doi: 10.1149/1.20960551988, Volume 135, Issue 6, Pages 1574-1578.J. Electrochem. Soc. 

 J. P. M. van Vliet, G. Blasse and L. H. Brixner 

3?)?3?(??IO? x Eu x?−1?Luminescence Properties of the System Gd

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1574 J. Electrochem. Soc.: S O L I D - S T A T E S C I E N C E A N D T E C H N O L O G Y J u n e 1988

8. J. M o n k o w s k i , J. S tach, and R. E. Tressler , This Jour- nal, 126, 1129 (1979).

9. J. M o n k o w s k i , R. E. Tressler , a n d J. S tach , ibid., 125, 1867 (1978).

10. K. H i r a b a y a s h i a n d J. I w a m u r a , ibid., 120, 1595 (1973). 11. W. C. S c h u m b and D. F. Hol loway, J. Am. Chem. Soc.,

63, 2753 (1941). 12. W. C. S c h u m b a n d A. J. S tevens , ibid., 69, 726 (1947). 13. C. A. S tea rns , F. J. Kohl , G. C. F rybu rg , a n d R. A.

Miller, N A S A T e c h n i c a l M e m o r a n d u m 73726, N A S A - L e w i s R e s e a r c h Center , Cleve land , Ohio 44135 (1977).

14. N. S. J a c o b s o n , N A S A T e c h n i c a l M e m o r a n d u m 87270, N A S A - L e w i s R e s e a r c h Center , Cleveland , Ohio 44135 (1986).

15. R. A. Mil ler a n d F. J. Kohl , N A S A TM X-3564, NASA- Lewis R e s e a r c h Center , Cleve land , Ohio 44135 (1977).

Luminescence Properties of the System Gdl-xEux(IO3)3

J. P. M. van Vliet and G. Blasse

Physicat Laboratory, State University of Utrecht, 3508 TA Utrecht, The Netherlands

k. H. Brixner

E. L du Pont de Nemours and Company, Central Research and Development Department, Experimental Station, Wilmington, Delaware 19898

A B S T R A C T

T h e l u m i n e s c e n c e and e n e r g y t r ans f e r p rope r t i e s of t he s y s t e m Gd, ~Eux(IO3)3 (0.005 < x < 1) h a v e b e e n inves t iga ted . I n Eu(IO3)3 d i f fus ion l imi t ed e n e r g y m i g r a t i o n occurs at T > 25 K. The d i f fus ion c o n s t a n t has on ly a w e a k t e m p e r a t u r e de- p e n d e n c e . T h e IO3- g roup does no t l uminesce . A c o m p a r i s o n is m a d e w i th r e l a t ed Eu 3§ c o m p o u n d s .

During recent years the energy migration properties of inorganic lattices with a high concentration of Eu 3. ions have been investigated extensively. In our laboratory, the dimensionality of this energy migration has received par- ticular attention (1-5). Analysis of the time development of the Eu 3t emission can yield information about the process by which energy migration takes place in the lattice under consideration. Some of these analyses are discussed in Ref. (6). An extensive discussion of the processes which can be responsible for excitation transfer among ions in solids has been given in Ref. (7) and (8).

A commonly encountered phenomenon in lattices which contain luminescent ions is the quenching of the lu- minescence when the concentration of these ions is in- creased. This may be due to cross relaxation, or to energy migration to quenching centers where the excitation energy is lost nonradiatively. The latter process occurs often in Eu 3~ compounds (6). These quenching sites, or killers, may be impurities or defects, which are inevitably present in the lattice. The rare earth system under consid- eration in this paper, Gd1_xEux(IO3)3, appears to show un- usually weak concentration quenching. The aim of our in- vestigations was to look into this feature.

The crystal structure of Gd(IO3)3 has been described in Ref. (9). The lattice is monoclinic with only one crystallo- graphic site with eightfold coordination and site symmetry CI for the rare earth ions. The shortest Gd-Gd distance is 5.9/~. In the pyramidal IO~ group the iodine ion has a for- mal oxidation state of +5, which makes it a 5s 2 ion. It has three oxygen neighbors at a distance of about 1.8/~; there are three further neighbors at distances varying from 2.74 to 3.19A. Since s 2 ions are known to luminesee, we looked also for iodate luminescence. As far as we are aware, the optical properties of the iodate group have not yet been in- vestigated.

Experimental All m e a s u r e m e n t s d e s c r i b e d in th i s p a p e r were per-

f o r m e d on p o w d e r samples . T he s am p l e s were p r e p a r e d a c c o r d i n g to ReL (10). They were c h e c k e d by x-ray p o w d e r d i f f rac t ion . T G A m e a s u r e m e n t s ind ica te t h a t Eu(IO3)3 is s t ab l e to 550~ The c o m p o u n d Tb(IO3)3 shows a s imi la r behav io r . B o t h c o m p o u n d s t u r n s l ight ly p i n k u p o n heat- ing in air.

