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XAS studies of 1:2:2 transition metal compounds
~
0038-1098/9356.00+ .00 Pergamon Press Ltd
Solid State Communications,Vol. 85, No. 4, pp. 291-296, 1993. Printed in Great Britain.
XA8
STUDIES
Department
Applied
OF 1 : 2 : 2
TRANSITION
METAL
COMPOUNDS
J. Chen, E. Kemly, a n d M. C r o f t of P h y s i c s and A s t r o n o m y , R u t g e r s P i s c a t a w a y , N. J., 08855, U S A
Physical
Y. J e o n Sciences Division, Laboratory, Upton, N. Y., 11973,
University,
Brookhaven
National
USA
X. Xu a n d S. A. S h a h e e n D e p a r t m e n t of Physics, C e n t e r of M a t e r i a l s F l o r i d a S t a t e U n i v e r s i t y , T a l l a h a s s e , Fla.,
Research, 32306, U S A
P. H. A n s a r i D e p a r t m e n t of Physics, S e t o n H a l l U n i v e r s i t y , S o u t h Orange, N. J., 07079, U S A (Received
June
3,
1992 b y A.
Pinczuk)
x-ray absorption spectroscopy (XAS) i n v e s t i g a t i o n s of the electronic structure of 4d transition metal (T) based materials are p r e s e n t e d . The s e n s i t i v i t y of t h e w h i t e line (WL) f e a t u r e at the T-L9 ~ e d g e s to t h e a b o v e EF, d - c o m p o n e n t of t h e e l e c t r o n i c s t a t e ~ ' I s first e m p h a s i z e d w l t h a s t u d y of t h e 4d row e l e m e n t s Mo to Ag. This W L b a s e d m e t h o d is then extended to a s y s t e m a t i c s t u d y of R T ~ X 2 c o m p o u n d s with: R G d or Ce; T - Ru, Rh, P d a n d Ag; a n d X = Si, Ge, a n d Sn. A central interpretation p r o p o s e d for the 1:2:2 c o m p o u n d s is t h e i d e n t i f i c a t i o n of a n e a r e d g e XAS f e a t u r e with T(4d)-X anti-bonding states split a b o v e Ew b y h y b r i d i z a t i o n . The s t r e n g t h a n d s p l i t t i n g of t h i s f e a t u r e are f o u n d to d e c r e a s e in the s e q u e n c e X = Si~Ge~Sn, c o n s i s t e n t w i t h the d e c r e a s i n g bonding interaction. A second interpretation proposed is the d e c r e a s e of T(4d) s t a t e s at E F in t h e s e c o m p o u n d s relat i v e to t h e elements.
pounds is the strongly bonded T-X planes. Thus, as we shall see, our XAS m e t h o d does i n d e e d d i r e c t l y a d d r e s s perhaps the key part of the electronic states. Below we will first define and review the L 9 ~ w h i t e line f e a t u r e sens i t i v i t y to t ~ ' ~ h e d - s t a t e distribution above E_ w i t h a s t u d y of the l a t t e r 4d r o w elements. We will then m o v e on to t h e s t u d y of the WL f e a t u r e v a r i a t i o n in the 4d row 1:2:2 compounds. 2. Experimental The c o m p o u n d samples were prepared by standard argon arc furnace techniques. X-ray powder diffraction measurements were used to verify the p r o p e r c r y s t a l s t r u c t u r e for the v a r i o u s compounds. The XAS m e a s u r e m e n t s were made on beam line X-19A at the Brookhaven National Synchrotron Light S o u r c e u s i n g a Si (111) double crystal monochromater. The a b s o r p t i o n m e a s u r e m e n t s were m a d e in the total electron yield ~ode to m i n i m i z e finite t h i c k n e s s effects .
i. Introduction Among ternary intermetallic compounds one of the largest and most d i v e r s e g r o u p s are R T 2 X 2 c o m p o u n d s with R - a rare earth, T = ~ t r a n s i t i o n m T t ~ al, a n d X = a G r o u p IV or V element. The f l e x i b i l i t y of such 1:2:2 c o m p o u n d s has m a d e t h e m h i g h l y useful in generating new f u n d a m e n t a l p h e n o m e n a in the mixed ~v~lent and heavy Fermion fields.--In these s t u d i e s the 1:2:2 c o m p o u n d has b e e n e x p l o i t e d m e r e l y as a vehicle for s t u d y i n g f - l o c a l i z a t i o n on the R-sites with the underlying electronic structure of the host b e i n g l a r g e l y ignored. Here we p r e s e n t a systematic x-ray absorption spectroscopy study of the e l e c t r o n i c s t r u c t u r e of a series of t h e s e 4 d - r o w 1:2:2 m a t e r i a l s . Our T-L_ 3 n e a r edge XAS study exp l i c i t l y Z ' v i e w s the e l e c t r o n i c s t r u c t u r e of t h e s e c o m p o u n d s as p r o j e c t e d onto the d-symmetry o r b i t a l s a ~ o u n d the T sites. H o f f m a n n a n d c o w o r k e r s - have emphasized that the central element in the e l e c t r o n i c s t r u c t u r e of t h e s e 1:2:2 com-
291
292 3.
1:2:2 TRANSITION METAL COMPOUNDS White
Line Faature and d-state Connection The L 9 ~ edges of t r a n s i t i o n m e t a l atoms is d o m i n a t e d by an intense white line (WL) f e a t u r e c a u s e d by t r a n s i t i o n s from the 2 p - c o r e level into empty dstates just above the F e r m i e n e r g y (EF). The s t r e n g t h and s t r u c t u r e of this WL f e a t u r e has b e e n u s e d in the past to est i m a t e b o t h the d-hole count and the distribution of d - s t ~ _ ~ above E_ in 4d and 5d row compounds. In th~ case of the latter 5d row e l e m e n t s a n e a r l i n e a r c o u p l i n g of the WL area tol2t~ ~ 5d-hole count has b e e n observed. This in t u r n has allowed quantitative estimation of the effective electron count c h a n g e s i n d u c e d by hybridization upon compound formation. We will ill u s t r a t e the g e n e r a l i z a t i o n of this WLarea to d - h o l e c o u p l i n g to 4 d - e l e m e n t s below. In figure 1 we .show the r e s u l t s (from a recent study I~ ) of the LR spectra for the 4d row e l e m e n t s f r o m M~ to A g (excepting the radioactive Tc) . The zero of e n e r g y is set at the inflect i o n p o i n t of the edge. The s p e c t r a are all normalized to u n i t y step h e i g h t at about 30 [eV]. The s t r e n g t h of the WL feature grows dramatically and m o n o t o n i c a l l y f r o m zero at A g (where the d-hole count is e s s e n t i a l l y zero) to a m a x i m u m at Mo. After. the m e t h o d u s e d in 5d row XAS s t u d i e s - - - ~ 4 we h a v e u s e d the A g s p e c t r a as an e m p i r i c a l estimate of the c o n t i n u u m onset and have s u b t r a c t e d the A g - b a c k g r o u n d s p e c t r u m from e a c h of the o t h e r s p e c t r a to e x t r a c t the WL feat u r e only. We h a v e then c a l c u l a t e d the area of this WL feature b e t w e e n app r o x i m a t e l y -3 and +5 to +8 leVI. These WL areas for b o t h the T(4d) L and L_ e dges are p l o t t e d in the inset o~ f i g u r ~ Mo Tc Ru Rh Pd Ag
'LIIX~'
4d-L 3
' ' ' 120 eL3
;°
f \
i
,
,
,
,
,<
-10
0 10 Energy (eV)
20
Figure i. The elemental 4d row L spectra illustrating the g r o w t h of th~ WL feature with increasing 4d-hole count. Inset, T ( 4 d ) - L 2 and LR WL areas versus atomic number ~llustrlting the proportionality of the d - h o l e count to W L area.
