Structural and magnetic characterization of ordered Sr2LnSbO6 (Ln=rare earth) perosvkite

The double perovskites A2LnMO6 (A = Sr2+ and Ba2+; Ln = trivalent lanthanide cation; M = pentavalent 4d or 5d transition elements) have been widely studied concerning their structure and properties [1]. If the Ln and M cations are ordered within the B-perosvkite sites the symmetry and size of the unit cell change when are compared to the ideal cubic aristotype. Woodward predicted 15 possible space groups for the ordered A2BB’O6 perovskites when the cation ordering and the octahedral tilting around the pseudo-cubic axes take place simultaneously [2]. The ordered double perovskites A2LnMO6 with only one of the two B-sites carrying magnetic moment, namely Ln, show a magnetic sublattice consisting of edge-sharing tetrahedral, which represents a frustrating magnetic geometry in three dimensions. More recently, the structure of double perovskites Sr2LnSbO6 (Ln= Dy, Ho, Gd, Y and In) has been investigated, and the monoclinic symmetry of the space group P21/n, with Ln and Sb elements ordered in the B-sites, was reported [3, 4]. We report the preparation of the whole family of double perovskites Sr2LnSbO6 (Ln = La-Lu), which crystallize with the P21/n space group, with lattice parameters p= a2a , p= a2b and p= a2c(β∼90 º), being the lattice parameter of the cubic aristotype. A progressive decreasing was observed in lattice parameters with the increasing of the atomic number of the Ln cation, according with the wellknown lanthanide contraction. pa Magnetic susceptibility measurements for this family of compounds reveal a paramagnetic behaviour in a very wide temperature range. From experimental spectroscopic data as well as from a semi-empirical estimation (Simple Overlap Model SOM [5]) of the crystal-field parameters corresponding to the point site symmetry of the magnetically active Ln, Oh, and using the wavefunctions associated with the energy levels obtained, the paramagnetic susceptibility and its evolution vstemperature is simulated according to the van Vleck formalism. The observed deviation from the Curie–Weiss behaviour at low temperature, very well reproduced in each case, reflects the splitting of the ground state of the corresponding Ln cation under the influence of the crystal field. Thus, magnetic frustration or cooperative interactions do not need to be considered to explain the mentioned low temperature deviation from the linearity of Curie-Weiss plots

Mesoporous Silica Matrix as a Tool for Minimizing Dipolar Interactions in NiFe2O4 and ZnFe2O4 Nanoparticles

NiFe2O4 and ZnFe2O4 nanoparticles have been prepared encased in the MCM (Mobile Composition of Matter) type matrix. Their magnetic behavior has been studied and compared with that corresponding to particles of the same composition and of a similar size (prepared and embedded in amorphous silica or as bare particles). This study has allowed elucidation of the role exerted by the matrix and interparticle interactions in the magnetic behavior of each ferrite system. Thus, very different superparamagnetic behavior has been found in ferrite particles of similar size depending on the surrounding media. Also, the obtained results clearly provide evidence of the vastly different magnetic behavior for each ferrite system

Determination of the crystal and magnetic structure of the DyCrO4-scheelite polymorph by neutron diffraction

Neutron diffraction data of DyCrO4 oxide, prepared at 4 GPa and 833 K from the ambient pressure zircon-type, reveal that crystallize with the scheelite-type structure, space group I41/a. Accompanying this structural phase transition induced by pressure the magnetic properties change dramatically from ferromagnetism in the case of zircon to antiferromagnetism for the scheelite polymorph with a T N= 19 K. The analysis of the neutron diffraction data obtained at 1.2 K has been used to determine the magnetic structure of this DyCrO4-scheelite oxide which can be described with a k = [0, 0, 0] as propagation vector, where the Dy and Cr moments are lying in the ab-plane of the scheelite structure. The ordered magnetic moments are 10 µB and 1 µB for Dy+3 and Cr+5 respectivel

