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MIXED VALENCE STATE OF Sm ON THE SURFACE OF AMORPHOUS La80-xSmxAu20 ALLOYS
G. Krill, A. Amamou, A. Berrada, J. Durand, N. Hassanain
To cite this version:
G. Krill, A. Amamou, A. Berrada, J. Durand, N. Hassanain. MIXED VALENCE STATE OF Sm ON THE SURFACE OF AMORPHOUS La80-xSmxAu20 ALLOYS. Journal de Physique Colloques, 1980, 41 (C8), pp.C8-799-C8-802. �10.1051/jphyscol:19808198�. �jpa-00220302�
JOURNAL DE PHYSIQUE CoZZoque C8, supple'ment au n08, Tome 41, aoCt 1980, pagecg-799
M I X E D VALENCE S T A T E OF Sm ON T H E SURFACE O F AMORPHOUS Lago-xSmxAU20 ALLOYS
G. Krill, A. Amamou, A. Berrada, J. Durand and N. Hassanain
L.A. M.S. E.S. IN0 306), Universite' Louis Pasteur, 67070 Strasbourg Ce'dex, France.
RGsum15.- Des mesures d'UPS analysGes 5 la lumigre des r6sultats obtenus pas XPS fournissent des 6lGments d4'information sur la nature (inhomoggne) de l'dtat de valence mixte du Sm 3 la surface des alliages amorphes LaSmAu. En outre, le caractgre quasi-ionique de ces alliages est mis en Gvidence par l'observation de la structure des Gtats 5d de l'or.
Abstract.- UPS measurements analyzed in the light of XPS spectra yield some information about the inhomogeneous nature of the Sm surface mixed-valence state in amorphous LaSmAu alloys. In addition, the quasi-ionic character expected for these materials is clearly evidenced by the observation of the structure of the Au 5d states.
In a recent letter / I / , we reported the re- letter / I / . Let us recall, first, what can be sults of XPS measurements on a series of splat- gained by the combined use of the XPS (A1 Ka =
cooled amorphous (a-) La80-xSmxA~20 ( X = 10, 20, 1486.6 eQ) and UPS (He I = 21.2 eV ; He I1 = 40, 80) alloys, giving evidence for a mixed valence 40.8 eV). The effective photoemission cross- state of Sm on the surface (2 or 3 first layers). section for 4f shells is known to vary conside- In addition, the Sm surlace valence was shown to rably as a function of energy of incident pho- become more divalent with increasing La content. tons /3/. In XPS experiments, the 4f effective Furthermore, by means of combined X-ray absorption cross-section is relatively large =
measurements, a change in electronic configura- 8.10-~ Y b ) , and then comparable with that for tion of Sm between the surface and the bulk of Au 5d states (05d512(Au) = 17. 'lb) ; it be-
I
a- Sm80A~20 was demonstrated. Moreover, it was comes rapidly negligible when hv tends to zero.
strongly suggested that the concentration depen- In He I experiments, one can assume that the 4f dence of the Sm volume valence does not follow shells are not ionized anymore. In the He I1
that of the surface when x decreases at least experiments, the situation is intermediate, since down to x = 20. This report present some comple- the 4f shells are still weakly ionized by the cor- mentary informations one can obtain on the same responding wave length. UPS experiments allow one amorphous samples from the U?S spectra in relation to define with a good accuracy (?. 0.1 eV) the va- with the XPS data. This analysis.enables us to
suggest that the Sm surface mixed valence state is likely to be inhomogeneous in nature, as has been deduced in crystalline SmB6 /2/. In addition, the 5d states of Au show evidence for the strongly ionic character of Au in these amorphous alloys.
For detail on sample preparation and experi- mental procedure, one can relrr to our previous
rious components of the valence band.
In Fig. 1 is shown the valence band of the a- LaA,3Sm40A~23 and a- Sm80A~20 alloys. In both spectra, the contributions arising from Au 5d
512 and 5d states are dominant. Nevertheless, the
312
struct~:~-es i r t the (1- Sm 80Au23 spectrum are less resolved. This is attributed to a larger quantity of Sm 3+ ions ( 4 f r ' ) whose final states overlap
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19808198
C8-800 JOURNAL DE PHYSIQUE
Fig. 1 : XPS spectra of valence electrons in a- La40Sm40A~20 and a- Sm Au
80 20'
partly with the Au states. It is then difficult to analyze the rnultiplet structure of the 4f5 and 4f states by using only the XPS spectra. 6
Since the photoemission 4f cross-sections are not zero for hv = 40.8 eV (He II), the multiplet structures associated with sm2+ (%, 6 ~ ) can be tentatively identified in the first eV range be- low the Fermi level. He I1 spectra for binding energies EB < 3 eV are reported in Fig. 2. In a- S I I I ~ ~ A U ~ ~ , the H' and F final states, separa- 6 ted by about 0.7 eV, are fairly well identified.