Di f fuse re f l ec tance spec t ra were r e c o r d e d u s i n g a P e r k i n - E l m e r L a m b d a 7 UV-VIS s p e c t r o p h o t o m e t e r . Gen- era l spec t ro scop i c m e a s u r e m e n t s at r o o m t e m p e r a t u r e

and liquid helium temperature (LHeT) were performed using a Perkin-Elmer MPF44B spectrofluorometer, equipped with a liquid helium flow cryostat. For the re- cording of high resolution excitation and emission spectra a setup was used consisting of a Molectron DL 200 dye laser pumped by a Molectron UV-14 N2 laser, in combina- tion w i t h a S p e x 1704-X m o n o c h r o m a t o r . The laser gener= a ted a pu l se w i t h a peak p o w e r of 30 kW and a w i d t h of 10 ns. The r epe t i t i on ra te of the laser was k e p t c o n s t a n t t h r o u g h all m e a s u r e m e n t s at a b o u t 30 Hz. The l i n e w i d t h of t he laser was 4 cm 1 at 530 rim. The d e t e c t i o n dev ice u sed was an RCA-C31034 fast p h o t o m u l t i p l i e r tube , w h i c h was k e p t in a coo led h o u s i n g at a b o u t -20~ Decay t i m e meas- u r e m e n t s were e x e c u t e d by p r o c e s s i n g the P M s ignal wi th a n ORTEC p h o t o n - c o u n t i n g sys tem, cons i s t i ng of a Mode l 9301 fast preampl i f ie r , a Mode l 574 fast t i m i n g ampli f ier , a M o d e l 436 100 MHz d i sc r imina to r , and a Mode l 7100 mul t i - c h a n n e l analyzer . D u r i n g the r eco rd ing of the h i g h resolu- t ion spec t r a a n d t he decay curves , the t e m p e r a t u r e of the s a m p l e cou ld be var ied b e t w e e n 1.7 K and r o o m t empera - t u r e b y u s i n g a T h o r c ryogen ics l iqu id h e l i u m b a t h cryo- s t a t e q u i p p e d w i th a Mode l 3020-II t e m p e r a t u r e control ler .

Results and Assignments Spectral properties.--All s amp le s show a s t rong red lu-

m i n e s c e n c e u p o n u l t rav io le t exc i t a t i on at r o o m t e m p e r a - t u r e a n d below. In the c o n c e n t r a t i o n series Gdl xEu~(IO3)3 (x = 0.005, 0.01, 0.05, 0.1, a n d 1) a n d the s a m p l e w i th x - 1 s h o w s t he m o s t i n t ense l u m i n e s c e n c e . F r o m x = 0.005 to x = 0.1, t he l u m i n e s c e n c e of t he s amp le s inc reases app rox - ima te ly l inear ly w i th x.

The exc i t a t i on s p e c t r u m of the Eu 3+ e m i s s i o n of Gd~ xEu~(IO3)3 cons i s t s of a w e a k b r o a d b a n d w i th a maxi - m u m at a b o u t 280 n m a n d s h a r p l ines in the r eg ion of 300- 600 nm. The b r o a d b a n d is due to t he cha rge - t r an s f e r t ran- s i t ion b e t w e e n t he Eu ~+ a n d O 2 ions. The s h a r p l ines cor- r e s p o n d to e lec t ron ic t r ans i t i ons w i t h i n t he 4f 6 conf igura- t ion of t he Eu 3~ ion.

The d i f fuse re f lec tance s p e c t r u m of Eu(IO3)3 shows two b r o a d b a n d s in add i t i on to t he sha rp l ines c o r r e s p o n d i n g to t he 4f 6 t r an s i t i ons of t he Eu 3+ ion. The i n t ens i t y of t he b r o a d b a n d s does no t d e p e n d on the e u r o p i u m concen t r a - t ion. T h e r e is a w e a k b a n d w i th a m a x i m u m at a b o u t 480 nm, w h i c h is p r o b a b l y due to a c e n t e r w i t h a d i f fe ren t va lency. A s imi la r o b s e r v a t i o n has b e e n m a d e for Eu3ReO8 (11). T h e r e f u r t h e r m o r e is a s t rong and b r o a d b a n d w i t h a m a x i m u m at a b o u t 250 rim, w h i c h m u s t be a sc r ibed to ab-

Vol. 135, No. 6 LUMINESCENCE PROPERTIES 1575

s o r p t i o n b y t h e IOs- group. A c c o r d i n g to M c G l y n n et al. (12), t h e IOs- a d s o r p t i o n s h o u l d be e x p e c t e d be low 300 nm. N o n e of t h e s e b r o a d b a n d s are p r e s e n t in t he exc i t a t i on s p e c t r u m of t he Eu 3+ emiss ion . This e x c l u d e s e n e r g y t r a n s f e r f rom the ioda te g r o u p to the Eu 3+ ion. T he weak- ne s s of t he cha rge - t r an s f e r b a n d in t he e x c i t a t i o n spec- t r u m m u s t be due to t h e fact t h a t t he g rea t e r pa r t of t he ex- c i t ing r a d i a t i o n is a b s o r b e d by t he o v e r l a p p i n g ioda te a b s o r p t i o n b a n d , so t h a t it does no t r each the Eu 3+ ions. S imi l a r o b s e r v a t i o n s were m a d e for EusReO8 (11) a n d Eu(NO3)3 �9 6H20 (13). We n e v e r o b s e r v e d any ioda te emis- sion, no t e v e n in Gd(IOs)~:Eu ~+ at LHeT.

