X-ray photoemission from Ce core levels of CePd3, CeSe, CeAl2 and CeCu2Si2

Physica 102B (1980) 360-366 (~) North-Holland Publishing Company

X-RAY PHOTOEMISSION FROM Ce CORE LEVELS OF CePd3, CeSe, CeAl2 AND CeCu2Si2 R. L,~SSER*, J.C. F U G G L E , M. BEYSS, M. C A M P A G N A Institut fiir Festk6rperforschung der KFA Ji~lich, D-5170 Jiilich, Fed. Rep. Germany

F. S T E G L I C H Universitiit Darmstadt, D-6100 Darmstadt, Fed. Rep. Germany

F. H U L L I G E R Laboratorium fiir Festk6rperphysik, ETH Ziirich, CH-8093 Ziirich, Switzerland

The central and new aspect of this paper is the interference in core level spectra between mixed valence features and screening mechanisms. We investigated the 3d and 4d X-ray photoelectron spectra (XPS) of Ce atoms in CePd3 (where configuration mixing has been established by other methods), CeSe (which is known to contain only Ce 3÷) with the spectra of CeCu2Si2 and CeAI2. Even in the cases with a stable 4f t configuration, the Ce 3d spectra are found to contain so-called shake down (4f2) contributions as a result of the change of the initial 4f configuration due to screening of the 3d final state hole. Therefore, only qualitative lower limits for the mixed valence ratio Ce 3+: Ce 4÷ can be derived from the XPS core level spectra. These ratios are ~ 10:1 for CeCu2Si 2 and ~ 5:1 for CeAI2. The multiplet splittings and shake down probabilities for core level XPS peaks of Ce are found to be strongly dependent upon chemical environment.

1. Introduction

contributions to the physics of rare earths via both core and valence level studies. This is despite some practical difficulties, related first to the extreme reactivity of many rare earth compounds, and secondly to possible intrinsic differences of valence between surface and bulk materials [2]. Studies of the 4f states themselves yield information on the value of the effective Coulomb interaction, Uea, within the 4f states [3], as well as on the average number of f electrons in the mixed valence material. However, for the specific case of Ce the cross sections for photoemission from the Ce 4f and 5d levels, or of other valence levels of chalcogenides and pnictides of Ce, are all comparable so that in many cases direct information on the 4f state cannot be obtained directly. Nevertheless, XPS studies of mixed valence are sensible for it has been shown that direct qualitative information on configuration fluctuations can be obtained by analysing the 3d and 4d core levels of rare earth spectra (see, for example, refs. 2-8). In this paper we present an analysis of these Ce d-level spectra

Mixed valence rare earth compounds (i.e. rare earth compounds with interconfiguration fluctuations) have been the subject of numerous investigations during the last ten years [1]. Within the temporal picture the characteristic fluctuation time in so-called homogeneous mixed valence rare earth compounds is of the order of 10 - u to 10 -14 S. Thus, "slow" techniques like M6ssbauer spectroscopy, magnetic susceptibility, and lattice constant measurements or N M R measure properties of the average rare earth configuration. Photoelectron spectroscopy is unique in that all transitions involved in the photoemission process are faster than the mean fluctuation time of these mixed valence materials and it yields information on the properties of the individual configurations. Photoelectron spectroscopy has made useful * Present address: Bell Telephone Labs., Murray Hill, N.J. 07974, USA. 360

R. Liisser et aL/X-ray photoemission from Ce core levels

from CePd3, CeSe, CeAI2 and CeCu2Si2. Howbver, quantitative information on the configuration fluctuations of Ce compounds can only be obtained from the XPS core level intensities in XPS by reaching a complete understanding of the mechanism of screening a core hole. We proceed with a discussion of this problem which is particularly poignant for Ce, but of much current interest in the whole field of electron spectroscopy. 2. Screening of Ce core levels

Core ionization of the Ce 3d or 4d levels in compounds with non-integral f-count leads to two sets of peaks separated by 8 - 10-12 eV with respect to each other and arising from atoms with "mixed valence" initial state configurations, as illustrated in fig. 1. Ionization of a core level in XPS of solids is often well described by an excited atom model in which an extra electron is transferred from the valence band to a screening orbital (see, for example, refs. 9-13). It is found experimentally that less energy is required to create a core hole in Ce, with a simultaneous increase in the number of 4f electrons by one, than to create the "MIXED VALENCE"

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Fig. 1. Schematic illustration of core-level pbotoemission from a cerium atom with fluctuating f-count in the initial state. The number of f electrons in the XPS final state after removal of an electron from the nth shell is given in brackets. nd represents the 3d or 4d hole of the ionized Ce and spin-orbit coupling is not included in this picture. For further explanation of symbols see text.

