Bremsstrahlung-Isochromat study of unoccupied electronic states in R.E.Pd3 compounds

~

Solid State Communications, Printed in Great Britain.

Vol.49,No.4,

BREMSSTRAHLUNG-ISOCHROMAT

pp.339-341,

1984.

0038-1098/84 $3.00 + .00 Pergamon Press Ltd.

STUDY OF UNOCCUPIED ELECTRONIC

STATES IN R.E.Pd 3 COMPOUNDS .

C. Laubsehat,

G. Kaindl, E,V. Sampathkumaran

Institut f~r Atom- und Festk6rperphysik; Freie Universit~t D-IO00 Berlin 33, Germany (Received

9 September

÷

, and W.D. Schneider Berlin,

1983 by P. Wachter)

Bremsstrahlung-lsochromat spectroscopy studies of R.E.Pd 3 compounds (R.E. = Eu, Yb, Lu) show - in addition to features from unoccupied 4f states - a narrow prominent peak 2.3 to 3.5 eV above EF, which is identified as due to a narrow R.E. d-band state typical for this class of materials. For EuPdq and YhPd 3 BIS spectra of 4f 7 and 4f 14 final states are reported, a~ energies in agreement with the 4f-configurational stability of these compounds.

The observed positions of the lowest hf_ multiplet state in EuPd 3 readily explains the previously observed surface-valence transition in this compound 7. For YbPd 3 the 4f14-groundstate level is shifted to higher energies, in agreement with stability calculations within Miedema's scheme s . The BIS experiments were performed with a modified VG-ESCA3 photoelectron spectrometer (base pressure low 10-10 torr), equipped in our laboratory with a specially designed electron gun, a Johann-type X-ray monochromator, and a photon detector 9. The electron g~m, based on an indirectly heated BaO cathode I°, is incorporated into the Faraday cup of the ESCA3 spectrometer. The X-ray monochromator, tuned to a photon energy of 1485 eV~ uses a spherically-bent quartz-(1010) crystal disc (0.3 mm thick) with lO0-mm diameter and a bending radius of 100 cm. Photons are detected with a channeltron via secondary electrons emitted from a CsI-coated gold photocathode. Typical counting rates for an electron current of 0.5 mA are 50 cts/s on a background of about 2 ets/s, with a total-system resolution of about 0.8 eV. The materials studied were prepared by At-arc melting and annealing, and their purity was checked by X-ray diffraction and MSssbauer spectroscopy (in the case of EuPd3). A diamond file was employed in order to prevent buildup even of minute quantities of oxygen on the surfaces. Fig. I shows BIS spectra obtained for the three R.E.Pd 3 samples studied. The BIS spectrum of LuPd~ is dominated by two peaks (designated A and B) at about 3 eV and 7 eV above EF, which we interpret as due to electronic transitions into empty Lu-Sd valence-band states (see below). In the least-squares fit procedure used these peaks were approximated by gaussian lines on an integral background, which in turn was set proportional to the integrated density of states below a certain energy. In addition a constant step at E F was added. This whole spectral function was convoluted by a gaussian to simulate the effects of instrumental resolution.

The early members of the transition metals (e.g. Sc, Y, La) as well as the rare-earth (R.E.) elements form highly stable and ordered binary compounds with palladium of the type MPd3, which crystallize in the simple cubic AuCu 3 structure I. Caused by a strong charge transfer from the M element to Pd and a reduction in the overlap of the M-d orbitals due to dilution with Pd, these compounds are expected to have an unusual electronic structure. As pointed out recently in several theoretical papers 2-~, the valence-band density of states of these compounds is characterized by a 2- to 3-eV wide bandgap separating the mostly Pd-4dlike states below E F from the predominantly M-d-like states above E F. Bremsstrahlung-Isochromat spectroscopy (BIS) s zs an ideal tool for studying the density of unoccupied electronic states. The only experimental data presently available for this class of materials are for CePd3, where the BIS spectrum, however, is further complicated by strong spectral features due to both 4f I- and 4f2-final states 6 . Particularly, a relatively narrow feature at an energy of ~2.3 eV above E F could not be explained in a satisfactory way, and was tentatively assigned to surfaceshifted Ce-4f states s. In the present work we present BIS spectra for EuPd3, YbPd3, and LuPd3, which exhibit quite similar structures at 2.3 to 3.5 eV above E F. Using the results of recent band-structure calculations for ScPd 3 and YPd 3 ~ we identify these structures as the image of the enhanced density of R.E.5d states above E F typical for these compounds. With the trivalent R.E. compounds EuPd 3 and YbPd 3 we can furthermore present BIS spectra for 4f7 and 4f 14 final-state configurations, which had not been observed previously.

