Some considerations on rare-earth activated phosphors

Some considerations on rare-earth activated phosphors


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JOURNAL OF LUMINESCENCE 1,2(1970) 766-777 © North-Holland Publishing Co.. Amsterdam

SOME CONSIDERATIONS ON RARE-EARTH ACTIVATED PHOSPHORS 0. BLASSE Philips Rnewrh Laboratories, N. V. PhilLos’ Glaeilampenfabrleken, EbuThovea, Netheriwuls

A review is given of some or the investigations on rarc~earthactivated phosphors carried out in the years 1966-1969. I. imt~ Nobody will doubt that an enormousamount ofwork has been performed on rare-earth activated materials since the Budapest meeting in l966. The present review can therefore only be limited. We will restrict ourselves to two problems: (a) how rare-earth ions can be excited in broad and strong excitation bands (4f-4f excitation bands will be neglected) and (b) how the excited rare-earth ion can lose its energy to other centres. 2. ExcItation mecbSmna 2.1. DIRECT EXCITATION OF RARE-EARTH IONS A number of rare-earth ions can be excited in broad cxcitation bands that correspond to transitions with high absorption strength belonging to the fluorescent centre itself. Two types of transitions can be distinguished. viz, a charge transfer transition from ligands (usually 02- ions) to central R.E. ion and intraionic 4f-5d transitions. The former will be found for R.E. ions that are easy to reduce, the latter is more common for R.E. ions that are easy to oxidize. In oxygen-containing host lattices with an absorption edge at wavelengths shorter than some 230 nm the Eu3 + ion can be excited in such a charge transfer band (fig. I). The position of this band depends strongly on the host lattice. As is to be expected. the band is at higher energies, if the potential field at the 02- ion due to the surrounding cations is higher’). It is found that the quantum efficiency for excitation in the charge transfer band increases at the same time. This must be due to a decreasing amount of radiationless deactivation of the charge-transfer level, since the quantum J.I












3t Curve E Fig. 1. Excitation of LiYO presents the excitation spectrum 2-Eu in of the the charge-transfer red Eu3~emission, absorption curves R band the of diffuse Eu reflection spectra of unactivated LiYO 2 (broken line) and LiYO2-Eu (full line).

3~ion is always efficiency high2). for excitation into the narrow 4f levels of the Eu A similar type of excitation has been observed for Sm3~3)• For a given host lattice the Sm3 + band is situated at a roughly 9.500cm~1higher position than the Eu3~band. Ions that can be excited in 4f-5d absorption bands are Tb34, Ce34 and Eu2 ~ The characteristics of these bands differ from those of charge-transfer bands. First the 4f-5d bands are narrower than charge-transfer bands4). Secondly we usually observe a number of 4f-5d bands due to crystal-field splitting of the excited d level. For Y 3~this is shown in fig. 2. 3Al5O12-Ce As expected, the splitting in the case of Y 3~is similar but 3Al5O12-Tb situated at higher energies. This has been found for Ce3~and Tb34 in a number of host lattices5). Activators with 4f-5d transitions show only high quenching temperatures










/,—.-.‘\J /\ I


~0~IJ~j 200




I 500

400 —p-





Fig. 2. Excitation and absorption bands of Y 3~ion. Full line presents 3A15012-Ce the excitation corresponding spectrum to the of the 5d crystalyellow emission, field components broken lines of thethe Ce diffuse reflection spectra of unactivated Y 3A15012 (straight line) and Y3A15012-Ce.


