EPR and photoluminescence properties of combustion-synthesized ZnAl<Subscript>2</Subscript>O<Subscript>4</Subscript>:Cr<Superscript>3+</Superscript> phosphors

An efficient red emitting ZnAl₂O₄:Cr³⁺ powder phosphor material was prepared at furnace temperatures as low as 500 °C by using the combustion method. The prepared powders were analyzed by X-ray diffraction and scanning electron microscopy techniques. The optical properties were studied using photoluminescence technique. The EPR spectra exhibit an intense resonance signal at g = 3.74 which is attributed to Cr³⁺–Cr³⁺ pairs, and the weak resonance signal of at g = 1.97 is attributed to Cr³⁺ single ion transition. The spin population (N) has been evaluated as a function of temperature. The excitation spectrum exhibits two broad bands in the visible region which are characteristic of Cr³⁺ ions in octahedral symmetry and the emission spectrum exhibits zero-phonon line frequencies along with vibronic frequencies. The crystal field parameter (Dq) and Racah parameters (B and C) have been evaluated and discussed.     

Studies of the EPR Parameters for Bi<Subscript>2</Subscript>Sr<Subscript>2</Subscript><Emphasis Type="Italic">Y</Emphasis>Cu<Subscript>2</Subscript>O<Subscript>8</Subscript>:Er<Superscript>3+</Superscript>

The electron paramagnetic resonance (EPR) parameters (the anisotropic g factors g _x , g _y , g _z , and the hyperfine structure constants A _x , A _y , and A _z ) of the two orthorhombic Er³⁺ centers in Bi₂Sr₂ YCu₂O₈ are theoretically studied from the perturbation formulas of these parameters for a 4f¹¹ ion in orthorhombic symmetry. In these formulas, the contributions due to the admixtures of various states are taken into account, and the orthorhombic field parameters are determined from the superposition model and the local geometry of Bi³⁺ site in Bi₂Sr₂ YCu₂O₈. The calculated EPR parameters show reasonable agreement with the observed values. The anisotropy g _z >g _x (g _y ) for the g factors may be attributed to the compression of the ligand octahedra in the Er³⁺ centers.     

Investigations on green-emitting,Mn<Superscript>2+</Superscript>: BaAl<Subscript>12</Subscript>O<Subscript>19</Subscript> phosphors obtained by solution combustion process

Manganese-doped BaAl₁₂O₁₉ green phosphor was prepared using a self-propagating (combustion) synthesis. Powder X-ray diffraction and scanning electron microscopy were used to characterize the as-prepared combustion product. A room temperature photoluminescence study shows an emission line at 513 nm corresponding to a transition from the upper ⁴T₁ → ⁶A₁ ground state of Mn²⁺ ions. The electron paramagnetic resonance (EPR) spectrum exhibits six line hyperfine structure at g = 1.981. From the EPR spectrum, the spin-Hamiltonian parameters have been evaluated. The g value indicates that the site symmetry around Mn²⁺ ions is distorted tetrahedral. The number of spins (N) participating in the resonance for g = 1.981 is measured as a function of temperature. The paramagnetic susceptibility (χ) is calculated from the EPR data at various temperatures. From the plot 1/χ versus T, the Curie constant (C) and Curie paramagnetic temperature (θ_p) have been evaluated and discussed.     

Structural,optical and non-linear investigation of Eu<Superscript>3+</Superscript>: Al(NO<Subscript>3</Subscript>)<Subscript>3</Subscript>-SiO<Subscript>2</Subscript> sol-gel glass

Optical absorption and fluorescence spectra of Eu³⁺ ions in Al(NO₃)₃-SiO₂ sol-gel glass have been investigated using the Judd- Ofelt theory. JO intensity parameters (ω _λ ) and subsequent radiative properties for⁵ D ₀ ↔⁷ F _1,2,4,6 transitions are determined. The lifetime (τ_r) of⁵ D ₀ state is computed and along with JO parameters are compared with their corresponding values in other glasses prepared by conventional technique. A structural analysis, using IR and XRD spectra and non- linear parametrization of the silica gel glass is carried out. The study reveals the glass to be a very good third order non- linear amorphous optical material.     

Spectral parameters of Er<Superscript>3+</Superscript> ion in Yb<Superscript>3+</Superscript>/Er<Superscript>3+</Superscript>:KY(WO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystal

The spectral parameters of Er³⁺ in Yb³⁺/Er³⁺:KY(WO₄)₂ crystal with space group C2/c have been investigated based on Judd-Ofelt theory. The spectral parameters were obtained: the intensity parameters are: ₂ = 6.33 × 10^–20 cm², ₄ = 1.35 × 10^–20 cm², ₆ = 1.90 × 10^–20 cm². The radiative lifetime and the fluorescence branch ratios were calculated. The emission cross section _e (at 1536 nm) is 2.0 × 10^–21 cm².     

