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HIGH ENERGY SPECTROSCOPY INVESTIGATION OF SOME TERNARY Ce SYSTEMS
E. Beaurepaire, J. Kappler, P. Lehmann, G. Krill, C. Godart
To cite this version:
E. Beaurepaire, J. Kappler, P. Lehmann, G. Krill, C. Godart. HIGH ENERGY SPECTROSCOPY INVESTIGATION OF SOME TERNARY Ce SYSTEMS. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-987-C8-990. �10.1051/jphyscol:19868190�. �jpa-00226097�
Colloque C8, suppl6ment au n o 12, Tome 47, dgcembre 1986
HIGH ENERGY SPECTROSCOPY INVESTIGATION OF SOME TERNARY Ce SYSTEMS
E. BEAUREPAIRE, J.P. KAPPLER, P. LEHMANN, G. KRILL* and C. GODART"
L.M.S.E.S. (U.A. C.N.R.S. 3 0 6 ) , Universite Louis Pasteur, 4, rue Blaise Pascal, F - 6 7 0 7 0 Strasbourg Cedex, France ' ~ a b o r a t o i r e de Physique du Solide, Universite de Nancy I , B P 2 3 9 , F-54506 Vandoeuvre-les-Nancy Cedex, France
ER C.N.R.S. 2 0 9 , place Aristide Briand, F - 9 2 1 9 0 Meudon, France
R6sum6 - Nous pr6sentons des mesures de s e u i l LIII e t de photoemission 3d dans l e s a l l i a g e s (Ce, La)Ru2Si2 e t (Ce, Y)Ru2S12. Nous montrons que l e s v a r i a t i o n s de v a l e n c e e t d ' h y b r ~ d a t i o n f-d s o n t peu importantes dans ce syst&me q u i r e s t e presque t r i v a l e n t . Dans ce c o n t e x t e , l a f o r t e v a r i a t i o n des p r o p r i 6 t 6 s macroscopiques de c e s a l l i a g e s ( t e l l e que s u s c e p t i b i l i t 6 magn6tique) s e p r o d u i t l o r s q u e l ' 6 c a r t e n t r e l e s niveaux de champ c r i s t a l l i n e t l a temperature Kondo s o n t du mzme o r d r e de grandeur.
A b s t r a c t - LIII a b s o r p t i o n and 3d photoemission r e s u l t s a r e presented f o r t h e a l l o y s (Ce, La)Ru2Si2 and (Ce, Y)Ru S i 2 . We show t h a t v a l e n c e and f-d h y b r i d i z a t i o n v a r i a t i o n s a r e modest i n t i i s system where Ce remains n e a r l y t r i v a l e n t . I n t h i s c o n t e x t , s t r o n g v a r i a t i o n s i n t h e macroscopic p r o p e r t i e s of t h e s e a l l o y s ( e g magnetic s u s c e p t i b i l i t i e s ) a r e a t t r i b u t a b l e t o a cross-over between c r y s t a l f i e l d l e v e l s s p l i t t i n g and Kondo temperature.
I - INTRODUCTION
Cerium i n t e r m e t a l l i c compounds C ~ Mw i t h t h e ThCr S i -type t e t r a g o n a l s t r u c t u r e ~ S ~ ~ 2 2
have r e c e n t l y focussed many s t u d i e s s l n c e a l l t h e v a r i o u s f a s c i n a t i n g p r o p e r t i e s of cerium systems can be found i n t h e s e r i e s : superconducting heavy fermion sys- tem (M = Cu), mixed v a l e n c e (M = Co, N i
...
) , magnetic o r d e r i n g (M = Au, Pd.. .).Within t h e s e compounds, CeRu S i i s p e c u l i a r s i n c e macroscopic measurements ( l a t - t i c e c o n s t a n t , s u s c e p t i b i l i t ? , Zhermopower.
. . ) have shown t h a t i t l i e s on t h e
b o r d e r - l i n e between a t r i v a l e n t and a mixed v a l e n t ground s t a t e , and p r e s e n t s a t low temperature t h e s o - c a l l e d Kondo l a t t i c e behaviour / 1 , 2 / . As a consequence a weak p e r t u r b a t i o n ( p r e s s u r e , a l l o y i n g , magnetic f i e l d . . . ) produces a t r a n s i t i o n e i t h e r t o mixed-valent o r t o t r i v a l e n t s t a t e .
