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Surface electronic structure of CeCo2, CeRh2 and CeRh3 probed by valence band resonant photoemission spectroscopy
surface science ELSEVIER
Surface Science 331-333 (1995) 1229-1232
Surface electronic structure of C e C o 2 , CeRh 2 and CeRh 3 probed by valence band resonant photoemission spectroscopy G. Chiaia a, p. Vavassori b, L. Dub b,. , L. Braicovich b, M. Qvarford a, I. Lindau
a
a Department of Synchrotron Radiation Research, Lund University S6lvegatan 14, S-22362 Luna~ Sweden b Dipartimento di Fisica, Politecnico di Milano, piazza Leonardo da Vinci 32, 1-20133 Milano, Italy
Received 28 July 1994; accepted for publication 25 November 1994
Abstract
The 4f surface versus bulk electronic structure of three strongly hybridized Ce intermetallic compounds (CeCo2, CeRh 2 and CeRh3) is investigated by exploiting the 4f photoemission resonant enhancement at the Ce 3d-4f and Ce 4d-4f thresholds. The much higher surface sensitivity at the shallow 4d threshold allows to extract the surface and bulk 4f electronic structure. Our results show a decreasing of the hybridization strength at the surface as compared to the bulk. The analysis of the whole set of results highlights the effect on the 4f spectral function obtained by varying the Ce metallic partner in the same crystal structure (CeCo 2 versus CeRh 2) and by changing the stoichiometry in the Ce-Rh phase diagram (CeRh 2 versus CeRh3). Keywords: Cerium; Cobalt; Polycrystalline surfaces; Rhodium; Soft X-ray photoelectron spectroscopy; Surface electronic phenomena
Ce and its transition metal (M) compounds exhibit many anomalous properties [1] related to the intermediate situation between localization and delocalization of the Ce 4f electrons [2]. Even if these electrons are rather localized on the rare earth site some hybridization between the Ce 4f electrons and the valence states takes place. The degree of hybridization of these Ce compounds strongly reflects both their low temperature and their magnetic properties [3] and has been found to vary depending on both the M partner and the compound stoichiometry [4]. Moreover in recent years the study of the surface effects on the correlated electronic structure o f these
* Corresponding author.
compounds has attracted considerable attention [5-7]. These effects are due to the lowering of the the C e - M coordination at the surface as compared with the bulk which results in a weakening of the Ce 4f hybridization with the valence states. The energy overlap, across the Fermi level (EF), between the f states and the more delocalized d states makes experimentally difficult to disentangle the two contributions [8]. To this purpose valence band resonant photoemission spectroscopy (VBRPS) has been shown to be a useful tool [3,9]. In fact, by exploiting the 4f photoemission resonant enhancement at the Ce 3 d - 4 f or Ce 4 d - 4 f thresholds, it is possible to sort out the w e a k f spectral contribution from the valence band emission. Moreover the different surface sensitivity between the V B R P spectra at the two thresholds allows to extract the 4f bulk
0039-6028/95/$09.50 © 1995 Elsevier Science B.V. All fights reserved SSDI 0039-6028(95)00273-1
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G. Chiaia et aL / Surface Science 331-333 (1995) 1229-1232