361

core hole and only allow screening by the 5d and 6s valence states [7, 14]. As illustrated in fig. 1, final states with both one and two 4f electrons can be reached by core ionization from Ce3+--4f1 ions with separation A = 4-5 eV. Because the 4f-states are strongly localized and their wave functions in the case of stable compounds are not mixed with those of the other valence electrons in any significant way, the integral (~bf(4f"+t)l~bi(4f")) between the initial state wave function $i(4P) with n 4f electrons and a final state, Sr(4P +t) with (n + 1) 4f electrons, is small. These 4f"+~ final states are thus represented by only small weights in the core level XPS spectra. Although energetically less favourable, the transitions involving (t~f(4f")l~bi(4f"))have no f-count change and hence larger cross-sections, so that they dominate the spectra. The spectrum from a single core level is thus as illustrated schematically in the lower part of fig. 1. In view of these effects we label the peaks in the XPS spectra by the nominal charge on the Ce atoms in the initial state followed in parenthesis by the number of f electrons in the final state. We must also consider spin-orbit and multiplet splittings which are observed in the 3d and 4d XPS spectra reported here. For the 3d levels in Ce the spin-orbit splitting (As o) is ~18.5 eV and is larger than 8, so that the contributions to the spectra, from Ce 3÷ and Ce 4+, overlap. The multiplet splitting of the spectra due to the interaction of the 3d core hole with the unpaired 4f electrons in the 4f1 and 4f2 final states is small compared to a .... or 8 [15]. For the 4d levels the spin--orbit splitting (A.... = 3 eV) is smaller than 8 so that the contributions from C e 3+ and Ce 4÷ are separated in the spectra. However, the multiplet splittings for the 4P and 41a final states are comparable with, or even larger than, the spin-orbit splittings and are also dependent upon the chemical environment so that the Ce 3+ region of the 4d spectra becomes very difficult to interpret in any detail, as experience from the literature so far has shown. We shall therefore primarily concentrate on discussion of the 3d core level spectra.

362

R. Liisser et al./X-ray photoemission [rom Ce core levels

3. Experimental

CePd3, CeCu2Si2 and CeAI2 samples were prepared by melting together pre-weighed samples of the pure elements in a cold crucible. CeCu2Si2 was then annealed at 900°C for 100 h in ultra high vacuum. The preparation of the CeSe single crystal samples was described earlier [16]. As a check on sample purity for CePd3 and CeA12 it was considered sufficient to check the absence of any weight loss during melting and the presence of only a single phase by X-ray diffraction. X-ray analysis indicated that the CeCu2Si2 sample had the proper (ThCr2Si2) structure and neither the X-ray diffraction pattern nor a microprobe analysis revealed the presence of any second phase. The CeCu2Si2 sample was the same as that used for ref. 17. The spectra were measured in an electron spectrometer custom built to our specifications by Kratos (U.K.) Ltd. Briefly, the spectrometer consists of separately pumped measurement, analyser, and preparation chambers with base vacua 2 x 10-H, 2 x 10-~° and I x 10-1° Torr (3 x 10-9, 3 x 10-s and 1.3 x 10-8 Pa), respectively. In its initial form, as used for this work, the spectrometer had a small commercial monochromator without a multidetector system which yielded ~-600 c/s at 0.6 eV FWHM for the Ag 3d XPS peaks. This is rather low sensitivity and as a result it took many hours to accumulate data, with good statistics, by digital counting methods. The samples of CePd3, CeCu2Si2 and CeAl2 were cleaned by scraping in the preparation chamber whilst the CeSe samples were cleaved in a differentially pumped Leybold Heraeus system and transferred to the measurement chamber. The samples were recleaned periodically to avoid build-up of contaminants. The oxygen contamination of the intermetallic samples could not be removed completely by scraping but was always less than one monolayer, as determined from the O ls XPS peak by published methods [18], for CePd3 and CeA12. A larger O ls signal found in some spectra from CeCu2Si2 is thought to have arisen from the sample holder as the CeCu2Si2 sample was rather small, but the

material of the sample holder produced no peaks in the region of the Ce 3d and 4d XPS peaks. No signal from Cu or Si oxides was detectable in the spectra of this sample, nor were any of the characteristic features of oxidized Ce [19] found in the spectra. 4. Results and discussion

