SPATIAL ENERGY TRANSFER IN MgO : Cr3+

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SPATIAL ENERGY TRANSFER IN MgO : Cr3+

A. Boyrivent, M. Ferrari, E. Duval, A. Monteil

To cite this version:

A. Boyrivent, M. Ferrari, E. Duval, A. Monteil. SPATIAL ENERGY TRANSFER IN MgO : Cr3+.

Journal de Physique Colloques, 1985, 46 (C7), pp.C7-95-C7-98. �10.1051/jphyscol:1985718�. �jpa-

00224967�

JOURNAL DE PHYSIQUE

Colloque C7, supplément au n°10, Tome 46, octobre 1985 page C7-95

SPATIAL ENERGY TRANSFER IN M g O : C r3 +

A. B o y r i v e n t , M. F e r r a r i , E. Duval and A. M o n t e i l

£44 442 au C.N.R.S., Vnivevsite de Lyon I, 69622 Villeurbanne, Franoe

Résumé - L'excitation sélective de la fluorescence d'ions Cr dans MgO montre que seul un direct et rapide transfert d'énergie existe entre différents sites. Cela est dû à l'agglomération des ions C r3 + autour des lacunes de Mg2+.

Abstract - Selective excitation of the fluorescence from Cr ions in MgO shows that only direct and rapid energy transfer exists between different sites. It is due to the aggregation of C r3 ions around of M g2 + vacancy.

I - INTRODUCTION

If in ruby, where the Cr sites are non-centrosymmetric the dipole-dipole energy transfer is important, in MgO:Cr3+ crystal where the C r3 + sites are octahedral or a little perturbed by the vacancies it is expected that the energy transfer is indu- ced principally by superexchange interaction.

F.L.N, on the R line in MgO showed that energy diffusion among Cr + ions in octa- hedral sites was negligible /l/. However MacCraith et al observed the lifetime of the emitting ?E level of octahedral sites in the luminescence decay of C r3 ions in rhombic sites / 2 / . This implies that energy diffusion would be present. Our experi- ments are not in agreement with the paper of MacCraith et al : for concentration higher than 750 ppm the 2E life time of C r3 + in octahedral sites does not appear - in the rhombic decay. That confirms the negligible energy diffusion in the MgO:Cr crystal.

On the other hand Mc Donagh et al /3/ found that there is spin memory during the energy transfer from cubic sites to rhombic sites. It implies that the transfer time from cubic to rhombic sites is short, in any case shorter than the spin-lattice rela- xation time, T-,, which is equal to 6 0 0 ^ . Even if energy diffusion is negligible, a rapid direct transfer from cubic or quasi-cubic to rhombic sites is possible.

The observation of the rapid transfer to rhombic sites, using time resolved fluores- cence, was unsuccessful. It is very difficult to observe a rise time in the rhombic fluorescence because it is impossible to excite cubic sites without exciting rhombic sites, and it i;s further complicated by the rapid decay of the luminescence (35/is).

Such difficulties don't exist for tetragonal sites where C r3 + ions are associated to a vacancy.

II - EXPERIMENTAL RESULTS AND DISCUSSION

Figure 1 shows the emission of Cr +-vacancy (N,) Cr +-vacancy-Cr + (N,) species. On figure 2 excitation spectra of N. and N , fluorescence are shown. The N., and N? lines correspond to the transitions from the Tower sublever Ev coming from trie splitting of the 2 E level by the tetragonal field. In the excitation spectrum of N, there

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985718

C7-96 JOURNAL DE PHYSIQUE

Fig.

1

- Fluorescence spectra Fig.

2

- Excitation spectra of

N1

and

N2

l i n e s .

appears e s s e n t i a l l y the t r a n s i t i o n from the ground level t o the upper sub-level

E U .

On the contrary i n the excitation spectrum of N2 there appear several l i n e s in addi- tion t o the t r a n s i t i o n from the ground level t o the upper sublevel

E u .

