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20 UNIVERSITÄT OSNABRÜCK, FACHBEREICH PHYSIK Spectroscopic investigation of 3d transition metal compounds Beginn des Projekts: 01. 11. 1997 Ende des Projekts: 31. 10. 2000 Dipl.Phys. Mihaela Carmen Demeter Betreuer: apl. Prof. Dr. M. Neumann Zusammenfassung The aim of our study was to investigate the electronic structure of recently reported CMR compounds, Mn- perovskites (La1-xAxMnO3) and Cr- spinels (Fe1-xCuxCr2S4), as well as the metal-insulator transition in vanadium oxides. A particular attention was paid to the effects induced by different dopants in the electronic structure of the investigated systems. The electronic structure of the parent compounds, LaMnO3 and FeCr2S4, respectively has been studied by X-ray photoelectron and X-ray emission spectroscopies. The role of the various dopants (Sr, Ba,Ca, Pb) and the changes induced in the electronic structure were discussed in relation to band structure calculations 11,14 . In addition to Fe1-xCuxCr2S4, other Cr- sulphides and selenides were investigated: ACr2S(Se)4 (A= Cu, Cd, Hg, Fe, Mn, Zn) 5-7, 10 . Many vanadium oxides, systems that exhibit metal to insulator transitions, were also studied. Besides the intensively investigated V2O5, VO2 and V2O3, we report new data on mixed-valence compounds, as V6O13, V3O5 and V4O7 in both phases, metallic and insulating 3 . The influence of the Mo-doping on the electronic structure and properties of the widely used catalyst V2O5 was determined by XPS and XES 1, 9 . Stand der Forschung The CMR (colossal magnetoresistance) phenomena observed in manganese perovskites La1-xAxMnO3 (A= divalent cation) have renewed the interest in these materials, not only because of the potential for technological applications, but also due to their interesting physical properties [1]. Recently, new materials exhibiting CMR effect have been reported: Cr- based chalcogenides Fe1-xCuxCr2S4 (x=0.0; 0.5) [2], layered manganites La2-2x (Sr, Ca, Sn)1+2xMn2O7 [3], Sr2-xNd1+xMn2O7 [4], Tl2Mn2O7 [5] with pyrochlore structure and Eu14MnBi11 [6]. The parent compound LaMnO3 is an A-type antiferromagnetic insulator with an orthorhombic crystal structure. When this compound is doped with divalent alkaline-earth cations A 2+ (A= Sr, Ba, Ca, Pb), a charge redistribution from Mn 3+ to Mn 4+ takes place. In a certain range of doping (0.2< x< 0.5) these manganites undergo a paramagnetic insulator to ferromagnetic metal phase transition upon cooling, giving rise to a sharp resistivity peak near the Curie temperature Tc. By applying an external magnetic field the resistivity is strongly suppressed and the sharp peak position shifts to higher temperatures, producing a large negative magnetoresistance. This effect has been firstly explained by the double-exchange model, which assumes that the conduction is achieved through the hopping of an electron from the eg state of the Mn 3+ ions into the unoccupied eg band of the Mn 4+ [7]. Later it has been shown that the electron-phonon interaction has to be taken into account in order to explain the high values of the resistivity at T> Tc or the large drop in resistivity just below Tc [8]. The microscopic origin of the strong electron-phonon coupling is the large Jahn-Teller effect, which occurs for the d 4 ions in an octahedral environment. The ferromagnetic/ antiferromagnetic superexchange interaction between the local spins, inter-site exchange interaction between the eg orbitals, intra-site and inter-site Coulomb repulsion among the eg electrons are other effects that play an important role in the CMR scenario [9]. Unlike the manganese perovskites, the spinels ACr2S4 are characterized by no mixed valency, an A-site cation that can contribute to states near the Fermi level and large deviations of the metal-anion-metal bond angle from 180°. Hence, other mechanisms, besides the double-exchange and electron-phonon interactions, have to be considered in order to understand the CMR effect observed not only in Fe1-xCuxCr2S4, but also in different other classes of materials (as mentioned above). Both, to explain the very interesting physics behind this effect and also the potential applications for reading heads, magnetic sensors and other devices, are strong reasons for scientists to perform further investigation of these compounds.
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
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