Synthesis and properties of AgTi<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>-based NASICON-type phosphates doped with Nb<Superscript>5+</Superscript>, Zr<Superscript>4+</Superscript>, and Ga<Superscript>3+</Superscript>

We have synthesized materials based on a silver titanium phosphate with partial substitution of tri-, tetra-, or pentavalent cations for titanium: Ag_1±x Ti_2−x M _x (PO₄)₃ (M = Nb⁵⁺, Ga³⁺) and AgTi_2−x Zr _x (PO₄)₃. The materials have been characterized by X-ray diffraction and impedance spectroscopy and have been shown to have small thermal expansion coefficients. Their ionic conductivity has been determined. Silver ions in these materials are difficult to replace with protons.     

Oxidation of tungsten in the W-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> system at temperatures from 2350 to 2500 K and pressures from 1 to 10<Superscript>5</Superscript> Pa

We examine chemical reactions that lead to the oxidation of tungsten in the W-Al₂O₃ system at T = 2350–2500 K and p = 1–10⁵ Pa. The results indicate that, for p ≥ 10 Pa, tungsten oxidizes through reactions with both Al₂O₃ vapor and dissociation products (Al₂O₂, Al₂O, AlO₂, and AlO). For p ≤ 10 Pa, oxidation is due to direct reaction of tungsten with O and/or O₂. For p ≤ 2 Pa, tungsten may react with molten Al₂O₃. A detailed analysis of tungsten oxidation processes is intended to optimize parameters of the melt growth of corundum crystals.     

Luminescence of SrAl<Subscript>2</Subscript>O<Subscript>4</Subscript>:Cr<Superscript>3+</Superscript>

Samples of SrAl₂O₄ and SrAl₂O₄:Cr³⁺ were prepared by mixing the powder materials SrCO₃, Al₂O₃, and Cr₂O₃. The crystal structures of the undoped and doped samples were analyzed by X-ray diffraction (XRD) measurements. The diffraction patterns reveal a dominant phase, characteristic of the monoclinic SrAl₂O₄ compound and another unknown secondary phase, in small amount, for doped samples. The data were fitted using the Rietveld method for structural refinements and lattice parameter constants (a, b, c, and β) were determined. Luminescence of Cr³⁺ ions in this host is investigated for the first time by excitation and emission spectroscopy at room temperature. Emission spectra present a larger band and a smaller structure associated to the and electronic transitions, respectively. The obtained results are analyzed by crystal-field theory and the crystal-field parameter, Dq, and Racah parameters, B and C, are determined from the excitation measurements.     

Mechanical and electrical switching of local ferroelectric domains of K<Subscript>0.5</Subscript>Bi<Subscript>4.5</Subscript>Ti<Subscript>4</Subscript>O<Subscript>15</Subscript> film

Ferroelectric/Piezoelectric K_0.5Bi_4.5Ti₄O₁₅ (KBT) film was fabricated by pulsed laser deposition method and confirmed by ferroelectric, dielectric measurements and local butterfly-type piezoresponse hysteresis loops. Importantly, ferroelectric domain switching by both electrical field and mechanical force in KBT film was demonstrated. The dark and bright contrast represents the PFM response of the up and down polarized domains, which can be written by a dc bias of ±12 V or a mechanical force of 40–50 nN. The successful demonstration of mechanical force switching of ferroelectric domain in KBT film other than electric field provides a novel mean for information storage and sensors.     

Comparative crystal field calculations of the Cr<Superscript>3+</Superscript> energy level schemes in ZnAl<Subscript>2</Subscript>S<Subscript>4</Subscript> and ZnGa<Subscript>2</Subscript>O<Subscript>4</Subscript>

The exchange charge model of crystal field (which includes the covalent effects) is used to analyze the energy level schemes of Cr³⁺ ion in ZnAl₂S₄ and ZnGa₂O₄ crystals with spinel structure. Calculations of the overlap integrals and crystal field parameters were performed before getting the Cr³⁺ energy levels. The calculated energy level schemes are compared with available experimental data; a new interpretation of the absorption peaks is suggested.     

Luminescence of Ce<Superscript>3+</Superscript> ion doped in SrZn<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>2</Subscript> phosphor under excitation of vacuum ultraviolet

Ce³⁺ doped SrZn₂(PO₄)₂ was prepared by high temperature solid-state reaction. The phosphor was investigated by X-ray powder diffraction, scanning electron microscope, and FT-IR measurements. Spectroscopic properties of the phosphor were characterized by vacuum ultraviolet spectroscopy. According to the excitation spectrum, the five 5d levels corresponding to the 4f ¹ → 4f ⁰5d ¹ transitions of Ce³⁺ ions were clearly identified. The observed excitation bands in the VUV region are due to the PO₄³⁻ anion groups of the host, in which energy transfer to Ce³⁺ ions is rather efficient. The emission bands corresponding to the 4f ¹ → 4f ⁰5d ¹ transitions of Ce³⁺ ions were analyzed. The barycenter of Ce³⁺ ions, host absorption bands, crystal field splitting, emission wavelength and Stokes shift were calculated and discussed.     

