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The bremsstrahlung isochromat spectra of d0 transition-metal oxides
Pergamon
Solid State Communications.
003% i~X(Y4)oo407-2 THE BR&~lSSTRAHLUNG
ISOCHROMAT
TRANSITION-METAL
Vol. Yl, No. 7, pp. 551-554. lYY4 Elsevia Science Ltd Printed in Great Britain 003x-10Yx/Y4 57.00+.00
SPECTRA OF d’
0XlDF.S
L Soriano’, M. Abbateb. D Alders’ and J M Sanz’ ’ Dept Fis’ca Aplicada, Institute Universitario N~colas Cabrera, Universidad Autonoma de Madrid Cantoblanco. E-28039 Madrid, Spain ’ Sol’d State Spectroscopy. University of N’jmegen, Toemooiveld. NL-6525 ED Nijmegen, The Netherlands
’ Materials Science Centre. University of Gronmgen. Nijenborgh 4. NL-9747 AG Groningen, The Netherlands (Received (accepted
20 May 1994 by F. Yndurain) for publication
I June IYY4)
We present and d’scuss the bremsstrahlung isochromat spectra (BIS) of four do transition-metal ox’des; namely ZrO,. HfO,. Nb,O,, and T%O, The spectra are related to the dennty of unoccupied states in the conduct’on band. They give directly the magnitude of the crystal-field sphttmg and the dispersion of the metal-d bands. The trends observed in the spectra are discussed and compared to those found in the 0 Is xray absorption spectra (XAS) of the same materials. Keywords:
A. thin films, A. insulators,
D. electronic
band structure,
ligand fields. E. X-ray and y-ray spectroscopies I lntroduct’on
D. crystal and
in these materials.’ Tbts appears experimentally confirmed in a recmt study of the 0 Is XAS spectra of these oxides.’ Other experimental studaes include valence band XPS and resonant photoemission (RPES) measurements on ZrOz.“~” In the BIS process. an electron of tunable kinetic mergy pmetrates into a solid and decays to an unoccupied electronic state by emitting a photon.” Therefore. a BIS spectrum gives the number of photons emitted with a given energy (‘sochromat mode) as a function of the kinetic energy of the mcoming electron. No simple selection rules apply to this process because the incidmt electron does not have a well defined angular quantum number f The shape of the spectra are related to the unoccupied density of states modulated by the appropriate matrix elemmt”. In general. these matrix elements depmd very strongly on both, the element and the angular quantum number.” and must be taken into account when the BIS spectra are compared with band-structure calculations. Another effect which can influence the BIS spectra is the mergy losses of the mcoming electrons. This effect is equivalent to a convolut’on with the characteristic spectrum of losses (response function) and may cause a significant redistnbution of the spectral weight.‘6 For the oxides cons’dered here. the main characteristic losses occur at merp’es 20-30 eV below the elastic peak.“.” therefore we do not expect any s’gnificant influmce on the BIS spectra shown m this work.
The purpose of this paper is to investigate the unoccupied electronic stata of several do trms’tion-metal oxides. To this end. we present and discuss the bremsstrahlung isochromat spectra (BIS) of ZrO,. HfO,. Nb,O, and TqO,. These spectra provide very useful information on the density of unoccupied states in the conduction band. In particular. the BIS spectra give directly the magnitude of the crystal-field splitting and the d’spersion of the d bands. The related 0 Is x-ray absorption spectra (XAS) of the same materials were already d’scussed in a recent paper’. The comparison of the BIS and 0 Is XAS spectra suggests that the core-hole potential in the XAS spectra could be affecting the position of the peaks. The materials studied in thus work are very ‘mponmt m m;ny different fields and find many technolog’cal applications because of their unique physical and cherrncai properties. For instance, ZrO, and HfO, are often used as high performance ceramic matenals. ZrO, is also an ionic conductor which founds applicat’ons in gas detectors and electro-optics. Nb,O, and TqO, are well known as Selectric materials. Nb,O, is also used in Josephson junctions as tunnel-barrier.a’ The electronic structure of these transition-metal oxides has been studied very seldom from both. the exper’mmtal and theoretical viewpo’nts. In fact. most of those studies refer to ZrO, due to its relat’vely s’mpler structure The theoretrcal studies include several bandstructure’J and cluster-modele’0 calculations These studms Indicate a significant covalmt contribut’on to the bonding
2 Experimental
Details
The BIS spectrometer used ‘n this work was similar to that designed by Lang and Baer’? The BIS spectra 551
552
BREMSS-IRWLUNGISOCHROMATSPECTRA
BIS
BIS
.-. : ..