Vol: 85 No. 4
1 v e r s e s a t o m i c number. The results of a l i n e a r fit to the data is also shown as a s o l i d line. The c h a n g e in d - h o l e count b e t w e e n s u c c e s s i v e 4d e l e m e n t s is unity, to within the p r e c i s i o n of our experiments. Hence these results indicate that t~e slope of the s o l i d line (3.6±0.2 [eV---]) r e p r e s e n t s the e x p e c t e d WL area change for a 4d hole count c h a r g e of one. 4. 1:2:2 C o m p o u n d R e s u l t s T. = Au, Pd. Rh. Ru and X = Si: In F i g u r e 2a a n d 2b the T-L^ . s p e c t r a of a • J , serles of G d T ~ X ~ c o m p o u n ~ w z t h T varying across th~ ~d row are shown. The W L f e a t u r e in t h e s e s p e c t r a is distinctly bimodal (with the e x c e p t i o n of the A g case) w i t h a F e r m i level a - f e a t u r e a n d a second b-feature split off at h i g h e r energy. The c e n t r a l variation between the spectra is the s y s t e m a t i c i n c r e a s e in the EF, a-feature strength with d e c r e a s i n g T-d e l e c t r o n count (i.e. as T v a r i e s f r o m A g to P d to Rh to Ru). As in the e l e m e n t a l study above, this effect is c o n s i s t e n t w i t h the i n c r e a s e in the number of T(4d) e l e c t r o n i c states a b o v e Ew in this s e q u e n c e a n d with the W L - i n t e h s i t y p r o p o r t i o n a l i t y to the numb e r of such states. As n o t e d b e l o w the a-feature in t h e s e c o m p o u n d s is w e a k e r t h a n in the a n a l o g o u s elements, e v i d e n c ing the loss of d - s t a t e s near E_ in the . E c o m p o u n d r e l a t z v e to the element. It is the h i g h e r e n e r g y b - f e a t u r e which is the central focus of this paper. We a s s o c i a t e this f e a t u r e w i t h a n t i - b o n d i n g T ( 4 d ) - S i states split up in e n e r g y by the h y b r i d i z a t i o n interaction. It s h o u l d be n o t e d that the b-feature for the A g b a s e d m a t e r i a l lies at significantly lower energy and is not d i s t i n c t f r o m the a-feature. As we will see below these features are most prominent in the Si b a s e d m a t e r i a l s and h e n c e the s p e c t r a in F i g u r e 2 best ill u s t r a t e the b - f e a t u r e . X = Si. Geo Sn Svstematics: In previous XAS studies of T-X b o n d i n g in b i n a r y compounds, e v i d e n c e was p r e s e n t e d for the systematic increase in t h e s t r e n g t h of the hybridization/ bonding interaction as1~ ~ r i e s in the s e q u e n c e Sn to Ge to Si. ~-'-v In those studies the i n c r e a s e of T-L^ _ WL area was t a k e n as a m e a s u r e of t h e Z ~ m b e r of antibonding T-X states shifted above Ew, a n d h e n c e as a m e a s u r e of the ~reng~h of the bonding interaction. Similar results, o b s e r v e d in the comparison of CeAu2X_ compounds with X = Ge and Si, provld~d preliminary evidence for analogo~ T-X bonding in 1:2:2 compounds. However, the split off bfeature, so prominent in the G d T _ S i ^ r e s u l t s above, m a k e s it clear that Zth~ situation in t h e s e m a t e r i a l s is in fact to c o m p l i c a t e d to be simply summarized by such a net W L area change. B e l o w we trace the systematics of T-4d-L-Z J . spectral v a r i a t i o n s in r e s p o n s e to co~pound component variations across the
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1:2:2 TRANSITION METAL COMPOUNDS
Vol. 85 No. 4
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Figure 2a. and 2b. The T-L 2 ~ s p e c t r a of G d T g S i 9 w i t h T v a r y i n g a c r o ~ the 4d row. ~n ~his a n d the f o l l o w i n g figures: the s p e c t r a are a p p r o x i m a t e l y a l i g n e d on the a - f e a t u r e ; a n d the g u i d e - t o - t h e - e y e v e r t i c a l lines i n d i c a t e the e n e r g i e s of the a and b features in the t o p m o s t s p e c t r u m in the figure.
4d-row and upon X= Si to Ge to Sn replacements. It s h o u l d be n o t e d that the Ce v a l e n c e in the Sn c o m p o u n d s d i s c u s s e d b e l o w is e s s e n t i a l l y p u r e t r i v a l e n t and h e n c e e q u i v a l e n t to that of Gd. T = A u and X = Si. Ge: T h e spectra of the e d A g 2 X ~ (X Si anoLt~e~ c o m p o u n d s are s h o w n in F i g u r e 3a a n d 3b. The b-feature for t h e s e m a t e r i a l s lies c l o s e e n o u g h to the weak a - f e a t u r e that it is essentially unresolved. The p r e s e n c e of an i n c r e a s e d W L i n t e n s i t y in the 1:2:2 compounds, r e l a t i v e to p u r e Ag, is clear from the spectra. Moreover, the W L f e a t u r e is also c l e a r l y m o r e p r o m i n e n t for the Si c o m p o u n d than
for the Ge compound. T h e s e r e s u l t s are in a n a l o g y to our p r e v i o u s observations for the Au-L 9 ~ WL ~ensity variation in A u - l : 2 : 2 c o ~ S u n d s . T = Pd and X = Si. Ge. Sn: In f i g u r e 4a and 4b we show the Pd-L 9 edges of e l e m e n t a l - P d , a l o n g w i t h t h ~ of the Si, Ge, and Sn b a s e d 1:2:2 compounds. We wish to note several f e a t u r e s in t h e s e spectra. In the compounds, spectral weight in the WL feature has been shifted to higher energy relative to e l e m e n t a l Pd. This shift entails: f i r s t l y a d e c r e a s e in the intensity of the a - f e a t u r e (relative to Pd) in all of the compounds; and
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3a. and 3b. The A g - L 9 ~ s p e c t r a and G d A g 2 X 2 w i t h X=Si ~ Ge.
