X-RAY ABSORPTION STUDIES OF MIXED-VALENT RARE EARTH CLUSTERS

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X-RAY ABSORPTION STUDIES OF

MIXED-VALENT RARE EARTH CLUSTERS

W. Niemann, W. Malzfeldt, P. Rabe, M. Lübcke

To cite this version:

W. Niemann, W. Malzfeldt, P. Rabe, M. Lübcke. X-RAY ABSORPTION STUDIES OF MIXED-

VALENT RARE EARTH CLUSTERS. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-931-C8-

935. �10.1051/jphyscol:19868179�. �jpa-00226084�

JOURNAL DE PHYSIQUE

Colloque C8, suppl6ment au n o 12, Tome 47, dbcembre 1986

X-RAY ABSORPTION STUDIES OF MIXED-VALENT RARE EARTH CLUSTERS

W. NIEMANN, W. MALZFELDT', P. R A B E * * and M. LUBCKE'"'

Haldor Topsoe Research Laboratories, DK-2800 Lyngby, Denmark

' I B M , Yorktown Heights, NY 10598, U.S.A.

**Fachhochschule Ostfriesland, 0-2970 Emden, F.R.G.

'"'c.H.F. Miiller, unternehmensbereich der Philips GmbH, 0-2000 Hamburg 6 3 , F.R.G.

A b s t r a c t The t r a n s i t i o n from atomic t o m e t a l l i c valences i n s m a l l m a t r i x i s o l a t e d c l u s t e r s o f Sm, Nd and Pr has been s t u d i e d by XANES- and EXAFS-measurements a t t h e corresponding L 3 -edges. The observed valence o f t h e Rare E a r t h c l u s t e r s depends on t h e atomic c o n c e n t r a t i o n s and on t h e t y p e o f m e t a l atom. H i n t s on t h e c l u s t e r s i z e a r e g i v e n by a n n e a l i n g t h e samples t o temperatures near t h e m e l t i n g p o i n t o f t h e Rare Gas m a t r i x .

I n Nd and Pr samples a complete valence change from 2 t o 3 has been observed w i t h - i n an atomic c o n c e n t r a t i o n range o f 0.1 t o 0.25 ?; m e t a l c o n t e n t . C l u s t e r s con- t a i n i n g more t h a n 4 - 6 atoms a r e t o t a l l y t r i v a l e n t . F o r Sm a valence t r a n s i t i o n i s observed a t 1 5 m e t a l c o n c e n t r a t i o n , t h e valence r i s i n g from 2.0 to-2.6. From EXAFS d a t a a c r i t i c a l c l u s t e r s i z e o f K = 13 atoms a t v = 2.6 has been elaborated.

Sm c l u s t e r s w i t h K > 13 a r e mixed v a l e n t .

1. I n t r o d u c t i o n The p r o p e r t i e s o f s m a l l m e t a l c l u s t e r s a r e known t o change d r a s t i c a l l y and d i s c o n t i n u o u s l y on i n c r e a s i n g t h e s i z e from monomers o r dimers t o u n i t s d i s p l a y i n g t h e f u l l c h a r a c t e r i s t i c s o f b u l k metals. T h i s has been shown e.g.

f o r t h e change o f t h e m e l t i n g temperature o f A u - c l u s t e r s ( 1 ) o r t h e decrease o f n e a r e s t neighbour d i s t a n c e s i n A g - c l u s t e r s ( 2 ) . Magic numbers i n t h e c l u s t e r s i z e d i s t r i b u t i o n o f Rare Gas c l u s t e r s a r e a l s o known ( 3 ) . The combination o f these f a c t s t o g e t h e r w i t h t h e phenomenon o f coordination-dependent mixed valences poses an e x t ~ e m e c h a l l e n g e t o s t u d i e s o f s m a l l Rare E a r t h c l u s t e r s . C o o r d i n a t i o n - dependent valences have been c l e a r l y demonstrated f o r b u l k Sm ( 4 ) , f o r Sm c l u s t e r s on s u b s t r a t e s (5,6) and f o r Tm m e t a l ( 7 ) .

I n t h i s paper, c l u s t e r s o f Sm, Nd and Pr i s o l a t e d i n m a t r i c e s o f Ne, Ar and Kr a r e s t u d i e d by X-ray a b s o r p t i o n spectroscopy a t t h e corresponding L -edges o f t h e metals. From near edge s t r u c t u r e s t h e valence i s determined, and %XAFS d a t a a r e used t o g a i n i n f o r m a t i o n on t h e mean c l u s t e r s i z e .

