Structure, nucleation and Eu3+ luminescence in alkaline earth rare earth borate glass

~TMCAL

OPTICAL MATERIALS 1(1992) 271-280 North-Holland

Structure, nucleation and Eu3~luminescence in alkaline earth rare earth borate glass R. Jagannathan, P. Jeyagopal and A. Mani Central Electrochemical Research Institute, Karaikudi-623006, India Received 24 June 1992

In La 203—MO—B2O3 ternary system, various glasses/glass ceramics with M = Ca, Sr and Ba have been prepared. In this ternary system, homogenous nucleation occurring in the B303 melt appears to be the cause for the formation of nanocrystallites, hence 5— tetrahedral oxygen, while the caseearth of bigger earths like Ba and Sr,earths (BO like 3— glass ceramics. The nucleation processcoordination is very muchwith dependent on theinalkaline usedalkaline viz., with smaller alkaline Ca, boron prefers triangular coordination (B04) seems to be predominant. Eu3~in this glass system yields intense 5D 7F, emission. A cursory 3) view on the dependence of the various Judd—Ofelt parameters (Qk) indicates that Q 0—. 3~)while £24 is not. Various results 2 parameter based on these is very aremuch discussed. dependent on the immediate vicinity of the luminescent ion (Eu

1. Introduction

Research on the luminescence of lanthanide ions (Eu3~,Nd3~) and transition metal ion (Cr3~)isacquiring much interest for their potential applications in pumped laser systems, luminescent solar concentrators (LSC) etc. [1—3]; in particular the various applications of luminescent glasses, glass ceramic materials have been extensively discussed by Boulon in ref. [2]. It should of the various lan3 + isbeofnoted greatthat technological interest thanide ions, Eu for its application in lamp phosphors [4] and laser systems [5]. Interestingly, Eu3~ion can serve as an excellent structural probe to explore the immediate vicinity of the host matrix for the reason that it has got several structure dependent hyper-sensitive 5D 7F~ [6]. In this investigation,tranthis sitionshas viz.,been 0-+ point gainfully utilized to have an insight in the formation of borate rich alkaline earth rare earth borate glass ceramics. Also various results pertaming to the formation of borate glass ceramics/ nano crystallites are discussed.

2. Experimental

prepared by the usual solid state reaction methods, which involves firing at a temperature around 900— 1000°Cwith starting materials taken being either analysed reagents or 3,9’s purity (Ln 203). The obtamed crystalline product was checked for phase purity and found to be phase pure conforming to the X-ray powder pattern reported in the literature [71. The corresponding glass/glass ceramic samples were prepared by homogenizing the rawmaterials in agate mortar and the mixand forthen 1—2 transferred h at 250— 300°Cto be preheating free of moisture into a graphite crucible covered with lid in order to minimize the volatilization loss of B 203 during firing. Firing was carried out at 1100—1200°Cand the melt was then poured into a graphite mould held under various conditions as given in table 1. The purpose of keeping the mould under different conditions was to see the influence of cooling rate on the formation of glass ceramics. The presence of nanocrystallites can be confirmed with the line(s) present in the X-ray diffraction pattern. XRD results were obtained using a computer controlled X-ray Powder Diffractometer JEOL JDX 8030 with CuIç (Ni filtered) radiation at a rating of 40 kY, 20 mA. The scan rate was 0.05—20°per step and the measuring time was 1 s per step. The

LaCaB 7O13 crystalline phosphor sample has been

scanning range was 10°to 85°for 20. The full width

0925-3467/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

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Table 1 Composition and heat treatment conditions for various lanthanide alkaline earth borate glasses. Glass Sample

Composition

JJ2

8La

JJ7

2O3—2Eu2O3—2OCaO—70B203 1OLa 2O3—2OCaO—70B2O3

JJ8

8La2O3—2Eu2O3—2OCaO—70B2O3

JJ9

8La2O3—2Eu2O3—2OCaO—70B2O3

JJ 10

8La2O3—2Eu2O3—2OCaO—70B203

JJ1 1

8La2O3—2Eu2O3—2OBaO—70B2O3

JJ12

8La2O3—2Eu2O3—2OSrO—70B2O3

Heat treatment conditions

900°C,4 hr, 3~(crystalline) —~LaCaB7O13: Eu 1100°C,3 hr, (melt poured into) +graphite mould kept at 300°C 1100°C,3 hr, -. (melt poured into) +graphite mould kept at 300°C (-do-)+ graphite mould kept at room temp (30°C) (-do-)+ melt in the crucible left in the furnace. (-do-) + graphite mould kept at 300°C (-do-)+graphite mould kept at 300°C