One-step Sustainable Preparation of Superparamagnetic Iron Oxide Nanoparticles Supported on Mesoporous SiO2

Superparamagnetic iron oxide nanoparticles (SPIONs) supported on high surface area mesoporous SiO2 are advanced materials of great interest in catalysis, adsorption and biomedicine. Here we present a new process to prepare SPION/SiO2 materials by the impregnation and insitu decomposition of Fe(NO3)3.9H2O on mesoporous SiO2 supports in a 25-50% mol ethanol + CO2 mixture at 523 K and 25.0 MPa. -Fe2O3 nanoparticles (NPs) of average size between 6-9 nm were distributed homogeneously on the supports. NPs deposited into the SBA-15 mesopores but mostly on the external surface of MCM-41. Materials prepared with the highest ethanol content were very homogeneous. Magnetic measurements confirmed the superparamagnetic nature of the materials at room temperature. The process proposed is sustainable and scalable, avoids tedious preparations and the additional high temperature treatment under a controlled atmosphere, as the metal decomposition is performed insitu in the CO2-expanded liquid mixture

Metodología de internacionalización de material docente basada en el uso de Markdown y Pandoc

La internacionalización de la docencia ofrece grandes oportunidades para la Universidad, pero también plantea retos significativos para estudiantes y profesores. En particular, la creación y mantenimiento efectivo del material docente de una asignatura impartida simultáneamente en varios idiomas y con alto grado de coordinación entre los distintos grupos de la misma (p.ej., examen final/prácticas comunes para todos los estudiantes) puede suponer un importante desafío para los profesores. Para hacer frente a este problema, hemos diseñado una estrategia específica para la creación y gestión de material docente en dual (p.ej., inglés-español), y desarrollado un conjunto de herramientas multiplataforma para ponerla en práctica. La idea general es mantener en un mismo fichero de texto el contenido del documento que se desee construir en ambos idiomas, proporcionando justo detrás de cada párrafo y título en uno de los idiomas su traducción al otro idioma, empleando delimitadores especiales. Para crear estos documentos duales se emplea Markdown, un lenguaje de marcado ligero, que dada su sencillez y versatilidad está teniendo una rápida adopción por un amplio espectro de profesionales: desde escritores de novelas o periodistas, hasta administradores de sitios web. A partir de los documentos duales creados con Markdown, es posible generar automáticamente el documento final para cada idioma en el formato deseado que se pondrá a disposición de los estudiantes. Para esta tarea, nos basamos en el uso de la herramienta Pandoc, que permite realizar la conversión de documentos Markdown a una gran cantidad de formatos, como PDF, docx (Microsoft Word), EPUB (libro electrónico) o HTML. Como parte de nuestro proyecto, hemos creado extensiones de Pandoc para permitir la creación de documentos duales en Markdown y para aumentar la expresividad de este lenguaje con construcciones comunmente utilizadas en documentos de carácter docente

Double double cation order in the high pressure perovskites MnRMnSbO6

International audienceCation ordering in ABO3 perovskites adds to their chemical variety and can lead to properties such as ferrimagnetism and magnetoresistance in Sr2FeMoO6. Through high-pressure and high-temperature synthesis, a new type of “double double perovskite” structure has been discovered in the family MnRMnSbO6 (R=La, Pr, Nd, Sm). This tetragonal structure has a 1:1 order of cations on both A and B sites, with A-site Mn2+ and R3+ cations ordered in columns and Mn2+ and Sb5+ having rock salt order on the B sites. The MnRMnSbO6 double double perovskites are ferrimagnetic at low temperatures with additional spin-reorientation transitions. The ordering direction of ferrimagnetic Mn spins in MnNdMnSbO6 changes from parallel to [001] below TC=76 K to perpendicular below the reorientation transition at 42 K at which Nd moments also order. Smaller rare earths lead to conventional monoclinic double perovskites (MnR)MnSbO6 for Eu and Gd

