LUMINESCENCE OF ALKALINE-EARTH PYROPHOSPHATES,. ACTIVATED WITH DIVALENT EUROPIUM

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1 R633 Philips Res. Rep!s ~-?' , 1967 " LUMINESCENCE OF ALKALINE-EARTH PYROPHOSPHATES,. ACTIVATED WITH DIVALENT EUROPIUM W. L. WANMAKER and J. W. ter VRUGT Abstract Eu 2 + -activated alkaline-earth pyrophosphates, such as Sr2P207 and Ca2P207, are efficient deep-blue or u.v.-emitting phosphors. The excitation spectra are very broad, extending from 200 nm to the long-wavelength u.v, part of the spectrum. Energy transfer from Eu2+ to Mn 2 + is possible, giving rise to a yellow or red Mn 2 + emission band in addition to the Eu 2 + emission. With u.v. excitation ( nm) a quantum efficiency of per cent was measured and with cathode-ray excitation a radiant efficiency of about 4 per cent. The phosphors are very well suited for application in fluorescent lamps used for photocopying. 1. Introduetion Although in recent years several papers have been published concerning the luminescence of phosphors activated with trivalent europium 1, 2, 3), little attention has hitherto been paid to the luminescence of phosphors activated with divalent europium. Kröger 4) gives in his book a list of 19 host lattices activated with Eu2+, e.g. CaS0 4 -Eu, NaBr-Eu and SrF 2 -Eu. Whereas the emission colour of Eu3+ phosphors is mostly orange or red, Eu 2 + phosphors generally show a blue emission. Moreover, in Eu3+ phosphors the emission spectrum consists of emission lines, whereas in Eu2+ phosphors emission bands are found. The absorption and fluorescence spectra of Eu2+ in crystals were studied in detail by Butement 5) and also by Freed and Katcoff 6). These authors observed that the absorption and fluorescence spectra of Eu2+ phosphors consist of broad bands. The absorption and fluorescence involve transitions from the 4f7 configuration to the 4f6 5d configuration, similar to the transitions found in Ce3+ phosphors, in which.the only 4fe1ectron can be excited to outer shells ofthe Ce3+ ion. These phosphors also show broad absorption bands. Allowed elec~ric-dipoletransitions occur in both Eu 2 + and Ce3+ phosphors, with a strong influence of the relevant host lattice, in contrast to the magnetic-dipole or the forced electric-dipole transitions 7) occurring in trivalent rare-earth phosphors, which show sharp emission lines with a weak interference of the surrounding host lattice. Sincetheionicradius ofeu2+ (1 17A) isquite similarto that of'srê" (1 13A) B), Sr salts are suitable host lattices for activation with Eu 2 +. Nazarova 9) ob-

2 356 W. L. WAN MAKER and J. W. ter VRUGT served cathodoluminescence in Sr3(P04)2-Eu, Sr2P207-Eu and et-ca2p207-eu. This last phosphor (emission maximurn at 425 nm) has an efficiency of 35 per cent with respect to that of ZnS-Ag. Gorbacheva 10) described 'energy transfer from Eu2+ to Mn2+ in (Sr, Mg)3(P04)2-Eu, Mn. Recently the luminescence of SrAI204-Eu 11) was, described: the emission spectrum -shows a band at 520 nm and the luminescence is due to Eu2+ ions, This paper describes the preparation and some properties of alkaline-earth pyrophosphates activated with divalent europium. ' 2. Experimental Method of phosphor preparation, '1 The ingredients used in the firing mixture were luminescent-grade SrHP04, CaHP04, BaHP04, ZnO, CdC03, MgNH 4 P0 4, or a mixture óf MgO and (NH4)2HP04; EU203 in a purity of 97,5-98,5 or of 99 9 per cent was used, as supplied by the Michigan Chemical Corporation. The purity of EU203 influences only the brightness of the phosphors and not the emission characteristics. The required amounts of the dry materials were thoroughly mixed and subsequently fired. Samples of 5-10 grammes were fired twice for a period of 2 h at a temperature of 1000 to 1200 C. Nazarova 9) states that the preparation of Eu2+ phosphors can be carried out in air, some reduction from Eu3+ to Eu2+ taking place at elevated temperatures. Although Eu 2 + phosphors can be made in this way, more efficient luminescence is obtained by firing in a slightly reducing atmosphere, e.g. nitrogen with 0 1 to 2 0 volume per cent of hydrogen. Another convenient method consists of heating the starting ingredients in air in the presence of carbon, the formation of CO being responsible for the reduction of Eu 3 + to Eu2+ 12). In most cases this was the method used by us Optical properties The excitation, diffuse-reflection and emission spectra, as well as the efficiencies and decay times, were measured by a procedure described earlier, 1). The radiant efficiency of the phosphors, with cathode-ray excitation, was determined bya method also reported before 13). 3. Results and discussion 3.1. Emission spectra It may be seen from fig. 1 that Ca:iP207-Eu shows a deep blue emission with an emission peak at 415 nm, whereas in Sr2P207-Eu this emission peak is located at 420 nm. X-ray analysis showed that both Ca2P207 and Sr2P207 have the orthorhombic et structure. The shifting of the position of the emission peaks may be attributed to a slight difference in the lattice dimensions 14), viz.

