CORE-HOLE SCREENING IN LIGHT RARE-EARTH INSULATING COMPOUNDS

HAL Id: jpa-00226090

https://hal.archives-ouvertes.fr/jpa-00226090

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

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.

CORE-HOLE SCREENING IN LIGHT RARE-EARTH INSULATING COMPOUNDS

E. Beaurepaire, F. Le Normand, G. Krill

To cite this version:

E. Beaurepaire, F. Le Normand, G. Krill. CORE-HOLE SCREENING IN LIGHT RARE-EARTH INSULATING COMPOUNDS. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-961-C8-964.

�10.1051/jphyscol:19868184�. �jpa-00226090�

Colloque C8, supplbment au n o 12, Tome 47, dgcembre 1986

CORE-HOLE SCREENING IN LIGHT RARE-EARTH INSULATING COMPOUNDS

E. BEAUREPAIRE, F. LE NORMAND* and G. KRILL" *

L.M.S.E.S. (u.A. C.N.R.S. 306), Universite Louis Pasteur, 4, rue Blaise Pascal, F-67070 Strasbourg Cedex, France

'~aboratoire de Catalyse & Chimie des Surfaces, 4 , rue Blaise Pascal, F-67070 Strasbourg Cedex, France

" ~ a b o r a t o i r e de Physique du Solide, Universite de Nancy I , BP 239, F-54506 Vandoeuvre-16s-Nancy Cedex, France

Resumk

-

3'

Abstract

-

(Ce(OH)4, Ce02..:) L and 3a photoemission data are consistent provided pos- sible conduct~on BEHte band structure effects and a recently suggested interaction mechanism between the photoexcited and 4f electrons are taken into account. In opposition, this conclusion does not hold for compounds

like La(OH)3.

I - INTRODUCTION

Strong satellites are frequently observed in the core photoemission (XPS) or photo- absorption (XAS) spectra of systems where nearly localized orbitals are located in the vicinity of the Fermi level (eg 3d levels in Transition Metal oxides, f levels in light Rare Earth or Actinides compounds). These satellites can be inter- preted within the framework of the one impurity Anderson model. It is then assumed that quasi-localized levels unoccupied in the initial state are pulled down below the Fermi level in the final state by a strong Coulombic attraction with the core hole and are hybridized with band states /I/.

In the particular case of formally tetravalent light Rare Earth insulating compounds (Ce02, CeF ) , the quantitative interpretation of spectroscopic experiments is a real challenge since some results are suggestive of 4 a true tetravalent ground state (eg N edges 1141) and others (like 3d XPS) can be interpreted, in agreement with band structure calculations 1131, by assuming the presence in the ground state of 0.5 4f electrons strongly hybridized with the 2p ligand electrons 12-51. More- over, the relation between the three peaks observed in 3d XPS (attributed to final state screening) and the two main LIII absorption structures is not obvious and can be achieved up to now only by introducing a repulsive Coulomb interaction between 4f electrons and the photoexicted 5d electrons (U % 4 eV) 1.51.

f-5d

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868184

C8-962 JOURNAL DE PHYSIQUE

I n o r d e r t o c l a r i f y t h i s problem, we p r e s e n t h e r e new experimental d a t a and d i s - c u s s t h e occurrenceof many-body and band s t r u c t u r e e f f e c t s i n t h e XAS s p e c t r a or l i g h t Rare E a r t h oxide and hydroxide systems.

I1

-

X Ray a b s o r p t i o n on t h e LIII edge of Rare E a r t h (RE) h a s been c a r r i e d o u t o n a labors.- t o r y XA-spectrometer(Ce(0H) Pro Pr 0 ) o r a t LURE u s i n g t h e X r a y beam d e l i v e r e d by t h e D C I s t o r a g e r i n g 4 ' ( ~ E ( 6 ; 1 ) 3 , 6 CeB ) w i t h an energy r e s o l u t i o n of r e s - p e c t i v e l y about 2 eV and 1 eV. P a r t i c u l a r a t t e n t i o n was paid t o t h e reproduci- b i l i t y of t h e energy s c a l e a t each run by r e c o r d i n g t h e same s p e c t r a (RE(0H) ) a t d i f f e r e n t times, s o t h a t we can e n s u r e a r e l a t i v e energy c a l i b r a t i o n k 0.2 eV.

