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Photoemission study of Pd Zr and Cu Zr alloys
Solid State Communications, Vol. 35, pp. 493—495. Pergamon Press Ltd. 1980. Printed in Great Britain. PHOTOEMISSION STUDY OF Pd Zr AND Cu Zr ALLOYS P. Steiner, M. Schmidt and S. Huffner Fachbereich Physik, Universität des Saarlandes, 6600 SaarbrUcken, Germany (Received 19 March 1980 by B. Muhlschiegel) The XPS valence bands and core levels of the alloys Pd1_~Zr~ (0
THE INVESTIGATION of metallic glasses is of considerable current interest for three reasons. First one wants to study the influence of the lack of periodicity on the properties of a solid. Secondly this modification of a solid offers the possibility to study concentrations, which may not be accessible in the crystaffine form, Thirdly these compounds seem to offer interesting possibilities for certain applications. The electronic structure as revealed by XPS (X-ray photoemission spectroscopy) of UPS (ultraviolet photoemission spectroscopy) has so far been studied in a number of metallic glasses [1—8].The conclusions reached from these studies differ and we do not want to elaborate upon them. It seems however established that in all systems studied so far the d-density of states (DOS) at the Fermi-energy is reduced in the glass as compared to the pure metals. It seems still an open question whether this is caused by a smearing out of the states due to the glass state of the sample or whether the d-states retreat from the Fermi energy. Of particular interest is the study of Oelhafen et ci. [7] because they state in their work that “The present photoemission study on the metallic glasses Pd—Zr and Cu—Zr provides clear evidence for a new type ofd-band alloy”, This is indeed a very exciting fmding, worthy of further investigation. Oelhafen et al. [7] give a number of reasons for their postulation of a new type of d-band alloy, which we shall deal with later. The strongest evidence however is a very large shift of the Pd valence band and core levels in the alloy as compared to the pure metal. The present investigation presents XI’S measurements on Pd—Zr and Cu—Zr alloys in the crystalline form over the whole composition range not accessible In the glass state. The results are as follows: The XPS spectra of the crystalline samples are very similar to those of the glassy ones, giving if anything even larger chemical shifts. These chemical shifts although 493
admittedly large are by no means of an unusual magnitude for alloys and a system with even larger shifts will be presented. By studying the whole composition range, we will show that the alloy valence band DOS is typical of other d-metal alloys, which has before been successfully explained by the coherent potential approximation (CPA) [9—11].Finally it will be shown that a phenomenological theory developed by Miedema [12] to explain the cohesive properties of alloys can very successfully be used to interpret alloy valence band and core level shifts. Thus, although one cannot rule out the existence of a special type of bonding in the metallic glasses, the present experimental material does not necessitate its postulation. XI’S spectra were measured with a modified Hewlett Packard 5950 A spectrometer operating in the 10-11 torr pressure range. The samples were prepared by coevaporation of the constituents and their concentrations were determined from a careful analysis of the core level intensities. In all samples the core lines were single lines with no shoulders or even a second component visible, indicating that one was dealing with homogeneous alloys. The structure of the samples were checked by electron diffraction in an electron microscope, where clear interference fringes were visible, indicating the crystalline nature of the samples. Figure 1 shows XPS valence band spectra, corrected for background, of Pd~Zr1..~ (0 ~x ~ 1) alloys over the whole composition range. (Those for Cu~Zr1 -x alloys are similar and will therefore not be reproduced here). The Pd d-band shifts continuously with decreasing Pd concentration to larger binding energies. The same shifts are observed in the Pd 3d core lines. The same behaviour is found for the Cu~Zr1...~ system. The spectra of Fig. 1 are typical of d-metal alloys [13—19] in that upon dilution of a constituent its band looses structure and intensity and fmally in the very dilute limit only the virtual bound state remains, —
494
PHOTOEMISSION STUDY OF Pd Zr AND Cu Zr ALLOYS
Vol. 35, No. 6
Pd, Zr
1.,
XPS
XPS and UPS Va’ence Band Spectra
Valence Bands
Background Subtracted
/
.1
2
Pd J
.c-’.