F i g u r e 1 shows t he e m i s s i o n s p e c t r u m of Eu(IO3)s at LHeT. Fo r low Eu ~+ c o n c e n t r a t i o n s th i s s p e c t r u m is essen- t ia l ly t he same. The 5D0 - ~F0 t r a n s i t i o n at a b o u t 580 n m is ve ry weak, t he ~D0 - VF~ a n d ~D0 - VF~ t r ans i t i ons c o n t a i n 3 a n d 5 l ines, respec t ive ly . T he la t t e r va lues are to be ex- p e c t e d for si te s y m m e t r y C~. T he l inea r c rys ta l field t e r m n e c e s s a r y to o b s e r v e t he 5D0 - VF0 t r a n s i t i o n (14) is p roba - b ly v e r y weak. S o m e w e a k l ines are o b s e r v e d in t he r eg ion

100

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625 620 6i5 610 <--- X{nm} Fig. 1. Emission spectrum of Eu(IO3)3, recorded at 4.2 K. (a) Spectral

region 580 -710 nm, X ~ = 396 nm. The inset shows the splitting of the two strongest lines in the 5D o ~ 7F 2 transition. (b) Spectral region 605- 625 nm enlarged, ~ r = 527.44 nm. Spectrum recorded 30 I~s after the laser pulse, e = Electronic transltlon,v = vibronic transition.

607-613 nm. T h e s e l ines are p r e s e n t in the spec t r a of all s a m p l e s i n v e s t i g a t e d a n d w i t h t he s ame re la t ive in tens i ty . T h e y are a sc r ibed to v i b r o n i c t r an s i t i ons w h i c h b e l o n g ei- t h e r to t h e 5D0 - 7F0 t r a n s i t i o n (coup l ing w i t h t he ioda te s t r e t c h i n g v ibra t ions) , or to the 5D0 - 7F1 t r ans i t i on ( coup l ing w i th t he ioda te b e n d i n g v ibra t ions) . The ioda te - v i b r a t i o n a l f r equenc i e s h a v e b e e n r e p o r t e d to lie in t he re- g ion 330-820 cm 1 (15). F u r t h e r m o r e , in t ime- re so lved spec- t ra a sma l l a m o u n t of e m i s s i o n f rom the 5D1 level (~D~ 7F0 is f o u n d at 624.54 a n d 626.95 nm. T h e s e l ines v a n i s h w i t h i n 50 ~s af te r t he laser pu l se due to m u l t i p h o n o n re- l axa t i on of t he lower 5D~ level to t he 5D0 level. The w e a k b r o a d b a n d area a r o u n d 625 n m is a sc r ibed to v i b r o n i c t r ans i t i ons . T h e s e m a y be 5D0 - 7F~ coup l ed w i t h t he IO3- s t r e t c h i n g v i b r a t i o n s or 5D0 - 7F2 coup l ed w i th Eu 3§ - O 2- b e n d i n g v ib ra t ions . The w e a k fea tu re at 650 n m is a sc r ibed to a v i b r o n i c t r a n s i t i o n i nvo lv ing ~D0 - 7F2 a n d t he IO3 s t r e t c h i n g v ib ra t ions . The v i b r o n i c l ines i nvo lv ing the io- da t e g r o u p s h o u l d be c o n s i d e r e d as coope ra t i ve v i b r o n i c t r a n s i t i o n s (16).

Decay measurements.--In orde r to i nves t i ga t e t he t i m e d e p e n d e n c e of t he e m i s s i o n of t he Eu 3§ ion, t he decay c u r v e s of t he 5D0 ~ 7F2 e m i s s i o n h a v e b e e n m e a s u r e d ; for Gd0.9~Eu0.01(IO~)~ at r o o m t e m p e r a t u r e a n d LHeT, a n d for Eu(IO3)3 in t he t e m p e r a t u r e r a n g e f rom 1.7 K to r o o m tem- pe ra tu re . The decay c u r v e of the d i lu ted s a m p l e is pu re ly e x p o n e n t i a l w i th a decay t i m e of 1.6 ms, b o t h at L H e T a n d at r o o m t e m p e r a t u r e . The decay cu rve of t he c o n c e n t r a t e d s a m p l e is e x p o n e n t i a l at low t e m p e r a t u r e s w i th a decay t i m e of 1.6 ms. At a b o u t 24 K, the cu rves b e c o m e n o n e x - p o n e n t i a l for sho r t t i m e s af te r t he laser pulse . The decay t i m e of the e x p o n e n t i a l tail dec reases u p to T = 100 K. F r o m 100 K to r o o m t e m p e r a t u r e the decay t i m e r e m a i n s c o n s t a n t w i th a va lue of 1.3 ms. F igu re 2 p r e s e n t s some s e m i l o g a r i t h m i c plots of t he Eu(IOs)s decay at va r ious t em- pe ra tu re s . The d r a w n l ines are fits to a o n e - e x p o n e n t i a l f u n c t i o n t a k i n g in to a c c o u n t the b a c k g r o u n d cor rec t ion .