This allows a d i r e c t comparison of s p e c t r a f o r d i f f e r e n t compounds ; by conven- t i o n , i n Fig. 1 and 2, t h e z e r o of energy r e f e r s t o t h e t o p of t h e main l i n e of RE(0H) compounds. The sample of nominal composition Ce(OH)4 has been o b t a i n e d i n an amorph8us s t a t e / 1 6 / .

Fig. 1 :

LIII edges of Ce and Pr i n some compounds ( d o t s ) t o g e t h e r w i t h a f i t of t h e d a t a and t h e t e n t a - ti& d e n s i t y of s t a t e s used f o r t h e f i t . The o r i g i n of t h e energy s c a l e i s s e t by t h e top of Ce(OIO3 and P ~ ( o H ) ~ edges ( n o t shown).

111

-

a ) " T e t r a v a l e n t i n s u l a t o r s "

Quite s u r p r i s i n g l y , and i n agreement w i t h 37-81, f o u r s t r u c t u r e s a r e observed on t h e white l l n e of CeO o r r e l a t e d compounds, denoted A, B1, B ~ : C on t h e Fig. 1 . The peak B appears a s a s h o u l d e r of t h e main l i n e and i s l o c a t e d n e a r l y a t t h e same energy a s t h e 1 main l i n e of t r i v a l e n t m e t a l l i c (CeB ) o r i n s u l a t i n g (Ce(OH)3) compounds. It can then 6 be s a f e y a t t r i b u t e d t o t r a n s i t i o n s of t h e

t i

type 2p 4 f 1 -+ 2p54fl 5Ed. I t i s q u i t e s t r i k i n g t h a t t h e same s e p a r a t i o n of t h e main l i n e i n t o peaks B and B2 (though t h e y a r e somewhat broadenj i s a l s o observed i n t h e spectrum of

and t h a t t h e peak B2 n e a r l y v a n i s h e s i n where t h e c r y s t a l synunetry i s lowered by ~ n t r o d u c t i o n of v a c a n c i e s on t h e oxygen s u b l a t t i c e .

For a n o t h e r f o r m a l l y " t e t r a v a l e n t " compound, Ce(OH)4, t h e peak B1 has a weak r e l a - t l v e i n t e n s i t y , though t h e 3d photoemission spectrum i s v e r y s i m l l a r t o Ce02 spec- trum, what i s i n d i c a t i v e of a s i m i l a r e l e c t r o n i c s t r u c t u r e . We t h e r e f o r e suggest t h a t t h e occurrence of t h i s s h o u l d e r i n t h e main peak B can be a t t r i b u t e d t o band s t r u c t u r e e f f e c t s and t h a t i n Ce02 o r Pro2, t h e 5d band i s s p l i t t e d , e g under t h e a c t i o n of c r y s t a l l i n e f i e l d . I n f a c t , f o r U02 t h a t c r y s t a l l i z e s i n t h e same CaF2 s t r u c t u r e , a s i m i l a r phenemenon h a s been observed by o p t i c a l r e f l e c t i v i t y / 9 / , b u t , i n t h i s c a s e , i t i s concealed spectroscopy by t h e l a r g e widths due t o i n s t r u m e n t a i r e s o l u t i o n and c o r e - ~ 1 ~ ' " l i f e t i m e . This i n t e r p r e t a t i o n i s a l s o i n agreement w i t h r e c e n t L d a t a on Tb02 /11/ where t h e shoulder B, (and a l s o t h e prepeak A) a r e o b s e r v e i .

i s pro osed on t h e Fig. 1 t h a & I 1 c o r r e c t l y reproduces peaks a t t r i b u t e d t o 4f1 (Bl-B2) and 4fg (C) c o n f i g u r a t i o n s (though t h e decomposition of t h e peak C proposed on t h e f i g u r e i s n o t unique). Let us mention t o o t h a t t h e prepeak A i s u s u a l l y observed i n L s p e c t r a of t e t r a v a l e n t i n s u l a t o r s w i t h a weak r e l a t i v e i n t e n s i t y (a few per- cent5IHnd i t s i n t e r p r e t a t i o n w i l l n o t be considered h e r e .

As a conclusion, t h e e s s e n t i a l f e a t u r e of t h e L s p e c t r a i n such compounds i s t h e occureace of two main l i n e s ( e v e n t u a l l y structuhH3 due t o a s p l i t t i n g of t h e 5d d e n s i t y of s t a t e s ) . These r e s u l t s a r e c o n s i s t e n t w i t h XPS d a t a provided t h e f i n a l s t a t e i n t e r a c t i o n U ^% 4 eV i s taken i n t o account. Within t h i s i n t e r p r e t a t i o n ,

f t 5 d

t h e low energy l i n e 1 s a t t r i b u t e d t o 4 f 1 c o n f i g u r a t i o n and t h e high energy l i n e t o t h e mixing of 4f0 and 4 f 2 c o n f i g u r a t i o n s i n t h e f i n a l s t a t e / 5 / .

