Combustion synthesis approach for spectral tuning of Eu doped CaAl2O4 phosphors

Despite being a particularly good emitter, use of divalent Eu has been seriously limited. This is because severe reducing environments or special hosts are needed during synthesis of divalent Eu containing phosphors. In this work we stabilize Eu in its 2+ state (in CaAl2O4) using an open-air solution combustion reaction. The impact of fuel (F) to oxidizer (O) molar ratios (F/O = 0.5-2.0) on luminescence properties is explored. Chromaticity of Eu:CaAl2O4 depends sensitively on the F/O ratio. In general, higher F/O inhibits Eu3+ and promotes Eu2+ formation, which in turn improves the quality of the blue phosphor. EPR spectra show inhomogeneous broadening effects with the increase in F/O ratio, which suggests that disorder creation is promoted when F/O is increase. This is also confirmed by an increase in emission line width in PL spectra, when F/O is increased. (C) 2013 Elsevier B.V. All rights reserved.

Brillouin Scattering by Phonons Renormalized by Two-Magnon Excitations in Ferromagnetic Dielectrics

Brillouin scattering by one-phonon-two-magnon interacting excitations in ferromagnetic dielectrics is discussed. The basic light scattering mechanism is taken to be the modulation of the density-dependent optical dielectric polarizability of the medium by the dynamic strain field generated by the longitudinal acoustic (LA) phonons. The renormalization effects arising from the scattering of phonons by the two-magnon creation-annihilation processes have, however, been taken into account. Via these interactions, the Brillouin components corresponding to the two-magnon excitations are reflected indirectly in the spectrum of the phonon scattered light as line-broadening of the otherwise relatively sharp Brillouin doublet. The present mechanism is shown to be dominant in a clean saturated ferromagnetic dielectric with large magneto-strictive coupling constant, and with the magnetic ions in the orbitally quenched states. Following the linear response theory, an expression has been derived for the spectral density of the scattered light as a function of temperature, scattering angle, and the strength of the externally applied magnetic field. Some estimates are given for the line-width and line-shift of the Brillouin components for certain typical choice of parameters involved. The results are discussed in relation to some available calculations on the ultrasonic attenuation in ferromagnetic insulators at low temperatures.

Approximate synthesis formulas for microstrip line with aperture in ground plane

Approximate closed-form expressions for the propagation characteristics of a microstrip line with a symmetrical aperture in its ground plane are reported in this article. Well-known expressions for the characteristic impedance of a regular microstrip line have been modified to incorporate the effect of this aperture. The accuracy of these expressions for various values of substrate thickness, permittivity and line width has been studied in detail by fullwave simulations. This has been further verified by measurements. These expressions are easier to compute and find immense use in the design of broadband filters, tight couplers, power dividers, transformers, delay lines, and matching circuits. A broadband filter with aperture in ground plane is demonstrated in this article. (c) 2011 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2012.

Study of Ion Transport in Lithium Perchlorate-Succinonitrile Plastic Crystalline Electrolyte via Ionic Conductivity and in Situ Cryo-Crystallography

Ion transport mechanism in lithium perchlorate (LiClO4)-succinonitrile (SN), a prototype of plastic crystalline soft matter electrolyte is discussed in the context of solvent configurational isomerism and ion solvation. Contributions of both solvent configurational isomerism and ion solvation are reflected in the activation energy for ion conduction in 0-1 M LiClO4-SN samples. Activation energy due to solvent configurational changes, that is, trans-gauche isomerism is observed to be a function of salt content and decreases in presence of salt (except at high salt concentrations, e.g. 1 M LiClO4-SN). The remnant contribution to activation energy is attributed to ion-association. The X-ray diffraction of single crystals obtained using in situ cryo-crystallography confirms directly the observations of the ionic conductivity measurements. Fourier transform infrared spectroscopy and NMR line width measurements provide additional support to our proposition of ion transport in the prototype plastic crystalline electrolyte.

ESR study of metal ion-guanosine monophosphate interaction

The addition of guanosine 5-monophosphate (5′-GMP) to an aqueous solution of Mn2+ ions results in a decrease in ESR signal intensity and an increase in line-width of Mn2+ ions. This can be interpreted in terms of stepwise formation of outersphere and inner-sphere complexes as When Mg2+ is added to a mixture of Mn2+ and 5′-GMP, ESR signal intensity increases, presumably due to the replacement of Mn2+ by Mg2+ in the complex. From the variation of ESR signal intensity as a function of concentration of Mg2+, the product K1K2 for the magnesium complex i s calculated as 125 M−1. This difference in stability constants may indicate that both phosphate group and guanine base are involved in the formation of Mn2+-5′-GMP complex.

