I. Álvarez-Serrano

Magnetic behaviour governed by Co spin transitions in LaCo1−xTixO3 (0 ≤ x ≤ 0.5) perovskite oxides

Perovskite-type cobaltites LaCo1−xTixO3 (0 ≤ x ≤ 0.5) synthesized by the liquid mix technique were characterized by x-ray diffraction, neutron diffraction, XPS, magnetic susceptibility and magnetization versus magnetic field measurements. Structural symmetry changes from rhombohedral (S.G. ) for 0.05 ≤ x ≤ 0.20 to orthorhombic (S.G. Pbnm) for 0.25 ≤ x ≤ 0.40. The bond distances dependence on the composition, x, has been analysed from x-ray and neutron diffraction data. Both the structural and the spectroscopic (from XPS) results suggest that Ti4+ substitution implies the stabilization of low-spin CoIII ions and the evolution of magnetic moments in the paramagnetic zone is also coherent with this assumption. For small values of x both FM responses and the absence of saturation in the M versus H curves at 2 K are interpreted in terms of a magnetic frustration. For x > 0.25 the cobalt cations seem to stabilize as CoIII (rather than Co3+) coexisting with Co2+. In these more substituted perovskites, AFM Co2+–O–Co2+ interactions become predominant, leading to a well established AFM ordering for x = 0.4 and 0.5.

Transport properties of new Ti manganites: Sr2?xLaxMnTiO6 (0.25 ? x ? 1)

Sr2−xLaxMnTiO6 (0.25 ≤ x ≤ 1) orthorhombic perovskite-like system has been synthesized by the ceramic method. Structural characterization from x-ray and neutron diffraction shows a random distribution of Sr/La and Mn/Ti cations, in A- and B-sublattices, respectively, in all the cases. These materials show complex magnetic behaviour which can be interpreted in terms of frustrated FM interactions, considering a cluster-glass picture. M–I transitions have been observed for x = 0.75, 1 and negative MR responses in the ranges of 1000% and 90% have been detected at 40 K and 210 K, respectively. Magnetization (ZFC and FC) measurements in ac and dc are interpreted considering the existence of superparamagnetic clusters embedded in a paramagnetic matrix.

Non-symmetric superparamagnetic clusters in the relaxor manganites Sr2−xBixMnTiO6 (0 ≤ x ≤ 0.75)

The structural, microstructural, electrical and magnetic characterization of microcrystalline powders of new Ti manganites Sr2−xBixMnTiO6 (0 ≤ x ≤ 0.75) is reported. Compositional characterization by means of EDS and XPS confirms the stoichiometry of all samples to be practically equal to the nominal ones. Neutron and X-ray diffraction data show that all the samples crystallize with cubic symmetry, space group Pmm, which is broken at the nanoscale as Raman spectroscopy and HRTEM/ED analysis suggest. The complex magnetic response is interpreted by considering the magnetic frustration derived from the presence of structural and atomic disorder; initial AFM behavior evolves to FM predominant interactions as x increases and superparamagnetic clusters stabilize above Tc. The variation of the permittivity with temperature for x ≥ 0.50, obtained from impedance measurements, seems to be related to a relaxor-type ferroelectric behaviour. The title materials could be regarded as a potential local multiferroic system, given that both the FE and FM responses seem to be dependent on the formation and development of BiMO3-type clusters, and therefore they are correlated.

Eco-Friendly Cavity-Containing Iron Oxides Prepared by Mild Routes as Very Efficient Catalysts for the Total Oxidation of VOCs

Iron oxides (FeOx) are non-toxic, non-expensive and environmentally friendly compounds, which makes them good candidates for many industrial applications, among them catalysis. In the present article five catalysts based on FeOx were synthesized by mild routes: hydrothermal in subcritical and supercritical conditions (Fe-HT, Few200, Few450) and solvothermal (Fe-ST1 and Fe-ST2). The catalytic activity of these catalysts was studied for the total oxidation of toluene using very demanding conditions with high space velocities and including water and CO2 in the feed. The samples were characterized by X-ray diffraction (XRD), scanning and high-resolution transmission electron microscopy (SEM and HRTEM), X-ray photoelectron spectroscopy (XPS) and nitrogen adsorption-desorption isotherms. It was observed that the most active catalyst was a cavity-containing porous sample prepared by a solvothermal method with a relatively high surface area (55 m2 g−1) and constituted by flower-like aggregates with open cavities at the catalyst surface. This catalyst displayed superior performance (100% of toluene conversion at 325 °C using highly demanding conditions) and this performance can be maintained for several catalytic cycles. Interestingly, the porous iron oxides present not only a higher catalytic activity than the non-porous but also a higher specific activity per surface area. The high activity of this catalyst has been related to the possible synergistic effect of compositional, structural and microstructural features emphasizing the role of the surface area, the crystalline phase present, and the properties of the surface.

Mapping chemical disorder and ferroelectric distortions in the double perovskite compound Sr 2-x Gd x MnTiO6 by atomic resolution electron microscopy and spectroscopy.

In this work we report a study of the chemical and structural order of the double perovskite compound Sr 2-x Gd x MnTiO6 for compositions x=0, 0.25, 0.5, 0.75, and 1. A noticeable disorder at the B-site in the Mn and Ti sublattice is detected at the atomic scale by electron energy-loss spectroscopy for all x values, resulting in Mn-rich and Ti-rich regions. For x ≥ 0.75, the cubic unit cell doubles and lowers its symmetry because of structural rearrangements associated with a giant ferroelectric displacement of the perovskite B-site cation. We discuss this finding in the light of the large electroresistance observed in Sr 2-x Gd x MnTiO6, x ≥ 0.75.

New Fe2O3-Clay@C Nanocomposite Anodes for Li-Ion Batteries Obtained by Facile Hydrothermal Processes

New iron-oxide-based anodes are prepared by an environmentally-friendly and low-cost route. The analysis of the composition, structure, and microstructure of the samples reveals the presence of a major hematite phase, which is accompanied by a certain concentration of an oxyhydroxide phase, which can act as a “lithium-reservoir”. By using sodium alginate as a binder, the synthesized anodes display superior electrochemical response, i.e., high specific capacity values and high stability, not only versus Li but also versus a high voltage cathode in a full cell. From these bare materials, clay-supported anodes are further obtained using sepiolite and bentonite natural silicates. The electrochemical performance of such composites is improved, especially for the sepiolite-containing one treated at 400 °C. The thermal treatment at this temperature provides the optimal conditions for a synergic nano-architecture to develop between the clay and the hematite nanoparticles. High capacity values of ~2500 mA h g−1 after 30 cycles at 1 A g−1 and retentions close to 92% are obtained. Moreover, after 450 cycles at 2 A g−1 current rate, this composite electrode displays values as high as ~700 mA h g−1. These results are interpreted taking into account the interactions between the iron oxide nanoparticles and the sepiolite surface through hydrogen bonds. The electrochemical performance is not only dependent on the oxidation state and particle morphology, but the composition is revealed as a key feature.