Fig. 2 :

LIII edges of t h e Rare E a r t h i n L ~ ( o H ) ~ , Ce(0H) and Sm(OH)3 ( d o t s ) and t h e f i t of t h e daGa ( l i n e ) . For v e r t i c a l b a r s , s e e t e x t .

b) T r i v a l e n t hydroxides

We d i s c u s s i n t h i s s e c t i o n r e s u l t s on t r i v a l e n t Rare E a r t h hydroxides i n o r d e r t o check t h e i m - p o r t a n c e of t h e term Uf-5d i n t h e p a r t i c u l a r case of La i n s u l a t i n g compounds, which i s c o n c e p t u a l l y t h e s i m p l e s t s i n c e only 4f0 and 4f1 c o n f i g u r a t i o n s play a s i g n i f i c a n t r o l e i n t h e i n i t i a l and f i n a l s t a t e s .

Fig. 2 shows LI I s p e c t r a f o r some t r i v a l e n t

'

i n a l l t h e s p e c t r a . Peaks

J2

*

The peak PI i s due t o dipoleallowed t r a n s i t i o n s 2p ⁺ 5 ~ d g i v i n g r i s e t o t h e "white l i n e " i n t h e LIII edge. The f u l l w i d t h a t h a l f maximum (FWHM) has been e s t i m a t e d by a f i t t a k i n g I n t o account core-hole l i f e t i m e broadening / l o / ; i t i s e q u a l t o 2.5

+

I f we assume t h a t t h e p h o t o e l e c t r o n does n o t i n t e r a c t w i t h other o r b i t a l s ,

the =I11 spectrum reduces t o a c o n v o l u t i o n of t h e 3d photoemission l i n e by t h e shape of t h e 5d band ( s i n c e U = U3d-f) and t h e s a t e l l i t e f e a t u r e would be preserved b u t broadened 12, 121. T&-gelatlve p o s i t i o n and i n t e n s i t y of t h e experimental XPS spectrum of La(OHl3 1 6 I a r e r e p r e s e n t e d by dashed b a r s f o r comparison. I t i s c l e a r from t h e Fig. 2 t h a t t h i s simple concept of "passive" p h o t o e l e c t r o n cannot be a p p l i e d .

Another mechanism can be proposed, a s mentionned i n t h e d i s c u s s i o n of 1 I I . a : t h e p h o t o e l e c t r o n may i n t e r a c t w i t h t h e o t h e r e l e c t r o n s v i a t h e r e p u l s i v e p o t e n t i a l

It i s p o s s i b l e t o c a l c u l a t e LIII s p e c t r a u s i n g a s i m p l i f i e d method d e s c r i b e d

%-5d;5b/. T h i s approximation allows t o s i m u l a t e t h e s p e c t r a o b t a i n e d by more e x a c t methods w i t h a c o r r e c t o r d e r of magnitude f o r t h e parameters. The f o l l o w i n g parameters have been used f o r La(OH)3 : U2p-f = -10 eV i n agreement with experiments on Ce systems, Uf-5d = 4 eV ( s e e above) and E~ = 11 eV, V = 1.5 eV (V i s t h e

C8-964 JOURNAL DE PHYSIQUE

mixing parameter between 4f s t a t e s and v a l e n c e s t a t e s ) . They have been chosen i n o r d e r t o s i m u l a t e XPS s p e c t r a of 161 i n agreement w i t h parameters known f o r o t h e r La compounds 12, 51. It can be e a s i l y shown t h a t t h e e f f e c t of Uf-5d i s t o push che s a t e l l i t e towards h i g h e r energy and t o reduce i t s r e l a t i v e i n t e n s i t y t o 'l. 12 %.

