- Email: [email protected]
Isochromat spectroscopy on CePd3
Solid State Communications, Printed in Great Britain.
Vol. 48, No. 3, pp. 257-260,
ISOCHROMAT
1983.
0038-1098183 $3.00 + .OO Pergamon Press Ltd.
SPECTROSCOPY ON CePds S. Hiifner
Laboratorium
fti Festkorperphysik,
ETH Zurich, Honggerbergf
Switzerland
P. Steiner Fachbereich
Physik, Universitat
des Saarlandes, D-6600 Saarbriicken, W. Germany and
V. Dose, D. Straub and A. Hart1 Physikalisches
Institut
der Universitit,
D-8700 Wiirzburg, W. Germany
(Received 11 May 1983 by P. Wuchter)
Isochromat spectra from CePds recorded at quantum energies of 9.7 and 72.7 eV are presented. The data obtained at 9.7 eV show three different peaks and are by and large identical to previously published data for a quantum energy of 1486 eV. Only two of those three peaks are present in the 72.7 eV spectrum. We conclude that the missing peak is bulk in origin and derives from a CeSd-Pd4d hybridization leading to a CeSd split off level from the filled Pd d-band.
discussion. Perhaps one should add, for clarity that there is a number of intermetallic Ce compounds (like, e.g. CeNi*, CeCoz, CeFe*) in which the 4fstate has similar properties as in c&e and these compounds are therefore called a-type compounds. The question as to the nature of the 4fstate (promoted to the (5d6s) conduction band or 4f itinerant) in these o-type compounds is the same as in a-Ce. Spectroscopic techniques like photoelectron spectroscopy (PES), Bremsstrahlen-isochromat Spectroscopy (BIS), X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) have been used to study the nature of the electronic structure of y-type, IV and a-type compounds. In all these techniques one measures however an excited state and it is not obvious, how the groundstate can be inferred from the measured spectra. Core level PES [5-71, XAS [8] and XES [9] on a-type compounds all point to a 4foccupation in the final state of - 0.75. This means in the PES and XAS data that the signal which can be attributed to a 4f” final state has an intensity of the order of only 25%. Although the final state contribution to the spectra is not clear it is difficult to conceive any model at all, that would be able to produce the spectra from a 4f’(5d6~)~ initial state. The situation with respect to the valence band photoemission spectra of Ce [IO] and its compounds [ 11,121 is also not clear. The valence band PES often shows two 4fsignals (in addition to the conduction band) below the Fermi energy which prevented an unambiguous determination of the energetic position of
METALLIC Ce COMPOUNDS can be grouped into three classes: y-type systems (e.g. y-Ce), intermediate valent (IV) systems (e.g. CePda) and o-type systems (e.g. c&e) [l-3]. In y-type Ce systems the 4felectron is localized giving rise, e.g. to a Curie paramagnetisim. IV systems are believed to have a partially occupied 4flevel. The electronic structure of a-type compounds is a matter of present controversy. Originally it was assumed that, e.g. the transition from y-Ce to a-Ce was achieved by promoting the 4felectron to the (5d6s) conduction band. Thus while the electronic configuration of y-Ce can be written formally as 4f’(Sd6~)~ that of a-Ce would be 4f’(Sd6~)~ in the promotional model. This is also the reason, why -y-type Ce systems are often called trivalent and o-type systems tetravalent. Johansson [4] argued that because the 4felectrons contribute little to the cohesive energy, which stems from the conduction band, and since the cohesive energy of y-Ce and cr-Ce are almost identical, the promotional model must be in error. He suggested, that the transition from y-Ce to a-Ce was thus one from a localized 4f’ configuration to a delocalized 4f’ configuration, with little promotion of the 4felectron into the (5d6s) conduction band. The question whether this assertion is correct and whether there are direct spectroscopic means to probe the electronic structure of a-Ce is a matter of present * Permanent address: Fachbereich Physik, Universitat des Saarlandes, D-6600 Saarbriicken, W. Germany. 257
258
ISOCHROMAT SPECTROSCOPY ON CePds
the 4flevel in the ground state (bare 4fposition). Hiifner and Steiner [ 13, 141 suggested that the two signals arise from two differently screened hole states similar to the situation encountered in nickel. While the signal at - 1 to - 3 eV below the Fermi energy corresponds to a localized hole state, the signal near the Fermi energy was assigned to an itinerant hole state. It is therefore difficult to identify the exact position of the bare 4flevel from photoemission data. However, it shows that the 4flevel in the ground state can be quite near to the Fermi energy even if a PES signal is seen 1-3 eV below it. Recently theories have been published [15-l 71 which show the importance of relaxation, and suggest that the core level PES spectra and the XAS measurements on o-type compound can only be reconciled with a sizeable 4foccupation in the initial state, which rules out the