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1:2:2 TRANSITION METAL COMPOUNDS
Vol. 85 No. 4
_
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Figure 4a. and 4b. The P d - L 2 ~ s p e c t r a of selected R P d 2 X~ compounds with [R=Gd; X=Si and Ge] a ~ d [R=Ce; X=Sn].
s e c o n d l y the f o r m a t i o n of a b - f e a t u r e at h i g h e r energies. This s p e c t r a l shift is most dramatic in the 3i c o m p o u n d w h e r e the r e s o l v e d W L b - f e a t u r e (noted above) is clear. In the G e - c o m p o u n d the bf e a t u r e occurs at lower energy and is less intense. Finally in the Sn comp o u n d a b - f e a t u r e is e v i d e n c e d only by a broadening and c e n t r u m shift to h i g h e r e n e r g y of the WL. T = Rh and X = Si. Ge. Sn: The r e s u l t s on the Rh c o m p o u n d s p e r h a p s b e s t i l l u s t r a t e the s p e c t r a l m o d i f i c a t i o n s in the 4d-L^ ~ edges in these 1:2:2 m a t e r i a l s ~ '~ A g a i n an a - f e a t u r e d e g r a d a -
RhL2 a
tion, r e l a t i v e to the e l e m e n t (Rh), is observed in all of the spectra. As in the P d - l : 2 : 2 r e s u l t s a c l e a r l y resolved split-off WL f e a t u r e (labeled b again) is present in the GdRh^Si 2 spectra (figures 5a and 5b). As b~fore, this bf e a t u r e is d i m i n i s h e d in intensity, and lowered in energy in the G e - c o m p o u n d where it appears as a shoulder. Finally, again as before, in the Snc o m p o u n d the b - f e a t u r e is e v i d e n c e d only by a h i g h e n e r g y b r o a d e n i n g of the WL. The p r i n c i p l e d i f f e r e n c e b e t w e e n the Rh and Pd compounds is that the first WL f e a t u r e (the a-feature), caused by 4d
%,
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-20
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Energy (eV) The R h - L 2 3 spectra X compounds with an~ [R--Ce; X=Sn].
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1:2:2 TRANSITION METAL COMPOUNDS
states near the Fermi level, is m u c h m o r e i n t e n s e due to the higher 4d-hole c o u l d of Rh. 5. Discussion T-X Electronic Structure: Our XAS r e s u l t s on 1:2:2 compounds have been discussed in t e r m s of two WL features. The l o w e r e n e r g y a - f e a t u r e has b e e n associated with the non-bonding T-4d s t a t e s just a b o v e the compound's Fermi energy. The s c a l i n g of the s t r e n g t h of this f e a t u r e across the 4d row (see f i g u r e 2) was r e m i n i s c e n t of the e l e m e n tal WL s t r e n g t h variation (see figure 1). M o r e o v e r the c o n s i s t e n t loss of afeature intensity in the compounds, relative to the c o r r e s p o n d i n g element, i n d i c a t e s the r e l a t i v e loss of d-states n e a r E_ in t h e s e compounds. T~e higher energy b-feature has been i n t e r p r e t e d h e r e as r e f l e c t i n g the energetic location and number of antibonding T(4d)-X states above E_. It s h o u l d be n o t e d that Hoffmann et ~ al's calculations e x p l i c i t l y i d e n t i f i e s such antibon~ing states in 3d-1:2:2 compounds. Within this interpretation a consistent empirical picture emerges f r o m our results. For a given X (say Si) as the centrum of the e l e m e n t a l 4d states fall f u r t h e r b e l o w E F (as in the l%h to P d to A g sequence) the a n t i - b o n d i n g T-X s t a t e s of the 1:2:2 c o m p o u n d fall with them. This is n i c e l y i l l u s t r a t e d by the m o v e m e n t of the b - f e a t u r e toward the edge (EF) in this s e q u e n c e in figure 2. The increasing population of these antibonding states in the Rh to P d to A g sequence must lower the structural energy. The Ag compound would appear f r o m this v i e w p o i n t to have lost subs t a n t i a l s t r u c t u r a l stability. F o r a g i v e n T (say Rh) the s e q u e n c e of X v a r y i n g from Sn to Ge to Si leads to the a n t i - b o n d i n g T ( 4 d ) - X states becoming better defined and moving to h i g h e r energy. In this s e q u e n c e the Xa t o m i c - s i z e d e c r e a s e s and the d e n s i t y of X - p - s t a t e s at the edge of the atomic sphere becomes larger. Thus a s t r e n g t h e n i n g of the X - h y b r i d i z a t i o n int e r a c t i o n w i t h the T(4d) o r b i t a l in this sequence is reasonable. Again the cohesive energy gain in the T-X b o n d e d l a y e r s s h o u l d scale with how high in e n e r g y (and h e n c e d e p o p u l a t e d ) the a n t i b o n d i n g o r b i t a l s are. The Sn compounds, with only a b r o a d - w e a k b - f e a t u r e , w o u l d appear to be short on structural stability. Rare Earth Valence Instabilities: The occurrence of a rare e a r t h v a l e n c e i n s t a b i l i t y in a series of compounds reflects, in some sense, the e l e c t r o n i c structure of the compounds. At this j u n c t u r e we w o u l d t h e r e f o r e like to m a k e contact between our 1:2:2 compound electronic structure observations and some r e l e v a n t s y s t e m a t i c s of rare earth v a l e n c e i n s t a b i l i t i e s in compounds. A g r e a t b o d y of results are available on rare earth, transition metal
295
