A potential green-emitting phosphor Ca8Mg(SiO4)4Cl2:Eu for white light emitting diodes prepared by sol-gel method

A potential green emitting phosphor Ca₈Mg(SiO₄)₄Cl₂:Eu²⁺ was prepared by modified sol-gel method. The factors those affect the photoluminescence intensity including heating temperature, the usage of the chlorine source CaCl₂ and the concentration of dopant Eu²⁺ were also investigated in detail. As comparison, the phosphor prepared by solid-state reaction was also prepared. The phosphors show intense absorption in the range of 375-450 nm, which makes it a potential candidate of green emitting phosphor used for near-UV or blue light excited white LEDs.     

Luminescent properties of KGd1−x(WO4)2:Eux and KGd1−x(WO4)2−y(MoO4)y:Eux phosphors in UV-VUV regions

KGd_1−x(WO₄)_2−y(MoO₄)_y:Eu³⁺_x(0.1?x?0.75, y=0 and 0.2) phosphors are synthesized through traditional solid-state reaction and their luminescent properties in ultraviolet (UV) and vacuum ultraviolet (VUV) regions are investigated. Under 147 nm excitation, these phosphors show characteristic red emission with good color purity. In order to improve their emission intensity, the MoO₄²⁻ (20 wt%) is introduced into the anion of KGd_1−x(WO₄):Eu³⁺_x. The Mo⁶⁺ and Eu³⁺ co-doped KGd(WO₄)₂ phosphors show higher emission intensity in comparison with the singly Eu³⁺-doped KGd(WO₄)₂ in VUV region. The chromaticity coordination of KGd_0.45(WO₄):Eu³⁺_0.55 is (x=0.669, y=0.331), while that of KGd_0.45(WO₄)_1.8(MoO₄)_0.2:Eu³⁺_0.55 is (x=0.666, y=0.334) in VUV region.     

Double emitting phosphor NaSr4(BO3)3:Ce, Tb for near-UV light-emitting diodes

Blue and green double emitting phosphor, Ce³⁺ and Tb³⁺ co-doped NaSr₄(BO₃)₃, was synthesized in a weak reducing atmosphere by a conventional high temperature solid-state reaction technique. For comparison, Ce³⁺ or Tb³⁺ singly doped NaSr₄(BO₃)₃ was also prepared. The emission and excitation spectra of all samples have been investigated. NaSr₄(BO₃)₃:Tb³⁺ excitation includes a strong absorption at about 240 nm and some weak sharp lines in near-ultraviolet (n-UV) spectral region. The excitation of Ce³⁺ and Tb³⁺ co-doped NaSr₄(BO₃)₃ shows a strong broad band absorption in the n-UV region from the contribution of Ce³⁺, which makes it suitable for excitation by a n-UV LED chip. The emission of NaSr₄(BO₃)₃:Ce³⁺,Tb³⁺ consists of a blue emission band from Ce³⁺ and a green emission from Tb³⁺ under the excitation of n-UV light. Energy transfer between Ce³⁺ and Tb³⁺ is also discussed, and the relative intensity of blue emission and green emission could be tuned by adjusting the concentration of Ce³⁺ and Tb³⁺. The phosphor NaSr₄(BO₃)₃:Ce³⁺,Tb³⁺ could be considered as a double emission phosphor for n-UV excited white light-emitting diodes.     

Luminescent properties of a novel red-emitting phosphor: Eu-activated Ca3Sr3(VO4)4

A new luminescent material, Eu³⁺ activated Ca₃Sr₃(VO₄)₄, was investigated. This compound shows a strong red emission centered at 618 nm under near-UV light with two distinct absorption bands; charge transfer state of VO₄³⁻ and f-f transitions of europium ions. As the europium concentration is increased, an additional red-emitting phosphor, EuVO₄, which is known to be a prominent luminescent material in the near-UV region can be traced. The UV excited luminescent properties of this material may find application in the production of red phosphors for white light-emitting diodes.     