Discussion Spectral properties.--The n u m b e r of l ines o b s e r v e d in

t he 5D0 - VFj t r an s i t i ons do no t c o n t r a d i c t w h a t is e x p e c t e d theo re t i ca l ly for a Eu 3+ ion at a s i te w i th s y m m e t r y C1. The crystal - f ie ld sp l i t t ings are re la t ive ly small , for e x a m p l e s o m e 180 a n d 130 cm 1 for t he 7F~ a n d 7F2 levels , respec- t ively. The v i b r o n i c l ines h a v e b e e n d i s c u s s e d above . T h e 5D1 e m i s s i o n cou ld no t b e o b s e r v e d u n d e r c o n t i n u o u s ex- c i ta t ion , i n d e p e n d e n t of t he E u 3+ c o n c e n t r a t i o n . This m e a n s t h a t t he 5D1 ~ ~D0 r e l a x a t i o n is no t due to cross re- l a x a t i o n b e t w e e n n e i g h b o r i n g Eu ~+ ions, s ince th i s ef fec t w o u l d b e c o n c e n t r a t i o n d e p e n d e n t . I t is t he r e fo re con- c l u d e d t h a t t he 5Dr - 5D0 r e l a x a t i o n t akes p lace by m e a n s of p h o n o n emiss ion . In v i ew of t he fac t t h a t t he 5D, - 5D0 s e p a r a t i o n is 1690 c m - ' , a n d t h a t t he m a x i m u m p h o n o n ene rg i e s ava i l ab le lie in t he r a n g e 330-820 cm -~ (15), th i s s e e m s a r e a s o n a b l e a s s u m p t i o n . This m e a n s t h a t t he io- da te g r o u p s are r e s p o n s i b l e for t he r ap id 5D~ ~ 5D0 relax- a t ion. Th i s is a s imi la r s i t ua t ion as ha s b e e n o b s e r v e d for b o r a t e s (17).

As s t a t ed above , t he iod ine ion of t he IO3- g r o u p has the e l ec t ron i c con f igu ra t ion 5s 2. The so-cal led s 2 ions are wel l k n o w n l u m i n e s c e n t spec ies in all k i n d s of i no rgan i c lat- t ices (18). In Gdl-xEu~(IO3)3, a b s o r p t i o n of t he IO3- g r o u p can b e o b s e r v e d in t he d i f fuse r e f l ec tance s p e c t r u m . The u n u s u a l l y w e a k c h a r g e t r a n s f e r a b s o r p t i o n ih t he exci ta- t i on s p e c t r u m of t he Eu 3+ e m i s s i o n was e x p l a i n e d b y as- s u m i n g t h a t m o s t of t h e exc i t a t i on e n e r g y in t he spec t ra l r eg ion of th i s t r a n s i t i o n (~270 rim) is a b s o r b e d b y t he IO3- g roup . U p o n e x c i t a t i o n in th i s reg ion no e m i s s i o n of t he IO3- g r o u p was obse rved . This i nd i ca t e s n o n r a d i a t i v e re- l a x a t i o n of t he exc i t ed s ta te of t he I ~+ ion. The 15+ ion ha s t h r e e o x y g e n n e i g h b o r s at a sho r t d i s t a n c e (1.8A), a n d t h r e e at a m u c h la rger d i s t a n c e (2.7-3.2A), so t h a t in t he g r o u n d s ta te t h e I s+ ion occup ie s def in i te ly a n of f -center pos i t ion . In t he exc i t ed s ta te the ion ha s t he poss ib i l i ty of o c c u p y i n g a pos i t i on w h i c h is m o r e s y m m e t r i c w i th re- s p e c t to t h e six o x y g e n ne ighbor s . Th i s causes t he pa rabo- las of t he g r o u n d a n d exc i t ed s ta te to s h o w a large offset. W h e n th i s offset is suf f ic ient ly large, n o n r a d i a t i v e re lax-

1576 J. Electrochem. Soc.: S O L I D - S T A T E S C I E N C E A N D T E C H N O L O G Y June 1988

Fig. 2. Decoy curves of the Eu 3+ emission intensity in Eu(103)3. Xem = 615.13 nrn, ;%~ = 527.44 nm. Drawn lines discussed in text. Curve 1, T = 4.2 K; curve 2, T = 24 K; curve 3, T = 75 K; curve 4, T = 1 6 0 K .

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0 2 i g g lb 17 TIME {ms} - - >

a t ion occu r s rapidly , e v e n at low t e m p e r a t u r e s . T he off- c e n t e r pos i t i on in t he ioda te g roup is e x t r e m e ( the ra t io be- t w e e n s h o r t a n d long I-O d i s t a n c e s is 0.6). I n BP + a n d P b 2+ c o m p o u n d s t h e s i t ua t ion is less e x t r e m e (rat io 0.8), a n d t h e s e c o m p o u n d s of ten s h o w a n eff ic ient l u m i n e s c e n c e ( 1 8 , 1 9 ) .

