Universität Osnabrück, Graduiertenkolleg Mikrostruktur oxidischer

16 UNIVERSITÄT OSNABRÜCK, FACHBEREICH PHYSIK
Using X-ray photoelectron spectroscopy we have investigated the cyclic hexanuclear cluster
[Li⊂Fe6[N(CH2CH2O)3]6]Cl. The Fe 3s and 2p photoelectron spectra recorded for the ferric-wheel and other Fecompounds,
where Fe has the formal valence state 3+ or 2+, were compared (Fig. 1).
Intensity (arb. units)
ferric-wheel
FeO
LiFeO2 CuFeO2 C
25 20 15 10 5 0 -5 -10
B
A
Relative binding energy (eV)
Intensity (arb. units)
ferric wheel
FeO
CuFeO 2
LiFeO 2
50 40 30 20 10 0 -10 -20
Relative binding energy (eV)
Fig 1. Fe 3s and 2p photoelectron spectra of [Li⊂Fe6L6]Cl, L=N(CH2CH2O)3. For comparison, the spectra of
FeO, LiFeO2 and CuFeO2 are presented.
The formal valence state of iron is 2+ in FeO and 3+ in the other compounds. The splitting of the 3s core-level
line originates from the exchange coupling between the 3s hole and the 3d electrons. The value of this splitting
is 6.5 eV for Fe 3+ and 5.4 eV for Fe 2+ in simple iron oxides.
In the case of the Fe(III) cluster, three peaks, marked A, B and C, can be distinguished. The distance between A
and B was found to be about 5.5 eV. Such position for the second peak would correspond, by simple comparison,
to a 2+ valence state. However, the magnetic properties of the sample can be well described by taking into
account a 3d 5 configuration for iron in the ground state. Thus, the interpretation of the 3s spectra should be consistent
with a 3d 5 electronic configuration. We have tried to explain these spectral features of the Fe 3s photoelectron
spectrum recorded for the ferric-wheel by considering a two-configuration model of the inter-atomic
configuration mixing. It seems, that the final state photoelectron spectrum differs from the spectra of simple
oxides. The population of the high-spin final state without a ligand-hole is favored. The 3s photoelectron spectra
exhibits particular features. We can conclude, that the relaxation effects in such organo-metallic systems could
lead to changes of the photoelectron lines of the metallic ions.
2. Rare-earth compounds
We have investigated the LaNi5-xCux series with x = 0; 0.5; 1.5; 1.5 which crystallizes in a hexagonal structure
of CaCu5 type. The XPS valence bands are in Fig.2. The band structure calculations were carried out using the
ab-initio tight-binding linear muffintin orbitals method in the atomic sphere approximation (TB-LMTO-ASA)
[14].There is a similarity of the Ni 3d bands for pure Ni with those recorded for LaNi5 which evidences that the
valence band of LaNi5 is mainly derived from Ni 3d-band. Alloying with Cu does not induce visible changes in
the Ni band. A rather independent Cu band is formed around 3.3 eV binding energy. The Cu states are probably
completely filled with electrons. The core-level lines of La, Ni and Cu show no chemical shift for the different x
values. Magnetic measurements were performed in the temperature range 1.7-400 K and the magnetic
susceptibility was corrected for possible magnetic impurities by using magnetization isotherms, according to a
Honda-Arrott plot [10]. The samples were generally shown to be free from magnetic impurities. For all samples,
the susceptibility increases up to a temperature Tmax and then decreases (Fig. 3). Above a characteristic
temperature T* a Curie-Weiss type behaviour is observed. In low temperature region (T