THERMAL SHIFT AND BROADENING OF THE 2E - 4A2 ZERO-PHONON LINES IN β-ALUMINA : CR3+ CRYSTALS

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THERMAL SHIFT AND BROADENING OF THE 2E

-4A2 ZERO-PHONON LINES IN β-ALUMINA : CR3+

CRYSTALS

G. Mariotto, M. Montagna, F. Rossi

To cite this version:

Colloque C7, supplkment a u nO1O, Tome 46, octobre 1985 page C7-343

THERMAL S H I F T AND BROADENING OF THE 2~

-

4 ~ 2 ZERO-PHONON L I N E S I N 6-ALUMINA

:

c r 3 + CRYSTALS

G. Mariotto, M. Montagna and F. Rossi

Vipartimento d i Fisica, Universitci d i Trento,

and Centro Studi deZ ConsigZio

NazionaZe deZZe Ricerehe,

38050

Povo, I t a l y

Resume

-

On reporte les spectres de luminescence des cristaux d'alumine

B

au 3+

Li, Na et Ag, dop6e par impuretes de Cr

.

On discute 1161argissement inhomo- gene des lignes

B

zdro phonons et leu? d6glaceme;lt en frequence avec la tein- p6rature en terms de la composition du cris'cal e-? du dBsordre dans le plan 2e conduction. Les experiments de FLN sur l'alumine

6

B

l'argent dans le doinaine de 50 - 250 X suggerent l'existence d'un fort cou2lage entre l'imguret6 e'c les modes de Sasse fr6quence du cristal.

3+

Abstract

-

Luminescence spectra of Li, Na and Ag 8-a1umina:Cr crystals are reported. Tile observed inhomogeneous broadenings of the ZPL and their thermal shifts are discussed in terms of crystal com~osition andof the degree of dis- order in the mobile cation sublattice. FLN e::periinents on Ag 8-alumina in the range 50 - 250

:<

support the hypothesis for a stroilg coupling of the irn.. purity with low frequency ntoties of the crystal.

I

-

INTRODUCTION

V-aluminas are very intersting systems for spectroscopic studies on impurities. In fait tiley show structural characteristic intermediate between t:iose of or6e:-ed crys- talline solids and of amorghous ones. This is due to their layered structure with spinel blocks (made of aluminum and oxygen atoms) separated by relatively low dense planar regions, which host monovalent mobile cations, disorderly distribu-ted among

,-, _{Lncee inequivalent sites /I/. They are, also, interesting from a dynamical point of} .

view, because of the possible interaction of the impurity ions with the monovalent cations, which are thermally pronoted to jump among the different mirror plane sites. In addition, the possibility to easily replace the monovalent ions of the conduction plane by simple ionic exchange technique, allows to vary the static and dynamic in- teractions of the impurity centres with crystal host lattice. In this paper we pre-

3+

sent the fluorescence spectra of 6-a1umina:Cr crystals, where the c:~i-omium ions enter substitutionally for the A13+ ions in the different octahedral sites of the spinel block structure.

I1 - EXPERIMENTAL

The sampleswere originally supplied by Union Carbide Co~poration as a boule of Na

8

-alumina containing ~ 2 3 % of sodiuin excess. Li and ,lg

6

-alumina crystals, obtained by means of the ionic exchange technique froiii ?latelets of sodium 6-alunina, were kindly su9glied to us by Professor Farrinp.ton. All of the sainples contain as natWal impurities some chromium traces, substitutional for aluminum ions. A crystal of Ag

3+

6-alumina, intentionally doped with Cr ions ( % 0.1%), and grown in Paris by Pro-

C7-344 JOURNAL

DE

PHYSIQUE

fessor Lucazeau group, was also available.

For the luminescence measurements we used the 5145

a

line of an ~ r + ion laser. Light pulses, with variable width, ranging from 1 ms to 50 ms, were produced by focusing the laser beam on a chopper wheel. The luminescence, recorded at different delays, was analyzed with a double monochromator and detected using photon counting technique. For the time-resolved Fluorescence-Line-Narrowing (FLN) experiments, a continuous dye

(DCM) laser, pumped by the ~ r + ion laser, was employed. Erickson geometry /2/ was em- ployed in order to avoid the collection of the elastically scattered laser light from the crystal.

