission is monitoredAbsorption (nm) Emission (nm) Stokes shift (cm 1 ) tðnsÞ T q ðKÞNaScO 2 208 322, 431 15 000 130 225NaLaO 2 262 } } } }LiScO 2 206 312, 448 16 000 190 180LiYO 2 214 355, 495 16 500 55 125LiLaO 2 252 } } } }Fig. 7. Reflection spectra of Y 2 O 3 (broken line) and Y 2 O 3doped with 3% Yb 3+ (solid line), measured at 10 K.3.3. AluminatesThe reflection spectra of the Y 3 Al 5 O 12 (YAG)host lattice and YAG doped with 2% Yb 3+ areshown in Fig. 9. At 178 nm the host latticeabsorption edge is observed. When Yb 3+ is incorporatedin the lattice, an extra absorption band isobserved at 209 nm, which is assigned to thetransition from the 2 F 7/2 ground state of Yb 3+ tothe charge transfer state.Fig. 10 shows the emission and excitationspectra of YAG and YAG : Yb 3+ . In the emissionspectrum of undoped YAG, host lattice luminescenceis observed at 256 nm. When exciting atlonger wavelengths defect emission around 450 nmis observed. In the Yb 3+ -doped samples strongemission bands are observed at 334 and 493 nmwhich are not present in the emission spectrum ofthe undoped sample. The energy separationbetween the bands is close to 10 000 cm 1 . Basedon these observations, the bands are assigned toCT luminescence from Yb 3+ . In the excitationspectrum of the 343 nm emission, the CT excitationband is observed at 210 nm, in agreement withthe position in the reflection spectrum. TheStokes shift of the charge transfer transition issome 17 500 cm 1 and the FWHM of both bandsis 6000 cm 1 . The decay time measured for theYb 3+ charge transfer luminescence is 30 5ns at10 K, much shorter than the decay times which aretypically observed for the Yb 3+ CT emission100–200 ns) at low temperatures. Probably, thecharge transfer luminescence is already partlyquenched at very low temperatures. The relativelyhigh intensity of the 2 F 5/2 ! 2 F 7/2 transitioncompared to the charge transfer luminescence alsosuggests that the CT emission is already partlyquenched at 10 K.In Fig. 11, the CT luminescence intensity and itsdecay time are given as a function of temperature.The immediate decrease in intensity in the lowtemperatureregion confirms that the luminescenceTable 5Positions of emission and absorption bands for undoped andYb 3+ -doped oxides RE 2 O 3 (RE=Sc, Y). For the undopedoxides, the absorption edge is given, for the Yb 3+ -doped oxidesthe CT absorption maximum is given. For decay time ðtÞmeasurements and determination of the quenching temperatureðT q Þ, the CT ! 2 F 7/2 emission is monitoredAbsorption(nm)Emission(nm)Stokesshift(cm 1 )tðnsÞ T q (K)Sc 2 O 3 196 Not measuredY 2 O 3 208 370Sc 2 O 3 :Yb 3+ 225 367, 485 14 000 13 530(very weak)Y 2 O 3 :Yb 3+ 227 368, 533 15 500 72 130

L. van Pieterson et al. / Journal of Luminescence 91 (2000) 177–193 185Table 6Positions of the emission and absorption bands for aluminatehost lattices and Yb 3+ -doped aluminates. For the undopedaluminates, the absorption edge is given, for the Yb 3+ -dopedaluminates the CT absorption maximum is givenAbsorption(nm)Emission(nm)Stokesshift(cm 1 )tðnsÞ T q ðKÞYAG 178 260, 460YAlO 3 158 Not measuredLaAlO 3 211 280, 600YAG : Yb 3+ 210 334, 493 17 500 28 580YAlO 3 :Yb 3+ 235 360, 533 14 000 100 100LaAlO 3 :Yb 3+ 244 } } } }Fig. 9. Reflection spectra of YAG (broken line) and YAGdoped with 2% Yb 3+ (solid line), measured at 10 K.Fig. 8. Emission (for l exc ¼ 170 nm) and excitation (forl em ¼ 370 nm) spectra of the Y 2 O 3 host lattice (a, b) andemission (for l exc ¼ 215 nm) and excitation (for l em ¼ 372 nm)spectra of Y 2 O 3 doped with 3% Yb 3+ (c, d), T ¼ 10 K.is already quenched at very low temperature.From this figure we can estimate that the quenchingtemperature is lower than 80 K.Fig. 12 shows the emission and excitationspectra of YAlO 3 doped with 2% Yb 3+ . Thebroad emission bands with maxima at 360 and533 nm are assigned to CT transitions of Yb 3+ .InFig. 