On leave of absence from Tata Institute of Fundamental Research, Bombay 400005, India Present address: Institut de Physique, Universit~ de Neuch~tel, CH-2000 Neuch~tel, Switzerland 339

UNOCCUPIED ELECTRONIC STATES IN R.E.Pd3 COMPOUNDS

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Fig. I. BIS spectra of LuPd3, YbPd3, and EuPd 3 together with the results of a leastsquares fit analysis (solid lines). Transitions to local 4f states give rise to the dashed subspectra. The dash-dotted lines represent the sums of transitions to valenceband states and the inelastic background. The dashed-bar diagram indicates positions and relative intensities of the individual multiplet components of the 4f7 final-state multiplat. The cross-hatched areas represent positions~ widths, and relative intensities of the unoccupied R.E. d-band states (distinguished by A and B).

The BIS spectra observed for YbPd 3 (Fig. ]b) end EuPd 3 (Fig. Ic) are characterized by similarly structured empty-valence-band states superimposed on intense signals from transitions into unoccupied 4f states. Since the Yb ion is known to be in its trivalent ionic state in YbPd31 , a single line due to the IS o groundstate of the 4f 14 final-state configuration is observed in the BIS spectrum (dashed line in Fig. Ib). In bulk-trivalent " EuPd~ 1,7 the 4f 7 flnal stat e is split into a multlplet with an 8S7/2 groundstate, which is indicated in Fig. Ic by the dashed-bar diagram. The relative intensities of the individual multiplet components were set equal to the results of intermediate-coupling calculations for the 4f7 configuration of Tb 11,s, while the relative separations of these lines had to be decreased by a fitted factor of 0.873 relative to Th. This is in line with expectations in view of the difference in the atomic

Vol. 49, No. 4

numbers of Eu (Z=63) and Tb (Z=65). In the least-squares fits of the data presented in Figs. Ib end Ic, the 4f multiplet components were described by Doniaeh-~unij6 lineshapes 12. In this way a good agreement is obtained between data end fitted curves (solid lines). The dashed subspectra in Figs. Ib and Ic describe the 4f signals, while the dash-dotted subspectra represent again the sum of transitions into empty R.E.-5d valence-band states and the integral background. The cross-hatched areas in Fig. I represent the positions, widths, and relative intensities of these 5d valenceband features. Table I gives a summary of the leastsquares fit results. The quoted 4f binding energy E4f stands for the position of the lowest 4f multiplet line above E F. Assuming a totally screened final state, E4f corresponds to the energy required to form an isolated divalent R.E. ion dissolved in a matrix of the trivalent R.E.Pd~ solid. Following the concept of Johensson and M~rtensson 13 E4f is given by the energy E2, 3 necessary to transform the trivalent compound into a divalent one plus the heat of solution, Eimp, of a divalent molecule dissolved in the trivalent matrix: E4f~E2,3+Eimp. The first term results form the difference A in the heats of formation AH of the divalent and trivalent compolmd plus the 4f promotion energy P 8: E2,3=(AH2+-AH3+) + P. Using the semiempirical scheme of Miedema 8, we obtain A~21 kcal/mol for EuPd 3 14 and A~25 kcal/mol for YbPd 3. The 4f promotional energy is P~22 kcal/mol for Eu and P~IO kcal/mol for Yb 14,~5. We therefore arrive at E 2 3~-I kcal/mol for EuPd 3 and ~+15 kcal/mol (corresponding to 0.7 eV/atom) for YbPd 3. This means that YbPd 3 should be a rather stable trivalent compound, while EuPd 3 is highly unstable in its trivalent state leading to a valence transition to the divalent state at the surface 7. It is more difficult to estimate Eimp, since Miedema's scheme cannot simply be applied to dissolving molecules in a matrix. For R.E. metals the heat of solution of a divalent ion in a trivalent matrix (or vice versa) is Elms=0.5 eV. If we take this value for Eim p we arrlve at energies of the lowest 4f multiplet component in the BIS spectra: E4f~0.5 eV for EuPd 3 end E4fml.2 eV for YbPd 3. These theoretical 4f energies are in good agreement with our experimental results (see Table I).

Table I. Summary of least-squares fit results. Energies are in eV, relative to EF. E4f = energy o~ lowest 4f multiplet state; r = Doniach-Suni~8 linewldth parameter; Esd(A) and E5d(B) are energy positions of features A and B, respectively; HWHM5d represents the half width half maximum of the prominent 5d peak (A)

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7.2(4)

LuPd 3

. . . .