4.1. The spectra of CeSe and CePd3 Fig. 2 shows the Ce 3d XPS peaks from CePda and CeSe. The observed binding energies and relevant splittings for these peaks are given in table I together with data for other Ce compounds from the literature. Both spectra exhibit strong peaks with binding energies ~903 and 884 eV which are assigned to the 3d3/2 and 3d5/2 peaks with a 4f 1 final state. We attribute the shoulders approximately 4 eV to lower B.E. in CeSe entirely to the 3d3/2 and 3ds/2 XPS peaks

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R. 1.2isser et al./X-ray photoemission from Ce core levels

363

Table I Binding energies and other relevant quantifies for some Ce compounds. All Ce peaks quoted are those where the f-count has not changed during core ionization and all energies are in e V . A l l B . E . s are given relative to EF. Ce

Compound CeSe CePd3 CeAI2 CeCu2Si2 C e S b s-s C e N 6"s C e N t a l c 3°

valence

B.E. 3d3/2

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3 3 4 3 3 3 3

903.9 903.1 914.7 901.9 902.1 902.8 ~905

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Other binding energies determined in this study were: for C e S e : Se 3ptc2 = 166.7, S e 3p312 = 161.0; for C e P d 3 : P d 3d3t2 -- 341.4, I ' d 3 d s n = 336.1; for CeAI2: A I 2s = 117.4, A1 2 p = 72.4, hwp = 14.0 e V ; for CeCu2Si2: C u 2pl/2 = 952.9, C u 2p3/2 = 932.9, C u 3s = 123.1, C u 3 d = 4.1, Si 2s = 150.2, Si 2 p = 99.0.

with a 4f 2 final state, in accordance with previous work on cerium and its pnictides [7, 19]. Similar shoulders are also observed for CePd3, but there is some overlap with peaks which we assign to transitions from a 3dr°... 4f0 (Ce 4+) initial state to a 3d9... 410 final state. The 3d3/2 peak from Ce 4+ is 11.6 eV to higher binding energy than the Ce 3+ 14f l) 3d3/2 peak. However, the Ce 4+ (410) 3din peak is submerged in the shoulder assigned to the Ce 3+ (4f2) 3d3/2 excitation. The most important feature to note here is the Ce 4+ (410) 3d3/2 peak at approximately 914eV, which is diagnostic of 4f valence instability with intensity 10% of that found in the Ce 3+ 3d3/2 peaks. A similar peak is found in CeN [7, 20], as well as CePd3, and both these compounds are unquestionably "mixed valence" compounds [1] in contrast to CeP, CeAs and CeSb [7, 20], and CeSe. The absence of such a peak labelled Ce 4+ (10) in CeAl2 and CeCu2Si2 would indicate the absence of valence fluctuations in both compounds. Note also that the Ce 4+ (410) 3d3/2 peak is narrower than the other 3d peaks because there is no multiplet broadening due to the interaction between the d hole and unpaired 4f electrons. A similar effect is found in lanthanum compounds which also have narrow 3d peaks. As a con-

sequence of these observations we expect XPS peaks due to Ce 4÷ to be narrow. Fig. 3 shows the Ce 4d XPS peaks from CeSe Ce 3. ~f') C¢ /.d XPS from CeSe

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obtain a rough correspondence of the peaks found the C¢S¢ spectrum has been moved ,=1 e V to lower B . E .

364

R. Li~ser et al./X-ray photoemission from Ce core levels

and CePd3. The main intensity is found in broad peaks, with approximately 111 eV binding energy. Here the peaks due to 4d3/2 and 4d5/2 ionization of Ce 3+, leading to 4f I and 4f2 final states, are poorly resolved due to multiplet effects. Note, however, that the sharp peaks due to Ce 4+ (4f°) 4d3/2 and 4d5/2 ionization are well resolved and separated from the Ce 3+ 4d peaks in CePd3. These Ce 4+ peaks are absent in the spectrum of CeSe where the Ce 4+ 4f° configuration plays no role in the initial state.

4.2. The spectra of CeCu2Sb We cannot use the Ce 4d XPS peaks from CeCu2Si2 to diagnose the presence or absence of Ce 4÷ because the Cu 3s peaks mask the region where the Ce 4÷ 4d XPS peaks are expected. Fig. 4 shows the XPS spectra from CeCu2Si2 in the region of the Ce 3d peaks. The 3d spectra are dominated by the "unscreened" Ce 3+ (4f 1) peaks with weak shoulders ~ 4 eV (=A) to lower B.E. due to transitions to the "screened" 4f2 final state. There are no peaks with binding energy approximately 914 eV as found for CePd3, and any such " p e a k " lying below the noise level is certainly less than 3% of the 3d3/2 peak (i.e. less than one-third as intense as that found for CePd3 in fig. 2). If we assume that the relative intensity of the peaks due to Ce 4÷ and Ce 3÷ is proportional to the amplitude associated with those states in the sample, then this must be less than 3% at

Ce 3d XPS Peaks :~ Ce3÷(/'f~)-w i from CeCu2Si2 "i: AS'0" '!'!~" il i i ..

