Thèse de doctorat : Physico-chimie : Paris 12 : 2003. / Titre provenant de l'écran-titre.

Stuttgart, Univ., Fakultät Chemie, Diplomarb., 1997.

Stuttgart, Univ., Diss., 2002.

No description available.

No description available.

In dieser Arbeit werden drei Lanthanmanganat-Systeme mittels SQUID-(Superconducting Quantum Interference Device) Magnetometrie und XMCD-(X-ray Magnetic Circular Dich-roism) Messungen an den jeweiligen Absorptionskanten (XANES: X-ray Absorption Near Edge Structure) sowie im kantenfernen Bereich (MEXAFS: Magnetic Extended X-ray Ab-sorption Fine Structure) im Hinblick auf die Klärung ihrer magnetischen (Unter-)Struktur untersucht. Bei Lanthanmanganaten wird sowohl im Verlauf des spingemittelten als auch spinabhängigen Absorptionskoeffizienten an der Mn K Kante immer eine energetisch über 40eV ausgedehnte Doppelstruktur beobachtet. Durch Vergleich mit theoretischen Bandstrukturrechnungen und Messungen an Referenzsystemen lassen sich diese Strukturen auf zwei energetisch getrennte, resonante Übergänge in leere Mn 4p Zustände zurückführen. Die Ursachen liegen in der Kristallstruktur der Lanthanmanganate und damit ihrer Bandstruktur begründet. XMCD-Messungen an den La L2,3 Kanten zeigen, dass dieses Element zur Gesamtmagnetisierung dieser Verbindungen nur ein unerhebliches Moment beiträgt und daher in einer Xenon-ähnlichen Elektronenkonfiguration vorliegt. Durch die interatomare Coulombwechselwirkung der nahezu unbesetzten La 5d Zustände mit den magnetisch aktiven Ionen im Kristall dienen XMCD-Messungen an den La L2,3 Kanten als Sonde für die magnetische Lanthanumgebung. Ähnliches gilt für die entsprechenden MEXAFS. Der proportionale Zusammenhang der Größe der MEXAFS mit dem Spinmoment der Nachbarionen besitzt auch bei den Lanthanmanganat-Systemen mit den stark hybridisierten Elektronen der Mn 3d Schale Gültigkeit. Der Spinmoment-Korrelationskoeffizient aSpin gilt auch hier, was eine weitere Bestätigung des MEXAFS-Modells auch für oxidische Systeme ist. Im dotierten System La1.2Nd0.2Sr1.6Mn2O7 koppelt das Neodymmoment innerhalb einer Doppellage antiferromagnetisch zum Mn-Untergitter. Durch die Neodym-Dotierung am La/Sr-Platz im Kristall ist die ferromagnetische Kopplung der Doppellagen untereinander abge-schwächt und die Rückkehr in die antiferromagnetische Phase nach dem Abschalten des äußeren Magnetfeldes damit erleichtert. Das Mn-Bahnmoment ist von nahezu verschwindender Größe („gequencht“). Das System La1.2Sr1.8Mn2-xRuxO7 zeigt mit zunehmendem Rutheniumgehalt eine Erhöhung der Curie-Temperatur, was bei Ruddlesden-Popper Phasen zum ersten Mal beobachtet wurde. Das Ru-Untergitter und das Mn-Gitter sind zueinander antiparallel gekoppelt. Durch Bestimmung der Valenzen von Mn und Ru wird ein dem Superaustausch ähnliches Kopplungsmodell entworfen, womit der Anstieg in der Curie-Temperatur erklärbar ist. Das neu entwickelte XMCD-Experiment auf Basis eines Phasenschiebers und digitaler Sig-nalaufbereitung durch eine Lock-In Software besitzt ein Signal-Rausch Verhältnis in der Nähe der Photonenstatistik und liefert einen großen Zeit- und Qualitätsgewinn gegenüber Messmethoden mit Magnetfeldwechsel. Auf teure analoge Lock-In Messverstärker kann verzichtet werden. Zukünftig erweitert sich mit diesem Aufbau die für XMCD-Experimente zugängliche