Photoluminescence studies on rare earth titanates prepared by self-propagating high temperature synthesis method

The laser-induced luminescence studies of the rare earth titanates (R2Ti2O7) (R = La, Nd and Gd) using 355 nm radiation from an Nd:YAG laser are presented. These samples with submicron or nanometer size are prepared by the self-propagating high temperature synthesis (SHS) method and there is no known fluorescence shown by these rare earths in the visible region. Hence, the luminescence transitions shown by the La2Ti2O7 near 610 nm and Gd2Ti2O7 near 767 nm are quite interesting. Though La3+ ions with no 4f electrons have no electronic energy levels that can induce excitation and luminescence processes in the visible region, the presence of the Ti3+ ions leads to luminescence in this region.

Martensitic transitions and the nature of ferromagnetism in the austenitic and martensitic states of Ni-Mn-Sn alloys

Structural and magnetic transformations in the Heusler-based system Ni0.50Mn0.50¿xSnx are studied by x-ray diffraction, optical microscopy, differential scanning calorimetry, and magnetization. The structural transformations are of austenitic-martensitic character. The austenite state has an L21 structure, whereas the structures of the martensite can be 10M , 14M , or L10 depending on the Sn composition. For samples that undergo martensitic transformations below and around room temperature, it is observed that the magnetic exchange in both parent and product phases is ferromagnetic, but the ferromagnetic exchange, characteristic of each phase, is found to be of different strength. This gives rise to different Curie temperatures for the austenitic and martensitic states.

Reply to "Comment on 'Nature and entropy content of the ordering transitions in RCo2'"

In the previous Comment, Forker and co-workers claim that perturbed angular correlation (PAC) data leave no alternative to the conclusion that the spontaneous magnetization of PrCo2 and NdCo2 undergoes a discontinuous, first-order phase transition at TC. We show here that their claim is in clear contradiction with a wealth of experimental evidence, including our own. Finally, we propose a possible origin for the disagreement between their interpretation of the PAC results and the literature on this subject.

Synthesis and Photoluminescence Properties of Novel Red Phosphors for White Light Emitting Diode Applications

Several series of Eu3+ based red emitting phosphor materials were synthesized using solid state reaction route and their properties were characterized. The present studies primarily investigated the photoluminescence properties of Eu3+ in a family of closely related host structure with a general formula Ln3MO7. The results presented in the previous chapters throws light to a basic understanding of the structure, phase formation and the photoluminescence properties of these compounds and their co-relations. The variation in the Eu3+ luminescence properties with different M cations was studied in Gd3-xMO7 (M = Nb, Sb, Ta) system.More ordering in the host lattice and more uniform distribution of Eu3+ ions resulting in the increased emission properties were observed in tantalate system.Influence of various lanthanide ion (Lu, Y, Gd, La) substitutions on the Eu3+ photoluminescence properties in Ln3MO7 host structures was also studied. The difference in emission profiles with different Ln ions demonstrated the influence of long range ordering, coordination of cations and ligand polarizability in the emission probabilities, intensity and quantum efficiency of these phosphor materials. Better luminescence of almost equally competing intensities from all the 4f transitions of Eu3+ was noticed for La3TaO7 system. Photoluminescence properties were further improved in La3TaO7 : Eu3+ phosphors by the incorporation of Ba2+ ions in La3+ site. New red phosphor materials Gd2-xGaTaO7 : xEu3+ exhibiting intense red emissions under UV excitation were prepared. Optimum doping level of Eu3+ in these different host lattices were experimentally determined. Some of the prepared samples exhibited higher emission intensities than the standard Y2O3 : Eu3+ red phosphors. In the present studies, Eu3+ acts as a structural probe determining the coordination and symmetry of the atoms in the host lattice. Results from the photoluminescence studies combined with the powder XRD and Raman spectroscopy investigations helped in the determination of the correct crystal structures and phase formation of the prepared compounds. Thus the controversy regarding the space groups of these compounds could be solved to a great extent. The variation in the space groups with different cation substitutions were discussed. There was only limited understanding regarding the various influential parameters of the photoluminescence properties of phosphor materials. From the given studies, the dependence of photoluminescence properties on the crystal structure and ordering of the host lattice, site symmetries, polarizability of the ions, distortions around the activator ion, uniformity in the activator distribution, concentration of the activator ion etc. were explained. Although the presented work does not directly evidence any application, the materials developed in the studies can be used for lighting applications together with other components for LED lighting. All the prepared samples were well excitable under near UV radiation. La3TaO7 : 0.15Eu3+ phosphor with high efficiency and intense orange red emissions can be used as a potential red component for the realization of white light with better color rendering properties. Gd2GaTaO7 : Eu3+, Bi2+ red phosphors give good color purity matching to NTSC standards of red. Some of these compounds exhibited higher emission intensities than the standard Y2O3 : Eu3+ red phosphors. However thermal stability and electrical output using these compounds should be studied further before applications. Based on the studies in the closely related Ln3MO7 structures, some ideas on selecting better host lattice for improved luminescence properties could be drawn. Analyzing the CTB position and the number of emission splits, a general understanding on the doping sites can be obtained. These results could be helpful for phosphor designs in other host systems also, for enhanced emission intensity and efficiency.