Preparation and luminescent properties of (Ca1-x,Srx)S:Eu red-emitting phosphor for white LED

The (Ca_1−x,Sr_x)S:Eu²⁺ red-emitting phosphors were prepared by using sulfide and sulfate in CO-reductive atmosphere, respectively, and their luminescent properties have been investigated. The XRD data showed that the doping of divalent Europium ion (Eu²⁺) enlarges the lattice parameters, and Sr/Ca ratio not only affects the lattice parameters, but also influences the emission peak. The excitation and emission spectra indicated that this phosphor can be effectively excited by visible light from 430 to 500 nm, and exhibits a satisfactory red performance. The annealing treatment process was also discussed in detail. Compared with the commercial afterglow sulfide phosphor, the present synthesized phosphor has higher emission efficiency and is a favorable choice for white light-emitting diode (LED). The white LED implanted by present phosphor shows desirable luminescence and chromaticity properties.     

Luminescence investigation of red phosphors Ca0.54Sr0.34−1.5xEu0.08Smx(MoO4)y (WO4)1−y for UV-white LED device

Ca_0.54Sr_0.34−1.5xEu_0.08Sm_x(MoO₄)_y (WO₄)_1−y red phosphors were prepared by solid-state reaction using Na⁺ as a charge compensator for light-emitting diodes (LED). The effects of Na⁺ concentration, synthesis temperature, reaction time and Eu³⁺ concentration were studied for the properties of luminescence and crystal structure of red phosphors. The results show that the optimum reaction condition is 6%, 900 °C, 2 h and 8%. The photoluminescence spectra show that red phosphors are effectively excited at 616 nm by 292, 395 and 465 nm. The wavelengths of 465 nm nicely match the widely applied emission wavelengths of blue LED chips.     

Photoluminescence of (Ba1−xEux)Si6N8O (0.005≤x≤0.2) phosphors

Eu²⁺-doped BaSi₆N₈O phosphors (Ba_1−xEu_xSi₆N₈O, 0.005≤x≤0.2) were synthesized by gas-pressure sintering of the powder mixture of BaCO₃, Si₃N₄, and Eu₂O₃ at 1750 °C under 0.5 MPa N₂. The fired powder consists of a major BaSi₆N₈O phase and a trace amount of impurity phases. The structural result of the BaSi₆N₈O powder, refined by the Rietveld method, agrees well with that of single crystals. A wide blue luminescence band peaking at about 500 nm is observed in BaSi₆N₈O:Eu²⁺, upon excitation with the ultraviolet light of 310 nm. Although Eu is covalently bonded to six nearest neighbor nitrogen atoms, the luminescence of Eu²⁺ is not significantly redshifted but shows a very narrow excitation spectrum at high energies. The origin of the short-wavelength luminescence is mainly ascribed to a small crystal-field splitting as a result of extremely long distances between europium and nitrogen ligands in BaSi₆N₈O:Eu²⁺.     

The synthesis and the magnetic properties of Gd-doped FexCo1−x/CoyFe3−yO4 micro-octahedrons composites

Gd³⁺-substituted micro-octahedron composites (Fe_xCo_1−x/Co_yGd_zFe_3−y−zO₄) in which the Fe-Co alloy has either a bcc or fcc structure and the oxide is a spinel phase were fabricated by the hydrothermal method. The X-ray diffraction (XRD) patterns indicate that the as-synthesized Gd³⁺-substituted micro-octahedron composites are well crystallized. Scanning electron microscopy (SEM) images show that the final product consists of larger numbers of micro-octahedrons with the size ranging from 1.3 to 5 μm, and the size of products are increased with increasing the concentration of KOH. The effect of the Co²⁺/Fe²⁺ ratio (0?Co²⁺/Fe²⁺?1) and substitution Fe³⁺ ions by Gd³⁺ ions on structure, magnetic properties of the micro-octahedrons composites were investigated, and a possible growth mechanism is suggested to explain the formation of micro-octahedrons composites. The magnetic properties of the structure show the maximal saturation magnetization (107 emu/g) and the maximal coercivity (1192 Oe) detected by a vibrating sample magnetometer.     