.-‘. i
a) 230,
Vol. 91,No.7
3 Nb,O,
-‘..,,
/
.
‘L... ‘.
..._._.e_---..
. .
-c ..
,‘-
..4
/d
‘e_-”
_..’
,..-.’ . b) Hfo,
.s..:. . .
.--. k.
:
b) $0,
*‘....
I
_A--
_-
_4-
:.
/
.._
L-c‘-
_.:
... 1
0
5
8
,
15
10
20
1
I
I
I
I
8
0
5
10
15
20
25
ENERGY ABOVE E, (ev)
ENERGY ABOVE E, (ev) L
Figure I:
Figure 2:
The BIS spectra of ZrO, and HfO,
were taken in the isochromat mode at a photon energy of approximately 1486.6 eV. The monochhromator contains 30
mA/cm’.
The BIS spectra of Nb,O,
and TqO,
More details on the preparation procedure can be
quartz crystals arranged in a modified Johansson geometry The overall energy resolution was approximately I. I eV.
found elsewhere.“” These anodic oxide films are particularly well suited for this experiment because both. the stoichiometry and the oxide film thickness can be
The energy scale of the spectra was cahbrated by using the Fermi level of a reference silver sample. All the spectra
oxide films are far less sensitive to the damage caused by
were normalized to the maximum peak intensity. The samples were anodic oxtde films (300
A)
grown on the corresponding metal substrate. Zr. Hf. Nb and Ta fotls were mealed. decarburized. deoxidized and
easily controlled.
the electron beam than thermally oxidized films. On the other hand, bulk oxides of these large band gap insulators cannot be used because they present severe charging effects.
degreased before they were submitted to anodizatton. The Nb wd Ta foils were anodized in a 0. I molar oxalic acid electrolyte
at a constatt current density of I-2 mA/cm*.
The Zr rmd Hf foils were anodized m a 30 g/l ammoma ptntaborate electrolyte at a constant current density of 2
3 Results and Discussion a) BIS Spectra Figures I and 2 show the BIS spectra of ZrO, and HfO, and of Nb,O, and TarO, respectively. The spectra are
Table I. Absorption threshold referred to the Fermi level (E,). sphttmg (M)
In addition. we found that the anodic
disperston of the metal d bands (Co). crystal-field
from the BIS spectra and from the XAS spectra (Ref.
I). relative cross-sectron of the nd and (n+/)sp
bands (u,,&J,,,.,,,). and band gSfX from the hterature (E,) for the respective oxides. References for Es are given in brackets. All energy values are given in eV.
