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251 results on '"Grenèche, J.-M."'

7. Coupled structural and magnetic properties of ferric fluoride nanostructures part I: a Metropolis atomistic study

Author

Fongang, B., Labaye, I., Zekeng, S., Calvayrac, F., and Greneche, J. M.

Subjects
Condensed Matter - Mesoscale and Nanoscale Physics, Condensed Matter - Materials Science
Abstract

A modified Metropolis atomistic simulation is proposed to model the structure of grain boundaries (GBs) and interfaces in ionic nanostructured systems and is applied to the magnetically interesting case of iron trifluoride (FeF3). We chose long-range interatomic potentials adjusted on experimental results, and adapted a previously established Monte Carlo scheme consisting of various modifications of the simulated annealing/ Metropolis algorithm. Atomic structures of twisted and tilted GBs as a function of the relative disorientation of the grains have been achieved yielding close to experimentally measured properties. This approach takes into account the structure of the grains far from the interface in order to constrain the relative orientation of the grains, without any periodic boundary conditions. One concludes that a long-range coulombic falloff of the interatomic potentials is necessary to obtain GB structures presenting a correct local topology but with a smooth transition from crystalline to amorphous states. The structural features are finally discussed in terms of topological aspects and local magnetic structure., Comment: submitted to JMMM

Published
2010
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8. Coupled structural and magnetic properties of ferric fluoride nanostructures: part II, a Monte-Carlo Heisenberg study

Author

Fongang, B., Labaye, I., Greneche, J. -M., Zekeng, S., and Calvayrac, F.

Subjects
Condensed Matter - Mesoscale and Nanoscale Physics, Condensed Matter - Materials Science
Abstract

We present a numerical study of the magnetic structure of nanostructured iron fluoride, using the Monte-Carlo-Metropolis simulated annealing technique and a classical Heisenberg Hamiltonian with a superexchange angle dependence. The parameters are adjusted on experimental results, and the atomic structure and topology taken from a previous atomistic model of grain boundaries in the same system. We find perfect antiferromagnetic crystalline grains and a disordered magnetic configuration (speromagnetic like) at the grain boundary, in agreement with experimental findings. Both the lowest magnetic energy and the rate of magnetic frustration are found to be dependent on the relative disorientation of crystalline grains, i.e. on the cationic topology. By simulating hysteresis loops, we find that the magnetization rotation is not spatially uniform. We conclude on possible extensions of the model., Comment: submitted to JMMM

Published
2010
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10. Assessing the reactivity of Fe(II) sorbed on smectite clays: U(VI) reduction

Author

Chakraborty, S., Banerjee, D., (0000-0002-6608-5428) Scheinost, A., Grenèche, J.-M., Favre, F., Géhin, A., Charlet, L., Chakraborty, S., Banerjee, D., (0000-0002-6608-5428) Scheinost, A., Grenèche, J.-M., Favre, F., Géhin, A., and Charlet, L.

Abstract

The reactivity of Fe(II) sorbed on three Ca-exchanged smectite clays has been probed via U(VI) reduction at pH 6.0 (± 0.2) under CO2-free, anoxic (O2 <1 ppmv) condition. The clays varied with regard to structural Fe content from Fe-free (0 wt.%) montmorillonite (MONT), to Fe-poor (2.6 wt.%) montmorillonite (Fe-MONT), to Fe-rich (25.8 wt.%) nontronite (NAu-2). U LIII-edge XANES spectra showed no reduction of U(VI) in presence of Fe(II) sorbed on either Fe-MONT or NAu-2 after 72 h but a partial reduction (21%) on MONT after 24 h. U LIII-edge EXAFS spectra further showed the formation of a soddyite-like surface precipitate on MONT in presence of sorbed Fe(II) after 72 h. The Mössbauer data further reveals that 10 (± 2)% of the total sorbed Fe(II) was oxidized in presence of MONT before and an additional 6 (± 2)% after U(VI) addition. The mechanism of U(VI) reduction involves Fe(II) specifically sorbed on oxidized and reduced strong edge sites of MONT. The non-reactivity of Fe(II) sorbed on Fe-MONT and NAu-2 towards U(VI) reduction is likely linked to the inter-valence charge transfer (IVCT) between surface Fe(II) and structural Fe(III). The present study demonstrates that the reduction capacity of surface bound Fe strongly depends on the nature of clay and correspondingly on the oxidation state of the sorbed Fe. Thus Fe(II) sorbed clay may not be always considered as “universal reductant” for U(VI).

Published
2023

33. Magnetic Aerogels

Author

Casas, Ll., Roig, A., Moner-Girona, M., Molins, E., Asenjo, J., Tejada, J., Grenèche, J. M., and Hadjipanayis, George C., editor

Published
2001
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35. Polyol-Made Spinel Ferrite Nanoparticles—Local Structure and Operating Conditions: NiFe2O4 as a Case Study

Author

GAUDISSON, T., Nowak, S., Nehme, Z., Menguy, N., Yaacoub, N., Grenèche, J.-M., Ammar, S., Interfaces, Traitements, Organisation et Dynamique des Systèmes (ITODYS (UMR_7086)), Institut de Chimie du CNRS (INC)-Centre National de la Recherche Scientifique (CNRS)-Université Paris Cité (UPCité), Institut des Molécules et Matériaux du Mans (IMMM), Le Mans Université (UM)-Institut de Chimie du CNRS (INC)-Centre National de la Recherche Scientifique (CNRS), Institut de minéralogie, de physique des matériaux et de cosmochimie (IMPMC), Muséum national d'Histoire naturelle (MNHN)-Institut de recherche pour le développement [IRD] : UR206-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS), and Institut de Chimie du CNRS (INC)-Centre National de la Recherche Scientifique (CNRS)-Université de Paris (UP)

Subjects
inorganic chemicals, 57Fe Mossbauer spectrometry, Technology, polyol-mediated annealing, [CHIM]Chemical Sciences, local structure analysis, 57 Fe Mossbauer spectrometry, ferrite nanoparticles, polyol process
Abstract

International audience; We report the effect of a polyol-mediated annealing on nickel ferrite nanoparticles. By combining X-ray fluorescence spectroscopy, X-ray diffraction, and 57 Fe Mössbauer spectrometry, we showed that whereas the as-prepared nanoparticles (NFO) are stoichiometric, the annealed ones (a-NFO) are not, since Ni 0-based crystals precipitate. Nickel depletion from the spinel lattice and reduction in the polyol solvent are accompanied with an important cation migration. Indeed, thanks to Mössbauer hyperfine structure analysis, we evidenced that the cation distribution in NFO departs from the thermodynamically stable inverse spinel structure with a concentration of tetrahedrally coordinated Ni 2+ of 20 wt-% (A sites). After annealing, and nickel demixing, originated very probably from the A sites of NFO lattice, the spinel phase accommodates with cation and anion vacancies, leading to the (Fe 3+ 0.84□0.16) A [Ni 2+ 0.80 Fe 3+ 1.16□0.04 ] B O 4-0.20 formula, meaning that the applied polyol-mediated treatment is not so trivial.

Published
2021

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