C;. aL*ns


of their emission (and therefore high efficiencies at room temperature) if they are introduced into lattices containing small ions with high charge (for example. silicates (Y2Si207-Ce3t). phosphates (Sr2P207-Eu2’) and borates (lnBO 3’). Lattices of this type will oppose changes of the 3-Tb of the luminescent centre upon excitation. This results equilibrium distance in a high quenching temperature6). In Tb3~phosphors the excitation energy is transferred from the excited 3d level to the lower 4f levels so that narrow line emissions with decay times of the order of milliseconds result. In Eu2’ and Ce3’ phosphors, however. the excited 3d level is deactivated radiatively with very short decay time (broad band emission). This is due to the fact that the excited 4f levels of Eu2’ are situated above the lowest component of the 3d level, whereas in the case of Ce34 no higher 4f levels are present. 2.2. INDIRECT EXCITATION OF R.E. IONS

The excitation energy can also be absorbed by absorbing centres different from the rare-earth ions. These centres may be intentionally added ions or host-lattice constituents themselves. There is no essential difference between these two possibilities. Examples ofthe first possibility include the sensitization ofTh3t emission by Cu’ (in Ca(P0 7)). Sn2’ (in LiSrPO 3’ (in Ca(PO 3)2YAI glass 4 8)) Ce 3)2 7)) and Bi3’ (in 3’ emission glass 3B4O,2 9)), and the sensitization of Eu by Bi3 + (in Y 3’ II)) 203 ~O) and YAI3B4O,2 9)) and the uranyl group (Cs2 U02C14EuHost-lattice excitation followed by energy transfer from host lattice groups to the rare-earth activator has especially been studied for Eu3’. Examples are YVO 4-Eu. Y2W06-Eu and YNbO4-Eu. Remarkably energy 3 + isenough, not efficient. transfer of this type from host-lattice groups to Th Neither is transfer from Ce3t to Eu3’ 9)• The processes occurring in materials of this type can be reasonably well explained qualitatively’2). Let S be the absorbing centre (sensitizer) and A the activator. For efficient emission from A upon excitation of S the probability of energy transfer between S ions mutually (Pss) and that of energy transferfrom S to A (~s~) must be larger than the radiative emission probability of S (P) (it is tacitly assumed that nonradiative losses in S and A do not occur as they do not for all examples mentioned). We will discuss some specific examples: a. Y,...~Eu 5Nb04l3)~ The compound YNbO4 shows an efficient blue emission due to the niobate group. From the absence of concentration quenching at such a high centre concentration it may be concluded that transfer between niobate groups mutually is not very effective (~sslow).

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Otherwise energy would be transported to killer sites, resulting in quenching’4). Only at high Eu34 concentrations (x > 0.1) does the quantum efficiency of the blue emission go to low values, and that of the red Eu34 emission obtains roughly the same value as that of the unactivated host lattice (see fig. 3). These results indicate that ~sA > P 3+ 5g. For low Eu SC


4~O~Nb04EU 0






Fig. 3. Quantum efficiency of the Eu3 and niobate emission of Yl_~Eu~NbO1as a function of the Eu~-concentration (254 nm excitation).

concentrations the efficiency of the Eu3 + emission upon host-lattice excitation is, therefore, low. For high Eu34 concentration it is high, since then the value of P 5s is not of importance. For such high activator concentrations, however, concentration quenching can be expected to decrease the emission efficiency. b. Y1_~Eu~P1_~V~O4 12. 13, 15. 16) Contrary to YNbO4 the emission of the host lattice YVO4 is concentration-quenched. This quenching can be reduced by diluting the vanadate 5’ lattice with groups or by using 16) In thephosphate system YP, _~V~O extremely pure starting materials’ the 4 efficiency and quenching temperature of the blue vanadate emission decreases for values ofj’ above 0.2. This points to an efficient transfer between 3 + vanadate emission groups, and is confirmed by the fact that the efficiency of the Eu in Y, _~Eu~P, _~V~O for 4 excitation into the vanadate is high for 0.2 j’ < I. For v < 0.2, however, blue vanadate emission is observed too (table I). The situation for y < 0.2 is comparable to that of YNbO4-Eu. TABLE


Ratio of efficiencies of V04 and Eu’ emission of (Y, Eu)(P, V)O1 v in Yo.s7Euo.o3P1~V,O4 0.01

