THE ELECTRONIC STRUCTURE AND ELECTRON
CORRELATION EFFECTS STUDIED BY XPS
G. Sawatzky, E. Antonides
To cite this version:
G. Sawatzky, E. Antonides. THE ELECTRONIC STRUCTURE AND ELECTRON CORRELATION EFFECTS STUDIED BY XPS. Journal de Physique Colloques, 1976, 37 (C4),
pp.C4-117-C4-123. <10.1051/jphyscol:1976419>. <jpa-00216533>
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JOURNAL DE PHYSIQUE
Colloque C4, supplkment au no 10, Tome 37, Octobre 1976, page C4-117
THE ELECTRONIC STRUCTURE AND ELECTRON CORRELATION
EFFECTS STUDIED BY XPS
G. A. SAWATZKY and E. ANTONIDES
Physical Chemistry Department of the Material Science Center
University of Groningen, The Netherlands
RQum6. - Les interactions importantes pour la comprehension des transitions mBtal-non metal
du type Mott-Hubbard sont les interactions Blectron-Blectron et les integrales B un electron. Les
premikres conduisent h des effets de correlation et B la localisation tandis que les dernikres donnent
la structure de bande B un electron et la dklocalisation. Dans cet article, nous etudions comment la
XPS peut fournir des informations sur ces interactions. L'application de la XPS a 1'Ctude de la
structure de bande a un electron est Bvidente et tout B fait directe. Comme exemple de ce cas, nous
prksentons des resultats recents sur TaS2 intercalk avec Sn et montrons comment la XPS peut donner une description correcte des changements de structure electronique en intercalant de l'etain.
Un autre exemple que nous discuterons est le changement de la nature de la bande d de V02 en
franchissant la transition serniconducteur-metal. L'application de la XPS a l'ktude des effets des
correlations 6lectroniques est moins Bvidente. Nous montrerons qu'a partir d'une Ctude dktaillk de
la spectroscopie Auger B haute resolution et de Ia XPS, on peut obtenir les interactions coulombiennes a un site reduites par une polarisation atomique et de bande ainsi que par des effets d'kcran. La
reduction peut atteindre 90 % des valeurs correspondantes B I'atome libre.
Nous montrerons aussi, comment une Ctude dktaillke de la forme de raie des raies dues au ceur
peut fournir des informations sur les effets des corr6lations. Par exemple, les raies de cceur de V02
sont fortement Blargies en franchissant la transition de l'Btat semiconducteur vers l't5tat mBtallique.
Cet Blargissement est bien interpret6 en termes de transition de Mott-Hubbard. Nous discutons les
dkcalages de la coupure du spectre de XPS a partir du niveau de Fermi en termes d'effets de correlation qui pourraient irnpliquerles 6lectronsou les phonons. A partir de ces decalages, une valeur approchke de 1'6nergie de liaison du polaron dans Fe304 est obtenue.
Nous discuterons egalement une nouvelle sorte dYexpMencemettant en jeu les intensites des
raies Auger, a partir desquelles on peut en principe obtenir le nombre de sites doublement occupes
et par-18 donner des conclusions concernant le degre de correlations t5lectroniques.
Abstract. - The basic interactions which are of importance for an understanding of metalnon metal transitions of the Mott-Hubard type are the electron-electron interactions and the one
electron integrals. The former leads to electron correlation effects and to localization and the latter
results in the one electron band structure and delocalization. In this paper we want to investigate
how XPS can yield information about these interactions. The application of XPS to a study of the
one electron band structure is quite obvious and straight-forward. As an example of this we present
recent results on Sn intercalated TaS2 and show XPS can give a very consistant picture of the
changes in the electronic structure upon intercalation. Another example which will be discussed
is the change in the d band of V02 upon going through the semiconductor-metal transition. The
application of XPS to a study of electron correlation effects is less obvious. We will show that from
a detailed study of high resolution Auger spectroscopy as well as XPS one can obtain the on site
coulomb interactions reduced by atomic as well as band polarization and screening effects. The
reductions can be as much as 90 % of the free atom values.