at half maximum (fwhm) of the most intense peak of the patterns were measured using the existing peak search software. Luminescence measurements for out the crystalline/glass samples developed were carried using a Hitachi 650-lOS Fluorescence Spectrophotometer equipped with a 150 W xenon lamp and a

crystalline counterpart are also presented. The purpose underlying is tocrystalline compare the ef3 + this in both and quantum glass matrificiency of Eu ces having the same chemical composition.

Hamamatsu R928F photomultiplier tube. The excitation spectra recorded were corrected for the beam intensity variation and the emission spectra were corrected for the spectral response of the photomultiplier tube to used. In order carry out relative quantum efficiency (?lrei) measurements for Eu3~(5D 7F 2)red phosemis3~ (2 m/o) 0—~ crystalline sion, was LaCaB7O13:Eu phor used as internal standard. For calculating the percentage ofreflectance (% ref), a high alumina pellet was used as the standard reflector. Infra-red spectra for various glasses were recorded with the help of a Perkin-Elmer 783 double beam IR spectrophotometer in a KBr mount.

crystalline system

3.1. Luminescence of Eu3 in LaCaB 7O13

3. Results and discussion Though the objective of this communication is to discuss about the luminescence of Eu3~in borate glass/glass ceramics system, results on the luminescence of Eu3~in the LaCaB 7O13, the corresponding 272

In La203—CaO—B203 ternary system, a new phase corresponding to monoclinic LaCaB7O13 been 3 + has yields an reportedred recently [7]. favouring In this system, Eu intense emission its prospects as UV conversion system. phase The emission trum of Eu3 phosphor in this crystalline is given inspecfig. 1. From the emission spectrum, the following observations can be made. The presence of various transitions of Eu3~,viz. 5D 7F~ (J=0 to 2, other higher f’s are not taken into 0—~ consideration) are quite significant owing to their structure dependent nature, in particular the J= 0, 2 cases. (i) Strictly speaking, 5D 7F 0—3~sites 0 is forbidden is having a but linear observable in the case of field Eu viz., C,,.. [8]. Neverfield term for the crystal theless, for a given Eu3~site, we can expect only one Stark component (2J+ 1 = 1 for J= 0). But we could observe the presence of two components (576 nm, 577.5 nm) of nearly equal intensity. 5D 7F (ii) In the case of the 0—~ 1magnetic dipole

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OPTICAL MATERIALS

________________________________________ T°iOU ~ —1--~

I I

580

575

3.2. Formation ofglass ceramics in La203—MO— B203 (M= Ca, Sr and Ba) ternary system

‘8 —

I

I 620

610

600

I 590 ri nm

580

570

Fig. 1. Emission spectrum of Eu3~in LaCaB 7F 5D 7O13 crystalline ma3~). The solid trix at T=line 100arrow K ~ in the 5Dnm:7F 0-. 2 transition of Eu 0-. 2 emission 5D indicates 7F the presence of the additional component (inset), 0-. 0 emission having two components as indicated by arrows.

transition (in the orange region around 590 nm), there are four Stark components which are too numerousJtofold be attributed a single site,i.e., even2J+ with1 = the3 entire degeneracytobeing lifted (maximum) 3 for J= 1. (iii) In the case of the 5D 7F 0—~ 2electric dipole transition in the red region around 620 nm, we observe three maj or Stark components, with a shoulder-like structure (indicated by an arrow in fig. 1) in the central component. It should also be noted that the line width of the central component 5D is about 7F 25% more than that of the other two 0—~ 2 Stark components. From these, it is apparent that the emission spec3~being trum (fig.than 1) may the manifestation of Eu in more one be site. It should be noted that the inhomogeneous shape ofthe 5D 7F 0 0 emission can not be categorically as3~sites pressigned to two types of inequivalent Eu —~

ent in the system. Alternatively, this can as well be attributed to multi-site distribution or structural disorder in the vicinity of Eu3~sites. However, with the emission spectra recorded at T= 100 K, the lowest temperature accessible to us, it is not possible to draw any precise conclusion’. Additional results on the quantum efficiency of Eu3~emission in the LaCaB 7O~3crystalline system is discussed in section 4.