O b s e r v a t i o n s s imi la r to Eu(IO3)3 u n d e r ioda te e x c i t a t i o n we re m a d e for Tb(IOa)3, Sm(IO3)3, Dy(IOa)3, a n d Tm(IOa)~. T h e Tb 3+ ion has a 4f a -* 4f75d t r a n s i t i o n in t he spec t ra l re- g ion of t he IO3- abso rp t ion . U p o n i r r ad ia t ion w i t h U V l ight , t h e Tb 3§ ion in th i s la t t ice does no t l u m i n e s c e be- c a u s e t he e x c i t a t i o n e n e r g y is a b s o r b e d b y t he ioda te g roups . The o t h e r ra re e a r t h ions do no t e v e n h a v e t rans i - t i ons w h i c h can c o m p e t e w i th t he ioda te abso rp t ion , so t h a t t he a b s e n c e of t he i r e m i s s i o n is obvious . However , Tb(IOa)a s h o w s eff ic ient Tb 3+ l u m i n e s c e n c e u n d e r x-ray ex- c i t a t i on at 300 K. Th i s i nd ica t e s t h a t t he e l ec t rons and ho l e s c r ea t ed u p o n x- ray exc i t a t i on r e c o m b i n e at t h e T b a+ ions , r e s u l t i n g in cha rac t e r i s t i c Tb 3+ l u m i n e s c e n c e . Appa r - en t ly t he ioda te a n i o n s do no t p lay a role in the x-ray lumi- n e s c e n c e process .

Energy transfer.--In t h e c o n c e n t r a t i o n ser ies of Gdl-~Eu:~(IO3)a w h i c h we h a v e m e a s u r e d (x = 0.005, 0.01, 0.05, 0.1, a n d 1) t h e s a m p l e w i th x - 1 shows t h e s t r o n g e s t l u t n i n e s c e n c e . Th i s i nd ica t e s t h a t c o n c e n t r a t i o n q u e n c h - ing is no t s t r o n g in th i s c o m p o u n d .

S i n c e t r a n s f e r of exc i t a t i on ene rgy is i m p o s s i b l e in t he d i l u t ed c o m p o s i t i o n s , t he m e a s u r e d decay t i m e of the di- l u t ed c o m p o s i t i e n is equa l to the r ad ia t ive decay t i m e of t he E u a§ ion in th i s lat t ice, viz., 1.6 ms.

The d e v i a t i o n f rom e x p o n e n t i a l behav io r , a n d t he t em- p e r a t u r e d e p e n d e n c e of t h e decay c u r v e s of t h e concen - t r a t e d sample , sugges t t h a t e n e r g y m i g r a t i o n t akes place, a l t h o u g h it is no t a s t rong effect. The exc i t a t ion e n e r g y mi- g ra te s ove r the E u a+ ions (donors) to q u e n c h i n g s i tes (ac- ceptors) , w h e r e it is los t nonrad ia t ive ly . In our sys tem, t he n a t u r e of t he accep to r s is no t k n o w n , b u t t hey m a y be cen- te r s w i t h dev i a t i ng va lency. The cha rac te r i s t i c s of the t r a n s f e r p roce s s can be d e t e r m i n e d by c o m p a r i n g our ex- p e r i m e n t a l decay cu rves w i th theo re t i ca l e x p r e s s i o n s for t h e t i m e d e v e l o p m e n t of the l u m i n e s c e n c e of d o n o r s in t he p r e s e n c e of acceptors . T h e s e e x p r e s s i o n s are d e s c r i b e d in Ref. (8). T h e s h a p e of our decay cu rves at T > 24 K, w h i c h are n o n e x p o n e n t i a l for sho r t t i m e s af ter the exc i t a t i on pulse , b u t e x p o n e n t i a l for l onge r t imes , po in t s to a diffu- s i on - l im i t ed e n e r g y t r a n s f e r process . T he d e c a y t i m e of t he e x p o n e n t i a l p a r t o f t he c u r v e is t h e n g iven b y

1 1 1 - + - - [ 1 ]

T T O 7 D

w h e r e r is t h e m e a s u r e d decay t ime, ~0 t h e r ad ia t ive decay t i m e of t he Eu 3+ ion, a n d rD the decay t i m e d u e to dif fusion�9 Y o k o t a a n d T a n i m o t o (20) h a v e de r ived a n e x p r e s s i o n for t h e to ta l d o n o r decay w h i c h is g iven b y

I(t) = I(0) exp(- tH0)

exp[-4/3"~31aNa(Ct)m( -1+10"87X+1 + 8.743-X15"50X~I3z4]/J [2]

in w h i c h

X - DC-t/3t 2~3 [3]

Here NA is t he accep to r concen t r a t i on , C is a p a r a m e t e r de- s c r i b i n g the d o n o r - a c c e p t o r in t e rac t ion , a n d D is a diffu- s ion cons t an t . For t --, ~ an e x p o n e n t i a l decay ra te is pre- d i c t ed

l / ' r D = ll.404NACV4D 3/4 [4]

T h e va lues of TD for the s y s t e m Eu(IO3)3 h a v e b e e n de r ived f r o m the e x p e r i m e n t a l decay cu rves u s i n g Eq. [I]. The ob- s e r v e d t e m p e r a t u r e d e p e n d e n c e of XD is in a first approx i - m a t i o n c a u s e d by the t e m p e r a t u r e d e p e n d e n c e of D, be- cause ~D -1 is on ly d e p e n d e n t o n C to t he p o w e r 1/4 (see Eq. [4]). Also, i t is no t ve ry l ikely t h a t t he spec t r a l ove r l ap of t h e a c e e p t o r s w i t h t he Eu ~+ e m i s s i o n var ies s t rong ly w i th t em- pe ra tu re . The va lues of TD-4/3~ w h i c h are t h e n p r o p o r t i o n a l to D, h a v e b e e n p lo t t ed as a f u n c t i o n of t e m p e r a t u r e in Fig. 3.