Such a s t r u c - ture has been observed by Mc Donagh and Henderson

/4/

who a t t r i b u t e d the s i x high energy l i n e s t o the t r a n s i t i o n s between magnetic sublevels of a pair. The

N3

l i n e

( f i g u r e

1)

would correspond t o the fluorescence of t h i s p a i r . The following experi- mental r e s u l t s do not confirm t h i s a t t r i b u t i o n .

When the N2 fluorescence i s detected, a f t e r excitation in the a,b,c,d ... l i n e s (Fig. 2 ) , a r i s e appears before the decay, as shown in Fig. 3. The r i s e time varies between

50

s and 00 s . The decay time i s equal to 8,3 ms, the l i f e t i m e of

E

level of Cr3+-va~ncy-Cr6 Z s o c i a t i o n , f o r a l l the excitation l i n e s . This l a s t rerul!

indicates t h a t the a,b,c,d ... l i n e s correspond t o t ansitions of special ~ r 3 + ions which d i r e c t l y t r a n s f e r t h e i r energy towards the CrSi-vacancy-Cr3+ pair.

Fig.

3

-

N 2

fluorescence as function Fig.

4

- Excitation spectra of N2

of the time a f t e r b-excitation fluorescence a t

T =

2K and

T =

90K.

I n f i g u r e 4 i t appears t h a t t h e e x c i t a t i o n spectrum does n o t change very much w i t h the temperature. Only a s l i g h t increase o f t h e f l i n e i s observed when t h e tempera- t u r e goes from 2 K up t o 80 K. I f a superexchange i n t e r a c t i o n constant J = 1.5 cm-1 i s assumed i n t h e ground l e v e l o f a h y p o t h e t i c a l p a i r the r e l a t i v e i n t e n s i t y v a r i a - t i o n o f t h e a,b,c,d,e,f l i n e s would be much b i g e r . Therefore i t i s much more l i k e l y t h a t these l i n e s correspond t o a b s o r p t i o n o f C r j ' i o n s forming t r i a d s w i t h t h e t e t r a g o n a l p a i r (N2) around a ~ g 2 + vacancy.

I n the luminescence spectrum ( f i g u r e 1) t h e appears a t h i r d l i n e , c a l l e d N3 by Mc Donagh e t Henderson /4/, between N2 and N1. F i g u r e 5 shows t h e e x c i t a t i o n spec- trum o f t h i s N3 luminescence. Two i n t e n s e l i n e s appear : t h e one a t h i g h energy corres onds t o the f l i n e ( f i g u r e 2) and t h e o t h e r a t lower energy i s separated by 93 cm-y from t h e N emission l i n e . I t i s the same s p l i t t i n g so t h a t observed b e t - ween t h e two N1 l i 8 e s and t h e two N l i n e s . A p o s s i b l e i n t e r p r e t a t i o n i s t h a t these two e x c i t a t i o n l i n e s and N3 l i n e befong t o a t r i a d o f ~ r 3 + ions which a r e n e x t neighbours o f a Mg2+ vacancy. Two o f them are s i t u a t e d on a (001) a x i s and form a p a i r , t h e t h i r d s i t u a t e d on a (010) a x i s . Obviously t h e energy t r a n s f e r between t h e p a i r and t h e t h r i d ~ r 3 + 'on o f t h e t r i a d i s expected t o be very f a s t . The f l i n e would correspond t o t h e aA2-+Eu t r a n s i t i o n o f t h e p a i r and t h e o t h e r e x c i t a t i o n l i n e corresponds t o t h e same t r a n s i t i o n o f t h e t h i r d

cr3+

i o n . The N3 emission l i n e would be t h e emission o f t h i s t h i r d ~ r 3 + i o n o f t h e t r i a d .

F i g . 5

-

E x c i t a t i o n spectra o f Ng fluorescence and p o s s i b l e s t r u c t u r e o f t h e t r i a d . It 's w e l l known t h a t h e a t i n g ( ~ 1 2 0 0 ° C ) and quenching decreases the aggregation of Cr3' i o n s around a MgZt vacancy because o f t h e thermodynamical e q u i l i b r i u m . For instance, t h e r a t i o N /N1 o f t h e luminescence l i n e s decreases a f t e r quenching.