Vol. 49, No. 4

UNOCCUPIED ELECTRONIC STATES IN R.E.pd 3 COMPOUNDS

In ~able I we have also listed the Donlaeh-~unlje llnewldth parameter for the 4f final-state multiplet components resulting from the fit analysis. It is striking that F is more than twice as wide in the case of YbPd3 as compared to EuPd 3. Intrinsic. 4f linewidths in BIS spectra are determlned by flnalstate lifetimes, particularly due to Auger decay to lower accessable unoccupied valenceband states. Such transition rates depend on the number of occupied 4f and unoccupied valence-band states as well as on transition matrix elements 6 . The large linewidth observed for YbPd 3 as compared to EuPd 3 is therefore mainly caused by the much higher number of occupied 4f states in Yb. EuPd 3 has a divalent surface layer known from ultraviolet photoemission experiments 7. Within experimental accuracy, however, no Eu2+-4f 8 multiplet components are visible in the BIS spectrum of EuPd 3. These spectral features from the divalent surface layer would be expected at an energy of ~9 eV above E F with a total width of ~3 eV. The absence of such features is actually not surprising in view of the relatively large mean-free path of 1485-eV electrons as well as the high inelastic background and large linewidths expected in the BIS spectrum at these energies. The spectra in Fig. I together with the given analysis clearly demonstrate that the densities of unoccupied d-band states have a very similar structure in common in the three compounds studied. In all three cases we observe a double structure with a relatively narrow prominent peak (A) 2.3 to 3.5 eV above E F plus a weaker peak (B) at about 3-eV higher energy. These structures are readily explained by the results of recent self-consistent bandstructure calculations for ScPd 3 and YbPd 3 ~ In this work it was shown that the density of unoccupied valence-band states in YPd 3 (and similarly in ScPd3) is dominated by two re-

341

latively narrow Y-hd (Sc-3d) like peaks above EF, separated by about 2.5 eV, while the occupied states below E F are almost completely Pd-hd like. The unoccupied and occupied d-band states are separated by a bandgap of ~2.5 eV width. This electronic structure seems to be quite general for compounds of the early transition elements (like Sc, Y, La, and the R.E. elements) with late transition elements (like e.g. Pd). In these compounds a large charge transfer occurs from the R.E. element to Pd caused by the sizeable difference in electronegativity. This pushes the R.E. d-states far above E F. In addition, the reduction in d-d overlap by dilution of the R.E. ion through Pd leads to a narrowing of the bandwldth~ in analogy to the occupied states. Since due to the structure of the R.E.Pd 3 compounds the R.E.-5d orbitals cannot directly overlap in these compounds the observed relatively narrow d states above E F may also be considered within the framework of the Friedel-Anderson model as virtual-bound states above E F 16,1~ The present results provide also a straightforward explanation for the previously unexplained peak at about 2.3 eV above E F in the BIS spectrum of CePd 3 ~. It is due to the same intense and narrow peak in the density of unoccupied d-band states as observed in the present work for the three R.E.Pd 3 compounds studied. Acknowledgement - This work was supported by the Sonderforschungsbereich 6, TP AI, of the Deutsche Forschungsgemeinschaft. One of the authors (E.V.S) is obliged to the Alexandervon-Humboldt Foundation for financial support. The authors thank Prof. Y. Baer for technical advice in building the X-ray monoehromator, C. Lange for his help in building the photon detector and Dr. B. Perscheid for providing the EuPd 3 sample.

References I.

2.

3.

h. 5.

6. 7.

8.

W.E. Gardner, J. Penfold, T.F. Smith, and I.R. Harris, J. Phys. F: Metal Phys. 2~ 133 (1972). A.R. Williams, R. Zeller, V.L. Moruzzi, C,D. Gelatt, Jr., and J. K~bler, J. Appl. Phys. 52~ 2067 (1981). P. Turchi, F. Dueastelle, and G. Treglia, J. Phys. C: Solid-State Phys. 3~, 2891 (1982). C. Koenig, Z. Physik, B - Condensed Matter 50, 33 (1983). P.A. Cox, J.K. Lang, and Y. Baer, J. Phys. F: Metal Phys. 11~ 113 (1981); J.K. Lang, Y. Baer, and P.A. Cox, ibid. 11~ 121 (1981). Y. Baer, H.R. Ott, J.C. Fuggle, and L.E. DeLong, Phys. Rev. B24, 5384 (1981). V. Murgai, L.C. Gupta, R.D. Parks, N. Mgrtensson, and B. Reihl, in Valence Instabilities, ed. by P. Wachter and H. Boppart, North-Holland (1982), page 299. A.R. Miedema, J. Less-Common Met. 46~ 167 (1976).

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C. Laubschat, PhD-Thesis, Freie Universit&t Berlin (1983), unpublished. The BaO cathode was obtained from AEGTelefunken, part No. 371.506.153.GZ. F. Gerken, PhD-Thesis, Universit&t Hamburg (1983), unpublished. S. Doniach and M. Sunij6, J. Phys. C3~ 285 (1970). B. Johansson and N. M~rtensson, Phys. Rev. B2._!, 4472 (1980). A.R. Miedema, J. Less-Common Met. 466~ 67 (1976). F.R. DeBoer, W.H. Dijkman~ W.C.M. Martens, J. Less-Common Met. 644~ 241 (1979). J. Friedel, Nuovo Cimento Suppl. 7, 287 (1958); P.W. Andersen, Phys. Rev. 124, 41 (1961). S. H~fner, G.K. Wertheim, and J.H. Wernick, Solid-State Commun. J!, 1585 (1975).