c o m p o u n d series in w h i c h the rare earth u n d e r g o e s a v a l e n c e i n s t a b i l i t y as the T v a r i e s in the Cu~Ni~Co, ~ R h ~ R u , or Au~Pt~Ir~Os series. Almost u n i f o r m l y in such c o m p o u n d s series the R-site valence increases monotonically (though o f t e n nonlinearly) ~ _ ~ e T delectron count increases. Indeed this p h e n o m e n o n is so u b i q u i t o u s that it has v i r t u a l l y r e a c h e d the status of emp i r i c a l "common law". This trend is c o n s i s t e n t w i t h the T d - b a n d e m p t y i n g or the fall of the F e r m i e n e r g y of the dband d e m a n d i n g c h a r g e t r a n s f e r from the R-site. For example, in the CeT 2 {T= P d ~ R h ~ R u } and ruT 2 {T=Pd~Rh} s e r l e ~ 3 s ~ h a s t r o n g v a l e n c e i n c r e a s e occurs. In the C e T g X 2 (T= Pd~Rh~Ru} series the Ce valenc~ follows a s i m i l a r T d e p e n d ence however, t h e 2 ~ a l e n c e variation is extremely small. Moreover, the Ce valence ~or a fixed T) moves toward pure Ce as X v a r i e s from Si~Ge~Sn. The d o n a t i o n of X c h a r g e to the R-sites would lead to such a s t a b i l i z a t i o n of the l o w e r v a l e n t rare earth state and this effect almost certainly plays a role in t h e s e m a t e r i a l s . The T-X i n t e r a c t i o n is also important in m o d u l a t i n g the e l e c t r o n demands of the T-d o r b i t a l s in t h e s e m a t e r i a l s . Like the X c h a r g e d o n a t i o n to the T-dorbitals mechanism noted above, the formation of T-X antibonding states a b o v e E F also c o n t r i b u t e s to r a i s i n g the e f f e c t i v e T - d F e r m i e n e r g y in the compound. In the latter of these two m e c h a n i s m s the t r a n s f e r of low l y i n g dstates into a n t i b o n d i n g states a b o v e E~ will n e c e s s i t a t e a F e r m i level rise t~ accommodate the same number of delectrons with a reduced number of states. Presumably this e f f e c t is most important in the Si based compounds where our r e s u l t s i n d i c a t e the T-X b o n d f o r m a t i o n is m o s t important. In the Sn c o m p o u n d the m o r e d i r e c t c h a r g e d o n a t i o n e f f e c t s p r e s u m a b l y d o m i n a t e w i t h the Ge case b e i n g i n t e r m e d i a t e . With the arguments of the last paragraph in m i n d it is i n t e r e s t i n g to c o n s i d e r p e r h a p s the sole exception to the above noted "common law" trend. N a m e l y in the E u T 2 ~ ' series at (5 2 o~ one observes: Eu 9 ..for T = Rh; E u ^ ' ~ . for T = Pd; and Eu -'±t for T = Ag. L z ' z l While the Pd~Ag change in Eu v a l e n c e obeys the "common law", that in the Pd~Rh replacement c l e a r l y v i o l a t e s it. At the same t i m e the e x i s t e n c e of a Eu valence transition at just 150 °K in EuPd^Si 2 clearly dictates that the energy balance associated with this a n o m a l y is q u i t e subtle. In c h a n g i n g T from Pd to Rh in the e l e c t r o n count on the T-Si sublattice d e c r e a s e s b y one and an i n c r e a s e d d e m a n d for c o m p e n s a t o r y charge transfer from the Eu sites is expected. R e f e r r i n g to f i g u r e 2 one s h o u l d note the b-feature has m o v e d to s u b s t a n t i a l l y h i g h e r e n e r g y in the Rh as c o m p a r e d to in the Pd com-
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1:2:2 TRANSITION METAL COMPOUNDS
pound. If the upward shift in the n u m b e r and s p l i t t i n g of the a n t i - b o n d i n g T(4d)-X states (using our b - f e a t u r e interpretation) is sufficient to over compensate the e l e c t r o n loss, t h e n the d - o r b i t a l c o n t r i b u t i o n to the compound Fermi energy shift c o u l d be u p w a r d in the P d ~ R h change. In such a case small net charge t r a n s f e r to (instead of away from) the Eu sites c o ~ d occur and the o b s e r v e d a n o m a l o u s E u - - s t a b i l i z a t i o n in the P d ~ R h change c o u l d result. Indeed the redistribution of anti-bonding states and s h i f t i n g of charge balance c o u l d be a n i s o t r o p i c and l o c a l i z e d near s p e c i f i c crystal sites so that the overall m o d i f i c a t i o n s n e e d no be too great. The central point is that our work motivates the n o t i o n that m o d i f i c a t i o n s in the T-X b o n d i n g c o u l d be countering conventional electron f i l l i n g effects.
Vol. 85.No. 4
To the authors knowledge this is the first formal i d e n t i f i c a t i o n of and attempt to explain this Eu valence anomaly. 6. S'1~T~Iry a n d C o n c l u s i o n XAS provides a useful electronic s t r u c t u r e p r o b e of the above E_ states which is c o m p l i m e n t a r y to the m o r e common below EF p h o t o e m i s s i o n methods. Moreover the s y m m e t r y - s e l e c t i v i t y , imp o s e d by the diploe selection rules, allows the m e t h o d to focus on s p e c i f i c p r o j e c t e d c o m p o n e n t s of the electronic structure. In this p a p e r this has b e e n well i l l u s t r a t e d by the t r a n s i t i o n metal-d focus p r o b e d by the L 2 3 edge XAS. Our results motivate mor~ specific t h e o r e t i c a l t r e a t m e n t of the role of the 4d states and 4d-X hybridization in these 1:2:2 compounds.
References I.
2.
3.
4. 5. 6. 7. 8. 9. i0.
11. 12.
13. 14.
15.
W. Rieger and E. Parthe, Monatsh. Chem., I00, 444, (1986); see numerous articles in J. Magn. Matr., 47&48 (1986). C. U. Segre, M. Croft, J. A. Hodges, V. Murgai, L. C. Gupta, and R. D. Parks, Phys. Rev. Lett., 49, 1947 (1982). F. Steglich, J. Aarts, C. D. Brendl, W. Lieke, D. Meschede, W. Franz, and H. Schafer, Phys. Rev. Lett., 43, 1992 (1982). R. H o f f m a n n and C. Zheng, J. Phys. Chem. 89, 4175 (1985). T. Guo and M. den Boer, Phys. Rev. B31, 6233 (1985). J. Horsley, J. Phys. Chem. 76, 1451 (1982). L. Matthesis and D. Dietz, Phys. Rev. B22, 1663 (1980). E. Stern and J. Rehr, Phys. Rev. B27, 3351 (1982). F. Lytle, J. Catalysis, 43, 376 (1976). F. Lytle, P. Wei, R. Gregor, G. Via and J. Sinfelt, J. Chem. Phys., 70, 4849 (1979). M. Brown, R. Peierls, and E. Stern, Phys. Rev. B15, 738 (1977). B. Qi, I. Perez, P. Ansari, F. Lu, and M. Croft, Phys. Rev. B36, 2972 (1987) . I. Perez, B. Qi, G. Liang, F. Lu, M. Croft, and D. Wieliczka, Phys. Rev., B38, 12233 (1988). Y. Jeon, B. Qi, F. Lu, and M. Croft, Phys. Rev., D40, 1538 (1989). T. Sham, Phys. Rev. B31, 1888 (1985).
16. 17.
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22. 23.
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26. 27.
