NATURE OF 4f ELECTRONS IN Tb OXIDES BY X-RAY ABSORPTION SPECTROSCOPY

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HAL Id: jpa-00226089

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Submitted on 1 Jan 1986

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NATURE OF 4f ELECTRONS IN Tb OXIDES BY X-RAY ABSORPTION SPECTROSCOPY

J. Esteva, R. Karnatak, H. Dexpert, M. Gasgnier, P. Caro, L. Albert

To cite this version:

J. Esteva, R. Karnatak, H. Dexpert, M. Gasgnier, P. Caro, et al.. NATURE OF 4f ELECTRONS IN

Tb OXIDES BY X-RAY ABSORPTION SPECTROSCOPY. Journal de Physique Colloques, 1986,

47 (C8), pp.C8-955-C8-959. �10.1051/jphyscol:19868183�. �jpa-00226089�

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NATURE OF 4f ELECTRONS IN Tb OXIDES BY X-RAY ABSORPTION SPECTROSCOPY

J.M. E S T E V A * . * * *

, R.C. K A R N A T A K * . * * * ,

H. DEXPERT*

M. GASGNIER*

,

P. E. CARO*

,

^{and L}

.

^ALBERT*

'LURE, Batiment 2090, Universitk Paris-Sud, F-91405 Orsay Cedex, France

" ~ d b o r a t o i r e des Elements d e Transition dans leg Solides, ER 210, CNRS, 1 , Place Aristide Briand, F-92195 Meudon Cedex, France

*

Laboratoire de Spectroscopic Atomique et Ionique, UA 040775, Bat. 350, Universite Paris-Sud, F-91405 Prsay Cedex.

France

RESUME

On Gtudie l ' d v o l u t i o n de l a valence du Terbium dans l e s oxydes Tb203, Tb407 e t Tb02

B

v a r t i r d e s s p e c t r e s MIV-V e t LIII. Les s p e c t r e s de Tb407 correspondent exactement 3 l a somme ponddrde (50%) d e s s p e c t r e s de Tb203 e t de Tb02. L'analyse d e s s p e c t r e s de Tb203 i n d i q u e que l e d e r n i e r e l e c t r o n 4f r e s t e l o c a l i s 6 dans l a couche 4 f . Dans l e c a s de Tb02 on i n t e r p r e t e l e s s t r u c t u r e s supplementaires p a r l a d d l o c a l i s a t i o n de l ' e l e c t r o n 4f. Ce comportement s p g c i f i a u e e t son i n f l u e n c e s u r l e s l i a i s o n s dans c e s oxydes f o n t l ' o b i e t de l a d i s c u s s i o n .

ABSTRACT

The MIV+ and LIII a b ~ 0 r ~ t i o t I s p e c t r a of Tb203, Tb407 and Tb02 r e v e a l a c l e a r e v o l u t i o n of Tb valence i n t h e s e oxides. The Tb407 s p e c t r a a r e found t o be 50%

weighted sum of Tbq03 and Tb02 i n d i v i d u a l s p e c t r a . These o b s e r v a t i o n s i n d i c a t e t h a t i n Tb203 t h e l a s t 4f e l e c t r o n remains l o c a l i z e d i n t h e 4f s h e l l whereas i t shows a s p e c i f i c behaviour i n Tb02. The d e l o c a l i z a t i o n of t h e l a s t 4f e l e c t r o n i n Tb02 l e a d s t o t h e o b s e r v a t i o n of complex s p e c t r a l f e a t u r e s . The n a t u r e of t h e 4f e l e c t r o n and i t s i n f l u e n c e on t h e t y p e of bonding i n t h e s e oxide i s discussed.

