Magnetism and the absence of superconductivity in the praseodymium–silicon system doped with carbon and boron

We searched for new structural, magnetic and superconductivity phases in the Pr-Si system using high-pressure high-temperature and arc melting syntheses. Both high and low Si concentration areas of the phase diagram were explored. Although a similar approach in the La-Si system produced new stable superconducting phases, in the Pr-Si system we did not find any new superconductors. At low Si concentrations, the arc-melted samples were doped with C or B. It was found that addition of C gave rise to multiple previously unknown ferromagnetic phases. Furthermore, X-ray refinement of the undoped samples confirmed the existence of the so far elusive Pr3Si 2 phase. © 2013 Elsevier B.V

Characterization of Serpentines from Different Regions by Transmission Electron Microscopy, X-ray Diffraction, BET Specific Surface Area and Vibrational and Electronic Spectroscopy.

[EN]Serpentinite powdered samples from four different regions were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), SBET and porosity measurements, UV-Vis and Infrared Spectroscopy of the skeletal region and surface OH groups. SEM micrographs of the samples showed a prismatic morphology when the lizardite was the predominant phase, while if antigorite phase prevailed, the particles had a globular morphology. The few fibrous-shaped particles, only observed by SEM and weakly detected by XRD on MO-9C and MO13 samples, were characteristic of the chrysotile phase. All diffraction XRD patterns showed characteristic peaks of antigorite and lizardite serpentine phases, with crystallite sizes in the range 310–250 Å and with different degrees and types of carbonation processes, one derived from the transformation of the serpentine, generating dolomite, and another by direct precipitation of calcite. The SBET reached values between 38–24 m2∙g−1 for the samples less crystalline, in agreement with the XRD patterns, while those with a higher degree of crystallinity gave values close to 8–9 m2∙g−1. In the UV region all electronic spectra were dominated by the absorption edge due to O2− → Si4+ charge transfer transition, with Si4+ in tetrahedral coordination, corresponding to a band gap energy of ca 4.7 eV. In the visible region, 800–350 nm, the spectra of all samples, except Donai, presented at least two weak and broad absorptions centred in the range 650–800 and 550–360 nm, associated with the presence of Fe3+ ions from the oxidation of structural Fe2+ ions in the serpentinites ((MgxFe2+1−x)3Si2O5(OH)4). The relative intensity of the IR bands corresponding to the stretching modes of the OH’s groups indicated the prevalence of one of the two phases, antigorite or lizardite, in the serpentinites. We proposed that the different relative intensity of these bands could be considered as diagnostic to differentiate the predominance of these phases in serpentinites.GIR CHARROC

Skeletal infrared spectra and structural properties of La2-xSrxCuO4 and La2-xBaxCuO4 cuprate powders in the 0 <= x <= 0.125 region

none5Several samples of the La2-xSrxCuO4 (or LaSCO) and La2-xBaxCuO4 (or LaBCO) cuprate families with stoichiometries within the 0 less than or equal to x less than or equal to 0.125 region have been synthesised by conventional solid state reaction at 1273 K. They were characterised by X-ray diffraction (XRD), Rietveld refinement analysis and FT-IR in the 1000-50 cm(-1) region. An expansion of the unit cell in the y direction is observed by increasing doping with Ba and Sr in the La2CuO4 structure. Doping with both Sr and Ba results in the progressive disappearance of the highest frequency IR band of La2CuO4. An empirical assignment of the absorption bands in IR spectra is proposed on the basis of the factor group analysis, and the internal vibrational structure of the CuOx structures is studied. (C) 1999 Elsevier Science B.V. All rights reserved.M. Napoletano;J. M. Gallardo;E. Magnone;G. Busca;M. FerrettiNapoletano, Myrta; J. M., Gallardo; Magnone, Edoardo; Busca, Guido; Ferretti, Maurizi

Is it possible to prepare olivine-type LiFeSiO4?. A joint computational and experimental investigation

Silicates LiMSiO4 are potential positive electrode materials for lithium ion batteries. In this work we analyse from first principles calculations the relative stability of possible LiFeSiO4-polymorphs within four structural types. Olivine-LiFeSiO4 is predicted to be more stable than the LiFeSiO4 prepared by delithiation of Li2FeSiO4; the latter being the only LiFeSiO4 compound reported so far. Attempts to prepare olivine-LiFeSiO4 from a mixture of reactants at ambient pressure (600-1100 °C) resulted in a mixture of quartz-SiO2, Li2SiO3, LiFe5O8 and LiFeSi2O6 phases. Conducting the reaction under HP conditions (40 kbar) leads to the formation of LiFeSi2O6 as a majority phase, regardless the nature of the reactants/precursors. First principles calculations indicate that the preparation of the olivine-LiFeSiO4 is thermodynamically hindered due to the competition with the more stable LiFeSi2O6 pyroxene, in the range of pressure/temperature investigated. © 2008 Elsevier B.V. All rights reserved