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Submitted on 1 Jan 1987
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SOFT X-RAY SCATTERING IN THE VICINITY OF THE 3d ABSORPTION EDGES OF Ce IN CeO2
J. Esteva, R. Karnatak, J. Connerade, P. Caro
To cite this version:
J. Esteva, R. Karnatak, J. Connerade, P. Caro. SOFT X-RAY SCATTERING IN THE VICINITY
OF THE 3d ABSORPTION EDGES OF Ce IN CeO2. Journal de Physique Colloques, 1987, 48 (C9),
pp.C9-1045-C9-1048. �10.1051/jphyscol:19879189�. �jpa-00227305�
SOFT X-RAY SCATTERING IN THE VICINITY OF THE 3d ABSORPTION EDGES OF Ce IN CeO,
J. M. ESTEVA, R. C. KARNATAK, J. P. CONNERADE' and P. E. CARO* ' Laboratoire d e Spectroscopic Atomique et Ionique and LURE, Universitb Paris-Sud, F-91405 Orsay Cedex, France
* ~ m p e r i a l College, GB-London SW7 ZBZ, Great-Britain
* *
Laboratoire des Elements d e Transition dans les Solides, CNRS ER-210, 1 , Place Aristide Briand, F-92195 Meudon Cedex, France
RESUME
Le spectre de d l f f u s ~ o n x de Ce02 au volslnage des seulls 3d du Ce presente des structures lntenses qul sont sltuees a une energle legerement plus elevee que celle des raies C9rri?5pOndantesdl.I spectre de ph0t9aDs0rDt;0n Ort dlscute l'oriylne de ces structures a l a lumlere d'un modele de potentlal a double pults
Abstract
The s o f t X-ray scattering spectrum of Ce02 I n the v i c i n i t y of the 3d absorptlon edges of Ce show intense new features appearing above the positions of the main lines
r ~ f tne corresponding absorptlon spectrum. The origin of these features i s discussed I n the light of a double w e l l potential model.
1 NTRODUCT 1 ON
The f l u o r i t e structure related CeO? presents an Increasing interest from the
L.
exnerimental as w e l l as from the theoretical point of view for understandinq the intermediate valence phonomenon. Much work has been done t o reconci ie the Intermediate valence w i t h the spectral features observed ~n different core levei spectra of CeO? i n the framework of Anderson impurity model. Nevertheless the controversy hlnges around the nature o f the 3 f electrons and the deflnrtron o f the lntermediate valence Recently the MI
"-,,
and L, I I edges of Ce02 , Pro2 and TbOZ were s t u d ~ e d ~ ~ ~ These Investigations revealed that the nature of the 4f electrons i n these oxides can be recognized I n thetr MIv-,, spectra2 The 3 d 9 4 f n t ~ m u l t ~ p l e t i d e n t ~ f ~ c a t ~ o n can dlstlngulsh the locallzed 4r states from the states ot f symmetry hybrldlzed w l th the conduction states Moreover, the orlgln ot the four structures observed I n the L , ,,
absorptlon edges of these oxides i s expialned by i n v o k ~ n g the ioexlstence of f symmetry locallzed and the extended stdtes These results were explained by postulating a double w e l l potentla1 model3Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19879189
C9-I 046 JOURNAL DE PHYSIQUE
X-ray spectroscopy by the photoyield method has become an efficient tool for the study of intermediate valnce rare earth materials. I t i s now well established that the electron yield as a function of the incident energy mimics faithfully the photoabsorption cross section. However, such correspondance between the yield and the photoabsorption implies that in the yield method the electron escape depth is much smaller than the photon penetration length. In the case of the 3d absorption 1 ines of the rare earth materials it. has been observed that the hoto on penetration length i s of the order of few hundred and the electron escape depth i s one order smaller than this. In fact, after photoexci tation, the main de-exci tation process i s electron emission through non-radiative transitions, while radiative de-exci t a t ion processes are rather weak. If, Instead of studying electron subsequent to photexcltation, one detects photon scattered by the sample, then one expects that the observed scatterlng cross section w i l l , to some extent, resemble the absorption cross section i n i t s structure.