promotion model. These theories start in essence from the work of Kotani and Toyozawa [ 181 but than proceed in very different ways to calculate the experimental spectra. We here comment briefly only on the work of [ 151 because it is the most sophisticated one. The calculations use a Hamiltonian suggested by Ramakrishnan [ 191 and Anderson [20]. In the calculation the degeneracy of the 4flevel is taken as a developing parameter. With sensible values for the f-d hybridization the PES core and valence band spectra can be explained. These authors [ 151 get two 4f derived signals in the valence band, one at the Fermi energy and one below the position of the bare 4f state. There is however a nonlinear relationship between peak intensity and 4f occupancy making it difficult to extract the latter number directly from the measured spectra. BIS spectra have also been calculated by Gunnarson and Schonhammer [ 151. The best suited method to investigate unoccupied electronic states in Ce and Ce-compounds appears to be Bremsstrahlung Isochromat Spectroscopy (BIS). This type of experiment is perhaps the most direct one in establishing the nature of the 4felectrons in IV intermediate valent and a-type systems because of the following reason: if there is an electronic configuration 4f’(5d6~)~, then the BIS spectrum should show only a 4f’ signal close to EF. If on the other hand the electronic configuration is described by e I 4f’(5d6~)~) -I(1 - E) 14f’(5d6~)~) a 4f1 signal and a 4fz signal at - 4 eV above EF should be visible. Thus the strength of the signal at - 4 eV should give an indication of the 4f1 component of the groundstate. Thus if a compound has indeed a configuration 4f’(5~?6s)~ (as suggested for o-type systems in the promotional model) the BIS spectra should show only a signal near EF but none at - 4 eV above EF, giving a good experimental discrimination of the two models. We note that there is of course according to [ 151 a
Vol. 48, No. 3
nonlinear relationship between experimental peak heights and 4foccupation in the groundstate. BIS spectra from y-Ce, CeSn3 and CeA13 exhibit structure at - 4 eV above the Fermi level corresponding to the 4f* electronic configuration [22]. BIS spectra from CePd3 are distinctly different since they exhibit additional peaks at the Fermi level and 2.5 eV above E,. While the emission at the Fermi level is probably due to 4f’ final configurations, the origin of the peak at 2.5 eV above EF is not clear at present. It has been tentatively proposed to be due to a surface state [22]. It was the purpose of the present work to advance the interpretation of the BIS data from CePd3. This is necessary in order to check whether the simple concept just mentioned to distinguish between the promotional and the delocalization model for o-type Ce systems can actually be used. Therefore, both PES and BIS measurements on CePd3 have been performed. The PES experiment employed Al-Ka radiation. Data were taken for the M, and MI, core levels with clean and oxidized samples in order to check on the importance of surface contamination. BIS experiments were performed at a quantum energy of 9.7 eV employing an energy selective Geiger Miiller counter as a radiation detector [23]. For measurements at a quantum energy of 72.7 eV, the absorption edge technique was used [24,25]. The choice of these quantum energies is motivated by the different penetration depths of the electrons at the respective energies. Consequently, a comparison of data taken at 9.7 eV and at 72.7 eV should enable to differentiate between surface and bulk contributions to the total emission. Figure 1 displays the 3d Ce PES lines obtained from clean CePd3 and from CePd3 after exposure to 11 L of oxygen. The spectrum of the clean sample is in agreement with published data [6], the structure being assigned to f”, f1 and f2 final state configurations [6]. Oxygen adsorption leads to an attenuation of the f ’ signal with respect to the f” and f2 signals and in addition introduces new structure due to Ce oxide. XPS has always shown an intensity ratio off’ vs (f’ + f2) which was difficult to reconcile with the f-counts from other experimental methods, e.g. X-ray absorption [7]. The data in Fig. 1 indicate that the weight of the different f” components can be different at the surface and in the bulk and that it is difficult to extract a quantitative f-count out of such data. Figure 2 shows BIS spectra for a quantum energy of 9.7 eV in comparison to measurements by Baer et al. [22] employing a quantum energy of 1486 eV. The electron penetration depth is similar at these two energies and is of the order of 25 A. Apart from a different background behaviour the two spectra are identical in every structural detail. Figure 2 also shows a BIS spectrum at
Vol. 48, No. 3
ISOCHROMAT SPECTROSCOPY
910
900 920 Binding Energy/N
880
6f
Fig. 1. The XPS 3d spectrum of clean CePds and of CePds after exposure to 11 L of 02. The various final states and the Ce oxide signal are indicated.