/ -~
I
.~
I .1
.
I’~.~.__
~
U’
t
Pd~Zro70—c~.-.
j
xPs
‘-1
2
Pd~302r070—c..
-~
UPS
:1 •1
c
•1
~80.
:.\~
L.)
:.
____
___ -~
•N
C-) 0 I•’...___.
S
Cu0~Lr040 ~
_____
~--~------—
X~0.30
.../
\~-
~
1
E
X.U.4~
E a
______
In
:
I/
.-
wo ~
/
-0.63
~,
aI
•1
/
.c
•1
~.
0
10
8
6
4
2
0
Binding Energy leVi ________________________________________________
x~0032
12
I
Fig.a typical 2. Comparison of Cu UPSZrspectra [7]their andcrystalline XPS spectra of Pd Zr and alloy, in (XPS) and metglass (UPS) state; the spectra are normalized at the Zr d-band.
~
8
4
Binding Energy [eV]
0
Fig. 1. XPS valence bands of Pd~Srj_~ alloys. The spectra for x ~ 0.63 are normalized to the same Zr content at EF.
oxidizing the samples slightly. From Figs. 1 and 2 we conclude that with respect to the shape of the valence bands there is nothing unique to the glassy samples. In Fig. 3 a few selected examples of Pd and Cu virtual bound states are shown. This is to indicate that
which of course in this case is obscured for Zr by the broad Pd band but is clearly visible for Pd on the chute Pd side. These spectra are similar to those presented by Oelhafen eta!. [7] as can be seen from Fig. 2 where such a comparison is performed. Quite arbitrarily the spectra have been normalized to each other at the peak of the Zr d-band. The difference in intensity is caused by different photoabsorption cross sections for XPS and UPS. If anything, the Pd and Cu bands show slightly lower binding energies in the crystalline state compared to the glassy one, which is in accordance with naive expectations attributing a stronger binding to the stable crystalline state as compared to the metastable glass state. The extra peak in the UPS spectra at about 6 eV binding energy is due to a slight oxidation of the samples in the UPS measurements, which could be verified in the present measurements by voluntarily
also the chemical shift of Pd Zr and Cu Zr is not unique, because those of Pd Al and Cu Al are even larger. The following points are given by Oelhafen et a!. [6] to show the uniqueness of the Pd~Zr1_~ and the Cu~Zrj -x alloys. (I) The integrity of the d-bands in the alloys; (II) a shift of both d-bands with respect to the Fermi energy; (III) a shift of the d-bands predominantly on one side of the d-bands; (IV) a change of shape of the d-bands upon alloying; (V) a reduction in the DOS near the Fermi energy of the alloys; (VI) a large core level shift of Pd. We note that all these fmdings also hold~for crystalline Pd~Zrj_~ and Cu~Zr1.~ alloys but also and more
Vol. 35, No. 6
PHOTOEMISSION STUDY OF Pd Zr AND Cu Zr ALLOYS
Virtual Bound States from XPS Difference Spectra i~
Pd~
I
~
PdZr
B
12’/.Pd
alloys are very similar. Furthermore the trend of alloy binding energy shifts can be calculated from the heats
I
of solution employing a Bom—Haber cycle.
I
a
• ,.~.~
CuZL
I I I I
• •
— —
I ,.-
•
~
cn
I
~ i
~
.