Compound
E*
f”
Ad (BIS)
Ad (XAS)
ZrOz
3.0
5.9
3.5
3.1
II.9
4.5 [24]
HfO*
3.7
64
4.0
3.7
25.0
5.7 [ZS]
Nb,O,
I.5
68
4.2
3.8
18.0
3.5 1261
-J-a@,
2.0
7.6
46
47
25.9
4.2 [27]
Q”&”
. I ,I
Es
vol.+91.No. 7
BREMSSTRAHLUNG
normalized to the maximum peak intensity ad the energy scales are referred to the Fernu level of a silver sample. We note first that the BIS spectra of these mater& present some general features in common. In paft~cular. all the spectra exhibit two strong bumps just above threshold (up to =I0 eV ) and a broad structure at higher energies (above IO eV). As mentioned above. the shape of each spectra is related to the correspondmg unoccupied electronic states in the conduction band, however as the 0 2p states are expected to be almost completely occupied. the observed spectrum is mainly determined by the transition-metal d md sp states The two strong bumps above threshold are attributed to 4d bands in ZrOz and Nb,O, and to 5d bands in the case of HfO, ;md T+O,. The broad structures at higher energies are assigned to transition-metal sp bands. The sp bands appear at higher energres as a consequence of the larger overlap with the 0 2p levels which results III stronger anttbondmg mteractlons. The splitting of the d bands observed in the BIS spectra IS caused by crystal-field effects The magnitude of the crystal-field splitting (AcI) IS very sensitive to the hgand coordination. Therefore. the values of Ad taken from the BIS spectra should be compared only for those oxides with the same ligand coordination. In our case, ZrOz and HfO, have a distorted fluorite structure with a pseudocubic ligand coordinatmn” whereas Nb,O, and T%O, have a complex crystallographic structure with a distorted octahedral oxygen coordinatton *‘The correylondmg values of Ad as dctermmed from the spectra of Figs I and 2 are glvcn in Table I. Accordmg to the expected larger overlap of the 5d orbltals with the 0 2p states which results m stronger Interactions. it IS observed that the crystal-field sphttmg IS larger for the 5d transmon-metal oxldcs The dispersion of the d band (r,,.) measured as the FWHM of the band. IS also given m Table I. The mam contrtbution to it IS given by mdlrect mteractlons through the ligands Therefore. the larger value of r, for the heavier transltion-metal oxides (cf Table I) appears agam to be caused by stronger interactions with the ligands. Since the BIS spectra shown in Figs. I and 2 are refened to the Fermi level. the absorption threshold (E,), measured as the energy at half of the maximum of the jump, gives directly the energy posItton of the bottom of the conduction band. The values of E, have also been collected in Table I They are consistent with the experimental values of the band gap (E,) found in the literature for these oxide?‘” (cf Table I) Finally. we would like to pomt out the Importance of these parameters (Ad, r, and E,) obtained from the BIS spectra for the understanding of the electronic structure of these oxides In fact. these parameters are vety difftcult to determme using more conventIonal techniques like optical spectroscopy. Table I includes also the relative cross-section of the d and sp states (q/u ,“.,,,) calculated by ustng the photoionlzatlon cross-sectlons given by Y eb and Lindau”. The contrlbutlon of the metal p states to the cross-sectlons IS very small and can be neglected In a first approxlmatlon The large values of u&,~.,,, (cf Table I) mdtcate that the contribution of the sp bands to the BIS spectra IS. at least. one order of magnitude weaker than that of the d bands 731s means that the contribution of the sp bands from the
ISOCHROMAT
553
SPECTRA
metal to the broad sbucture at high energies in the BIS spectra (cf. Table I) is very small for all the oxides. In fact. this spectral region is mostly due to the background coming from the adjacent d bands b) BJS versus XAS Figure 3 compares the Nb,O, BIS spectrum of Fig. Zw&theOIsXASspectrumreportedinRef l.TheO Is XAS spectra of transition-metal oxides reflect the unoccupied states of 0 2p character which are mlxed in the conduction band”. In fact. the double feature near the threshold is attributed to the contribution of the 0 2p states hybridized with the Nb 4d bands. whereas the broad feature at high energy corresponds to those 0 2p states hybridized with Nb 5sp bands.’ Therefore, the remarkable similarity between the BIS and 0 Is XAS spectra is not unexpected. The d bands are better reproduced by the 0 Is XAS spectra because the energy resolution (0 25 eV) IS much better ln addition, the sp bands are more clearly observed in the 0 Is XAS spectra. Both spectra reproduce the band structure of the unoccupied states but. whereas BIS shows mamly the contribution of the d bands from the cation due to the cross section values. the 0 Is XAS spectrum pomts out the 0 tp character of those bands due to the selection Nk3 which affect the process A similar comparison between the XAS and BIS spectra can be done for the other oxldes The values of the crystal-field sphtting (w obtained from the 0 Is XAS spectra (Ref. I) are also given In Table I for comparison W&I those estimated from BIS. The XAS values of Ad result slightly smaller than
1
Nb*os ;’ ‘.