..
*....v..-.*
..-
Ce Pd,
Ii
’
..*._**_._* ‘....:.-
hw 9.7eV q
_..?. ....
I
nw=73ev
i .*J 0
2
4
6
ON CePds
259
72.7 eV where the electron penetration depth is only of the order of 5 A. Common features to the other spectra in Fig. 2 are the emission peaks at 5 and 4.5 eV and an inflection point at about 6.5 eV. The peak at 2.5 eV, visible in the 9.7 and 1486 eV data, is hardly discernible in the 72.7 eV surface sensitive data. Note also that the 4.5 eV emission is shifted by 0.5 eV to higher binding energy analogous to surface core level shifts. The peak at EF in the 72.7 eV data shows that the surface of the sample is still in the IV state. The absence of the 2.5 eV emission in the 72.7 eV spectrum proves that this peak is a typical bulk feature of CePds. It can be explained by (Ce-5d) * (Pd4d) hybridization. Such a hybridization would lead to a Ce-5d state split off from the full Pd d-band. A similar effect for occupied electronic states is observed upon alloying Pd with hydrogen [26]. The remaining two peaks in the BIS spectrum from CePds at 0.5 eV and 4.5-5 eV which show up in ail three spectra are attributed to 4f’ and 4f2 final state configurations respectively of Ce. From a comparison of their physical properties one can conclude that a-cerium systems are expected to be similar to IV systems. The XPS data of CePds in fact resemble closely those of cu-cerium systems in that they show, e.g. in the PES core level spectra a similar strength of the f” components of - 25%. The same is true for the f” signal in the XAS data. In fact a quantitative analysis of the XPS spectra indicates if anything the hybridization is larger in IV CePds than in o-type CeNi2 [7]. This then would suggest that also their electronic structure is very similar and that also in a-type Ce compounds there is a 4f’ contribution of - 75% to the electronic structure. This in turn would mean that in a-type compounds one would expect a 4f’ signal (close to EF) and a 4f 2 signal (- 4 eV above EF) in the BIS experiments. Barth et aZ.[27] have recently measured the susceptibility of very dilute Ce Impurities in a great number of hosts. They find y-type, IV- and a-type behaviour meaning a constant susceptibility. They conclude that in the hosts in which Ce impurities show the temperature independent susceptibility Ce is tetravalent. One can however also argue that the delocalization of the 4f electron leads to the observed magnetic behaviour. However these experiments show that the delocalization in a-type Ce compounds cannot be due to 4f-4f overlap but has to be caused by 4f-5d hybridization.
(E-E,llev Fig. 2. BIS spectra at quantum energies of 9.7 and 72.7 eV from this work compared to earlier data by Baer et al. [22]. Except for the peak at 2.5 eV in the 72.7 eV spectrum the three sets of data agree in every structural detail, even the small inflection indicated by the arrow.
Acknowledgement - This work was supported by the Deutsche Forschungsgemeinschaft. One of the authors (S.H.) wants to thank Y. Baer, J. Fuggle, F.U. Hillebrecht, F. Steglich, P. Wachter and D. Wohlleben for many helpful discussions. We are indebted to Prof. B. Elschner for providing the sample for this experiment.
260
ISOCHROMAT SPECTROSCOPY ON CePds REFERENCES
1.
2.
3. 4. 5. 6.
7. 8.
9. 10. 11.