Le t us n o w c o n s i d e r f rom w h e r e t he o b s e r v e d t e m p e r a - t u r e d e p e n d e n c e of D or iginates . I n t he ioda te c rys ta l lat- t ice t h e s h o r t e s t E u - E u d i s t a n c e is re la t ive ly long, viz., 5.9A, w h i c h m a k e s e x c h a n g e i n t e r ac t i on b e t w e e n t he Eu 3§ ions i m p r o b a b l e . At low t e m p e r a t u r e s e n e r g y m i g r a t i o n m u s t t ake place via t he 7F0 - 5D0 t r a n s i t i o n w h i c h is ex- t r e m e l y w e a k in th i s lat t ice, as can be seen in t he spect ra . Th i s exp l a in s w h y t h e r e is no m i g r a t i o n at T ~ 24 K: all t ypes of i n t e r ac t i on van ish . The t e m p e r a t u r e d e p e n d e n c e of D ar ises f rom the t h e r m a l p o p u l a t i o n of the 7F1 and per- h a p s t he 7F2 levels. T h e r m a l l y ac t iva t ed m i g r a t i o n t h e n t akes p lace v ia t h e 7F1.2 - 5D0 t rans i t ions . S imi la r eases h a v e b e e n d e s c r i b e d b y W e b e r [Eu 3+ in p h o s p h a t e glass, Ref. (21)] a n d K e l l e n d o n k a n d Blasse [EuAl~B4012, Ref. (17)]. I n o rde r to ca lcu la te t he t e m p e r a t u r e d e p e n d e n c e of D t h e y u s e d the fo rmu la

gr exp( -Er /kT) gf f~f2 D �9 - . [ 5 ]

~ gr exp( -Ef /kT) g, vifAv,r f

Vol . 135, No. 6 L U M I N E S C E N C E P R O P E R T I E S 1577

.

f %

C3 F~

r ...J

.

.6z .oz, l IT (l/K) >

Fig. 3. Temperature dependence of ~o -4/~. Drawn line discussed in text.

where Ef is t he e n e r g y of level f; vif the f r equency , f,f t he os- c i l l a tor s t r e n g t h a n d h,,if t he l i n e w i d t h of t h e t r a n s i t i o n be- t w e e n levels i a n d f c o n s i d e r e d a n d g t he d e g e n e r a c y of t he levels . In a first a p p r o x i m a t i o n on ly t he VF1 level is t a k e n in to a c c o u n t (as a d e g e n e r a t e level) a n d fif and vif are sup- p o s e d to be cons t an t s . F o r m u l a [5] t h e n r e d u c e s to

3 exp( - AEI/kT) 1 D = �9 [6]

1 + 3 e x p ( - h E J k T ) exp( -hE2/kT)

w h e r e AE1 is the e n e r g y d i f f e rence b e t w e e n the 7F0 a n d 7F1 levels , a n d Avis is a s s u m e d to b e h a v e exponen t i a l l y . F r o m ou r spec t r a i t can be de r ived t h a t t he 7F~ levels lie at 240, 320, a n d 420 c m -1 a b o v e t he g r o u n d state. I f we t ake AE, to be 300 c m -l, t h e d r a w n l ine in Fig. 3 r e p r e s e n t s a fit of t he e x p e r i m e n t a l p o i n t s to Eq. [6] w i t h AE2 = 250 cm-L Con- s i d e r i n g t he e x p e r i m e n t a l u n c e r t a i n t y in t he va lues of ~D, t h e fit c u r v e fol lows t he t r e n d of t he t e m p e r a t u r e d e p e n - d e n c e r e a s o n a b l y wel l a t t e m p e r a t u r e s a b o v e 25 K. B e l o w th i s t e m p e r a t u r e e n e r g y m i g r a t i o n van i shes . Also, t he h E va lues are in t h e o r d e r of m a g n i t u d e to be e x p e c t e d . The m e n t i o n e d u n c e r t a i n t y in t he va lues of ' r D 1, w h i c h are t he re la t ive ly smal l d i f f e rences b e t w e e n ~-~ a n d v0 1, p r o h i b i t s a de t a i l ed ana lys i s of t he t e m p e r a t u r e d e p e n d e n c e . Never- t he l e s s we can c o n c l u d e t h a t in Eu(IO3)3 t h e r e is a re- s t r i c t ed a m o u n t of t h e r m a l l y ac t iva t ed e n e r g y m i g r a t i o n over t he E u 3§ subla t t ice .