Three-electron radiative transitions

A new type of many-electron radiative transitions involving three electrons is predicted. The results of their investigation by many-body perturbation theory are presented. New spectral lines observed in the wavelength range of 37.5 to 54.0 nm by means of photon-induced fluorescence spectroscopy (PIFS) following the excitation of the Kr I 3d{^-1}np resonances are reported and compared with the predictions.

A clear atomic example for the surface sensitivity of penning ionization

Using a crossed-beam apparatus with a double hemispherical electron spectrometer, we have studied the spectrum of electrons released in thermal energy ionizing collisions of metastable He^*(2^3S) atoms with ground state Yb(4f^14 6s^2 ^1S_0) atoms, thereby providing the first Penning electron spectrum of an atomic target with-4f-electrons. In contrast to the HeI (58.4nm) and NeI (73.6/74.4nm) photoelectron spectra of Yb, which show mainly 4f- and 6s-electron emission in about a 5:1 ratio, the He^*(2^3S) Penning electron spectrum is dominated by 6s-ionization, acoompnnied by some correlation- induced 6p-emission (8% Yb+( 4f^14 6p^2P) formation) and very little 4f-ionization (<_~ 2.5%). This astounding result is attributed to the electron exchange mechanism for He^*(2^3S) ionization and reflects the poor overlap of the target 4f-electron wavefunction with the 1s-hole of He^*(2^3S), as discussed on thc basis of Dirac-Fock wave functions for the Yb orbitals and through calculations of the partial ionization cross sections involving semiempirical complex potentiale. The presented case may be regarded as the elearest atomic example for the surface sensitivity of He^*(2^3S) Penning ionization observed so far.

Beam-foil-excited auger transitions in neon

Energy spectra of electrons ejected from collisions between a carbon foil and Ne projectiles with energies between 1.4 and 20 MeV have been measured. Continuous and discrete electron energy distributions are observed. Auger transitions of foil-excited Ne have been studied. Using relativistic Dirac-Fock multiconfiguration calculations, most of the measured Auger transitions have been identified.

Relativistic effects in physics and chemistry of element 105. [Part] IV.

Results of relativistic (Dirac-Slater and Dirac-Fock) and nonrelativistic (Hartree-Fock-Slater) atomic and molecular calculations have been compared for the group 5 elements Nb, Ta, and Ha and their compounds MCl_5, to elucidate the influence of relativistic effects on their properties especially in going from the 5d element Ta to the 6d element Ha. The analysis of the radial distribution of the valence electrons of the metals for electronic configurations obtained as a result of the molecular calculations and their overlap with ligands show opposite trends in behavior for ns_1/2, np_l/2, and (n -1 )d_5/2 orbitals for Ta and Ha in the relativistic and nonrelativistic cases. Relativistic contraction and energetic stabilization of the ns_1/2 and np_l/2 wave functions and expansion and destabilization of the (n-1)d_5/2 orbitals make hahnium pentahalide more covalent than tantalum pentahalide and increase the bond strength. The nonrelativistic treatment of the wave functions results in an increase in ionicity of the MCl_5 molecules in going from Nb to Ha making element Ha an analog of V. Different trends for the relativistic and nonrelativistic cases are also found for ionization potentials, electronic affinities, and energies of charge-transfer transitions as well as the stability of the maximum oxidation state.