INTRODUCTION

The physico-chemical p r o p e r t i e s of t h e f l u o r i t e s t r u c t u r e r e l a t e d ROX(') (R=Ce, P r , Tb ; 1.5< x (2 ) a r e t r a d i t i o n a l l y understood i n an i o n i c model. I n t h i s d e s c r i p t i o n , t h e d i f f e r e n t s i t e s of t h e f l u o r i t e l a t t i c e a r e supposed t o be occupied by R ~ + and R ~ + ions. I n t h e s e extreme c a s e s t h e R i o n s a r e c o o r d i n a t e d by 6 and 8 oxygen atoms r e s p e c t i v e l y f o r t h e 3+ and 4+ i o n i c charges. The l o c a l probes l i k e X-ray a b s o r p t i o n spectroscopy (XAS) y i e l d s a somewhat d i f f e r e n t p i c t u r e ( 2 ) of t h e

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868183

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C8-956 JOURNAL

DE

PHYSIQUE

n a t u r e o f t h e l a s t 4 f e l e c t r o n when t h e r a r e e a r t h atom i s c o o r d i n a t e d by 8 oxyaen atoms. I n t h e p r e s e n t p a p e r we w i l l b r i e f l y d e s c r i b e t h e LIIT and MIV+ a b s o r p t i o n s t u d y o f Tb203, Tb407 and Tb02 i n r e l a t i o n of t h e n a t u r e o f t h e 4 f e l e c t r o n i n t h e s e o x i d e s .

EXPERIMENTS AND RESULTS

The MIV-V and t h e LIII a b s o r p t i o n s p e c t r a were o b t a i n e d by u s i n a t h e s y n c h r o t r o n r a d i a t i o n from ACO and D C I r i n g s r e s p e c t i v e l y a t LURE, ORSAY. The d e t a i l s of t h e sample p r e p a r a t i o n and t h e s p e c t r o s c o p i c methods a r e d e s c r i b e d e l s e w h e r e . The MIV-V and t h e LIII a b s o r p t i o n s p e c t r a o h t a i n e d from t h e ~ o w d e r samples of Tb203, Tb407 and Tb02are p r e s e n t e d i n f i a . 1 and 2.

DISCUSSION

MIV+ s p e c t r a o f Tb203 i n f i g . 1 show f e a t u r e s which c o r r e s p o n d w e l l t o t h e 3d94f9 m u l t i p l e t l i n e s ( 3 ) . T h i s c l e a r l y i n d i c a t e s t h a t t h e f 8 around s t a t e c o n f i g u r a t i o n of Tb i s w e l l l o c a l i z e d i n t h e t r i v a l e n t o x i d e . _A c l e a r e v o l u t i o n i n t h e m u l t i p l e t f e a t u r e s and t h e i r p o s i t i o n s i s n o t e d i n Tb407 and f i n a l l y , f o r Tb02

-

we o b s e r v e t h a t t h e m u l t i p l e t s t r u c t u r e i s a u i t e d i f f e r e n t from t h a t o f t h e Tb203.

We o b s e r v e a n o v e r a l l s h i f t of 1.2 eV t o w a r d s h i g h e r e n e r g y i n t h e Tb02 m u l t i p l e t

p o s i t i o n s a s compared t o t h o s e of

1

^I ^-⁷^.

1

Tbg03. The Tb02 m u l t i p l e t s a r e found t o b e u n u s u a l l y b r o a d e r t h a n t h e

n o r m a l 3 d - 4 f r a r e e a r t h

: \ /

i

m u l t i ~ l e t s ( ~ ) . V a r i o u s m u l t i p l e t F i g u r e 1

.. - .

^;

f e a t u r e s a p p e a r i n g i n t h e Tb02

_:.:

:> _{' I .}

. ^. ^. .

_:

s p e c t r a c a n be i d e n t i f i e d ( ' ) w i t h

. . . :. . . ^. .

^:

t h o s e o b s e r v e d f o r Gd203. One

a d d i t i o n a l s t r u c t u r e ( i n d i c a t e d by

_{. :} . :

m u l t i p l e t l i n e s . ( i i i ) The p r e s e n c e a r r o w s i n f i g . 1 ) w i t h i n e a c h m u l t i p l e t g r o u p ( 6 ) i n Tb02 s p e c t r u m c a n e a s i l y be r e c o g n i z e d . On t h e b a s i s o f t h e s e o b s e r v a t i o n s we c o n c l u d e t h a t i n Tb02: ( i ) t h e l a s t 4 f e l e c t r o n d e l o c a l i z e s i n t h e v a l e n c e band, ( i i ) t h e remaining

. .

:.3 '?

. . I

n

. ^.

.. . .

. .

:. .

:

; .\

. \..