T. Sham, Phys. Rev. B31, 1903 (1985). M. D e C r e s c e n z i , E. Colavita, U. Del Pinnino, P. Sassaroli, S. Valeri, C. Rinaldi, L. Sorba, and S. Nannarone, Phys. Rev., B~2, 612 (1985) . O. Bisi, O. Jepsen, and O. Andersen, Phys. Rev., B~6, 9439 (1987). J. Chen, Thesis work, Rutgers U n i v e r s i t y (1992). Y. Jeon, J. Chen, and M. Croft, to be p u b l i s h e d . P. Ansari, B. Qi, G. Liang, I. Perez, F. Lu, and M. Croft, J. App. Phys., 63, 3503 (1988). I. F e l n e r and I. Nowik, J. Magn. Matr., 47&48, 420 (1985). R. Niefeld, M. Croft, T. Mihalisin, C. U. Segre, M. Madigan, M. T o r i k a c h u i l i , M. B. Maple, and L. Delong, Phys. Rev. B, 32, 6928 (1985) and r e f e r e n c e therein. E. Kropp, E. Dormann, K. Buschow, Sol. St. Comm., 32, 507 (1979); E. Banminger, I. Felner, 0. Levron, I. Nowik, and S. Ofer, Sol. St. Comm., 18, 1073 (1976). I. Perez, G. Liang, J. B. Zhou, H. Jhans, S. A. Shaheen, and M. Croft, Physica B, 163, 618 (1990) and r e f e r e n c e therein. R. V i j a y a r a g h a v a n , J. Magn. Matr., 47&48, 561 (1985). E. Kemly, M. Croft, V. Murgai, L. Gupta, C. Godart, R. Parks, and C. Segre, J. Magn. Matt., 47&48, 403 (1985).
0038-1098/9356.00+ .00 Pergamon Press Ltd
Solid State Communications,Vol. 85, No. 4, pp. 291-296, 1993. Printed in Great Britain.
XA8
STUDIES
Department
Applied
OF 1 : 2 : 2
TRANSITION
METAL
COMPOUNDS
J. Chen, E. Kemly, a n d M. C r o f t of P h y s i c s and A s t r o n o m y , R u t g e r s P i s c a t a w a y , N. J., 08855, U S A
Physical
Y. J e o n Sciences Division, Laboratory, Upton, N. Y., 11973,
University,
Brookhaven
National
USA
X. Xu a n d S. A. S h a h e e n D e p a r t m e n t of Physics, C e n t e r of M a t e r i a l s F l o r i d a S t a t e U n i v e r s i t y , T a l l a h a s s e , Fla.,
Research, 32306, U S A
P. H. A n s a r i D e p a r t m e n t of Physics, S e t o n H a l l U n i v e r s i t y , S o u t h Orange, N. J., 07079, U S A (Received
June
3,
1992 b y A.
Pinczuk)
x-ray absorption spectroscopy (XAS) i n v e s t i g a t i o n s of the electronic structure of 4d transition metal (T) based materials are p r e s e n t e d . The s e n s i t i v i t y of t h e w h i t e line (WL) f e a t u r e at the T-L9 ~ e d g e s to t h e a b o v e EF, d - c o m p o n e n t of t h e e l e c t r o n i c s t a t e ~ ' I s first e m p h a s i z e d w l t h a s t u d y of t h e 4d row e l e m e n t s Mo to Ag. This W L b a s e d m e t h o d is then extended to a s y s t e m a t i c s t u d y of R T ~ X 2 c o m p o u n d s with: R G d or Ce; T - Ru, Rh, P d a n d Ag; a n d X = Si, Ge, a n d Sn. A central interpretation p r o p o s e d for the 1:2:2 c o m p o u n d s is t h e i d e n t i f i c a t i o n of a n e a r e d g e XAS f e a t u r e with T(4d)-X anti-bonding states split a b o v e Ew b y h y b r i d i z a t i o n . The s t r e n g t h a n d s p l i t t i n g of t h i s f e a t u r e are f o u n d to d e c r e a s e in the s e q u e n c e X = Si~Ge~Sn, c o n s i s t e n t w i t h the d e c r e a s i n g bonding interaction. A second interpretation proposed is the d e c r e a s e of T(4d) s t a t e s at E F in t h e s e c o m p o u n d s relat i v e to t h e elements.
pounds is the strongly bonded T-X planes. Thus, as we shall see, our XAS m e t h o d does i n d e e d d i r e c t l y a d d r e s s perhaps the key part of the electronic states. Below we will first define and review the L 9 ~ w h i t e line f e a t u r e sens i t i v i t y to t ~ ' ~ h e d - s t a t e distribution above E_ w i t h a s t u d y of the l a t t e r 4d r o w elements. We will then m o v e on to t h e s t u d y of the WL f e a t u r e v a r i a t i o n in the 4d row 1:2:2 compounds. 2. Experimental The c o m p o u n d samples were prepared by standard argon arc furnace techniques. X-ray powder diffraction measurements were used to verify the p r o p e r c r y s t a l s t r u c t u r e for the v a r i o u s compounds. The XAS m e a s u r e m e n t s were made on beam line X-19A at the Brookhaven National Synchrotron Light S o u r c e u s i n g a Si (111) double crystal monochromater. The a b s o r p t i o n m e a s u r e m e n t s were m a d e in the total electron yield ~ode to m i n i m i z e finite t h i c k n e s s effects .
i. Introduction Among ternary intermetallic compounds one of the largest and most d i v e r s e g r o u p s are R T 2 X 2 c o m p o u n d s with R - a rare earth, T = ~ t r a n s i t i o n m T t ~ al, a n d X = a G r o u p IV or V element. The f l e x i b i l i t y of such 1:2:2 c o m p o u n d s has m a d e t h e m h i g h l y useful in generating new f u n d a m e n t a l p h e n o m e n a in the mixed ~v~lent and heavy Fermion fields.--In these s t u d i e s the 1:2:2 c o m p o u n d has b e e n e x p l o i t e d m e r e l y as a vehicle for s t u d y i n g f - l o c a l i z a t i o n on the R-sites with the underlying electronic structure of the host b e i n g l a r g e l y ignored. Here we p r e s e n t a systematic x-ray absorption spectroscopy study of the e l e c t r o n i c s t r u c t u r e of a series of t h e s e 4 d - r o w 1:2:2 m a t e r i a l s . Our T-L_ 3 n e a r edge XAS study exp l i c i t l y Z ' v i e w s the e l e c t r o n i c s t r u c t u r e of t h e s e c o m p o u n d s as p r o j e c t e d onto the d-symmetry o r b i t a l s a ~ o u n d the T sites. H o f f m a n n a n d c o w o r k e r s - have emphasized that the central element in the e l e c t r o n i c s t r u c t u r e of t h e s e 1:2:2 com-
291
292 3.