57

/ ~ ‘4

6~J

November 1992

Glass matrices prepared from borate rich La203— MO—B203 ternary system have got scope for applications in pumped laser system, LSC, etc. In this ternary system, B203 for its low melting point and high bond strength (496 kJ/mole) can act as good glass former (forming oxide), while La203, MO can act as modifiers of the glass network because of their lower bond strengths viz., 240, 135 kJ/mol, respectively. In our earlier observation [9],found the addition for3 + were to induceofcrys3 tallization, but it has now been found to be incorrect. eign ions like Eu Gd .

.

.

~,

Surprisingly, like Eu3~,theeven formation in the absence of nano of crystallites any foreign (hence ions the glass ceramics) has been found to be prevalent (fig. 2). Indeed, the sharp line(s) present in the XRD pattern (fig. 2) characterising the presence of nano crystallites corresponds to well known B203 line(s) [10]. In striking contrast to our expectations, the degree of crystallinity is found to be more in the un3~. doped sample thanofwith the one doped EuproThe percentage crystallinity (% C) with is the portion of nano crystallites (in the total matrix consisting of both crystalline and amorphous regions) observed in the experimental sample to that of internal standard (undoped glass sample). The percentage of crystallinity and crystallite size (/1) have been calculated for various samples and the results are given in table 2. The identification of the nano crystalline structure exhibited by the various glass samples was characterized by sharp lines occurring at lowindiffraction angles. has Thebeen percentage of crystallinity a glass ceramics determined as the percentage of integrated X-ray intensity of the nano crystallite peak with respect to that of an internal standard JJ7determined (table 1). Moreover,viz., the undoped crystalliteglass sizesample (fi) was 273

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November 1992

(for CuK,,= 1.5418

175 i~ ~

A) 0 is the Bragg’s angle of the

peak considered, where and B Bmeas is theis half-width given as 2B~easB~td, the measured halfB width and B 55d is the standard half-width at the par-

JJ 12

100

ticular Bragg angle 0 due to experimental broadening. I

From table 2, it can be seen that the glass sample prepared3~without addition of anyhas foreign impurities (undoped glass-JJ7) got the highest like Eu crystallinity (relative percentage of crystallinity being 100). But for the other glass samples doped with Eu3~,the %C decreases. Higher crystallinity in the former case points to the conclusion that the homogenous nucleation in B 203 melt is the active phenomenon that is responsible for the formation of nano crystallites in the undoped glass samples. In the 3~tends to reduce the latter, the substitution accumulating effect of with anionic Eu complex viz., borate network. Furthermore, it should be noted that the intense diffraction lines observed in the XRD pat-

I ii

150

____________________________ I

-

—

250

-

too

-

350 I

150 I

550 I

tern correspond to B

400

203 lines. Interestingly, in La203—MO—B2O3 ternary system, the role of alkaline earth as modifier of the glass network has got a profound effect on the formation of nano crystallites as evidenced from table 2 and XRD patterns for various glass samples prepared with different alkaline

-

U

I

I 25.0

10.0

400

I 550

28

Fig. 2. XRD patterns of various glasses/glass ceramics. B2O3 incompositions various glasses aretogiven in table 1). lines (the dicates XRD of lines corresponding prominent B203

using Scherrer formula [11], which is normally employed for an ordered system, given by 0 92 B 0

(A)

(1)

,

where 2 is the wavelength of the X-ray radiation used

earth ions (fig. 2). Using the same arguments advanced by Boulon 3~ et ions in the will play the role network al. [12], it glass can besystem reasonably expected thatofthe Eu modifier. Also it is interesting to see that the role of alkaline earths (MO=CaO, SrO and BaO) as network modifiers is more pronounced than that of the dopant viz., Eu3t This can be explained as due to (i) the influence of Eu3~as network modifiers will be less pronounced owing to its small content in the glass composition which is limited to a few mole per-

Table 2 Percentage of crystallinity (%C) and crystallite size (/J) of nano crystallites in the borate glass system.

