Interaction argiles-polluants métalliques dans le cadre des centres de stockage de déchets: Expériences d'infiltration sous pression et en batch d'une smectite de référence SWy-2 avec des solutions de zinc

International audienceLe stockage des déchets ménagers et assimilés est devenu un enjeu important pour l'environnement. La réglementation exige la présence d'une barrière de sécurité avec une étanchéité optimale afin d'éviter la contamination du sous-sol. Les smectites, ayant une faible perméabilité et une forte capacité à retenir les polluants, sont donc fréquemment utilisées comme barrière de sécurité passive au fond des alvéoles de stockage. Cependant ces argiles doivent assurer la pérennité de ces barrières en contact avec le lixiviat et particulièrement avec les polluants métalliques qu'il contient. Ce travail consiste à simuler au laboratoire l'infiltration des solutions de polluants métalliques dans les argiles, d'analyser les mécanismes d'interaction et donc de prévoir le comportement hydraulique et physico-chimique de ces argiles. Pour cela, nous utilisons la smectite naturelle de référence SWy2 échangée avec Na+ ou Ca2+ notée Na-SWy2 ou Ca-SWy2. Les échantillons d'argile Na-SWy2 et Ca-SWy2 ont été compactés dans des cellules oedométriques jusqu'à 0,5 MPa, puis infiltrés à une pression constante de 0,3 MPa avec de l'eau pure. La conductivité hydraulique, calculée à partir des volumes injectés et de la loi de Darcy, est très faible, de l'ordre de 10-13 m/s pour les deux types d'argiles, preuve de l'importante étanchéité de cette argile. En complément de ces expériences oedométriques de longue durée (plusieurs mois), des expériences en batch où l'équilibre est atteint rapidement permettent de tester les principaux paramètres qui influent sur les mécanismes d'adsorption. Des essais ont été menés sur Na-SWy2 avec des solutions de Zn(NO3)2 tamponnées à pH=5 avec un rapport solide/liquide de 10 g/L et à force ionique contrôlée mais à différentes concentrations initiales de Zn(NO3)2. La variation du taux de Zn adsorbé en fonction de sa concentration à l'équilibre dans la solution montre que l'adsorption du zinc varie avec la concentration initiale de la solution mère. 80% du zinc initialement mis en solution est adsorbé par l'argile Na-SWy2 à une concentration initiale de 10-5 mol.L-1, représentative des teneurs moyennes dans les lixiviats des centres de stockage. Plus on augmente la concentration initiale de Zn, plus le taux d'adsorption diminue jusqu'à atteindre une valeur de 15% pour la plus forte concentration initiale de 10-2 mol.L-1

Magnetodielectric coupling and multi-blocking effect in the Ising-chain magnet Sr2Ca2CoMn2O9

We have demonstrated magnetodielectric (MD) coupling in an Ising-chain magnet Sr2Ca2CoMn2O9, via detailed investigation of ac susceptibility and dielectric constant as a function of temperature, magnetic field and frequency. Sr2Ca2CoMn2O9 consists of spin-chains, made of the regular stacking of one CoO6 trigonal prism with two MnO6 octahedra. The (Co2+ Mn4+ Mn4+) unit stabilizes a (up-down-up) spin-state along the chains which are distributed on a triangular lattice. This compound undergoes a partially disordered antiferromagnetic transition at TN ~ 28 K. The dielectric constant exhibits a clear peak at TN only in presence of an external magnetic field (above 5 kOe), evidencing the presence of MD coupling, which is further confirmed by field-dependent dielectric measurements. We argue that spatial inversion symmetry can be broken as a result of exchange-striction along each spin chain, inducing uncompensated local dipoles. At low temperatures, a dipolar relaxation phenomenon is observed, bearing strong similarities with the blocking effect typical of the spin dynamics in this compound. Such a spin-dipole relationship is referred to as a multi-blocking effect, in relation with the concept of magnetodielectric multiglass previously introduced for related materials.Comment: Multiferroicity, Magnetoelectric coupling, Complex exchange interaction, spin-chain oxid

