EFFECT OF THE ONE-DIMENSIONAL STRUCTURE ON THE ENERGY TRANSFER IN Li6Gd (BO3)3

HAL Id: jpa-00225051

https://hal.archives-ouvertes.fr/jpa-00225051

Submitted on 1 Jan 1985

HAL is a multi-disciplinary open access

archive for the deposit and dissemination of

sci-entific research documents, whether they are

pub-lished or not. The documents may come from

teaching and research institutions in France or

abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est

destinée au dépôt et à la diffusion de documents

scientifiques de niveau recherche, publiés ou non,

émanant des établissements d’enseignement et de

recherche français ou étrangers, des laboratoires

publics ou privés.

EFFECT OF THE ONE-DIMENSIONAL STRUCTURE

ON THE ENERGY TRANSFER IN Li6Gd (BO3)3

C. Garapon, B. Jacquier, Y. Salem, R. Moncorge

To cite this version:

EFFECT OF THE ONE-DIMENSIONAL STRUCTURE ON THE ENERGY TRANSFER IN L i6G d ( B 03)3

C.T. Garapon, B. J a c q u i e r , Y. Salem and R. Moncorge

Labovatoiice de Physico-Chimie des Matériaux Luminescents, VA 442 C.N.R.S., Université Lyon I, 43 Bd du 11 novembre 1918, 69622 Villeurbanne, France

Résume - L'étude de la fluorescence des ions Gd + dans LigGd(B0,), montre

qu'il y a diffusion de l'énergie parmi les ions Gd3+ et piégeage par des impuretés à haute température et par des ions G d3 + perturbés à basse

tempé-rature. Le passage d'un régime de diffusion rapide à un régime de transfert limité par la diffusion à basse température est attribué à la structure quasi-unidimensionnelle du cristal.

Abstract - The Gd + fluorescence has been studied in LigGd(B03)3. Energy

migration among G d3 + and trapping by impurities at high temperature and

perturbed G d3 + ions at low temperature are observed. The cross-over from a

fast diffusion regime to a diffusion-limited regime at low temperature is attributed to the quasi one-dimensional crystal structure.

INTRODUCTION

It has often been observed that energy migrates easily among Gd ions in Gd compounds and this property has been used in phosphors materials to transfer energy from a sensitizer to an activator /1,2/. The aim of the study of Li6Gd(B03)3 is to

see how the energy transfer among G d3 + ions is influenced by the crystal structure

which is0here one-dimensional. G d3 + ions belong to chains separated by a distance

of 6,65 A for an intrachain distance of 3.88 A /3/. The intrachain interactions should be dominant over the interchain interactions and the transfer might be expec-ted to be one-dimensional. No magnetic measurements at very low temperature have been performed at the present time. However, the shortest interatomic distance along the chain is comparable to the ones in magnetic order material such as GdCl3 or Gd(0H)3.

EXPERIMENTAL

The synthesis and single crystal growths have been achieved in the laboratoire de Chimie du Solide de Bordeaux /3/. Most of the results have been obtained with crys-tal 1 prepared with Gd203, 3N. Some additional results concern crystal 2 prepared

with Gd203 4N. The fluorescence spectra and decays have been obtained after selec-tive pulsed excitation with.a doubled dye laser pumped by a doubled neodymium YAG laser. Details of the apparatus have been described elsewhere / 3 / .

RESULTS

1) High temperature (T> 40 K) - The fluorescence spectrum of the transition from the~first"excited"manyfold 6P7/2 to the ground state 8S7/2 contains four lines as

expected for G d3 + ions in a low symmetry site ((4). The fluorescence decay of the

line I4 lying at lowest energy is exponential with a time constant at room tempe-rature of about 360 fcs for the crystal 1 and 820 j*s for crystal 2. These values are much smaller than the radiative life time which in this material is estimated as 4.35 ms (see later).

C7-142 JOURNAL

DE

PHYSIQUE

From t h i s exponential and f a s t decay we conclude t h a t f a s t diffusion of the energy

takes place among the Gd3+ ions nd that the energ gets trapped by impurities

( E U ~ +

5

f o r example whose l i n e s a t 5980

1

due t o emission 0 0 - + 7 ~ 1 are observed). Up t o now

the behavior of LigGd(B03)3 i s very similar t o t h a t of other Gd3+ compounds and no

information about the dimensionality of the energy diffusion may

be

obtained a s we

are dealing with a f a s t diffusion regime.

As t h e temperature i s decreased from 40

K

t o

1 . 5 K

the i n t r i n s i c fluorescence

decreases a t the advantage of new emissions a t lower energy labelled

Po

t o P10

( f i g u r e 1 ) .

Fig. 1

-

I n t r i n s i c

(14)

and t r a p (Pa) fluorescences from the

6

t r a n s i t i o n of Gd3+ a t 1.5

K

( c r y s t a l 1 ) .

be r e l a t e d t o d e f e c t centers s i n c e they behave d i f f e r e n t l y f o r c r y s t a l s 1 and 2. I n t h i s temperature range t h e energy mostly absorbed by t h e i n t r i n s i c ions i s spread over t h e whole c r y s t a l before g e t t i n g trapped i n perturbed ions. This i s a l s o confirmed by t h e fluorescence decay a n a l y s i s . The decay o f t h e i n t r i n s i c f l u o - rescence 14 i s no more exponential. We have checked t h a t t h i s i s n o t due t o t h e pump i n t e n s i t y o r o t h e r sources /3/. A simple t r a p p i n g model assuming f a s t d i f f u s i o n among i n t r i n s i c i o n s and t r a p p i n g by t h e most i m p o r t a n t t r a p s (P7 and P g down t o

10 K and P1 and

P2

f o r 4.4 K and 1.5 K) has been used.