and surface contribution from the photoemission spectra [5,6] of strongly hybridized Ce compounds. In this work we present a VBRPS analysis at the two thresholds (Ce 3 d - 4 f and 4d-4f) of three different Ce compounds, i.e. CeCo2, CeRh 2 and CeRh 3. For CeCo z [3,10,11] and CeRh 3 [5,12,13] considerable information is presently available and the new results given herein extend the experimental data basis. Moreover the analysis on CeRh 2 allows, on the one hand, to highlight the effect of the variation of M partner between isostructural CeCo 2 and CeRh 2 and, on the other hand, to follow the variations of the 4f spectral function by changing the stoichiometric ratio in the same phase diagram (CeRh 2 versus CeRh3). While the 4f signals related to the bulk emission indicate a large hybridization for all three compounds, the surface spectral functions display a decreased hybridization strength. Po!ycrystalline samples have been prepared by induction melting from stoichiometric amounts of the components in Ta crucibles after Ar purging. They were annealed at 800-900°C for several days to obtain homogeneous polycrystals. The quality has been checked by X-ray diffraction and microprobe analysis. The crystal structure types are for CeCo 2 and CeRh 2 that of MgCu 2 and for CeRh 3 that of Cu3Au. The photoemission measurements have been performed at the soft X-ray beam line 22 at the MAX synchrotron radiation laboratory in Lund [14] in normal emission geometry. An overall (FWHM) energy resolution of about 0.1 and 0.7 eV was achieved at the Ce 4 d - 4 f ( h v ~ 120 eV) and Ce 3 d 4f ( h v ~ 880 eV) thresholds, respectively. The samples were cleaned by scraping with a diamond file in vacuum (base pressure 7 × 10 -11 mbar) immediately before the measurements. The samples were continuously kept at ~ 100 K during the scraping and the measurements in order to avoid surface contamination due to segregation of impurities from the bulk. Sample cleanliness has been checked via the O ls, C ls and O 2p signals. The surface stoichiometry has been accurately checked by use of core-level peak intensities of XPS spectra and was found to be in excellent agreement with the bulk stoichiometry. Fig. 1 displays the 4f extracted line-shapes across the Ce 4 d - 4 f resonance for the three compounds. These are obtained by evaluating the difference spectra between the background subtracted on- and off-
f
Ce-4f
~
/ A
~
.
CeRh3
.Q
Rh2 "-..
j.
] J
CeCo 2
j , t ~ I , i p i r i ~ ~"='-" -6 -5 -4 -3 -2 -1 E F Binding Energy (eV) F i g. 1. Ce 4f extracted spectral functions of CeC02, CeRh 2 and CeRh 3 obtained by subtracting the hv = 112 eV off-resonance from the on-resonance at hv = 122 eV spectra. An integral background has been subtracted from all the spectra.
resonance measurements ( h v = 122 and 112 eV', respectively) following the procedure described in literature [3]. In this way the weakly resonating non-f part of the valence band emission [15], mainly related to the M d-like spectral weight [3,10], is strongly reduced. In the subtraction procedure the different stoichiometry between CeRh 2 and CeRh 3 has been taken into account. In this way the areas of the off-resonant spectra (mainly Rh d-like) and the 4fextracted spectra are proportional to the different Ce versus Rh concentrations. All the spectra exhibit the typical double peaked structure of the Ce 4f spectra in which the higher binding energy feature (at ~ 2 eV below the Fermi level (EF)) is assigned to the 4f ° final state configuration and the more intense peak located close to E r has a dominant 4f 1 final state character. This spectral shape results from the correlated nature of the 4f electronic structure showing an increase of the f l / f 0 intensity ratio at larger hybridization degrees [16]. Looking more closely, the CeRh 2 spectrum seems to show the two spinorbit split components of the fl peak [12]. On the contrary these two components are not displayed by the CeCo 2 and CeRh 3 spectra. For CeCo 2 and CeRh 3 these findings are in good agreement with previously published results in Refs. [3,10] and Ref. [12] respectively.
