ELECTRONIC DISTRIBUTIONS OF UO2 BY X-RAY SPECTROSCOPY

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ELECTRONIC DISTRIBUTIONS OF UO2 BY X-RAY SPECTROSCOPY

G. Lachere, C. Bonnelle

To cite this version:

G. Lachere, C. Bonnelle. ELECTRONIC DISTRIBUTIONS OF UO2 BY X-RAY SPECTROSCOPY.

Journal de Physique Colloques, 1980, 41 (C5), pp.C5-15-C5-17. �10.1051/jphyscol:1980503�. �jpa-

00219938�

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JOURNAL DE PHYSIQUE Colloque C5, supplkment au n o 6 , Tome 41, juin 1980, page C5-15

E L E C T R O N I C D I S T R I B U T I O N S OF U 0 2 B Y X-RAY SPECTROSCOPY

G. L a c h e r e and C. B o n n e l l e

L a b o r a t o i r e d e C h i m i e P h y s i q u e

+

, U n i v e r s i t S P i e r r e e t M a r i e C u r i e , 7 5 2 3 1 P a r i s C e d e x 0 5

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F r a n c e .

Resume.- A partir des spectres d'absorption et d16mission X et des r6sultats obtenus par photo6mission X, nous proposons un diagramme des niveaux df6nergie pour U 0 2 . Les caracteristiques des 6tats Sf de lluranium dans ce compose sont discut6es. Nous observons 2 la fois des Etats Sf m6langes aux orbitales 2p de lloxygSne, qui traduisent un effet de covalence, et des etats 5f purs localis6s remplis ou vides et situes dans la bande interdite.

Abstract.- From X-ray absorption and emission spectra and X-ray photoemission results, an energy level diagram is proposed for U02. Emphasis is placed on the properties of the 5f states in this solid. A covalent mixing of uranium 5f and oxygen 2p orbitals is found. Pure localized uranium 5f levels are also present;

these are either filled or empty excitation levels situated in the band gap.

Information on the electronic distribution of solids can be obtained by means of various spectrographic methods. It is best to combine several such techniques, for example, soft X-ray emission and absorption, photoelectron and optical spectroscopies. Here we propose an energy level scheme for UO, obtained from the results by soft X-ray and photoelectron spectroscopies carried out on uranium in the metal and the dioxide.

The soft X-ray 3d emission and absorption and 3p absorption of uranium have been analyzed in our laboratory /I/. The resolu- tion is limited by fhe life time of the inner hole, here 3d or 3p, and the intensity is governed by the dipolar transition proba- bilities.

Thus, from the 3d spectra, it is possible to obtain the 5f distributions.

Two types of U 5f-3d emissions are observed for a-U and U 0 2 /2/ :

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the resonance lines which are in coincidence with the absorption transitions and provide information on the normally empty 5f excited states in the solid

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the normal lines which, in the independent electron model, correspond to the transition of a 5f electron to a 3d hole and give the distribution of the Sf

occupied states in the solid.

The 3p absor~tion threshold corresponds to the transition of a 3p electron to the d-s states at the Permi ievel (EF) in the metal of at the bottom of the conduction band ( C B ) in the oxide.

The distance between the 3p and 3d levels can be obtained from the X-ray atomic emissions. By combining the 3p-3d energy separation, the energies of the 3p absorption threshold and of the Sf-3d emissions, it is possible to locate the 5f filled and excited states with respect to the Fermi level or to the bottom of the conduction band separately in the metal and the oxide.

In the figure 1 the 5f occupied states determined from the normal 3d lines are labelled 5fn in the metal and Sf in the oxide, the 5f excited states obtained from the resonant lines are labelled 5fn+'. The separation between the 5fn (or 5f) and 5fni1 states is determined to better than 0 . 2 eV because the normal and resonant lines are observed simultaneously~on the same 3d emission spectrum. This separation increases strongly from 2 . 8 eV for a-U to 7.9 eV for u02.

The Sf

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^{EF or 5f}

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^{C B}distances are known with a clearly less good precision ( 0 . 8 ev) because they involve three different measure- ments : those of the 3p-3d distance. the 3p absorption threshold and the 3d lines.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980503

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JOURNAL DE PHYSIQUE

Moreover, the correlation terms can be different for the various transitions.

The energy of an X-ray transition, 5f-3d for example, is the difference between the energies of 5f and 3d levels. The energy of the 3d level, as that of 5f level, is dependent on the chemical binding and is different in the metal and its oxide. Then to determine the chemical shift of a particular level, for example 5f, it is necessa- ry to dispose of supplementary data. From photoelectron spectroscopy, it is possible to obtain the shift of a single level. Only the 4f and the outer levels have been measured for a-U and UO,. The influence of

the chemical bond has been shown for each of theq /3/. The chemical shift is approxi- matively the same for all inner levels and it is possible to deduce that the 3p and 3d levels shift by about 2 eV towards higher energies in the oxide. From these values, we have situated the energy diagrams

of a-U and UO2 one with respect to other as presented in .the figure 1.