3 emission ratio of the quantum efficiencies of V01 and Eu 1.5



0.10 0.20 0.40

0.3 <0.1 ~0.l





The energy transfer from vanadate (niobate, etc.) to Eu

occurs by

exchange interaction. The strongest experimental evidence for this comes from the dependence of transfer efficiency on the angle between the centre of the host lattice group, the oxygen anion and the Eu3~ion’2). If this angle is large (e.g. Y 7W06-Eu. 180) the transfer efficiency is high, if the angle is smaller (e.g. Gd,WO,,-Eu. 90) the transfer efficiency is less. The energy transfer through the host lattice (e.g. YVO4) may he decreased by the introduction of phosphorus (increase of average V-V distance) as described above, or by decreasing the temperature. At low 3temperatLire ~ enlission Y~,~9Eu00 V04large shows the vanadate wellforasexample, the redYP Eu (now ~ is not compared to P~)as as does, 0~~V01O4-Eu at room temperature. This situation is comparable to similar well—known results for CaWO4-Sm described many years ago by Boiden I c. Y —~ 0Bi~Eu~Al3B4Oi 2 ~ The phosphor YAI3B4O, shows con3 + 7-Bi concentrations centration quenching of to its SS ultraviolet emission Bi3 for+ Bi above 0.5 °~,. This points transfer between ions over large dis-

tances. The Bi3~ Eu3~transfer is also very efficient since the UV enlission band of the Bi3~ion overlaps the Eu3r charge-transfer absorption band (fig. 4). This implies strongelectric dipole-dipole interaction. In YVO 4-Eu and —*


/ \\ /I


/~~i3 B~012—E+~

~ so







I 250










Fig. 4. Spectral overlap of the excitation band of YAl2B4Oi~-Fuand the emission hand of YAI3B 012-Bi (I gises the radiant power per wavelength interval in arbitrary units).

YNbO4-Eu the sensitizer emission overlaps onlyofthe 4t’-4f absorption 3~ emission Y,weak ~._VBi~,Eu.VAl bands. The efficiency of the red Eu 3 + ions is found to be high for low values of .v3B+Ol2 and

for of the from Bi the high values of ~ and PSA in this case. as isexcitation to be expected In table 2 we have compiled the results of an estimate of the numerical values of ~ and ~SA 18) and the reciprocal decay time of the S fluorescence (i.e. P~) for the three cases mentioned above. These quantitative data agree with the arguments given above. All these cases of energy transfer can be understood in terms of the

J. t



Values of P

58 and PSA (for r Phosphor system Pss



1) 4

A) and PSA

p5r (sec p 5r





~tO~ 7

YAI YVO4-Eu 3B4O12-Bi, Eu




~t0 ~IO~


57 >>~ l0~ lO~


t9). The absence of energy transfer in the following Förster-Dexter theory cases Ce3~ Eu3~ or Sm34 and vanadate (or tungstate, niobate, etc.) Tb3 + cannot be explained by this theory. It is noteworthy that in all these cases the energy of the “electron-transferred” state will not be very high above that of the ground state. The energy difference between the combination Ce3~ + Eu3~,for example, will not differ much from that of the combination Ce44 + Eu2~.In view of this it has been proposed9) that a decreasing correlation energy will result in a decreasing probability of transfer by exchange. In agreement with this no energy transfer was found from Eu2+ to Eu3~20) Recently investigations have been started by Kingsley and Ludwig to elucidate the excitation mechanisms for higher-energy excitation (e.g. vacuum UV or cathode-rays)21). Quantum efficiencies far above I00~ have been observed for excitation energies above 15 eV (e.g. YVO 4-Eu has q = I50~for 21 eV excitation). —*