In addition we will show how a detailed line shape study of core lines can yield information about
electron correlation effects. For example the core lines of V02 are strongly broadened as one goes
through the transition from semiconductor to metal. This broadening is shown to be consistant
with an interpretation in terms of a Mott-Hubbard type of transition. Shifts of the XPS cut off
from the Fermi energy are discussed in terms of correlation effects which could involve either
electrons or phonons. From these shifts an approximation to the polaron binding energy in Fe304
is obtained.
Also a new kind of experiment involving Auger intensities will be discussed from which one can
in principal obtain the number of doubly occupied sites and thereby reach conclusions concerning
the amount of electron correlation.
For a detailed understanding of metal-non metal
transitions one has to have detailed knowledge of the
electron-electron interactions, the one electron' band
structure and the electron phonon interaction. Starting with the Hubbard hamiltonian for example it is
the relative importance of the on site coulomb interac-
tion (U) and the band width ( W ) which is the determining factor for the material being a metal o r
non metal. For an s band and for U $- W the material
will be a non metal for a half filled band and a strongly
correlated metal for other fillings. For U 4 W the
material will behave like an ordinary metal. In the
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1976419
C4-118
G. A. SAWATZKY AND E. ANTONIDES
Hubbard hamiltonian we should use an effective
interaction (Ue,3 which can be written as I-A where I
is the ionization potential and A the electron affinity
of a singly occupied site. It is however difficult to
obtain a good estimate of U,,, since it is strongly
reduced from the free ion value by polarization and
screening effects. We will show below that a value for
U,,, for transition metal compounds can be obtained
from a combination of XPS and Auger spectroscopy
(AS) studies. As far as the one electron band width is
concerned one can obtain this from XPS measurements. Here however one must be careful to correct for
all symmetry splittings in for example d bands caused
by crystal Jields, spin-orbit coupling and exchange
splittings in order to obtain the one electron band
width which is to be compared to U,,,. In 3d transition
metals for example like Cu the major portion of the
observed d band width comes from symmetry splittings and only a small portion is due to one electron
transfer integrals. This is quite obvious when one
looks at the very narrow lines appearing in the LMM
Auger spectra.
A very important property of XPS measurements is
that they usually involve very short characteristic
measurement times, typically of the order of 10-l6
seconds depending somewhat on the nature of the
measurement and on the interactions involved. This
means that all processes which take place in much less
than 10-16 s are essentially static in an XPS measurement. In a metal then with a one electron band width
of less than one electron volt the conduction electrons
can be considered as frozen during the course of the
measurement. If the conduction electrons are uncorrelated as for U,,, 4 W the core lines will be broadened
or even split because of the statistical distribution of
conduction electrons on the ions. On the other hand
if the conduction electrons are strongly correlated as
for example in a Mott insulator the core lines will be
narrow because each ion will have the qverage number
of conduction electrons. It should be noted here that
the binding energy of core electrons are quite strongly
dependent on the occupation of the outer shell.
Because of the above mentioned effect one should see
a large change in the core line width if a material
undergoes a Mott-Hubbard transition.
Another point worth mentioning about XPS measurements in conjunction with the fast measuring time
concerns the Fermi level. The Fermi level is defined
as the thermodynamic potential for all electrons and is
therefore only defined in a thermodynamic equilibrium
state. If the material to be measured is in good electrical
contact with a good metal the Fermi levels will coincide.
In an XPS measurement one measures only the
occupied'states. This means that we should see a
sharp cut off at the Fermi energy of a good metal. This
is the case for metals like Ag, Au and many others.
If however correlation effects in the material are
important the sudden removal of one electron will
leave the material behind in an excited state of the
N - 1 electron system and the cut off in the XPS
measurement need not coincide with the Fermi level.