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G. Chiaia et aL / Surface Science 331-333 (1995) 1229-1232
In Fig. 2 the same results are shown (dotted) for the Ce 3 d - 4 f threshold (on- and off-resonance at h v = 882 and 872 eV, respectively) together with the Ce 4 d - 4 f resonance results of Fig. 1 (line) after a Gaussian broadening ( F W H M = 0.7 eV) accounting for the different resolutions. For each compound clear differences between the two thresholds in the 4f spectral weight are found. These differences mainly reflect the different surface sensitivity of the two measurements. For photon energy tuned to the Ce 4d threshold the surface-to-bulk photoemission intensity ratio is remarkably larger than at the Ce 3d threshold ( 4 - 5 times). The larger f 0 / f l intensity ratios, which is evident at the more surface sensitive Ce 4d threshold, is observed in all compounds and indicates a lower degree of the f - d hybridization in the surface as compared to the bulk. This reflects the mentioned lowering of the C e - M coordination at the surface which favours a more localized behavior for the 4f electrons• In Fig. 3 the surface and bulk spectral contributions for each compound are shown separated and normalized to the same area. These results have been achieved by a decomposition of the 3 d - 4 f and 4 d - 4 f spectra in terms of a linear combination of the
Ce-4f
.~
- - 4d -~ 4f ~ 3d -* 4f
~...~.,-""
/.~,~ ~_
i
CeRh3
eRh2
E
-6
-4
-2
EF
Binding Energy (eV)
Fig. 2. Comparisonbetween the Ce 4f spectral functions of CeCo2, CeRh2 and CeRh3 extracted as described in the text at the 4d-4f threshold (solid line) and at the 3d-4f threshold (dots; on- and off-resonance at hv =882 and 872 eV, respectively). All the spectra are background subtracted and the 4d-4f spectra have been broadened with a 0.7 eV FWHM Gaussian function.
Ce-4f Surface
i~'
Bulk : ; .
CeRh8
,.,..,,,. : : . :.
..~"
..,.,...CeRh= ,,~,,
~,
-
j' -6
t~.~ ,: " ~
•.
OeO,, .j"
, i , i , i -4 -2 EF
~
.
I , i , i -4 -2 E,
Binding Energy (eV)
Fig. 3. 4f surface (left side) and bulk (right side) spectral weights of CeCo2, CeRh2 and CeRh3 normalized to the same area.
surface and the bulk spectral functions taken with the proper weights. These weights have been obtained by evaluating the different surface/bulk intensity ratios for each compound at the two thresholds taking into account the variation of the photoelectron mean-free path (A) as a function of the kinetic energy [17] ( h is about 4.5 and 15 A at the Ce 4d and Ce 3d thresholds, respectively). In normal emission geometry the surface/bulk intensity ratio ( I = / I b) depends on A and on the thickness of the surface emission layer (T). Assuming an exponential extinction law for the photoemission signal this ratio is given by I = / I b = exp(T/A) - 1. The dependence of the decomposed profiles on T indicates a short range of possible T values for which the surface and bulk contributions are fully decoupled. In all the three cases we have found T ~ 7.5 A [18], which correspond to about one stoichiometric layer for isostructural CeCo 2 and CeRh 2 and to about two stoichiometric layers for the more packed crystal structure of CeRh 3. For all the compounds the bulk photoemission signal shows, if any, a negligible f0 signal (similarly to CeIr 2 [6]) which indicates a largely hybridized character for the 4f electrons in the bulk. This behavior is consistent with what previously found for other strongly hybridized Ce compounds and for c~-Ce [5,6]. While the lineshape of the bulk contribution is quite similar for all the three compounds, large line-shape variations can be seen in the surface