13, the decay curves of the CT emissionsmeasured at 350 and 516 nm are shown. The decaytime of these emissions is 100 10 ns. In theexcitation spectrum of the 350 nm emission theCT band can be observed at 235 nm.In LaAlO 3 no evidence for CT luminescencecould be obtained. Table 6 summarizes theluminescence properties of the undoped andYb 3+ -doped aluminates.3.4. OxysulfidesThe charge transfer absorption for Yb 3+ in theoxysulfides is at relatively low energies andluminescence spectra could be recorded with the

186L. van Pieterson et al. / Journal of Luminescence 91 (2000) 177–193Fig. 11. Decay time (*), on the left axis, and CT luminescenceintensity (&), on the right axis, of YAG doped with 2% Yb 3+as a function of temperature.Fig. 12. Emission (a, l exc ¼ 220 nm) and excitation (b,l em ¼ 350 nm) spectra of YAlO 3 doped with 2% Yb 3+ atT ¼ 10 K.Fig. 10. Emission (for l exc ¼ 160 nm) and excitation (forl em ¼ 255 nm) spectra of the YAG host lattice (a, b) andemission (for l exc ¼ 170 nm) and excitation (for l em ¼ 345 nm)spectra of YAG doped with 2% Yb 3+ (c, d). In Fig. 10 (c), thebroken line represents measurements performed with a SEYAmonochromator, the solid line represents measurements performedwith a SPEX monochromator equipped with aTektronics ccd-array.Fig. 13. Decay curves (at 10 K) of the Yb 3+ CT emissions at350 (dotted) and 516 nm (broken line) in YAlO 3 .

L. van Pieterson et al. / Journal of Luminescence 91 (2000) 177–193 187Fig. 15. Decay time (*), on the left axis, and CT luminescenceintensity at 420 nm (&) and at 822 nm (n), on the right axis, ofLa 2 O 2 S doped with Yb 3+ as a function of temperature.SPEX 1680 spectrofluorometer. Fig. 14 shows theemission and excitation spectra of Y 2 O 2 S:Yb 3+and La 2 O 2 S:Yb 3+ at 10 K. In Y 2 O 2 S, Yb 3+charge transfer luminescence is observed at 390and 612 nm. The energy difference between theCT bands is 9500 cm 1 , close to the expected10 000 cm 1 . The FWHM of the bands is about3500 cm 1 . In the excitation spectrum the chargetransfer band is observed at 309 nm so the Stokesshift is relatively small, about 6000 cm 1 . Thedecay time measured at 390 nm is about 250 ns.The Y 2 O 2 S host lattices luminesce at 318 nm whenexciting into the conduction band (l5260 nm).This emission quenches already below 50 K. Also abroad defect-related emission around 590 nm isobserved which quenches at about 275 K.In La 2 O 2 SYb 3+ CT luminescence is observed at439 and 759 nm, the energy difference betweenthese bands is 9500 cm 1 . The CT excitation bandis observed at 317 nm, giving a Stokes shift ofapproximately 8500 cm 1 . In Fig. 15, the intensityof the Yb 3+ CT luminescence in La 2 O 2 S and itsdecay time are given as a function of temperature.From this figure we can estimate the quenchingtemperature at 150 K. Table 7 summarizes theluminescence properties of the oxysulfide hostlattices and Yb 3+ -doped samples.Fig. 14. Emission (for l exc ¼ 240 nm) and excitation (forl em ¼ 393 nm) spectra of Y 2 O 2 S doped with 1% Yb 3+ (a, b)and emission (for l exc ¼ 270 nm) and excitation (for l em ¼460 nm) spectra of La 2 O 2 S (c, d) doped with 1% Yb 3+ ,T ¼ 10 K.3.5. FluoridesFig. 16 shows the emission and excitationspectrum of LiYF 4 doped with 3% Yb 3+ . Theemission spectrum has been recorded using gateddetection (delay 5 ns, gate 10 ns) and shows two