BIS
‘. ‘.-...,, ‘..
.;..
‘._ . .
I
Figure 3:
,,.,._. .‘.--
...--..., ,,..... .......-
... .... .
,;” ‘x’
..,.” ,...
I
The 0 Is XAS and BIS spectra of Nb,O,
554
Vol. 91. No. 7
BREMSSTRAHLUNG ISOCHROMAT SPECTRA
those obtained from the BIS spectra The discrqanaes could
be due to the influence
of the 0 Is core-hole
pot&al in the XAS spectra”” As no core-level is involved m the BIS process, this effect cannot play a role in the BIS spectra. Therefore.
the values of Ad obtained
each group The mergy of the respective absorption threshold is conslstcnt with the magnitude of the bad gaps. The
comparison
of BIS
aad 0 Is XAS
useful mformatlon (ti. r+d structure of these mater&..
from the BIS spectra should be preferred III 011s case.
spectra
suggests that the core-hole potmtial could be a&ctmg the peak posmons. In conclusion. the BIS spectra provide very E,)
on the electronrc
4 Summary sod Conclusions In summary. we have presented and discussed BIS spectra of ZrO,, HfO*, Nb,O, and TgO,. The spectra are interpreted in terms of the contribution of the cation d and sp bmds
to the
unoccupied
states. The
crystal-field
splitting and dispersion of the d bands have been determined. both increase with the atomic number Z within
Acknowledgements-
This work was partially supported by
Fundamenteel Onderzoek der Materie (FOM),
Schakunde
Onderzoek Nederland (SON), the European Union Human Capital and Mobility program under contract # ERBCHRXCT930358 and the CICYT of Spain under contract MAT9310805.
References
I L. Soriano, M. Abbate. J.C. Fuggle. M.A. Jtmtiez, Sanz. C.S. Mythen and Commun. 87,699 (1993). 2 P A Cox. m Tmsrrlon Oxford.
H.A. Padmore.
Solid
J.M. State
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1992).
3 V E Hennch and P. A. Cox. in The Surface Science of Metal
Oxides. (Cambridge
University
Press. Cambridge.
J.M. Sam. E. Elizalde and L. Gal&t. Surf.
I7 F. Yubero.
Sci. 237, I73 (1990). I8 J M Sanz. M.A. Bail6n. E. Elizalde and F. Yubero, J. Electron Spec. Relat. Phmom. 48, 143 (1989). I9 J K. Lang and Y. Baer. Rev. Sci. lnstnun. (1979).
51, 221
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20 Y. Baer, G. Busch and P. Cohn, Rev. Sci. Instrum. 46. 466 (1975).
4 F Zandiehnadcm, R.A. Murray and W Y. Ching. Physica B 150. I9 (1988).
21 L. Young, in Anodic New York. 1961).
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V.P. Zhukov, M.Y.
Khodos and V.A.
22 S Hofmann
Oxide Films (Academic
and J M. Saz,
Press.
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Guh‘anov. Phys. Stat. Sol. (b) 160. 517 (1990).
Techniques
6 R. Orlando, C. Pisam. C. Roctti and E. Stefanovich, Phys. Rev B 45. 592 ( 1992)
(Clarendon.
7 L. Sormno. M
24 R.S. Sokolova, Sov J. Appl. Phys. 41, 454 (1974).
Abbate. J Faber and J M
S.anz (to be
publrshcd) 8 tl J F Jansen. Phys. Rev B 43, 7267 M Mormaga, H Ad&i Chem Sohds 44. 301 (1983)
9
10 A B
Sobolev,
Khalmmov,
A.N
and M
Varaksin.
