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Surface and bulk valence in Yb3Si5
Physica 130B {19~51 141- 143 North-Holland, Amsterdam
S U R F A C E AND BULK V A L E N C E IN Yb3Sis 1. A B B A T I +, I.. B R A I { ' O V I C H ~, U. DEL P E N N I N O ~+, A. I A N I ) E L L I ~'~*, G.I,. OI_,CESE~ ~ A. P A I , E N Z O N A " ' , C. ( ' A R B O N E°, J. N O G A M I°. ].J. YEH ° and I. I ] N D A U° ' Istituto di Fi,sica del Politecnico, M i l a n o , I t a l y ' " Istituto di Fisica dell" I,lnieersitfi, M o d e n a , I t a l y • " " Istituto di C h t m i c a h i s i c a dell" IJniver,sitd, ( ; e n o v a . I t a l y °.glanford Fle('tronics l,aboralory, S t a n f o r d ( Iniversity, C a l . . U S A
We present the first photoemission study of mixed valence Yb~Sis. A monophasic, [xdycrystalline sample has been studied with synchrotron radiation in the r a n g e 6 5 - 4 5 0 eV and with MgK,, radialion. The wide hz, r a n g e allows the surface sensitivity to be c h a n g e d strongly due to the e n e r g y dependence of the escape depth. The ratio of the intensities from Yb:' and Yb ~ is higher when the surface sensitivity increases; this trend can be fitted accurately with a simple m o d e l which gives valence -- 2.6 at rc~m temperature in the bulk and at least one surface layer with divalent Yb. The fitting gives also the e n e r g y dependent escape depth. We give also the shifts of the Yb bulk and surface f states u[xm Yb~Si~ formation which explain the different valence in the bulk and at the surface. Si2p photoemission shows the effect of inequivalent Si sites in YbxSi,.
B i n a r y Rare E a r t h (RE) silicides are interesting per se and for S i - R e interfaces. We summarize some results on a n g l e integrated p h o t o emission from Yb3Si5 whose structure is g i v e n in fig. I ; Yb and Si a t o m s are in alternate p l a n e s and Si r i n g s have v a c a n t places f o r m i n g a r e g u l a r net. All Yb sites are e q u i v a l e n t differently from o t h e r h e a v y Re silicides of RSi2 ~ type with r a n d o m
0\o.5 i
°lo ',
Q). ,, Fig. I. The structure of Yb.~Sis projected along c axes. ( ) p e n circles Yb; solid circles Si.
v a c a n c i e s [1]. Two Si families are present: 3Si with 2Si nearest n e i g h b o u r s and 2 with 3 n e i g h b o u t s . The s a m p l e p r e p a r e d as in [i] was c l e a n e d by scraping in situ (4 x 10 i i T o r r ) . S y n c h r o t r o n radiation 65 <~ hv <~ 450 eV (4° line at Stanford) and M g K , , radiation were used. The a b s e n c e of any segregation was c h e c k e d with ad hoc A u g e r experiment. A selection of 4f spectra (fig. 2) shows the m i x e d v a l e n c e b e h a v i o u r i.e. Ix)th Yb 2÷ (al'x)ve - 3 eV) and Yb 3+ (below - 4 e V ) . The 2+ p e a k s show at lower hv (smaller linewidth) surface p e a k s at h i g h e r binding e n e r g y (0.9 eV). The e n h a n c e m e n t of 3+ a r o u n d 181 eV is a Fano r e s o n a n c e at the crossing of the 4d level. The ratio R of 2~ and 3+ emission decreases with hv (at h i g h e r e s c a p e depths) indicating l o w e r surface valence. This is b e t t e r s h o w n in the interpolation of R on all spectra g i v e n in fig. 2 (solid line) w h e r e the r e s o n a n c e has been suppressed and m i n o r corrections due to photoemission cross sections (from 2' and 34) and to o c c u p a t i o n n u m b e r s have been d o n e . The fitting of R in a wide hv interval gives reliable information on the surface v a l e n c e . If n are the c o n c e n t r a t i o n s and B the relative weight of bulk
0 3 7 8 - 4 3 6 3 / 8 5 / $ 0 3 . 3 0 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
142
.,~hhati et al. / .'.;urf(i('c and h u l k /'.h'nce i,'l Yb :Ni.
I
R F. I
.
.
.
.
.
.