PdAI
CoAL
I I
30/.Pd
~4__
I
:
~
I
I
-
-
~
-
~
.1, 4
In conclusion it has been shown that photoelectron spectra of glassy and crystalline Pd~Zr1—x and Cu~Zr1 —x
I
•
_
0
8
“
Acknowledgements We thank Prof. H. Gleiter for performing the structure determination of the samples used in this work. This work was supported by the Deutsche Forschungsgemeinschaft. —
I
I, °
Binding Energy LeVI Fig. 3. Virtual bound states of Pd Ag, Pd Zr, Pd Al, Cu Zr and Cu Al as obtained from XPS difference spectra, where the contributions of the pure host metals have been subtracted. Table 1. XPS—core line shifts ofthe 2p lines of Cu and of the 3d lines of Zr and Pd in different dilute alloys. = E~0~ ~ The calculations were performed using the semi-empirical heats ofsolution from Miedema [12]. _____________________________________________
REFERENCES 1. 2. 3. 4. 5.
—
*
System ~ eV Ec~~ [eV] PdAl 2.15 2.44 Pd Zr 1.65 2.62 PdAg 035 0.46 Cu Al 0.80 0.78 Cu Zr 0.35 —0.24 ZrPd 1.15 1.18 Zr Cu 0.85 0.98 _________________________________________ *
495
The accuracy of the experimental data is ±0.10eV.
importantly to standard d-metal alloys like CUxNiO...X and Ag~Pd1_~. Thus it does not seem necessary on the basis of the experimental material available so far to postulate a new type of d-metal alloy. It seems worthwhile to elaborate upon the chemical shifts in the alloys studied here and in general. Employing the heats of solution determined by Miedema [12] and using a Bom—Haber cycle one can calculate the binding energies of metals [20] and also their shifts in dilute alloys ZB; the final state of the photoemission process is represented in the dilute alloy by (Z + 1 )B where Z and (Z + 1) are the atomic numbers of the dilute elements. Table I gives the results relevant to this work and one can see a good agreement between the ~. culated and measured binding energy shifts [21]. ThiS indicates that Pd~Zr1~ and CuxZri_x are by no means unique.
6. 7. 8. 9. 10. 11.
S.R. Nagel & J. Tauc,Ploys. Rev. Lett. 35, 380 (1975). S.R. Nagel, G.B. Fisher, J. Tauc & B.G. Bagley, Ploys. Rev. B13, 3284 (1976). S.R. Nagel, J. Tauc & B.C. Giessen, Solid State Commun. 22,471(1977). J.D. Riley, L. Ley, J. Azoulay & K. Terakura, PlOys. Rev. B20, 776 (1979). p. Terzieff, K. Lee & N. Heiman, J. App!. Phys. 50,1031(1979). P. Oelhafen, M. LAard, H.-J. Guntherodt, K. Berresheim & H.D. Polaschegg, Solid State Commun. 30, 641 (1979). P. Oelhafen, E. Hauser, H.-J. Güntherodt & K.H. Bennemann, Ploys. Rev. Lett. 43, 1134 (1979). See also Y. Takasu, R. Unwin, B. Tesche & A.M. Bradshaw, Surf ScL 77, 219 (1978). 5. Kirkpatrick, B. Velicky & H. Ehrenreich, Phys. Rev. Bi, 3250 (1970). G.M. Stocks, R.W. Williams & J.S. Faulkner, J.Phys.F3,l688(1973). G.M. Stocks, R.W. Williams & J.S. Faulkner,
Ploys. Rev. B4, 4390 (1971). A.R. Miedema, J. Less Common Metals 46, 67 (1976). 13. P.O. Nilsson,Phys. Kondens. Mater. 11, 1(1970). 14. D.H. Seib & W.E. Spier, Ploys. Rev. B2, 1976 (1970). 15. S. Hufner, G.K. Wertheim and J.H. Wernick, Phys. Rev. B8, 4511(1973). 16. rC.Norris&H.P.Myers,J.Phys. Fl,62 (1971). 17. K.Y. Yn, C.R. Helms, W.E. Spicer & P.W. Chye, Ploys. Rev. B15, 1629 (1977). 18. A.D. McLachlan, J.G. Jenkin, R.C. G. Leckey & J. Liesegang,J. Phys. F5, 2415 (1975). 19. P. Steiner, H. Hochst & S. Hufner, J. Ploys. F7, L145 (1977). 20. N. Martensson & B. Johansson, Solid State Commun. 32, 791 (1979). 21. A detailed comparison of XPS line shifts for a large number of dilute alloys with the semiempirical calculations heat solutions of Miedema [12] will of be the given in aofforthcoming communication. 12.