1:2:2 TRANSITION METAL COMPOUNDS White
Line Faature and d-state Connection The L 9 ~ edges of t r a n s i t i o n m e t a l atoms is d o m i n a t e d by an intense white line (WL) f e a t u r e c a u s e d by t r a n s i t i o n s from the 2 p - c o r e level into empty dstates just above the F e r m i e n e r g y (EF). The s t r e n g t h and s t r u c t u r e of this WL f e a t u r e has b e e n u s e d in the past to est i m a t e b o t h the d-hole count and the distribution of d - s t ~ _ ~ above E_ in 4d and 5d row compounds. In th~ case of the latter 5d row e l e m e n t s a n e a r l i n e a r c o u p l i n g of the WL area tol2t~ ~ 5d-hole count has b e e n observed. This in t u r n has allowed quantitative estimation of the effective electron count c h a n g e s i n d u c e d by hybridization upon compound formation. We will ill u s t r a t e the g e n e r a l i z a t i o n of this WLarea to d - h o l e c o u p l i n g to 4 d - e l e m e n t s below. In figure 1 we .show the r e s u l t s (from a recent study I~ ) of the LR spectra for the 4d row e l e m e n t s f r o m M~ to A g (excepting the radioactive Tc) . The zero of e n e r g y is set at the inflect i o n p o i n t of the edge. The s p e c t r a are all normalized to u n i t y step h e i g h t at about 30 [eV]. The s t r e n g t h of the WL feature grows dramatically and m o n o t o n i c a l l y f r o m zero at A g (where the d-hole count is e s s e n t i a l l y zero) to a m a x i m u m at Mo. After. the m e t h o d u s e d in 5d row XAS s t u d i e s - - - ~ 4 we h a v e u s e d the A g s p e c t r a as an e m p i r i c a l estimate of the c o n t i n u u m onset and have s u b t r a c t e d the A g - b a c k g r o u n d s p e c t r u m from e a c h of the o t h e r s p e c t r a to e x t r a c t the WL feat u r e only. We h a v e then c a l c u l a t e d the area of this WL feature b e t w e e n app r o x i m a t e l y -3 and +5 to +8 leVI. These WL areas for b o t h the T(4d) L and L_ e dges are p l o t t e d in the inset o~ f i g u r ~ Mo Tc Ru Rh Pd Ag
'LIIX~'
4d-L 3
' ' ' 120 eL3
;°
f \
i
,
,
,
,
,<
-10
0 10 Energy (eV)
20
Figure i. The elemental 4d row L spectra illustrating the g r o w t h of th~ WL feature with increasing 4d-hole count. Inset, T ( 4 d ) - L 2 and LR WL areas versus atomic number ~llustrlting the proportionality of the d - h o l e count to W L area.
Vol: 85 No. 4
1 v e r s e s a t o m i c number. The results of a l i n e a r fit to the data is also shown as a s o l i d line. The c h a n g e in d - h o l e count b e t w e e n s u c c e s s i v e 4d e l e m e n t s is unity, to within the p r e c i s i o n of our experiments. Hence these results indicate that t~e slope of the s o l i d line (3.6±0.2 [eV---]) r e p r e s e n t s the e x p e c t e d WL area change for a 4d hole count c h a r g e of one. 4. 1:2:2 C o m p o u n d R e s u l t s T. = Au, Pd. Rh. Ru and X = Si: In F i g u r e 2a a n d 2b the T-L^ . s p e c t r a of a • J , serles of G d T ~ X ~ c o m p o u n ~ w z t h T varying across th~ ~d row are shown. The W L f e a t u r e in t h e s e s p e c t r a is distinctly bimodal (with the e x c e p t i o n of the A g case) w i t h a F e r m i level a - f e a t u r e a n d a second b-feature split off at h i g h e r energy. The c e n t r a l variation between the spectra is the s y s t e m a t i c i n c r e a s e in the EF, a-feature strength with d e c r e a s i n g T-d e l e c t r o n count (i.e. as T v a r i e s f r o m A g to P d to Rh to Ru). As in the e l e m e n t a l study above, this effect is c o n s i s t e n t w i t h the i n c r e a s e in the number of T(4d) e l e c t r o n i c states a b o v e Ew in this s e q u e n c e a n d with the W L - i n t e h s i t y p r o p o r t i o n a l i t y to the numb e r of such states. As n o t e d b e l o w the a-feature in t h e s e c o m p o u n d s is w e a k e r t h a n in the a n a l o g o u s elements, e v i d e n c ing the loss of d - s t a t e s near E_ in the . E c o m p o u n d r e l a t z v e to the element. It is the h i g h e r e n e r g y b - f e a t u r e which is the central focus of this paper. We a s s o c i a t e this f e a t u r e w i t h a n t i - b o n d i n g T ( 4 d ) - S i states split up in e n e r g y by the h y b r i d i z a t i o n interaction. It s h o u l d be n o t e d that the b-feature for the A g b a s e d m a t e r i a l lies at significantly lower energy and is not d i s t i n c t f r o m the a-feature. As we will see below these features are most prominent in the Si b a s e d m a t e r i a l s and h e n c e the s p e c t r a in F i g u r e 2 best ill u s t r a t e the b - f e a t u r e . X = Si. Geo Sn Svstematics: In previous XAS studies of T-X b o n d i n g in b i n a r y compounds, e v i d e n c e was p r e s e n t e d for the systematic increase in t h e s t r e n g t h of the hybridization/ bonding interaction as1~ ~ r i e s in the s e q u e n c e Sn to Ge to Si. ~-'-v In those studies the i n c r e a s e of T-L^ _ WL area was t a k e n as a m e a s u r e of t h e Z ~ m b e r of antibonding T-X states shifted above Ew, a n d h e n c e as a m e a s u r e of the ~reng~h of the bonding interaction. Similar results, o b s e r v e d in the comparison of CeAu2X_ compounds with X = Ge and Si, provld~d preliminary evidence for analogo~ T-X bonding in 1:2:2 compounds. However, the split off bfeature, so prominent in the G d T _ S i ^ r e s u l t s above, m a k e s it clear that Zth~ situation in t h e s e m a t e r i a l s is in fact to c o m p l i c a t e d to be simply summarized by such a net W L area change. B e l o w we trace the systematics of T-4d-L-Z J . spectral v a r i a t i o n s in r e s p o n s e to co~pound component variations across the
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Vol. 85 No. 4
4d L3, o
4d L2 ~GdRu2Si2
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Figure 2a. and 2b. The T-L 2 ~ s p e c t r a of G d T g S i 9 w i t h T v a r y i n g a c r o ~ the 4d row. ~n ~his a n d the f o l l o w i n g figures: the s p e c t r a are a p p r o x i m a t e l y a l i g n e d on the a - f e a t u r e ; a n d the g u i d e - t o - t h e - e y e v e r t i c a l lines i n d i c a t e the e n e r g i e s of the a and b features in the t o p m o s t s p e c t r u m in the figure.