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room temperature, implying that CeCu2Si: is not a mixed valence compound. This conclusion contrasts with that of Sampathkumaran et al. who considered CeCu2Si2 as "mixed valenr' [20, 21]. These authors do not, however, give an estimate of the average valence needed to explain the results of their magnetic susceptibility measurements. We shall have reason below to consider the validity of the assumption that the intensity ratio of the Ce 4+ (4f°):Ce 3+ (4fl)+Ce 3+ (4f2) XPS peaks is quantitatively proportional to the amplitude of the Ce 4÷ and Ce 3÷ states in the initial state.

4.3. The spectra of CeAl2 The 4d and 3d XPS peaks of CeAI2 are accompanied by strong satellites on the low kinetic energy sides of the main peaks which can all be explained as losses, or multiple losses, of a 14 eV quantum due to plasmon creation. Although these losses complicate the spectrum we can say that the weight in the spectrum due to photoemission from Ce 4+ in the initial state to a 4f° final state is less than 10%. This is in agreement with L6wenhaupt's conclusion that CeA12 does not exhibit mixed valence [23].

4.4. The relative intensity of the "screened" peaks As explained in the introduction, the creation of a core hole in Ce may result in a change of the f-count between the initial and final states. As a consequence, even in Ce with a stable 4f 1 configuration we see features due to a "screened" final state with an extra 4f electron (4f2) and a final state (4f 1) with less perfect screening by the 5d and 6s valence electrons. We argue that some mixing of the 4f wave functions with those of the 5d and 6s states is necessary in order for an f-count to increase during core-level photoemission. Fig. 5 shows the 3d5/2 XPS peaks of the compounds investigated on an expanded energy scale in which the relative intensities of the Ce 3+ (4f 1) and Ce 3+ (4f2) peaks are clearly seen to be dependent upon chemical environment. The peak is largest in the mixed valence, CePd3 compound indicating that the mixing of the 4f and 5d, 6s states is largest here. A similar

R. Lf~.sser et aL/X-ray photoemission from Ce core levels

C¢ 3ds/a XPS Peaks Ce3*(&f I) • i ~ C3~ (4f a)

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Fig. 5. The 3d5/2 XPS peaks from CePd3, CeSe, CeAI2, CeCu2Si2. To line up the Ce 3+ (4f l) contribution the CeSe and CePd3 peaks have been shifted to lower B.E. by ~0.8 and ~1.0 eV, respectively.

effect was found in the Ce pnictides where the mixed valence compound CeN has larger Ce 3÷ (4f2) peaks than CeP, CeAs, and CeSb [6-8]. There is insufficient data available to say that it is a universal trend, but the simplest interpretation of these results is that the 4f levels are more strongly mixed with the 5d-6s states in mixed valence Ce compounds. 4.5. Determination of valence from Ce core level intensities in XPS The relative intensity of the Ce 4÷ and Ce 3÷ peaks in CePd3 is approximately 1:10 so that if the XPS peak intensities were really representative of the Ce 4+ and C e 3+ amplitudes then the valence of Ce in CePd3 would be 3.1. It is actually 3.5 [24] and XPS from core levels has underestimated the Ce 4+ amplitude in CePd3 by a factor of about 5. When we examine the literature we find similar discrepancies for CeN (Ce valence estimated from published XPS spectra [6-8] =3.1, optical measurements 3.4 [25]) and Ce3Th (Ce valence from published XPS spectra [26] < 3.1, by lattice constant measurements 3.23.4 [27]). We do not believe that surface effects

365

cause this discrepancy because under the conditions used most of the XPS signal does not come from the top monolayer of material which is most likely to have anomalous structure and valence. One possible explanation of the discrepancy between XPS and other measurements is that core ionization of Ce 4÷ 4f° leads predominantly to a "screened" nd_ 4f ~ final state. This is then indistinguishable from the "unscreened" final state resulting from ionization of the Ce 3÷ 4f1 initial state. The present study has shown that only if the detailed screening mechanism of core holes in Ce is quantitatively understood precise statements can be made about valence mixing of the 4f levels from 3d/4d XPS peak intensities.