Canted antiferromagnetism in high purity NaFeF3\mathrm{NaFeF_3} prepared by a novel wet-chemical synthesis method

We report a novel synthesis method for, and structural and magnetic characterization of the fluoroperovskite $\mathrm{NaFeF_3}$. We have developed a wet-chemical method that allows preparation of large volumes of air-sensitive fluoroperovskites with high purity. $\mathrm{NaFeF_3}$ has a N\'eel temperature ($T_N$) of 90 K and a Weiss constant ($\theta$) of -124 K, corresponding to dominant antiferromagnetic interactions. Below $T_N$, a slight difference is observed between zero-field and field cooled samples, indicating spin-canting and weak ferromagnetism. AC magnetometry confirms that weak ferromagnetism is inherent to $\mathrm{NaFeF_3}$ and not due to impurities. From powder neutron diffraction data, we describe the magnetic structure precisely as a weakly canted G-type (magnetic space group $Pn'ma'$). A ferromagnetic component is allowed in $Pn'ma'$, however, this component may be absent in zero magnetic fields and is too small to be confirmed on the basis of powder neutron diffraction data.Comment: 9 pages, 10 figure

Interaction of a synthesized Na-montmorillonite compacted and percolated with pollutants (Pb, Zn): application to subsurface waste landfill

International audienceDue to their low permeability and their capacity to retain pollutants, smectites are of great importance for industrial applications, in particular as engineered barrier in landfills. One of the main challenges is to predict the long-term behaviour of these clays, especially the evolution of their properties at different scales depending on interactions with pollutant metals. The originality of this study was to reproduce at a laboratory scale the in-situ compaction of clays soaked by metallic solutions coupled with the use of synthesized smectites whose chemical composition and charge location could be fixed. For this purpose, hydrothermal syntheses were carried out at 350°C and 120 MPa for 28 days, using the gelling method of Hamilton (1968) (Lantenois et al., 2008). As expected, the synthesized product contained one well-crystallized dioctahedral smectite phase (montmorillonite), as shown by X-ray diffraction, and had the following structural formulae, confirmed by ICP-ES and microprobe analyses: Na0.66[Al3.34Mg0.66][Si8]O20(OH)4.nH2O. Percolation experiments were performed with œdometer cells equipped with an injection system (Jullien et al., 2002). Clay samples were compacted up to 0.5 MPa then percolated under a constant pressure of 0.3 MPa, either with water or with metallic (Zn2+ or Pb2+) nitrate solutions at 2.10-3 mol/L. The hydraulic conductivity, k, was calculated from the measured injected volume and using the Darcy’s law. Low k values in the order of 10-12 m/s were obtained, either with water or with Pb2+ or Zn2+ solutions. After a percolation of nine months, neither Pb2+ nor Zn2+ was detected in leachates, which show their total retention in the compacted montmorillonite. Moreover, the measured Na+ contents in leachates coupled with the geochemical modelling (Phreeqc2) highlight the cationic exchange as the main mechanism for this pollutant retention. At the end of these percolation experiments, the compacted clay samples will be investigated along a vertical profile by using X-ray diffraction, scanning and transmission electron microscopy, and electron microprobe.Hamilton D.L., Henderson C.M.B. (1968). The preparation of silicate compositions by a gelling method. Mineralogical Magazine, 36, 832-838.Jullien A., Proust C., Le Forestier L., Baillif P. (2002). Hydro-chemio-mechanical coupling effects on permeability and swelling behaviour of a Ca smectite soaked by Cu solutions. Applied Clay Science, 21, 143-153.Lantenois S., Champallier R., Bény J.M., Muller F. (2008). Hydrothermal synthesis and characterization of dioctahedral smectites: A montmorillonites series. Applied Clay Science, 38, 165-178

Raman spectroscopic and magnetic properties of Europium doped nickel oxide nanoparticles prepared by microwave-assisted hydrothermal method