Anzahl an Synchrotronstrahlplätzen. Diese Experimente sind jetzt auch mit linear polarisierter Röntgenstrahlung an Wiggler/Undulator Strahlplätzen und zukünftigen XFELs (X-ray Free Electron Laser) durchführbar. / In this work three Lanthanum Manganate systems are investigated in terms of their magnetic (sub) structures. These investigations are done with SQUID- (Superconducting Quantum Interference Device) magnetometry and XMCD- (X-ray Magnetic Circular Dichroism) measurements at the respective absorption edges (XANES: X-ray Absorption Near Edge Structure) as well as in the MEXAFS (Magnetic Extended X-ray Absorption Fine Structure) range. For Lanthanum Manganates at the Mn K edge there is always seen a double peak structure in the shape of the spin dependent and spin averaged absorption coefficient, which is energetically expanded over more than 40eV. These structures are ascribed to two energetic separated, resonant transitions into empty Mn 4p states by comparing with theoretical band structure calculations and measurements of reference systems and are caused in the crystal structure of the Lanthanum Manganates and with it their band structure. XMCD-measurements at the La L2,3 edges show that this element adds only a negligible magnetic moment to the total magnetisation and La is therefore in a Xenon-like electronic configuration. These measurements probe the magnetic neighbourhood of the Lanthanum in the crystal due to the interatomic Coulomb interaction of the almost empty La 5d states with the magnetic active ions like the MEXAFS. The proportionality of the MEXAFS amplitude with the spin-moment of the neighboring ions is even valid here for Lanthanum Manganate systems with their strongly hybridized Mn 3d shell electrons. The validity of the correlation coefficient of the spin-moment aSpin confirms the MEXAFS-model also for oxide systems. In the doped system La1.2Nd0.2Sr1.6Mn2O7 the Neodymium moment couples antiferromagnetically with the Mn-sublattice within a double layer. The ferromagnetic coupling of the double layers is weak among each other due to the Nd doping at the La/Sr crystal position. Therefore the reversion into the antiferromagnetic phase is relieved after switching off the external magnetic field. The orbital moment of Mn is almost vanishing (“quenched”). The system La1.2Sr1.8Mn2-xRuxO7 shows an increasing of the Curie-temperature with an increase of the Ruthenium doping level, observed for the first time for Ruddlesden-Popper phases. The Ru-sublattice is antiparallel coupled to the Mn-sublattice. A superexchange like coupling model is composed through determination of the valences of Mn and Ru, also explaining the increase of the Curie-temperature. A new XMCD-experiment is developed with phase retarder and digital signal processing through Lock-In software with signal to noise ratio nearby photon statistics. This experiment provides a huge benefit in time and quality compared to XMCD-measurement with changing the external magnetic field. Also there is no need of expensive Lock-In analog amplifiers. Now the number of synchrotron beamlines for XMCD-measurements are increased and XMCD-experiments are realizable also at Wiggler/Undulator beamlines with linear polarized radiation and in future at XFEL (X-ray Free Electron Laser).