Transitions between quintet states of doubly excited four-electron ions

Following an earlier observation in F VI we identified the line pair 1s2s2p^2 {^5P}-1s2s2p3d {^5P^0} , {^5D^0} for the elements N, O, Mg, and tentatively for A1 and Si in beam-foil spectra. Assignment was established by comparison with Multi-Configuration Dirac-Fock calculations along the isoelectronic sequence. Using this method we also identified some quartet lines of lithium-like ions with Z > 10.

Programa Helios, Acci??n Modelo Local de Oviedo : informe evaluador de resultados

Emitir una primera valoraci??n de los resultados apreciados en la Acci??n Modelo Local (AML) desarrollada en Oviedo dentro del programa Helios de la Comunidad Europea. Hip??tesis de partida: 1) La AML de Oviedo ha elevado el nivel de coordinaci??n existente entre los diversos equipos, centros y servicios que atienden a las personas minusv??lidas. 2) Los profesionales valoran positivamente la utilidad de su acci??n coordinadora. 3) Las instituciones valoran positivamente sus logros. Equipos de salud hospitalarios y de centros, equipos pedag??gicos, servicios sociales regionales y municipales, INSERSO, centros escolares y asociaciones de minusv??lidos. Las acciones evaluadoras se dise??aron en cuatro fases: contexto, entrada, proceso y producto. Se analizaron los efectos o impactos que han provocado las acciones desarrolladas en diferentes niveles. 4 cuestionarios creados ad hoc sobre la valoraci??n de eficacia de la red de coordinaci??n: evaluaci??n de resultados, evaluaci??n de resultados en centros educativos, equipo t??cnico y equipo consultivo. An??lisis de la base de datos elaborada por los investigadores utilizando los m??dulos 1D, 2D y 4F, de car??cter descriptivo, del paquete estad??stico BMDP. La calificaci??n de la AML, en cuanto a efectos totales conseguidos, puede estimarse de media-baja. Esta apreciaci??n valorativa se debe a la inmovilidad observada en cuanto al intercambio de informaci??n. Los grupos de salud y de servicios sociales regionales y municipales parecen ser los que m??s han activado sus mecanismos de coordinaci??n, obteniendo una confirmaci??n del conjunto de participantes respecto a su leve incremento de remisi??n de informes. Los equipos psicopedag??gicos parecen remitir m??s informes que los que el total de participantes aprecia recibir. En los dem??s centros y equipos no se aprecia apenas variaci??n con respecto al informe emitido el 15 de febrero de 1990. La valoraci??n y adaptaci??n psicopedag??gica, la atenci??n m??dico-preventiva y las ayudas sociales son los tres factores que m??s han alterado su nivel habitual, causando las peque??as variaciones de intercambios coordinados de informaci??n. Todos los equipos psicopedag??gicos han hecho aportaciones para ayudar a mejorar la coordinaci??n. Entre ellas hay que destacar la necesidad que ven de institucionalizar las acciones del programa Helios y de establecer una comunicaci??n m??s fluida. La AML debe: ofrecer m??s informaci??n, mejorar sus acciones de coordinaci??n, consolidar un equipo t??cnico interinstitucional, actualizar la gu??a de recursos, potenciar su actividad y hacer un seguimiento continuo de sus logros.

Molecular computations for reactions and phase transitions: applications to protein stabilization, hydrates and catalysis

