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Shake-up satellites in the U4f E.S.C.A. spectra of NaCl-type uranium monocompounds : UN, UP, UAs
and US
E. Thibaut, J. Verbist, R. Trok
To cite this version:
E. Thibaut, J. Verbist, R. Trok. Shake-up satellites in the U4f E.S.C.A. spectra of NaCl-type uranium
monocompounds : UN, UP, UAs and US. Journal de Physique Colloques, 1979, 40 (C4), pp.C4-77-
C4-78. �10.1051/jphyscol:1979425�. �jpa-00218821�
JOURNAL DE PHYSIQUE Colloque C4, supplément au n° 4, Tome 40, avril 1979, page C4-77
Shake-up satellites in the U4f E.S.C.A. spectra of NaCl-type uranium monocompounds: UN, UP, UAs and US
E . T h i b a u t (*), J. Verbist a n d R. T r o c CO
Facultes Universitaires, Laboratoire de Spectroscopic Electronique, rue de Bruxelles, 61, B-5000 Namur, Belgium (•) Institute of Low Temperatures and Structural Research, PI. Katedralny, 1, P.O. Box 937, PL-50-950 Wroclaw, Poland
Résumé. — Les spectres E.S.C.A. des niveaux U4f de quatre composés d'uranium, tous de structure NaCl, sont décrits et discutés en vue d'une approche du caractère de la liaison chimique.
Abstract. — The U4f E.S.C.A. spectra of four NaCl-type uranium monocompounds are reported, and discussed in an attempt to relate them to the character of the chemical bond.
1. Introduction. — The study of uranium mono- compounds with group Va and Via elements was undertaken in order to extend the understanding of chemical bonding of this metal with elements of different electronegativities.
All the studied compounds : U N [1], U P [2], UAs [3] and US [4], have cubic (fee, NaCl-type) structure and present semi-metallic electrical pro- perties. The uranium electronic state in these solids is still questioned. On one side indeed, neutron diffraction experiments on UN [5], UP [6], UAs [7], confirm the existence of the U(III) state with [Rn]
5f3 configuration ; but on the other hand, a discus- sion of magnetic properties [8] leads to the conclu- sion that uranium is rather in the + 4 state. For U S , Suski [9] recently noted that the electronic configu- ration of uranium is still a matter of discussion.
It was therefore tempting to tackle these problems with a technique which produces a direct look at electronic states : X-ray photoelectron spectroscopy (Electron Spectroscopy for Chemical Analysis) of core and valence levels.
2. Results. — All spectra were obtained on a Hewlett-Packard 5950 A instrument, using mono- chromatized AlKa radiation (hv = 1 486.6 eV). The spectrometer's sample inlet chamber is equipped with a stainless steel glove box, in which a dry nitrogen atmosphere is maintained by recycling the gas over appropriate catalysts which eliminate resi- dual water and oxygen.
In spite of drastic operating conditions, all pow- dered samples presented severe oxidation, marked by the presence of OIs and 0 2 s peaks around 532 and 22 eV in the spectra, and by a marked broade- ning of the normally sharp U4f peaks.
Attempts to sputter the oxidized layer off the sample by Ar+ (1 kV) ions were unsuccessful or
worse. Better experimental data could probably be obtained by studying single crystals, which could be cleaved and scraped in vacuo. Powders indeed have high reactivity due to their large specific area in contact with the surrounding atmosphere.
Of all core and valence spectra, which have been obtained, we will only discuss here a noteworthy fraction : the U4f levels show indeed much stronger shake-up satellites than those already known for
i
RELATIVE BINDING ENERGY (eV)
Fig. 1. — E.S.C.A. spectra of U4f peaks and their satellites in NaCl-type uranium compounds.
(*) Holder of a specialization grant from I.R.S.I. A. (Belgium).
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979425
C4-78 E. THIBAUT, J. VERBIST AND R. TROC
Table I.