4d-row and upon X= Si to Ge to Sn replacements. It s h o u l d be n o t e d that the Ce v a l e n c e in the Sn c o m p o u n d s d i s c u s s e d b e l o w is e s s e n t i a l l y p u r e t r i v a l e n t and h e n c e e q u i v a l e n t to that of Gd. T = A u and X = Si. Ge: T h e spectra of the e d A g 2 X ~ (X Si anoLt~e~ c o m p o u n d s are s h o w n in F i g u r e 3a a n d 3b. The b-feature for t h e s e m a t e r i a l s lies c l o s e e n o u g h to the weak a - f e a t u r e that it is essentially unresolved. The p r e s e n c e of an i n c r e a s e d W L i n t e n s i t y in the 1:2:2 compounds, r e l a t i v e to p u r e Ag, is clear from the spectra. Moreover, the W L f e a t u r e is also c l e a r l y m o r e p r o m i n e n t for the Si c o m p o u n d than
for the Ge compound. T h e s e r e s u l t s are in a n a l o g y to our p r e v i o u s observations for the Au-L 9 ~ WL ~ensity variation in A u - l : 2 : 2 c o ~ S u n d s . T = Pd and X = Si. Ge. Sn: In f i g u r e 4a and 4b we show the Pd-L 9 edges of e l e m e n t a l - P d , a l o n g w i t h t h ~ of the Si, Ge, and Sn b a s e d 1:2:2 compounds. We wish to note several f e a t u r e s in t h e s e spectra. In the compounds, spectral weight in the WL feature has been shifted to higher energy relative to e l e m e n t a l Pd. This shift entails: f i r s t l y a d e c r e a s e in the intensity of the a - f e a t u r e (relative to Pd) in all of the compounds; and
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3a. and 3b. The A g - L 9 ~ s p e c t r a and G d A g 2 X 2 w i t h X=Si ~ Ge.
40
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1:2:2 TRANSITION METAL COMPOUNDS
Vol. 85 No. 4
_
-20
0
20
40
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Figure 4a. and 4b. The P d - L 2 ~ s p e c t r a of selected R P d 2 X~ compounds with [R=Gd; X=Si and Ge] a ~ d [R=Ce; X=Sn].
s e c o n d l y the f o r m a t i o n of a b - f e a t u r e at h i g h e r energies. This s p e c t r a l shift is most dramatic in the 3i c o m p o u n d w h e r e the r e s o l v e d W L b - f e a t u r e (noted above) is clear. In the G e - c o m p o u n d the bf e a t u r e occurs at lower energy and is less intense. Finally in the Sn comp o u n d a b - f e a t u r e is e v i d e n c e d only by a broadening and c e n t r u m shift to h i g h e r e n e r g y of the WL. T = Rh and X = Si. Ge. Sn: The r e s u l t s on the Rh c o m p o u n d s p e r h a p s b e s t i l l u s t r a t e the s p e c t r a l m o d i f i c a t i o n s in the 4d-L^ ~ edges in these 1:2:2 m a t e r i a l s ~ '~ A g a i n an a - f e a t u r e d e g r a d a -
RhL2 a
tion, r e l a t i v e to the e l e m e n t (Rh), is observed in all of the spectra. As in the P d - l : 2 : 2 r e s u l t s a c l e a r l y resolved split-off WL f e a t u r e (labeled b again) is present in the GdRh^Si 2 spectra (figures 5a and 5b). As b~fore, this bf e a t u r e is d i m i n i s h e d in intensity, and lowered in energy in the G e - c o m p o u n d where it appears as a shoulder. Finally, again as before, in the Snc o m p o u n d the b - f e a t u r e is e v i d e n c e d only by a h i g h e n e r g y b r o a d e n i n g of the WL. The p r i n c i p l e d i f f e r e n c e b e t w e e n the Rh and Pd compounds is that the first WL f e a t u r e (the a-feature), caused by 4d
%,
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Energy (eV) F i g u r e 5a. and 5b. of selected RRh^ [R=Gd; X=Si and Ge~
-20
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Energy (eV) The R h - L 2 3 spectra X compounds with an~ [R--Ce; X=Sn].
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1:2:2 TRANSITION METAL COMPOUNDS
states near the Fermi level, is m u c h m o r e i n t e n s e due to the higher 4d-hole c o u l d of Rh. 5. Discussion T-X Electronic Structure: Our XAS r e s u l t s on 1:2:2 compounds have been discussed in t e r m s of two WL features. The l o w e r e n e r g y a - f e a t u r e has b e e n associated with the non-bonding T-4d s t a t e s just a b o v e the compound's Fermi energy. The s c a l i n g of the s t r e n g t h of this f e a t u r e across the 4d row (see f i g u r e 2) was r e m i n i s c e n t of the e l e m e n tal WL s t r e n g t h variation (see figure 1). M o r e o v e r the c o n s i s t e n t loss of afeature intensity in the compounds, relative to the c o r r e s p o n d i n g element, i n d i c a t e s the r e l a t i v e loss of d-states n e a r E_ in t h e s e compounds. T~e higher energy b-feature has been i n t e r p r e t e d h e r e as r e f l e c t i n g the energetic location and number of antibonding T(4d)-X states above E_. It s h o u l d be n o t e d that Hoffmann et ~ al's calculations e x p l i c i t l y i d e n t i f i e s such antibon~ing states in 3d-1:2:2 compounds. Within this interpretation a consistent empirical picture emerges f r o m our results. For a given X (say Si) as the centrum of the e l e m e n t a l 4d states fall f u r t h e r b e l o w E F (as in the l%h to P d to A g sequence) the a n t i - b o n d i n g T-X s t a t e s of the 1:2:2 c o m p o u n d fall with them. This is n i c e l y i l l u s t r a t e d by the m o v e m e n t of the b - f e a t u r e toward the edge (EF) in this s e q u e n c e in figure 2. The increasing population of these antibonding states in the Rh to P d to A g sequence must lower the structural energy. The Ag compound would appear f r o m this v i e w p o i n t to have lost subs t a n t i a l s t r u c t u r a l stability. F o r a g i v e n T (say Rh) the s e q u e n c e of X v a r y i n g from Sn to Ge to Si leads to the a n t i - b o n d i n g T ( 4 d ) - X states becoming better defined and moving to h i g h e r energy. In this s e q u e n c e the Xa t o m i c - s i z e d e c r e a s e s and the d e n s i t y of X - p - s t a t e s at the edge of the atomic sphere becomes larger. Thus a s t r e n g t h e n i n g of the X - h y b r i d i z a t i o n int e r a c t i o n w i t h the T(4d) o r b i t a l in this sequence is reasonable. Again the cohesive energy gain in the T-X b o n d e d l a y e r s s h o u l d scale with how high in e n e r g y (and h e n c e d e p o p u l a t e d ) the a n t i b o n d i n g o r b i t a l s are. The Sn compounds, with only a b r o a d - w e a k b - f e a t u r e , w o u l d appear to be short on structural stability. Rare Earth Valence Instabilities: The occurrence of a rare e a r t h v a l e n c e i n s t a b i l i t y in a series of compounds reflects, in some sense, the e l e c t r o n i c structure of the compounds. At this j u n c t u r e we w o u l d t h e r e f o r e like to m a k e contact between our 1:2:2 compound electronic structure observations and some r e l e v a n t s y s t e m a t i c s of rare earth v a l e n c e i n s t a b i l i t i e s in compounds. A g r e a t b o d y of results are available on rare earth, transition metal