In this work we have made significant contributions in three different areas of interest: therapeutic protein stabilization, thermodynamics of natural gas clathrate-hydrates, and zeolite catalysis. In all three fields, using our various computational techniques, we have been able to elucidate phenomena that are difficult or impossible to explain experimentally. More specifically, in mixed solvent systems for proteins we developed a statistical-mechanical method to model the thermodynamic effects of additives in molecular-level detail. It was the first method demonstrated to have truly predictive (no adjustable parameters) capability for real protein systems. We also describe a novel mechanism that slows protein association reactions, called the “gap effect.” We developed a comprehensive picture of methioine oxidation by hydrogen peroxide that allows for accurate prediction of protein oxidation and provides a rationale for developing strategies to control oxidation. The method of solvent accessible area (SAA) was shown not to correlate well with oxidation rates. A new property, averaged two-shell water coordination number (2SWCN) was identified and shown to correlate well with oxidation rates. Reference parameters for the van der Waals Platteeuw model of clathrate-hydrates were found for structure I and structure II. These reference parameters are independent of the potential form (unlike the commonly used parameters) and have been validated by calculating phase behavior and structural transitions for mixed hydrate systems. These calculations are validated with experimental data for both structures and for systems that undergo transitions from one structure to another. This is the first method of calculating hydrate thermodynamics to demonstrate predictive capability for phase equilibria, structural changes, and occupancy in pure and mixed hydrate systems. We have computed a new mechanism for the methanol coupling reaction to form ethanol and water in the zeolite chabazite. The mechanism at 400°C proceeds via stable intermediates of water, methane, and protonated formaldehyde.

Innovación educativa : el currículum de Ética en Enseñanzas Medias.

Reconstrucción de la innovación curricular por el profesorado. Valoración de la innovación y su desarrollo en los centros. Relación entre actitud del profesorado y uso y conceptualización. Vivencia, percepción y valoración del currículo por los alumnos. Papel de ausencia de dinámicas de apoyo institucionales en el desarrollo innovador. Primero: cuantitativo (cuestionario): profesores (n: 105) de Enseñanzas Medias (BUP y FP). Alumnos (n: 490) de primero, segundo y tercero de BUP y FP. Segundo: cualitativo (entrevistas): 10 profesores (5 de BUP y 5 de FP). Enfoque exploratorio, con un diseño abierto, combinación de diferentes técnicas cuantitativas y cualitativas, con un carácter procesual, buscando una complementariedad metodológica o triangulación metodológica; análisis estadísticos para determinar 'representantes típicos' a los que realizar un análisis cualitativo con entrevistas semiestructuradas. Cuestionario a profesores: 54 ítems (147 variables). Cuestionario a alumnos: 26 ítems (31 variables). Doble entrevista semiestructurada a 10 profesores. Análisis descriptivo y multivariante utilizando el paquete estadístico 'Biomedical' (BMDP): programas: 1D (análisis descriptivo de frecuencia), 2D (porcentajes categoriales), 4F (tablas de contingencia), 2M (análisis Cluster), 6M (correlación canónica) y 7M (análisis discriminante). Análisis cualitativo de entrevistas: mapa cognitivo de contenidos semánticos del pensamiento del profesor y categorías de conocimiento práctico de Elbaz. Determinar carácter diferencial y específico del currículo en los centros. Relación positiva entre actitud del profesorado ante la innovación y receptividad-rechazo, conceptualización y uso de la innovación. Relación y diferencias entre percepción y vivencias de los alumnos y profesores. Desarrollo de la innovación con carácter diferencial en centros de BUP y FP. La complementariedad metodológica se muestra potencialmente explicativa. Limitaciones internas del enfoque cultural: necesidad de criterios normativo-críticos. La materia curricular debe tener entidad curricular propia, con un específico perfil docente. Necesidad para una implementación exitosa de articular dinámicas de apoyo al desarrollo de la innovación.

Conoce la naturaleza de Aragón.

Programa de educación ambiental promovido por la Fundación para la conservación del quebrantahuesos cuyo objetivo es dar a conocer el medio natural y la actuación del ser humano en relación con el territorio. El programa se ha desarrollado por educadores ambientales profesionales en el entorno de la comarca del Sobrarbre (Huesca). Se trata de una propuesta de conocimientos, experiencias y juegos orientados a fomentar en los asistentes su implicación personal en la protección y disfrute de la naturaleza, así como a invitar a la reflexión sobre las relaciones que existen entre el medio natural y la intervención humana, desde el concepto de desarrollo sostenibleaSeleccionado en la convocatoria: Ayudas para proyectos de temática educativa, Gobierno de Aragón 2009-10. Resumen basado en el del autor.