Seuils d'absorption des rayons X : un outil de caractérisation indispensable en chimie de coordination.

National audienceSynchrotron radiation allows to perform spectroscopic experiments in an energy range from infra-red to hard X-rays. The paper shows, through particular examples (role of the geometry, of the oxidation and spin state of the absorber ...) how it is possible to extract unique informations from X-ray absorption edges to characterize the electronic structure of the absorbing elements whatever the state of the sample (crystalline, amorphous, liquid, solution, gaseous). The interpretation of the data is proposed at a double level : qualitative, thanks to symmetry theory and more quantitative, thanks to quantum calculations

STRUCTURE MOLECULAIRE ET ELECTRONIQUE DES HEXACYANOFERRATES D'ACTINIDES

ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Spectroscopie d'absorption X et dichroïsme circulaire magnétique (sondes locales de la structure électronique et du magnétisme de matériaux moléculaires inorganiques)

Le Magnétisme Moléculaire est un domaine de recherche pluridisciplinaire récent comprenant des thématiques variées. Dans ce cadre, nous montrons comment la Spectroscopie d'Absorption des rayons X (SAX), le Dichroïsme Circulaire Magnétique (DCMX), et les outils théoriques qui les accompagnent, permettent d'obtenir des informations uniques sur les structures électroniques et magnétiques locales des ions porteurs de spins dans des systèmes représentatifs des nouvelles orientations de ce domaine de recherche. Le DCMX a permis, pour la première fois, de caractériser la nature de l'interaction d'échange dans des analogues cobalt-fer photomagnétiques du bleu de Prusse. En parallèle, nous avons quantifié par SAX, couplée à des calculs de multiplet, le champ cristallin des ions Co(II), paramètre à l'origine des propriétés magnétiques dans ces systèmes, et étudié son évolution au cours de l'irradiation. Ces études nous ont amenés à proposer une hypothèse sur l'évolution structurale qui accompagne le transfert d'électron photo induit. Ce couplage expérience-théorie nous a également permis de développer une nouvelle approche permettant d'avoir accès à la cartographie de la densité de spin dans des systèmes moléculaires. Nous l'illustrons sur l'exemple de composés formés d'ions Mn(II) et Cu(II) en interaction d'échange antiferromagnétique qui présentent un effet quantique de perte d'aimantation locale. Un premier signal DCMX a été obtenu sans champ magnétique, à la rémanence, dans des aimants à base moléculaire. Ce travail ouvre la voie à l'étude de l'origine de la rémanence dans des molécules aimants comme le Mn(12) aux propriétés magnétiques originales. Enfin, nous avons sondé, en utilisant la sélectivité orbitalaire du DCMX, les différents chemins d'interaction d'échange à l'origine du phénomène de frustration de spin dans des chaînes où alternent des ions Gd(III) et des radicaux organiques.Molecular Magnetism is a recent pluridisciplinary field of research on various topics. We show, in this thesis, how X-ray Absorption Spectroscopy (XAS), X-ray Magnetic Circular Dichroism (XMCD) and their theoretical tools give information on the electronic and magnetic structures of spin carrier ions in systems representative of the new trends in this domain. For the first time, XMCD has enabled to characterize the nature of the exchange interaction in photomagnetic cobalt-iron Prussian blue analogs. Beside, we have quantified by XAS, coupled with multiplet calculations, the Co(II) cations crystal field in these systems, which governs the magnetic properties, and studied its evolution during the irradiation. These studies led us to propose an hypothesis on the structural evolution that comes with the light induced electron transfer. We have developed a new approach through the coupling between experiment and multiplet calculations giving access to the spin density in molecular systems. We illustrate this point on the example of clusters of Mn(II) and Cu(II) cations antiferromagnetically coupled and presenting a quantic effect of local magnetization loss. A first remnant XMCD signal was obtained in molecular-based magnets. This works is the first step in the study of the origins of remanence in single molecule magnets presenting original magnetic properties such as Mn(12). Finally, using the orbital selectivity of XMCD, we probed the different exchange interaction pathways responsible for the spin frustration phenomenon observed in Gd(III) and organic radicals alternating one-dimensional chains.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Spin Transition Evidenced by Soft X-ray Absorption Spectroscopy

Iron L2,3 X-ray absorption spectra have been recorded in order to follow the thermal reversible spin interconversion of the FeII(phen)2(NCS)2 (phen = o-phenanthroline). Important differences are observed which show that this spectroscopy is a fingerprint of the spin state. The evolution of the branching ratio is discussed and found to be consistent with the expected value obtained by multiplet calculations.

Structural Rearrangements Induced by Photoexcitation in a RbCoFe Prussian Blue Derivative

International audienceSystems that undergo a reversible and controlled change of their physical properties offer appealing perspectives for the elaboration of electronic devices. Evidence of a photomag- netic effect in a CoFe Prussian blue analogue was a milestone in this research area and showed that molecular excitation can induce long-range magnetic order.[1] In the face-centered cubic structure of the CoFe Prussian blue analogues of chemical formula MxCo4[Fe(CN)6](8+x)/3&(

A chemical model of intermediate states implied in the switching properties of CoFe Prussian blue analogues: how a cell parameter lengthening can cause a crystal field parameter increase

International audienceA series of CoFe Prussian blue analogues of chemical formula Rb2Co4 xZnx[Fe(CN)6]3.3 11H2O (x = 0, 1, 1.95 and 2.7) has been synthesized along which the MII/CoIII ions ratio at the Co site has been tuned. The long range order and the electronic structure of the Co ions have been investigated by combined powder X-ray diffraction and X-ray absorption spectroscopy (XAS) measurements. The cell parameter of the face-centered cubic structure lengthens as the MII/CoIII ions ratio increases without phase demixing. The study of the electronic structure of the Co ions by XAS shows that the coordination polyhedra of the CoII(HS) ions play an important role in the flexibility of the cubic structure. The variation of the cell parameter in the series of compounds is accompanied by the variation of the CoII-NC bond angle which allows the expansion or contraction of the cubic structure accompanying the electronic switch without phase demixing. Due to this structural re-arrangement, a lengthening of the cell parameter unusually produces an increase of the Co ion crystal field. Such a re-arrangement occurs in the course of the photo-induced electron transfer

Introduction

Introduction générale de l'ouvrag