See Valence Fluctuations in Solids (Edited by L.M. Fahcov, W. Hanke & M.B. Maple). North Holland, Amsterdam (198 1). See Valence Instabilities (Edited by P. Wachter & H. Boppart). North Holland, Amsterdam (1982). J.M. Lawrence, P.S. Riseborough & R.D. Parks, Rep. Prog. Phys. 44,1 (1981). B. Johansson, Phil. Mag. 30,469 (1974). Y. Baer & Ch. Ziircher, Phys. Rev. Lett. 39,956 (1977). R. I..%sser,J.C. Fuggle, M. Beyss, M. Campagna, F. Steglich & F. Hulliger, Physica 102B, 360 (1980). J.C. Fuggle, F.U. Hillebrecht, Z. Zolnierek, R. Lesser & Ch. Freiburg, Phys. Rev. B (in press). K.R. Bauchspies, W. Boksch, E. Holland-Moritz, H. Lannois, R. Pott & D. Wohlleben, Valence Fluctuations in Solids (Edited by L.M. Falicov, W. Hanke & M.B. Maple), p. 417. For a summary see 0.1. Sumbaev, Sov. Phys. Usp. 21,141 (1978). Y. Baer & G. Busch, Phys. Rev. Lett. 31,35 (1973). J.W. Allen, S.J. Oh, I. Lindau, J.M. Lawrence, L.I. Johansson & S.B. Hagstrom, Phys. Rev. Lett. 46,llOO (1981).
12. 13. 14. 15. 16. 17. 18. 19. 20.
21. 22. 23. 24. 25. 26.
Vol. 48, No. 3
M. Croft, J.H. Weaver, D.J. Peterman & A. Franciosi, Phys. Rev. Lett. 46,1104 (1981). S. Htifner & P. Steiner, Z. Phys. B46,37 (1982). S. Hiifner & P. Steiner, Valence Instabilities (Edited by P. Wachter & H. Boppart), p. 263. 0. Gunnarsson & K. Schonhammer, Phys. Rev. Lett. 50,604 (1983). S.H. Liu & K.M. Ho,Phys. Rev. B26,7052 (1982). A. Fujimori, Phys. Rev. B27,3992 (1983). A. Kotani & Y. Toyozawa, J. Phys. Sot. Japan 35, 1073 (1973); 37,912 (1974); 46,488 (1979). T.V. Ramakrishnan, Valence Fluctuations in Solids (Edited by L.M. Falicov, W. Hanke & M.B. Maple), p. 13. P.W. Anderson, Valence Fluctuations in Solids (nEdi;;d by L.M. Falicov, W. Hanke & M.B. Maple), . . J.K. Lang, Y. Baer & P.A. Cox, J. Phys. F; Met. Phys. 11,121 (1981). Y. Baer, H.R. Ott, J.C. Fuggle & L.E. De Long, Phys. Rev. B24,5384 (1981). G. Denninger, V. Dose & H. Scheidt, Appl. Phys. 18,375 (1979). M. Conrad, V. Dose, Th. Fauster & H. Scheidt, Appl. Phys. 20,37 (1979). V. Dose, H.J. Gossmann & D. Straub, Surf. Sci. 117,387 (1982). M. Gupta & A.J. Freeman, Phys. Rev. B17,3029 (1978).
Vol. 48, No. 3, pp. 257-260,
ISOCHROMAT
1983.
0038-1098183 $3.00 + .OO Pergamon Press Ltd.
SPECTROSCOPY ON CePds S. Hiifner
Laboratorium
fti Festkorperphysik,
ETH Zurich, Honggerbergf
Switzerland
P. Steiner Fachbereich
Physik, Universitat
des Saarlandes, D-6600 Saarbriicken, W. Germany and
V. Dose, D. Straub and A. Hart1 Physikalisches
Institut
der Universitit,
D-8700 Wiirzburg, W. Germany
(Received 11 May 1983 by P. Wuchter)
Isochromat spectra from CePds recorded at quantum energies of 9.7 and 72.7 eV are presented. The data obtained at 9.7 eV show three different peaks and are by and large identical to previously published data for a quantum energy of 1486 eV. Only two of those three peaks are present in the 72.7 eV spectrum. We conclude that the missing peak is bulk in origin and derives from a CeSd-Pd4d hybridization leading to a CeSd split off level from the filled Pd d-band.