THE INVESTIGATION of metallic glasses is of considerable current interest for three reasons. First one wants to study the influence of the lack of periodicity on the properties of a solid. Secondly this modification of a solid offers the possibility to study concentrations, which may not be accessible in the crystaffine form, Thirdly these compounds seem to offer interesting possibilities for certain applications. The electronic structure as revealed by XPS (X-ray photoemission spectroscopy) of UPS (ultraviolet photoemission spectroscopy) has so far been studied in a number of metallic glasses [1—8].The conclusions reached from these studies differ and we do not want to elaborate upon them. It seems however established that in all systems studied so far the d-density of states (DOS) at the Fermi-energy is reduced in the glass as compared to the pure metals. It seems still an open question whether this is caused by a smearing out of the states due to the glass state of the sample or whether the d-states retreat from the Fermi energy. Of particular interest is the study of Oelhafen et ci. [7] because they state in their work that “The present photoemission study on the metallic glasses Pd—Zr and Cu—Zr provides clear evidence for a new type ofd-band alloy”, This is indeed a very exciting fmding, worthy of further investigation. Oelhafen et al. [7] give a number of reasons for their postulation of a new type of d-band alloy, which we shall deal with later. The strongest evidence however is a very large shift of the Pd valence band and core levels in the alloy as compared to the pure metal. The present investigation presents XI’S measurements on Pd—Zr and Cu—Zr alloys in the crystalline form over the whole composition range not accessible In the glass state. The results are as follows: The XPS spectra of the crystalline samples are very similar to those of the glassy ones, giving if anything even larger chemical shifts. These chemical shifts although 493
admittedly large are by no means of an unusual magnitude for alloys and a system with even larger shifts will be presented. By studying the whole composition range, we will show that the alloy valence band DOS is typical of other d-metal alloys, which has before been successfully explained by the coherent potential approximation (CPA) [9—11].Finally it will be shown that a phenomenological theory developed by Miedema [12] to explain the cohesive properties of alloys can very successfully be used to interpret alloy valence band and core level shifts. Thus, although one cannot rule out the existence of a special type of bonding in the metallic glasses, the present experimental material does not necessitate its postulation. XI’S spectra were measured with a modified Hewlett Packard 5950 A spectrometer operating in the 10-11 torr pressure range. The samples were prepared by coevaporation of the constituents and their concentrations were determined from a careful analysis of the core level intensities. In all samples the core lines were single lines with no shoulders or even a second component visible, indicating that one was dealing with homogeneous alloys. The structure of the samples were checked by electron diffraction in an electron microscope, where clear interference fringes were visible, indicating the crystalline nature of the samples. Figure 1 shows XPS valence band spectra, corrected for background, of Pd~Zr1..~ (0 ~x ~ 1) alloys over the whole composition range. (Those for Cu~Zr1 -x alloys are similar and will therefore not be reproduced here). The Pd d-band shifts continuously with decreasing Pd concentration to larger binding energies. The same shifts are observed in the Pd 3d core lines. The same behaviour is found for the Cu~Zr1...~ system. The spectra of Fig. 1 are typical of d-metal alloys [13—19] in that upon dilution of a constituent its band looses structure and intensity and fmally in the very dilute limit only the virtual bound state remains, —
494
PHOTOEMISSION STUDY OF Pd Zr AND Cu Zr ALLOYS
Vol. 35, No. 6
Pd, Zr
1.,
XPS
XPS and UPS Va’ence Band Spectra
Valence Bands
Background Subtracted
/
.1
2
Pd J
.c-’.
/ -~
I
.~
I .1
.
I’~.~.__
~
U’
t
Pd~Zro70—c~.-.
j
xPs
‘-1
2
Pd~302r070—c..