.
.
.
.
.
.
~..\
~sk
i i
i
M g
K ,
i
x\
,
'~
t
t
/
1815
I
/
~
/
eV
\,...
i
h v = 195 e V
hv=
I I
!
/
~
I ,_
,
x
0
500
I ' i g . 3.
/
h v = 1 7 5 eV
f ' - - ' ' -
t x
"
t
^
1
',
, h v = l O O e V
.
/"~
~',,
t
I
I
-
|
[
,
-15
t
,
1
eV
1
l
I
-10
l
I
I
I
I
i
,
,
-5
[
l
EF
Fig. 2. 4f photoemissiLm spectra from Yb,Si. al vaN, ms I1~'
and s u r f a c e skin e m i s s i o n (B d e p e n d s on c',capc d c p t h l,) R can be w r i t t e n as Ib and s mean bulk and s u r f a c c skin r e s p c c t i v c l y l
R ( h~,) =
ht' teV,
p, / !
hv=13OeV ~
1000
n , a l i , J ,1~ c'o111['u, H I C l l t ~ , i l l p h o t o
cmis,;iou] ,,pec'tra a,~ a functi~m of hp ~',olid l i n e l : l i l t i n g v. ith cxpresq.n (l) tdot..dasllcd line~
I
--
Ihe r n l i o R of Ihe
divalent Yb at the surface [3" weight. Itr',,.. within the errors): I.(hp) is of the known traditional type alld special (21 i . ( t , , ) for Yh is strollgIv questioned also bv :tssuming a thicker surf':lee skin. I'he bulk and surface valence arc understood by comparing 4f peaks in Yh~Si, and Yt~ (lig. 4a). l'he bulk peaks shift 1.1 eV upon cornpound formation: with the above Yh~gis surface shift and with ().6eV in Yb (ref. 2) one gets at surface peak shift upon compound formation of ().b~ eV, in agreement with other Yb compounds (3). Thus bulk 2~ peaks are close to E'~ allowing valence fluctuations which arc prevented at the ',urface where 4f peaks arc ().3 eV deeper. l h c
n 2 , B ( h z , ) 4. n2~ n~l.B(hv) + n~,
_ [ ( R ~ + I ) / ( R , + l ) ] R , B ( h z . ) + R. [(R~+ I)/(Rb+ l)]B(hv)+ 1
a:
(I)
h v = 1 5 0 eV
Yb 2"
4f
b
hv = 140 eV
2p
S,
2 i J
2 corresponding
r e a s o n a b l e 1 . ( - 5 ,g,). B~ a s s u m i n g a s u r f a c e skin one m o n o l a v e r t h i c k , one
J
to a
o b t a i n s Rb, R,, B ( I w ) , i.e. 1 . ( h u ) with an e x c e l l e n t
litting (fig. 3). T h e results are R , - - I L 7 1 4 in a g r e e m e n t with i I) and R , = ~ i.e. p u r e l y
iI
ii/h f III / !
w h e r e the c o n d i t i o n nb = n, h o l d s for n = n2 ~ n3. The fitting of e x p e r i m e n t a l R is i t e r a t i v e up to s e l f c o n s i s t e n c y : the initial RI, is t a k e n from Mg s p e c t r u m (basically bulklike) and initial B ( 6 5 ) • L
/ I iI j! p
i I
J
-4 BE feV ~,
"I.~-ILJ V EF
1
l
35
I
I
- - - - i
38 EK , e V ,
Fig. 4. a) ('(,mparison of Y h - " emission from Y b ~ g i , and ~b: b) Si2p photoemission m Yb~Si~.
1. Abbati et al. / Surface and bulk t~alence in Y b l S K
shift difference stems from reduced Yb coordination at the surface. Finally, fig. 4b g i v e s S i - 2 p spectra which are due to the superposition of two spin-orbit splitted terms; clearly the 3Si with coordination 2 contribute at lower binding energy ( - 0 . 4 eV).
143
Reterences [1] A. landelli, A. Palenzona and G.L. Olcese, J. Less Comm. Met. 64 (1979) 213. [2] F. Gerken, J. Barth, R. Kammerer, L.I. Johansson and A. Fk,,dstr6m, Surface Sci. 117 (1982) 468. [3] G. Kaindl, W.D. Schneider, C. Laubshat, B. Reihl and N. Martensson, Surface Sci. 126 (1983) 105: and references quoted therein.