5. Concluding remarks In this paper we have shown that in core level XPS studies of Ce compounds, peaks in the spectra can be attributed to Ce with 4f° ( C e 4+) and 4f I (Ce 3÷) initial state configurations. We find no peaks due to 4f° Ce in CeCu2Si2. We have shown that peaks found in the Ce 4÷ region of the CeAI2 XPS spectrum are due to plasmon losses. We note for the first time that the XPS technique apparently underestimates the "average valence" of Ce in mixed-valence compounds and attribute this to the screening of core holes by the 4f electrons.

Acknowledgements We thank J. Keppels for able technical assistance and A. Bringer, Y. Jafet and D. Wohlleben for informative discussions.

Reterences [1] For general reviews see C.M. Varma, Rev. Mod. Phys. 48 (1976) 219; Valence Instabilities and Related Narrow Band Phenomena, R.D. Parks, ed. (Plenum, New York, 1977). [2] G.K. Wertheim, J.H. Wernick and G. Crecelius, Phys. Rev. B18 (1978) 875. [3] M. Campagna, E. Bucher, G.K. Wertheim and L. Longinotti, Phys. Rev. Lett. 32 (1974) 885. [4] M. Campagna, G.K. Wertheim and E. Bucher, Structure and Bonding, vol. 30 (Springer, Berlin, 1976) p. 99. [5] J.-N. Chazaeviel, M. Campagna, G.K. Wertheim and P.H. Schmidt, Solid State Commun. 19 (1976) 725.

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R. Liisser et al./X-ray photoemission from Ce core levels

[6] Y. Baer and Ch. Ziircher, Phys. Rev. Lett. 39 (1977) 956. [7] M. Campagna, G.K. Wertheim and Y. Baer, Topics Appl. Phys. 27' (1979) 217. [8] Y. Baer, R. Hauger, Ch. Ziircher, M. Campagna and G.K. Wertheim, Phys. Rev. B18 (1978) 4433; J. Electron. Spectrosc. 15 (1979) 27. [9] C.D. Wagner and P. Biloen, Surf. Sci. 35 (1973) 82. [10] S.P. Kowalczyk, R.A. Pollak, F.R. McFeely, L. Ley and D.A. Shirely, Phys. Rev. B8 (1973) 2387; S.P. Kowalczyk, L. Ley and F.R. McFeely, Phys. Rev. B9 (1974) 381. [11] R. Hoogewijs, L. Fiermans and J. Vennik, Chem. Phys. Lett. 38 (1976) 192, 471; Surf. Sci. 69 (1977) 273. [12] N.D. Lang and A.R. Williams, Phys. Rev. B20 (1979) 1369. [13] R. L~isser and J.C. Fuggle, Phys. Rev. B22 (1980) 2237. [14] G.K. Wertheim and M. Campagna, Solid State Commun. 26 (1978) 553. [15] See, for example, N. Spector, C. Bonnelle, G. Dufour, C.K. J~rgensen and H. Berthou, Chem. Phys. Lett. 41 (1976) 199. [16] F. Hulliger, in: Handbook on Physics and Chemistry of the Rare Earths, K.A. Gschneidner Jr. and L.R. Eyring, eds. vol. II (North-Holland, Amsterdam, 1980).

[17] F. Steglich, J. Aarts, C.D. Bredl, W. Lieke, D. Meschede, W. Franz and H. Schiller, Phys. Rev. Lett. 43 (1979) 1892. [18] T.E. Madey, J.T. Yates Jr. and N.R. Erickson, Chem. Phys. Lett. 19 (1975) 487. [19] A. Platau, L.I. Johansson, A.L. Hagstr6m, S.E. Karlsson and S.B.M. Hagstr6m, Surf. Sci. 63 (1977) 153; A. Platau, private communication. [20] B.C. Sales and R.V. Viswanthan, J. Low Temp. Phys. 23 (1976) 449. [21] E.V. Sampathkumaran, L.C. Gupta and R. Vijayaraghavan, Phys. Lett. 70A (1979) 356. [22] M. Loewenhaupt, B. Rainford and F. Steglich, Phys. Rev. Lett. 42 (1979) 1709. [23] I.R. Harris, M. Norman and W.E. Gardner, J. Less. Common Metals 29 (1972) 299. [24] A. Schlegel, E. Kaldis, P. Wachter and Ch. Ziircher, Phys. Lett. 66A (1978) 125. [25] R.A. Pollak, S.P. Kowalczyk and R.W. Johnson, in: Valence Instabilities and Related Narrow Band Phenomena, R.D. Parks, ed. (Plenum, New York, 1977). [26] S.M. Shapiro, J.D. Axe, R.J. Birgeneau, J.M. Lawrence and R.D. Parks, Phys. Rev. B16 (1977) 2225.