Superconducting fluctuations and anomalous phonon renormalization much above superconducting transition temperature in Ca4Al2O5.7Fe2As2

Raman studies on Ca4Al2O5.7Fe2As2 superconductor in the temperature range of 5K to 300 K, covering the superconducting transition temperature T-c = 28.3 K, reveal that the Raman mode at similar to 230 cm(-1) shows a sharp jump in frequency by similar to 2% and linewidth increases by similar to 175% at T-o similar to 60 K. Below T-o, anomalous softening of the mode frequency and a large decrease by similar to 10 cm(-1) in the linewidth are observed. These precursor effects at T-0 (similar to 2T(c)) are attributed to significant superconducting fluctuations, possibly enhanced due to reduced dimensionality arising from weak coupling between the well separated (similar to 15 angstrom) Fe-As layers in the unit cell. A large blue-shift of the mode frequency between 300 K and 60 K (similar to 7%) indicates strong spin-phonon coupling in this superconductor. (C) 2012 American Institute of Physics. http://dx.doi.org/10.1063/1.4724206]

Anomalous optical phonons in Cs-0.9(NH4)(0.1)H2AsO4: a temperature-dependent Raman study

We determine the nature of coupled phonons in mixed crystal of Cs-0.9(NH4)(0.1)H2AsO4 using inelastic light scattering studies in the temperature range of 5 K to 300 K covering a spectral range of 60-1100 cm(-1). The phase transition in this system are marked by the splitting of phonon modes, appearance of new modes and anomalies in the frequency as well as linewidth of the phonon modes near transition temperature. In particular, we observed the splitting of symmetric (v(1)) and antisymmetric (v(3)) stretching vibrations associated with AsO4 tetrahedra below transition temperature (T-c(*) similar to 110 K) attributed to the lowering of site symmetry of AsO4 in orthorhombic phase below transition temperature. In addition, the step-up (hardening) and step-down (softening) of the AsO4 bending vibrations (v(4) (S9, S11) and v(2) (S6)) below transition temperature signals the rapid development of long range ferroelectric order and proton ordering. The lowest frequency phonon (S1) mode observed at similar to 92 cm(-1) shows anomalous blue shift (similar to 12 %) from 300 K to 5 K with no sharp transition near T-c(*) unlike other observed phonon modes signaling its potential coupling with the proton tunneling mode. (C) 2013 Author(s).

Synthesis, characterization and photoluminescence properties of Gd2O3:Eu3+ nanophosphors prepared by solution combustion method

Gd2O3:Eu3+ (0.5-8.0 mol%) nanophosphors have been prepared by low temperature solution combustion method using metal nitrates as oxidizers and oxalyl dihydrazide (ODH) as a fuel. The phosphors are well characterized by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and photoluminescence (PL) techniques. PXRD patterns of as-formed and calcined (800 degrees C, 3 h) Gd2O3 powders exhibit monoclinic phase with mean crystallite sizes ranging from 20 to 50 nm. Eu3+ doping changes the structure from monoclinic to mixed phase of monoclinic and cubic. SEM micrographs shows the products are foamy, agglomerated and fluffy in nature due to the large amount of gases liberated during combustion reaction. Upon 254 nm excitation the photoluminescence of the Gd2O3:Eu3+ particles show red emission at 611 nm corresponding to D-5(0)-> F-7(2) transition. It is observed that PL intensity increases with calcination temperature. This might be attributed to better crystallization and eliminates the defects, which serve as centers of non-radiative relaxation for nanomaterials. It is observed that the optical energy gap (E-g) is widened with increase Eu3+ content. (C) 2010 Elsevier B.V. All rights reserved.