In contrast, when La substitutes for Sm, this iden- tification is made difficult by i) the decrease in energy of the 6~ and F levels, ii) and the on- 6 set at the Fermi level of a rather broad band ari- sing from the La d states. Even for the case of a- Sm80Au20, the 6~ level is found to be located at a minimum of about 0.5 eV below EF. If the Sm surface mixed valence state were homogeneous, the 6 H level should pin the Felmi level in order
5 1
to allow the 4f6 5d0 Z 4f 5d rransitions. The Sm surface valence state of a- SmF,,Auz0 is then
I
a. LamxSrnx AU 20 UPS HenI
Fig. 2 : UPS (He I1 = 40.8 eV) spectra of valence electrons in a- Lago-xSmxA~20 alloys.
very similar to that described for the surface of c- SmB6 121. In both cases, the admixture of Sm
electronic configuration on the surface is static (inhomogeneous) in nature. Unfortunately, owing to the poor resolution (2. 1 eV) of the XPS spectra, it is not possible to estimate the exact position of the 6~ level with respect to OF, and, therefore, the XPS experiments fail to yield informations as to the nature of the mixed valence state in the layers ju$t below ti,? surface.
In Fig. 3 and 4, we plotted the series of spectra obtained from the He I and He I1 experi- ments, respecti\-ly. The Au 5d and 5d StruC-
5/2 3/2
tures, located near 4.9 and 6.5 eV with respect
Fig. 3 : UPS (He I = 21.2 eV) spectra for the a- La80~xSmxA~20 alloys.
to EF, respectively, are very well defined. From the sharpness of these transitions together with the valence of the spin-orbit splitting
( A (5d5/2 - 5d3/2) 2 1.6 eV, as compared with the free-ion value A = 1.5 eV), the strongly ionic character of these a- LaSmAu alloys is clearly evidenced. Similar conclusions were drawn for the crystalline CsAu and YbAu2 compounds /4,5/ as well as for some amorphous GdFe alloys / 6 / . Such a result for our a- LaSmAu alloys was quite ex- pected, knowing from structural studies on a- La80A~20 that the Au atoms have no Au atoms in their first surrounding atomic shell / 7 / . Never- theless, the Au 5d transitions in a- Sm80A~20
Fig. 4 : UPS (He I1 = 40.8 eV) spectra for the a- La80-xSmxA~2a alloys.
appear to be slightly broadened, which might be due to some tiny differences in atomic-scale structure between a- La80A~20 and a- Sm Au
8 0 20'
In conclusion, UPS measurements, in addition to XPS spectra, yield complementary information about the nature of the Sm surface mixed-valence state in a- LaSmAu alloys. They ascertain also the quasi-atomic character expected for these materials.
References
/ I / Krill, G., Durand, J., Berrada, A., Hassanain, N. and navet, M.F., Solidstate Com.(in print).
/2/ Allen, J.W., Johansson, L.I., Lindau, I. and
~a~strgrn, S.B., Phys. Rev. B 2 (1980) 1335.
c8-802 JOURNAL DE PHYSIQUE
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/ 4 / W e r t h e i m , G.K., B a t e s , C.W. and B u c h a n a n , D . N . E . , S o l i d S t a t e Comm. 30 ( 1 9 7 9 ) 4 7 3 . 1 5 1 I d e r t h e i m , G.K., I , f e r n i c k , J . H . a n d C r e c e l i u s ,
G., P h y s . Rev. R 18 ( 1 9 7 8 ) 8 7 5 .
161 G c n t h e r o d t , G. a n d S h e v c h i k , N . J . , A . I . P . C o n f . P r o c . 9 ( 1 9 7 5 ) 1 7 4 .
/ 7 / L o g a n , J . , S c r i p t a Y e t 2 ( 1 9 7 5 ) 3 7 9 .