3. Energy transfer from rare-earth ions to other centres In the preceding section we have discussed how rare-earth ions can be excited to give emission. In this section we will see how excited rare-earth ions can transfer their energy to other centres. It is well known that rare-earth ions with broad band emission can transfer 3~to Mn2~ 22) and trivalent their excitation energyrecently to otherit has ions,been e.g. found Ce that the Eu24 ion can also rare-earth ions. More transfer its energy to other ions, e.g. to Mn24 23) and Ho3+ 24) A definite decision on the mechanism of these transfer processes does not seem feasible at the moment. Transfer from trivalent rare-earth ions with line emission to other centres is also possible and has been studied extensively by Van Uitert and coworkers25). Energy transfer from trivalent rare-earth ions to Cr3~has been found, for example, from Eu34 to Cr34 in EuA1 26)if Emission 2Eg as well as from the 4T2g level of Cr3 3B4O12-Cr ~ is observed the Eu34 from the





ions are excited in their charge-transfer absorption hand. The critical distance for this transfer is estimated to be about IS A. This distance excludes transfer by exchange interaction. In this connection it is interesting to note that the emission of EuAIO,—Cr 2E 3 + a number of’ contains in addition to the emission from the 5 levelresLilt of Crfrom an optical strong lines at longer wavelengths27). These lines process in which a Cr3 + ion, initially in the 2E 53excited simultaneously ion tostate, an excited7 F emits a photon and excites a neighbouring Eu 1 level 3 + -ELt3 coupling is ascribed to exchange interaction. (J = I to 5). This Cr These effects seem to be related to the excitation of ion pairs as observed for the first time in PrCI 3 28) We will close this section with a discussion of the concentration quenching of rare-earth ion fluorescence. Several different mechanisms are necessary to explain the concentration quenching of all2~rare-earth . Ce3~)it ions. is assumed that the a. In the case of broad-hand emission (Eu excitation energy is transferred from the activator via other activator ions to energy-sinks in the lattice .~ I ~ The probability of energy transfer between the activator ions determines the concentration above which the quantum efficiency starts to drop (the critical concentration). Typical values of the critical distance for this type of energy transfer are 15—20 A. so that the critical concentration is a few atomic per cent’8). b. Energy transfer from one ion to a lower-lying level of another ion by transitions that are matched in energy causes concentration quenching of a number of trivalent rare—earth ions with line emission. This iS shown for Sm3 in fig. 5. The Sm3 ion fluoresces from the 4G 2 manifold. For high

20 10~ cm1








6H 5~




ions. level scheme of Sm~ showing concentration 2quenching 01 the emission h~’cross-,-elaxation between two Sm





Sm36 concentration the following transfer occurs Sm(4G5/2) + Sm(6H512) 6F 4G —~ 2 Sm( 912) so that the orange emission from 512 is quenched (crossrelaxation). Independent of the the corresponds critical concentration is 3~ion per 4600host A3 lattice 29) This to a critical found at one Sm distance of about 20 A or a critical concentration of a few atomic percent. A similar mechanism occurs for Dy3~30). c. Concentration quenching of Eu3~and Tb34 usually occurs at much

higher concentrations and depends strongly on the crystal structure29). Some compounds can even contain the total possible amount of these activators without any concentration quenching, e.g. EuA1 3B4O12 31, 32) KEu (W04)2 29) and TbAI3B4O12 31) 34 and Tb34 described above (fIg. are not applicable itfor in The viewmechanisms of their energy level scheme 6). Therefore hasEubeen proposed by Van Uitert et al. that the quenching mechanism is associated with

exchange interaction between a number of Eu34 or Tb34 ions. En the case of Eu34 the emission from the 5D t) is quenched by a 0 level (17,250 cm transfer process involving the excited Eu3~ion and two of its neighbours. Since the width of the lower 7F manifold is only 5000 cm a 2250 cm excess energy has to be taken up by the lattice. Emission from the 5D 4


,~3crir’ ______






5 __ 03







Fig. 6. Energy level schemes of Euii and Tb°~showing possible ways in which the °D 1 and 5D 3~may be quenched. 2 and °D1emission of Eu 3 emission of Tb