3 LUMINESCENCE OF Eu2+-ACTIVATED ALKALINE-EARTH PY~OPHOSP!iATES ~50 Fig.1. Spectral-energy distribution of the emission of'. Cal.9SP2Û7-Û 02 Eu (curve 1), of Srl'9SP2Û7-Û'û2 Eu (curve 2) and of Sro.78Mgl.2P207-Û û2 Eu (curve 3). Mainly 254-nm excitation'. ex-ca2p207: a = 8 25A, b = 12 65A, c = 5 31 A, «-Sr2P207: a = 8 98A, b = 13 13A, c = 5 40 A, where the hitherto unknown lattice parameters of ex-sr2p207have been calculated from the powder diagrams of the phosphors here described. From the other Eu2+-activated pyrophosphates (viz. those of Ba, Zn, Cd, Mg) a weak luminescence was observed only in Mg2P207-Eu with Àmax = 424 nm. Compounds with substituted cations Efficient phosphors are made in the system Ca2P207-Sr2P207-Eu, whereas partial substitution of Ba for Ca or Sr in the Eu 2 +-activated Ca and Sr pyrophosphates leads to a gradual decrease of the brightness, practically no luminescence being found with about' a 60 atom per cent substitution. It is interesting to note that both,in SrBaP207-Eu and CaBaP207-Eu the emission peak shifts to 435 nm. This may be related to the fact that X-ray analysis shows an initial expansion of the lattice due to the incorporation of Ba. With increasing amounts of Ba substituted for Ca, a separate Ba2P207 phase is formed. As the X-ray powder diagram of Cao:4Bal.6P Eu (table I) shows, the Ba2P207 in turn may dissolve Ca. This diagram agrees very well

4 358 w. L. WAN MAKER and J. W." Ier VRUGT TABLE I X-ray powder pattern of Cao.4Ba106P Eu(copper radiation) dca) relative peak height dca) relative peak height I with that of o-ba2p2û7 as given by McKeag and Steward 15). Whereas the partial substitution of Mg for Ca in Ca2P2û7-Eu results in a pronounced decrease in brightness, efficient phosphors can be made in the system Sr2P2û7- Mg2P2Û7-Eu. The introduetion of Mg into Sr2P207-Eu gives riseto a second emission peak, located at 394 nm. The 394-nm peak dominates when more than about 40 per cent of the Sr is replaced by Mg, as is shown in fig. 2 (see also figs 1 and 6). X-ray analysis of Sr2-xMgxP207-Eu shows the presence of a distinct phase, differing from both Sr2P207 and Mg2P207. The powder diagram of this separate phase is given in table 11.This diagramstrongly resembles that of BaMgP207 16). When Eu and Mn are added simultaneously and the phosphor is :fired in such a way as to form Eu2+, an Mn2+ emission band appears, next to the Eu2 + emission. This band is located between 570 and 680 nm, e.g. in Ca2P207-Eu,Mn at 610 nm and in Sr2P207-Eu, Mn at 570 nm. Since pyrophosphates-mn-+ are not excited by u.v. radiation and the excitation spectra of the Eu2+ emission in pyrophosphates-euê+ and ofthe Mn 2 + emission in pyrophosphates-euê",mn2+ are quite similar, energy transfer from Eu 2 + to Mn2+ probably takes place. This energy transfer also follows from the fact that the intensity of the Eu2 + emission. decreases, when increasing amounts of Mn2 + are added. From the fact that even with a high molar ratio of Mn/Eu the intensity of the Eu band is still appreciably higher than that of the Mn 2 +. band, it may be concluded that the energy transfer from Eu 2 + ~o Mn 2 + in this type of phosphor is not very