Fig. 1.- Energy level diagrams for UO, and a-U; energies are in eV.

From these diagrams, we prove that the states labelled 5f in UO, are strongly bound : they are positionned well below the conduction band and the UO, 5fn+' and a-U 5fn states. This suggests that the 5f

states are not pure in the oxide but ener- getically mixed with the valence states. TO confirm this interpretation, we have consi- der the results obtained from the low binding energy part of the photoelectron spectrum.

Whatever their symmetry, the valence and localized electrons participate in the spectrum. The intensity of a photoelectron peak is a function of the photoionisation probability of electrons contributing to the peak. This probability is nctknown in the solid. Thus, it is difficult to attri- bute a low binding energy structure to the electrons of a particular symmetry. However, by analogy with the neighbouring elements, the photoelectron peak located immediately below the Fermi level in the metal has been attributed the 5f-6d hybridized electrons /3/. This peak is also present in the UO, spectrum shifted by about 1 or 2 eV towards higher binding energies; it has been attributed to the 5f non hybridized electrons /4/. In the UO, 3d spectrum, the correspon- ding transition is difficult to resolve from the resonance line which is very intense and it has not been measured from our X-ray spectra. A large photoelectron peak at 5 eV towards higher binding energies relative to the 5f peak is attributed to the valence band. Using these data, we have located the 5f pure states, labelled 5f in the diagram (see Fig. 1)

.

From this energy level scheme, the UO, band gap width corresponds to the distance between the 5f2 level and the bottom of conduction band and the sf-6d transitions start above about 5 eV. The filled 5f2 states are separated from the excited 5fn+2 states by 2.9 eV; thus, 5f- 5f transitions should be present in the optical spectra at this energy. Indeed, the excited 5fn+" states have the 3d.'5fn+.' configuration; as a consequence of the interaction between the 3d and 5f shells, the observed maximum in the absorption and resonant emission is the barycentre of

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multiplets associated with the confiquraticm.

The 5f-5f optical transitions are also demultiplied and their energies can vary about the values mentioned above. Moreover, the distance between the 5f character states present in the valence band and the bottom of the conduction band is % 10 eV.

These results confirm the interpretation of optical spectra given by Naegele et a1./5/ and disagree with the recent paper of J. Schoenes /6/.

The other information which can be deduced from the UO, diagram and the soft X-ray emission spectrum is as follows :

Firstly, the observation of a line in the 3d emission spectrum at the UO2 valence band position shows that the valence band possesses a faint 5f character.

In fact, the valence band is formed prin- cipally from the 2p oxygen and 6d-7s uranium states. But because of the dipolar selection rules, the 6d or 7s uranium states do not contribute to the 3d emission spectrum and only the 5f uranium states are susceptible to give an emission. A faint covalent mixing of uranium 5f and oxygen 2p levels is present which indicates that the binding is not purely ionic. In Tho2

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we have found similarly that states having a faintly 5f character are present mixed with the 2p oxygen orbitals.

Secondly a resonance line is present only if the excited state is very strongly localized /7/. These lines are analogous to the optical resonant lines which are present in the spectra of atoms. In this process, the emission corresponds to the direct radiative recombination of the excited electron with the inner hole, here 3d. It is observable only if this direct recombination is more rapid than the inter- actions of ,the 3d-I5fnfi excited state with the extended states in the solid. As a consequence of the presence of a resonant

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^!5fnf

'.

line, the localization of the 3d state must be very strong.

Lastly, the 5f filled and excited states are more localized in UO, than a-u because their binding energies are larger

(see Fig.1) and especially the width of X-ray transitions is smaller and the resonance lines are more intense.

Similar experiments are in progress for PuO,

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References

/1/ Lacher@,G., Thsse de Doctorat dWEtat, Paris, Juin 1979.

/2/ Bonnelle, C., Lachers, G., J. Physique

2

(1974) 295.

/3/ Fuqqle, J.C., Burr, A.F., Watson, L.M.

Fabian, D.J., Lang, W., J. Phys. F, 4 (1974) 335.

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/4/ Veal, B.W., Lam, D.J., Phys. Rev.

B

10

(1974) 4902.

/5/ Naegele, J., Manes, L., Birkholz, U., Plutonium and other actinides, gds.

H. Blank and R. Lindner, North- Holland, Amsterdam, 1976, p.393.

/6/ Schoenes, J., J. Appl. Phys., 49

(1978) 1463.

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/7/ Bonnelle, C., Structure and Bonding, 31 (1976) 23.

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