As an extreme example we could consider a small
polaron material. In a slow measurement the phonon
or lattice polarization cloud will have a chance to
relax during the measurement and a cut off corresponding to the Fermi energy will be found. In a fast
measurement however the polarization cloud will not
have a chance to relax and the material will be left in
an excited state and the cut off will be shifted from the
Fermi energy by an amount of the order of the polaron
binding energy. The same sort of shift can also occur
if electron correlations are important. The amount of
shift and the width of the cut off will depend on the
interaction of the electron with the polarization cloud
and the relaxation time of the polarization cloud
which in turn is determined by the excitation spectrum
of the polarized system.
With attention to these rather general introductory
remarks we will now discuss in more detail and with the
aid of several examples how one can obtain information about the band structure and electron correlation
effects from XPS.
We start with the valence and conduction band
structures. As an example we take the 2 H phase
of TaS, and various Sn intercalates. These materials
exhibit superconductivity and the superconducting
transition temperature can be influenced by intercalation. By intercalation we mean the addition of atoms
or molecules in positions inbetween the TaSz layers.
The structures of these compounds are described in
detail in other papers of this conference. It is of fundamental importance to know how the electronic structure of the materials change upon intercalation and in
which ionization state the intercalates appear.
In figure 1 we show XPS spectra, obtained with
monochromatised A1-Ka radiation of the valence band
region for TaS2, Sn,,,TaS, and Sn,TaS2. The sharp
peak close to the Fermi level is the Ta 5d band and the
FIG. 1. - XPS spectra of the valence band region of 2 H-TaS2
(drawn line) and SnzTaS2 (X = 113... ; x = 1 - - -). The zero of
the energy scale is the Fermi energy.
THE ELECTRONIC STRUCTURE AND ELECTRON CORRELATION EFFECTS STUDIED BY XPS C4-119
broad peak below it is the S 3p band. It is quite obvious
from these spectra that the number of Ta 5d electrons
increases upon Sn intercalation. In fact if we compare
the areas under the 5d part of the spectrum to the S 3p
part we find that the number of 5d electrons I for
TaS, 1.8 $- 0.2 for Snl,,TaS, and 2.2 0.2 for
Sn,TaS,.
We can conclude from this that every Sn donates
2 electrons to the 5d band of Ta up to an upper limit of
a total of about I electron per Ta. More Sn intercalation does not increase the number of 5d electrons so
that the rest of the Sn probably goes into the lattice as
neutral Sn. This can be checked by looking at the
Sn 3d core lines shown in figure 2. We see indeed that
+
l
22 0
U0
I
26 0
BINDING ENERGY l E V l
I
I
280
300
I
320
FIG. 3. - XPS spectra of the Ta 4f region in 2 H-TaS2 (drawn
line) and SnzTaSz (X = 113 ... ; x = 1 - -). The energy scale
was shifted so as to make the peaks coincide.
-
obtain the excited states of the systems which we have
shown to be in good agreement with the band structure
as calculated by Mattheiss [3].
From these experiments we can also obtain the
shift of the 4f lines as a function of the number of 5d
electrons. This is of importance for determining the
amplitude of the charge density waves in these materials at low temperature [4]. We find a shift of about
0.8 eV per 5d electron. For a more detailed description
of these experiments we refer you to ref. [5].
l
I
I
I
I
l
I
I
l
Another quite interesting material is VO,. As is well
L81 0
L830
L85 0
L870
L91 0
BINDING ENERGY l EVl
known this material exhibits a semiconductor-metal
FIG. 2. - XPS spectra of the Sn 3d,,, region of SnzTaS~ transition at about 70 OC. A very important question
which has not yet been satisfactorally answered is that
( X = 113 ... ; X = l - - -).
of the driving force for the transition and what is the
nature of the ground state in the two phases. The
for Snl,,TaS2 there is only one kind of Sn present question really is are the electron phonon or the elecwhereas for SnlTaS, there are 2 kinds of Sn present tron electron interactions the most important for
corresponding approximately to Sn2 and Sn neutral. describing the properties of this system.
