A different look at the spin state of Co3+^{3+} ions in CoO5_{5} pyramidal coordination

Using soft-x-ray absorption spectroscopy at the Co-$L_{2,3}$ and O-$K$ edges, we demonstrate that the Co$^{3+}$ ions with the CoO$_{5}$ pyramidal coordination in the layered Sr$_2$CoO$_3$Cl compound are unambiguously in the high spin state. Our result questions the reliability of the spin state assignments made so far for the recently synthesized layered cobalt perovskites, and calls for a re-examination of the modeling for the complex and fascinating properties of these new materials.Comment: 5 pages 3 figure

Electric-field-induced charge-transfer phase transition: a promising approach toward electrically switchable devices

Much research has been directed toward the development of electrically switchable optical materials for applications in memory and display devices. Here we present experimental evidence for an electric-field-induced charge-transfer phase transition in two cyanometalate complexes: Rb₀․₈Mn-[Fe(CN)₆]₀․₉₃•1.62H₂O and Co₃[W(CN)₈]₂(pyrimidine)₄•6H₂O, involving changes in their magnetic, optical, and electronic properties as well. Application of an electric field above a threshold value and within the thermal hysteresis region leads to a transition from the high- to the low-temperature phase in these compounds. A model is proposed to explain the main observations on the basis of a para-ferroelectric transition. Our observations suggest that this new concept of electrical switching, based on materials exhibiting charge-transfer phase transitions with large thermal hysteresis loops, may open up doors for novel electro-optical devices

Temperature- and Light-Induced Spin Crossover Observed by X-ray Spectroscopy on Isolated Fe(II) Complexes on Gold

Using X-ray absorption techniques, we show that temperature- and light-induced spin crossover properties are conserved for a submonolayer of the [Fe(H2B(pz)2)2(2,2′-bipy)] complex evaporated onto a Au(111) surface. For a significant fraction of the molecules, we see changes in the absorption at the L2,3 edges that are consistent with those observed in bulk and thick film references. Assignment of these changes to spin crossover is further supported by multiplet calculations to simulate the X-ray absorption spectra. As others have observed in experiments on monolayer coverages, we find that many molecules in our submonolayer system remain pinned in one of the two spin states. Our results clearly demonstrate that temperature- and light-induced spin crossover is possible for isolated molecules on surfaces but that interactions with the surface may play a key role in determining when this can occur

Wettability Contrasts in Kaolinite and Illite Clays : Characterization by Infrared and X-ray Absorption Spectroscopies

A reservoir rock is a porous geological formation in contact with two liquids : brine and oil. An improved knowledge of rock wettability is of primary importance to estimate the amount of crude oil in underground resources. The petroleum industries have observed that wettability contrasts in sedimentary reservoir rocks are largely correlated to the presence of clays (illite and/or kaolinite) in the rock's intergranular space. More precisely, the grain surfaces of illite show a preference to brine. Kaolinite preferentially adsorbs oil, which imparts its hydrophobic characteristics to the mineral surface. Using X-ray absorption and Fourier Transform Infrared spectroscopies, we studied at the atomic level the adsorption process of asphaltenes in the presence of water. We show experimentally that the wettability contrasts observed in kaolinite and illite are related to structural differences between these two clays and we show the primordial role of the grain surface hydroxyls. At last, the role of the water in the asphaltenes adsorption process has been investigated, using the same spectroscopies

New synthetic route to rare earth doped halide clusters

We illustrate in this work the use of free atoms of Er to elaborate solvent soluble NbCl5 target molecules into increasingly extended condensed phase compounds. Reductive condensation of NbCl5 by dechlorination with atomic Er gives molecular cluster antecedents resembling fragments of infinite solid NbCl4. In the light of XPS and EXAFS measurements, we propose a condensation mechanism consistent with these data

In Situ EXAFS Characterisation of the Synthesis of Polynuclear Chloroniobates from Sm Atom Reactions with Niobium Pentachloride

A new method to prepare solid state chlorometallates by reductive dechlorination of solvated niobium pentachloride with vaporisation of nominally free atoms of samarium is described. Products were prepared in an original set-up by evaporation of samarium into a mixed toluene solution of NbCl5, at 180 K. The local order around the Nb atoms was probed by EXAFS spectroscopy at the Nb K edge, using an in situ cryostat coupled to the metal vapour oven, in order to better understand the condensation mechanisms. These in situ measurements confirm our previous ex situ EXAFS studies with solid state products obtained by Sm vaporisation : condensation of NbCl, by face-sharing octahedra

Molecular and electronic structure of AnIV FeII (CN)6 xH2O (AN = Th, U, Np) compounds an X-ray absorption spectroscopy investigation

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XMCD of a single layer of single molecule magnets

A detailed report on the X-ray Magnetic Circular Dichroism (XMCD) investigation of monolayers of Mn12-based single molecule magnets (SMMs) deposited on gold Au(111) is presented. A semi-quantitative analysis of data is provided in order to extract chemical and magnetic information on Mn ions, by comparison with XMCD on bulk samples. This work points that XMCD is a key-tool for the characterization of SMMs-based nanostructured systems. XMCD surface sensitivity and element-specificity will play a fundamental role in the identification of good candidates for SMMs based devices

La molécule, précurseur du solide magnétique

L’obtention et l’étude de nouveaux objets magnétiques pour répondre à de nouvelles applications ou à de nouvelles questions théoriques est un des domaines de coopération fructueuse entre chimistes et physiciens. Nous donnons ici quelques exemples de nouveaux systèmes magnétiques moléculaires synthétisés de manière rationnelle afin de développer certaines de leurs propriétés physiques