Structural characterisation and spin glass behaviour of the new oxide Ba2ScCoO5

The new oxide Ba2ScCoO5 has been synthesised by solid state reaction and characterised by using both X-ray and electron diffraction techniques. It crystallises in a perovskite-type structure, showing cubic symmetry, space group Pm39m [a=4.1400(9)A°]. xdc and xac magnetic susceptibility have been measured down to 1.6 K and analysed, indicating the existence of a freezing phenomenon corresponding to a spin glass state. In the high temperature range, xdc shows a Curie–Weiss like behaviour, with magnetic correlations starting around TC=50 K. Below this temperature, a strong irreversibility in x vs. T curves obtained by zero field cooled and field cooled processes (ZFC/FC) has been detected. A second magnetic transition to a spin-glass phase is observed at TF=35 K in the ZFC curve. xac measurements performed at different frequencies and magnetic fields also show all the common features characteristic of re-entrant spin-glasses. A possible origin of this magnetic behaviour is proposed and discussed.Depto. de Química InorgánicaFac. de Ciencias QuímicasTRUEpu

Discussion on the Interparticle Interactions in NiFe2O4 and ZnFe2O4 Nanosized Systems Based on the Matrix Effects in the Magnetic Behavior

ZnFe2O4 and NiFe2O4 particles ranging from 5 to 8 nm have been prepared inside the channels of SBA-15 mesoporous material and nanowires were recovered after dissolving the silica matrix. For both ferrite compositions a hardening of the magnetic behavior has been obtained when using the mesoporous matrix. Thanks to the comparison of the magnetic behavior of the nanoparticles when contained and not in the matrix, it was possible to elucidate not only the matrix effect but also the kind of interparticle interactions depending on the ferrite composition. Thus, nickel ferrite particles are characterized by intense dipolar interactions that are responsible for the so high superparamagnetic response and that can be avoided by matrix effects only at high temperatures. On the contrary, the inherent low-intense dipolar interactions of the zinc ferrite system lead it to present lower superparamagnetic moments, and in the case of encapsulated particles, the superparamagnetic behavior would correspond to almost not-interacting particles. In addition, interactions occurring between surface spins of different particles are more visible in zinc ferrite system as consequence of its so low intense dipolar interactions and are prevented thanks to the use of the matrix.Depto. de Química InorgánicaFac. de Ciencias QuímicasTRUEpu

Infrared spectra of mixed oxides of the type Ln₂BaCoO₅

Ln₂BaMO₅ oxides (Ln = trivalent lanthanide; M = Co, Ni, Cu, Zn) present very interesting structural, magnetic and spectroscopic properties and, when MII= Cu, they are generated as byproducts or impurities (the so-called "green phases") during the syntheses of the LnBa₂Cu₃0₇_δ-type superconductors. Despite their identical stoichiometries, the Ln₂BaMO₅ materials belong to four different structural types, depending on the LN(III) and/or M(II) cations present in the lattices. Besides, during the past few years, it has also been shown that some of them present temperature dependent dimorphism.Facultad de Ciencias Exacta

Magnetic and crystal structure determination of Mn2FeSbO6 double perovskite

The perovskite form of the Mn2FeSbO6 oxide has been obtained at 5.5 GPa and 1523 K. X-ray and neutron diffraction data reveal that this compound crystallizes in the monoclinic P21/n space group with a = 0.5234 nm, b = 0.5389 nm, c = 0.7642 nm and β = 90.372° lattice parameters. AC and DC magnetic susceptibility measurements suggest the existence of a complex magnetic behavior below 200 K. The magnetic structure has been determined and can be described on the basis of an elliptical spiral with an incommensurate propagation vector, k = [0,0.426,0], where the magnetic moments of 2Mn2+ and Fe3+ cations are confined to the ac-plane.Comunidad de MadridMICINNDepto. de Química InorgánicaFac. de Ciencias QuímicasTRUEpu