The l i n e s we predicted by such a c a l c u l a t i o n a r e r e p r e s e n t e d by v e r t i c a l b a r s i n Fig. 2. However, i t i s c l e a r from experimental d a t a of Fig. 2 t h a t no s a t e l l i t e can be evidenced i n La(0H) spectrum w i t h t h e suggested p o s i t i o n and r e l a t i v e i n t e n s i t y . I n t h i s c a s e , t i e c o n c l u s i o n i s d e c i s i v e i n o p p o s i t i o n t o CeO, where t h e

2 ^L

ambiguity remains due t o t h e p o s s i b l e o v e r l a p of t h e "4f s a t e l l i t e " and t h e 4f 0 peak , and t o t h e g r e a t e r i n t r i n s i c width of t h e l i n e s . As a consequence, none of t h e s e schemes propose t o compute t h e L s p e c t r a ("passive" o r i n t e r a c t i n g p h o t o e l e c t r o n ) can e x p l a i n quantitatively1'' t h e l o s s of s a t e l l i t e i n t e n s i t y i n LIII a b s o r p t i o n .

I V

-

I n agreement w i t h t h e r e s u l t s on o t h e r systems 17, 81, we have observed t h a t t h e f i n a l s t a t e e f f e c t s encountered i n 3d photoemission a r e s t r o n g l y suppressed i n L a b s o r p t i o n . These r e s u l t s suggest t h a t i n t e r a c t i o n s between t h e p h o t o e l e c t r o n aBilf o r b i t a l s a r e important, though a s i m p l i f i e d model based on a c l u s t e r l i k e approach i s not s u f f i c i e n t 151. This u n d e r l i n e s t h e need of more e l a b o r a t e d mo- d e l s i n o r d e r t o s i m u l a t e t h e p o s i t i o n s and i n t e n s i t i e s of t h e s a t e l l i t e l i n e s w i t h parameters c o n s i s t e n t f o r XPS and XAS experiments and f o r d i f f e r e n t compounds.

We have a l s o suggested t h a t t h e LIII s p e c t r a of " t e t r a v a l e n t " i n s u l a t i n g compounds (Ce(0H) , CeO Pro ) c a be r e p r e s e n t e d by t h e s u p e r p o s i t i o n of two l i n e s repre- s e n t a t i 3 e of

6L3+

ACKNOWLEDGEMENTS :

Thanks a r e due t o t h e f o l l o w i n g persons : P.E. Caro and J . M . Heintz f o r providing r e s p . Pro2 and Ce(OH)4 samples, A. Kotani and R. K a m a t a k f o r f r u i t f u l d i s c u s s i o n s and F. G a u t i e r f o r c o n s t a n t i n t e r e s t i n t h i s work.

REFERENCES

/ 1 / A. Kotani, Y. Toyozawa, J . Phys. Soc. Japan 37 (1974) 912.

K. Sch6nhammer, 0. Gunnarson, Z. Phys. B 30 E 9 7 8 ) 297.

/2/ W.D. Schneider, B. Delley

,

2

(1985) 6819.

/ 3 / A. F u j i m o r i , Phys. Rev. B

8

/4/ A. Kotani, H. Mizuta, T. Jo, J.C. P a r l e b a s , S o l i d S t a t e Corn.

53

/5/ T. J o , A. Kotani,

a ) S o l i d S t a t e Comm.

2

b) J . Mag. Mag. Mat.

2

/6/ D.F. ~ u l l i c a , C.K.C. Lok, H.O. P e r k i n s , V. Young, Phys. Rev. B x (1985) 4039.

/7/ A. Bianconi, A. M a r c e l l i , M. T o m e l l i n i , I . Davoli, J . Mag. Mag. Mat.

47-48 (1985) 209.

181 K ~ a r n a t a k , M. Gasgnier, H. Dexpert, J.M. Esteva, P.E. Caro, L. A l b e r t , J. Less Common Met.

110

191 J. Schoenes, 0. Vogt, S o l i d S t a t e Comm.

28

/ l o / A d e s c r i p t i o n of t h e l i n e s h a p e used f o r t h e f i t s can be found i n J. RGhler, i b i d 171, p. 175.

/ I 1 1 H. Dexpert e t a l , submitted t o Phys. Rev. B (1986).

1121 Y. Hammoud, J . C . P a r l e b a s , F. G a u t i e r , J . Phys. F ( i n p r e s s ) . 1131 D.D. K o e l l i n g , S o l i d S t a t e Comm.

5

1141 G. Kalkowski, C. Laubschat, W.D. Brewer, E.V. Sampathkumaran, M. Domke, G. Kaindl, Phys. Rev. B 32 (1985) 2717.

1151 A. Bianconi, A. ~ a r c e l l i 7 1 . Davoli, S. S t i z z a , M. Campagna, S o l i d S t a t e Comm.

2

1161 J.M. Heintz, D.E.A. U n i v e r s i t k Louis P a s t e u r , S t r a s b o u r g (1984).