Upconversion Luminescence of Gd<Subscript>2</Subscript>O<Subscript>3</Subscript> Nanocrystals Doped with Er<Superscript>3+</Superscript> and Yb<Superscript>3+</Superscript> Ions

The upconversion luminescence (UCL) of nanocrystalline gadolinium oxide (Gd₂O₃) doped with Er³⁺ and Yb³⁺ ions has been studied in the temperature range of 90–400 K. The nanocrystals were synthesized by chemical vapor deposition and possessed a cubic crystalline structure with an average particle size within 48–57 nm. It is established that the USL intensity in the red (⁴F_9/2 → ⁴I_15/2 transition in Er3+ ion) and green (⁴S_3/2 → ⁴I_15/2 transition) spectral regions depends on the sample temperature and concentration of dopant ions, as well as on the additional structural defects (anion vacancies) created in the crystal lattice by the introduction of Zn²⁺ ions or irradiation with high-energy (10 MeV) electrons. The luminescence efficiency and spectrum of the upconversion phosphor are determined by energy transfer processes.     

Preparation and Properties of MPMG YBCO Bulk with Y<Subscript>2</Subscript>O<Subscript>3</Subscript> Layer

In this study, four kinds of melt-processed YBCO samples were fabricated with the MPMG procedure. The compacted powders were located on a crucible with a buffer layer of Y₂O₃ to avoid liquid to spread on the furnace plate. Their microstructures were defined by XRD analysis and polarized light optical microscopy. The microstructure investigations indicated that the 123 grains were very big and fine and dispersed 211 particles remained in the samples. Resistivities of the samples were measured by a standard continuous dc four-probe method. Magnetization measurements were made and flux jumps were observed at a relatively higher temperature for Y1060. The critical current density, J _c , values of the samples, measured by VSM in 5 T magnetic field, exceeded 0.6×10³ A⋅cm⁻² at 77 K and 4 T.     

Synthesis,tunable luminescence and thermal stability of Mg<Subscript>2</Subscript>Al<Subscript>4</Subscript>Si<Subscript>5</Subscript>O<Subscript>18</Subscript>:Ce<Superscript>3+</Superscript>/Mn<Superscript>2+</Superscript> phosphors

Ce³⁺/Mn²⁺ singly doped and codoped Mg₂Al₄Si₅O₁₈ phosphors were synthesized by a solid state reaction. The phase, luminescent properties and thermal stability of the synthesized phosphors were investigated. Ce³⁺ and Mn²⁺ singly doped Mg₂Al₄Si₅O₁₈ phosphors show emission bands locating in blue and yellow–red regions, respectively. In Ce³⁺ and Mn²⁺ codoped Mg₂Al₄Si₅O₁₈, tunable luminescence was obtained because of the energy transfer from Ce³⁺ to Mn²⁺. In Mg₂Al₄Si₅O₁₈:Ce³⁺/Mn²⁺ phosphors with a fixed Ce³⁺ concentration, energy transfer efficiency increases with the increasing Mn²⁺ concentration, which is confirmed by the continually decreasing intensity and shortening decay time of Ce³⁺ emission. Moreover, the luminescent properties and thermal stability provide a great significance on the applications in the field of light emitting diodes.     

Luminescence studies on Eu<Superscript>2+</Superscript> and Tb<Superscript>3+</Superscript> doped Ca<Subscript>2</Subscript>MgSi<Subscript>2</Subscript>O<Subscript>7</Subscript> phosphors

Eu²⁺ and Tb³⁺ doped Ca₂MgSi₂O₇ phosphors were synthesized by conventional solid-state reaction. The phase formation was confirmed by X-ray powder diffraction technique and refined lattice parameters were calculated by rietveld refinement process using Celref v3. The photoluminescence (PL) excitation and emission spectra were investigated. The phosphors exhibited broaden green emitting luminescence peaking at 520 nm when excited at 374 nm source. Morphological studies were carried out using Scanning electron microscopy (SEM) images of the sample with optimum PL emission. The dependence of photoluminescence intensity on co-dopant concentration and the kinetic parameters were also reported. Time resolved fluorescence spectroscopy (TRFS) is used to investigate the decay in luminescence signals with respect to time. The sample proved to be a good long lasting material, which makes it useful in emergency signs, textile printing, textile exit sign boards and electronic instrument dial pads etc.     