5 LUMINESCENCE OF Eu2 +-ACfrVATED ALKALINE-EARTH PYROPHOSP~ATES 359 Fig.2. Relative intensity of the 419-nm and 394-nm emission in Srl.98_2xMg2xP2Û Eu phosphors. Mainly 254-nm excitation. _X TABLE II X-ray powder pattern of Sro.sMg1-2P207-Ö 015Eu (copper radiation); a large number of very weak diffuse lines are not sufficiently resolved to be' measured with accuracy ', relative dca) I relative dca) relative peak height peakheight peak height 6 96 " , 4 16 f , ' ,1'9, , ' ' ', I, "

6 360 w. L. WANMAKER and J. W. ter VRUGT efficient. The 'utensity of the Mn band (with a given concentration of Eu2+ and MÎl2+ added) increases in the sequence Sr1.6MgO.4P207-Srl.6Bao.4P20. - Sru rimgl.2p207 - Sr2P207 - Ca2P207 Energy transfer from Eu2+ to Mn2+ was reported (Sr,MgMP04)~ 10) Excitation and diffuse reflection earlier by Gorbachcva in Tne wavelength dependence of the quantum output and of the diffuse reflection was determined for several Eu2+-activated pyrophosphates. As may be seen from figs 3 and 4, the excitation spectrum extends to the long-wavelength part ofthe ultraviolet spectrum. As the host lattices hardly show any absorption between 200 and 400 nm, the excitation does not take place via the host lattice. Excitation in the 4flevels of the rare-earth ions can also be excluded, because the excitation bands are broad and intense. Presumably the excitation takes place due to the transition 4f _,. 5d, which is similar to that of the Ce3+ ion, since Ce 3 + -activated phosphors also show a broad excitation spectrum, which extends to the long-wavelength u.v. part of the spectrum, e.g. in Ca3(P04)z-Ce 17). In many Eu2+-activated alkaline-earth pyrophosphates absorption bands are found at wavelengths of 240 and 340 nm, corresponding to peaks in the excitation spectrum. As follows from figs 3 and 4, the Eu2+ phosphors prepared by us are very well suited!or excitation with short-wavelength (254 nm) and longwavelength ultraviolet radiation (365 nm) of the type present in a mercuryvapour discharge. l(r) À(nm) Fig.3. Excitation and diffuse-reflection spectra of Cal.9SP Eu (curve 1) and of Srl.9SP Eu (curve 2).

7 LUMINESCENCE OF Eu 2 +-ACTIVATED' ALKALINfj-EAP ~11 PYROI'HOSPHATES ,-~-._-_._.-_._._- '--'_- ---_._-_._ '----2{r) À.(nm) Fig. 4. Excitation and diffuse-reflection spectra of Srl.SsMgO.40P20,-O 02 Eu (curve 1) and of Sru 7~Mgl 20P207-0'02 Eu (curve 2) Efficiency Table III gives the efficiencies of the Eu2+ emission of some pyrophosphates at room temperature, both for excitation in the nm range and for excitation with cathode rays. It may be seen from these data that the Eu2+-activated pyrophosphates are efficient phosphors with u.v, excitation. TABLE III Efficiencies of Eu2+-activated pyrophosphates; r = reflection (%),?l = radiant efficie-ncy ~%), q = quantum efficiency (%) ( nm excitation); ïlc«= energy conversion efficiency (%) with cathode-ray (20 kv) excitation phosphor nm exc. I C.R. exc. r n q ïlc«srlo9sp207-0'02 Eu Sr!.5sMgo.40P Eu s-; '!lmg].20p Eu I Srl'47Bao'36P207-0'02 Eu In table IV the values of rand q are given for Ca2_XP207-xEu and Sr 2 - X P xeu. Itmay be seen that with x :::;;; a quantum efficiency of about 65 per cent has been found in Ca2P207-Eu and of about 85 per cent in Sr 2 P Eu. With a higher Eu+ 2content, the quantum efficiency decreases most pronounced-