2. Experimental The experiments have been performed a t t h e EXAFS I 1 beamline o f t h e Hamburger Synchrotronstrahlungslabor HASYLAB ( 8 ) . Three i o n i s a t i o n chambers a r e mounted i n a row w i t h two sample chambers p l a c e d between them which c o n t a i n t h e m a t r i x and t h e r e f e r e n c e sample, r e s p e c t i v e l y .

I n t h e f i r s t sample chamber t h e m a t r i c e s a r e prepared on a He-cryostat by t h e co- e v a p o r a t i o n technique a t T = 5 K and p = T o r r ( 9 ) . The condensation r a t e s o f t h e m e t a l atoms which a r e evaporated from a r e s i s t a n c e heated furnace a r e con- t r o l l e d by a q u a r t z m o n i t o r . The Rare Gas atoms emanated from a t h i n p i p e i n - s t a l l e d below t h e sample. The pressure i n t h e sample chamber t h e n determines t h e condensation r a t e o f t h e Rare Gas atoms.

The mean c l u s t e r s i z e i s known t o be a f u n c t i o n o f t h e e v a p o r a t i o n c o n d i t i o n s , o f t h e m a t e r i a l s used and o f t h e m e t a l c o n c e n t r a t i o n C. U s u a l l y t h e t e r m M/R ( = M a t r i x / R a d i c a l ) = 1/C i s used i n m a t r i x i s o l a t i o n spectroscopy f o r c h a r a c t e r i z i n g t h e sample.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868179

C8-932 JOURNAL DE PHYSIQUE

3. R e s u l t s I n f i g . 1 a s e r i e s o f a b s o r p t i o n s p e c t r a f o r Sm c l u s t e r s i s o l a t e d i n s o l i d Kr is shown (T = 5 K). For t h i s m a t r i x t h e i n v e s t i g a t i o n c o v e r e d M/R r a t i o s from 3 . 5 : l t o 1 3 5 : l r e s u l t i n g i n a v a l e n c e r a n g e 2 < ^v < 3. The v a l e n c e s a r e d e t e r m i n e d from t h e i n t e n s i t i t e s o f t h e two c o r r e s p p n d i n g White L i n e s f o r d i v a l e n t and t r i v a l e n t s t a t e s , r e s p e c t i v e l y , a s d e s c r i b e d i n r e f . ( 1 0 ) . S i m i l a r s e r i e s f o r Sm c l u s t e r s i n m a t r i c e s o f Ne and Ar ( 9 , l O ) and f o r c l u s t e r s o f Nd and P r i n m a t r i c e s o f Ar ( 1 1 ) h a v e been s t u d i e d .

F i g . 1 XANES o f Sm c l u s t e r s -Fig. 2 XANES o f a Srn/Kr s a m p l e i s o l a t e d i n s o l i d Kr a t v a r i o - Y s w i t h M/R = 78 a t v a r i o u s a n n e a l i n g m e t a l c o n c e n t r a t i o n s = (M/R) . t e m p e r a t u r e s .

The a b s o r p t i o n s p e c t r u m o f t h e M/R = 1 3 5 s a m p l e i n f i g . 1 i s i n d i s t i n g u i s h a b l e from t h e a b s o r p t i o n s p e c t r u m o f a t o m i c Sm (12). From p h o t o e m i s s i o n e x p e r i m e n t s i t is known t h a t Sm i s t r i v a l e n t i n t h e b u l k s o l i d s t a t e , b u t shows d i v a l e n c y i f t h e c o o r d i n a t i o n is l o w e r e d (4-6). H i n t s on t h e c o o r d i n a t i o n number o f t h e Sm atoms i n t h e m a t r i x i s o l a t e d c l u s t e r s c a n b e found from c a l c u l a t i o n s o f t h e s t a t i s t i c a l c l u s t e r s i z e d i s t r i b u t i o n ( 1 3 ) , b u t i n p r a c t i c e t h e mean c l u s t e r s i z e is a l - ways b i g g e r . I t i s w e l l known t h a t t h e a n n e a l i n g o f t h e s a m p l e s may r e s u l t i n t h e c o n s o l i d a t i o n o f s m a l l a g g r e g a t e s t o b i g g e r o n e s o r i n t h e r e - a r r a n g e m e n t o f p a r t i c l e s t o s t r u c t u r e s which a r e e n e r g e t i c a l l y more s u i t a b l e . I n b o t h c a s e s t h e r e s u l t s h o u l d b e a n i n c r e a s e o f t h e v a l e n c e . A n n e a l i n g s t u d i e s a r e shown f o r a Sm/Kr s a m p l e w i t h M/R = 78 i n f i g . 2.