Another f a c t i s obser6ed which i s c e r t a i n l y r e l a t e d t o t h e thermodynamical e q u i l i - brium. I n f i g u r e 6 t h e e x c i t a t i o n spectra o f N1 fluorescence l i n e o f non quenched and quenched c r y s t a l s a r e compared. For the non quenched c r y s t a l o n l y the N1 e x c i - t a t i o n l i n e appears c l e a r l y . For t h e quenched c r y s t a l small supplementary l i n e s a r e v i s i b l e , which a r e s i m i l a r t o the a,b,c,d,e,f e x c i t a t i o n l i n e s o f N2 luminescen ce. This phenomenon can be explained as f o l l o w s . A t low temperatur (lower than 1200°C and h i g h e r than room temperature) a nearby ~ t - 3 ~ around a

CrR ^-

^vacancy

a s s o c i a t i o n i s unstable. It w i l l m i g r a t e t o form t h e cr3+

-

vacancy

-

^{~ r 3 + p a i r}

which i s r e s p o n s i b l e o f t h e N2 luminescence. A t h i g h temperature (1200°C)or i n 3+

quenched c r y s t a l a nearby Cr3+ i o n can e x i s t i n a metastable p o s i t i o n around a C r vacancy association, s i m i l a r t o

cr3+

ions around t h e ~ r 3 +

-

vacancy

-

c r 3 + p a i r i n a non quenched c r y s t a l .

JOURNAL DE PHYSIQUE

F i g . 6

-

E x c i t a t i o n spectra o f N1 fluorescence : ( a ) non quenched c r y s t a l ( b ) quenched c r y s t a l I 1 1

-

CONCLUSION

This study shows t h a t t h e s p a t i a l ener y t r a n s f e r i n

M ~ o : c ~ ~ +

i s governed by t h e c l u s t e r i n g o f

cr3+

ions around t h e Mgzq vacancy and t h e short-range superexchange i n t e r a c t i o n . That i s t h e reason why o n l y d i r e c t and r a p i d t r a n s f e r i s observed.

The t r a n s f e r from octahedral s i t e s t o rhombic i s c e r t a i n l y s i m i l a r t o t h e t r a n s f e r from octahedral t o t e t r a g o n a l s i t e s . Only d i r e c t and r a p i d t r a n s f e r e x i s t s from nearby

cr3+

ions. However i f t h e t r a n s f e r from quasi-octahedral t o t e t r a g o n a l s i t e s i s a s s i s t e d by one ( e m i t t e d ) phonon o f a 1 0 0 cm-1, t h e t r a n s f e r from quasi-octa- hedral t o rhombic can be resonant s i n c e t h e ~ E - & A ~ t r a n s i t i o n o f octahedral s i t e s overlaps t h e 4 ~ 2 + ? ~ 2 a b s o r p t i o n band o f rhombic s i t e s . Therefore a t r a n s f e r time l e s s than 1 0 0 ~ s can be expected f o r t h e t r a n s f e r from quasi-octahedral t o rhombic s i t e s . Such a r a p i d t r a n s f e r e x p l a i n s t h e s p i n memory observed by Mc Donagh e t a1 d u r i n g t h i s t r a n s f e r /3/.

REFERENCES

/1/ Rabia, K., Boyrivent, A., Duval, E., J. Phys. C S o l i d S t a t e Phys. 18 (1985) 1975.

/2/ Mc C r a i t h , B.D., Glynn, T.J., Imbusch, G.F., Mc Donagh, C., J. P h y c C S o l i d S t a t e Phys. 13 (1980) 4211.

/3/ Mc DonagK C., Dawson,

P.,

Henderson, B., J. Phys. C 13 (1980) 2191.

/4/ Mc Donagh, C., Henderson, B., J : Lum.

- -

31-32 (1984) 2 6 K