We have studied the electronic structure of VO, in
The presence of 2 types of Sn in Sn,TaS, seems to be in
both
phases with XPS [6]. The spectra of the valence
contradiction to Mossbauer effect [ l ] and NMR [2]
band
region are shown in figure 4 for both phases. In
results in which only one type of Sn was found. This
apparent discrepancy can be explained when one
considers the effective measurement times. For Mossbauer effect and NMR this is of the order of 10-' S.
while for XPS it is of the order of 10-l6 S. It is also
concluded from the Mossbauer effect experiments that
the Sn atoms are quite loosely bound having a large
mean square displacement in the plane of the layers
and that the isomer shift lies between Sn2+and neutral
Sn. We conclude therefore that there are two types of
Sn which however exchange rapidly in a characteristic
time between I O - ~ and 10-16.
XPS experiments can also give information about the
I
non occupied states of the system. In figure 3 we show
L
2
0
-2
the 4f lines of Ta in the 3 above mentioned samples.
b~ndlngenergy (eV)
These lines are very asymmetric which is a result of the FIG. 4. - XPS spectra of the V 3d band above and below the
material being left behind in an excited state. From the transition temperature. The insert shows these spectra after corshape of the asymmetry one can at least qualitatively rection for the background including the tail of the 0 2p line.
I
I
+
-
(3-120
G. A. SAWATZKY AND E. ANTONIDES
general the band structure consists of a V 3d band close
to the Fermi level and a deeper lying broad 0 2p band.
These spectra were obtained with MgKa radiation
which has a width of about 0.7 eV. If we correct for this
width the 3d band of the semiconducting phase is
about 1 eV wide and lies about 2 eV above the top of
the 0 2p band.
The 3d band gap was found to be slightly temperature dependent and is about 0.3 eV at 300 K if we
assume the Fermi level to be pinned to the bottom of
the conduction band. 'Upon going through the transition to the metallic phase the band gap closes and the
highest occupied states now are at the Fermi level. In
addition the 3d band is seen to broaden somewhat
although the total number of occupied d states stays
the same as can be concluded from the areas under the
3d portion of the spectrum. From these results we still
cannot distinguish between the two possible mechtinisms mentioned above for the semiconductor-metal
transition. A broadening and shift of the band is
expected in both cases.
Perhaps we can learn more from the core lines. In
figure 5 we show the V 2p and 0 1s core lines in both
TABLEI
Peak positions and Widths of XPS lines of V 2p3,,
and 0 1s lines of various V compounds
Material
v02
v205
V
T (K)
273
293
393
393
293
293
(uppercurve) and below (lower curve) the transition temperature,
after correction of the background. The arrows indicate the
positions of the V and the 0 lines and also the positions of the
0 1s lines of some possible surface contaminants.
phases. A careful measurement of the peak positions
shows that the binding energy difference between the
V 2p3,, and the 0 1slines is slightly larger in the metallic
phase indicating a slightly more ionic bond. The
differences are however very small and could also be
due to Madelung energy changes. In table I we have
listed some of the peak positions and widths of these
core lines. For comparison we also listed the corresponding lines for V205 and V metal. We see that the
VO, lines are first of all much broader and secondly
width
2.4
2.35
3.0
3.0
1.7
position
width
515.7
515.7
515.75
515.7
516.8
512.7
3.2
3.1
4.2
4.25
2.0
2.0
-
that the V 2p lines broaden considerably upon going to
the metallic phase. A possible explanation of this
could be the following. The binding energy of a core
electron depends on the oxidation state of the ion and
therefore of the number of 3d electrons. The shift of
the V 2p lines per 3d electron added is about 0.8 eV.