The i n t r i n s i c and t r a p fluorescence decays are c o r r e c t l y f i t t e d r e s p e c t i v e l y by a sum and a d i f f e r e n c e o f two exponential f u n c t i o n s as i l l u s t r a t e d i n f i g u r e 2 a,b.

C7-144

JOURNAL

DE

PHYSIQUE

(say P i ) t o f l u o r e s c i n g t r a p s such as ~ b o r ~ E U ~ + + which a r e 1 s t nn o r 2nd nn. I n t h i s model t h e decay r a t e o f t h e s h o r t time f a s t component i s equal t o t r a p p i n g r a t e t o t h e t r a p . From t h e i n t e n s i t y o f t h e t r a p l i n e s r e l a t i v e t o t h e i n t e n s i t y o f the i n t r i n s i c l i n e a t v e r y low temperature an e s t i m a t i o n o f t h i s t r a p p i n g r a t e i s obtained ( K - 2 0 000 s - 1 a t 1.5 K f o r c r y s t a l 1 ) . This value i s n o t c o n s i s t e n t w i t h the value obtained from t h e decay r a t e (K- 6000 S-1 f o r c r y s t a l 1). T h i s x i s c r e - pancy, which i s observed f o r b o t h samples, leads us t o t h e conclusion t h a t t h i s 3+ s h o r t time component does n o t correspond t o t r a p p i n g b u t t o d i f f u s i o n between Gd i n t r i n s i c i o n s which i s then t h e l i m i t i n g s t e p i n t h e t r a n s f e r . To account f o r t h e i n t e n s i t i e s we expect a s t i l l f a s t e r component a t s h o r t e r times t h a t those sampled i n our experiment t h a t i s l e s s than 2 s. P r e l i m i n a r y r e s u l t s a t s h o r t e r time ( t i m e s c a l e 0 , 2 p s ) seem t o i n d i c a t e

tkt

the decay r a t e becomes indeed f a s t e r . This cross over from a f a s t d i f f u s i o n r e g i ~ e t o a d i f f u s i o n - l i m i t e d E g i m e a t low temperature has never been observed i n ~ d ~ compounds t o our knowledge and m i g h t be a t t r i b u t e d t o t h e one-dimensional s t r u c t u r e .

DISCUSSION

It has been proposed t h a t f o r a one-dimensional energy m i g r a t i o n t h e fluorescence decay should be exp(-A \rt) and exp(-A

fi

-

K t ) i f a t h r e e dimensional component i s present /4/. I n our case t h e s h o r t component d e v i a t e s o n l y s l i g h t l y from an exponential time dependence as shown i n f i g u r e 3.

The r e s u l t s may be explained as f o l l o w :

-

a t h i g h temperature t h e perturbed ions a r e i n e f f i c i e n t and f a s t d i f f u s i o n o f the energy takes place, probably along t h e chains as s t r o n g l y suggested by t h e c r y s t a l s t r u c t u r e .

-

a t low temperature t h e perturbed ions a c t as energy t r a p s t h e p r o b a b i l i t y o f back t r a n s f e r t o t h e i n t r i n s i c i o n s decreases as temperature decreases and t h i s h i n d e r s t h e d i f f u s i o n along t h e chains which becomes as slow as t o be t h e l i m i t i n g step i n t h e t r a n s f e r so t h a t d i f f u s i o n l i m i t e d t r a n s f e r takes p l a c e and as t o be o f the same order o f magnitude t h e slow i n t e r c h a i n energy t r a n s f e r so t h a t 3D d i f f u s i o n i s observed. This cross over from f a s t d i f f u s i o n t o d i f f u s i o n l i m i t e d t r a n s f e r might be considered as an i n d i c a t i o n t h a t t h e f a s t d i f f u s i o n c o u l d be ID. I f i t were 3D i t could n o t be prevented by such a small amount o f shallow t r a p s (which has been evaluated as about 10-3 / 3 / ) and f a s t d i f f u s i o n should remain probably as l o n g as i n t r i n s i c ~ d 3 + c o n c e n t r a t i o n i s more than 30 % ( o r d e r o f magnitude o f a p e r c o l a t i o n t h r e s h o l d a t 3D). Our understanding o f t h i s system i s l i m i t e d by t h e f a c t t h a t t h e perturbed ~ d i o n s a c t both as b l o c k i n g and t r a p p i n g ions depending on temperature ~ + and have n o t c l e a r l y d e f i n e d concentrations. I n o r d e r t o improve i t should be i n t e - r e s t i n g t o use b l o c k i n g ions l i k e ~ 3 + i n known concentrations and acceptors, i n t h e chains o r o u t t h e chains, such as the t r a p p i n g r a t e i s very f a s t as t o f a v o r t h e d i f f u s i o n l i m i t e d regime which i s t h e o n l y one where d i m e n s i o n a l i t y e f f e c t s can be seen.

ACKNOWLEDGEMENTS

We wish t o thank J.P. CHAMINADE f o r p r o v i d i n g new samples and G. BLASSE f o r several discussions.

REFERENCES

/1/ Mahiou,R., Jacquier, 6. and Linares, C., J.O.S.A.

73

(1983) 1383. /2/ Blasse, G., Phys. S t . S o l . (a) 73 (1982) 205.

/3/ Garapon, C.T., Jacquier, B., C h z i n a d e , J.P. and Fouassier, C., J. o f Lum. (accepted paper).