R661 Philips Res. Repts 23, 189-200, 1968

FLUORESCENCE OF Eu2+-ACTIVATED SILICATES

by G. BLASSE, W. L.WANMAKER, J. W. ter VRUGT and A. BRIL

AbstractThe fluorescence of Eu2+ -activated binary and ternary silicates isreported. The host lattices are of the types Me2Si04, Me3SiOs,Me2MgSi207 and Me3MgSi20s (Me = Ca,Sr,Ba). Many of thesephosphors are very efficient under ultraviolet excitation. Their efficiencyunder cathode-ray excitation is lower. The emission spectra are foundin the region between 435 and 600 nrn. The excitation spectra are verybroad and extend even into the visible in some cases. These spectra aresensitive to the choice of the host lattice and the alkaline-earth ion. Anapproximate energy-level scheme is derived to explain these facts. Thetemperature dependence of the luminescence intensity of the phosphorsdepends also markedly on the host lattice and the alkaline-earth ioninvolved. The decay time of the fluorescence is short, but not as shortas that of Ce3+-activated phosphors.

1. Introduetion

Recently the fluorescence of alkaline-earth pyrophosphates activated by di-valent europium was reported by two of us 1). In this paper data on Eu2+-activated binary and ternary silicates are presented and discussed. The fluores-c~nce of Eu2+ -activated Ca, Sr and Ba silicates was described many years agoby Jenkins ·and McKeag 2). Our results differ, however, from theirs and aremore complete. The use of binary and especially ternary silicates as host lattices

, in fluorescence is well known from the work of Klasens, Hoekstra and Cox 3).These authors used the Pb2+ ion as an activator and reported a number ofefficient, u.v.-emitting phosphors (for example Sr2MgSi207 - Pb).

2. Experimental

Powder samples were prepared by heating intimate mixtures of Ca, Sr and/orBa carbonate, MgC03 or ZnO, EU203 and very fine grain-size Si02 (Mallinck-rodt) in the desired proportions at temperatures between 1100 and 1300°C.The reduction of trivalent to divalent europium was achieved by heating in areducing atmosphere, for example in a mixture of N2 and H2 in the ratio20 : 1. The final PFoducts were checked by X-ray analysis using a Philips dif-fractometer (CuKa radiation).

The performance of the optical measurements has been described previous-ly 4).

TABLE I

Some data on Eu2+-activated silicates

quantum efficiency (%)composition position radiant Tso b) crystal structure

(Eu2+ cone. 2 at. %) maximum reflection efficiency (OK)emission 250-270 nm maximum (%; 254 nm) for cathode-band exc. value in ray excitation(nrn)") u.v. region (%; 20 kV)

p-Ca2Si04-Eu 505 15 30 12 2·0 365 K2S04-like C)Sr2Si04-Eu 560 + sh 30 30 18 2·0 390 K2S04-like C)Ba2Si04-Eu 505 50 75 8 4·0 425 K2S04-like C)Ca3SiOs-Eu 510 5 10 12 0·5 360 related to Cs3CoCIs d)Sr3SiOs-Eu 545 + sh 10 20 24 1·0 400 Cs3CoCIs d)Ba3SiOs-Eu 590 + sh 5 30 27 0·2 455 Cs3CoCIs d)BaSi2OS-Eu 505 55 60 13 3·5 440 ?Ba2Si30a-Eu 485 30 30 15 1·0 300 ?CaMgSi04-Eu 475 10 15 29 0·5 325 monticellite C)BaMgSi04-Eu 440 15 20 20 0·5 360 stuffed tridymite. C)Ca2MgSi207-Eu 535 5 5 24 1·0 285 akermannite f)Sr2MgSi207-Eu 470 35 40 20 2·0 305 akermannite f)Sr2ZnSi207-Eu 470 35 40 17 - 280 akermannite f)SrO.SBal.sMgSi207-Eu 440 35 40 22 1·0 350 akermannite f)Ba2MgSi207-Eu 500 60 75 26 3·0 460 own type ')Ba2ZnSi20,-Eu 505 55 70 24 - 460 Ba2MgSi207 f)Ca2AI2Si07-Eu 440 20 35 36 0·5 280 akermannite ')Ca3MgSizOa-Eu 475 40 40 15 . 0·5 505 merwinite f)Sr3MgSi20a-Eu 460 35 60 11 3·0 520 merwinite f)Ba3MgSizOa-Eu 440 50 80 8 0·5 545 merwinite ') ,

