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

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

Cluster ordering in synthetic ultramarine pigments

Blue ultramarine pigments with composition Na7.7[(Al,Si)12O24](SO4)0.9(S3)0.8 and Na6.5K0.5[(Al,Si)12O24](SO4)0.9(S3)0.7 were synthesized in order to get a deep knowledge of its structure and microstructure. These pigments were prepared following the current industrial method synthesis, which consists of the calcination of the metakaolin, sulphur, sodium carbonate and charcoal under controlled atmosphere. The structural characterization of the obtained pigments was performed in detail by the complementary use of different techniques such as X-ray powder diffraction and transmission electron microscopy (SAED and HRTEM) together with 29Si MAS-NMR and IR spectroscopies. The obtained results have allowed to propose a structural model with the lowest symmetry for ultramarine pigments. The suitable space group to describe this structure is P23 (No.195), which takes account the observed b-cage cluster ordering at the unit cell level and the framework adaptation to the different size of the b-cage clusters.Depto. de Química InorgánicaFac. de Ciencias QuímicasTRUEpu

Structural Characterization and Magnetic Properties of Divanadates ZnMV2O7 (M: Co, Ni and Cu).

Divanadates of general formula ZnMV2O7 (M = Co, Ni and Cu) have been prepared as powdered samples and the X-ray structural determination and magnetic studies have been carried out. Both ZnCoV2O7 and ZnNiV2O7 crystallize with the so-called dichromate structure type, showing monoclinic symmetry, space group P21/c, while the third member of this family of vanadates ZnCuV2O7 shows the thortveitite structure type with monoclinic symmetry, space group C2/c. Magnetic susceptibility and specific heat measurements reveal that these derivatives behave as antiferromagnets and the estimated Néel temperatures are 4.5, 3.5 and 5.3K for cobalt, nickel and copper vanadates, respectively. In all cases, the one-dimensional magnetic behavior that present the M2V2O7 vanadates is broken down when the magnetic cation M is half replaced by the diamagnetic Zn2+ to give the ZnMV2O7 samples studied in the present work.Depto. de Química InorgánicaFac. de Ciencias QuímicasTRUEpu

Unusual Magnetic Behaviors and Electronic Configurations Driven by Diverse Co(II) or Mn(II) MOF Architectures

Five novel metal organic frameworks were obtained by hydrosolvothermal reactions using the hexafluorisopropylidenebis(benzoic) acid (H2hfipbb) as linker and Co(II) or Mn(II) ions as connectors. [Co2(Hhfipbb)-(TEA)], compound 1 (TEA = triethanolamine trianion) with a threedimensional (3D) framework, and a tpu net; [Co1.5(hfipbb)2]·HN(CH2CH3)3 and [Co3(hfipbb)2]·2{HN(CH2CH3)3}, compounds 2 and 2-a, respectively, both with two-dimensional structure, sql topologies, and different layer packings. Compounds 3 and 4, having the general formula [M2(hfipbb)2]·C7H8,where M = Co (3) or Mn (4), have 3D frameworks with an sqc topology. A deep analysis of the magnetic measurements reveals different striking magnetic behaviors resulting from diverse secondary building unit and framework architectures. Compound 1 presents canted antiferromagnetic chains, compound 2 contains ferromagnetic linear trimeric clusters, and compound 3 exhibits ferromagnetic chains. For the three compounds, a 3D canted antiferromagnetic structure takes place at ∼8 K by means of weak magnetic interactions between the mentioned magnetic units. Such long-range magnetic order is precluded with the application of a high enough magnetic field. Compound 4 evidenced intrachain antiferromagnetic interactions.Depto. de Química InorgánicaFac. de Ciencias QuímicasTRUEpu