OA
J
Phys
Keda and A.P
Phys. Stat. Sol. (b) 162. I65 (1990).
I I C. Morant. J M 1391 (1992)
Sanz and L. Galti,
I3
JC
Fuggle
and
J E.
Phys. Rev. B 45.
lnglesfield,
in
Wells,
in
Oxford.
Sttuctuml
Inorganic
Unoccupred
1992)
I4 W Speier. J C. Fuggle. R. Zeller. B Ackermann, K. Szot. F U. Hillebrecht and M Campagna. Phys Rev B 30. 6921 (1984). I5 W Spewer. J C. Fuggle. P Durham. R Zeller. Blake and P Steme, J Phys C 21. 2621 (1988)
RJ
I6 K W. Goodman and V. E. Hennch, Phys. Rev. B 49. 4827 (1994).
Chemistry
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and S. Patia, Thin
26 M T. Duffy. C.C. Wang. A. Waxman and Zaininger. J. Electrochem. Sot. 116. 234 (1969)
K.H.
27 W.H. Knausenberger and R.N. Tauber. J. Electrochem. Sot. 120.927
It J.M Sanz. A.R. Gontilez-Elipe, A. Femtidez. D. Leinen. L. Gal&t. A Stampfi and A M. Bradshaw, Surf. Sci 307/309. 848 (1994)
Elecrrunrc Stares (Springer, Berlin.
A F.
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( 199 I). Tsukada.
23
I. 213 (1982183).
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F.M F. de Groat.
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Czy*k, M. Abbate and J.C. Fuggle. Phys. Rev. B 48. 2074 (1993). 30 P J W. Weijs. M.T. Czy*k.
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J.B Goedkoop. H. van Leuken. H.J M. Hendrix. R.A. de Groot. G. van der Laan, K.H.J. Buschow, G. Wiech and J.C Fuggle. Phys. Rev. B 41. 11899 (1990). 3 I M. Grlonl. M T. Czytik. F.M F. de Groot. J.C. Fuggle and B E. Watts. Phys. Rev B 39. 4886 (1989).
Solid State Communications.
003% i~X(Y4)oo407-2 THE BR&~lSSTRAHLUNG
ISOCHROMAT
TRANSITION-METAL
Vol. Yl, No. 7, pp. 551-554. lYY4 Elsevia Science Ltd Printed in Great Britain 003x-10Yx/Y4 57.00+.00
SPECTRA OF d’
0XlDF.S
L Soriano’, M. Abbateb. D Alders’ and J M Sanz’ ’ Dept Fis’ca Aplicada, Institute Universitario N~colas Cabrera, Universidad Autonoma de Madrid Cantoblanco. E-28039 Madrid, Spain ’ Sol’d State Spectroscopy. University of N’jmegen, Toemooiveld. NL-6525 ED Nijmegen, The Netherlands
’ Materials Science Centre. University of Gronmgen. Nijenborgh 4. NL-9747 AG Groningen, The Netherlands (Received (accepted
20 May 1994 by F. Yndurain) for publication
I June IYY4)
We present and d’scuss the bremsstrahlung isochromat spectra (BIS) of four do transition-metal ox’des; namely ZrO,. HfO,. Nb,O,, and T%O, The spectra are related to the dennty of unoccupied states in the conduct’on band. They give directly the magnitude of the crystal-field sphttmg and the dispersion of the metal-d bands. The trends observed in the spectra are discussed and compared to those found in the 0 Is xray absorption spectra (XAS) of the same materials. Keywords:
A. thin films, A. insulators,
D. electronic
band structure,
ligand fields. E. X-ray and y-ray spectroscopies I lntroduct’on
D. crystal and
in these materials.’ Tbts appears experimentally confirmed in a recmt study of the 0 Is XAS spectra of these oxides.’ Other experimental studaes include valence band XPS and resonant photoemission (RPES) measurements on ZrOz.“~” In the BIS process. an electron of tunable kinetic mergy pmetrates into a solid and decays to an unoccupied electronic state by emitting a photon.” Therefore. a BIS spectrum gives the number of photons emitted with a given energy (‘sochromat mode) as a function of the kinetic energy of the mcoming electron. No simple selection rules apply to this process because the incidmt electron does not have a well defined angular quantum number f The shape of the spectra are related to the unoccupied density of states modulated by the appropriate matrix elemmt”. In general. these matrix elements depmd very strongly on both, the element and the angular quantum number.” and must be taken into account when the BIS spectra are compared with band-structure calculations. Another effect which can influence the BIS spectra is the mergy losses of the mcoming electrons. This effect is equivalent to a convolut’on with the characteristic spectrum of losses (response function) and may cause a significant redistnbution of the spectral weight.‘6 For the oxides cons’dered here. the main characteristic losses occur at merp’es 20-30 eV below the elastic peak.“.” therefore we do not expect any s’gnificant influmce on the BIS spectra shown m this work.