-~
UPS
:1 •1
c
•1
~80.
:.\~
L.)
:.
____
___ -~
•N
C-) 0 I•’...___.
S
Cu0~Lr040 ~
_____
~--~------—
X~0.30
.../
\~-
~
1
E
X.U.4~
E a
______
In
:
I/
.-
wo ~
/
-0.63
~,
aI
•1
/
.c
•1
~.
0
10
8
6
4
2
0
Binding Energy leVi ________________________________________________
x~0032
12
I
Fig.a typical 2. Comparison of Cu UPSZrspectra [7]their andcrystalline XPS spectra of Pd Zr and alloy, in (XPS) and metglass (UPS) state; the spectra are normalized at the Zr d-band.
~
8
4
Binding Energy [eV]
0
Fig. 1. XPS valence bands of Pd~Srj_~ alloys. The spectra for x ~ 0.63 are normalized to the same Zr content at EF.
oxidizing the samples slightly. From Figs. 1 and 2 we conclude that with respect to the shape of the valence bands there is nothing unique to the glassy samples. In Fig. 3 a few selected examples of Pd and Cu virtual bound states are shown. This is to indicate that
which of course in this case is obscured for Zr by the broad Pd band but is clearly visible for Pd on the chute Pd side. These spectra are similar to those presented by Oelhafen eta!. [7] as can be seen from Fig. 2 where such a comparison is performed. Quite arbitrarily the spectra have been normalized to each other at the peak of the Zr d-band. The difference in intensity is caused by different photoabsorption cross sections for XPS and UPS. If anything, the Pd and Cu bands show slightly lower binding energies in the crystalline state compared to the glassy one, which is in accordance with naive expectations attributing a stronger binding to the stable crystalline state as compared to the metastable glass state. The extra peak in the UPS spectra at about 6 eV binding energy is due to a slight oxidation of the samples in the UPS measurements, which could be verified in the present measurements by voluntarily
also the chemical shift of Pd Zr and Cu Zr is not unique, because those of Pd Al and Cu Al are even larger. The following points are given by Oelhafen et a!. [6] to show the uniqueness of the Pd~Zr1_~ and the Cu~Zrj -x alloys. (I) The integrity of the d-bands in the alloys; (II) a shift of both d-bands with respect to the Fermi energy; (III) a shift of the d-bands predominantly on one side of the d-bands; (IV) a change of shape of the d-bands upon alloying; (V) a reduction in the DOS near the Fermi energy of the alloys; (VI) a large core level shift of Pd. We note that all these fmdings also hold~for crystalline Pd~Zrj_~ and Cu~Zr1.~ alloys but also and more
Vol. 35, No. 6
PHOTOEMISSION STUDY OF Pd Zr AND Cu Zr ALLOYS
Virtual Bound States from XPS Difference Spectra i~
Pd~
I
~
PdZr
B
12’/.Pd
alloys are very similar. Furthermore the trend of alloy binding energy shifts can be calculated from the heats
I
of solution employing a Bom—Haber cycle.
I
a
• ,.~.~
CuZL
I I I I
• •
— —
I ,.-
•
~
cn
I
~ i
~
.
PdAI
CoAL
I I
30/.Pd
~4__
I
:
~
I
I
-
-
~
-
~
.1, 4
In conclusion it has been shown that photoelectron spectra of glassy and crystalline Pd~Zr1—x and Cu~Zr1 —x
I
•
_
0
8
“
Acknowledgements We thank Prof. H. Gleiter for performing the structure determination of the samples used in this work. This work was supported by the Deutsche Forschungsgemeinschaft. —
I
I, °
Binding Energy LeVI Fig. 3. Virtual bound states of Pd Ag, Pd Zr, Pd Al, Cu Zr and Cu Al as obtained from XPS difference spectra, where the contributions of the pure host metals have been subtracted. Table 1. XPS—core line shifts ofthe 2p lines of Cu and of the 3d lines of Zr and Pd in different dilute alloys. = E~0~ ~ The calculations were performed using the semi-empirical heats ofsolution from Miedema [12]. _____________________________________________
REFERENCES 1. 2. 3. 4. 5.