S U R F A C E AND BULK V A L E N C E IN Yb3Sis 1. A B B A T I +, I.. B R A I { ' O V I C H ~, U. DEL P E N N I N O ~+, A. I A N I ) E L L I ~'~*, G.I,. OI_,CESE~ ~ A. P A I , E N Z O N A " ' , C. ( ' A R B O N E°, J. N O G A M I°. ].J. YEH ° and I. I ] N D A U° ' Istituto di Fi,sica del Politecnico, M i l a n o , I t a l y ' " Istituto di Fisica dell" I,lnieersitfi, M o d e n a , I t a l y • " " Istituto di C h t m i c a h i s i c a dell" IJniver,sitd, ( ; e n o v a . I t a l y °.glanford Fle('tronics l,aboralory, S t a n f o r d ( Iniversity, C a l . . U S A
We present the first photoemission study of mixed valence Yb~Sis. A monophasic, [xdycrystalline sample has been studied with synchrotron radiation in the r a n g e 6 5 - 4 5 0 eV and with MgK,, radialion. The wide hz, r a n g e allows the surface sensitivity to be c h a n g e d strongly due to the e n e r g y dependence of the escape depth. The ratio of the intensities from Yb:' and Yb ~ is higher when the surface sensitivity increases; this trend can be fitted accurately with a simple m o d e l which gives valence -- 2.6 at rc~m temperature in the bulk and at least one surface layer with divalent Yb. The fitting gives also the e n e r g y dependent escape depth. We give also the shifts of the Yb bulk and surface f states u[xm Yb~Si~ formation which explain the different valence in the bulk and at the surface. Si2p photoemission shows the effect of inequivalent Si sites in YbxSi,.
B i n a r y Rare E a r t h (RE) silicides are interesting per se and for S i - R e interfaces. We summarize some results on a n g l e integrated p h o t o emission from Yb3Si5 whose structure is g i v e n in fig. I ; Yb and Si a t o m s are in alternate p l a n e s and Si r i n g s have v a c a n t places f o r m i n g a r e g u l a r net. All Yb sites are e q u i v a l e n t differently from o t h e r h e a v y Re silicides of RSi2 ~ type with r a n d o m
0\o.5 i
°lo ',
Q). ,, Fig. I. The structure of Yb.~Sis projected along c axes. ( ) p e n circles Yb; solid circles Si.
v a c a n c i e s [1]. Two Si families are present: 3Si with 2Si nearest n e i g h b o u r s and 2 with 3 n e i g h b o u t s . The s a m p l e p r e p a r e d as in [i] was c l e a n e d by scraping in situ (4 x 10 i i T o r r ) . S y n c h r o t r o n radiation 65 <~ hv <~ 450 eV (4° line at Stanford) and M g K , , radiation were used. The a b s e n c e of any segregation was c h e c k e d with ad hoc A u g e r experiment. A selection of 4f spectra (fig. 2) shows the m i x e d v a l e n c e b e h a v i o u r i.e. Ix)th Yb 2÷ (al'x)ve - 3 eV) and Yb 3+ (below - 4 e V ) . The 2+ p e a k s show at lower hv (smaller linewidth) surface p e a k s at h i g h e r binding e n e r g y (0.9 eV). The e n h a n c e m e n t of 3+ a r o u n d 181 eV is a Fano r e s o n a n c e at the crossing of the 4d level. The ratio R of 2~ and 3+ emission decreases with hv (at h i g h e r e s c a p e depths) indicating l o w e r surface valence. This is b e t t e r s h o w n in the interpolation of R on all spectra g i v e n in fig. 2 (solid line) w h e r e the r e s o n a n c e has been suppressed and m i n o r corrections due to photoemission cross sections (from 2' and 34) and to o c c u p a t i o n n u m b e r s have been d o n e . The fitting of R in a wide hv interval gives reliable information on the surface v a l e n c e . If n are the c o n c e n t r a t i o n s and B the relative weight of bulk
0 3 7 8 - 4 3 6 3 / 8 5 / $ 0 3 . 3 0 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
142
.,~hhati et al. / .'.;urf(i('c and h u l k /'.h'nce i,'l Yb :Ni.
I
R F. I
.
.
.
.
.
.
.
.
.
.
.
.