C,. iiuvssr

3~ (20,500 cm’) is quenched by cooperative interaction

manifold of Tb

Tb34 ions (the width of the 7F manifold is 5500 cm )29) This theory also explains the dependence of the critical activator concentration on crystal structure2~ 33)~ In EuAI 3B4O, 2~for example. the shortest between four

Eu-Eu distance is 6 A. This excludes exchange interaction of’ reasonable strength, and therefore also rules out concentration quenching. In Y3AI ~O, 2~ 2 —ELI each rare-earth ionabout has four rare-earth neighbours with R.E.-0 R.E. angle equal to 100nearest . The critical concentration is at 13 Eu3~

per 4600


3-Eu each rare-earth ion has 18The nearest rare-earth 2-R.E. angles of l50~ or larger. critical concenneighbours with R.E.-0 tration is at 3 Eu34 ions per 4600 A3 (compare Sm3~ with I Sm3~ per 4600 A3 for all host lattices). Since the exchange interaction involves porbitals of oxygen. 180 is an optimum angle f’or exchange. For the 90 case the interaction strength is greatly reduced, in agreement with the experimental results. This angle dependence goes parallel to the angle dependence of transfer from host lattice groups to rare-earth ions and is based on the same physical principle.

Concentration quenching of ELI34- and Tb3~emission from higher levels usually occLtrs at much lower activator concentrations. The Tb34 5D 3 emission is quenched by the following cross-relaxation process (compare fig. 6): S D3 —s 5 D4 and 7 F(, —s 7 F0 or a D3 7 F11 and 7 F1~ a D4 34 30 levels (5D,. 5D 5D The emission from higher ELi 2, 3) may be quenched 3~ con—~


in two ways, either by cross relaxation (depending the ELi centration) or viz, by multiphonon emission (depending on theon phonon spectrum of the host lattice)3~). In Y 3~a high activator concentration is 7O3-ELi needed to quench the 6D, emission. Here cross-relaxation dominates, since it has been shown that the D, — D 035). transition (1750 cm The 5D~ 5D — ) occLirs with emission ol’ f’our 430 cm — phonons 1> multiphonon emission is a forbidden transition. At room temperature this l’our-phonon 5D, to the

process can therefore compete decay from phonons3°). 7F manifold. The same not holds for Y with radiative 3~with still smaller 2O2S-Eu In silicates. horates and chelates, however, a two-phonon emission is possible due to the presence of much larger phonons37 38) In these host lattices multiphonon emission dominates and even at low Eu3

concentration only

the 5D 0 emission is observed. In many cases the dependence of the

efficiency of R.E.34line emission on the ion, for example. host lattice can he explained in a similar way. The Er fluoresces green from the 4S 3,, 4F level. This emission can be quenched by a nonradiative transition to the 92 level, which is 3000 cm’five-phonon lower. In 3~1)~ This

Y3Ga5O~ 2 five phonons are required this process~ process competes successfully with the for radiative emission, resulting in a low

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efficiency of the emission. In silicates and borates with larger phonons a three-phonon process with much higher probability is possible. Indeed it is impossible to fInd any green emission from Er3 4-activated silicates and borates40). In lattices with heavy ions, however, this ion may act as an efficient fluorescent centre (e.g. La 2O2S-Er 36), LaOCl-Er 40)), 4. Conclusions The numerous investigations on rare-earth activated phosphors during the last three years have yielded on the one hand new and/or improved materials and on the other hand a qualitative insight into the physical processes that play a role in their luminescence. For the future we may expect a deepening of the knowledge at present available and a serious attempt to solve problems that are incompletely understood, as for example the mechanisms for high-energy excitation sources, the non-occurrence of

energy transfer in some special cases and the influence of host lattice on luminescent properties. References 1) G. Blasse, J. Chem. Phys. 45(1966) 2356. 2) A. Bril, G. Blasse and J. A. A. Bertens, J. Electrochem. Soc. 115 (1968) 395.