The purpose of this paper is to investigate the unoccupied electronic stata of several do trms’tion-metal oxides. To this end. we present and discuss the bremsstrahlung isochromat spectra (BIS) of ZrO,. HfO,. Nb,O, and TqO,. These spectra provide very useful information on the density of unoccupied states in the conduction band. In particular. the BIS spectra give directly the magnitude of the crystal-field splitting and the d’spersion of the d bands. The related 0 Is x-ray absorption spectra (XAS) of the same materials were already d’scussed in a recent paper’. The comparison of the BIS and 0 Is XAS spectra suggests that the core-hole potential in the XAS spectra could be affecting the position of the peaks. The materials studied in thus work are very ‘mponmt m m;ny different fields and find many technolog’cal applications because of their unique physical and cherrncai properties. For instance, ZrO, and HfO, are often used as high performance ceramic matenals. ZrO, is also an ionic conductor which founds applicat’ons in gas detectors and electro-optics. Nb,O, and TqO, are well known as Selectric materials. Nb,O, is also used in Josephson junctions as tunnel-barrier.a’ The electronic structure of these transition-metal oxides has been studied very seldom from both. the exper’mmtal and theoretical viewpo’nts. In fact. most of those studies refer to ZrO, due to its relat’vely s’mpler structure The theoretrcal studies include several bandstructure’J and cluster-modele’0 calculations These studms Indicate a significant covalmt contribut’on to the bonding
2 Experimental
Details
The BIS spectrometer used ‘n this work was similar to that designed by Lang and Baer’? The BIS spectra 551
552
BREMSS-IRWLUNGISOCHROMATSPECTRA
BIS
BIS
.-. : ..
.-‘. i
a) 230,
Vol. 91,No.7
3 Nb,O,
-‘..,,
/
.
‘L... ‘.
..._._.e_---..
. .
-c ..
,‘-
..4
/d
‘e_-”
_..’
,..-.’ . b) Hfo,
.s..:. . .
.--. k.
:
b) $0,
*‘....
I
_A--
_-
_4-
:.
/
.._
L-c‘-
_.:
... 1
0
5
8
,
15
10
20
1
I
I
I
I
8
0
5
10
15
20
25
ENERGY ABOVE E, (ev)
ENERGY ABOVE E, (ev) L
Figure I:
Figure 2:
The BIS spectra of ZrO, and HfO,
were taken in the isochromat mode at a photon energy of approximately 1486.6 eV. The monochhromator contains 30
mA/cm’.
The BIS spectra of Nb,O,
and TqO,
More details on the preparation procedure can be
quartz crystals arranged in a modified Johansson geometry The overall energy resolution was approximately I. I eV.
found elsewhere.“” These anodic oxide films are particularly well suited for this experiment because both. the stoichiometry and the oxide film thickness can be
The energy scale of the spectra was cahbrated by using the Fermi level of a reference silver sample. All the spectra
oxide films are far less sensitive to the damage caused by
were normalized to the maximum peak intensity. The samples were anodic oxtde films (300
A)
grown on the corresponding metal substrate. Zr. Hf. Nb and Ta fotls were mealed. decarburized. deoxidized and
easily controlled.
the electron beam than thermally oxidized films. On the other hand, bulk oxides of these large band gap insulators cannot be used because they present severe charging effects.
degreased before they were submitted to anodizatton. The Nb wd Ta foils were anodized in a 0. I molar oxalic acid electrolyte
at a constatt current density of I-2 mA/cm*.
