THE OXIDATION OF CERIUM : A SXAPS STUDY

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THE OXIDATION OF CERIUM : A SXAPS STUDY

H. Hinkers, C. Westphal, H. Merz

To cite this version:

H. Hinkers, C. Westphal, H. Merz. THE OXIDATION OF CERIUM : A SXAPS STUDY. Journal de Physique Colloques, 1987, 48 (C9), pp.C9-1041-C9-1044. �10.1051/jphyscol:19879188�. �jpa-00227304�

JOURNAL DE PHYSIQUE

C o l l o q u e C9, s u p p l i S m e n t a u n012, Tome 48, dbcembre 1987

THE OXIDATION OF CERIUM : A SXAPS STUDY

H. HINKERS, C. WESTPHAL a n d H. MERZ

Physikalisches Institut der Universitdt, Wilhelm-Klemm-Strasse 10, 0-4400 Munster, F.R.G.

Abstract

The valence transition of trivalent y-Ce to tetravalent cerium with oxidation has been studied with Soft X-ray Appearance Potential Spectroscopy (SXAPS). The spectra are discussed within the framework of the Gunnarsson-Schonhammer-model. The inclusion of multiplet effects allows the determination of the 4f-count of cerium in Ce02.

In recent years the valence of cerium in Ce02 has been subject to a variety of spectroscopic investigations leading to controversial discussions concerning the 4f- count nf i.e. the occupation number of the 4f-level in the ground state of Ce02.

Different nf-values between 0 and 0.7 have been derived from these measurements /1,2/. In order to clear up this apparent discrepancy Schneider et al. /3/ proposed a model for the electronic structure of Ce02 which was based on band structure calculations of Koelling et al. /4/ and allows to explain a variety of spectroscopic results. According to this model there exist two types of 4f-states. The first one is of extended nature and forms a part of the valence band of the insulator. These states are occupied with about 0.5 electronslce-atom. The second type of 4f-states is of nearly pure 4f -character i.e. of localized character. They are found in the 1 band gap of the insulator and are not occupied in the ground state. It is the aim of this paper to demonstrate that our SXAPS results are in full agreement with this mode 1.

In SXAPS the sample is excited with electrons and the total X-ray yield is detected as a function of the excitation energy. Near the excitation threshold of a core level the X-ray yield increases abruptly due to the creation and radiative decay of a core hole (within this paper the interpretation of the spectra is based on a two step model for the SXAPS process: (i) excitation step i.e. the creation of

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19879188

C9-1042 JOURNAL DE PHYSIQUE

a core hole and (ii) decay step i.e. the radiative decay of the hole state). In the case of the 3d-spectra of light rare earth compounds the SXAPS features are due to electronic transitions in the excitation step where near threshold both the incident electron and the excited 3d-core electron are promoted into empty 4f-states / 5 / . Neglecting the hybridization of 4f-states with itinerant states (a=O) one expects

10 n

transitions 3d 4f

+

^e-

-

^{3d 4f n+2} with n being the number of 4f-electrons in the ground state. Transitions involving extended states do not contribute to the intensity of the main SXAPS features 151. SXAPS is thus an appropriate method to investigate the 4f-electron structure of Ce02. According to the model of Schneider et al. only transitions

are expected for Ce02. The SXAPS fine structure should thus be dominated by the Ce

9 2

3d 4f multiplet structure.

The experiments were carried out in a vacuum of about 1*10-10 mbar. The samples were prepared in situ by vacuum evaporation and the oxidation was performed in an oxygen atmosphere of about mbar during evaporation. It is known that Ce02 is reduced to Ce203 during electron bombardment 161. The SXAP spectrum of Ce02 was

2 *

therefore taken with low primary current density of about 0.1 mA/cm and the sample was exposed to an oxygen atmosphere of 2*10-~ mbar during the measurement. The Ce203 spectrum was taken with a current density

of about 2 m ~ / c m ~ in the absence of an oxygen r ^{I I I 1} I I I

model parameters taken from ref.131. For Ce203

M

8 8 0 9 0 0 9 2 0

atmosphere; under these conditions Ce02 is com-

pletely reduced to Ce203 in the surface region

,

of the sample.

- -

In Fig.1 the 3d-SXAP spectra of 7-Ce, Ce203 W D

and Ce02 are shown in the derivative mode. The spectra are normalized to equal height of the 3d5/ 2-structures. They have been deconvoluted with the energy window of the apparatus. The two bar diagrams indicate the energy positions of

9 m

the 3d 4f -configurations for 7-Ce (m=2,3,4) and Ce02 (m=1,2,3). The energy positions have been calculated from the uncoupled version of the Gunnarsson-Schonhammer model 171 (a=O) with no parameters were available.