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G. Chiaia et aL // Surface Science 331-333 (1995) 1229-1232
electronic structure. A c o m p a r i s o n b e t w e e n C e C o 2 and C e R h 2 s h o w s a larger f0 intensity for C e C o 2 indicating a l o w e r degree o f hybridization. Considering the s a m e crystal structure o f the two c o m p o u n d s and the i s o e l e c t r o n i c character o f the C o and Rh ions ( o c c u p a n c y in the solids d ~ 8) this difference can be e x p l a i n e d b y m e a n s o f the larger radial extension o f the R h 4d orbitals as c o m p a r e d to the Co 3d, w h i c h favours the f - d hybridization. A c o m p a r i s o n b e t w e e n C e R h 2 and C e R h 3 is less straightforward due to the different crystal structures o f the two c o m p o u n d s . T h e larger f 0 / f l intensity ratio disp l a y e d by the C e R h 3 surface is the e v i d e n c e o f a larger reduction o f the hybridization on g o i n g f r o m the b u l k to the surface. This effect m a y be related to the C e R h 3 crystal structure w h i c h i m p l i e s a reduction o f coordination o f Ce atoms and a relaxation o f the C e - R h nearest neighbors distance at the surface that m o r e h e a v i l y reflect on the hybridization [19]. Similarly, a larger hybridization for the bulk- and surface-like electronic structure o f C e R h 2 as c o m pared to C e R h 3 has b e e n o b s e r v e d by m e a n s o f C e 3d X - r a y spectroscopy [20]. In c o n c l u s i o n w e h a v e studied the 4 f surface v e r s u s b u l k electronic structure o f three Ce interm e t a l l i c c o m p o u n d s ( C e C o 2 , C e R h 2 and CeRh3); by e x p l o i t i n g the different surface sensitivity b e t w e e n 4 f p h o t o e m i s s i o n resonant e n h a n c e m e n t at the C e 3 d - 4 f and C e 4 d - 4 f thresholds it is possible to extract the surface and b u l k 4 f electronic structure. T h e analysis o f the results s h o w s the effects on the Ce 4 f spectral function u p o n variation o f the Ce transition m e t a l partner in the s a m e crystal structure (i.e. C e C o 2 versus C e R h 2) and by c h a n g i n g the s t o i c h i o m e t r y in the C e - R h phase d i a g r a m (i.e. C e R h 2 versus CeRh3). This w o r k w a s supported by the S w e d i s h Natural S c i e n c e R e s e a r c h C o u n c i l and by the Ministero d e l l ' U n i v e r s i t h e della R i c e r c a Scientifica e T e c n o l o g i c a through the " I s t i t u t o N a z i o n a l e di Fisica della Materia".
References [1] See for example: A. Iandelli and A. Palenzona, in: Handbook on the Physics and Chemistry of Rare Earths, Vol. 2, Eds.
K.A. Gschneidner, Jr. and L. Eyring (Elsevier, Amsterdam, 1979) p. 1. [2] K.A. Gschneidner, Jr., L. Eyring and S. Hiifner, Eds., Handbook on the Physics and Chemistry of Rare Earths, Vol. 10, (Elsevier, Amsterdam, 1987) p. 103. [3] J.W. Allen, S.J. Oh, O. Gunnarsson, K. Schfnhammer, M.B. Maple, M.S. Torikachvili and I. Lindau, Adv. Phys. 35 (1986) 275. [4] J.C. Fuggle, F.U. Hillebrecht, Z. Zolnierek, R. L~isser, Ch. Freiburg, O. Gunnarsson and K. Sch/Snhammer, Phys. Rev. B 27 (1983) 7330. [5] C. Laubschat, E. Weschke, C. Holtz, M. Domke, O. Strebel and G. Kaindl, Phys. Rev. Lett. 65 (1990) 1639. [6] E. Weschke, C. Lanbschat, T. Simmons, M. Domke, O. Strebel and G. Kaindl, Phys. Rev. B 44 (1991) 8304; C. Lanbschat, E. Weschke, M. Domke, C.T. Simmons and G. Kaindl, Surf. Sci. 269//270 (1992) 605. [7] L.Z. Lin, J.W. Allen, O. Gunarsson, N.E. Christensen and O.K. Andersen, Phys. Rev. B 45 (1992) 8934. [8]P. Vavassori, L. Dub, L. Braicovich and G.L. Olcese, Phys. Rev. B 50 (1994) 9561. [9] L. Braicovich, C. Carbone, O. Gunnarsson and G.L. Olcese, Phys. Rev. B 44 (1991) 13756. [10] Y.