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VALENCE CHANGE IN THE Pd-Ce SYSTEM
D. Malterre, G. Krill, J. Durand, G. Marchal
To cite this version:
D. Malterre, G. Krill, J. Durand, G. Marchal. VALENCE CHANGE IN THE Pd-Ce SYSTEM.
Journal de Physique Colloques, 1987, 48 (C9), pp.C9-1029-C9-1032. �10.1051/jphyscol:19879185�.
�jpa-00227301�
Colloque C9, suppl6ment au n 0 1 2 , Tome 48, dgcembre 1987
VALENCE CHANGE I N THE P d - C e SYSTEM
D. MALTERRE, G. KRILL, J. DURAND and G. MARCHAL
Laboratoire d e Physique du Solide, Universit6 de Nancy 1, BP 239, F-54506 Vandoeuvre-les-Nancy Cedex, France
and LURE, B s t . 209 D , F-91405 Orsay Cedex, France
Resume.- Ues mesures d'absorptlon X et de pnotokmlsslon ont ete errectukes sur les alliages Pdl-,CeX cristallins et amorphes. Dans ce systeme, l e cerium presente une transition de valence avec la concentration. Une correlation entre l a valence et la structure de bande a ete mise en evidence. Les anomalies de valence semblent pouvoir e t r e correlees a l a f o r t e densite d'ktats d du palladium prks du niveau de Fermi. L'effet du desordre s t r u c t u r a l sur I'instabilite de valence est egalement discute.
Abstract.- We report X-Ray Absorption and X-Ray Photoemission measurements on crystalline and amorphous Pdl-xCex alloys. I n this system, cerium presents a concentration-induced valence change. A correlation between cerium electronic configuration and density of occupied states have been shown.Valence anomalies appear t o be due t o the high palladium 4d density of states near the Ferrni level. The effect of structural disorder on the cerium valence instability i n amorphous alloys i s also discussed.
I n many compounds, cerium exhibits electronic instabilities resulting from the hybridization between the 4f and the conduction electrons /I/. I n recent years, high energy spectroscopies like X-Ray absorption (XAS) o r photoemlssion (XPS) have been used t o obtain information on the electronic ground state, though these spectroscopies image excited final states. The aim of the present study i s t o follow the behaviour of the valence of Cerium i n the Pd-Ce system, i n which the electronic parameters can be changed continuously. Indeed Pd and Ce form a binary solution on a broad concentration range (up t o x=O. 125) and i t i s possible t o extend this limit up to x=0.3 by the formation of amorphous alloys. Ce i s known t o be 'tetravalent' (saturated mixed valent) i n dilute Pd-Ce alloys /2/, wheras crystalline CePdg i s a typical weak intermediate valent compound (v=3.15) /3/. XAS and XPS experiments are used to obtain the cerium average valence and the evolution of the density of occupied states with composition i n the Pdl-xCex alloys. Then we examine the correlatlon between the Ce electronic configuration and the band structure, and discuss the r o l e of the d band f i l l i n g and the
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19879185
C9-1030 JOURNAL DE PHYSIQUE
influence of structural disorder on the valence instabilities. Sample preparation and experimental details are described i n a previous paper /4/.
11-11 LIII edge spectroscopy.
Figure 1 shows the normalized LIII edges of cerium f o r several Pd-Ce alloys. The data were taken at 300K with an energy resolution of about 1 eV. The low concentration alloys spectra exhibit a two-bump structure with a peak separation of about 10 eV which i s characteristic of the mixed-valent character of the Ce ions. Whereas no significant change of the LIII edge shape is observed i n the crystalline alloys (x40.125), a growing up of the low energy feature i s shown with increasing Ce content i n the amorphous range.
The average valence i s evaluated through a standard fitting procedure used and discussed elsewhere /5/.The concentration dependence of the cerium valence i s shown on figure 2.
Two regimes can be underlined:
i) I n the crystalline alloys, the valence of cerium i s saturated t o a V=3.3 value.
Such a saturation effect have previously been observed i n pseudobinary compounds /6/
and i n binary amorphous alloys /2/.
ii) I n the amorphous phase, the sample average valence decreases progressively with increasing cerium content. The amorphous counterpart of CePd3 compound i s trivalent.
Fig. 1 : Ce
till
edge absorption spectra.11-2) XPS measurements.
-
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I n contrast to heavy Rare-Earth elements (Sm,Eu,Yb) cerium exhibits the same valence both at the surface and i n the bulk /7/. Then reliable information about electronic configuration of cerium can be deduced from such XPS experiments.
m _,/ :
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5700 5'20 5760 Pdl-xCex alloys.
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Fig.3 : Cerium 3d core level spectra.
The cerium 3d core level spectra are reported on figure 3 f o r several concentrations. The intensity of the high energy structure (EB=914.5eV), which i s associated t o 4f0 final states, qualitatively reproduces the composition dependence of the cerium valence deduced from the LIII edges. I n addition, we observe on a l l spectra the well known shake-down structure which reflects the 4f2 final state. The fntensity of this satelllte which provides a measure of the hybridization between 4f and conduction electrons /8/, exhibits roughly the same concentration dependence as f o r the 4 P structure. As already observed on the XAS spectra, two concentration ranges can be c l e a r l y distinguished:
i)In the crystalline state, no significant changes occur on the 4f0 and 4f2 intensities reflecting that valence and hybridization remain roughly constant.
ii) I n the amorphous state i n contrast, both intensities decrease with increasing Ce concentration, confirming the progressive stabilization of the trivalent state.