188L. van Pieterson et al. / Journal of Luminescence 91 (2000) 177–193Table 7Positions of emission and absorption bands for oxysulfide host lattices and for Yb 3+ -doped RE 2 O 2 S (RE=Y, La) host lattices. Forthe undoped lattices, the absorption edge is given, for the Yb 3+ -doped lattices the CT absorption maximum is given. For decay timeðtÞ measurements (at 10 K) and determination of the quenching temperature ðT q Þ, the CT ! 2 F 7/2 emission is monitoredAbsorption edge (nm) Emission (nm) Stokes shift (cm 1 ) t (ns) T q (K)Y 2 O 2 S 263 318, 590 550 resp 275La 2 O 2 S 276 377, 560 60 resp 250Y 2 O 2 S:Yb 3+ 309 390, 612 6000 250 140La 2 O 2 S:Yb 3+ 317 439, 759 8500 223 150band between 148 and 163 nm observed bySzcezurek and Schlesinger for Yb 3+ in CaF 2 [18],which could be assigned to the CT transition ofYb 3+ .3.6. Other latticesFig. 16. Emission (a, l exc ¼ 155 nm) and excitation (b, l em ¼181 nm) spectra of LiYF 4 doped with 3% Yb 3+ at 10 K.very weak bands with maxima at 182 and 228 nm.The energy separation between both bands is11 000 cm 1 . The two emission bands have thesame excitation spectrum and also the decay timeis approximately the same: 13 2 ns. Therefore,these bands are assigned to transitions from thecharge transfer state to the 2 F 7/2 resp. 2 F 5/2 state ofYb 3+ . The fact that the emission is very weak andthe decay time is short suggests that the luminescenceis already quenched at very low temperatures.At 250 nm, the onset of the host latticeemission band can be observed.In the excitation spectrum, the charge transferband is observed at 159 nm, so the Stokes shift isca. 8000 cm 1 . The position of the CT excitationband is in agreement with the broad absorptionIn the previous sections the charge transferluminescence of Yb 3+ in many compounds hasbeen described. In view of the very limited numberof compounds in which the CT luminescence ofYb 3+ has been reported before, a large numberof compounds has been added. Yet, it is notsurprising that until now the number of compoundsin which CT luminescence from Yb 3+ hasbeen reported, is so small. In many compounds inwhich CT luminescence is observed, the Yb 3+ CTluminescence has a quenching temperature belowroom temperature. In many other compounds noCT luminescence is observed at all, not even at4 K. In addition to the compounds discussed inthe previous sections, we looked for Yb 3+ CTluminescence in the orthoborates REBO 3(RE=Sc, Y, La) and the oxyhalogenides REOX(RE=Y, La; X=F, Cl, Br) by comparing theluminescence spectra of undoped host lattices andYb 3+ -doped samples. For none of the borates oroxyhalogenides evidence for Yb 3+ CT luminescencewas obtained, indicating that the luminescenceof Yb 3+ is already quenched at 10 K in thesehost lattices.3.7. Comparison with Eu 3+For comparison, the luminescence of Eu 3+under charge transfer excitation was measured

L. van Pieterson et al. / Journal of Luminescence 91 (2000) 177–193 189Table 8Position of the Eu 3+ and Yb 3+ charge transfer band and quenching temperature of the 5 D 0 ! 7 F J luminescence after excitation in thecharge transfer band resp. the quenching temperature of the Yb 3+ CT luminescenceEu 3+ CT excitation band (nm) Eu 3+ T q (K) Yb 3+ CT absorption band (nm) Yb 3+ T q (K)ScPO 4 205 760 a 195 225LuPO 4 215 850 a 210 250YPO 4 218 >850 210 290LaPO 4 251 400 228 510NaScO 2 225 650 208 225NaLaO 2 264 320 262 510LiScO 2 223 >850 206 180LiYO 2 247 600 214 125LiLaO 2 274 475 252 510Y 2 O 3 250 >850 227 130Y 2 O 2 S 334 725 310 140La 2 O 2 S 347 480 317 150YAG 237 >850 212 580LaAlO 3 306 500 244 510ScBO 3 218 420 } 510YBO 3 228 >850 216 510LaBO 3 275 350 } 510a The luminescence did not quench completely at high temperatures, but stayed at more than a quarter of the maximum intensity.for all host lattices described above. In Table 8, theposition of the Eu 3+ charge transfer band and thequenching temperature of the 5 D 0 ! 