The 3d band width in the metallic phase is about 1 eV.
As discussed above in an instantaneous picture the 3d
electrons will be distributed statistically over the sites
in a manner given by the binomial distribution.
This means that 0.5 of the V ions will be 4+ and
0.25 will be 3 and 5 . This would result in a symmetrically broadened line. The amount of braodening will
depend on the measurement time as compared to the
one electron band width and will also be reduced by
correlation effects. In the semiconducting phase the
lines are relatively narrow which indicates that here
each V ion has the average number of 3d electrons even
in an instantaneous picture. This interpretation would
be in excellent qualitative agreement with a picture in
which VO, is a Mott insulator at low temperatures
and goes through a Mott-Hubbard transition to a
high temperature metallic phase. We see from this
example that core line shapes can yield information
about electron correlation effects and the nature
of metal non metal transitions.
Fe304is another material which has caused a lot of
controversy. Photoelectron studies of this material
have been done by Bishop and Kemeny [7]. Among
other things there is something quite interesting
happening at the Fermi level. The photoelectron cut
off seems to be about 0.4 eV below the Fermi level.
As mentioned before this is a strong indication that
the high temperature phase of Fe,O, is not a simple
metal.
We have also done studies below the transition
temperature and find no shift in this cut off. This
probably means that the conductivity in the high
temperature phase is due to small polarons and that the
transition has to do with an ordering or localization of
these polarons. Such an ordering will of course also
result in a change in the crystal structure.
Before going on there is another rather general
feature of valence band spectra of transition metal
compounds which shows that the old ideas about
+
FIG. 5. - XPS spectra of the V 2p and 0 Is region taken above
position
529.6
529.6
529.35
529.45
529.9
+
THE ELECTRONIC STRUCTURE AND ELECTRON CORRELATION EFFECTS STUDIED BY XPS (24-121
covalency effects in these compounds must be modi- where E,, and E,, are the one electron energies and E,
fied to include the band structure effects. Covalency is the kinetic energy of the Auger electron. From this
effects and the consequences thereof like super- we can determine U,,,. In Auger spectroscopy it is
exchange, transfered hyperfine interactions and crystal customary to split up U,,, into an atomic coulomb
field splittings have usually been described in ti local interaction and a reduction of this due to relaxation
cluster approximation neglecting all band structure or polarization effects as follows :
effects. An examination of XPS spectra of TaS,
(Fig. l), V 0 2 (Fig. 4), VS [g] and many others clearly
shows that the valence band region is composed Here F ( X ) is the atomic coulomb and exchange inte-,
of a rather broad (5-8 eV) ligand band structure and a grals corresponding to the various final state terms
rather sharp transition metal d band (1 eV). The local and R is the relaxation or polarization reduction. The
cluster model is only valid if the band width of the relaxation energy R is a result of the interaction of the
states to be covalently mixed is small compared to the one hole with the polarization cloud of the other and
separation in energy of the bands. This is clearly not visa versa. The interaction of a hole with its own
the case in these compounds. We have shown [9] that polarization cloud is included in the measured values
band structure effects will lead to long range super- for the one partical energies E,,, and
Before we go
exchange interactions. In these materials a better on a word about the measurement time. The polariapproach would seem to be to consider the anions zation correction measured in this way is the high
to form a lattice with rather broad energy bands frequency correction corresponding to hw I. l eV.
resulting in a semiconductor. The cations are then Any relaxation effects which are much slower than this
introduced in the large holes in the anion lattice and will not contribute. This means that for example
have sharp d levels. The material would then behave relaxation of the lattice will not be included completely.
like a high impurity concentration semiconductor. This would then be comparable to the polarization
The d levels will broaden into bands because of the energy reduction expected for d electrons moving in a
covalent mixing and long range effects will be observed band of width 1 eV. We could call this the electronic
in the superexchange and transfered hyperfine inter- pdarization reduction to distinguish it from the
actions.
static mainly lattice polarization reduction in the
We now turn to the problem of how one can mea- Anderson bi-polaron [12].
sure the effective on site electron-electron interaction.