-S. Kang, J.H. Hong, J.I. Jeong, S.D. Choi, C.J. Yang, Y.P. Lee, C.G. Olson, B.I. Min and J.W. Allen, Phys. Rev. B 46 (1992) 15689. [11] L. Braicovich, E. Puppin, P. Vavassori, G.L. Olcese, A. Nordstrtim and B. Johansson, Solid State Commun. 89 (1994) 651. [12] E. Weschke, C. Laubschat, R. Ecker, A. HShr, M. Domke and G. Kaindl, Phys. Rev. Lett. 69 (1992) 1792. [13] D. Malterre, M. Grioni, Y. Baer, L. Braicovich, L. Dub, P. Vavassori and G.L. Olcese, Phys. Rev. Lett. 73 (1994) 2005. [14] J.N. Andersen, O. Bj/Srneholm, A. Sandell, R. Nyholm, J. Forsell, L. Th~nell, A. Nilsson and N. M[irtensson, Synchr. Rad. News 4 (1991) 15. [15] R.D. Parks, S. Raaen, M.L. den Boer, Y.-S. Chaug and G.P. Williams, Phys. Rev. Lett. 52 (1984) 2176. [16] O. Gunnarsson and K. Sch6nhammer, in: Handbook on the Physics and Chemistry of Rare Earths, Vol. 10, Eds. K.A. Gschneidner, Jr., L. Eyring and S. Hiifner (Elsevier, Amsterdam, 1987) p. 103. [17] S. Tanuma, C.J. Powell and O.R. Penn, Surf. Sci. 192 (1987) L849. [18] Similar T values can be extracted for LuCo 2 and LuRh 2 which have the same crystal structure as CeCo 2 and CeRh 2. See: P. Vavassori, L. Dub, L. Braicovich and G.L. Olcese, Surf. Sci. 307/309 (1994) 863. [19] Note that in CeRh 3 Ce atoms have twelve Rh nearest neighbors at 2.85 A, while in CeRh2 Ce atoms have twelve Rh nearest neighbors at 3.15 A. [20] L. Braicovich, L. Dub, P. Vavassori and G.L. Olcese, Surf. Sci., this Proceedings.
Surface Science 331-333 (1995) 1229-1232
Surface electronic structure of C e C o 2 , CeRh 2 and CeRh 3 probed by valence band resonant photoemission spectroscopy G. Chiaia a, p. Vavassori b, L. Dub b,. , L. Braicovich b, M. Qvarford a, I. Lindau
a
a Department of Synchrotron Radiation Research, Lund University S6lvegatan 14, S-22362 Luna~ Sweden b Dipartimento di Fisica, Politecnico di Milano, piazza Leonardo da Vinci 32, 1-20133 Milano, Italy
Received 28 July 1994; accepted for publication 25 November 1994
Abstract
The 4f surface versus bulk electronic structure of three strongly hybridized Ce intermetallic compounds (CeCo2, CeRh 2 and CeRh3) is investigated by exploiting the 4f photoemission resonant enhancement at the Ce 3d-4f and Ce 4d-4f thresholds. The much higher surface sensitivity at the shallow 4d threshold allows to extract the surface and bulk 4f electronic structure. Our results show a decreasing of the hybridization strength at the surface as compared to the bulk. The analysis of the whole set of results highlights the effect on the 4f spectral function obtained by varying the Ce metallic partner in the same crystal structure (CeCo 2 versus CeRh 2) and by changing the stoichiometry in the Ce-Rh phase diagram (CeRh 2 versus CeRh3). Keywords: Cerium; Cobalt; Polycrystalline surfaces; Rhodium; Soft X-ray photoelectron spectroscopy; Surface electronic phenomena
Ce and its transition metal (M) compounds exhibit many anomalous properties [1] related to the intermediate situation between localization and delocalization of the Ce 4f electrons [2]. Even if these electrons are rather localized on the rare earth site some hybridization between the Ce 4f electrons and the valence states takes place. The degree of hybridization of these Ce compounds strongly reflects both their low temperature and their magnetic properties [3] and has been found to vary depending on both the M partner and the compound stoichiometry [4]. Moreover in recent years the study of the surface effects on the correlated electronic structure o f these
* Corresponding author.