7 F J luminescencewhen excited in the Eu 3+ charge transferband, are given for all lattices. For some lattices, inTable 8 marked with a , the luminescence did notquench completely at high temperatures but stayedat more than a quarter of the maximum intensity,although the quenching curve of the Eu 3+luminescence had the usual S-shape.4. Discussion4.1. Charge transfer absorptionIf we compare the position of the Yb 3+ chargetransfer absorption bands in the lattices weinvestigated, we can identify three classes ofcompounds. In LiYF 4 :Yb 3+ charge transferabsorption takes place at very high energy(7.5 eV), in the oxidic lattices (phosphates,oxides and aluminates) at lower energies(5.5 eV) and in the oxysulfides, charge transferabsorption takes place at lowest energies (3 eV).Jrgensen [2] has given an expression to estimatethe position of charge transfer transitions, viz.s ¼½wðXÞwðMÞŠ 30 kK:Here s gives the position of the charge transferband in kK (1000 cm 1 ), wðXÞ the optical electronegativityof the anion and wðMÞ that of thecentral metal ion. Using wðFÞ ¼3:9 [19] andwðOÞ ¼3:2 [20], we can estimate the value of w(Yb)at 1.68, somewhat higher than the value given byJrgensen (1.6) [21]. For Eu 3+ w has been determinedto be about 1.75 [21]. The slightly lowervalue for Yb 3+ is in agreement with the observationof Jrgensen that w uncorr varies in the orderEu(III)>Yb (III)>Sm (III). When for S we usewðSÞ ¼2:8 [19], the calculated positions of thecharge transfer bands of the S 2 –RE 3+ , O 2 –RE 3+ and F –RE 3+ transitions are: s(Yb 3+ –S 2CT) ffi 33 000 cm 1 , s(Eu 3+ –S 2 CT) ffi30 000 cm 1 ,s(Yb 3+ –O 2 CT) ffi 45 000 cm 1 , s(Eu 3+ –O 2CT) ffi 42 000 cm 1 , s(Yb 3+ –F CT) ffi 66 000 cm 1and s(Eu 3+ –F CT) ffi 63 000 cm 1 . These calculatedvalues are in good agreement with theexperimental values. The shift of the CT bandof about 3000 cm 1 to higher energies for Yb 3+in comparison to Eu 3+ is in agreement withthe average value in our observations, although

190L. van Pieterson et al. / Journal of Luminescence 91 (2000) 177–193there is a spread in the experimentally observeddifferences.The position of the CT band is not onlydetermined by the nature of the ligand and themetal ion. Also the coordination and the size ofthe cation site influence the energy of the CTtransition. In isostructural host lattices the Yb 3+and Eu 3+ charge transfer absorption band shiftsto longer wavelengths when the rare earth ion isincorporated on a larger cationic site. Forexample, in ScPO 4 (cation radius is 0.87 A˚ forVIII-coordination), the Yb 3+ charge transfer bandis located at 195 nm, whereas in LaPO 4 (cationradius is 1.18 A˚ for VIII-coordination) the Yb 3+charge transfer band is observed at 228 nm. Theshift to lower energies is in agreement with thework of Hoefdraad who showed that for VIII andXII coordination the position of the Eu 3+ chargetransfer band in oxides shifts to longer wavelengthswhen the RE–O distance increases [22].4.2. Charge transfer luminescenceIn a number of host lattices Yb 3+ chargetransfer luminescence can be observed, i.e., inorthophosphates, in oxides, in oxysulfides, inaluminates and in LiYF 4 . The Yb 3+ chargetransfer luminescence observed in LiYF 4 is thehighest energetic charge transfer luminescenceobserved till now as far as we are aware. In allcases the CT emission bands are broad and theStokes shift is large, between 