discussion. Perhaps one should add, for clarity that there is a number of intermetallic Ce compounds (like, e.g. CeNi*, CeCoz, CeFe*) in which the 4fstate has similar properties as in c&e and these compounds are therefore called a-type compounds. The question as to the nature of the 4fstate (promoted to the (5d6s) conduction band or 4f itinerant) in these o-type compounds is the same as in a-Ce. Spectroscopic techniques like photoelectron spectroscopy (PES), Bremsstrahlen-isochromat Spectroscopy (BIS), X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) have been used to study the nature of the electronic structure of y-type, IV and a-type compounds. In all these techniques one measures however an excited state and it is not obvious, how the groundstate can be inferred from the measured spectra. Core level PES [5-71, XAS [8] and XES [9] on a-type compounds all point to a 4foccupation in the final state of - 0.75. This means in the PES and XAS data that the signal which can be attributed to a 4f” final state has an intensity of the order of only 25%. Although the final state contribution to the spectra is not clear it is difficult to conceive any model at all, that would be able to produce the spectra from a 4f’(5d6~)~ initial state. The situation with respect to the valence band photoemission spectra of Ce [IO] and its compounds [ 11,121 is also not clear. The valence band PES often shows two 4fsignals (in addition to the conduction band) below the Fermi energy which prevented an unambiguous determination of the energetic position of
METALLIC Ce COMPOUNDS can be grouped into three classes: y-type systems (e.g. y-Ce), intermediate valent (IV) systems (e.g. CePda) and o-type systems (e.g. c&e) [l-3]. In y-type Ce systems the 4felectron is localized giving rise, e.g. to a Curie paramagnetisim. IV systems are believed to have a partially occupied 4flevel. The electronic structure of a-type compounds is a matter of present controversy. Originally it was assumed that, e.g. the transition from y-Ce to a-Ce was achieved by promoting the 4felectron to the (5d6s) conduction band. Thus while the electronic configuration of y-Ce can be written formally as 4f’(Sd6~)~ that of a-Ce would be 4f’(Sd6~)~ in the promotional model. This is also the reason, why -y-type Ce systems are often called trivalent and o-type systems tetravalent. Johansson [4] argued that because the 4felectrons contribute little to the cohesive energy, which stems from the conduction band, and since the cohesive energy of y-Ce and cr-Ce are almost identical, the promotional model must be in error. He suggested, that the transition from y-Ce to a-Ce was thus one from a localized 4f’ configuration to a delocalized 4f’ configuration, with little promotion of the 4felectron into the (5d6s) conduction band. The question whether this assertion is correct and whether there are direct spectroscopic means to probe the electronic structure of a-Ce is a matter of present * Permanent address: Fachbereich Physik, Universitat des Saarlandes, D-6600 Saarbriicken, W. Germany. 257
258
ISOCHROMAT SPECTROSCOPY ON CePds
the 4flevel in the ground state (bare 4fposition). Hiifner and Steiner [ 13, 141 suggested that the two signals arise from two differently screened hole states similar to the situation encountered in nickel. While the signal at - 1 to - 3 eV below the Fermi energy corresponds to a localized hole state, the signal near the Fermi energy was assigned to an itinerant hole state. It is therefore difficult to identify the exact position of the bare 4flevel from photoemission data. However, it shows that the 4flevel in the ground state can be quite near to the Fermi energy even if a PES signal is seen 1-3 eV below it. Recently theories have been published [15-l 71 which show the importance of relaxation, and suggest that the core level PES spectra and the XAS measurements on o-type compound can only be reconciled with a sizeable 4foccupation in the initial state, which rules out the promotion model. These theories start in essence from the work of Kotani and Toyozawa [ 181 but than proceed in very different ways to calculate the experimental spectra. We here comment briefly only on the work of [ 151 because it is the most sophisticated one. The calculations use a Hamiltonian suggested by Ramakrishnan [ 191 and Anderson [20]. In the calculation the degeneracy of the 4flevel is taken as a developing parameter. With sensible values for the f-d hybridization the PES core and valence band spectra can be explained. These