274

Glass sample

d

JJ7 JJ8 JJ9 JJ1O JJ11 JJ 12

3.318 7125 0.330 0.00513 3.386 2017 0.291 0.00430 3.348 1859 0.234 0.00310 3.411 350 0.284 0.00420 3.348 923 0.423 0.00690 No characteristic XRD line (absence of nano crystallites)

A

Intensity (counts)

FWHM

Bmea, (radian)

%C(rel)

/3 (A)

100 28 26 5 13

303 360 500 368 225

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OPTICAL MATERIALS

cent; (ii) lower bond strengths of the alkaline earths and their presence in higher proportions in the glass composition. These factors will greatly influence the formation of nano crystallites in the glass system. It should be noted that, of the three alkaline earths used, is the smallest 1.37 A rsr2+ =Ca 1.50 A and rBa2+ in = ionic 1.66 Asize for(rca2± a ten=coordinated system). This should enable the Ca2 + ion to be an intermediate in the glass network during the formation of nano crystallites through homogeneous nucleation process. But, in the cases of bigger alkaline earths like Ba2~and Sr2~,this seems to be not possible. This can be attributed to the higher coordinative need of oxygen in the nucleation process. This would increase the viscosity of the melt, which would hamper the nucleation process resulting in poor crystallinity in these cases. Incompatibility of the bigger alkaline earths as intermediates can be explained by considering the coordination of B3~with 02_ ions. Boron can coordinate with oxygen in two ways either (a) in tetrahedral coordination as (BO 4) 3. or (b) in triangular coordination as (B03) In the case of smaller alkaline earth (Ca), the case (a) should be possible while for Ba and Sr, case (b) seems to be more possible. These can be schematically illustrated as given in fig. 3. The latter scheme can be contemplated for Ba and Sr analogues to account for poor crystallinity. Interestingly, this is verifiable with the knowledge of the IR spectrum, for the fact that (B0 3 ) ~ and (B04 ) ~ have 3their maxima in different regions. Foror-a group, the stretching vibrations of the (B03)

November 1992

tho borate group is around 1300 cm_i while for the (B04 ) ~ group, it is around 1100 cm [13]. Indeed, the IR spectra recorded for different glass samples, lend support to our argument. Using IR spectroscopy, a structural analysis of different borate groups 5,such triangular ) ~ tetrahedral etc., asis possible with(B03 the knowledge of the (B04) various fundamental frequencies involved. However, this becomes difficult for systems involving both triangular and tetrahedral coordinations, as the various fundamental modes of vibration involved tend to overlap thereby making the process of identifying different groups difficult. Especially, this effect is much pronounced in disordered systems-glasses [14]. This may well account for the shapes of the IR spectra obtained for different glasses in fig. 4. —

~,

3.3. Luminescence ofEu3~in borate glass ceramics Luminescence spectra of Eu3~in various oxidic glasses are different from that of their crystalline counterpart in that the spectra observed due to var-

~

,...

+

I

JIll

4~

(B~

(a) L~°~1m~ ~

o /

3

1P\ (b) [8203]

(B03)

+ Ba0/Sr0~ 0/--~--\0

~

1400

12140

1080

920

760

600

n,ett irregular layered structure

Fig. 3. The possible schemes for the formation of nano crystallites in the La2O3—MO—B2O3 ternary system,

...—

)~in

nm

Fig. 4. IR spectra of different rare earth alkaline earth borate glasses (for JJ7 glass M = Ca and for JJ 11 glass M = Ba).

275

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ious transitions are not well defined owing to in homogenous spectral broadening. This can be illustrated with the spectra of Eu34’ emission in crystalline LaCaB 7Oi ~and its corresponding glass modification (figs. 1 luminescence and 5). Spectral broadening observed the various transitions of Eu34’ in theinglass matrix can be attributed to an ensemble of sites distributed over a narrow energy region. Based on ionic size considerations and local electro-neutrality, Eu3 + ion can be expected to occupy La3 ± sites rather than Ba24’ sites. Lack of definition in the various Stark levels of Eu34’ observed in the emission spectrum, points to the conclusion that the luminescent ion can be located in the glass phase with more probability than in nano crystallites. The luminescence of Eu3 + in various oxidic glasses like borates, silicates, germanates and phosphates has been extensively studied [15]. These studies were focussed on the influence ofthe glass composition on the various transitions between various Eu3’4 levels. In the glass system, owing to lack of long range ordering, almost all the transitions of Eu3~,in particular 5D 7F,, (J= 0, 2, 4, 6) electric dipole transi0—+ tions can be expected, with their relative intensities being muchThe dependent on the relaxation particular from glass systemvery chosen. multi-phonon higher 5D,, (J= 1, 2) to 5D 0 levels makes the 3’4’ emis5D lusion from the 0 level dominant. A typical Eu minescence spectrum in lanthanum calcium borate glass is given in fig. 5. In this spectrum, one could observe all the 7F 1 manifolds. Of the various transitions observed, the most important one that can serve as5Da probe inant 7F to study the glass system is the dom0—~ 2transition around 612 nm corresponding to a red region called as hypersensitive transition in a familiar way [61. It should be noted that emission around nm in the yellow region 5D 5907F~ transition is relatively structure independent and can be used as reference corresponding to 0—s for further studies. 3.4. Judd—Ofelt parameters and transition probabilities for various Eu3’4’ transitions