C o m p a r i n g Eu(IO3)3 w i t h EuA13B4O12 (17), i t can be s e e n t h a t in t he case of Eu(IO3)3 t he t e m p e r a t u r e depen- d e n c e of D is less p r o n o u n c e d . I f we e s t i m a t e the re la t ive r a d i a t i v e ra te for t he 5D0 --~ 7F1 t r a n s i t i o n s (P0 1) in t h e s e la t t i ces u s i n g t he r ad ia t ive decay t i m e a n d t he re la t ive in- t en s i t i e s oft :he t r a n s i t i o n s in t he e m i s s i o n s p e c t r u m , we ar- r ive at P0-1 -- 200 s -1 in EuA13B4012 a n d P0-1 -~ 130 s - ' in Eu(IO~)3. A l t h o u g h t h e d i f f e rence is n o t large, th i s m i g h t e x p l a i n w h y e n e r g y m i g r a t i o n in EuAl~B40~2, w h e r e t he E u 3 + - E u 3+ d i s t a n c e is t h e s a m e ( -5 .9A) as in Eu(IO3)3, is m o r e effect ive. In EuA13B40~2 t he Eu 3+ ions are in t r i gona l p r i s m a t i c coo rd ina t ion , so t h e r e is no i n v e r s i o n s y m m e t r y . I n Eu(IO3)~ t h e E u 3§ ions o c c u p y a d i s t o r t ed d o d e c a h e d r a l site w h i c h is c lose r t-o i n v e r s i o n s y m m e t r y t h a n t he tri- g o n a l prismatic site. T h e 5D0-TF ~ t r a n s i t i o n is m a i n l y mag- ne t i c d ipo le in charac te r . Th i s c h a r a c t e r c a n n o t b e ex- p e c t e d to depend s t rong ly on coord ina t ion . T h e r e f o r e t he h i g h e r P0-, v a l u e of EuA13B4012 is a s c r i b e d to a h i g h e r a m o u n t of e lec t r ic d ipo le c h a r a c t e r in t h e 5D0-TF~ t rans i - t ion , w h i c h in t u r n leads to a s t r o n g e r E u ~ + - E u ~§ dipole- d ipo le i n t e r a c t i o n n e c e s s a r y fo r t he migra t ion . In th i s con- n e c t i o n it is i n t e r e s t i n g to note that in GdAlaB~O~ t h e r e is sti l l a r e s t r i c t e d a m o u n t of e n e r g y m i g r a t i o n in t h e G d ~§

s u b l a t t i c e at 300 K. However , in LiGdP4012 w h e r e t he Gd ~+ ions a re in d o d e c a h e d r a l c o o r d i n a t i o n w i th s h o r t e s t G d 3§ G d 3+ d i s t ance equa l to 5.6A, t he m i g r a t i o n is absen t , e v e n at 300 K (22). This was e x p l a i n e d by a s s u m i n g t h a t G d 3 + - G d 3+ e n e r g y t r a n s f e r occurs m a i n l y b y e x c h a n g e w i t h a sma l l c o n t r i b u t i o n f rom elect r ic d ipo le -d ipo le inter- ac t ions . In LiGdP4012 th i s c o n t r i b u t i o n is m u c h sma l l e r t h a n in GdA13B4012, s ince t he s i te s y m m e t r y in t he phos - p h a t e is c loser to i n v e r s i o n s y m m e t r y t h a n in t h e bora te . Th i s r u n s para l le l w i th t he a r g u m e n t g iven above . T h e G d 3+ e x a m p l e is m o r e impress ive , b e c a u s e t h e c o m p o u n d w i t h t he l a rger G d S + - G d 3+ d i s t ance (GdA13B40,2) shows m o r e m i g r a t i o n t h a n t h a t w i t h t h e s h o r t e r d i s t a n c e (LiGdP40~2). In LiEuP4012 c o n c e n t r a t i o n q u e n c h i n g of t he E u 3+ l u m i n e s c e n c e at 300 K does no t occu r (23), w h i c h sug- ges t s t h a t in L iEuP40 ,z t he e n e r g y m i g r a t i o n is a lso of a re- s t r i c t ed na tu re . U n f o r t u n a t e l y t he l u m i n e s c e n c e of G d 3+ in t he i oda t e s was h a r d to i nves t i ga t e due to a b s o r p t i o n b y t h e h o s t la t t ice in t he spec t ra l r eg ion of in te res t .

In ana logy w i th EuAl~B4Oi2, we can t ry to e s t i m a t e t he v a l u e of t h e d i f fus ion c o n s t a n t D at r o o m t e m p e r a t u r e by u s i n g Eq. [4]. Wi th th i s va lue we can t h e n e s t i m a t e t he dif- fu s ion l e n g t h l, t h e h o p p i n g t i m e tH, a n d t he n u m b e r of s t eps n in t he m i g r a t i o n p roces s (24)

= (6D~0) lj2 [7]

tH = a2/6D [8]

n = ~oltH [9]

He re a is t h e s h o r t e s t d i s t a n c e b e t w e e n two n e a r e s t ne igh- bors . The fac tor C in Eq. [4] c an be e s t i m a t e d w i th (17)

1 C [10]

.r 0 R06

The cr i t ica l t r a n s f e r d i s t ance R0 a n d NA can be e s t i m a t e d f rom the d i f fuse re f l ec tance s p e c t r u m to be a b o u t 16A a n d 10 TM c m -3, respec t ive ly . With t h e s e va lues we c o m e to: D = 10 -1' cm2s -1, C = 10 -38 cm6s - ' , l = 30A, tu = 6.10-5s, n = 30. One m u s t bea r in m i n d t h a t t h e s e va lues are no m o r e t h a n r o u g h es t imates . Still, t h e s e va lues are i n d e e d m u c h sma l l e r t h a n for EuA13B4012 [n = 1400, Ref. (17)].