8 W. L. WANMAKER and J. W. ter VRUGT ly 'in Ca2P207-Eu. The decrease in brightness is smaller than the decrease in quantum efficiency, 'since the absorption of the exciting u.v. radiation increases, with x exceeding a value ofo 015.As the preparation conditions ofthe phosphors mentioned in table IV were not optimized, the data give only a general impression of the efficiencies of these types of phosphor. For instance, we succeeded in a more thorough investigation in preparing a sample of Sr1.98P207-0'02Eu which has a quantum efficiency of about 95 per cent. This sample has been cited in fig. 6. TABLEIV Efficiencies of Ca2_XP207-xEu and Sr2_XP20rxEu q = quantum efficiency (%), r. reflection'( %) ( nm excitation); Ca2_XP207-xEu,. Sr2_XP207-xEu x q r q I r I With cathode-ray excitation the phosphors have a much lower efficiency than that of the blue-emitting ZnS-Ag, for which phosphor Bril measured a radiant efficiency of 21 per cent 18). Nevertheless, the efficiencies are equal to those found in Ce-activated silicates, i.e. an efficiency of 4 % with Ca2A12Si07-Ce and Ca2MgSi207-Ce as reported by Bril and Klasens 13). ~ Decay times (ultraviolet excitation). The decay times of some of the Eu2+-activated pyrophosphates were determined and ljè values of about s were found. So the decay time is of the same order of those of Ce3+-activated phosphors 19), in accordance with what has been stated in the introduction Temperature dependence For the Eu2 + -activated pyrophosphates the quenching temperature (i.e. the

9 LUMINESCENCE OF Eu 2 +-ACfIVATED ALKALINE-EARTH PYROPHOSPHATI:S 363 temperature in "C at which the brightness decreases to 50 per cent of its value measured at room temperature) lies between 125 and 200 C, as may be seen from fig. 5. This quenching temperature is much lower than those found for most Eu 3 +-activated phosphors. This difference is reasonable, as, in Eu3+ phosphors, in contrast to Eu2+ phosphors, no excitation of the 41electrons to Fig. 5. Variation tion. of brightness with temperature of various phosphors. Mainly 254-nm excita- curve phosphor Cal.9SP Eu, 2 Srl'9SP207-0'02 Eu, 3 Srl.5sMgo.40P Eu, 4 Sro.78Mgl.20P Eu. outer shells takes place. Excitation to outer shells renders the phosphors more susceptible to the influence of the surroundings of the activator ion, e.g. the crystal structure of the phosphor has a strong influence on the temperature dependence. For example both ct-sr 3 (P04h-Ce3+ 13) (measured with cathode-ray excitation) and ct-sr3(p04)z-eu2+ 10) show a high quenching temperature. The introduetion of small amounts of Mg into Sr 3 (P04)2 causes a transformation to the (:J crystal structure and Gorbacheva 10) noticed a much lower quenching temperature, similar to those of the pyrophosphates shown in fig. 5. In both Ca2P207-Eu and Sr2P207-Eu the effect of the activator concentration on the quenching temperature is relatively small, e.g. the quenching temperature Tso COC) decreases for Ca2P20 7 -xeu from 135 to 120 C and for Sr:iP xeu from 225 to 185 C, when increasing x from to 0'12.

10 364 w. L. WAN MAKER, and J: W. ter VRUG~ 4. Application The Eu2+-activated phosphors described above are very suitable for application in lamps (both low- and high-pressure mercury vapour) for photocopying, as most light-sensitive papers show a maximum in their sensitivity between 350 and 450 nm 20).It follows clearly from fig. 6 that the emission curves of the Eu2+ phosphors are narrower thanthose of other phosphors emitting in this region of the spectrum, viz. CaW04, (Sr,Ga)J(P04)2-Sb,K 21) and Sr,Ba,Mg silicate-pb 22). Moreover, the peak intensities of the Eu2+-activated pyrophosphates are higher and therefore the whole emission spectrum falls within the range of maximum sensitivity of the available photocopying papers. Consequently the use of the new Eu2+-activated pyrophosphates makes a higher printing speed possible. Another attractive point is that with this new type of phos- 5 Î Fig. 6. Spectral-energy distribution of various phosphors suitable for photocopying. Mainly 254-nm excitation. curve phosphor Sr, Ba, Mg silicate - Pb, 2 Sr, Ga phosphate - Sb, K, 3 CaW0 4, 4 Sr21'207 - Eu, 5 Sro.sMg1.2P207 - Eu.