Observability of conduction electron spin resonance (CESR) in superconductors

The condition for the observability of CESR in superconducting thin films is analysed taking into account the finiteness of the flux penetration depth. We have explicitly evaluated the path-dependent phase mixing factor occuring in the expression for power absorption. The calculated line width turns out to be of the order of, or larger than, the nominal resonance frequency for the experimentally realisable choice of parameters.

EPR Evidence for Premonitory Charge-Ordering Fluctuations in Hydrothermally Grown Pr0.57Ca0.41Ba0.02MnO3 Nanowires

Electron paramagnetic resonance (EPR) and magnetic properties of nanowires of Pr0.57Ca0.41Ba0.02MnO3 (PCBMO) are studied and compared with those of the bulk material. PCBMO nanowires with diameter of 80-90 nm and length of similar to 3.5 mu m were synthesized by a low reaction temperature hydrothermal method and the bulk sample was prepared following a solid-state reaction route. The samples were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The bulk PCBMO manganite exhibits charge order at 230 K along with a ferromagnetic transition at 110 K. However, superconducting quantum interference device measurements on the PCBMO nanowires show a complete `melting' of charge ordering and a ferromagnetic transition at 115 K. This result is confirmed by the EPR intensity behavior as well. However, the EPR line width, which is reflective of the spin dynamics, shows a shallow minimum for nanowires at the temperature corresponding to the charge-ordering transition, i.e., 230 K. We interpret this result as an indication of the presence of charge-ordering fluctuations in the nanowires even though the static charge order is absent, thus heralding the occurrence of charge order in the bulk sample.

EIT as a tool to observe dual wavelength operation of a semiconductor laser

We propose a simplified technique for dual wavelength operation of an extended cavity semiconductor laser, and its characterization using electromagnetically induced transparency (EIT). In this laser cavity scheme light beam is made converging before it incidences on the cavity grating. The converging angle of the beam creates two longitudinal oscillating modes of resonating cavity. Frequency separation between the longitudinal modes are measured with the help of beat frequency generation in a photodiode and creating pair of EIT spectra in Rb vapor. The pair of EIT dips that are generated due to dual wavelength of this laser (that is used as control laser) can be used to estimate frequency difference between the generated wavelengths. Width of EIT spectra can be used to estimate line width of individual wavelength components.

Force-free equilibria of magnetized jets

Force-free equilibrium configurations of magnetic-pressure-dominated magnetized supersonic jets confined by slowly varying external pressure are investigated analytically. For the case where internal dissipation mechanisms are active, the lowest-energy field configuration is found to be the superposition of an axisymmetric mode and a helical mode with a wavelength equal to 5 times the jet radius, and the pressure below which the nonaxisymmetric mode becomes energetically favorable is given as 2700 times the product of the 4th power of the magnetic helicity per unit length and the -6th power of the magnetic flux. A model of the total and polarized emission of such a configuration is developed and applied to the extended well-collimated astronomically resolved jet NGC 6251. The model is shown to reproduce significant features such as transverse oscillations of the ridge line, width oscillations and emission knots, the projected magnetic-field configuration, oscillations of the degree of polarization, and the distribution of the Faraday rotation measure.

Structure and optical properties of Cd-substituted ZnO (Zn1-xCdxO) nanostructures synthesized by the high-pressure solution route

We report the synthesis of Cd-substituted ZnO nanostructures (Zn1-xCdxO with x up to approximate to 0.09) by the high-pressure solution growth method. The synthesized nanostructures comprise nanocrystals that are both particles (similar to 10-15 nm) and rods which grow along the [002] direction as established by transmission electron microscope (TEM) and x-ray diffraction (XRD) analysis. Rietveld analysis of the XRD data shows a monotonic increase of the unit cell volume with the increase of Cd concentration. The optical absorption, as well as the photoluminescence (PL), shows a red shift on Cd substitution. The line width of the PL spectrum is related to the strain inhomogeneity and it peaks in the region where the CdO phase separates from the Zn1-xCdxO nanostructures. The time-resolved photoemission showed a long-lived (similar to 10 ns) component. We propose that the PL behaviour of the Zn1-xCdxO is dominated by strain in the sample with the red shift of the PL linked to the expansion of the unit cell volume on Cd substitution.