~..\
~sk
i i
i
M g
K ,
i
x\
,
'~
t
t
/
1815
I
/
~
/
eV
\,...
i
h v = 195 e V
hv=
I I
!
/
~
I ,_
,
x
0
500
I ' i g . 3.
/
h v = 1 7 5 eV
f ' - - ' ' -
t x
"
t
^
1
',
, h v = l O O e V
.
/"~
~',,
t
I
I
-
|
[
,
-15
t
,
1
eV
1
l
I
-10
l
I
I
I
I
i
,
,
-5
[
l
EF
Fig. 2. 4f photoemissiLm spectra from Yb,Si. al vaN, ms I1~'
and s u r f a c e skin e m i s s i o n (B d e p e n d s on c',capc d c p t h l,) R can be w r i t t e n as Ib and s mean bulk and s u r f a c c skin r e s p c c t i v c l y l
R ( h~,) =
ht' teV,
p, / !
hv=13OeV ~
1000
n , a l i , J ,1~ c'o111['u, H I C l l t ~ , i l l p h o t o
cmis,;iou] ,,pec'tra a,~ a functi~m of hp ~',olid l i n e l : l i l t i n g v. ith cxpresq.n (l) tdot..dasllcd line~
I
--
Ihe r n l i o R of Ihe
divalent Yb at the surface [3" weight. Itr',,.. within the errors): I.(hp) is of the known traditional type alld special (21 i . ( t , , ) for Yh is strollgIv questioned also bv :tssuming a thicker surf':lee skin. I'he bulk and surface valence arc understood by comparing 4f peaks in Yh~Si, and Yt~ (lig. 4a). l'he bulk peaks shift 1.1 eV upon cornpound formation: with the above Yh~gis surface shift and with ().6eV in Yb (ref. 2) one gets at surface peak shift upon compound formation of ().b~ eV, in agreement with other Yb compounds (3). Thus bulk 2~ peaks are close to E'~ allowing valence fluctuations which arc prevented at the ',urface where 4f peaks arc ().3 eV deeper. l h c
n 2 , B ( h z , ) 4. n2~ n~l.B(hv) + n~,
_ [ ( R ~ + I ) / ( R , + l ) ] R , B ( h z . ) + R. [(R~+ I)/(Rb+ l)]B(hv)+ 1
a:
(I)
h v = 1 5 0 eV
Yb 2"
4f
b
hv = 140 eV
2p
S,
2 i J
2 corresponding
r e a s o n a b l e 1 . ( - 5 ,g,). B~ a s s u m i n g a s u r f a c e skin one m o n o l a v e r t h i c k , one
J
to a
o b t a i n s Rb, R,, B ( I w ) , i.e. 1 . ( h u ) with an e x c e l l e n t
litting (fig. 3). T h e results are R , - - I L 7 1 4 in a g r e e m e n t with i I) and R , = ~ i.e. p u r e l y
iI
ii/h f III / !
w h e r e the c o n d i t i o n nb = n, h o l d s for n = n2 ~ n3. The fitting of e x p e r i m e n t a l R is i t e r a t i v e up to s e l f c o n s i s t e n c y : the initial RI, is t a k e n from Mg s p e c t r u m (basically bulklike) and initial B ( 6 5 ) • L
/ I iI j! p
i I
J
-4 BE feV ~,
"I.~-ILJ V EF
1
l
35
I
I
- - - - i
38 EK , e V ,
Fig. 4. a) ('(,mparison of Y h - " emission from Y b ~ g i , and ~b: b) Si2p photoemission m Yb~Si~.
1. Abbati et al. / Surface and bulk t~alence in Y b l S K
shift difference stems from reduced Yb coordination at the surface. Finally, fig. 4b g i v e s S i - 2 p spectra which are due to the superposition of two spin-orbit splitted terms; clearly the 3Si with coordination 2 contribute at lower binding energy ( - 0 . 4 eV).
143
Reterences [1] A. landelli, A. Palenzona and G.L. Olcese, J. Less Comm. Met. 64 (1979) 213. [2] F. Gerken, J. Barth, R. Kammerer, L.I. Johansson and A. Fk,,dstr6m, Surface Sci. 117 (1982) 468. [3] G. Kaindl, W.D. Schneider, C. Laubshat, B. Reihl and N. Martensson, Surface Sci. 126 (1983) 105: and references quoted therein.