The 3d scattering and the yield spectra of some normal rare earths and Ce02 , Pro2 and TbD7 were obtained for the comparative study of the shapes and widths of
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the 3d absorption lines in these materials. In this communication we w i l l briefly describe and discuss the 3d scattering and the photoyield spectra of Ce02. The details of the line width measurements and elaborate discussion on the rare earth dioxides w i l l be given in a futur publication.
RESULTS AND DISCUSSIONS
The 3d spectra from normal RE oxides like La203 and Gd203 show features i n the scattering spectra whose positions are coincident w i t h those appearing In thelr corresponding absorption spectra The scatterlng spectra llnes are much broader than the corresponding absorption lines This increase i n the width I s easlly unaerstooa i n terms of the much larger escape depth for photons than tor electrons tor a given penetration depth ot the lncldent photon In the scattering spectra of the normal rare earths the fluorescence contributes to the rising background and as a result the
lntensl ty ot the h~gner energy structures i s enhanced
The 3d scattering and the yield spectra of Ce02 obtalned by uslng synchrotron radiation emitted by the ACO storage ring at Orsay are given in Fig. 1. The photon intensity scattered from a Ce09 sample powder spread on a rotatible aluminium plate
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was detected by a proportional counter placed perpendicular to the direction of the incident monochromatic beam. A channel tron detector placed opposi te to the proportional counter was used to detect the electrons by suitably orienting the sample.
i n the absorption spectrum appear as more or less weak strucutres on a slightly rislng background In the scattering spectrum. We note a coincidence tn the spectral positions of the absorption structures and the scattering features but. the relative Intensity of these features i s inverted. Besides this, we observe, a new type of intense structure situated a t about 1.5 eV above each MIV and MV absorption line. The corresponding structure i n the yield spectrum i s barely observable. Similar trend i s observed i n
P a 2
and Tb02. This new observation seems to be characterstic of these materials.Photon Energy (eV)
Figure 1. Soft X-ray Scattering and Yield Spectra of Ce02 i n the vicinity of the 3d absorption edges o f Ce
C9-1048 JOURNAL DE PHYSIQUE
In order to understand the new feature appearing i n the scattering spectra or tne aioxides we examinea the w l a t h s of the lines or m u l t i p l e t s i n t h e i r f a
spectrai
The case ot the La I ive lines oaserved rn Get).?
-
1s part~cularly
interesting For CeO7 we &deternl~ned the w i d t h of the lowest energy fdlnt line observed a t 881 eV and compared it w i th chat of the mdln
MV
llrlc a t 89 1 eV We observed that the w i d t h ( 1 I eV! of the raint line remains equal t o the w l d t h of the 3 ~ 1 , faint line observed I n La compounds In La comp~unds the w i d t h of the 'P, and '0, are found t o have equal widths ( I 1 eVi wnerea? in Leo2 the S ~ , like line broadens t o 1 7 eV Thus we conclude that the 'D?-
l i k e line observed i n ieG2 i s above a threshold. The broadeninu o f i h e 3 ~ 1 like line is due to irlteraction w i t h the associated contitluurn.
The observatiort of the new feature in the scattering spectrum of Leo-;,
-
can be qualitatively explained by postulating a double w e l l potential model. I t seems that the o \ ~ t e r m o s t 4f electron of Ce i n Ce02 remains i n the outer w e l l and does not contribute t o any recoqnizable 3n-4f rnultiplet The singularity ooserveb i n the jcatterinq cross sectlon rnay then tje related t o the presence of a .If electron i n the ollter w e l l H o ~ e v e r , a t present we have no tneor-etical calculatior~s available or1 t h i Dehaviour of the observed scattering cross section whlch would take into account the presence of 4f electrons i n the outer extended w e l l it i s interestlnq t o mention that i n normal rare eartr6 ( ekceptinq La) the l o i a l i z e a 4 i electrons rn the Inner w e l i contribute only t o monotonically r i s i n u fluorescence background due t o I k and Mp type ernissiorrREFERENCES
I . T.Jo. A.Kotani. J.Phvs. Soc Japan, 55,2457 ( 1 986) ana references tnereln 2 R.C.Karnatak. J.M.Esteva. H. Dexpert. M.Gasqnier. P.E.Caro. L.Albert
Phys. Rev B 36, 1745 i 1987)
3 H. Dexpert, R,C.Karnatak, J.M.Esteva. J P Conneracle, M.Gasgnier, F.E Caro, ~ . A l u e r t