I11 - RESULTS AND DISCUSSION

In figure 1 we report the luminescence spectra, obtained by continuous excitation of

0 3+

the 5145 A line at 78 K from Li, Na and Ag 6-a1umina:Cr crystals. The spectra show a rather complex structure, with common bands whose relative intensities are strongly monovalent-cation dependent. More information is gained from time-resolved lumines- cence spectra. In this way it is possible to show that the actual luminescence spec- tra are the sum of the emissions from different centres, with characteristic decay times. At least three different sets of centres have been identified previously /3/.

Fig. 1

-

Luminescence spectra of Li (a), Fig. 2

-

Time-resolved luminescence spec; 3+

Na (b)

,

and Ag (c) 6-a1umina:Cr crys- tra at 78 K of Li (a), Na (b)

,

and A& ( c )

3+

tals at 78 K, obtained by 5145

1

line con- 6-a1umina:Cr crystals: pulse width =42 tinuous excitation. ms, delay = 22

+

50 ms.

In figure 2 we show the spectra corresponding to the centres with longest decay times ( ~ 2 ' 5 0 ms at 78 K, with small dependence on the crystal composition). The observed

2 4

luminescence was attributed to the E

-

A transition of the cr3+ ion in Al(4) site 2

R1 and

R2,

originate from the splitting of the cubic Eg doublet due to the spin-orbit interaction and to the low symmetry crystal field. In Na

B

-aluminaqcrystal the lines

-1

do not show any internal structure, and their width,

I',

is 1.40 cm at 78 K and 2 4 5 -1

cm at room temperature. On the contrary, in Li and Ag

B

-alumina they show some structures, which should be related to the particular arrangement of the mobile ca- tions in the conduction plane of these crystals. The broadening of the lines is inho- mogeneous in character, as it results from FLN experiments at low temperature. In fact, the luminescence after resonant laser excitation at 4.2

K

has the same shape

-1

of the laser pulse (full width at half maximum = 0.5 cm ) . The observed inhomoge-

-1

neous width (r=40 cm ) should be compared with the ty ical one observed in ordered

-1

-P

crystals ( E 1 cm or less) and in glasses (r1.100 cm or more). In our case the crystals present a strongly disordered ionic distribution in their conduction planes, out the emitting long living center occupies a site which is relatively far from these planes ( = 5.65

i),

and the structure of its ligands is quite ordered. On the other hand, the main features of figure 1 have shorter decay times (a few ms), and are assigned to emissions from crJ+ ions in Al(1) sites / 3 / , which have lower sym- metry and are closer to the disorderd conduction plane. As a consequence, the struc- tural disorder has stronger effects, and the linewidths turn out to be larger than

-1

100 cm

.

Here we want to focusour attention on the emissions from long lived centres, whose lines are sharp enough to allow us to measure their shifts as a function of thg, temperature. In figure 3 we plot the luminescence-delayed spectra of Na B-alumina: croT

I

--.

, \

. .. 35

TEMPERATURE

(K>

14450 14250 14050

ENERGY

(cm-'1

Fig.

4

- Thermal shifts of the R1 line in Li (triangles), Na (open circles), and

Ag

Fig. 3

-

Delayed luminescence spectra of (full circles) B-alumina:~r~+ crystals,

3+

Na 6-a1umina:Cr crystal at different computed with the respect of the line po- temperatures. Pulse width = 42 ms, and sition in the spectra at low temperature, delay = 22 i 50 ms. as described in the text.

C7-346 JOURNAL

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PHYSIQUE

t a k e i n account t h e i n t e r n a l s t r u c t u r e of each l i n e . Both R 1 and R2 l i n e s s h i f t with temperature toward lower energy, and t h e i r s p l i t t i n g i n c r e a s e s very slowly ( % 5 % of t h e s h i f t v a l u e ) . The s h i f t of t h e lower energy l i n e ( R 1 ) a s a f u n c t i o n o f t h e tempe- r a t u r e i s r e p o r t e d i n f i g u r e 4 f o r t h e t h r e e d i f f e r e n t c r y s t a l s . A s z e r o s h i f t we took t h e mean p o s i t i o n o f t h e low t e m p e r a t u r e s p e c t r a (T < 7 0 K ) because, w i t h i n ex- perimental accuracy ( r 2 cm-', i.e.: %1/20 of