November 1992

-__________________________________________ ==

276

2

j~~

/

JJ11

5~.~F

3°°~

I JJ8

I

I 680

,_1\!,_,hi~’~’ I

______________

620

680

620

~

~3

620

560

—

Fig. 5. Emission spectra of Eu34’ in various glass hosts of the La 2O3—MO—B2O3 ternary system nmT=300 corresponding 34’—O2 charge transfer band (3~-.260 excitation at K). The nos. 0, 1, 2, 3, 4 indicate various Eu34’ transitions involved in to Eu the emission.

/=3. However, these transitions are allowed as forced electric dipole transitions for this ion being surrounded by ligands of the glass system in continuous vibration facilitating the admixture of opposite par5D 7F~is ity states in 4f levels. But the transition 0—+ allowed as magnetic dipole transition governed by selection rules /=0 and J= 0, ±1 (with J=O-i-~0).The intensity of this magnetic dipole transition is almost constant. Using Judd—Ofelt theory [6,16], the transition probability for electric dipole transitions can be found out using the familiar relation AED(w, —~) 1

64ir4a3e2a

—

For Eu34’ ion involving intra-configurational f~f transition, the various 5D 7F~ (J=0, 2, 4, 6) tran0—~ dipole process, regsitions are forbidden by electric ulated by Laporte’s selection rules LiJ= ±1, J°~ 2/with

:~—~

—

~

41r6~

3h

QklI

k=2,4,6

(2) where AED is the electric dipole transition probabil-

Volume 1, number 4

OPTICAL MATERIALS

ityforatransitionfrom5D0(~~)-+7F~(~)withJ=O, 2, 4, 6. 2+2)2/9 is the correction for the refraca=n (n material with n being refractive index of tivity ofthe the material, a is the wave number corresponding to a given transition. are the Judd—Ofelt parameters and I U(k) is the reduced matrix element for a given transition, which is constant for a given transition and can be considered to be independent of the host matrix, The Judd—Ofelt parameter Qk is the most important parameter and is characteristic of the system in which the 12k rare(k= earth ion6)is will present. The 2, 4, enable oneknowledge to get an of various idea about the transition probability of a given electric dipole transition. Based on the arguments given by Oomen and van Dongen [15], it can be reasonably said that the relative integrated intensities of 5D 7F 5D 7F 5D0-+ 7F2 and 0—+ 4 transitions with respect to 0—~ more important transitions and can 1betransition taken to are represent the Q 2 and Judd— Ofelt parameters respectively for a given Q4 glass systern with no major difference in refractive indices, 34’ The relativeare integrated transitions denotedintensities as RI (Ffor various Eu 2) and RI(F4), respectively, 34’ emission 3.5. ~ovalency related effects on Eu intensity in borate glasses The Eu34’ emission in various borate glasses is fig. 5. The various transitions corresponding to different J manifolds of 7Ff, viz. 5F 0 (578 5D 7F~(590 nm), 5D 7F 5D nm), 7F 0—~ 0—~ 3 5D 7F 0-~+ 2(612 nm), (650 nm) and 0—~5D 4(695 7F nm) are observed. The other transitions like 0—+ 5~6are so weak that they are not taken into consideration and hence are not presented in the figure. By calculating the relative integrated intensities for various Eu34’ emission lines with respect to 5D 7F 0—~ 1emission intensity, table 3 can be obtained, In eq. 3(2) giving thethough transition probability ± transition, the exact physicalfor sig-a given Eu nificance ofthe familiar Judd—Ofelt parameters (Qk) remains unresolved [17], on closer examination of the emission spectra, some broad conclusions are forthcoming (fig. 5 and table 3). given in

