PHOTOIONIZATION OF DIVALENT RARE EARTH IONS IN FLUORITE-TYPE CRYSTALS

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PHOTOIONIZATION OF DIVALENT RARE EARTH

IONS IN FLUORITE-TYPE CRYSTALS

D. Mcclure, C. Pedrini

To cite this version:

D. Mcclure, C. Pedrini.

PHOTOIONIZATION OF DIVALENT RARE EARTH IONS IN

J O U R N A L DE PHYSIQUE

Colloque C7, suppl6ment a u nOIO, Tome

46,

octobre 1985 page C7-397

PHOTOIONIZATION OF DIVALENT RARE EARTH IONS IN FLUORITE-TYPE CRYSTALS

D.S. McClure and C. ~ e d r i n i '

gepartmen

t

of Chemist*,

Princeton Universi&y, Princeton,

N.

J . 08544, U . S . A . E. R. A.

No.

1003 C . N . R. S . ,

Universitd Lyon

I , 69622

Villeurbanne, France

Rdsumd

-

Les v a l e u r s d e s e n e r g i e s d e p h o t o i o n i s a t i o n d ' i o n s t e r r e s r a r e s d i v a l e n t s dans l e s c r i s t a u x f l u o r d s s o n t r e v u e s e t compardes aux s e u i l s d ' d n e r g i e c a l c u l d s p a r un s i m p l e modale d l e c t r o s t a t i q u e . Un i n t s r e s s a n t ph6nomSne r d s u l t a n t de l ' i o n i s a t i o n e s t l a f o r m a t i o n d ' u n e x c i t o n p i s g d au s i t e impuretd. Dans p l u s i e u r s c a s c e t e x c i t o n e s t m d t a s t a b l e e t respon- s a b l e d ' u n e luminescence anormale q u i Q t a i t r e s t d e j u s q u ' 8 p r g s e n t i n e x p l i q u d e . A b s t r a c t

-

The magnitudes of t h e p h o t o i o n i z a t i o n e n e r g i e s of d i v a l e n t r a r e e a r t h i o n s i n f l u o r i t e type c r y s t a l s a r e reviewed and compared t o a s i m p l e e l e c t r o s t a t i c model f o r t h e t r e s h o l d e n e r g y . An e s p e c i a l l y i n t e r e s t i n g o c c u r r e n c e r e s u l t i n g from i o n i z a t i o n i s . t h e f o r m a t i o n of an i m p u r i t y - t r a p p e d e x c i t o n . T h i s e x c i t o n i s m e t a s t a b l e i n s e v e r a l c a s e s and i s t h e c a u s e of a n anomalous luminescence which h a s h e r e t o f o r e remained unexplained.

The d i v a l e n t r a r e e a r t h i o n s can be chemically s t a b i l i z e d a s i m p u r i t i e s i n CaF2, SrF2, BaF2 and o t h e r c r y s t a 1 s . l But i n CdF2, only E U ~ + can be o b t a i n e d as a s t a b l e s p e c i e s as shown by Kingsley

6;

prener.* An a t t e m p t t o reduce t h e i o n s i n CdF2 o t h e r t h a n E U ~ + l e a d s t o t h e p r o d u c t i o n of semiconducting c r y s t a l s . 3 We have found t h a t t h e p h o t o i o n i z a t i o n e n e r g i e s of t h e d i v a l e n t r a r e e a r t h s i n CaF2, SrF2 and BaF2 a r e on t h e o r d e r of a few e V t h u s showing t h a t t h e i r chemical s t a b i l i t y i n t h e s e l a t - t i c e s is not g r e a t . Furthermore, t h e p h o t o i o n i z a t i o n l e a d s , i n some c a s e s t o a t r a ped e x c i t o n s t a t e which may be a model f o r t h e semiconducting s t a t e of doped CdF2.

g

-

The p r e s e n t l y known v a l u e s of t h e p h o t o i g n i z a t i o n e n e r g i e s have r e c e n t l y been

reviewed and compared t o a s i m p l e model. I n t h i s model we assume t h a t t h e t h i r d i o n i z a t i o n p o t e n t i a l of t h e d i v a l e n t r a r e e a r t h atom is reduced i n t h e c r y s t a l by t h e l o c a l e l e c t r o s t a t i c f i e l d a t t h e i o n ' s s i t e and by t h e e l e c t r o n a f f i n i t y of t h e c r y s t a l . Removal of t h e e l e c t r o n a l s o produces an e l e c t r o n i c p o l a r i z a t i o n of t h e c r y s t a l which reduces t h e energy needed. The r e s u l t is :