295
c o m p o u n d series in w h i c h the rare earth u n d e r g o e s a v a l e n c e i n s t a b i l i t y as the T v a r i e s in the Cu~Ni~Co, ~ R h ~ R u , or Au~Pt~Ir~Os series. Almost u n i f o r m l y in such c o m p o u n d s series the R-site valence increases monotonically (though o f t e n nonlinearly) ~ _ ~ e T delectron count increases. Indeed this p h e n o m e n o n is so u b i q u i t o u s that it has v i r t u a l l y r e a c h e d the status of emp i r i c a l "common law". This trend is c o n s i s t e n t w i t h the T d - b a n d e m p t y i n g or the fall of the F e r m i e n e r g y of the dband d e m a n d i n g c h a r g e t r a n s f e r from the R-site. For example, in the CeT 2 {T= P d ~ R h ~ R u } and ruT 2 {T=Pd~Rh} s e r l e ~ 3 s ~ h a s t r o n g v a l e n c e i n c r e a s e occurs. In the C e T g X 2 (T= Pd~Rh~Ru} series the Ce valenc~ follows a s i m i l a r T d e p e n d ence however, t h e 2 ~ a l e n c e variation is extremely small. Moreover, the Ce valence ~or a fixed T) moves toward pure Ce as X v a r i e s from Si~Ge~Sn. The d o n a t i o n of X c h a r g e to the R-sites would lead to such a s t a b i l i z a t i o n of the l o w e r v a l e n t rare earth state and this effect almost certainly plays a role in t h e s e m a t e r i a l s . The T-X i n t e r a c t i o n is also important in m o d u l a t i n g the e l e c t r o n demands of the T-d o r b i t a l s in t h e s e m a t e r i a l s . Like the X c h a r g e d o n a t i o n to the T-dorbitals mechanism noted above, the formation of T-X antibonding states a b o v e E F also c o n t r i b u t e s to r a i s i n g the e f f e c t i v e T - d F e r m i e n e r g y in the compound. In the latter of these two m e c h a n i s m s the t r a n s f e r of low l y i n g dstates into a n t i b o n d i n g states a b o v e E~ will n e c e s s i t a t e a F e r m i level rise t~ accommodate the same number of delectrons with a reduced number of states. Presumably this e f f e c t is most important in the Si based compounds where our r e s u l t s i n d i c a t e the T-X b o n d f o r m a t i o n is m o s t important. In the Sn c o m p o u n d the m o r e d i r e c t c h a r g e d o n a t i o n e f f e c t s p r e s u m a b l y d o m i n a t e w i t h the Ge case b e i n g i n t e r m e d i a t e . With the arguments of the last paragraph in m i n d it is i n t e r e s t i n g to c o n s i d e r p e r h a p s the sole exception to the above noted "common law" trend. N a m e l y in the E u T 2 ~ ' series at (5 2 o~ one observes: Eu 9 ..for T = Rh; E u ^ ' ~ . for T = Pd; and Eu -'±t for T = Ag. L z ' z l While the Pd~Ag change in Eu v a l e n c e obeys the "common law", that in the Pd~Rh replacement c l e a r l y v i o l a t e s it. At the same t i m e the e x i s t e n c e of a Eu valence transition at just 150 °K in EuPd^Si 2 clearly dictates that the energy balance associated with this a n o m a l y is q u i t e subtle. In c h a n g i n g T from Pd to Rh in the e l e c t r o n count on the T-Si sublattice d e c r e a s e s b y one and an i n c r e a s e d d e m a n d for c o m p e n s a t o r y charge transfer from the Eu sites is expected. R e f e r r i n g to f i g u r e 2 one s h o u l d note the b-feature has m o v e d to s u b s t a n t i a l l y h i g h e r e n e r g y in the Rh as c o m p a r e d to in the Pd com-
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1:2:2 TRANSITION METAL COMPOUNDS
pound. If the upward shift in the n u m b e r and s p l i t t i n g of the a n t i - b o n d i n g T(4d)-X states (using our b - f e a t u r e interpretation) is sufficient to over compensate the e l e c t r o n loss, t h e n the d - o r b i t a l c o n t r i b u t i o n to the compound Fermi energy shift c o u l d be u p w a r d in the P d ~ R h change. In such a case small net charge t r a n s f e r to (instead of away from) the Eu sites c o ~ d occur and the o b s e r v e d a n o m a l o u s E u - - s t a b i l i z a t i o n in the P d ~ R h change c o u l d result. Indeed the redistribution of anti-bonding states and s h i f t i n g of charge balance c o u l d be a n i s o t r o p i c and l o c a l i z e d near s p e c i f i c crystal sites so that the overall m o d i f i c a t i o n s n e e d no be too great. The central point is that our work motivates the n o t i o n that m o d i f i c a t i o n s in the T-X b o n d i n g c o u l d be countering conventional electron f i l l i n g effects.
Vol. 85.No. 4
To the authors knowledge this is the first formal i d e n t i f i c a t i o n of and attempt to explain this Eu valence anomaly. 6. S'1~T~Iry a n d C o n c l u s i o n XAS provides a useful electronic s t r u c t u r e p r o b e of the above E_ states which is c o m p l i m e n t a r y to the m o r e common below EF p h o t o e m i s s i o n methods. Moreover the s y m m e t r y - s e l e c t i v i t y , imp o s e d by the diploe selection rules, allows the m e t h o d to focus on s p e c i f i c p r o j e c t e d c o m p o n e n t s of the electronic structure. In this p a p e r this has b e e n well i l l u s t r a t e d by the t r a n s i t i o n metal-d focus p r o b e d by the L 2 3 edge XAS. Our results motivate mor~ specific t h e o r e t i c a l t r e a t m e n t of the role of the 4d states and 4d-X hybridization in these 1:2:2 compounds.
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