authors [ 151 get two 4f derived signals in the valence band, one at the Fermi energy and one below the position of the bare 4f state. There is however a nonlinear relationship between peak intensity and 4f occupancy making it difficult to extract the latter number directly from the measured spectra. BIS spectra have also been calculated by Gunnarson and Schonhammer [ 151. The best suited method to investigate unoccupied electronic states in Ce and Ce-compounds appears to be Bremsstrahlung Isochromat Spectroscopy (BIS). This type of experiment is perhaps the most direct one in establishing the nature of the 4felectrons in IV intermediate valent and a-type systems because of the following reason: if there is an electronic configuration 4f’(5d6~)~, then the BIS spectrum should show only a 4f’ signal close to EF. If on the other hand the electronic configuration is described by e I 4f’(5d6~)~) -I(1 - E) 14f’(5d6~)~) a 4f1 signal and a 4fz signal at - 4 eV above EF should be visible. Thus the strength of the signal at - 4 eV should give an indication of the 4f1 component of the groundstate. Thus if a compound has indeed a configuration 4f’(5~?6s)~ (as suggested for o-type systems in the promotional model) the BIS spectra should show only a signal near EF but none at - 4 eV above EF, giving a good experimental discrimination of the two models. We note that there is of course according to [ 151 a
Vol. 48, No. 3
nonlinear relationship between experimental peak heights and 4foccupation in the groundstate. BIS spectra from y-Ce, CeSn3 and CeA13 exhibit structure at - 4 eV above the Fermi level corresponding to the 4f* electronic configuration [22]. BIS spectra from CePd3 are distinctly different since they exhibit additional peaks at the Fermi level and 2.5 eV above E,. While the emission at the Fermi level is probably due to 4f’ final configurations, the origin of the peak at 2.5 eV above EF is not clear at present. It has been tentatively proposed to be due to a surface state [22]. It was the purpose of the present work to advance the interpretation of the BIS data from CePd3. This is necessary in order to check whether the simple concept just mentioned to distinguish between the promotional and the delocalization model for o-type Ce systems can actually be used. Therefore, both PES and BIS measurements on CePd3 have been performed. The PES experiment employed Al-Ka radiation. Data were taken for the M, and MI, core levels with clean and oxidized samples in order to check on the importance of surface contamination. BIS experiments were performed at a quantum energy of 9.7 eV employing an energy selective Geiger Miiller counter as a radiation detector [23]. For measurements at a quantum energy of 72.7 eV, the absorption edge technique was used [24,25]. The choice of these quantum energies is motivated by the different penetration depths of the electrons at the respective energies. Consequently, a comparison of data taken at 9.7 eV and at 72.7 eV should enable to differentiate between surface and bulk contributions to the total emission. Figure 1 displays the 3d Ce PES lines obtained from clean CePd3 and from CePd3 after exposure to 11 L of oxygen. The spectrum of the clean sample is in agreement with published data [6], the structure being assigned to f”, f1 and f2 final state configurations [6]. Oxygen adsorption leads to an attenuation of the f ’ signal with respect to the f” and f2 signals and in addition introduces new structure due to Ce oxide. XPS has always shown an intensity ratio off’ vs (f’ + f2) which was difficult to reconcile with the f-counts from other experimental methods, e.g. X-ray absorption [7]. The data in Fig. 1 indicate that the weight of the different f” components can be different at the surface and in the bulk and that it is difficult to extract a quantitative f-count out of such data. Figure 2 shows BIS spectra for a quantum energy of 9.7 eV in comparison to measurements by Baer et al. [22] employing a quantum energy of 1486 eV. The electron penetration depth is similar at these two energies and is of the order of 25 A. Apart from a different background behaviour the two spectra are identical in every structural detail. Figure 2 also shows a BIS spectrum at
Vol. 48, No. 3
ISOCHROMAT SPECTROSCOPY
910
900 920 Binding Energy/N
880
6f
Fig. 1. The XPS 3d spectrum of clean CePds and of CePds after exposure to 11 L of 02. The various final states and the Ce oxide signal are indicated.
..
*....v..-.*
..-
Ce Pd,
Ii
’
..*._**_._* ‘....:.-
hw 9.7eV q
_..?. ....