6000 and 8000 cm 1(in oxysulfides and LiYF 4 ) and up to 17 000 cm 1in the aluminates. The observation of large Stokesshifts and broad bands is expected for chargetransfer transitions. Often, the charge transfertransition is described as the transfer of an electronfrom the ligands to the central metal ion. Inreality, the transition will not involve the transferof one electron, but it certainly involves aconsiderable reorganization of the charge densitydistribution around the metal ion. This reorganizationis accompanied by an expansion of themetal-ligand bonds in the excited state, whichgives rise to the observation of large Stokes shiftsand broad bands.Also the size of the cationic site for which Yb 3+substitutes is expected to influence the Stokes shift.Smaller Stokes shifts are expected for Yb 3+ onsmaller sites. This is indeed observed. When welook at the orthophosphates, the lithium rare earthoxides and the oxysulfides, for which Yb 3+ chargetransfer luminescence is measured as a function ofthe size of the cation site, we see that also thecharge transfer emission shifts to longer wavelengthswith increasing cation size. From Sc to La,the radius increases: 0.87 A˚ for Sc 3+ , 0.97 A˚ forLu 3+ , 1.01 A˚ for Y 3+ and 1.18 A˚ for La 3+ ineightfold coordination [23]. The size of the siteavailable to the Yb 3+ ion influences the relaxationin the charge transfer state. When Yb 3+ isincorporated in a lattice on a larger cationic site,the relaxation in the excited charge transfer state islarger and therefore the Stokes shift is larger.Estimation of the Stokes shift of the chargetransfer luminescence (ScPO 4 , LuPO 4 and YPO 4 ,resp. 14 300, 14 900 and 15 100 cm 1 , LiScO 2 andLiYO 2 , resp. 16 200 and 16 500 cm 1 ,Y 2 O 2 S andLa 2 O 2 S, resp. 6000 cm 1 and 8500 cm 1 ) seems toconfirm that relaxation in the excited state is largerwhen Yb 3+ has been incorporated on a largercationic site. However, the Stokes shifts have beenestimated from uncorrected (emission) spectra,and therefore the error may be large. In Fig. 17,the influence of the size of the cation site for Yb 3+on the position of the charge transfer state and therelaxation (Stokes shift) is represented in aconfiguration-coordinate diagram.The radiative lifetime of the Yb 3+ CT emissionis typically between 100 and 200 ns. This is longerthan what one would expect for a fully allowedtransition. In view of the large change in theelectronic configuration, a large transition dipolemoment is expected. Typically, lifetimes of theorder of 10–20 ns are measured for such fullyallowed transitions. To understand why the presentlyobserved lifetimes are typically 10 timeslonger, one would need to calculate the electronicconfiguration of the ground state and the excitedcharge transfer state and determine the transitiondipole moment. Without these calculations, onecan only speculate about the reason for the relativelong lifetime. For example, the wave functionoverlap between the ‘hole’ on the ligands and theelectron on the metal ion may be small due todelocalization of the hole over the ligands. This