We p r e s e n t h e r e 3d c o r e l e v e l photoemission and LIII a b s o r p t i o n on Ce Y Ru S i 1 - x x 2 2 (where Ce i s non-magnetic) and Ce La Ru S i (where Ce i s magnetic f o r
y 2 0.2) and compare t h e r e s u l t s w i t h t h o s e o b t a i n e d from macroscopic measurements/ 2 / (magnetic s u s c e p t i b i l i t y , s p e c i f i c h e a t . . . ) .
I1 - EXPERIMENTAL
Ce La Ru S i and Ce -xYxRu2Si samples w i t h y 0, 0.1, 0.2, 0.3, 0.5, 0.7, 0.9 a n r y x 0?1 , 2 ~ - 2 , 0.1, 0.5, o.? and 0 . 9 have been prepared by a r c m e l t i n g . LIII a b s o r p t i o n experiments have been c a r r i e d o u t a t LURE on t h e EXAFS I1 s t a t i o n u s i n g t h e S i (31 1) double c r y s t a l monochromator. UHV (P = t o r r ) photoemission
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868190
C8-988 JOURNAL DE PHYSIQUE
have been performed in a VG ESCA 111 apparatus using A1 K radiation with an energy a resolution of 1 eV. Samples were cleaned by cycling argon ion bombardment and heating to 400°C in order to limit oxygen contamination (& 5 %) and preferential sputtering. For the XPS study, we used polycrystalline CeRu2Si and Ce
samples and a monocrystalline Ce .7La0. 3R~2Si2 ( (001) surface?. 0 . 6 ~ 0 . 4 ~ ~ 2 ~ ~ 2 111 - RESULTS
a) &I-LII absorption
The L spectra of (Ce,T)Ru2Si2 alloys exhibit a main resonance near 5720eV due to 2p64£I1: ~ ~
ansi sit ions
~ 4 f and a shoulder at 10 eV highe energy characteristic ~ of 2p64f0 I 2p54fO 5 ~ d transitions. The relative weight of 4 f r and 91 contribu- 0 tions C, and Co have been estimated by a fit of the "white line" 131. For metallic:e systems, the values obtained by this method agree qualitatively with those used Co fit other spectroscopic experiments (3d XPS, UPS, BIS, MV absorption) /7/ so that, as a first approximation, we shall identify C1 with the fractional occupation of 4f states in the ground state (nf).
These values of nf are reported on Fig. 1 as a function of the reduced volume Log V/V (V refers to CeRu Si ) . It is remarkable that in the whole concentration
2 2
range, ?he 8f occupancy vanes smoothly from nf = 0.825 (Ce0.1Y0.9R~2Si2) to nf =
0.95,(Ce0.1La0.9Ru2Si2) with a slight knee corresponding to the crltlcal concentration for the onset of magnetism (y = 0.15). It can be concluded that the chemical pressure in this system is not strong enough for a valence saturation as observed eg in Ce(Pd,Rh)g alloys /6/. This valence satura- tion regime is observed in the series for the compounds CeNi2Si2, CeCo2Si2 and CeFe2Si2 (see inset of fig 1 and 151). As a consequence, nf is not far
from unity in CeRuLSiL alloys and justifies a posterior1 a Kondo type descrip- tion of the macroscopic properties 121. These results also confirm that an integer valence is not a necessary condition for a magnetically ordered ground state at
low temperature since samples with y _> 0.2 antiferromagnetically order below
T < 5 K ; this effect is also known for some Eu or Yb compounds (for a theoretical
N %
discussion see 171).
Log (Y/V0lx 1 0 2
Fig. 1 :
Zero temperature extrapolated magnetic susceptibility (full symbols) and 4f count deduced from LILI (open symbols) in Cel-xYxR~2Si2 (squares) and Cel-yLayRu2Si2
(circles) as a function of the reduced volume (Vo for CeRu2Si2). The llnes are a guide to the eyes. Typical spectra and variations of nf as a function of the unit cell volume for CeM2Si2 and Ce La Y Ru,Si, in insek.
1-x-y y x L L
Fig. 2 :
3d photoemission spectra for Ce
0.4'0. 6Ru2Si2' CeRu2Si2 and Ce0.3La0-7Ru2Si2. The 4f2 satellite is indicated by an arrow ; lines are a guide to eyes.
A small peak is also found 1 1 eV above the main 3d photoemission line in CeRu Si Ce La Ru Si and Ce
0.7 0.2 ,2 $ Ru !S "fjnd can
be attrl u ed o the 0.6'0.4 2 2 4f
i- guration. Itsrelative intensity Co < 10 %
980 990 agrees qualitatively with L edge results.
