Photomagnétisme dans les analogues cobalt-fer du bleu de prusse (rôle des espèces du réseau tridimensionnel)

Le photomagnétisme est un axe de recherche en pleine expansion dans le domaine des matériaux inorganiques moléculaires, motivé en partie par les éventuelles perspectives d application pour le stockage de l information. C est dans ce cadre que s inscrit ce travail de thèse. En 1996, l équipe du professeur Hashimoto, à Tokyo, met en évidence un cas de magnétisme photoinduit dans un analogue cobalt-fer du bleu de Prusse. Un transfert d électron photoinduit CoIII(BS)-NC-FeII(BS) CoII(HS)-NC-FeIII(BS) dans des paires diamagnétiques, formées au cours de la synthèse du matériau, est à l origine du gain d aimantation après irradiation, au-dessous de 20 K. Les analogues CoFe du bleu de Prusse restent de bons modèles très intéressants à étudier car ils reproduisent parfaitement la fonction de mémoire des composants traditionnellement utilisés dans l industrie électronique. Pour pouvoir un jour espérer utiliser les analogues CoFe du bleu de Prusse pour stocker une information, il faut impérativement augmenter la température d observation de l effet photomagnétique. Pour cela nous allons chercher à mieux comprendre le rôle joué par les différentes espèces du réseau tridimensionnel dans un analogue photomagnetique de composition chimique Rb2Co4[Fe(CN)6]3,3 11H2O pour but de trouver les conditions chimiques nécessaires pour observer l effet photomagnétique et moduler les propriétés. Les espèces chimiques du réseau 3D sont le ligand cyanure, les espèces composant le réseau tridimensionnel (les ions Co et les entités [Fe(CN)6] et le cation alcalin. La determination du paramètre de champ cristallin des ions Co dans l analogue photo-commutable avant et après irradiation par spectroscopie absorption des rayons X au seuil L2,3 a permis de définir le rôle joué par le ligand cyanure sur les propriétés photomagnétiques. Les études des effets de la substitution/dilution ont permis de comprendre les rôles joués par les espèces Co, les entités [Fe(CN)6] et les cations alcalins sur les propriétés photomagnétiques.One of the challenges in the field of molecular magnetism is the design of optically and thermally switchable magnetic materials for which various kinds of applications may be feasible. The long-known photomagnetic CoFe Prussian blue analogs (PBA) appear to have great potentials for these technological applications. The photomagnetic properties in Co-Fe PBA were evidenced by Hashimoto et al. ten years ago. They are due to a photo-induced electron transfer CoIII(LS)-FeII(LS) CoII(HS)-FeIII(LS). Some significant results concerning the formation of photomagnetic CoIII-FeII pairs and the effect of the amount and nature of inserted alkali-metals on the photomagnetic properties of these compounds have been reported. However the phenomenon is not yet totally understood. In order to understand the switching properties in the Co-Fe Prussian blue analogues, we have studied the role play by each chemical species forming the 3D network in the photomagnetic Prussian blue analogue of chemical formula Rb2Co4[Fe(CN)6]3,3 11H2O. The chemical species forming the 3D network are Co ions, [Fe(CN)6] entities and alkali metal ions. The determination of the Co ligand field parameter in the rubidium compound has allowed the understanding of the role of the cyanide ligand. The effect of the substitutions of the Co, [Fe(CN)6] entities and the alkali metal ions have allowed to understand the role played by these species in the photomagnetic properties.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Optical and magnetic properties of the photo-induced state in the coordination network Na₂Co₄[Fe(CN)₆]_3.3.14H₂O.

The study of the metastable state, obtained by light irradiation of Na2Co4[Fe(CN)6]3.3.14H2O is reported using reflectivity and magnetic measurements. This compound is characterized by a charge transfer phenomenon occurring between a high-temperature phase (HT phase) formed by paramagnetic..

CoFe Prussian Blue Analogues under Variable Pressure. Evidence of Departure from Cubic Symmetry: X-ray Diffraction and Absorption Study

International audienc

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&(

XMCD at the Transition Metal K‑Edges as a Probe of Small Pressure-Induced Structural Distortions in Prussian Blue Analogues

Prussian blue analogues (PBAs) and derivatives have been extensively studied for the wide variety of their properties (magnetic, optical, electrochemical, thermal, ...), and slight structural distortions have been repeatedly called on in order to explain them. Variable pressure X-ray magnetic circular dichroism (XMCD) at the Ni and Fe K-edges of a series of four NiFe PBAs (C_<i>x</i>Ni₄[Fe­(CN)₆]_{(8+<i>x</i>)/3}·<i>n</i>H₂O (<i>x</i> = 0, 1, 2 for C = Cs and <i>x</i> = 2 for C = Rb)) at 10 K over the 0–7 GPa pressure range was used here to get new insights into these slight structural distortions. XMCD signals have been found to exhibit a high sensitivity to slight structural distortions and to offer promising perspectives for their quantifications. Furthermore, XMCD signals at the transition metal K-edges are not yet well understood and PBAs appear as particularly well-suited model compounds in order to disentangle the components at the origin of these XMCD signals. The pressure dependence of the XMCD signals can be qualitatively explained in terms of 3d–4p and 4p–4p orbital overlapping changes and symmetry change of the transition metal sites. Therefore, this work also shed new light on exchange interactions between 3d localized and 4p delocalized electrons as well as on the interpretation of XMCD signals at the TM K-edges

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

Absence of Ce³⁺ Sites in Chemically Active Colloidal Ceria Nanoparticles

The catalytic performance of ceria nanoparticles is generally attributed to active sites on the particle surface. The creation of oxygen vacancies and thus nonstoichiometric CeO_2−δ has been proposed to result in Ce³⁺ sites with unpaired f electrons which can be oxidized to spinless Ce⁴⁺ ions during catalytic reactions. We monitored the Ce electronic structure during the synthesis and catalase mimetic reaction of colloidal ceria nanoparticles under <i>in situ</i> conditions. By means of high-energy resolution hard X-ray spectroscopy, we directly probed the Ce 4f and 5d orbitals. We observe pronounced changes of the Ce 5d bands upon reduction of the particle size and during the catalytic reaction. The Ce 4f orbitals, however, remain unchanged, and we do not observe any significant number of spin-unpaired Ce³⁺ sites even for catalytically active small (3 nm) particles with large surface to bulk ratio. This confirms strong orbital mixing between Ce and O, and the Ce spin state is conserved during the reaction. The particles show an increase of the interatomic distances between Ce and O during the catalytic decomposition of hydrogen peroxide. The redox partner is therefore not a local Ce³⁺ site, but the electron density that is received and released during the catalytic reaction is delocalized over the atoms of the nanoparticle. This invokes the picture of an electron sponge