11-2-21 Valence band spectra.
Information about the 4f distribution are difficult t o obtain from usual (A1
Kci
o r Mg Kci) XPS measurements, especially i n RE-Transition compounds, because of the high transition element d contribution arising near the Fermi level (EF).The He I (h9=20.1 eV), , , , ,
1
Fig. 4 : UPS valence band spectra for different Pdl-XCeXX- alloys.
--..-
E , 1 2 3 4
Binding Energy [ 2 V 3
UPS spectra are reported on figure 4. These spectra c l e a r l y show that the D.0.S at the
5
-
c ~l-
Fermi level strongly decreases with increasing cerium content i n the solid solution range, but remains roughly constant f o r the amorphous alloys. This behaviour can be explained by a charge t r a n s f e r from cerium t o palladium and thus, by the progressive f i l l i n g of the palladium 4d band which i s f i l l e d up f o r x=0.125, the upper limit of the solid solution range. The shoulder which appears at EF f o r l a r g e r concentration, results from the extended s band whereas the narrow palladium d band progressively moves away from EF when the cerium content increases. Comparison with the LIII and core level results suggests that the cerium average valence and the band structure are correlated. This w i l l be dicussed i n the following section.
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C9-1032 JOURNAL DE PHYSIQUE
Comparison between XAS and XPS measurements shows the contrasted effects of the composition on the Ce average valence and the band structure. I n the crystalline binary solid solution (Odxt0.125) alloys, the average valence of cerium remains saturated (V=3.3), whereas the D. 0.S at EF deduced from valence band spectra strongly decreases.
On the contrary, i n the amorphous alloys (0.125dxc0.30) the trivalent configuration of cerium i s progressively stabilized whereas no significant change of the D.O.S. at EF i s observed. Then, the valence state seems to depend on the possibility of transferring Ce electrons towards the Pd 4d band. I n dilute alloys, the Pd 4d band remains unfilled and Ce exhibits a 'tetravalent' state. A t about x=O. 125, the Pd d band i s full, no t r a n s f e r i s then possible and the trivalent state should be stabilized. Moreover, f o r higher concentration, the d density moves away from EF leading to a reduction of the density of conduction electron i n the one eV range below EF. I n the case of cerium intermetallics, the 4f level lies i n this energy region (0.9 eV f o r crystalline CePd3 compounds /9/) as shown by resonnant photoemission studies. Therefore, as the d band moves away from EF, mixing between 4f and conduction electron i s strongly reduced, leading t o a progressive stabilization of trivalent state. Such a decrease of the transition metal derived D.O.S.
has already been observed i n going from CeRh3 t o CePd3 and invoked t o explain the valence variation i n the pseudo-binary Ce(Pdl -xRhx)3 system
/lo/.
Let us conclude by discussing b r i e f l y the effect of structural disorder on the Ce valence instabilities. Whereas crystalline CePd3 i s a weak mixed-valent compound (Cea type), i t s amorphous counterpart i s found t o be trivalent. This stabilization of the normal electronic state of cerium i n the amorphous CePd3 alloy confirm the general t r e n d we have observed on the Cea type compounds/6/. Correlatively, a modification of the valence band spectra i s observed i n the amorphous CePd3 alloy. The high d density of states i n crystalline CePd3 compound i s found t o be closer t o the Fermi level than that i n the corresponding amorphous alloy, ascertaining the correlation we discuss above, between electronic configuration and valence band spectrum.
References
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/2/- B. Lengeler, J.E. Muller and G. Materlik, i n EXAFS and Near Edge Structure, edited by A. Bianconi, L. Incoccia and S. Stipcich (Spinger -Verlag, 1983) 151.
/3/- D. Wohlleben and J. Rohler, J. Appl. Phys. 55, 1904 (1984).
/4/- D. Malterre, 5. Krill, J. Durand, G. Marchal and M.F. Ravet, Phys. Rev. B 34, 2176 ( 1 986).
/5/- J. Rohler, J. Mag. Mag. Mat. 47-48, 175 (19851175.
/6/- S. Raaen, M. L. den Boer, V. Murgai and T. Mihalisin, Phys. Rev. B 28, 3556 (1983).
/7/- B. Johansson, Phys. Rev. B 19,6615 (1979).
/8/- 0. Gunnarsson and K. Schonhammer, Phys. Rev. B 28, 4315 (1983).
/9/- J.W. Allen, S. -J. Oh, 1. Lindau, J.M. Lawrence, L.I. Johansson and S.B. Hagstrom, Phys. Rev. Lett. 46, 1100 (1981).
/lo/-S. Raaen, M. L. den Boer, V. Murgai and R. D. Parks, Phys. Rev. B 27, 5139 (1983).