November 1992

From table 3, it can be seen that the relative intensity of 5D 7F 0—~ 2{RI(F2)} is glass, very much dependent on the composition of the in particular the alkaline earth content. Interestingly, the relative intensity of the 5D 7F 0-+ 4 transition {RI(F4)} is not markedly dependent on the composition ofthe glass system. In the glass system, as the alkaline earth is changed from Ca to Ba and Sr, there is a marked decrease in RI(F2), while the variation in RI(F4 is insignificant. These values correspond to Judd—Ofelt parameters Q2 and Q4, respectively [15]. The marked decrease in RI (F2), representing Q2 on substitution with bigger alkaline earths (Ba and Sr) can be explained as thenetwork/polyhedra manifestation of a in structural change in the borate the immediate vicinity of the Eu34’ ion. The incompatibility of a (BO 5 tetrahedral network to accomodate the bigger4) alkaline earths (as modifiers) as intermediates can be envisaged. It is reasonable to expect that the substitution with bigger modifier ions, 34’—02 distance leading to would increase the Eu 34’ and 02_ ions. This weaker covalency between Eu argument can be advanced to contemplate the lower value of RI (F 2) observed in the Ba and Sr glasses. Another important consequence of decrease in covalency with the substitution of bigger alkaline earths is the shift in the maximum of CT excitation band. of excitation spectra given in fig. Careful 6 leads examination to the following. (i) Forglasses, the CTB excitation maximum shifts by a few nm towards longer wavelength, as the % C increases (fig. 6a). (ii) In the glass samples with Ca being replaced by Sr, there is a shift by a few nms in the CTB maximum towards higher energy while for the Ba glass sample, the shift is not pronounced (fig. 6b). It should be noted that replacement of Ca as modifier by bigger alkaline earths like which Sr and in Baturn results in in34’—O2~distance leads to. creased Eu weaker covalency of the ligands. This should result in a shift in the maximum of the CT band to a higher energy. These results only point to the conclusion that covalency cannot independently influence the position ofalone CT band maximum and a detailed study is needed (taking into consideration crystallite size and percentage of crystallinity) in this direction.

277

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November 1992

Table 3 34’ in various borate glasses. Relative integrated emission intensites for Eu Glass

RI F

sample

I(5D 7F 5D 0 -. 7F 2) RI(F2)= I( 0—. 1)

5D

JJ2 JJ8 JJ9 JJ1O JJ1 1 JJ12

4.00 5.33 4.33 3.29 3.2i

0.63 0.70 0.66 0.53 0.59

I(

7F 0 —.

4)

% Ref

98 50 12 5 30 41

100% ~ 1% ~ 1% ~ 1% ~ 1%

HH

JJ8

HH

~uz jjq.~...

~

—

.1J8

,jjio—o—o—o—

,JJ 11 JJ12————-

I

p~

‘.x30

,j

f

/

/

!i I,

‘\,

/

(a)

/

~

I

f ~

(b)

,~7s’\~

fl/I,

I!,,

l\

Ii

a

:3

/

_

~‘

I.)’

\~

:.,TIT---”

I ~—

300

A

in

‘I I

I.

_iiIi/~ 340

I

1

_~.

~ iif~\\ / I ~\\~

380

‘\

I

Ii’,

Ij(

~\

260

//

I 220

380

nm

34’ emission monitoring the 5D

340 ~—

1 I

I

300

A

260

220

in nm

7F

Fig. 6. Excitation spectra of Eu 0-. 2 transition (—.612 nm) in various borate glasses at T= 300 K. This figure shows the dependence of position of CT band maximum on (a) percentage of crystallinity and (b) glass modifier-alkaline earth ion used.

4. Lower quantum efficiency in glasses It is not surprising that quantum efficiency for Eu3 + emission in a glass matrix is considerably lower, by orders of magnitude as compared to that of its crystalline counter-part (table 3). This can be attributed to pronounced non-radiative transfer from 278

3 ±_02 charge transfer the excited (CT) state,state, in theusually loose Eu surroundings of the glass modification [18]. From fig. 7a, it can be seen that, for the case of a crystalline system (LaCaB 3’4’), the excitation in the CT band 7Oi3: Eufeed (Pi) various 5D,, levels which in can effectively turn can lead to an intense 5D 7F.,Eu3’4’ emission. 0—~

Volume 1, number 4

OPTICAL MATERIALS

_________________________________________ 3~o2..

Icharge transfer band)