C b 4 O 7 -

j ^. : 1 ^. ^' ^~ ^j -

. . . .

^I.

. . . .

. .

j :,

.. '., ; i\

seven l o c a l i z e d 4f e l e c t r o n s g i v e

\ ^{T b 2 4}

r i s e t o Gd l i k e f i n a l 3 d 9 4 f 9 I

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i n d i c a t e s t h a t t h e l a s t 4f e l e c t r o n p a r t i c i p a t e s i n a p a r t i a l c o v a l e n t type of bonding. ( i v ) The unusual of t h e m u l t i p l e t l i n e s i s a c o n s e q u e n c e o f t h e presence of t h e f e l e c t r o n i n t h e v a l e n c e band. This broadeninn i s due t o t h e i n t e r a c t i o n of t h e e x c i t e d 4f e l e c t r o n with t h e f i n i t e continuum s t a t e s .

The e v o l u t i o n of t h e valence t r a n s i t i o n i s c l e a r l y evidenced i n t h e LIII n e a r edge s t r u c t u r e ( f i ~ . 2 ) of Tb407 and Tb02 a s compared t o t h e s i n g l e peaked edge s t r u c t u r e of

Tb203. The Tb02 edge shows some

complex f e a t u r e s . The main high e n e r g y a b s o r p t i o n l i n e h a s a prominant low energy shoulder. These f e a t u r e s a r e followed by a n o t h e r weaker shoulder about 10 eV below t h e main l i n e and f u r t h e r by a

I

F i g u r e 2

Photon Energy ( e V )

p r e a b s o r p t i o n s t r u c t u r e on t h e low energy s i d e . These f o u r f e a t u r e s a r e a l s o s y s t e m a t i c a l l y observed i n t h e LIII edges of Ce02 and ~ r 0 ~ ( ~ ) . The h i g h e r energy peak and t h e prominant s h o u l d e r i n t h e LIII edge of Tb02 a r e i n t e r p r e t e d a s mainly due t o t h e ground s t a t e c o n f i g u r a t i o n . According t o t h e m u l t i p l e t i d e n t i f i c a t i o n of t h e MIV+ s p e c t r a of Tb02, t h e h i g h e r energy peak i s assigned t o t h e f 7 ground s t a t e c o n f i g u r a t i o n . The prominent shoulder about 7 eV below t h e main l i n e i n Tb02 i s considered t o be due t o t h e s c r e e n i n g of t h e 2p h o l e by t h e 4f admixture i n t h e oxygen valence band. The weaker shoulder i n Tb02, which i s c o i n c i d e n t with t h e main a b s o r p t i o n l i n e i s i n t e r p r e t e d a s a f 8 shake down f e a t u r e . The p r e a b s o r p t i o n l i k e s t r u c t u r e which i s s i t u a t e d a t about 18 eV below t h e main a b s o r p t i o n l i n e i n TbOg needs f u r t h e r c o n s i d e r a t i o n s r e l a t e d t o t h e s p e c i f i c n a t u r e of 4f e l e c t r o n . p r e s e n t i n t h e valence band. Taking i n t o account i t s s e p a r a t i o n ( 10 eV) from t h e prominant s h o u l d e r and t h e s i m i l a r s e p a r a t i o n observed between t h e f ' main l i n e and t h e weaker f 8 s h o u l d e r , t h i s s t r u c t u r e may probably be assigned t o a shake down f e a t u r e . This s u g g e s t i o n needs some experimental c o n f i r m a t i o n which may demonstrate t h a t t h e 4f s t a t e s i n t h e valence band a r e somewhat l o c a l i z e d i n t h e f i n a l s t a t e .