Ep1 =

I3

-

Em

-

AEm

-

Epol

-

EA

Here E, is t h e e l e c t r o s t a t i c p o t e n t i a l at t h e u n d i s t u r b e d m e t a l i o n s i t e and AEm is t h e c o r r e c t i o n due t o t h e d i s t o r t i o n i n t r o d u c e d by t h e impurity. Epol i s t h e pol- a r i z a t i o n c o r r e c t i o n and EA t h e e l e c t r o n a f f i n i t y . These q u a n t i t i e s can be d e t e r - mined w i t h s u f f i c i e n t a c c u r a c y f o r a test of t h e model a g a i n s t experiment, a s noted i n t h e r e f e r e n c e s 4-8.

The v a l u e of AEm is i m p o r t a n t but n o t w e l l known s i n c e i t i s t h e e l e c t r o s t a t i c e f f e c t of t h e l o c a l d i s t o r t i o n due t o t h e i m p u r i t y . I n t h e c a s e of SrF2 where t h e r a d i u s of

sr2+

i s n e a r l y t h e same a s t h a t of t h e d i v a l e n t r a r e e a r t h s , t h e agree- ment w i t h t h e model i s f a i r l y good a s shown i n f i g u r e 1. The t h r e e e x p e r i m e n t a l c a s e s shown by an X, a r e ce2+, Tm2+ and ~ b 2 + . The p o i n t s marked w i t h a s q u a r e a r e t h e f u l l y c a l c u l a t e d v a l u e s from eq 1, and f o r t h o s e marked w i t h t r i a n g l e s AEm has been omitted. The l a t t e r c o r r e c t i o n i s , t h e r e f o r e , s i g n i f i c a n t . For BaF2 and CaF2

C7-398 JOURNAL

DE

PHYSIQUE

F i g u r e 1. Comparison of c a l c u l a t e d and observed p h o t o i o n i z a t i o n t h r e s h o l d s of d i v a l e n t r a r e e a r t h i o n s i n SrF2 h o s t c r y s t a l . Squares r e p r e s e n t v a l u e s c a l c u l a t e d by eq. 1, o m i t t i n g EA. T r i a n g l e s repre- s e n t v a l u e s a l s o o m i t t i n g AEm. The X p o i n t s , a r e observed v a l u e s : ~ e 2 + , ~ m ~ + , yb2+. - 2 1 : : :

: : . : : . : : : I

CePrNdPmSmEuGdTbDy HoGTmYb t h e c o r r e c t i o n s a r e m c h l a r g e r , and t h e r e i s l e s s agreement w i t h eq 1. It i s , t h e r e f o r e , an i m p o r t a n t c o n c l u s i o n t h a t t h e l o c a l d i s t o r t i o n has a s i g n i f i c a n t e f f e c t on t h e p h o t o i o n i z a t i o n t h r e s h o l d .

The d i v a l e n t r a r e e a r t h s u s u a l l y have a ground s t a t e a r i s i n g from t h e f n con- f i r a t i o n , and e x c i t e d s t a t e s which arise from t h i s and t h e fn-ld c o n f i g u r a t i o n . Ceg, ~ d and ~ b ~ ~ have ground s t a t e s from t h e l a t t e r , however. ~ + I n t h e c a s e s of

E U ~ + and yb2+ t h e lowest e x c i t e d s t a t e i n most l a t t i c e s belongs t o fn-Id, and nor- mally s t r o n g s h o r t - l i v e d f l u o r e s c e n c e occurs from t h i s t o t h e f n ground s t a t e a f t e r e x c i t a t i o n i n t o any of t h e e x c i t e d l e v e l s . I n c e r t a i n c r y s t a l l a t t i c e s t h e s e i o n s emit an "anomalous luminescence". Table 1 compares s p e c t r a l p r o p e r t i e s of yb2+ i n SrFg, SrC12 and NaC1. I n t h e f i r s t of t h e s e only a ~ o m a l o u s e m i s s i o n is observed w h i l e i n t h e o t h e r s normal e m i s s i o n , t h e r e v e r s e of t h e l o c a l i z e d absorp- t i o n p r o c e s s , is observed. The anomalous e m i s s i o n o c c u r s from a d e l o c a l i z e d s t a t e , a s is shown by t h e d a t a g i v e n i n f i g u r e 2.