Structure and photoluminescence of ZnGa<Subscript>2</Subscript>Se<Subscript>4</Subscript>:Eu<Superscript>2+</Superscript>

The structure of undoped and europium-doped ZnGa₂Se₄ has been studied, and the interplanar spacings, the Miller indices of the observed reflections, and their relative intensities have been determined. The photoluminescence spectra of ZnGa₂Se₄ and ZnGa₂Se₄:Eu²⁺ crystals have been measured at temperatures from 77 to 230 K. The 566-nm band in the spectrum of ZnGa₂Se₄:Eu²⁺ is assigned to the Eu²⁺4f ⁶5d→4f ⁷(⁸ S _7/2) electronic transition, and the bands at 591 and 646 nm are attributed to transitions of donor-acceptor pairs.     

Luminescence properties and electronic structure of Sm<Superscript>3+</Superscript>-doped YAl<Subscript>3</Subscript>B<Subscript>4</Subscript>O<Subscript>12</Subscript>

The luminescence properties of Sm³⁺ ions in YAl₃B₄O₁₂ were studied upon synchrotron excitation in the 3.8–11 eV region. In addition to the 4f → 4f excitation bands, the excitation spectra of the Sm³⁺ emission contain broad bands at 6.1 and ~7.0 eV. These bands are attributed to charge transfer transition in Sm³⁺–O²⁻ complexes and 4f → 5d transition of Sm³⁺ ions, respectively. The optical absorption edge of YAl₃B₄O₁₂ was determined at 7.3 eV. A comparison with the results of electronic structure calculations on YAl₃B₄O₁₂ is also made.     

Synthesis and luminescence properties of Eu<Superscript>2+</Superscript>-doped Li<Subscript>2</Subscript>SrSiO<Subscript>4</Subscript>

We have studied the luminescence spectra of Li₂Sr_{1 − x} Eu _x SiO₄ (x = 0.0001–0.01) solid solutions prepared by solid-state reactions and a sol-gel process in a reducing atmosphere. The spectra show a broad band in the range 500–700 nm, centered at 578 nm, which is due to the 4f ⁶5d → 4f ⁷ transition. The luminescence excitation spectrum shows, in addition to bands due to Eu²⁺ 4f ⁷ → 4f ⁶5d transitions, a strong band centered at 174 nm, attributable to absorption in the SiO₄⁴⁻ group.     

Synthesis and photoluminescence properties of Eu<Superscript>3+</Superscript>-activated SrZnO<Subscript>2</Subscript>

Synthesis, X-ray diffraction, and photoluminescence (PL) investigations of SrZnO₂ doped with Eu³⁺ were carried out in order to characterize the material. The emission spectra showed a broad band emission at 525 nm attributed to oxygen defect centers in the host matrix, along with peaks corresponding to the ⁵D₀ → ⁷F _j (j = 1, 2) transitions of Eu ion under 250 nm excitation. PL decay time studies were done to confirm these investigations. Time-resolved emission spectrometric (TRES) study was carried out to extract the emission spectra of the Eu ion which was buried under the broad band emission. After giving suitable delay times and by choosing a proper time gate, transitions due to ⁵D₀ → ⁷F _j (j = 0, 1, 2, 3, and 4) could be observed. Judd–Ofelt intensity parameters and other radiative properties for the system were evaluated from this emission spectrum and decay time data by adopting standard procedure. The color coordinates of the system were also evaluated and plotted on a standard CIE index diagram. The observations showed that the SrZnO₂:Eu³⁺material has near white light emission (also considering the emission from host) whereas, the extracted emission spectrum due to only Eu ions has a near red emission.     

Synthesis and properties of laser-active liquids POCl<Subscript>3</Subscript>-SbCl<Subscript>5</Subscript>-<Superscript>235</Superscript>UO<Stack><Subscript>2</Subscript><Superscript>2+</Superscript></Stack>-Nd<Superscript>3+</Superscript>

The effect of the synthesis conditions on the properties of inorganic laser-active liquids POCl₃-SbCl₅-²³⁵UO ₂ ²⁺ -Nd³⁺ is considered. The kinetic dependences of the U(IV) content and decay time of the Nd³⁺ luminescence in POCl₃-SbCl₅-²³⁵UO ₂ ²⁺ -Nd³⁺ solutions for various synthesis procedures at 380 K have been obtained. In POCl₃-SbCl₅-²³⁵UO ₂ ²⁺ -Nd³⁺ solutions, nonradiative energy transfer Nd³⁺ → U⁴⁺ is observed, and quenching of the Nd³⁺ luminescence is described by the Stern-Volmer law: k _q = (6.4 ± 0.6) × 10⁵ l mol^?1 s^?1. Laser liquids POCl₃-SbCl₅-²³⁵UO ₂ ²⁺ -Nd³⁺ with neodymium concentration of up to 0.7 M, uranyl concentration of up to 0.1 M, and decay time of the Nd³⁺ luminescence of up to 220 μs have been prepared for the first time.