n) The position of the bands are rounded to 5 nm; sh: shoulder.b) Tso is the temperature at which the luminescence intensity of the phosphor has decreased

to 50% of the value at Iiquid-Nj temperature.

C) Ref. 11.d) Ref. 19.C) Ref.20.f) Ref.3.

.....\0o

Pb:l

~en!'l~1"'

~

i!:-<

~~

êQ""""?>~

FLUORESCENCE OF Eu2+-ACTIVATED SILICATES 191

TABLE 11

Decay time of some Eu2+-activated silicates at room temperature (lIe value)

composition(Eu2+ concentration 2 at. %) -r (microseconds)

Ca2Si04-EuSr2Si04-EuBa2Si04-EuBaSi20S-EuBaMgSi04-EuCa2MgSi207-EuSr 2MgSi2 07- EuBa2MgSi207-EuCa3MgSi20 s-EuSr3MgSi20s-EuBa3MgSi20s-Eu

0'70·50·61·40·20·20·30·80·20·20'3

3. Results

In the first place the host lattices used are summarized.In the CaO-Si02 system: Ca2Si04 and Ca3SiOs.In the SrO-Si02 system: Sr2Si04 and Sr3SiOs.In the BaO-Si02 system: BaSi20s, Ba2Si30 s, BazSi04 and Ba3SiO 5'

In the CaO-MgO-Si02 system: Ca~gSi04' CazMgSi207 'and Ca3MgSi20s.In theSrO-MgO(ZnO)-SiOz system: SrzMgSiz07, SrzZnSiz07 and Sr3MgSi20s.In the BaO-MgO(ZnO)-SiOz system: BaMgSi04, BazMgSiz07, BazZnSiz07

and Ba3MgSizOs.In addition, we investigated solid solutions between these compounds mutually.The host lattice CazAlzSi07 was also studied.

The Eu2+-activated phosphors ofthis type are in general very efficient underu.v. excitation and less efficient under cathode-ray excitation. Some ofthem areeven excited by blue radiation. These phosphors show a bright-yellow bodycolour.

Table I gives the wavelength position of the maximum of the emission band,the quantum efficiency for u.v. excitation, the reflection at 254 nm, the radiantefficiency for cathode-ray excitation, the temperature at which the fluorescenceintensity has decreased to 50% of the value at liquid-N, temperature and thecrystal structure of the materials studied. Table 11 shows the decay time of thefluorescence of a number of Eu 2+-activated silicates. The spectral-energy distri-bution of a number of the phosphors studied is given in figs 1-5. Most emission

192 G. BLASSE. W. L. WANMAKER. J. W. ter VRUGTand A. BRIL

bands are rather steep on the short-wavelength side due to self-absorption. Thisis most clearly shown by BaMgSi04-Eu (fig. 3).

I

.t5or-------r-~~---r---~~~------~

gOO~----~45~O~----~500~----~5~~~----~600-À (nm)

Fig. 1. Spectral-energy distribution of the emission of phosphors Me2Si04-Eu2+ (Me =Ca,Sr,Ba as indicated in the figure); 254-nm excitation. In figs 1-5 the radiant power perconstant wavelength interval (1) is plotted along the ordinate in arbitrary units.

