VALENCE CHANGE IN THE Pd-Ce SYSTEM

We report X-Ray Absorption and X-Ray Photoemission measurements on crystal l ine and amorphous Pdl-xCex alloys. I n this system, cerium presents a concentration-induced valence change. A correlat ion between cerium electronic configuration and density of occupied states have been shown.Valence anomalies appear to 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 instabilit ies resulting from the hybridization between the 4f and the conduction electrons /I/. I n recent years, high energy spectroscopies l ike X-Ray absorption (XAS) or photoemlssion (XPS) have been used to obtain information on the electronic ground state, though these spectroscopies image excited final states. The aim of the present study i s to 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 to x=O. 125) and i t i s possible to extend this l imit up to x=0.3 by the formation of amorphous alloys. Ce is known to be 'tetravalent' (saturated mixed valent) i n dilute Pd-Ce alloys /2/, wheras crystal l ine 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 correlat lon between the Ce electronic configuration and the band structure, and discuss the ro le of the d band f i l l ing 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 for 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 crystal l ine alloys (x40.125), a growing up of the low energy feature is shown with increasing Ce content i n the amorphous range. The average valence is 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 crystal l ine 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) In the amorphous phase, the sample average valence decreases progressively with increasing cerium content. The amorphous counterpart of CePd3 compound is trivalent. Fig. 1 : Ce till edge absorption spectra. 11-2) XPS measurements. :--?--I--'t Pdl-X CeX 't \ \ I\ \ \ I \ \ +