Synthesis, characterization, thermo- and photoluminescence properties of Bi3+ co-doped Gd2O3:Eu3+ nanophosphors

Gd2O3:Eu3+ (4 mol%) co-doped with Bi3+ (Bi = 0, 1, 3, 5, 7, 9 and 11 mol%) ions were synthesized by a low-temperature solution combustion method. The powders were calcined at 800A degrees C and were characterized by powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), Fourier transform infrared and UV-Vis spectroscopy. The PXRD profiles confirm that the calcined products were in monoclinic with little cubic phases. The particle sizes were estimated using Scherrer's method and Williamson-Hall plots and are found to be in the ranges 40-60 nm and 30-80 nm, respectively. The results are in good agreement with TEM results. The photoluminescence spectra of the synthesized phosphors excited with 230 nm show emission peaks at similar to 590, 612 and 625 nm, which are due to the transitions D-5(0)-> F-7(0), D-5(0)-> F-7(2) and D-5(0)-> F-7(3) of Eu3+, respectively. It is observed that a significant quenching of Eu3+ emission was observed under 230 nm excitation when Bi3+ was co-doped. On the other hand, upon 350 nm excitation, the luminescent intensity of Eu3+ ions was enhanced by incorporation of Bi3+ (5 mol%) ions. The introduction of Bi3+ ions broadened the excitation band of Eu3+ of which a new strong band occurred ranging from 320 to 380 nm. This has been attributed to the 6s(2)-> 6s6p transition of Bi3+ ions, implying a very efficient energy transfer from Bi3+ ions to Eu3+ ions. The gamma radiation response of Gd2O3:Eu3+ exhibited a dosimetrically useful glow peak at 380A degrees C. Using thermoluminescence glow peaks, the trap parameters have been evaluated and discussed. The observed emission characteristics and energy transfer indicate that Gd2O3:Eu3+, Bi3+ phosphors have promising applications in solid-state lighting.

Photoluminescence properties of Eu3+-activated CaMoO4 phosphors for WLEDs applications and its Judd-Ofelt analysis

A series of scheelite-type Eu3+-activated CaMoO4 phosphors were synthesized by the nitrate-citrate gel combustion method. All the compounds crystallized in the tetragonal structure with space group I4(1)/a (No. 88). FESEM results reveal the spherical-like morphology. The CaMoO4 phosphor exhibited broad emission centered at 500 nm under the excitation of 298 nm wavelength, while Eu3+-activated CaMoO4 shows an intense characteristic red emission peak at 615 nm at different excitation wavelengths, due to D-5(0) -> F-7(2) transition of Eu3+ ions. The intensities of transitions between different J levels depend on the symmetry of the local environment of Eu3+ ions and were estimated using the Judd-Ofelt analysis. The high asymmetric ratio revealed that Eu3+ occupies sites with a low symmetry and without an inversion center. The CIE chromaticity co-ordinates (x, y) were calculated from emission spectra, and the values were close to the NTSC standard. Therefore, the present phosphor is highly useful for LEDs applications.

Synthesis of Eu3+-activated BiOF and BiOBr phosphors: photoluminescence, Judd-Ofelt analysis and photocatalytic properties

A series of Bi1-xEuxOX (X = F and Br; x = 0, 0.01, 0.03 and 0.05) phosphors were synthesized at relatively low temperature and short duration (500 degrees C, 1 h). Rietveld refinement results verified that all the compounds were crystallized in the tetragonal structure with space group P4/nmm (no. 129). Photoluminescence spectra exhibit characteristic luminescence D-5(0) -> F-7(J) (J = 0-4) intra-4f shell Eu3+ ion transitions. The magnetic dipole (D-5(0) -> F-7(1)) transition dominates the emission of BiOF:Eu3+, while the electric dipole (D-5(0) -> F-7(2)) peak was stronger in BiOBr:Eu3+ phosphors. The evaluated CIE color coordinates for Bi0.95Eu0.05OBr (0.632, 0.358) are close to the commercial Y2O3:Eu3+ (0.645, 0.347) and Y2O2S:Eu3+ (0.647, 0.343) red phosphors. Intensity parameters (Omega(2), Omega(4)) and various radiative properties such as transition rates (A), branching ratios (beta), stimulated emission cross-section (sigma(e)), gain bandwidth (sigma(e) x Delta lambda(eff)) and optical gain (sigma(e) x tau) were calculated using the Judd-Ofelt theory. It was observed that BiOBr:Eu3+ phosphors have a long lifetime (tau) and better optical gain (sigma(e) x tau) as compared to reported Eu3+ doped materials. Furthermore, these compounds exhibit excellent photocatalytic activity for the degradation of rhodamine B dye under visible light irradiation. The determined radiative properties and photocatalytic results revealed that BiOBr:Eu3+ phosphors have potential applications in energy and environmental remedies, such as to develop red phosphors for white light-emitting diodes, red lasers and to remove toxic organic industrial effluents.