L. van Pieterson et al. / Journal of Luminescence 91 (2000) 177–193 191Fig. 17. Configurational-coordinate diagrams for Yb 3+ indifferent types of host lattices. For Yb 3+ on a larger cationsite (e.g. La 3+ ) the CT state shifts to lower energies and therelaxation in the excited state is larger, yielding a larger Stokesshift and lower quenching temperatures, or even quenching atthe lowest temperatures by fast cross-over (curved arrow).would result in a smaller transition probability forthe emission from the CT state.4.3. Quenching mechanismBesides the position of the charge transferabsorption and emission bands, it is interestingto discuss the quenching of the charge transferluminescence. Based on a simple configurationcoordinatemodel, one would expect the lowestquenching temperature for Yb 3+ incorporated onthe largest site. Due to the large relaxation in theexcited state, quenching by fast cross-over to theground state can occur (see also Fig. 17). Indeed,in most lattices in which Yb 3+ occupies the site ofthe La 3+ -ion, charge transfer luminescence isquenched at the lowest temperatures. An exceptionis La 2 O 2 S, which also has a relative small Stokesshift (8500 cm 1 ). However, not in all lattices thebehavior for the quenching temperature of thecharge transfer luminescence is in agreement withwhat is expected on the basis of the simpleconfiguration-coordinate diagram. For example,in the orthophosphates, the Yb 3+ charge transferluminescence in ScPO 4 (smallest site) is quenchedat lower temperatures (225 K) than in the LuPO 4(250 K) and YPO 4 (290 K) host lattices. The sametrend is observed for Eu 3+ in the orthophosphates.The reason for the increase of T q for Yb 3+ orEu 3+ on larger sites is not clear. It may be due to astrong distortion of the local environment of Yb 3+on the much smaller Sc 3+ site. Another explanationmay be that the quenching mechanism is notthermally activated cross-over from the excitedstate to the 4f 13 ground state, but photoionization.If this is the case, the position of the chargetransfer state in the band gap determines thequenching temperature and this is hard to predict.The CT excited state may very well be more stablefor Yb 3+ on a larger site, which would give rise toa higher quenching temperature. Thermally activatedphotoionization is well known for4f n ! 4f n1 5d (fd) transitions on lanthanides. Inthe 4f n1 5d state the 5d electron is delocalized overthe ligands and if the fd state is close to the edge ofthe conduction band (CB), the electron can bethermally excited from the fd state to the conductionband and the fd emission is quenched. Insome cases (e.g. BaF 2 :Eu 2+ and SrF 2 :Yb 2+ ) thelowest fd excited state is situated above the CBedge and no fd emission is observed at all [24,25].In Fig. 18(a), this well-known situation of thermallyactivated photoionization is depicted schematically.For Yb 3+ the photoionization processis expected to be different. The (excited) chargetransfer state corresponds to a situation in which ahole is delocalized over the ligands [26]. Theenergy of the hole will be close to the energy fora hole in the valence band (which usually has alarge contribution of the 2p wave function ofligands like O 2 and F ). Depending on the energyof the excited CT state, the hole can escape atelevated temperatures to the valence band (VB)and the CT luminescence is quenched [27]. InFig. 18(b), the suggested situation for thermallyactivated photoionization from a CT state of alanthanide is depicted. In host lattices in which noCT luminescence from Yb 3+ is observed at all,photoionization may occur without thermal activation(CTS in VB) and quenching already occursat the lowest temperatures. In Fig. 18, the energy