The Zr rmd Hf foils were anodized m a 30 g/l ammoma ptntaborate electrolyte at a constant current density of 2
3 Results and Discussion a) BIS Spectra Figures I and 2 show the BIS spectra of ZrO, and HfO, and of Nb,O, and TarO, respectively. The spectra are
Table I. Absorption threshold referred to the Fermi level (E,). sphttmg (M)
In addition. we found that the anodic
disperston of the metal d bands (Co). crystal-field
from the BIS spectra and from the XAS spectra (Ref.
I). relative cross-sectron of the nd and (n+/)sp
bands (u,,&J,,,.,,,). and band gSfX from the hterature (E,) for the respective oxides. References for Es are given in brackets. All energy values are given in eV.
Compound
E*
f”
Ad (BIS)
Ad (XAS)
ZrOz
3.0
5.9
3.5
3.1
II.9
4.5 [24]
HfO*
3.7
64
4.0
3.7
25.0
5.7 [ZS]
Nb,O,
I.5
68
4.2
3.8
18.0
3.5 1261
-J-a@,
2.0
7.6
46
47
25.9
4.2 [27]
Q”&”
. I ,I
Es
vol.+91.No. 7
BREMSSTRAHLUNG
normalized to the maximum peak intensity ad the energy scales are referred to the Fernu level of a silver sample. We note first that the BIS spectra of these mater& present some general features in common. In paft~cular. all the spectra exhibit two strong bumps just above threshold (up to =I0 eV ) and a broad structure at higher energies (above IO eV). As mentioned above. the shape of each spectra is related to the correspondmg unoccupied electronic states in the conduction band, however as the 0 2p states are expected to be almost completely occupied. the observed spectrum is mainly determined by the transition-metal d md sp states The two strong bumps above threshold are attributed to 4d bands in ZrOz and Nb,O, and to 5d bands in the case of HfO, ;md T+O,. The broad structures at higher energies are assigned to transition-metal sp bands. The sp bands appear at higher energres as a consequence of the larger overlap with the 0 2p levels which results III stronger anttbondmg mteractlons. The splitting of the d bands observed in the BIS spectra IS caused by crystal-field effects The magnitude of the crystal-field splitting (AcI) IS very sensitive to the hgand coordination. Therefore. the values of Ad taken from the BIS spectra should be compared only for those oxides with the same ligand coordination. In our case, ZrOz and HfO, have a distorted fluorite structure with a pseudocubic ligand coordinatmn” whereas Nb,O, and T%O, have a complex crystallographic structure with a distorted octahedral oxygen coordinatton *‘The correylondmg values of Ad as dctermmed from the spectra of Figs I and 2 are glvcn in Table I. Accordmg to the expected larger overlap of the 5d orbltals with the 0 2p states which results m stronger Interactions. it IS observed that the crystal-field sphttmg IS larger for the 5d transmon-metal oxldcs The dispersion of the d band (r,,.) measured as the FWHM of the band. IS also given m Table I. The mam contrtbution to it IS given by mdlrect mteractlons through the ligands Therefore. the larger value of r, for the heavier transltion-metal oxides (cf Table I) appears agam to be caused by stronger interactions with the ligands. Since the BIS spectra shown in Figs. I and 2 are refened to the Fermi level. the absorption threshold (E,), measured as the energy at half of the maximum of the jump, gives directly the energy posItton of the bottom of the conduction band. The values of E, have also been collected in Table I They are consistent with the experimental values of the band gap (E,) found in the literature for these oxide?