T a b l e 1. Peak p o s i t i o n of allowed and f o r b i d d e n a b s o r p t i o n and e m i s s i o n , and anomalous emission of yb2+ i n t h r e e h o s t c r y s t a l s , i n eV. T=80K.

Forbidden: Allowed: Anomalous

System Abs

.

Em. Abs

.

Em. Em. Ref S ~ F ~ : Y ~ Z + 3.22

-

3.45

-

1.69

src12:yb2+ 3.15 3.04 3.41 3.28 i a , b Ilac1:yb2+ 2.93 2.86 3.25 3.10

-

c

*

I n a d d i t i o n t o t h e normal e m i s s i o n s , src12:yb2+ shows unexplained e m i s s i o n bands under some c o n d i t i o n s . See r e f . a.

a. H. Witzke, D.S. McClure and B. M i t c h e l l , i n Luminescence of C r y s t a l s , Molecules. and s o l u t i o n s , (F.E. Williams, ed) Plenum P r e s s , New York 1973, p. 598.

F i g u r e 2. s r F 2 : ~ b 2 + : P h o t o c o n d u c t i v i t y (P), a t 10K, l i n e a r s c a l e a t l e f t ; absorp- t i o n (A) a t 10 K, absorbance s c a l e a t r i g h t ; f o r b i d d e n a b s o r p t i o n (A"), anomalous e m i s s i o n (F) a t 80K; e s t i m a t e d t r a p p e d e x c i t o n a b s o r p t i o n (A") a t 80K ( s e e t e x t ) ; e s t i m a t e d z e r o phonon l i n e ( 0 ) . The f l u o r e s c e n c e s p e c t r u m shown i s t h a t g i v e n by Reut, r e f

.lo.

e m i s s i o n spectrum is a l s o shown. It is an example of anomalous emission s i n c e t h e normal e m i s s i o n s would be much narrower and 'would o c c u r j u s t t o t h e r e d of t h e allowed and f o r b i d d e n a b s o r p t i o n bands.

. - -

-

7

F

.Sr~:l'b~;-

F i g u r e 3. Energy l e v e l diagrams f o r CaF2 and f o r yb2+ i n SrF2. At l e f t i s

I

-

6 " c o r r e c t e d " band s t r u c t u r e f o r CaF2 of A l b e r t e t a l . ( r e f . 9 ) ,

showing t o p of v a l e n c e band and

-

5 c o n d u c t i o n band ( u s e l e f t s c a l e ) . At r i g h t i s energy d i a ram f o r l o c a l i z e d l e v e l s of Yb5+ i n SrF2, a V b u t w i t h ground l e v e l placed 3 eV below t h e r l - p o i n t of conduction band; shown i n same s c a l e as CaF2,

-

\

'\

-

2

and i n magnified s c a l e a t r i g h t .

JOURNAL

DE

PHYSIQUE

n e a r 3 eV, but a l l of t h e l o c a l i z e d l e v e l s l i e above 3 eV. T h e r e f o r e , e x c i t a t i o n i n t o any of t h e l o c a l i z e d l e v e l s l e a d s t o t h e p o s s i b i l i t y of p h o t o i o n i z a t i o n . Furthermore, e l e c t r o n i c r e l a x a t i o n , which normally l e a d s t o p o p u l a t i n g t h e lowest e x c i t e d l o c a l i z e d l e v e l followed by f l u o r e s c e n c e w i l l l e a d t o a new kind of r e s u l t i n t h i s case. The e x c i t a t i o n can i n s t e a d l e a d t o an e l e c t r o n i n t h e conduction band, i.e. yb2+ + yb3+

+

e. The e l e c t r o n i n i t s lowest energy s t a t e s would occupy

t h e 4s o r b i t a l s of t h e 12 n e a r e s t neighbor ca2+ i o n s i n <110> d i r e c t i o n s from t h e i m p u r i t y . R e l a x a t i o n of t h e i o n i c p o s i t i o n s would cause t h e n e a r e s t F- neighbors t o c o l l a p s e toward t h e Yb by about 0.2

8.