Synthesis, photoluminescence and Judd-Ofelt parameters of LiNa3P2O7:Eu3+ orthorhombic microstructures

We report, for the first time, the photoluminescence properties of Eu3+-doped LiNa3P2O7 phosphor, synthesized by a facile solid-state reaction method in air atmosphere. The crystal structure and phase purity of the phosphors were analyzed by X-ray diffraction analysis. Orthorhombic structural morphology was identified by scanning electron microscopy. The phosphate groups in the phosphor were confirmed by Fourier transform infrared analysis. Bandgap of the phosphor was calculated from the diffuse reflectance spectra data using Kubelka-Munk function. Under 395-nm UV excitation, the phosphors show signs of emitting red color due to the D-5(0) -> F-7(2) transition. In accordance with Judd-Ofelt theory, spectroscopic parameters such as oscillator intensity parameter Omega(t) (t = 2), spontaneous emission probabilities, fluorescence branching ratios and radiative lifetimes were calculated and analyzed for the first time in this system.

Synthesis of Eu3+-activated BaMoO4 phosphors and their Judd-Ofelt analysis: Applications in lasers and white LEDs

Eu3+-activated BaMoO4 phosphors were synthesized by the nitrate citrate gel combustion method. The Rietveld refinement analysis confirmed that all the compounds were crystallized in the scheelite-type tetragonal structure with I4(1)/a (No. 88) space group. Photoluminescence (PL) spectra of BaMoO4 phosphor reveals broad emission peaks at 465 and 605 nm, whereas the Eu3+-activated BaMoO4 phosphors show intense 615 nm (D-5(0) -> F-7(2)) emission peak. Judd-Ofelt theory was applied to evaluate the intensity parameters (Omega(2), Omega(4)) of Eu3+-activated BaMoO4 phosphors. The transition probabilities (A(T)), radiative lifetime (tau(rad)), branching ratio (beta), stimulated emission cross-section (sigma(e)), gain bandwidth (sigma(e) x Delta lambda(eff)) and optical gain (sigma(e) x tau(rad)) were investigated by using the intensity parameters. CIE color coordinates confirmed that the BaMoO4 and Eu3+-activated BaMoO4 phosphors exhibit white and red luminescence, respectively. The obtained results revealed that the present phosphors can be a potential candidate for red lasers and white LEDs applications. (C) 2015 Elsevier B.V. All rights reserved.

Comparative study of Eu3+ -activated LnOCl (Ln=La and Gd) phosphors and their Judd-Ofelt analysis

Eu3+-activated layered LnOCl (Ln=La and Gd) phosphors were synthesized by the conventional solid-state method at relatively low temperature (700 degrees C) and shorter duration of 2 h. The structural parameters were refined by the Rietveld refinement analysis and confirmed by the high resolution transmission electron microscopy (HRTEM). Both the compounds were crystallized in the tetragonal structure with space group P4/nmm (No. 129). The homogeneity of the elements were analyzed by TEM mapping and found to be uniformly distributed. The photoluminescence spectra revealed that the intensity of D-5(0)-> F-7(2) transition (619 nm) was more intense in Eu3+-activated GdOCl compared to LaOCl. This was due to the property of Gd3+ ions to act as an intermediate sublattice to facilitate the energy transfer to Eu3+ ions. Intensity parameters and radiative properties such as transition probabilities, radiative lifetime and branching ratio were calculated using the Judd-Ofelt theory. The CIE color coordinates result revealed that the Eu3+-activated GdOCl (0.641, 0.354) phosphor was close to the commercial red phosphors like, Y2O3:Eu3+ (0.645, 0.347), (Y2OS)-S-2:Eu3+ (0.647, 0.343) and National Television System Committee (NTSC) (0.67, 0.33). The results suggest that the present GdOCl:Eu3+ compound acts as a potential candidate for red phosphor materials.