I

I500 550 600 650

-il.(nm}

550 600--À (nm)

Fig. 3. Spectral-energy distributionofthe emission of CaMgSi04-Eu2+· and BaMgSi04-Eu2+ ;254-nm excitation. .

Fig. 2. Spectral-energy distribution of the emission of phosphors Me3SiOs-Eu2+ (Me =Ca,Sr,Ba as indicated in the figure); 254-nm excitation.

FLUORESCENCE OF Eu2+-ACTIVATED SILICATES - 193

I

t50~-+--~~~--~~----~~--~~

550 600-À (nm)

Fig. 4. Spectral-energy distribution of the emission of phosphors MezMgSiz07-Eu2+(Mej = Caz. Srz. SrO.SBal'S' Baz as indicated in the figure); 254-nm excitation.

550 600-À (nm)

Fig. 5. Spectral-energy distribution of the emission of phosphors MeaMgSizOs-Eu2+(Mea = Caj, Sra. Ba, as indicated in the figure)..

I

t5Or-~~r+Pr--~~~----~----~

£boo

1Ioo.-----.-----~----~~--~--~s

i~ 50r--------r.~~----~-------,--------~~--t-1.s.r!

t11s0 300 250 220

Àexc(nm)-350400

Fig. 6. Relative-excitation spectra (quantum output) of MeMgSi04-Eu2+ (Me = Ca or Baas indicated in the figure).

Figures 6-8 give the excitation spectra of the light output of the Eu2+ fluores-cence of some phosphors. These figures illustrate the broad excitation regionofthe Eu2+-activated silicates very well. For clarity the diffuse-reflection spectrahave been drawn for one case only (fig. 7), the reflection spectra of the otherphosphors being similar.

194 G. BLASSE. W. L. WANMAKER. J. W. Ier VRUGT and A. BRIL-----------------TOOlr"---r-----:::-r:-::::----=-:~----~___,;;:___==~----

Reflection (%);relativequantumoutput

tRefl.O~~~----~~ ~~ ~ ~ __~

450 400 350 300 250-).exc(nm)

Fig. 7. Diffuse-reflection spectra and relative-excitation spectra (quantum output) of phos-phors Me2MgSi207-Eu2+ (Me = Ca, Sr, Ba as indicated in the figure).

/

i II

O~-L------~ ~ ~ ~ ~450 400 350 300 250 220

Àexc (nm)-Fig. 8. Relative-excitation spectra (quantum output) of phosphors Me3MgSi20s-Eu2+(Me = Ca, Sr, Ba as indicated in the figure).

Relativequantum

'''IejY51J1----+---\--Plr---I---I---l

Fig. 9. The relative quantum efficiency for 250-270-nm excitation of the ft uorescence 0Sr2MgSi207-Eu2+ (I) and Sr3MgSi20s-Eu2+ (2) as a function of the activator concentra-tion (in atomic per cent relative to the alkaline-earth metal).

The optimum activator concentration amounts to a few atomic per cent ofeuropium (relative to the alkaline-earth metals). This is illustrated in fig. 9,where the relative quantum efficiency of the phosphors Sr2MgSi207-Eu andSr3MgSi20s-Eu is plotted against the activator concentration. These data werenot corrected for self-absorption. This effect may be more important for higher

FLUORESCENCE OF Eu2+-ACTIVATED SILICATES 195

than for lower Eu2+ concentrations, but will not affect the curves of fig. 9drastically.

Figures 10 and 11 show the temperature dependence of the fluorescenceintensity of the phosphors Me2MgSi207-Eu and Me3MgSi20-sEu.

100·r----r-=:::::::--"""""'=-=-=:-r-----,r----,----,-,

Int.

t

°OL_--~---L---L--~L--~L-~~-u400 500 500

-T(OK)

Fig. 10. Temperature dependence offluorescence intensity ofphosphors Me2MgSi207-Eu2+(Me2 = Ca2' Sr2' SrO.SBal.S' Ba2 as indicated in the figure); 254-nm excitation.