Eu

,/ \ / I /

/

I,

//

I

//

-

~

p

2

T~ 0/ Lu

yields intense red emission. The formation of glass ceramics (nano crystallites) in this system is found to occur through a homogeneous nucleation process. Substitution with bigger alkaline earths (Ba and Sr) induces the propensity to be more glassy unlike the

casesbe case centage can with ofexplained of bigger smaller crystallinity modifier inalkaline terms (~ ions of earth 10the in%)the incompatibility (Ca). observed ternary A low system in perthe of

alternatively the borate network the triangular to have tetrahedral coordination coordination, seems to be rameters (Qk), the Q2 parameter is very much deapendent possibleonmechanism. the and immediate variousof Judd—Ofelt the luminespa34’, theOf Q thevicinity cent ion-Eu 4 parameter appears to be independent of the immediate surroundings of the luminescent ion. This study has opened up avenues

J Ia) __________________

5. Conclusions Eu34’ in a La 203—MO—B203 ternary glass system

~ I

November 1992

Ib

___________________

Fig. 7. Configurational coordinate diagram 34’ emission explaining in the (a) low the quantum efficiency crystalline system and in (b) a glass the system. corresponding Eu glass modification. p~and P2 represent CTB-.5DJ and CTB—sground state non-radiative transfer rates, respectively.

to the extension of this kind of behaviour of the alkaline earths as modifiers in other glass systems too and detailed study is needed in this direction to verify this. References [1] J.W.M. Verwey and G. Blasse, Mater. Chem. Phys. 25

Interestingly in the corresponding glass modification, the CT band feeding 5D~levels have less scope than the previous case as the loose surroundings of the glass modification will encounter less resistance for the expansion of the excited state viz., the CT state, resulting in a pronounced non-radiative transfer to the ground state (P2) than feeding 5D~levels Cm). This leads to a lower quantum efficiency for Eu3’4’ emission in the glass system. It can be explained with the help of a configurational coordinate diagram as shown in fig. 7b. Another important aspect that leads to lower quantum efficiencyin the glass system is the scattering due to nano-crystallites present in the glass matrix, From table 3, it can be seen that scattering losses (higher reflectance ) is pronounced forglasses with higher % C, resulting in poor quantum efficiency.

(1990) 91. [2]G. Mater. (1987) 301. [3] L.J.Boulon, Andrews, G.H.Chem. Beau Phys. and A.16Lempicki, J. Lumin 36 (1986) 65. [41B.M.J. Smets, Mater. Chem. Phys. 16 (1987) 283. [5] M.J. Weber, Handbook on the physics and chemistry of rare earths, eds. K.A. Gshneidner and L. Eyring, Chap. 35 (North Holland, Amsterdam, 1979). [6] R.D. Peacock, Structure and Bonding 22 (1975) 83. [7] Chen Risheng, Huang Jinggen, Lu Ling and Xu Yan, Mater. Res. Bull 23 (1988) 1699. [8] G. Blasse and A. Bril, Philips. Res. Repts. 22 (1967) 481. [9] R. Jagannathan, P. Jeyagopal, M. Avudaithai and R.L. Naryanan, Mater. Res. Bull, submitted. [10]Fang Youngxing, in: New frontiers in rare earth science and applications, Vol. II, eds. Xu Guangxian and Xiao Jimei (Science Press, China, 1985) p. 1469. [11] B.D. Cullity, in: Elements of X-ray diffraction (Addison Wesley, Massachusetts, 1956) p. 99. [12] J.L. Adam, V. Poncon, L. Lucas and G. Boulon, J. Non’ Cryst. Solids 91 (1987) 191. [13] S. Tanabe, K. Hirao, N. Soga and T. Hanada, J. Solid St. Chem. 91 (1992) 481.

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OPTICAL MATERIALS

[14] JO. Edwards and V.F. Ross, The chemistry of boron and its compounds, ed. EL. Muetterties (Wiley, 1967) Chap. 3. [15] E.W.J.L. Oomen and A.M.A. van Dongen, J. Non. Cryst. Solids 111 (1989) 205, and references 1—11 therein. [16] R. Reisfeld, Structure and Bonding 22 (1975)123.

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[17] R. Reisfeld and C.K. Jorgensen, Handbook on the physics and chemistry ofrare earths, Vol. 9, eds. K.A. Gschneidner and L. Eyring (North Holland, Amsterdam, 1987) Chap. 58. [18] G. Blasse, Advances in Inorganic Chem. 35 (1990)319.