Universität Osnabrück, Graduiertenkolleg Mikrostruktur oxidischer

GRADUIERTENKOLLEG MIKROSTRUKTUR OXIDISCHER KRISTALLE 21
The Fe1-xCuxCr2S4 (x=0.0; 0.5) are ferrimagnetic semiconductors, whereas the end member of the series,
CuCr2S4, is a ferromagnetic metal. In FeCr2S4 the Fe 2+ and Cr 3+ ions have the 3d 6 and 3d 3 electronic configurations,
respectively. For CuCr2S4 the question of the Cu valence state has been a longstanding problem. In connection
with this, two models were proposed by Lotgering [10] and Goodenough [11]. The first model claimed
that the Cu valence state should be monovalent (Cu 1+ ) whereas the second one claimed that Cu is divalent (Cu 2+ )
. For the Fe0.5Cu0.5Cr2S4 system the situation becomes even more complicated, since several possibilities can
occur for the substitution of Cu for Fe, depending on whether Cu is present as Cu 1+ or Cu 2+ .
The vanadium oxides have been extensively studied over the past few decades due to their interesting
electric and magnetic properties. It is well known that many compounds may form in the vanadium-oxygen
system, such as V2O5, V2O3, VO2, VO, VnO2n-1 (Magneli phases) and V2nO5n-2 (Wadsley phases). Most of the
vanadium oxides exhibit metal-to-insulator transitions (MIT) and many experimental and theoretical studies
have been done in order to investigate the mechanism behind MIT [12]. Despite this, there are still open questions
related to the surface characterization of V2O5, the phase transitions mechanism in V2O3 and VO2, which is
related by some authors to a Mott-Hubbard scenario [13], whereas others attribute it to the electron-phonon
coupling on the basis of the change in the crystal symmetry [14].
Ergebnisse
We have investigated the changes induced in the electronic structure of LaMnO3 by various dopants, as
Sr, Ba, Ca, Pb by using XPS and XES. From the XPS valence band (VB) and XES Mn Lα and O Kα spectra
the O 2p and Mn 3d electronic states distribution in the VB was determined. A strong hybridization between the
Mn 3d and O 2p states was observed (see Fig.1), in good agreement with the band structure calculations [15].
The wide band in the VB spectra was associated with the Mn t2g↑ hybridized with O 2p states and the structure
close to the Fermi level was identified as mainly reflecting the Mn eg↑ states.
In order to estimate the valence state of manganese ions we have analyzed the Mn 2p and 3s core level
spectra. The splitting of the 3s XPS spectra of transition metals and their compounds originates from the exchange
coupling between the 3s core hole created during the XPS process and its magnitude is proportional to
the local spin of the 3d electrons in the ground state [16]. We have investigated La1-xSrxMnO3 single crystals in a
doping regime 0.1