The spectra consist mainly of two features

SXAPS

:L

^f:llf:

^If:

^{f:l[f: If:}

Ce203

4-+ -k-+

^&2f$

E x c i t a t i o n E n e r g y (eV1

according to the spin orbit splitting of the 3d- FIG.1. S x A P s of y-Ce and Ce-oxides. The

level. The fine structure of these features re-

~ ~ ~ ~ ~ ~ f ~ a f l ~ r r ~ ~ i o ~ ~ s i ~ ~ o n ~ n ~ ~ c a ~ ~ ~ ~ e ~ e i ~ ~

flects the multiplet splitting of the excited the presence o f t h e Jd-hole)

10 1

configurations i.e. 36 4f

+

^e-

-

^3d94f3 for trivalent 7-Ce and Ce203 and 10 0

according to the model of Schneider et al. 3d 4f

+

^e-

-

3d94f2 for tetravalent Ce02. Neglecting hybridization effects one expects therefore similar spectra for y- Ce and Ce203 and a spectrum with somewhat different fine structure for Ce02. This is indeed confirmed by our measurements: the spectra of Ce and Ce203 both consist of a rather broad main line and a shoulder on the low energy side whereas for Ce02 the width of the main line is drastically reduced, the shoulder is clearly separated from the main 1 ine and an additional feature appears on the low energy side of the 3d3/2-line indicating clearly the valence change in going from trivalent cerium to Ce02. It should however be mentioned that there are significant differences between the spectra of Ce and Ce203, too. First of all there is a chemical shift to higher energies in going from Ce to the oxides which is in agreement with X-ray absorption measurements of Yagci /2/ and Bonnelle et al. /8/. Another interesting point concerns the fine structure of the spectra which in the case of Ce203 is somewhat smeared out as compared with Ce. We assume that this difference is caused by hybridization effects: From Fig.1 it is evident that for trivalent cerium the two

9 3

neighboring configurations 3d94f2 and 3d 4f are nearly degenerate so that the coup1 ing of 4f-states to itinerant states due to finite A produces considerable mixing of these configurations. Differences in coupling strength caused by slightly different sets of model parameters for y-Ce and Cei03 will then lead to differences in the corresponding SXAP spectra.

9 2 On the other hand the Ce02 spectrum seems to be dominated by the 3d 4f - configuration since both neighboring configurations 3d94f and 3d94f3 are well

9 2

separated from 3d 4f

.

This offers the possibility to carry out a more detailed 9 2

analysis of the SXAPS fine structure on the basis of the Ce 3d 4f -multiplet structure. In an earlier paper it has been pointed out that only a selected part of the excited multiplet terms contributes to the SXAPS signal /9/. This selection mechanism was attributed to the fact that there is one X-ray emission channei which is by far the dominating decay channel in SXAPS. It is this -the X-ray 'recombination channel where one of the excited 4f-electrons recombines with the 3d-core hole leading to a 2 F final state.

9 2

As a consequence only those Ce 3d 47 -multiplet terms with 2 ~ - , 'F- or '6- character are contributing significantly to the SXAPS intensity. In this sense the X-ray recombination process (of tetravalent cerium) behaves like an inverse X-ray absorption process (of trivalent cerium).

In Fig.2 the SXAPS result for Ce02 is shown in the nonderivative mode (the spectrum is the same as in Fig.1 but has been integrated numerically). It is compared with that part of the Ce 3d94f2-multiplet structure /lo/ which is reached

10 1 by the X-ray absorption process from the ground state 3d 4f

.

C9-1044 JOURNAL DE PHYSIQUE

The overall agreement is quite good supporting the assumption that the Ce02 SXAPS structure is

X

dominated by the excitation of the 3d94f2-

5

configuration.

".. -

_C ⁰

In conclusion SXAPS has turned out to be an appropriate method for the investigation of

valences and valence changes in Ce-compounds. 880 900 920

The oxidation of y-Ce under UHV conditions leads Excitation E n e r g y IeVl

to tetravalent CeO, which is reduced to

L

trivalent Ce203 by electron bombardment. The o f CeO2. The spectrum i s

0 the same a s i n F i g . ? but has been in-

ground state of CeO? is of 4f -character which t e q r a t e d n u m e r i c a l l y . T h e m u l t i p l e t

means that no localized 4f-states are occupied s t i u c t u r e h a s been taken from / l o / .

in the ground state.

ACKNOWLEDGEMENTS

This work has been supported by the Deutsche Forschungsgemeinschaft.

REFERENCES

P. Wachter in: Valence Instabilities, P. Wachter, H. Boppart (eds.), North- Holland Publ. Comp., Amsterdam 1982, p.145

A. Kotani, H. Mizuta, T. Jo, J.C. Parlebas, Solid State Comm. 53, 805 (1985) K.R. Bauchspiess, W. Boksch, E. Holland-Moritz, H. Launois, R. Pott, D.

Wohlleben in: Valence Fluctuations in Solids, L.M. Falikov, W. Hanke, M.P.

Maple (eds.), North-Holland Publ. Comp., Amsterdam 1981, p.417 0. YaGci, J. Phys. C: Solid State Phys. 19, 3487 (1986)

W.-D. Schneider, B. Delley, E. Wuilloud, J.-M. Imer, Y. Baer, Phys. Rev. B32,

6819 ( 1985)

D.D. Koelling, A.M. Boring, J.H. Wood, Solid State Comm. 47, 227 (1983) H. Hinkers, Thesis, University of Munster, 1987

E. Wuilloud, B. Delley, W.-D. Schneider, Y. Baer, Phys. Rev. Lett. 53, 202

( 1984)

0. Gunnarsson, K. Schonhammer, Phys. Rev. 828, 4315 (1983) C. Bonnelle, R.C. Karnatak, 3. Sugar, Phys. Rev. A9, 1920 (1974)

H. Hinkers, R. Stiller, H. Merz, W. Drewes, H.-G. Purwins, Physica Scripta 36, 166 ( 1987)

/lo/ B.T. Thole, G. van der Laan, J.C. Fuggle, G.A. Sawatzky, R.C. Karnatak, J.-M.

Esteva, Phys. Rev. 832, 5107 (1985)