-
Comparison of E.S.C.A. results on NaC1-type uranium compounds. Description o f U4f peaks and their satellites (symbols used are the same as in figure 1).U4f peaks Shake-up satellites
Separation (eV) from Intensity ratio to
E, shift (eV) main peak main peak
relative to
UX
-
UO2 - U4f,/2 U4f7, U4f5,2 U4f,/2-
- --
UN A ' = 4.3 A = 4.8 (0.61) (1)
- 1.2
B' = 9.6 B = 8.8 (0.83) (1.48) UP - 1.6 C' = 6.3 C = 5.1 0.08 0.01 UAs - 1.3 D' = 6.6 D = 6.8 0.25 0.06
% ionic character
(Pauling model [" I)
Atomic polariza- bilities [ b ]
cm3)
r ]
PAULING, L., The Nature of the Chemical Bond, 3rd ed. (Cornell University Press) 1960, chap. 111.[ b ] MILLER, M. T., BEDERSON, B., Adv. Atom. Molec. Phys. 13 (1977) 1.
uranium oxides [lo]. We wanted to investigate the possible relations between position and intensity of such satellites and nature of the anion and bonding.
Failing at present to have oxygen-free spectra, we have analysed the existing data, using standard '
mathematical procedures. The U4f main peak components were first located by reconstructing the spectrum from Gaussian curves. The scaled U 0 2 (and if necessary UO,) original spectra were then substracted at their identified positions in binding energy. The results are illustrated in figure 1, and the main characteristics are summarized in table I.
The spectra were not calibrated individually in binding energy ; however the U4f binding energy values relative to the UO, component is listed in the table.
It should be noted here that the UN spectrum is the worse example of the problems we encountered : the oxide component could not be completely sub- stracted for an unknown reason related to the back- ground line shape. For this reason, the corres- ponding numerals in table I have to be taken carefully.
3. Discussion.
-
Although very distinct, the U4f shake-up satellites d o not show simple correlations with the nature of the anion, neither by their num-ber, nor in position, and not either in relative intensi- ties. This conclusion may seem rather disappointing at this point ; forthcoming results on single crystals, eventually cleaned in vacuo, should confirm or correct these observations.
Comparing the compounds of highly covalent cha- racter (UP and UAs) the explanation might be sought in the different polarizabilities of the anion.
This property is known indeed to affect in several ways the aspect of E.S.C.A. spectra, as observed in the series of M'UO, alkali metal hypouranates [I 11.
On the reverse, the ionic fraction of the chemical bond certainly is one of the determining factors of shake-up satellites. Asada and Sugano [l2] have calculated in the case of transition-metal compounds that their intensity is a result of the relative increase of covalency upon departure of a metal core elec- tron. Finally, the satellite-to-main peak separations are to be associated with energy gap values in the valence band, observing the monopole selection rule [lo]. Band structure calculations would in this .view be particularly welcome.
Acknowledgments.
-
The authors want to thank Dr. H. Noel (University of Rennes, France) for stimulating discussions. One of us (E. Thibaut) is grateful t o I.R.S.I.A. (Belgium) for financial sup- port.References
[l] COUNSELL, 3. F., DELL, R. M., MARTIN, J. F., Trans.
Faraday Soc. 62 (1966) 1736.
[2] BASKIN, Y., 3. Am. Ceram. Soc. 49 (1966) 541.
[3] TRZEBIATOWSKI, W., SEPICHOWSKA, A., ZYGMUNT, A., Bull.
Acad. Pol. Sci. Ser. Sci., Chim. 12 (1964) 687.
[4] SHALEK, P. D., 3. Am. Ceram. Soc. 46 (1963) 155.
[5] CURRY, N. A., R O C . Phys. Soc. (London) 86 (1965) 1193.
[6] CURRY, N. A,, Proc. Phys. Soc. (London) 89 (1966) 427.
[7] TROC, R., MURASIK, A., ZYGMUNT, A., LECIEJEWCZ, J . , Phys. Status Solidi 23 (1967) K123.
[8] GRUNZWEIG-GENOSSAR, J., KUZNIETZ, H., FRIEDMAN, F., Phys. Rev. 173 (1968) 562.
[9] SUSKI, W., Uranium Chalcogenides, Fizyka i chernia ciala stralego, PAN Wroclaw (1976) 43.
[lo] PIREAUX, J. J., RIGA, J., THIBAUT, E., TENRET-NOEL, C:, CAUDANO, R., VERBIST, J. J., Chem. Phys. 22 (1977) 113.
[I 11 MOUSTY-DESBUQUOIT, C., MBrnoire de Licence, FacultBs Universitaires de Namur (1978).
1121 ASADA, S., SUGANO, S., J. Phys. Soc. Japan 41 (1976) 1291.