r

) , no s h i f t was observed below 70 K . Thermal s h i f t i s known t o be due t o t h e i n t e r a c t i o n between tHe i o n i c system and t h e normal modes of t h e l a t t i c e by Raman and d i r e c t p r o c e s s e s /4/. Since t h e c o n t r i b u t i o n of d i r e c t p r o c e s s e s i n n e g l i g i b l e a t high temperatures / 4 , 5 / , i t r e s u l t s t h a t t h e thermal s h i f t i s p r o p o r t i o n a l t o t h e thermal energy:

where n(w T ) i s t h e thermal p o p u l a t i o n f a c t o r of t h e k-th mode, and

a

i s a c o n s t a n t . k

:

This r e l a t l o n i s v a l i d i n t h e h y p o t h e s i s t h a t t h e phonon-impurity i n t e r a c t i o n i s t h e same f o r a l l t h e phonons, and depends only on t h e l o c a l l a t t i c e d i s t o r t i o n s . Equation

2 4 2+

(1) was w e l l v e r i f i e d f o r t h e E - A t r a n s i t i o n of t h e simple system Mg0:V 161. 2

However, f o r ruby high frequency o p t i c a l phonons were shown t o be l e s s coupled t o t h e i m p u r i t i e s . I n f a c t , i n o r d e r t o f i t t h e s h i f t i t was necessary t o use an e f f e c t i v e Debye temperature lower than t h a t f i t t i n g t h e s p e c i f i c h e a t / 4 / . Data of s p e c i f i c h e a t of d i f f e r e n t c r y s t a l s o f 8-alumina show a behaviour which seems t o be i n agreement with t h e s h i f t s r e p o r t e d i n f i g u r e 4. A s a m a t t e r of f a c t , t h e s p e c i f i c h e a t of Ag 8-alumina i s h i g h e r t h a n t h a t of t h e Na 8-alumina, i n t h e whole temperature range

s t u d i e d ( T

<

300 . K ) / 7 , 8 / . These d a t a were q u a n t i t a t i v e l y i n t e r p r e t e d i n terms of a s e p a r a t i o n of modes between s p i n e l block v i b r a t i o n s (assumed t o be t h e same a s f o r a - A 1 0 ) , and t h o s e of t h e 2-dimensional l a t t i c e formed by monovalent c a t i o n oxydes.

2 3

.

These i o n i c s p e c i e s were assumed t o v i b r a t e a s E i n s t e i n o s c i l l a t o r s with t h e same f r e -

-1 -1

quency a s t h a t measured s p e c t r o s c o p i c a l l y ( %30 cm f o r Ag, and '%O cm f o r Na B-alumina, r e s p e c t i v e l y / 9 , 1 0 / . However, t h e d i f f e r e n c e of s p e c i f i c h e a t between Ag and Na @-alumina i n t h e measurements of r e f s . 7 and 8 were s i g n i f i c a n t only i n t h e low temperature range (T<120 K ) . A t higher temperatures t h i s d i f f e r e n c e became l e s s s i g n i f i c a n t , a l s o because of an i n c r e a s i n g c o n t r i b u t i o n t o t h e t o t a l s p e c i f i c h e a t o r i g i n a t i n g from modes of t h e s p i n e l block, which c o n t a i n eleven A 1 0 u n i t s . On t h e

h i g h t e m p e r a t u r e s ( 4 5 % a t 600 K ) . Moreover it d o e s n o t s a t u r a t e a s i t i s e x p e c t e d f o r t h e c o n t r i b u t i o n s from low e n e r g y phonons. I n o u r o p i n i o n t h e measured l a r g e e f - f e c t i s t o be a t t r i b u t e d t o t h e e x t r e m e l y s t r o n g i n t e r a c t i o n s o f t h e i m p u r i t y i o n w i t h t h e motions o f t h e mobile c a t i o n s : I seems p o s s i b l e t h a t t h e c o u p l i n g c o n s t a n t ( a o f t h e e q u a t i o n 1 ) i s v e r y s e n s i t i v e t o t h e mobile c a t i o n s p e c i e s , and t o t h e i r tempera- t u r e - d e p e n d e n t d i s t r i b u t i o n among t h e d i f f e r e n t s i t e s o f t h e m i r r o r p l a n e .