3) G. Blasse and A. Bril, Phys. Letters 23 (1966) 440. 4) C. K. Jørgensen, Mol. Phys. 5 (1962) 271. 5) G. Blasse and A. Bril, J. Chem. Phys. 47 (1967) 5139. 6) G. Blasse, J. Chem. Phys. 51(1969) 3529. 7) S. Shionoya and F. Nakazawa, AppI. Phys. Letters 6(1965)118. 8) W. L. Wanmaker, A. Bril and J. W. ter Vrugt, AppI. Phys. Letters 8 (1966) 260. 9) G. Blasse and A. Bril, J. Chem. Phys. 47 (1967) 1920. 10) R. K. Datta, J. Electrochem. Soc. 114 (1967) 1137. 11) M. E. Zhabotinskii, Yu. P. Rudnitskii, V. V. Tsapkin and G. V. Ellert, Soy. Phys.

JETP 22(1966)1155. 12) G. Blasse and A. Bril, J. Electrochem. Soc. 115 (1968) 1067. 13) W. L. Wanmaker, A. Bril, J. W. ter Vrugt and J. Broos, Philips Res. Repts 21(1966) 270. 14) D. L. Dexter and J. H. Schulman, J. Chem. Phys. 44 (1966) 3514. 15) G. Blasse, Philips Res. Repts. 23 (1968) 344. 16) H. F. Ivey and T. J. Isaacs, Electrochem. Soc. Spring Meeting, New York (1969), Late news paper 328. 17) Th. P. J. Botden, Philips Res. Repts. 6 (1951) 425. 18) G. Blasse, Phys. Letters 28A (1968) 444; Philips Res. Repts. 24 (1969) 131. 19) Tb. Förster, Ann. Phys. Leipzig, 2 (1948) 55; D. L. Dexter, J. Chem. Phys. 21(1953) 836. 20) G. Blasse and A. Bril, J. lnorg. Nuel. Chem. 31(1969)1521. 21) J. D. Kingsley and G. W. Ludwig, Bull. Am. Phys. Soc. Ser. II 13 (1968) 421; Electrochem. Soc. Spring Meeting, New York (1969) p. 151. 22) Th. P. J. Botden, Philips Res. Repts. 7 (1952) 197. 23) N. A. Gorbacheva, Izv. Akad. Nauk. SSSR, Ser. Fiz. 30 (1966) 1521.


(1. IIL/1551


24> T. F. E’,sanizky, J. App!. Phys. 38 (1967) 4338. 25) See e.g.: L. G. Van Outeri. J. Elecirochem. Soc. 114 (1967) 1048: L. G. Van Uitert, E. F. Dearborn and J. J. Rubin, J. Cheni. Phys. 46 (1967) 420, and references cited therein. 26) G. Blasse and A. Bril, Phys. Status Solidi 20 (1967) 55!. 27> P. van der Zie! and L. G. Van (uterI, Phys. Rev. Letters, 21(1968) 1334. 28) F. Varsanvi and G. H. Diekc, Phys. Rev. Letters, 7 (1961) 442. 29) L. U. Van L’itert and L. F. .lohnson. J. Chem. Phys. 44 (19661 3514. 30) L. G . Van ti itcrt, lot. (oof. on L,onine,scence, Budapest (I 966). Fd . (1. Szigei i (A kad KiadO, Budapest, 1968) p. 588. 31) A. A. Ballnian, Am. M iieralogist 47 11962) 1380. 32) U. Blasse, J. Oiem. Phys. 46 (1967) 2583. 33) L. U. Van tiitert, R. C. [mares. R. R. Soden and A. A. Ballman, J. Chem. Phys. 36(1962) 702. 34> A. D. Pearson, G. E. Peicrson and W. R. Northover, J. App!. Phys. 37 (1966) 729. 35> M. J. Weher, Phys. Rev. 171 (1968) 283: and Optical P,spertie.s of Ions oi C~rr.sta/.s. Ed.: I-I. M. Crosswhite and H. W. Moos (Inlerscience, New York, 1967> p. 467. 36) M. R. Royce and A. I_. Smith, Extended Alsrtroets. Electrocheo,. Soc. Spring Meeting Boston (1968> p. 94. 37) M. L. Bhaumik and L. J. Nugent. J. Chcm. Phys. 43 (1965> 1680. 38) U. Blasse and J. de Vries, J. Electrochem. Soc. 114 (1967) 875: G. Blasse and A. Bril, J. Inorg. Nuc!. (Them. 29 (1967) 2231. 39) J. R. Chamberlain, D. H. Paxman and J. L. Page. Proc. Phys. Soc. 89 (1966>143. 40> U. Blasse, unpublished.