Fig. 11. Temperature dependence of the fluorescence intensity of phosphors Me3MgSi20s-Eu2+ (Me = Ca, Sr, Ba as indicated in the figure); 254-nm excitation.

Solid-solution series between isomorphous phosphors were also studied, forexample (Ca,SrhMgSi20s-Eu, (Sr,Ba)zMgSi207-Eu and (Ca,Sr)2MgSi207-Eu.In all cases it was found that the fluorescent properties varied regularly withcomposition. It is thus possible to obtain phosphors with a specific wavelengthposition of the emission band by an appropriate choice of the chemical com-position.The alkaline-earth magnesium silicates are ideal compounds for studying the

energy transfer from Eu2+ to Mn2+ as observed earlier in (Sr,Mg)3(P04)2 5)and alkaline-earth pyrophosphates 1). However, the transfer probability in thesilicates was found to be low.

Comparing our results on the binary silicates with the data on these phos-phors reported by Jenkins and McKeag 2), it is seen that there are serious

",

196 G. BLASSE. W. L. WANMAKER. J. W. Ier VRUGT and A. BRIL

discrepancies in the case of the barium silicates. They report a green emissionfor Ba3SiOs-Eu (we find yellow), a green or a blue emission for Ba2Si308-Eudepending on firing temperature (we find blue-green), and a blue emission forBaSi20s-Eu (we find green).

4. Discussion

It is nowadays generally agreed 6) that the absorption of the Eu2+ ion(4j1 configuration) in solids in the near ultraviolet region is due to the tran-sition 417 -)0 4f65d. We are only concerned with the lowest term of the 4f con-figuration, viz. 8S1/2' For the free ion the terms 6PDFGH and 8PDFGH areexpected from the excited configuration 4f6(1 F)5d. In solids the crystal-fieldsplitting of the 5d level will be roughly 10000 cm-I, so that the overall pictureof the excited states is very complicated. Therefore, we followan approachgiven by Wood and Kaiser 7) for the Sm2+ ion in alkaline-earth fluorides. The5d orbitals extend further into the space of the crystals than the 4f orbitals.Therefore the strong-field formalismis used to describe the 5d levels and theweak-field formalism to describe the 41 levels. Putting these descriptions to-gether we arrive at an approximate energy-level diagram, which gives a reason-able understanding of the phenomena observed.If one of the 41electrons is promoted to the 5d level, it leaves six electrons

behind in the lower 4f configuration. This is a 7Fo term (as in the case ofEu3+ (416». This term always gives the total-symmetric representation of thesite-symmetry group involved. The promoted electron occupies a 5d level, whichis split by the crystal field. Let us assume a cubic field in which the eg level isthe lower level, e.g. a cubic eight coordination. The other crystal-field level isthe t2g level. Since the 41 electrons give a 7Fo term, the excited 4[65d con-figuration will consist of two levels the separation between which is the cubiccrystal-field splitting. That such a simplification works was shown before byBlasse and Bril for Tb3+- and Ce3+-activated phosphors 8.9).

An investigation of the phosphor BaZr03: Eu2+ has shown the merits ofthis simplification for the Eu2+ ion 10). BaZr03 has the cubic perovskitestructure 11), in which the Eu2+ ion is regularly surrounded by twelve02- ions.The site symmetry is Oh' In this cubic field the eg level is expected to be the lowerone 12). The excitation spectrum of the (green) fluorescence of BaZr03-Eu2+shows two separate excitation peaks (see fig. 12). These correspond to the twoexcited levels of the 4f65d configuration expected for this symmetry. Data ofthis type are known to exist for the Eu2+ ion in alkaline-earth fluorides andalkali halides 13.14.1S) and have been collected together with those forBaZr03"Eu2+ in table Ill. Note that the centre of the excited configuration liesat a much higher energy in fluorides than in the chlorides, bromide and oxide.The same result was observed previously for Ce3+, Tb3+ and BP+ 8.9.16).