I
nw=73ev
i .*J 0
2
4
6
ON CePds
259
72.7 eV where the electron penetration depth is only of the order of 5 A. Common features to the other spectra in Fig. 2 are the emission peaks at 5 and 4.5 eV and an inflection point at about 6.5 eV. The peak at 2.5 eV, visible in the 9.7 and 1486 eV data, is hardly discernible in the 72.7 eV surface sensitive data. Note also that the 4.5 eV emission is shifted by 0.5 eV to higher binding energy analogous to surface core level shifts. The peak at EF in the 72.7 eV data shows that the surface of the sample is still in the IV state. The absence of the 2.5 eV emission in the 72.7 eV spectrum proves that this peak is a typical bulk feature of CePds. It can be explained by (Ce-5d) * (Pd4d) hybridization. Such a hybridization would lead to a Ce-5d state split off from the full Pd d-band. A similar effect for occupied electronic states is observed upon alloying Pd with hydrogen [26]. The remaining two peaks in the BIS spectrum from CePds at 0.5 eV and 4.5-5 eV which show up in ail three spectra are attributed to 4f’ and 4f2 final state configurations respectively of Ce. From a comparison of their physical properties one can conclude that a-cerium systems are expected to be similar to IV systems. The XPS data of CePds in fact resemble closely those of cu-cerium systems in that they show, e.g. in the PES core level spectra a similar strength of the f” components of - 25%. The same is true for the f” signal in the XAS data. In fact a quantitative analysis of the XPS spectra indicates if anything the hybridization is larger in IV CePds than in o-type CeNi2 [7]. This then would suggest that also their electronic structure is very similar and that also in a-type Ce compounds there is a 4f’ contribution of - 75% to the electronic structure. This in turn would mean that in a-type compounds one would expect a 4f’ signal (close to EF) and a 4f 2 signal (- 4 eV above EF) in the BIS experiments. Barth et aZ.[27] have recently measured the susceptibility of very dilute Ce Impurities in a great number of hosts. They find y-type, IV- and a-type behaviour meaning a constant susceptibility. They conclude that in the hosts in which Ce impurities show the temperature independent susceptibility Ce is tetravalent. One can however also argue that the delocalization of the 4f electron leads to the observed magnetic behaviour. However these experiments show that the delocalization in a-type Ce compounds cannot be due to 4f-4f overlap but has to be caused by 4f-5d hybridization.
(E-E,llev Fig. 2. BIS spectra at quantum energies of 9.7 and 72.7 eV from this work compared to earlier data by Baer et al. [22]. Except for the peak at 2.5 eV in the 72.7 eV spectrum the three sets of data agree in every structural detail, even the small inflection indicated by the arrow.
Acknowledgement - This work was supported by the Deutsche Forschungsgemeinschaft. One of the authors (S.H.) wants to thank Y. Baer, J. Fuggle, F.U. Hillebrecht, F. Steglich, P. Wachter and D. Wohlleben for many helpful discussions. We are indebted to Prof. B. Elschner for providing the sample for this experiment.
260
ISOCHROMAT SPECTROSCOPY ON CePds REFERENCES
1.
2.
3. 4. 5. 6.
7. 8.
9. 10. 11.
See Valence Fluctuations in Solids (Edited by L.M. Fahcov, W. Hanke & M.B. Maple). North Holland, Amsterdam (198 1). See Valence Instabilities (Edited by P. Wachter & H. Boppart). North Holland, Amsterdam (1982). J.M. Lawrence, P.S. Riseborough & R.D. Parks, Rep. Prog. Phys. 44,1 (1981). B. Johansson, Phil. Mag. 30,469 (1974). Y. Baer & Ch. Ziircher, Phys. Rev. Lett. 39,956 (1977). R. I..%sser,J.C. Fuggle, M. Beyss, M. Campagna, F. Steglich & F. Hulliger, Physica 102B, 360 (1980). J.C. Fuggle, F.U. Hillebrecht, Z. Zolnierek, R. Lesser & Ch. Freiburg, Phys. Rev. B (in press). K.R. Bauchspies, W. Boksch, E. Holland-Moritz, H. Lannois, R. Pott & D. Wohlleben, Valence Fluctuations in Solids (Edited by L.M. Falicov, W. Hanke & M.B. Maple), p. 417. For a summary see 0.1. Sumbaev, Sov. Phys. Usp. 21,141 (1978). Y. Baer & G. Busch, Phys. Rev. Lett. 31,35 (1973). J.W. Allen, S.J. Oh, I. Lindau, J.M. Lawrence, L.I. Johansson & S.B. Hagstrom, Phys. Rev. Lett. 46,llOO (1981).
12. 13. 14. 15. 16. 17. 18. 19. 20.
21. 22. 23. 24. 25. 26.
Vol. 48, No. 3
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