compounds has attracted considerable attention [5-7]. These effects are due to the lowering of the the C e - M coordination at the surface as compared with the bulk which results in a weakening of the Ce 4f hybridization with the valence states. The energy overlap, across the Fermi level (EF), between the f states and the more delocalized d states makes experimentally difficult to disentangle the two contributions [8]. To this purpose valence band resonant photoemission spectroscopy (VBRPS) has been shown to be a useful tool [3,9]. In fact, by exploiting the 4f photoemission resonant enhancement at the Ce 3 d - 4 f or Ce 4 d - 4 f thresholds, it is possible to sort out the w e a k f spectral contribution from the valence band emission. Moreover the different surface sensitivity between the V B R P spectra at the two thresholds allows to extract the 4f bulk
0039-6028/95/$09.50 © 1995 Elsevier Science B.V. All fights reserved SSDI 0039-6028(95)00273-1
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G. Chiaia et aL / Surface Science 331-333 (1995) 1229-1232
and surface contribution from the photoemission spectra [5,6] of strongly hybridized Ce compounds. In this work we present a VBRPS analysis at the two thresholds (Ce 3 d - 4 f and 4d-4f) of three different Ce compounds, i.e. CeCo2, CeRh 2 and CeRh 3. For CeCo z [3,10,11] and CeRh 3 [5,12,13] considerable information is presently available and the new results given herein extend the experimental data basis. Moreover the analysis on CeRh 2 allows, on the one hand, to highlight the effect of the variation of M partner between isostructural CeCo 2 and CeRh 2 and, on the other hand, to follow the variations of the 4f spectral function by changing the stoichiometric ratio in the same phase diagram (CeRh 2 versus CeRh3). While the 4f signals related to the bulk emission indicate a large hybridization for all three compounds, the surface spectral functions display a decreased hybridization strength. Po!ycrystalline samples have been prepared by induction melting from stoichiometric amounts of the components in Ta crucibles after Ar purging. They were annealed at 800-900°C for several days to obtain homogeneous polycrystals. The quality has been checked by X-ray diffraction and microprobe analysis. The crystal structure types are for CeCo 2 and CeRh 2 that of MgCu 2 and for CeRh 3 that of Cu3Au. The photoemission measurements have been performed at the soft X-ray beam line 22 at the MAX synchrotron radiation laboratory in Lund [14] in normal emission geometry. An overall (FWHM) energy resolution of about 0.1 and 0.7 eV was achieved at the Ce 4 d - 4 f ( h v ~ 120 eV) and Ce 3 d 4f ( h v ~ 880 eV) thresholds, respectively. The samples were cleaned by scraping with a diamond file in vacuum (base pressure 7 × 10 -11 mbar) immediately before the measurements. The samples were continuously kept at ~ 100 K during the scraping and the measurements in order to avoid surface contamination due to segregation of impurities from the bulk. Sample cleanliness has been checked via the O ls, C ls and O 2p signals. The surface stoichiometry has been accurately checked by use of core-level peak intensities of XPS spectra and was found to be in excellent agreement with the bulk stoichiometry. Fig. 1 displays the 4f extracted line-shapes across the Ce 4 d - 4 f resonance for the three compounds. These are obtained by evaluating the difference spectra between the background subtracted on- and off-
f
Ce-4f
~
/ A
~
.
CeRh3
.Q
Rh2 "-..
j.