The i n c r e a s e o f t h e v a l e n c e f o r Sm s a m p l e s i s

6195 6200 6205 6210

a l w a y s l i m i t e d t o Av (0.15

EIkeVl i f t h e t e m p e r a t u r e is

F i g . 3 XANES of a Nd/Ar s a m p l e w i t h M/R = 580 a t k e p t below t h e m e l t i n g

v a r i o u s a n n e a l i n g t e m p e r a t u r e s .

p o i n t o f t h e m a t r i x m a t e r i a l . F o r K r t h e e v a p o r a t i o n o f s i g n i f i c a n t amounts o f f r o z e n g a s s t a r t s a t T c r . t = 50 K . The i n c r e a s e o f v a l e n c e f o r s a m p l e s o f Nd o r P r i s much b i g g e r . T h i s i s shown i n f i g . 3 f o r a Nd/Ar s a m p l e w i t h N/R = 5 8 0 : l . I f t h e s e m e t a l a t o m s had a r r a n g e d s t a t i s t i c a l l y t h e c l u s t e r s s h o u l d o n l y be monomers and d i m e r s [ I 3 1

o f 6 - 8 atoms. he

l o w e r M/R v a l u e f o r t h e mixed v a l e n t Sm c l u s t e r s E n e r g y c o n s i d e r a t i o n s i n

2 0 L h ~ ~ ~ ~ I 3 I t ~ ~ l ' ~

0 0 2 OL 06 0 8 1 12 T I T , , i n d i c a t e s a b i g g e r

Annealing Temperature c l u s t e r s i z e t h a n f o r Nd a n d P r . T h i s s t a t e m e n t is

28-

26-

-

g 24-

h a r d e n e d by t h e s m a l l F i g . 4 Mean v a l e n c e o f R a r e E a r t h c l u s t e r s i s o l a t e d maximum i n c r e a s e o f i n R a r e Gas m a t r i c e s a s a f u n c t i o n o f t h e v a l e n c e Av = 0.15. Re- a n n e a l i n g t e m p e r a t u r e s . s u l t s o f t h e a n n e a l i n g

p r o c e d u r e s a r e shown i n f i g . 4.

3 0 1 , , , , , , , 1 , , , , t h e p a i r b o n d i n g model

The v a l e n c e o f t h e o r i g i n a l s a m p l e s a t T = 5 K is shown i n f i g . 5 a s a f u n c t i o n o f t h e i n v e r s e m e t a l c o n c e n t r a t i o n l o g M/R. C l u s t e r s o f Sm show a v e r y d i f f e r e n t v a l e n c e c u r v e compared t o t h a t o f P r a n d Nd.

EXAFS m e a s u r e m e n t s h a v e b e e n p e r f o r m e d t o g a i n i n f o r m a t i o n on t h e mean c l u s t e r s i z e . T h i s h a s b e e n d o n e s u c c e s s f u l l y f o r Sm c l u s t e r s w i t h v 2 2.6 ( 1 6 ) . A t l o w e r v a l e n c e s no r e l i a b l e EXAFS c o u l d b e o b s e r v e d . A series o f a b s o r p t i o n s p e c t r a ex- t e n d i n g t o t h e Sm L2-edge i s shown i n f i g . 6 f o r t h e c a s e o f Sm/Kr. A t 7 . 1 keV t h e K-edge o f t h e Fe I m p u r i t i e s o f t h e A 1 f o i l w h i c h was u s e d a s s u b s t r a t e f o r t h e m a t r i x a p p e a r s .

The Sm L -EXAFS o b t a i n e d from t h e a b s o r p t i o n s p e c t r a i n f i g . 6 is d i s p l a y e d i n f i g . 7. ?t c a n b e s t be f i t t e d i n a s i n g l e s h e l l model by Sm-Sm EXAFS. No Sm-Rare Gas c o n t r i b u t i o n s h a v e b e e n o b s e r v e d ( 1 6 ) .

The r e s u l t s o f t h e f l t t i n g p r o c e d u r e a r e p r e s e n t e d i n t a b l e I .