T h i s type of e x c i t a t i o n could be c a l l e d a n impurity-trapped e x c i t o n , s i n c e i t c o n s i s t s of a h o l e on t h e Yb and an e l e c t r o n i n t h e l a t t i c e .

The emission spectrum of t h e t r a p p e d e x c i t o n is t h e anomalous luminescence. I t s r e d - s h i f t compared t o t h e expected p o s i t i o n of t h e normal luminescence (Table 1 ) is due: a ) t o t h e lower energy of t h e conduction band r e l a t i v e t o t h e lowest loca- l i z e d e x c i t e d s t a t e s , b) t o t h e binding energy of t h e e x c i t o n and c ) t o t h e Franck-Condon f a c t o r r e s u l t i n g from t h e 0.2

A

F s h i f t . Its band width of 3200 cm-l, f i v e times t h e normal band width, i s a l s o due t o t h e 0.2 8 F- s h i f t . I n f i g u r e 2, t h e emission spectrum p u b l i s h e d by ~ e u t l O i s shown. A s t u d y of t h e ano- malous emission of yb2+ and E U ~ + was done by Reut

,

'

O K a p l y a n s k i i , l 1 ~ e o f i l o v l and t h e i r coworkers,,but without t h e p h o t o c o n d u c t i v i t y d a t a , a d e f i n i t i v e i n t e r p r e t a - t i o n was not p o s s i b l e . Other d a t a published by t h e s e workers is i n agreement with t h e p r e s e n t i n t e r p r e t a t i o n .

The emission l i f e t i m e was measured by Reut t o be 10 vsec a t 8 0 ~ . l ' Mancuso

measured t h e quantum y i e l d of t h e emission t o be 0.035

*

.01. l 3 Thus t h e r a d i a t i v e l i f e t i m e is about 300 vsec, l e a d i n g t o an o s c i l l a t o r s t r e n t h of 10-5. This is about of t h e v a l u e f o r t h e allowed 4f+5d

transition^.'^

I f we " r e f l e c t " t h e f l u o r e s c e n c e around a p o i n t on t h e energy s c a l e i n f i g u r e 2 near 2.5 eV, we g e t a r e a s o n a b l e looking p o s s i b l e a b s o r p t i o n spectrum a s shown i n t h e f i g u r e . This spectrum a p p e a r s t o peak n e a r t h e low energy p h o t o c o n d u c t i v i t y peak. Its maximum e x t i n c t i o n c o e f f i c i e n t can be found from t h e o s c i l l a t o r s t r e n g t h and t h e band shape t o be E = 0.2 l/mole-cm. An a b s o r p t i o n c o e f f i c i e n t could be e s t i m a t e d from t h e yb2+ c o n c e n t r a t i o n of about 0.1 mole p e r c e n t i n our sample: i t would be 0.006 cm-l, s u f f i c i e n t f o r a l a s e r f l u o r e s c e n c e e x c i t a t i o n experiment. Indeed, we have s e e n i n d i c a t i o n s of t h i s weak a b s o r p t i o n and p l a n t o i n v e s t i g a t e i t thoroughly. The binding energy of t h e e x c i t o n ought t o be s i m i l a r t o t h e a c t i v a t i o n energy f o r conduction i n semiconducting CdF2. E i s e n b e r g e r and p e r s h a n L S showed t h a t a l i k e l y e l e c t r o n i c s t r u c t u r e i n t h i s m a t e r i a l a t low temperatures is t o have an e l e c t r o n s p r e a d over t h e 12 m e t a l i o n s i t e s around t h e t r i v a l e n t impurity ion. An a c t i v a - t i o n energy of 0.16 eV t o f r e e t h e e l e c t r o n h a s been found f o r t h i s p r o c e s s by Weller.16 E i s e n b e r g e r and Pershan showed t h a t a Bohr model f o r t h e h o l e - e l e c t r o n p a i r g i v e s a good f i r s t approximation t o t h e binding energy and e l e c t r o n o r b i t a l r a d i u s . At lOOK t h e s t a t i c d i e l e c t r i c c o n s t a n t of CdF2 is 7.85, and t h a t of SrF2 i s 6.21, g i v i n g r = 4.158, Eb = 0.22 eV f o r CdF2 and r = 3.298 and Eb = 0.35 eV f o r SrF2. These numbers a r e c l o s e t o t h e v a l u e s of t h e impurity-metal i o n d i s t a n c e 3.80 and 4.09 r e s p e c t i v e l y , and t h e binding energy of 0.16 eV i n CdF2.