E~ (ev)
Because of the overlap of t&$g peak with the 3d line, we only represent on the Fig. 2 XPS spectra around the main 3d peak?lg satellite is clearly seen 6 eV below the main line with roughly the5kime weight in the three alloys. The occurence of this satellite i.s a common feature of Ce (and other light R2re Earth) compounds and is due to the possible screening of the 3d core-hole by 4f orbitals /8/.
An interpretation of this phenomer.on, based on the one impurity Anderson model has been proposed and applied to various intermetallics 171. A comparison with computed spectra for systems similar to CeRu2Si2 is consistent with an hybridiza- tion parameter A % 80 meV for the three alloys of Fig. 2.
IV - DISCUSSION
Macroscopic measurements on the system (magnetic susceptibility and specific heat)/2/
allow to distinguish two cases : (i) samples y _> 0.2 where antiferromagnetic ordering competes with Kondo coupling (T
2
3 TK) ; these alloys will not be considered in the following ; (ii) in the other cases, a nearly constant magnetic susceptibilityxO
is found at low temperature, and, as one goes towards Y,CeRu2Si2 the occurrence of a free ion magnetic behaviour is gradually suppressed asxO
decreases by one order of magnitude (see Fig. I), what has been interpreted by a rlsing of TK (10 K -t 300 K).Let us now relate the previous spectroscopic results with these macroscopic measure- ments. Using an 1/N expansion of the one impurity Anderson model (N is the dege- neracy of 4f statesf, the values of nf can be approximately related fo A and to x0, the zero temperature extrapolated susceptibility (also represented in Fig. 1 and of course defined only when the ground state is non-magnetic) /7/ :
~ I C nf2
N A = -
-
f
x0
1-n (1)f
C is the Curie constant relevant to N, ; n, should be obtained at 0 K for consistency, however, we observed on CeRu S i L onl; a slight decrease of nf (2 %) in the temperature range 300 K - 4.2 K. so ?ha$, for simplicity, we neglec- ted these valence variations. A simple numerical application with our experimental data suggests that A varies from 60 meV for Ceo lYo gRu2Si2 to 17 meV for CeRu2SiZ.
This concentration variation of A is much larger'thari those estimated for the a-y transition of metal Ce /9/ and is apparently conflicting with those deduced from photoemission data. However, it is important to note at this stage that the formula
(1) does not take into account the eventual splitting of 4f1 levels by the crystal-
C8-990 JOURNAL DE PHYSIQUE
lyne field. In a tetragonal solid, the ground state of trivalent Ce is splitted into three doublets as a result of crystal field interaction. But the effective degenera- CY Nf that enters in (1) is not obvious to define when the hybridization parameter reaches the same order of magnitude as the crystal field levels splittings A1 andA2.
From an experimental point of view, we must consider separately the following situations :
- Kondo s stems : (CeRu Si2, CeCu 6...) the formation of a Kondo singlet at low temperatu:e can be ideneifled (eg by specific heat measurements), and at higher temperature, crystal field excitations occur /2/. In such a case the "effective"
degeneracy is N = 2 for T << A1,A2 and Nf = 6 for T >> A1.82-
- Mixed-valent faystems : (CeSn3, CeNi, CeiVi2.. .) crystal field excitations are not clearly observed 112, 131. The use of N = 6 and 1/N expansions even at low tempe- f rature seems to be a valuable approximaEion for these systems.
In the present situation, the characteristic temperature TK can be related to the three-level splitting by the following approximate relation /Ill :
D is an energy cut off and pJ can be expressed as a function of the Anderson Hamiltonian parameters 0 :
with a set of parameters relevant for Ce alloys (E = 2 eV, Uff = 5 eV, Do = 10 K, 4 A = 220 K and A = 600 K /2/), one gets : A = 75 meV for TK f = 24 K (i.e. CeRu Si ) and 1 A = 105 meV ?or TK = 300 K (i.e. Y,CeRu2Si2). 2 2
V - CONCLUSION
As a conclusion we suggest that the transition observed in the magnetic behaviour of CeRu2Si2 alloys can be interpreted by a cross-over between the crystal field le- vel spllttmg and the Kondo temperature, due to a slight increase of the hybridiza- tion parameter as the unit cell volume decreases. This cross-over produces a rising of the effective degeneracy of 4f states from 2 to 6. However, at room temperature
(kT > A 1, the effective degeneracy that enters for the interpretation of 3d
photoem!?gsion can be thought to be close to 6, and the conclusions deduced from comparison with 1/N expansion calculation (weak variations of A % 80 meV) are valid and agrees wlth the above discussion. f
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