Before we can actually measure U,,, however we
We will show that a combination of Auger spectros- must be able to interpret the Auger spectra in detail
copy and XPS provide a quite simple way of doing and identify the peaks with the various final state
this for transition metal compounds. By the effective terms. We have recently done this for Cu, Zn, Ga,
coulomb interaction (U,,,) we mean the interaction and Ge with remarkable success [13]. To display this
with all the screening and polarization corrections we show the part of the Auger spectrum of Ga corresmade. We will show that the polarization corrections ponding to 2 holes in the' 3d shell in figure 6. The
are especially important as. has been suggested in terms which can occur with two holes in 3d are 'S,
several other contributions in this conference [IQ, 1l]. 'G, ,P, 'D, ,F. We have calculated the transition
The most important Auger processes in the 3d transi- probabilities and fit the spectrum using the theoretically
tion metal ions are the LMM processes. In Auger determined transition probabilities. The fit is shown as
spectroscopy one first ionizes say a 2p,,, electron. the solid line and it is obvious that we now fully
The ion will now decay to a lower energy state either
43L
by x-ray fluorescence or by the Auger process. In the
L3 M45 M45
Auger decay we can distinguish 3 dominant processes :
- flt
exper>mentol
1) The 2p hole is filled up with a 3p electron and
another 3p electron is ejected the final state being 2 holes
30in the 3p shell. 2) The 2p hole is filled up with a 3p
electron and a 3d electron is ejected the final state
being I hole in 3p and one in 3d. 3) The 2p hole is
3
c
filled up with a 3d electron and another 3d electron
8 17is ejected the final state being 2 holes in the 3d shell.
The kinetic energy of the ejected Auger electron
will depend on the energy of the 2p hole and on the
energy of the final state of the ion left behind. The
energy of the 2p hole as well as other one electron or
K ~ n e t ~ cEnergy i eV l
hole states can be measured with XPS. We can then
RG. 6. - The L3M45M45 Auger spectrum of Ga. The dots are
write for the Auger process with 2 holes in the 3d the experimental values and the curve is the fit to five final state
shell :
terms 'S 1G, 3F, 3P, and ID. The intensities were taken from
ooo
X
-
U)
theoretical transition probability calculations and the widths of
the terms were constrained to be equal.
G. A. SAWATZKY AND E. ANTONIDES
C4-122
understand these spectra. In table I1 we have listed the
free atom coulomb integrals involved in the 'G final
state as well as the relaxation energy and the U,,.
We see indeed that the U,,, is strongly reduced from the
TABLEI1
The coulomb interaction as determined
from atomic calculations and the measured relaxation
energies and U,,, for the L3M45M45Auger lines
F
Cu
Zn
Ga
Ge
27.0
30.2
33.3
36.2
Fe304
v02
R
19.0
22.5
22.2
23.3
U,,,
U,,,
U,,,
-
8.0
7.7
11.1
12.9
= 3-4 eV
1-2 eV
free atom value. We have only just begun the measurements on the transition metal compounds so only a
few results are known. From these however we can
conclude that for the beginning of the 3d series Ti and
V the polarization reduction can reduce U to about
10% of the free atom value. An interesting question
now is : where does this relaxation come from ?
We can distinguish 2 processes : 1) atomic relaxation coming from the core electrons of the atoms,
2) extra atomic relaxation coming from mainly the
valence electrons. We can determine the atomic
contribution from free ion electronic structure calculations ; for Zn we find a value of only 6 eV. The
dominant contribution to the polarization reduction
is therefore from the valence electrons which are delocalized into a band in the solid. The U,,, will therefore
be quite sensitive to the state of the material. For a
metal we expect U,,, to be lower than for a semiconductor because of the lower dielectric constant of the
latter due to the band gap. This has been observed
in Zn as compared to ZnO. In ZnO U,,, is a b ~ u t
2.0 eV higher than in Zn metal. This kind of polarization reduction is exactly what we proposed in a recent
paper in which it was argued that U,,, will be different
in the semiconducting state than in the metallic state of
a material undergoing a semiconductor-metal transition [14].