F i n a l l y , we want t o c o n s i d e r t h e t e m p e r a t u r e dependence o f t h e homogeneous l i n e w i d t h . I n f i g u r e 5 we p l o t t h e r e s u l t s o f t h e FLN measurements on t h e Ag 0-alumina c r y s t a l , doped w i t h %0.1% o f chromium. The o t h e r samples d i d n o t g i v e s u f f i c i e n t s i g n a l f o r r e l i a b l e a n a l y s i s . The dye l a s e r was t u n e d i n o r d e r t o match t h e e n e r g y o f t h e more i n t e n s e ZPL o f t h e l o n g l i v i n g c e n t r e s . U n f o r t u n a t e l y , i n t h e s e e x p e r i m e n t s , t h e con- t r i b u t i o n s from o t h e r s h o r t - l i v i n g c e n t r e s were n o t n e g l i g i b l e , because o f t h e i r s t r o n g a b s o r p t i o n ; a l s o we had t o u s e t o o s h o r t d e l a y s ; moreover, changes i n t h e l i f e - t i m e o f t h e d i f f e r e n t c e n t r e s o c c u r w i t h t e m p e r a t u r e . A f t e r d e c o n v o l u t i o n of t h e expe- r i m e n t a l l i n e s h a p e ( l a s e r p l u s s p e c t r o m e t e r f u n c t i o n s ) , t h e d a t a were f i t t e d w i t h l o - r e n t z i a n c u r v e s . I n f i g u r e 6 we p l o t t h e h a l f - w i d t h s v e r s u s t e m p e r a t u r e . The w i d t h s f o l l o w a T' t r e n d , w i t h

a"

2 . 3 . T h i s s t r o n g l y s u p p o r t s t h e h y p o t h e s i s o f a predominant c o u p l i n g o f t h e i m p u r i t y

I

_{i o n w i t h t h e low f r e q u e n c y modes: i n} D _{f a c t , a T 2 -b e h a v i o u r is e x p e c t e d i n} G t h e h i g h t e m p e r a t u r e regime ( T > e ) , D I? whereas a T7 dependence i s e x p e c t e d a t T < < e D f o r Raman p r o c e s s e s / 4 / .

o Thus i n s i l v e r 8-alumina t h e more e f -

%

I

I

; i c i e n t modes t u r n o u t t o have a n e-

';Im

,

l l

1

n e r g y l e s s t h a n 50 K , t h a t means j u s t t h e v i b r a t i o n a l modes o f s i l v e r i o n . Z _I

1

F i g . 6 - P l o t o f t h e l i n e w i d t h o f t h e

50

100

150

250

FLN s p e c t r a on Ag B-a1umina:Cr 3+ c r y s -

LN T

(K)

t a l v e r s u s T ( K ) . REFERENCES

/1/ P e t e r s , C . R . , Bettman, M., Moore, J.W. and G l i c k , M.D., Acta C r y s t a l l o g .

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/2/ E r i c k s o n , L.E., 0pt.Commun. 1 5 ( 1 9 7 5 ) 246.

/3/ P.lariotto, G., Montagna, M. and R o s s i , F . , S o l i d S t a t e I o n i c ( t o be p u b l i s h e d ) . /4/ McCumber, D.E. and S t u r g e , M.D., J.Appl.Phys.

34

( 1 9 6 3 ) 1682.

/5/ Nuramoto, T. and H a s h i , T . , P h y s . L e t t .

g

( 1 9 7 5 ) 423.

/6/ Imbusch, G . F . , Yen, W.I.f., Schawlcw, D . E . , McCumber, D.E. and S t u r g e , N.D., Phys.Rev.

133A

( 1 9 6 4 ) 1029.

/7/ B j o r k s t a m , J . L . , F e r l o n i , P . and V i l l a , M . , J.Chem.Phys.

3

(1980) 2932. /8/ ;lcr;Jhan, D.B., Varma, C.M., Hsu, F.L.S. and Remeika, J . P . , Phys.Rev. (1977)

553.

/9/ A l l e n , S . J . , J r . , Remeika, J . P . , DeXosa, F. and U l a s i , S.X., Phys.Rev.

a17

(1973; 4031.