Let us now consider the Eu2+-activated silicates. The absorption bands of

FLUORESCENCE OF Eu2+-ACTIVATED SILICATES 197

I

t

Relativequantum

50~---r--+-+------r'_--~~-r--~---+~--'r--450 '"fP~

~OO 20~

Fig. 12. Spectral-energy distribution of the emission of BaZr03-Eu2+ at 365-nm excitation(left-hand side) and relative-excitation spectrum (quantum output) of this phosphor (right-hand side). Note the change in the wavelength scale.

TABLE III

Absorption bands, crystal-field splitting and centre of gravity of the 5d levelof Eu2+ in several host lattices (all values in 103 cm-I)

host coordination absorption crystal- centre ref.lattice Eu2+ ion bands field 5d level

splitting

CaF2 cube 27·1 45·0 17·9 37·9 13

SrF2 cube 27·9 43·5 15·6 37·3 13

BaF2 cube 28·5 42·7 14·2 37·0 13NaCI octahedron 29·4 41·5 12·1 34·2 14KCI octahedron 29·1 40·1 11·0 33·5 15

29·7 41·4 10·7 34·4 14

KBr octahedron 29·0 40·2 11·2 33·5 1529·7 38·1 8·4 33·1 14

BaZr03 regular 25·2 38·2 13·0 33·012 coordina-tion

the Eu2+ ion cannot be determined accurately from the reflection spectra ofthe powder materials, since the minima in the reflection curves are not verypronounced due to the strong absorption. Therefore, the excitation spectra areused to derive the absorption bands of the Eu2+ ions. From figs 6-8 it is seenthat the excitation spectra always contain a number of broad bands, extendingfrom 220 to 450 nm. Unfortunately the crystal structures of the host latticesused are-not known in all cases and, as far as they have been determined, they

198 G. BLASSE. W. L. WANMAKER. J. W. Ier VRUGT and A. BRIL

offer low-symmetrical sites to the large, divalent ions. From the discussionabove it follows that:(a) for low-symmetrical fields the first excited configuration of the Eu2+ ion

consists of levels up to a maximum of five (the d level is fivefold degenerate);if the host lattice contains more than one type of Eu2+ ion, this number maybe even higher;(b) for isomorphous host lattices with different alkaline-earth ions the ex-

citation spectra may be quite different, since the crystal fields on the Eu2+ ionmay be different.In all the host lattices mentioned, the symmetry of the Eu2+ site is extremely

low. Some examples are fJ-Ca2Si04, Sr2Si04 and Ba2Si04 with K2S04 struc-ture (containing a nine- and a ten-coordinated cation site), CaMgSi04 withmonticellite structure (strongly distorted octahedron around the Ca2+ ion),BaMgSi04 with BaAl204 structure (strongly distorted hexagonal 12 coordi-nation), Ca2MgSi207, Sr2MgSi207 with akermannite structure (8 coordina-tion of the alkaline-earth ion consisting of a cube with the bottom plane turned45° relative to the upper plane). The large number of Eu2+-absorption bandsis therefore ascribed to the low symmetry of the crystallographic sites involved.From the figures it follows that the excitation spectra are not similar for iso-morphous host lattices. Note, for example, the excitation spectra of the iso-morphous phosphors Ca2MgSi207-Eu2+ and Sr2MgSi207-Eu2+; that ofBa2MgSi207-Eu2+ with another structure is again different (fig. 7).Due to the low symmetry of the crystallographic sites, the total crystal-field

splitting of the 5d level is very large. As a result the absorption bands extendinto the visible region and the emission bands, which originate from the lowestexcited level, are situated at relatively long wavelengths (blue-green and evenyellow emission). Similar effects were observed recently for the Ce3+ ion 9).The decay time of the Eu2+-activated silicate phosphors is short, but not as