Red-emitting LaOF:Eu3+ phosphors: Synthesis, structure and their Judd-Ofelt analysis for LED applications

In the present study, we have synthesized a series of La1-xEuxOF (0.01 <= x <= 0.09) phosphors by the conventional solid-state reaction route at relatively low temperature (500 degrees C) and shorter duration of 2 h. The compounds were crystallized in the rhombohedral structure with the space group R-3m (No. 166). Upon UV excitation (254 nm), the photoluminescence spectra exhibit characteristic luminescence D-5(0) -> F-7(J) (J= 1, 2, 3, and 4) intra-4f shell Eu3+ ion transitions. An intense red emission peak at 610 nm was observed due to electric dipole (D-5(0) -> F-7(2)) transition. Judd-Ofelt theory was employed to evaluate various radiative parameters such as radiative emission rates, lifetime, branching and asymmetry ratios. CIE color coordinates confirmed the red emission of the phosphors. The luminescent results reveal that LaOF:Eu3+ phosphor can be used as potential candidate for developing red component in white LED applications. (C) 2015 Elsevier Ltd. All rights reserved.

Photoluminescence of Sr(2-x)Ln(x)CeO(4+x/2) (Ln = Eu, Sm or Yb) prepared by a wet chemical method

A wet chemical route is developed for the preparation of Sr2CeO4 denoted the carbonate-gel composite technique. This involves the coprecipitation of strontium as fine particles of carbonates within hydrated gels of ceria (CeO2.xH(2)O, 40<x<75) by the addition of ammonium carbonate. During calcination, CeO2.xH(2)O dehydroxylation is followed by the reaction with SrCO3 to form Sr2CeO4 with complete phase purity. Doping of other rare-earths is carried out at the co-precipitation stage. The photoluminescence (PL) observed for Sr2CeO4 originates from the Ce4+-O2- charge-transfer (CT) transition resulting from the interaction of Ce4+ ion with the neighboring oxide ions. The effect of next-nearest-neighbor (NNN) environment on the Ce4+-O2- CT emission is studied by doping with Eu3+, Sm3+ or Yb3+ which in turn, have unique charge-transfer associated energy levels in the excited states in oxides. Efficient energy transfer occurs from Ce4+-O2- CT state to trivalent lanthanide ions (Ln(3+)) if the latter has CT excited states, leading to sensitizer-activator relation, through non-resonance process involving exchange interaction. Yb3+-substituted Sr2CeO4 does not show any line emission because the energy of Yb3+-O2- CT level is higher than that of the Ce4+-O2- CT level. Sr2-xEuxCeO4+x/2 shows white emission at xless than or equal to0.02 because of the dominant intensities of D-5(2)-F-7(0-3) transitions in blue-green region whereas the intensities of D-5(0)-F-7(0-3) transitions in orange-red regions dominate at concentrations xgreater than or equal to0.03 and give red emission. The appearance of all the emissions from D-5(2), D-5(1) and D-5(0) excited states to the F-7(0-3) ground multiplets of Eu3+ is explained on the basis of the shift from the hypersensitive electric-dipole to magnetic-dipole related transitions with the variation in site symmetry with increasing concentration of Eu3+. White emission of Sr2-x SmxCeO4+x/2 at xless than or equal to0.02 is due the co-existence of Ce4+-O2- CT emission and (4)G(4)(5/2)-H-6(J) Sm3+ transitions whereas only the Sm3+ red emission prevails for xgreater than or equal to0.03. The above unique changes in PL emission features are explained in terms of the changes in NNN environments of Ce4+. Quenching of Ce4+-O2- CT emission by other Ln(3+) is due to the ground state crossover arising out of the NNN interactions.