Structure and optical properties of Cd-substituted ZnO (Zn1-xCdxO) nanostructures synthesized by the high-pressure solution route

We report the synthesis of Cd-substituted ZnO nanostructures (Zn1-xCdxO with x up to approximate to 0.09) by the high-pressure solution growth method. The synthesized nanostructures comprise nanocrystals that are both particles (similar to 10-15 nm) and rods which grow along the [002] direction as established by transmission electron microscope (TEM) and x-ray diffraction (XRD) analysis. Rietveld analysis of the XRD data shows a monotonic increase of the unit cell volume with the increase of Cd concentration. The optical absorption, as well as the photoluminescence (PL), shows a red shift on Cd substitution. The line width of the PL spectrum is related to the strain inhomogeneity and it peaks in the region where the CdO phase separates from the Zn1-xCdxO nanostructures. The time-resolved photoemission showed a long-lived (similar to 10 ns) component. We propose that the PL behaviour of the Zn1-xCdxO is dominated by strain in the sample with the red shift of the PL linked to the expansion of the unit cell volume on Cd substitution.

Ion Transport in a Polymer-Plastic Solid Soft Matter Electrolyte in the Light of Solvent Dynamics and Ion Association

Ion transport in a recently demonstrated promising soft matter solid plastic-polymer electrolyte is discussed here in the context of solvent dynamics and ion association. The plastic-polymer composite electrolytes display liquid-like ionic conductivity in the solid state,compliable mechanical strength (similar to 1 MPa), and wide electrochemical voltage stability (>= 5 V). Polyacrylonitrile (PAN) dispersed in lithium perchlorate (LiClO4)-succinonitrile (SN) was chosen as the model system for the study (abbreviated LiClO4-SN:PAN). Systematic observation of various mid-infrared isomer and ion association bands as a function of temperature and polyme concentration shows an effective increase in trans conformer concentration along with free Li+ ion concentration. This strongly supports the view that enhancement in LiClO4-SN:PAN ionic conductivity over the neat plastic electrolyte (LiClO4-SN) is due to both increase in charge mobility and concentration. The ionic conductivity and infrared spectroscopy studies are supported by Brillouin light scattering. For the LiClO4-SN:PAN composites, a peak at 17 GHz was observed in addition to the normal trans-gauche isomerism (as in neat SN) at 12 GHz. The fast process is attributed to increased dynamics of those SN molecules whose energy barrier of transition from gauche to trans has reduced under influences induced by the changes in temperature and polymer concentration. The observations from ionic conductivity, spectroscopy, and light scattering studies were further supplemented by temperature dependent nuclear magnetic resonance H-1 and Li-7 line width measurements.

NMR study of ammonium halides under high pressure

Wide-line c.w. proton resonance investigations have been carried out on the ammonium halides, namely, ammonium chloride, ammonium bromide and ammonium iodide in the temperature range between 77 and 300 K and in the pressure range between 1 bar and 14 kbar. It has been found that the narrow iodide spectrum at 77 K broadens under the application of hydrostatic pressure. The barrier height for the ammonium ion motion in ammonium iodide under pressure has been estimated by carrying out a temperature variation study. The rotational potential for the motion of ammonium ion in ammonium iodide at 1 bar and 14 kbar has been calculated using earlier theoretical models and compared with values calculated for ammonium chloride and bromide. The barrier height in the case of ammonium iodide under pressure is found to be of the same order of magnitude as the value obtained in the case of ammonium bromide at atmospheric pressure indicating that the high pressure phase of ammonium iodide is likely to have the same structure as the low temperature ordered CsCl phase found in the case of the chloride and the bromide. The increase in the potential barrier height in the case of ammonium iodide under pressure indicates that the reorientational motion executed by the ammonium ions is inhibited by the application of pressure. This is also confirmed by the broadening of the spectral line at 77 K under the application of pressure.

Fe-57 Mössbauer and Mo K-EXAFS Investigations of MoxFe3-xO4, an Interesting Mixed-Valent Oxide System

Ferrites of the formula MoxFe3-xO4, prepared by a soft-chemistry route, show mixed valence states of both iron and molybdenum cations. Mössbauer studies show that Fe2+ and Fe3+ ions are present on both the A and B sites, giving Fe an average oxidation state between 2+ and 3+. Molybdenum is present in the 3+ and the 4+ states on the B sites. The presence of Mo in the 3+ state has been established by determining the Mo3+-O distance (2.2 Å), for the first time, by Mo K-EXAFS. The mixed valence of Fe on both the A and B sites and of Mo on the B sites is responsible for the fast electron transfer between the cations. All the Mössbauer parameters including the line width show a marked change at a composition (x ? 0.3) above which the concentration of Fe2+A increases rapidly.

High Pressure NMR Study of Ammonium Thiocyanate

Wide-line proton NMR spectra of ammonium thiocyanate have been recorded at 77 K as a function of external hydrostatic pressure. Contrary to expectations the line-width and the second moment decrease with the increase of pressure. This, however, is in accordance with the anomalous behaviour observed in other magnetic resonance studies of this compound and can be understood in terms of the change of electron density around the nitrogen atom of the SCN- group.