11 LUMINESCENCE OF Eu 2 +-ACTIVATED ALKALINE"EARTH PYROPHOSPHATES 365 phor, reflector lamps can now be made using a layer of Ti0 2 as the reflecting material ê"). It has hithertobeen impossible to make these lamps with Sr,Ba,Mg silicate-pb, as the Ti0 2 absorbs a large part of the phosphor emission. An important factor conc~rning the application of a phosphor in a fluorescent lamp is the stability of the activator, since during lamp processing a burning-out occurs at about 600 C in order to remove the organic binder required for the stabilization of the phosphor suspension. This factor causes some difficulties with Sn 2 + -activated phosphors 24), and is so pronounced in Cu+-activated phosphors, that it has prevented the application of these efficient phosphors, such as of SrLi P0 4 -Cu 25) which, because of the high quantum efficiency and the position of the emission band, would also have heen well suited to photocopying. The Eu 2 + -activated phosphors show some oxidation of Eu2+ to Eu3+ at 600 C, but this oxidation is much smaller than that found for Cu+ phosphors and even for Sn2+ phosphors, so that the application of Eu 2 + phosphors in lamps presents no problems. For example, if Sr 2 P 2 0 r O 02 Eu is heated in air for 1 h at 600 C a decrease in brightness of only 6 per cent occurs; conven- tionallamp-making procedures may therefore be used. Acknowledgement The authors wish to express their thanks to Mr J. G. C. M. de Bres, Mr J. G. Verlijsdonk and Mr G. J. R. A. Tops for preparing the phosphors and measuring the temperature dependence of the phosphors; to Mr C. Bakker for measuring the reflection and excitation spectra and to Dr A. Bril for measuring the efficiencies and the decay times of the phosphors. Eindhoven, 9th May 1967 REFERENCES 1) A. Bril and W. L. Wanmaker, J. electrochem. Soc. Hl, 1363, ) A. K. Levine and F. C. Palilla, Appl. Phys, Letters 5, 118, ) A. Bril and W. L. Wanmaker, Philips tech. Rev. 27, 22, ) F. A. Kröger, Some aspects of the luminescence of solids, Elsevier Publishing Co., Amsterdam-New York, 1948, p ) F. D. S. Butement, Trans. Faraday Soc. 44,617, ) S. Freed and S. Katcoff, Physica 14,17, ) G. Blasse, A. Bril and W. C. Nieuwpoort, J. Phys. Chem. Solids 27,1587, ) L. Pa ui ing and H. N. McCoy, J. Am. chem. Soc. 59, 1131, ) v. P. Nazarova, Bull. Acad. Sci. USSR, Phys. Ser. 25, 323, ) N. A. Gorbacheva, Izvest. Akad. Nauk SSSR, Phys. Ser. 30, 1521, ) U. S. Patent , ) G. E. Cr o sby, The electrochem. Society, Enlarged Abstracts of Papers presented by the Electronics Division, Spring Meeting, San Francisco, April 29-May 3,1956, Abstract nr ) A. Bril and H. A. Klasens, Philips Res. Repts 7,401, ) ASTM powder diffraction file ) A. H. McKeag and E. G. Steward, Brit. J. appl. Phys, Suppl. 4, 526, ) M. V. Hoffman, J. electrochem. Soc. 110, 1227, 1963.

12 366 w. L. WANMAKER and J. W. ter VRUGT 17) Th. P. J. Botden, Philips Res. Repts 7, 197, ) A. Bril in Kallmann-Spruch (ed.), Luminescence of organic and inorganic materials, John Wiley and Sons, 1962, p ) A. Bril and H. A. Klasens, Philips Res. Repts 7, 421, ) K. Hoffmann and L. Fischer, Angew. Chem. 66, 593, ) W. L. Wanmaker, J. G. C. M. de Bres and J. W. ter Vrugt, Paper presented at the international Conference on Luminescence, Budapest 1966, to be published in the Proceedings of the Conference. 22) H. A. Klasens, A. H. Hoekstra and A. P. M. Cox, J. electrochem. Soc. 104, 93, ) Neth. Patent , ) H. Koelmans and A. P. M. Cox, J. electrochem. Soc. 104, 442, ) W. L. Wanmaker and H. L. Spier, J. electrochem. Soc.109, 109,