192L. van Pieterson et al. / Journal of Luminescence 91 (2000) 177–193Fig. 18. Thermal quenching due to photoionization for a4f n ! 4f n1 5d transition (a) and for a CT transition (b). VB andCB are the valence band and conduction band, respectively,E e and E h represent the energy of an electron and a hole,respectively, RE=rare earth ion and L=ligand. The position ofthe excited CTS for Eu 3+ (broken line in (b)) is at slightly lowerenergies than for Yb 3+ (drawn line in (b)).levels are shown in an energy diagram as iscommonly used for semiconductors. In this pictureinformation on lattice relaxation in the excitedstate is lost. An alternative picture for photoionizationfrom a charge transfer state has beenused in Refs. [24,25], where a configurationalcoordinate diagram like Fig. 17 is shown. Thecharge transfer state, the ionized state and the freeelectron–hole pair are represented by parabolaewith a large (CT) or small (electron–hole) off-setwith respect to the ground state. In this representationthe information on relaxation is evident, butthe mobility of the charge carriers (holes in the VBor electrons in the CB) is less evident. Regardlessof which picture is used, one should always beaware of the limitations in each representation,especially when both localized and delocalizedprocesses are shown in one picture.Evidence supporting the explanation of quenchingby thermally activated photoionization comesfrom the results on the quenching of the Eu 3+emission under CT excitation. The Eu 3+ emission( 5 D 0 ! 7 F J ) under charge transfer excitationquenches at much higher temperatures. The lowerquenching temperature for the CT luminescencefrom Yb 3+ can be related to the lower stability ofYb 2+ compared to Eu 2+ (the most stable divalentlanthanide). In the excited CT state the lanthanideis (formally) 2+ and due to the lower stability ofYb 2+ the CTS will be closer to the valence bandand quenching due to photoionization can beexpected at lower temperatures.The order in quenching temperatures is similarfor Yb 3+ and Eu 3+ . In the host lattices whereEu 3+ is incorporated on a La 3+ -site, the quenchingtemperature is much lower than in latticeswhere Eu 3+ occupies a Sc 3+ or Y 3+ site. This is inagreement with the low-quenching temperature forYb 3+ in these lattices. In the host lattices with veryhigh-quenching temperatures for the Eu 3+ emissionunder charge transfer excitation, Yb 3+ CTluminescence is observed, but has a quenchingtemperature below room temperature. An exceptionis YBO 3 . In this lattice, the Eu 3+ luminescenceafter CT excitation quenches at very hightemperature (>850 K), but no Yb 3+ CT luminescenceis observed at all. In general, CT luminescencefrom Yb 3+ is only observed in host latticesin which the quenching temperature for Eu 3+ ishigher than about 500 K.An alternative explanation for the higherquenching temperatures of Eu 3+ compared toYb 3+ is that for Yb 3+ radiative decay from thecharge transfer state (t 150 ns) has to competewith non-radiative decay to the 2 F 5/2 and 2 F 7/2states. In Eu 3+ , fast non-radiative decay (t ps)to the 5 D states has to compete with non-radiativedecay to the ground state 7 F levels, and once the5 D 0 state is populated, radiative decay occurs. Atpresent, new experiments are planned to obtainbetter insight in the quenching mechanism.5. ConclusionsThe charge transfer luminescence of Yb 3+ hasbeen investigated in a large variety of host lattices.In some host lattices (LiYF 4 , orthophosphates,aluminates, oxides and oxysulfides) charge transferluminescence has been observed, while in otherhost lattices (orthoborates, oxyhalogenides) thecharge transfer luminescence is quenched even at10 K. Besides quenching due to cross-over fromthe CT excited state to the ground state, alsoindications for quenching by (thermally activated)photoionization have been obtained. Most likely,