It would be interesting to do this kind of study on
impurities in glasses. As discussed by Anderson [l21
there should be a bap in the one electron spectrum
which would show up in the XPS of the valence
band. In the two electron spectrum however no gap
should appear because U,,, is attractive. Perhaps an
example of an attractive U,,, is the one dimensional
compound K2Pt(CN),Br,,,2.3 H20. In figure 7
we show the 4f core lines of Pt in this compound.
The spectrum is fit with 2 doublets and can be fit very
satisfactorally in this way. The peak positions of the
I
62
l
l
68
7L
t
80
ELECTRON B I N D I N G EDERGY ( e V )
FIG.7. - XPS spectra of the Pt 4f and the Br 3d region.
1) K2Pt(CN)4n(H20);2a) KzPt(CN)4Bro,sn(HzO)at - 120 C ;
2b) same as 2a) but at -50 C so without crystal water;
3) K2Pt(CN)4Brz. The lines drawn are the least squares fits to
the spectra.
two doublets correspond to Pt2+ and Pt4+ and the
intensities are in the expected ratio corresponding to
the chemical formula. This is indicative of a strongly
correlated system with an attractive Hubbard U the
electrical conductivity being provided by 2 electron
processes as well as one electron processes but then
with an energy gap as given by Anderson. Another
indication that this is the case is the XPS valence band
spectrum. In this spectrum we find a gap of about
1.5 eV as would be expected in the one electron
excitation spectrum. A word of caution however.
We have not as yet been able to conclusively exclude
the possibility that the spectrum arises from a surface
layer of a frozen solution in which case Pt2+ and
Pt4+ would be expected. A detailed discussion of this
work can be found in ref. [15].
There is another rather interesting application of
Auger spectroscopy to the problem of the metal
non metal transition which is especially suitable for
compounds containing V and Ti and perhaps Cr. We
consider a system like VO, where the V is 4 + and
therefore in a d' configuration. We consider the
extreme case of a Mott-Hubbard transition in which at
THE ELECTRONIC STRUCTURE AND ELECTRON CORRELATION EFFECTS STUDIED BY XPS C4-123
low temperature the electrons are localized so that
each V has the average number of d electrons namely
one. In the high temperature metallic phase we consider for illustration purposes the material to be an
uncorrelated metal. As discussed above an instantaneous picture will then show V with zero, one or two
3d electrons with a probability ratio 1 : 2 : 1.
It is obvious that in a fast measurement the Auger
process corresponding to 2d holes in the final state will
not be present in the low temperature phase but will
appear in the high temperature metallic phase. By
measuring the intensity of this Auger line one can
obtain the average number of doubly occupied sites
and reach conclusions concerning the amount of
electron correlation in the two phases. The experiment is however not as simple as it may appear. Aside
from the usual surface contamination and oxidation
problems nature has for V02 and Vz03 provided for
another more serious problem. The V Auger line of
interest falls almost exactly under a very strong 0
Auger line. This means long measurement times and a
very careful subtraction of the 0 Auger contribution.
These experiments are now in progress on VOz,
V203, V4O9, V and V20, as well as a series of Ti
oxides.
In conclusion we can say that a careful study of line
shapes, Fermi level shifts and Auger peak positions
and intensities can yield quite a lot of information
concerning the nature of metal non metal transitions
and electron correlation effects.
This investigation was supported by the Netherlands
Foundation for Chemical Research (S. 0. N.) with
financial aid from the Netherlands Organization for
the Advancement of pure Research (Z. W. 0.).
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