short as that of Ce3+-activated phosphors (~ 10-8 S 17)). This indicates thatthe fluorescent transition of the Eu2+ ion is slightly forbidden. The reason forthis is probably the existence of sextets in the excited configuration 4f6jd.A transition from an octet term ofthe 4f65d configuration to the 8S7/2 groundstate is completely spin-allowed. However, due to spin-orbit coupling the octetsand sextets ofthe excited configuration are mixed, so that the transition becomespartly spin-forbidden. This accounts for.the longer decay time of Eu2+ phos-phors as compared with that ofthe Ce3+ phosphors. In the case ofthe Ce3+ionwe .are involved with one electron only (and a number of completely filledshells).The temperature dependence differs from case to case. A strong influence of

the temperature on the fluorescence intensity was found, for example, for thephosphors Ca2MgSi207-Eu and CaMgSi04-Eu (see table I and also fig. 10).Consequently at room temperature these phosphors show only ~ relatively

FLUORESCENCE OF EuZ+-ACTIVATED SILICATES

weak emission. In the case ofthe phosphors Me3MgSi20s-Eu (Me = Ca,Sr,Ba)the influence of the temperature is less pronounced (fig. 11). It is noteworthythat the Stokes shift of the fluorescence of the latter phosphors amounts to1000-2000cm-1• This is a rough estimate, because the position of the lowestabsorption level can only be estimated from the broad bands in the excitationspectra. The phosphors with a low quenching temperature have a much largerStokes shift, viz. some 6000 cm-I. This accords with the general observationthat a large displacement between ground and excited state in the configurationcoordinate diagram (giving a large Stokes shift) will involve a high probabilityof non-radiative transitions (i.e. poor temperature dependence) IS).We further note that the influence of the temperature on the Eu2+ fluores-

cence becomes more pronounced in the sequence Ba, Sr, Ca (host lattices). Thisis especially clear for isomorphous or structurally related series of host lattices,like p-Ca2Si04, Sr2Si04 and Ba2Si04 with K2S04 structure (table I),Me3MgSi20s (Me = Ca,Sr,Ba) with merwinite structure (fig. 11) andCa2MgSi207, Sr2MgSi207 and Sro.sBal.sMgSi207 with akermannite structure(fig. 10).The ternary silicates Me2MgSi207-Eu2+ illustrate the marked influence of

the crystal structure and within one crystal structure the influence of the choiceof Me on the fluorescent properties. Note, for example, the regular shift of theemission band to shorter wavelengths in the series of isomorphous phosphorsCa2MgSi207-Eu2+, Sr2MgSi207-Eu2+ and Sro.sBal.sMgSi207-Eu2+ (fig.4).The phosphor Ba2MgSi207-Eu2+ (with another structure) has its emission atconsiderably longer wavelengths than would be expected from an extrapolationof the series mentioned above. The phosphor Ca2AI2Si07-Eu2+, which is iso-morphous with Ca2MgSi207-Eu2+, shows a blue emission peaking at 440 nm.The emission of Ca2MgSi207-Eu2+ peaks at 535 nm, showing the influenceof the replacement Mg2+ + Si4+ -->- 2AI3+. The quenching temperature offluorescence of'Ca-MgêijOj-Eu?", Sr2MgSi207-Eu2+ and Srx, sBal. sMgSi207-Eu2+ increases in this sequence, that of Ba2MgSi2 07-Eu2+, however, is con-siderably higher (fig. 10).

Acknowledgement

The authors are greatly indebted to Miss M. P. Bol and Miss D. J. M.Trumpie and to Messrs C. J. Looyen, J. A. de Poorter, G. J. R. A. Tops,J. G. Verlijsdonk and J. de Vries for their assistance in the experimental partof the work.

Eindhoven, November 1967

-,-,

199

200 G. BLASSE, W. L. WANMAKER, J. W. Ier VRUGT and A. BRIL

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