—
*
System ~ eV Ec~~ [eV] PdAl 2.15 2.44 Pd Zr 1.65 2.62 PdAg 035 0.46 Cu Al 0.80 0.78 Cu Zr 0.35 —0.24 ZrPd 1.15 1.18 Zr Cu 0.85 0.98 _________________________________________ *
495
The accuracy of the experimental data is ±0.10eV.
importantly to standard d-metal alloys like CUxNiO...X and Ag~Pd1_~. Thus it does not seem necessary on the basis of the experimental material available so far to postulate a new type of d-metal alloy. It seems worthwhile to elaborate upon the chemical shifts in the alloys studied here and in general. Employing the heats of solution determined by Miedema [12] and using a Bom—Haber cycle one can calculate the binding energies of metals [20] and also their shifts in dilute alloys ZB; the final state of the photoemission process is represented in the dilute alloy by (Z + 1 )B where Z and (Z + 1) are the atomic numbers of the dilute elements. Table I gives the results relevant to this work and one can see a good agreement between the ~. culated and measured binding energy shifts [21]. ThiS indicates that Pd~Zr1~ and CuxZri_x are by no means unique.
6. 7. 8. 9. 10. 11.
S.R. Nagel & J. Tauc,Ploys. Rev. Lett. 35, 380 (1975). S.R. Nagel, G.B. Fisher, J. Tauc & B.G. Bagley, Ploys. Rev. B13, 3284 (1976). S.R. Nagel, J. Tauc & B.C. Giessen, Solid State Commun. 22,471(1977). J.D. Riley, L. Ley, J. Azoulay & K. Terakura, PlOys. Rev. B20, 776 (1979). p. Terzieff, K. Lee & N. Heiman, J. App!. Phys. 50,1031(1979). P. Oelhafen, M. LAard, H.-J. Guntherodt, K. Berresheim & H.D. Polaschegg, Solid State Commun. 30, 641 (1979). P. Oelhafen, E. Hauser, H.-J. Güntherodt & K.H. Bennemann, Ploys. Rev. Lett. 43, 1134 (1979). See also Y. Takasu, R. Unwin, B. Tesche & A.M. Bradshaw, Surf ScL 77, 219 (1978). 5. Kirkpatrick, B. Velicky & H. Ehrenreich, Phys. Rev. Bi, 3250 (1970). G.M. Stocks, R.W. Williams & J.S. Faulkner, J.Phys.F3,l688(1973). G.M. Stocks, R.W. Williams & J.S. Faulkner,
Ploys. Rev. B4, 4390 (1971). A.R. Miedema, J. Less Common Metals 46, 67 (1976). 13. P.O. Nilsson,Phys. Kondens. Mater. 11, 1(1970). 14. D.H. Seib & W.E. Spier, Ploys. Rev. B2, 1976 (1970). 15. S. Hufner, G.K. Wertheim and J.H. Wernick, Phys. Rev. B8, 4511(1973). 16. rC.Norris&H.P.Myers,J.Phys. Fl,62 (1971). 17. K.Y. Yn, C.R. Helms, W.E. Spicer & P.W. Chye, Ploys. Rev. B15, 1629 (1977). 18. A.D. McLachlan, J.G. Jenkin, R.C. G. Leckey & J. Liesegang,J. Phys. F5, 2415 (1975). 19. P. Steiner, H. Hochst & S. Hufner, J. Ploys. F7, L145 (1977). 20. N. Martensson & B. Johansson, Solid State Commun. 32, 791 (1979). 21. A detailed comparison of XPS line shifts for a large number of dilute alloys with the semiempirical calculations heat solutions of Miedema [12] will of be the given in aofforthcoming communication. 12.