Precise measurement of hyperfine structure in the 2P(1/2) state of Li-7 using saturated-absorption spectroscopy

We report a precise measurement of the hyperfine interval in the 2P(1/2) state of Li-7. The transition from the ground state (D-1 line) is accessed using a diode laser and the technique of saturated-absorption spectroscopy in hot Li vapor. The interval is measured by locking an acousto-optic modulator to the frequency difference between the two hyperfine peaks. The measured interval of 92.040(6) MHz is consistent with an earlier measurement reported by us using an atomic-beam spectrometer Das and Natarajan, J. Phys. B 41, 035001 (2008)]. The interval yields the magnetic dipole constant in the P-1/2 state as A = 46.047(3), which is discrepant from theoretical calculations by > 80 kHz.

Structural, Magnetic, and Electrical Properties of Microwave-Sintered Cr3+-Doped Sr Hexaferrites

SrCrxFe12-xO19 (x = 0.0, 0.1, 0.3, 0.5, 0.7, 0.9) hexaferrites were prepared by a microwave-hydrothermal method and subsequently sintered at 950 degrees C for 90 min using the microwave sintering method. The results show that, with increasing Cr3+ content, the lattice parameters changed anisotropically. The average grain sizes of sintered samples were in the range of 280 nm to 660 nm. The saturation magnetization systematically decreased with increasing Cr3+ doping, but the coercivity values increased. The electrical resistivity (log rho) decreased linearly with increasing temperature up to a certain temperature known as the transition temperature (T-c), and T-c decreased with further increase (x>0.5) of the Cr3+ content. This decrease in log rho and the activation energy (E-g) is due to electron hopping and occupancy of doped ions at different lattice sites. We found that the dielectric constant and dielectric loss for all the samples decreased with the Cr3+ content. The structural, magnetic, and electrical properties of Cr3+-doped SrFe12O19 hexaferrites have thus been investigated.

Electric field induced short range to long range structural ordering and its influence on the Eu+3 photoluminescence in the lead-free ferroelectric Na1/2Bi1/2TiO3

Eu+3 was incorporated into the lattice of a lead-free ferroelectric Na1/2Bi1/2TiO3 (NBT) as per the nominal formula Na0.5Bi0.5-xEuxTiO3. This system was investigated with regard to the Eu+3 photoluminescence (PL) and structural behaviour as a function of composition and electric field. Electric field was found to irreversibly change the features in the PL spectra and also in the x-ray diffraction patterns below the critical composition x = 0.025. Detailed analysis revealed that below the critical composition, electric field irreversibly suppresses the structural heterogeneity inherent of the host matrix NBT and brings about a long range ferroelectric state with rhombohedral (R3c) distortion. It is shown that the structural disorder on the nano-scale opens a new channel for radiative transition which manifests as a new emission line branching off from the main D-5(0)-> F-7(0) line along with a concomitant change in the relative intensity of the other crystal field induced Stark lines with different J values. The study suggests that Eu+3 luminescence can be used to probe the relative degree of field induced structural ordering in relaxor ferroelectrics and also in high performance piezoelectric alloys where electric field couples very strongly with the lattice and structural degrees of freedom. (C) 2015 AIP Publishing LLC.

Comparative high pressure Raman studies on perfluorohexane and perfluoroheptane

High pressure Raman spectroscopic studies on perfluorohexane and perfluoroheptane have performed up to 12 GPa. Perfluorohexane under goes two pressure induced transitions: (1) liquid-solid transition at 1.6 GPa and (2) solid-solid transition at 8.2 GPa. On the contrary, perfluoroheptane under goes three phase transitions, they are as follows: (1) liquid-solid transition at 1.3 GPa, (2) intermediate solid I transition at 3 GPa, (3) solid II transition at 7 GPa. The change in slope (d omega/dP) shows that the solid I transition at 3.0 GPa could be the conversion of mid-gauche defect into trans conformers for perfluoroheptane. The pressure induced Raman spectra and the behavior of individual band with pressure shows that the solid phase comprises more than one conformer beyond crystallization. The intensity ratio for both the compounds shows that the high pressure phase beyond 8.2 and 7.0 GPa tends to have close packing with distorted all-trans conformers. (C) 2015 Elsevier B.V. All rights reserved.