] J
CeCo 2
j , t ~ I , i p i r i ~ ~"='-" -6 -5 -4 -3 -2 -1 E F Binding Energy (eV) F i g. 1. Ce 4f extracted spectral functions of CeC02, CeRh 2 and CeRh 3 obtained by subtracting the hv = 112 eV off-resonance from the on-resonance at hv = 122 eV spectra. An integral background has been subtracted from all the spectra.
resonance measurements ( h v = 122 and 112 eV', respectively) following the procedure described in literature [3]. In this way the weakly resonating non-f part of the valence band emission [15], mainly related to the M d-like spectral weight [3,10], is strongly reduced. In the subtraction procedure the different stoichiometry between CeRh 2 and CeRh 3 has been taken into account. In this way the areas of the off-resonant spectra (mainly Rh d-like) and the 4fextracted spectra are proportional to the different Ce versus Rh concentrations. All the spectra exhibit the typical double peaked structure of the Ce 4f spectra in which the higher binding energy feature (at ~ 2 eV below the Fermi level (EF)) is assigned to the 4f ° final state configuration and the more intense peak located close to E r has a dominant 4f 1 final state character. This spectral shape results from the correlated nature of the 4f electronic structure showing an increase of the f l / f 0 intensity ratio at larger hybridization degrees [16]. Looking more closely, the CeRh 2 spectrum seems to show the two spinorbit split components of the fl peak [12]. On the contrary these two components are not displayed by the CeCo 2 and CeRh 3 spectra. For CeCo 2 and CeRh 3 these findings are in good agreement with previously published results in Refs. [3,10] and Ref. [12] respectively.
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G. Chiaia et aL / Surface Science 331-333 (1995) 1229-1232
In Fig. 2 the same results are shown (dotted) for the Ce 3 d - 4 f threshold (on- and off-resonance at h v = 882 and 872 eV, respectively) together with the Ce 4 d - 4 f resonance results of Fig. 1 (line) after a Gaussian broadening ( F W H M = 0.7 eV) accounting for the different resolutions. For each compound clear differences between the two thresholds in the 4f spectral weight are found. These differences mainly reflect the different surface sensitivity of the two measurements. For photon energy tuned to the Ce 4d threshold the surface-to-bulk photoemission intensity ratio is remarkably larger than at the Ce 3d threshold ( 4 - 5 times). The larger f 0 / f l intensity ratios, which is evident at the more surface sensitive Ce 4d threshold, is observed in all compounds and indicates a lower degree of the f - d hybridization in the surface as compared to the bulk. This reflects the mentioned lowering of the C e - M coordination at the surface which favours a more localized behavior for the 4f electrons• In Fig. 3 the surface and bulk spectral contributions for each compound are shown separated and normalized to the same area. These results have been achieved by a decomposition of the 3 d - 4 f and 4 d - 4 f spectra in terms of a linear combination of the
Ce-4f
.~
- - 4d -~ 4f ~ 3d -* 4f
~...~.,-""
/.~,~ ~_
i
CeRh3
eRh2
E
-6
-4
-2
EF
Binding Energy (eV)
Fig. 2. Comparisonbetween the Ce 4f spectral functions of CeCo2, CeRh2 and CeRh3 extracted as described in the text at the 4d-4f threshold (solid line) and at the 3d-4f threshold (dots; on- and off-resonance at hv =882 and 872 eV, respectively). All the spectra are background subtracted and the 4d-4f spectra have been broadened with a 0.7 eV FWHM Gaussian function.
Ce-4f Surface
i~'
Bulk : ; .
CeRh8
,.,..,,,. : : . :.
..~"
..,.,...CeRh= ,,~,,
~,
-
j' -6
t~.~ ,: " ~
•.
OeO,, .j"
, i , i , i -4 -2 EF
~
.