The only evidence t h a t t h e e x c i t o n binding energy is of t h i s magnitude i n CaF2 SrF2 and BaF2 i s g i v e n by t h e occurrence of anomalous luminescences i n C ~ F * : Y ~ ~ + . No p h o t o c o n d u c t i v i t y 4 h a s been d e t e c t e d i n t h i s system, but i t s t h r e s h o l d should be somewhat below 4 eV. The z e r o phonon l i n e of t h e lowest l o c a l i z e d t r a n s i t i o n i n t h i s system is 3.28 eV. T h e r e f o r e , i n o r d e r f o r t h e system t o r e l a x i n t o t h e exci- t o n s t a t e , t h e p h o t o i o n i z a t i o n t h r e s h o l d minus t h e binding energy would have t o f a l l below 3.28 eV. T h i s r e q u i r e s a v a l u e on t h e o r d e r of 0.5 eV.

a b l e t o " t u n n e l " i n t o t h e e x c i t o n s t a t e b e f o r e t h e r a p i d allowed e m i s s i o n p r o c e s s o c c u r s .

The p r e s e n t l y known systems i n which an impurity-trapped e x c i t o n p r o b a b l q forms a r e : caF2:yb2+, srF2:yb2+, BaF2:yb2+, BaF2:~u2+, cdc12:cu+, cdBr2:cu+. A l l of t h e s e i m p u r i t y i o n s have f i l l e d o r h a l f - f i l l e d s h e l l ground s t a t e s , and t h e i r f i r s t e x c i t e d s t a t e s belong t o a d i f f e r e n t e l e c t r o n c o n f i g u r a t i o n from t h e ground s t a t e ( i . e . , f 1 3 d , f 6 d o r d 9 s ) . The c r y s t a l L a t t i c e s p r o v i d e t h e n e g a t i v e p o t e n t i a l t o h e l p e x p e l t h e e l e c t r o n , and i n t h e c a s e of Cd-compounds, a l s o p r o v i d e a l a r g e e l e c t r o n a f f i n i t y t o a c c e p t t h e e l e c t r o n i n t o t h e c o n d u c t i o n band. The extreme c a s e of t h e l a t t e r is CdF2 c o n t a i n i n g t h e r a r e e a r t h s where t h e e l e c t r o n a f f i n i t y of t h e l a t t i c e i s g r e a t e r t h a n t h a t of t h e t r i v a l e n t i m p u r i t y atom.

The impurity-trapped e x c i t o n i s an i n t e r e s t i n g o b j e c t f o r s t u d y . T r a n s i t i o n s from i t t o h i g h e r p a r t s of t h e c o n d u c t i o n band may g i v e u s e f u l i n f o r m a t i o n about t h e l a t t e r s i n c e t h e t r a p p e d e l e c t r o n o c c u p i e s a f a i r l y l a r g e volume of t h e c r y s t a l ; and a l s o because i t w i l l have d i f f e r e n t o v e r l a p w i t h c o n d u c t i o n band s t a t e s t h a n does t h e v a l e n c e band. D i r e c t t r a n s i t i o n s from t h e i m p u r i t y t o t h e e x c i t o n s t a t e w i l l g i v e i n f o r m a t i o n on o v e r l a p between atoms s e p a r a t e d by about 48, i n t h e

c r y s t a l . It w i l l be i n t e r e s t i n g t o compare t h e s e o v e r l a p s w i t h t h e p a r a m e t e r s of o t h e r e l e c t r o n t r a n s f e r systems, p a r t i c u l a r l y m o l e c u l a r ones.

The t u n n e l i n g r a t e between l o c a l and e x c i t o n s t a t e s may be measureable, a s Chase was a b l e t o o b s e r v e e l e c t r o n s p i n r e s o n a n c e of t h e lowest l o c a l i z e d s t a t e of E U ~ +

i n BaF i n s p i t e of t h e f a c t t h a t t h i s s t a t e i s decaying i n t o a t r a p p e d e x c i t o n l e v e l . 3 8

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2. J.D. K i n g s l e y and J. S. P r e n e r , Phys. Rev. L e t t e r s

8,

315 (1962). 3. J.S. P r e n e r and J.D. K i n g s l e y , J. Chem. Phys.

38,

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