L. van Pieterson et al. / Journal of Luminescence 91 (2000) 177–193 193the photoionization involves the escape of a holefrom the charge transfer state to the valence band.The position of the charge transfer absorptionand emission bands shifts to longer wavelengthswith increasing covalency of the host lattice andwith increasing size of the cation site. The radiativelifetime of the charge transfer emission is typicallybetween 100 and 200 ns, which is relatively long fora fully allowed transition. A comparison has beenmade with the luminescence of Eu 3+ . The chargetransfer absorption band is higher in energy forYb 3+ than for Eu 3+ in the same host lattice. Thereis a clear relation between the quenching temperatureof the Eu 3+ emission under CT excitation andthe CT emission from Yb 3+ . Only for host latticesin which the quenching temperature of the Eu 3+emission is higher than about 500 K charge transferluminescence from Yb 3+ could be observed,but this emission is already quenched below roomtemperature.AcknowledgementsThe authors are grateful to Dr. P. Gu¨ rtlerand Mr. S. Petersen from HASYLAB for theopportunity to use the excellent facilities for VUVspectroscopy at the DESY synchrotron, Hamburg(Germany) and their support whenever needed.The authors are also grateful to Mr. E. Mix andDr. S. Ku¨ ck of the ‘Institut fu¨ r Laser-physik’ of theUniversity of Hamburg for supplying the RE 2 O 3 ,YAlO 3 and YAG crystals. The financial support ofPhilips Lighting is gratefully acknowledged.References[1] G. Blasse, B.C. Grabmaier, Luminescent Materials,Springer, Berlin, 1994.[2] C.K. Jrgensen, Progr. Inorg. Chem. 12 (1970) 101.[3] E. Nakazawa, Chem. Phys. Lett. 56 (1978) 161.[4] E. Nakazawa, J. Lumin. 18/19 (1979) 272.[5] E. Danielson, M. Devenney, D.M. Giaquinta, J.H. Golden,R.C. Haushalter, E.W. McFarland, D.M. Poojary,C.M. Reaves, W.H. Weinberg, X. Di Wu, Science 279(1998) 837.[6] A.T. Aldred, Acta Crystallogr. B 40 (1984) 569.[7] G. Chadeyron, M. El-Ghozzi, R. Mahiou, A. Arbus,J.C. Cousseins, J. Solid State Chem. 128 (1997) 261.[8] M. Kottaisamy, R. Jagannathan, Ravi P. Rao,M. Avudaithai, L.K. Srinivasan, V. Sundaram, J. Electrochem.Soc. 142 (1995) 3205.[9] R.W.G. Wyckoff, Crystal Structures, 2nd Ed., vol. 1–3,Wiley, New York, 1960.[10] G. Blasse, J. Inorg. Nucl. Chem. 28 (1966) 2444.[11] F. Stewner, R. Hoppe, Z. Anorg. Allgem. Chem. 380(1971) 250.[12] R.E. Thoma, C.F. Weaver, H.A. Friedman, H. Insley,L.A. Harris, H.A. Yakel Jr., J. Phys. Chem. 65 (1961)1096.[13] L. Fornaseiro, E. Mix, V. Peters, K. Petermann, G. Huber,Cryst. Res. Technol. 34 (1999) 255.[14] E. Mix, L. Fornasiero, V. Peters, K. Petermann, G. Huber,Proceedings of the Twelfth International Conference onCrystal Growth, July 26–31, Jerusalem, 1998.[15] U. Hahn, N. Schwentner, G. Zimmerer, Nucl. Instrum.and Meth. 152 (1978) 261.[16] R.T. Wegh, H. Donker, A. Meijerink, Phys. Rev. B 56(1997) 13 841.[17] T. Tomiki, T. Shikenbaru, Y. Ganaha, T. Futemma,H. Kato, M. Yuri, H. Fukutani, T. Miyahara, S. Shin,M. Ishigame, J. Tamashiro, J. Phys. Soc. Japan 61 (1992)2951.[18] T. Szcezurek, M. Schlesinger, in: B. Jezowska-Trzebiatowska, J. Legendziewic, W. Strek (Eds.), RareEarths Spectroscopy, World Scientific, Singapore, 1985,p. 309.[19] C.K. Jrgensen, Modern Aspects of Ligand-Field Theory,North-Holland, Amsterdam, 1971.[20] N. Vugt, T. Wigmans, G. Blasse, J. Inorg. Nucl. Chem. 35(1973) 2602.[21] C.K. Jrgensen, Molec. Phys. 5 (1962) 271.[22] H.E. Hoefdraad, J. Solid State Chem. 15 (1975) 175.[23] R.D. Shannon, C.T. Prewitt, Acta Crystallogr. B 25 (1969)925.[24] C. Dujardin, B. Moine, C. Pedrini, J. Lumin. 54 (1993)259.[25] B. Moine, B. Courtois, C. Pedrini, J. Phys. (France) 50(1989) 2105.[26] T. Hoshina, S. Imanaga, S. Yokono, J. Lumin. 15 (1977)455.[27] C.W. Struck, W.H. Fonger, Phys. Rev. B 4 (1971) 22.