I , i , i -4 -2 E,
Binding Energy (eV)
Fig. 3. 4f surface (left side) and bulk (right side) spectral weights of CeCo2, CeRh2 and CeRh3 normalized to the same area.
surface and the bulk spectral functions taken with the proper weights. These weights have been obtained by evaluating the different surface/bulk intensity ratios for each compound at the two thresholds taking into account the variation of the photoelectron mean-free path (A) as a function of the kinetic energy [17] ( h is about 4.5 and 15 A at the Ce 4d and Ce 3d thresholds, respectively). In normal emission geometry the surface/bulk intensity ratio ( I = / I b) depends on A and on the thickness of the surface emission layer (T). Assuming an exponential extinction law for the photoemission signal this ratio is given by I = / I b = exp(T/A) - 1. The dependence of the decomposed profiles on T indicates a short range of possible T values for which the surface and bulk contributions are fully decoupled. In all the three cases we have found T ~ 7.5 A [18], which correspond to about one stoichiometric layer for isostructural CeCo 2 and CeRh 2 and to about two stoichiometric layers for the more packed crystal structure of CeRh 3. For all the compounds the bulk photoemission signal shows, if any, a negligible f0 signal (similarly to CeIr 2 [6]) which indicates a largely hybridized character for the 4f electrons in the bulk. This behavior is consistent with what previously found for other strongly hybridized Ce compounds and for c~-Ce [5,6]. While the lineshape of the bulk contribution is quite similar for all the three compounds, large line-shape variations can be seen in the surface
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G. Chiaia et aL // Surface Science 331-333 (1995) 1229-1232
electronic structure. A c o m p a r i s o n b e t w e e n C e C o 2 and C e R h 2 s h o w s a larger f0 intensity for C e C o 2 indicating a l o w e r degree o f hybridization. Considering the s a m e crystal structure o f the two c o m p o u n d s and the i s o e l e c t r o n i c character o f the C o and Rh ions ( o c c u p a n c y in the solids d ~ 8) this difference can be e x p l a i n e d b y m e a n s o f the larger radial extension o f the R h 4d orbitals as c o m p a r e d to the Co 3d, w h i c h favours the f - d hybridization. A c o m p a r i s o n b e t w e e n C e R h 2 and C e R h 3 is less straightforward due to the different crystal structures o f the two c o m p o u n d s . T h e larger f 0 / f l intensity ratio disp l a y e d by the C e R h 3 surface is the e v i d e n c e o f a larger reduction o f the hybridization on g o i n g f r o m the b u l k to the surface. This effect m a y be related to the C e R h 3 crystal structure w h i c h i m p l i e s a reduction o f coordination o f Ce atoms and a relaxation o f the C e - R h nearest neighbors distance at the surface that m o r e h e a v i l y reflect on the hybridization [19]. Similarly, a larger hybridization for the bulk- and surface-like electronic structure o f C e R h 2 as c o m pared to C e R h 3 has b e e n o b s e r v e d by m e a n s o f C e 3d X - r a y spectroscopy [20]. In c o n c l u s i o n w e h a v e studied the 4 f surface v e r s u s b u l k electronic structure o f three Ce interm e t a l l i c c o m p o u n d s ( C e C o 2 , C e R h 2 and CeRh3); by e x p l o i t i n g the different surface sensitivity b e t w e e n 4 f p h o t o e m i s s i o n resonant e n h a n c e m e n t at the C e 3 d - 4 f and C e 4 d - 4 f thresholds it is possible to extract the surface and b u l k 4 f electronic structure. T h e analysis o f the results s h o w s the effects on the Ce 4 f spectral function u p o n variation o f the Ce transition m e t a l partner in the s a m e crystal structure (i.e. C e C o 2 versus C e R h 2) and by c h a n g i n g the s t o i c h i o m e t r y in the C e - R h phase d i a g r a m (i.e. C e R h 2 versus CeRh3). This w o r k w a s supported by the S w e d i s h Natural S c i e n c e R e s e a r c h C o u n c i l and by the Ministero d e l l ' U n i v e r s i t h e della R i c e r c a Scientifica e T e c n o l o g i c a through the " I s t i t u t o N a z i o n a l e di Fisica della Materia".
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