Charge Transfer Luminescence of Yb3+

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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
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Charge Transfer Luminescence of Yb3+

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