‘” (cf Table I) Finally. we would like to pomt out the Importance of these parameters (Ad, r, and E,) obtained from the BIS spectra for the understanding of the electronic structure of these oxides In fact. these parameters are vety difftcult to determme using more conventIonal techniques like optical spectroscopy. Table I includes also the relative cross-section of the d and sp states (q/u ,“.,,,) calculated by ustng the photoionlzatlon cross-sectlons given by Y eb and Lindau”. The contrlbutlon of the metal p states to the cross-sectlons IS very small and can be neglected In a first approxlmatlon The large values of u&,~.,,, (cf Table I) mdtcate that the contribution of the sp bands to the BIS spectra IS. at least. one order of magnitude weaker than that of the d bands 731s means that the contribution of the sp bands from the
ISOCHROMAT
553
SPECTRA
metal to the broad sbucture at high energies in the BIS spectra (cf. Table I) is very small for all the oxides. In fact. this spectral region is mostly due to the background coming from the adjacent d bands b) BJS versus XAS Figure 3 compares the Nb,O, BIS spectrum of Fig. Zw&theOIsXASspectrumreportedinRef l.TheO Is XAS spectra of transition-metal oxides reflect the unoccupied states of 0 2p character which are mlxed in the conduction band”. In fact. the double feature near the threshold is attributed to the contribution of the 0 2p states hybridized with the Nb 4d bands. whereas the broad feature at high energy corresponds to those 0 2p states hybridized with Nb 5sp bands.’ Therefore, the remarkable similarity between the BIS and 0 Is XAS spectra is not unexpected. The d bands are better reproduced by the 0 Is XAS spectra because the energy resolution (0 25 eV) IS much better ln addition, the sp bands are more clearly observed in the 0 Is XAS spectra. Both spectra reproduce the band structure of the unoccupied states but. whereas BIS shows mamly the contribution of the d bands from the cation due to the cross section values. the 0 Is XAS spectrum pomts out the 0 tp character of those bands due to the selection Nk3 which affect the process A similar comparison between the XAS and BIS spectra can be done for the other oxldes The values of the crystal-field sphtting (w obtained from the 0 Is XAS spectra (Ref. I) are also given In Table I for comparison W&I those estimated from BIS. The XAS values of Ad result slightly smaller than
1
Nb*os ;’ ‘.
BIS
‘. ‘.-...,, ‘..
.;..
‘._ . .
I
Figure 3:
,,.,._. .‘.--
...--..., ,,..... .......-
... .... .
,;” ‘x’
..,.” ,...
I
The 0 Is XAS and BIS spectra of Nb,O,
554
Vol. 91. No. 7
BREMSSTRAHLUNG ISOCHROMAT SPECTRA
those obtained from the BIS spectra The discrqanaes could
be due to the influence
of the 0 Is core-hole
pot&al in the XAS spectra”” As no core-level is involved m the BIS process, this effect cannot play a role in the BIS spectra. Therefore.
the values of Ad obtained
each group The mergy of the respective absorption threshold is conslstcnt with the magnitude of the bad gaps. The
comparison
of BIS
aad 0 Is XAS
useful mformatlon (ti. r+d structure of these mater&..
from the BIS spectra should be preferred III 011s case.
spectra
suggests that the core-hole potmtial could be a&ctmg the peak posmons. In conclusion. the BIS spectra provide very E,)
on the electronrc
4 Summary sod Conclusions In summary. we have presented and discussed BIS spectra of ZrO,, HfO*, Nb,O, and TgO,. The spectra are interpreted in terms of the contribution of the cation d and sp bmds
to the
unoccupied
states. The
crystal-field
splitting and dispersion of the d bands have been determined. both increase with the atomic number Z within
Acknowledgements-
This work was partially supported by
Fundamenteel Onderzoek der Materie (FOM),
Schakunde
Onderzoek Nederland (SON), the European Union Human Capital and Mobility program under contract # ERBCHRXCT930358 and the CICYT of Spain under contract MAT9310805.
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