In c o n c l u s i o n we can s ta te t h a t t he e n e r g y m i g r a t i o n in Eu(IO3)3 occu r s b y t h e r m a l p o p u l a t i o n of t he 7F~ levels o f t h e Eu 3§ ions. The w e a k c o n c e n t r a t i o n q u e n c h i n g e n c o u n - t e r e d in t he c o m p o u n d Eu(IO3)3 is due to t he fact t h a t t he e n e r g y mig ra t ion , w h i c h is a b s e n t at low t e m p e r a t u r e s , ha s on ly a w e a k t e m p e r a t u r e d e p e n d e n c e . This is c o n n e c t e d w i t h t he w e a k i n t e r a c t i o n s b e t w e e n t he Eu ~ ions, e v e n via

1578 J. Elec trochem. Soc.: S O L I D - S T A T E S C I E N C E A N D T E C H N O L O G Y J u n e 1988

the 7FI levels. Therefore energy migration to quenching centers remains an inefficient process even when the tem- perature increases.

Acknowledgments The investigations were supported by the Netherlands

Foundation for Chemical Research (SON) with financial aid from the Netherlands Organization for Advancement of Pure Scientific Research (ZWO).

Manuscript received July 2, 1987.

The E. I. du Pont de Nemours and Company assisted in meeting the publication costs of this article.

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Nucleation of Precipitates of ZnS and (Zn, Cd)S for Phosphor Synthesis

M. S. Abrahams* and C. J. Buiocchi

David Sarnof f Research Center, Princeton, New Jersey 08543-5300

ABSTRACT

Monodisperse particles of ZnS and (Zn, Cd)S were grown by precipitation from an acidified water solution. We have examined these materials by analytical electron microscopy. The typical spherical precipitates have diameters ~ 5 p.m; in addition, the volume fraction of voids is about 35%. At high magnifications, (~50,000•215 using SEM, a precipitate resembles a ball of tangled string. Thinned precipitates were examined by STEM and TEM. A central plane of (Zn, Cd)S shows a central region with a large concentration of Cd + S; this region has a diameter ~ 2.3 ~m. The diameter of the outer region is about 4.0 ~m and contains mostly Zn + S. This happens because the solubility product of CdS is smaller than that of ZnS by several orders of magnitude. Suitably thinned samples of both ZnS and (Zn, Cd)S show the development of small holes, which are found predominantly at the center of the precipitates; the precipitates are thought to nucleate on bubbles, (e.g., H2S or air), which are revealed as holes upon ion-thinning. The microstructure shows lamellae, (width ~ 40 nm), arrayed in spoke-like fashion about the center of the precipitate. It is likely that these crystals represent a new exam- ple of spherulitie growth.

Despite the fact that sulfide precipitates have been pro- duced for many years, little is known about their mecha- nism of growth. Also, little is known about the growth of the precipitate after nucleation or its detailed crystalline morphology. The precipitate is the precursor of the final phosphor particle. After adding fluxes, activators, etc., the precipitate is subjected to heat-treatment for its change to the final phosphor. As will be discussed later, the open structure of the original precipitate becomes quite dense upon such conversion. During the characterization of ZnS and (Zn, Cd)S precipitates grown from solution, several new observations were made. For instance, the structure of the precipitate is not dense; rather, it is an open struc- ture with a volume fraction of fibrils of about 65% for this particular material. Also, the formation of precipitates very likely occurs via spherulitic growth. These two items are the subjects of this paper.

* Electrochemical Society Active Member.

Procedure Crystal growth.--The specimens were prepared by Wil-

liams, Yocom, and Stofko. The method of crystal growth used is based on precipitation from acidified aqueous solu- tion and has been used for the preparation of ZnS and (Zn, Cd)S materials (1). The sulfur is provided by continuous hydrolysis of thioacetamide that is present in the reaction mixture. A solution of Zn § and H2SO4 [for ZnS] and Zn § Cd § and H2SO4 [tbr (Zn, Cd)S] was held at 80~ in the presence of dissolved thioacetamide (CH3CSNH2). The thioacetamide hydrolyzes slowly and generates H2S. The result is a slow precipitation of uniform particles of ZnS or (Zn, Cd)S over a period of about 0.5h. These particles are found to be monodisperse aggregates.

Specimens.--Due to the small diameters (~5 ~tm) of the precipitate particles, those selected to be examined were first formed into a slab-like wafer, held together by a con- ducting epoxy. Specimens for TEM and STEM were then