Thus we s e e t h a t t h e MTV-" and t h e

LIII

a b s o r p t i o n s p e c t r a of Tb02 a r e q u i t e d i f f e r e n t from t h o s e of Tb203.The simple s p e c t r a l f e a t u r e s of Tb203 correspond w e l l

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C8-958 JOURNAL DE PHYSIQUE

t o t h e i o n i c d e s c r i p t i o n of t h e r a r e e a r t h s i t e s i n t h i s oxide. I n Tb407 t h e d i f f e r e n t s i t e s i n t h e f l u o r i t e l a t t i c e a r e occupied by t r i v a l e n t and t e t r a v a l e n t Tb and t h e Tb407 s p e c t r a appear t o c o n t a i n s p e c t r a l c o n t r i b u t i o n s from t h e s e s i t e s . This i s i l l u s t r a t e d i n f i g . 3 and 4 r e s p e c t i v e l y f o r t h e I-$, and MIV s p e c t r a of Tb407. The sumZ(so1id c u r v e s ) of Tb02 and Tb203 s o e c t r a weighted t o 50% a r e a b l e

Photon Energy (eV) Photon Energy (eV)

t o y i e l d t h e MV and MIV s p e c t r a which correspond w e l l t o t h e observed s p e c t r a of Tb407. S i m i l a r r e s u l t s a r e obtained f o r t h e i n t e r m e d i a t e oxides of ~ r ( ~ ) . This o b s e r v a t i o n f i n d s an important a p p l i c a t i o n i n t h e q u a n t i t a t i v e a n a l y s i s of unknown i n t e r m e d i a t e and mixed r a r e e a r t h o x i d e s ( l O ) . These r e s u l t s i n d i c a t e t h a t t h e valence i n such oxide i s inhomogenious. A s p e c i f i c valence b e h a v i o u r ( l l ) i s observed a t t h e t e t r a v a l e n t s i t e s only.

The s t u d y of Tb oxides by XAS i s i n t e r e s t i n a from t h e o o i n t of view of t h e valence t r a n s i t i o n i n ROx. The p r e s e n t r e s u l t s i n d i c a t e t h a t i n a heavy r a r e e a r t h l i k e Tb, i n s p i t e of t h e d e c r e a s e i n t h e 4f r a d i u s , t h e 4f-2p oxynen o v e r l a p i s s t i l l a p p r e c i a b l e . I n t h e c a s e of t h e i n t e r m e d i a t e oxide Tbq07 t h e Tb atoms on d i f f e r e n t s i t e s of t h e f l u o r i t e l a t t i c e g i v e d i f f e r e n t s p e c t r a l f e a t u r e s . The n a t u r e of t h e 4f e l e c t r o n i n t h e s e o x i d e s i s r e v e a l e d i n t h e XAS and t h e i r s t u d y g i v e s f u r t h e r i n s i ~ h t i n t o t h e valence problem i n r a r e e a r t h s .

REFERENCES

1. L.Eyring i n "Non Stoechiometric o x i d e s W , e d i t e d b 0.T.Sorensen (Academic P r e s s , New York, 1981) p 337.

2. R.C.Karnatak, M.Gasnnier H.Dex e r t , J.M.Esteva, P.E.Caro, L.Albert J.Less Comm. Met. 110 (1485) 377.

3. B.T.Thole, G.van d e r Laan J.-M.Esteva Phys.Rev. ~ 3 2

4. R.C.Karnatak, J.-M.Esteva and J.P.Connerade J. Phys. B 14 (1981) 4747.

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are found to be well separated from the main multiplet lines.

7. J.-M.Esteva, R.C.Karnatak in "Giant Resonances in Atoms, Molecules and Solids"

,

to be published by Plenum Press

8. H.Dexpert et a1 (submitted to Phys. Rev. B) 9. M.Gasgnier,L.E ring, R.C.Karnatak, H.Dexpert,

J.M.Es teva, ~.z.~aro, L.Albert

17 th Rare Earth Research Conference, Hamilton, Canada, June 1986.

10. G.van der Laan, J.C.Fu~p;le, M.P.van Dijk, A.J.Burggraaf J.-M.Esteva, R.C.Karnatak, J.Phys.Chem.Solids 47(1986)4i3

11. R.C.Karnatak J.M.Esteva, H.Dexpert, M.Gasgnier, P.E.Caro, L.Albert

~nternationai Conference of "Anomalous Rare Earths and Actinides", Grenoble, France, July 1986

NATURE OF 4f ELECTRONS IN Tb OXIDES BY X-RAY ABSORPTION SPECTROSCOPY

HAL Id: jpa-00226089

https://hal.archives-ouvertes.fr/jpa-00226089

Submitted on 1 Jan 1986

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.