,,+ ,., ^SmlAr -

,,

= + NdlAr _-

A

PrlAr T,,,,= 32K

-

-

A t v = 2.6 a mean c o o r d i n a t i o n number N = 5 . 5 i s o b t a i n e d r e s u l t i n g i n a c l u s t e r s i z e o f 1 3 a t o m s . A c o n t r a c t i o n o f t h e n e a r e s t n e i g h b o u r d i s t a n c e w i t h i n c r e a s i n g c l u s t e r s i z e i s o b t a i n e d . T h i s is d u e t o t h e l a r g e r a t o m i c r a d i u s o f d i v a l e n t S coiiipared t o t r i v a l e n t Sm. An i n c r e a s e o f t h e mean s q u a r e r e l a t i v e d i s p l a c e m e n t o I

w i t h d e c r e a s i n g c l u s t e r s i z e h a s a l s o b e e n o b s e r v e d . T h i s f a c t s h o u l d b e e x p l a i n e d by s t a t i c d i s o r d e r and by e n h a n c e m e n t o f t h e r m a l d i s o r d e r d u e t o t h e i n c r e a s e o f t h e r e l a t i v e f r a c t i o n o f s u r f a c e a t o m s ( 1 7 ) .

( 1 4 ) show t h a t Nd a n d P r c l u s t e r s s h o u l d b e t o t a l l y d i v a l e n t f o r a c l u s t e r s i z e < 5 a t o m s ( 1 5 ) . So t h e l a r g e i n - c r e a s e o f v a l e n c e a f t e r a n n e a l i n g i n d i c a t e s t h e f o r m a t i o n o f b i g g e r c l u s t e r s c o n s i s t i n a

The most s u r p r i s i n g r e s u l t o f t h e s e s t u d i e s i s t h e f a c t t h a t t h e mean c l u s t e r s i z e f o r Sm c l u s t e r s h a v i n g a v a l e n c e V - 2 . 5 i n 1 3 a t o m s . Assuming t h e s u r f a c e t o b e d i v a l e n t and t h e b u l k t o be t r i v a l e n t a mean c l u s t e r s i z e o f a b o u t 1000 a t o m s would h a v e been e x p e c t e d . So we must c o n c l u d e t h a t t h i s s i m p l e model c a n n o t be a p p l i e d t o t h e s e m a t r i x i s o l a t e d c l u s t e r s b e c a u s e t h e v a l e n c e may be i n f l u e n c e d by t h e m a t r i x . We a l s o t h i n k t h a t t h e u n i x e d v a l e n c e o f Sm c l u s t e r s w i t h ~ 2 1 3 a t o m s i s inhomogenous.

A d d i t i o n a l e x p e r i m e n t s a t t h e e d g e o f t h e R a r e Gas m a t e r i a l h a v e been p e r f o r m e d .

F o r Sm/Ar a c o n t r a c t i o n o f t h e A r l a t t l c e by Ar = 0 . 0 5 fl compared t o p u r e Ar is

r e p o r t e d ( 1 6 ) . T h i s m u s t b e due t o t h e embedding o f t h e Sm c l u s t e r s . The mixed

v a l e n c e o f Sm c l u s t e r s w i t h K > 1 3 a t o m s h a s t o b e e x p l a i n e d by t h e i n t e r n a l

p r e s s u r e e x e r t e d on t h e c l u s t e r s by t h e d i s t o r t e d R a r e Gas l a t t i c e .

JOURNAL DE PHYSIQUE

v Matrix r ( 1 ) 12 N

3.0 evaporated 3.59

2.91 Ar 3.62 7.5+1.5

2.85 A r 3.62 7.5-

2.85 K r 3.61 6 . 0

2.83 Kr 3.64 6.8

2.83 Ne 3.65 7.0

2.70 Kr 3.67 5.3

2.67 Ne 3.66 5.3

2.62 Kr 3.68 6.0

Table 1 R e s u l t s of t h e EXAFS-fits.

F i q . 5 Mean valence of Rare Earth c l u s t e r s i s o l a t e d i n Rare Gas m a t r i c e s a s a function of t h e i n v e r s e metal c o n c e n t r a t i o n M/R = 1/C.

3.0

28-

26

5 z ^{2 L -}

2 2 -

20

- _'? ^-

SmIAr -

'% ' _' . _-.. ^,'+i . ^{NdQl -}

o PrlAr

*'+.. ^{n PrIKr -}

- f I ^-

- I

I ^{T=5K -}

1

-

- I $ . , \

'i -

\

-

\--+-

Fig. 6 Sm L -absorption s p e c t r a of 3 F i g . 7 Sm L3-EXAFS o f Sm/Kr samples.

Srn/Kr samples.

1 15 2 25 3

log (MIRI

1 l I I I

Srnl Kr

m

- 2 8

I - I I I I I I I

6 5 70 E l k e V 1 7 5

Acknowledeement

T h i s work has been supported by t h e BMFT under c o n t r a c t No 0 5 254 RA. The a u t h o r s g r a t e f u l l y acknowledge d i s c u s s i o n s w i t h Professor 8 . SONNTAG and P r o f e s s o r R . HAENSEL

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