Discussion on paper J 1 Quest/ott /.~ R. C. Ropp I was very interested in your efficiency values for various 110sf lattices 34. You seem to imply that the efficiency of Eu34 charge containing ELI transfer state luminescence (lr —s 4f) depends primarily upon structure and not upon any other f’itctors. We have not found this to be the case. For example. we can prepare Y 203, YVO4 (or Y(P, V)04). YNbO4. 34, all having quantLtm efficiencies between 80—90°c, (at activated 2537 A by ELI excitation, for example).

All have approximately the same Eu34 content. In addition. yttrium tungstate has nearly the same quantum efficiency. One cannot mention Gd34 hosts (GdNbO 4 or tungstate) in the same capacity because there are magnetic coupling processes which affect the luminescence process. How do our results fit in with your conception of the energy processes involved? I personally believe because that all the hosts transfer 3~efficiency hostwith CT charge center involves bands willoxygen produce high active Eu in the ELI3 0 CT center (as nearest neighbors). the same atoms Thus host absorption is sufficient to give high efficiency, and ilot only structLtrc. YPO 4 hasisnolow host whereasbutYVO4 does structure). 34 efficiency in absorption the former (25°~) 90°~ QE in(same the latter. ELi

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Anstt’er: G. Blasse It is not true that host absorption or CT absorption alone is sufficient 3 + efficiencies. Some experimental examples of phosphors

to have high Eu with low efficiency are: La

203 (CT); LaAlO3 (CT) and Gd2WO6-Eu (host); Y2MoO6-Eu (host); Y2Ti2O7 NaYTiO4 Y6W012. In the former two compounds the CT band is at relatively low energies. It seems probable that nonradiative decay occurs from the CT state (G. Blasse, J. Chem. Phys 51 (1969) 3529). the other there is no efficient transfer Eu34 from 34. inIn your own compounds, examples the conditions for efficient host to Eu emission are satisfied so that it is not surprising to find high quantum efficiencies.

Question 2: 5. Shionoya Do you have any interpretation for the fact that host materials like Y 2Si2O7, which involve light atoms, produce high efficiency luminescence of rare earth ions? Answer: G. Blasse

44, P54)

lattices small the andtendency highly-charged cations (Si center to willHost be very rigidcontaining and will oppose of the luminescent expand (or contract) upon excitation. As a consequence the variation of the equilibrium distance of the center will be small and, therefore, the quenching temperature high (see also: G. Blasse, J. Chem. Phys. 51 (1969) 3529). Comment 1: Dr. M. Bancie-Grillot

Concerning the energy transfer, there is also another situation which occurs for example in ZnS. Some authors use copper as a co-dopant for rare earth ions. But, on the other hand, the well-known emission bands of copper are much more intense than the fluorescence lines of lanthanide ions, often making the observation of these latter more difficult. The introduction into the lattice of minute amounts of cobalt ions, usual killer of the emission of copper centers, suppresses it but does not decrease the intensity of the fluorescence of the lanthanide ions which appear now much more clearly in the blue and green region of the spectrum (Journal de Physique 27 (1966), C2-l 16). Then the energy transfer from the lattice to the impurity ions becomes forbidden for copper when, at the same time, it remains not disturbed as far as the lanthanide ions are concerned.