An overview of synchrotrons and synchrotron radiation is presented, along with the theory and practical considerations behind several types of X-ray spectroscopy. The theory and practical considerations of density functional theory are also given, with direct reference to some specific software packages.<p> Some synchrotron-excited X-ray spectroscopy measurements and density functional theory calculations of selenium and arsenic-doped selenium films are then outlined. The physical structure of crystalline and amorphous selenium and the electronic structure of amorphous selenium are discussed and comparison is made to the experimental results.<p> A weak feature in the conduction band is identified as a "fingerprint" of the degree of crystallization in amorphous selenium from X-ray absorption measurements. Similarly, a weak feature corresponding to lone-pairs in the valence band is identified as a "fingerprint" of the arsenic concentration from X-ray emission measurements.<p> Finally a detailed model of the structure of amorphous selenium is explained, and compared to experiment. This model is tested both by direct calculations and by a reverse Monte Carlo approach. The implications of this model with respect to the structure of amorphous and arsenic-doped amorphous selenium are discussed. Calculations suggest that simply randomizing the arrangement of "perfect" trigonal selenium is unable to reproduce the measurements of amorphous selenium; a moderate variation in the bond angle of "perfect" trigonal selenium is also necessary.

Cu is an important trace element in living systems, including animals. It is a part of many enzymes responsible for different processes occurring in the body. However, antagonists can severely decrease the amount of Cu available to the animal, which results in numerous health issues caused by Cu deficiency. The main antagonists of the Cu are thiomolybdates whose effect is especially harsh in ruminants due to their high potential to synthesize these compounds. Thiomolybdates (the ions MoOxS4-x2- ,where x = 0-3) are formed in the rumen from molybdate and sulfide (from degradation of sulfate), both of which come from the food and water ingested by the animal.<p> The first part of the project described in this thesis was to develop reproducible preparative methods of CuTM samples in solution and solids, under conditions as biologically relevant as possible. This crucial information is missing in the literature related to Cu deficiency. The second part studied the structure of the adduct formed between Cu and two of the thiomolybdates: tetrathiomolybdate, TM4 and trithiomolybdate, TM3, as these are the most important in Cu deficiency.<p> In solution, the appropriate solution medium and the concentration of reactants were chosen to study the stoichiometry of Cu:TM in solution before precipitation occurs.<p> Conditions for preparation of solid CuTM adducts were chosen to obtain the amount of solid necessary for further studies and to eliminate the formation of unwanted side-products. In the solid phase, the stoichiometry of Cu:TM was investigated by elemental analysis. EPR spectroscopy of solid CuTM samples revealed the presence of diamagnetic species which were partially identified.<p> EXAFS studies were performed on both the Mo and Cu K edges. Spectra defined the oxidation states of both main metal atoms, Cu and Mo. Fitting of the measured EXAFS spectra of solid CuTM samples prepared at three different Cu:TM ratios determined the interatomic distances involved in these compounds. This data was used to determine the structure of the CuTM units which polymerize to form the final product. IR spectroscopy was also used to confirm the presence of some of the groups in these adducts.<p> All the results from the various spectroscopic techniques were then combined to proposed a complete reaction scheme for the two-step formation of the CuTM4 and CuTM3 adduct species.

Cu is an important trace element in living systems, including animals. It is a part of many enzymes responsible for different processes occurring in the body. However, antagonists can severely decrease the amount of Cu available to the animal, which results in numerous health issues caused by Cu deficiency. The main antagonists of the Cu are thiomolybdates whose effect is especially harsh in ruminants due to their high potential to synthesize these compounds. Thiomolybdates (the ions MoOxS4-x2- ,where x = 0-3) are formed in the rumen from molybdate and sulfide (from degradation of sulfate), both of which come from the food and water ingested by the animal.<p> The first part of the project described in this thesis was to develop reproducible preparative methods of CuTM samples in solution and solids, under conditions as biologically relevant as possible. This crucial information is missing in the literature related to Cu deficiency. The second part studied the structure of the adduct formed between Cu and two of the thiomolybdates: tetrathiomolybdate, TM4 and trithiomolybdate, TM3, as these are the most important in Cu deficiency.<p> In solution, the appropriate solution medium and the concentration of reactants were chosen to study the stoichiometry of Cu:TM in solution before precipitation occurs.<p> Conditions for preparation of solid CuTM adducts were chosen to obtain the amount of solid necessary for further studies and to eliminate the formation of unwanted side-products. In the solid phase, the stoichiometry of Cu:TM was investigated by elemental analysis. EPR spectroscopy of solid CuTM samples revealed the presence of diamagnetic species which were partially identified.<p> EXAFS studies were performed on both the Mo and Cu K edges. Spectra defined the oxidation states of both main metal atoms, Cu and Mo. Fitting of the measured EXAFS spectra of solid CuTM samples prepared at three different Cu:TM ratios determined the interatomic distances involved in these compounds. This data was used to determine the structure of the CuTM units which polymerize to form the final product. IR spectroscopy was also used to confirm the presence of some of the groups in these adducts.<p> All the results from the various spectroscopic techniques were then combined to proposed a complete reaction scheme for the two-step formation of the CuTM4 and CuTM3 adduct species.

An overview of synchrotrons and synchrotron radiation is presented, along with the theory and practical considerations behind several types of X-ray spectroscopy. The theory and practical considerations of density functional theory are also given, with direct reference to some specific software packages.<p> Some synchrotron-excited X-ray spectroscopy measurements and density functional theory calculations of selenium and arsenic-doped selenium films are then outlined. The physical structure of crystalline and amorphous selenium and the electronic structure of amorphous selenium are discussed and comparison is made to the experimental results.<p> A weak feature in the conduction band is identified as a "fingerprint" of the degree of crystallization in amorphous selenium from X-ray absorption measurements. Similarly, a weak feature corresponding to lone-pairs in the valence band is identified as a "fingerprint" of the arsenic concentration from X-ray emission measurements.<p> Finally a detailed model of the structure of amorphous selenium is explained, and compared to experiment. This model is tested both by direct calculations and by a reverse Monte Carlo approach. The implications of this model with respect to the structure of amorphous and arsenic-doped amorphous selenium are discussed. Calculations suggest that simply randomizing the arrangement of "perfect" trigonal selenium is unable to reproduce the measurements of amorphous selenium; a moderate variation in the bond angle of "perfect" trigonal selenium is also necessary.