International audiencePure and 5 mol% Eu doped nickel oxide (NiO) nanoparticles were synthesized by microwave-assisted hydrothermal method. X-ray diffraction and Rietveld refinement analysis confirmed the phase and purity of prepared nanoparticles. Scanning electron microscopy analysis revealed that the undoped nanoparticles are porous flake-like structures while the doped samples were aggregated. X-ray photoelectron spectroscopy analysis disclosed that Eu doping increases hole concentration, Ni2+ vacancies, and defective oxygen concentration in the nanoparticles. The magnetic analysis shows the presence of weak ferromagnetism in lower fields of the undoped nanoparticles. On doping with Eu, the ferromagnetic behaviour increases due to increased oxygen defects and uncompensated surface spins. Raman spectroscopy investigation shows the absence of 2 M vibrational mode and reduced antiferromagnetic coupling in the prepared samples. It also substantiated the presence of defects in the prepared samples. Eu doping produced local vibrations that overlap with SO mode of NiO and shifting of 1 M band due to its higher mass. Eu dopant can modify the magnetic property of NiO nanoparticles that can be used in spintronic applications

Raman spectroscopic and magnetic properties of Europium doped nickel oxide nanoparticles prepared by microwave-assisted hydrothermal method

International audiencePure and 5 mol% Eu doped nickel oxide (NiO) nanoparticles were synthesized by microwave-assisted hydrothermal method. X-ray diffraction and Rietveld refinement analysis confirmed the phase and purity of prepared nanoparticles. Scanning electron microscopy analysis revealed that the undoped nanoparticles are porous flake-like structures while the doped samples were aggregated. X-ray photoelectron spectroscopy analysis disclosed that Eu doping increases hole concentration, Ni2+ vacancies, and defective oxygen concentration in the nanoparticles. The magnetic analysis shows the presence of weak ferromagnetism in lower fields of the undoped nanoparticles. On doping with Eu, the ferromagnetic behaviour increases due to increased oxygen defects and uncompensated surface spins. Raman spectroscopy investigation shows the absence of 2 M vibrational mode and reduced antiferromagnetic coupling in the prepared samples. It also substantiated the presence of defects in the prepared samples. Eu doping produced local vibrations that overlap with SO mode of NiO and shifting of 1 M band due to its higher mass. Eu dopant can modify the magnetic property of NiO nanoparticles that can be used in spintronic applications

Enhanced magnetic frustration in a new high entropy diamond lattice spinel oxide

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Origin of large magnetocapacitance in K 0.5 Na 0.5 NbO 3 / La 0.67 Sr 0.33 MnO 3 superlattices

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Long Range Magnetic Ordering and Magneto-(di) electric Effect in a New Class of High Entropy Spinel Oxide

International audienceHerein, a new class of high entropy spinel material with composition (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)Cr2O4 is introduced. Detailed structural and microstructural characterizations highlight that the sample crystallizes in a cubic spinel structure with an excellent chemical homogeneity at the nanoscale. A peak in the dielectric measurement is observed at the antiferromagnetic ordering temperature (35 K), indicating the possibility of magneto-electric coupling at that temperature. Our study, for the first time, highlights the feasibility to accommodate multiple elements onto a single sublattice in a complex spinel structure and opens new possibilities to design and tailor functional properties in high entropy stabilized correlated electron systems

Enhanced magnetic frustration in a new high entropy diamond lattice spinel oxide

International audienceHigh‐entropy stabilization chemistry offers a new paradigm in the design of functional materials stabilized by the high configurational entropy. In the present work, a new high entropy spinel material with composition (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)Al2O4 is introduced, detailed structural and microstructural characterizations highlight that the material crystallizes in a cubic spinel structure with an excellent chemical homogeneity at the nanoscale. Magnetic studies show that the frustration (magnetic) parameter (f =│θCW│/TM> 27) is significantly enhanced in this new diamond lattice spinel material