A potential reddish orange emitting phosphor Ca2BO3Cl:Eu for white light-emitting diodes

A series of phosphors Ca₂BO₃Cl:Eu³⁺ were synthesized by using a high-temperature solid-state reaction technique, and their UV–vis luminescence properties were investigated. The f–f transitions of Eu³⁺ in the host lattice were assigned and discussed. The excitation and emission spectra indicate that this phosphor can be effectively excited by ultraviolet (394 nm), and exhibit reddish orange emission corresponding to the ⁵D₀→⁷F_J (J=0, 1, 2) transitions of Eu³⁺. The influence of the doping concentration and charge compensators on the relative emission intensity of Eu³⁺ was investigated, and the optimum doping concentration is 0.04. The critical distance R_c was estimated to be 17.1 Å in terms of the concentration quenching data. The present study suggests that Ca₂BO₃Cl:Eu³⁺ can be a potential candidate as an UV-convertible phosphor for white light-emitting diodes (LEDs).     

Electrochemical luminescence of Mg1−xCaxIn2O4:Er electrodes

Improvement for electrochemical luminescence (ECL) property of MgIn₂O₄ is attempted by partial exchange of Mg²⁺ ion in MgIn₂O₄ to Ca²⁺ ion. Mg_1−xCa_xIn₂O₄ solid solution was obtained in the region 0<x<0.4. Efficiency for ECL per unit current for Mg_1−xCa_xIn₂O₄:Er³⁺ increased with the increase in the ratio of Ca²⁺ ion, and showed a peak at x=0.25 and then decreased steeply. ECL efficiency for other rare-earth ion-(RE:Sm, Eu, Ho) doped Mg_1−xCa_xIn₂O₄ also increased comparing with those for MgIn₂O₄:RE. This Ca²⁺ addition effect on the ECL efficiency seems to be caused by the improvement of the efficiency for the impact activation.     

VUV-UV luminescence of magnetoplumbite: (Sr0.96−xBa0.04)Al12−yMgyO19:Tbx

The title compounds (Sr_0.96−xBa_0.04)Al_12−yMg_yO₁₉:Tb_x (0<x<0.4; 0<y<0.18) are single-phase magnetoplumbite determined by X-ray powder diffraction analysis. The characteristic emission lines of ⁵D₃→⁷F_j (j=2, 3, 4, 5) and ⁵D₄→⁷F_j (j=4, 5, 6) of Tb³⁺ are recorded under the VUV excitation. The intensive luminescence mainly comes from ⁵D₃→⁷F_j transition when the concentration of Tb³⁺ is low. However, when the concentration of Tb³⁺ starts to increase from very low concentration, ⁵D₄→⁷F_j transition is becoming dominant. Three broad excitation bands at 165, 193 and 233 nm have been observed. The band at 165 nm originates from the overlap between the host absorption and the charge transfer of Tb³⁺-O²⁻. The other two broad bands are the first spin-allowed and the spin-forbidden of 4f-5d transition, respectively. The experimental observation of the 4f-5d transition of Tb³⁺ is consistent well with the theoretical expectations.     

Effects of Zn on the grain boundary properties of La2−xSrxCu1−yZnyO4 superconductors

The properties of the grain boundaries (GBs) are of significant importance in high-T_c cuprates. Most large scale applications of cuprate superconductors involve usage of sintered compounds. The critical current density and the ability to trap high magnetic flux inside the sample depend largely on the quality of the GBs. Zn has the ability to pin vortices but it also degrades superconductivity. In this study we have investigated the effect of Zn impurity on the intergrain coupling properties in high-quality La_2−xSr_xCu_1−yZn_yO₄ sintered samples with different hole concentrations, p (≡x), over a wide range of Zn contents (y) using field-dependent AC susceptibility (ACS) measurements. The ACS results enabled us to determine the superconducting transition temperature T_c, and the temperature T_gcp, at which the randomly oriented superconducting grains become coupled as a function of hole and disorder contents. We have analyzed the behavior of the GBs from the systematic evolution of the values of T_gcp(p, y), T_c(p, y), and from the contribution to the field-dependent ACS signal coming from the intergrain shielding current. Zn suppresses both T_c and T_gcp in a similar fashion. The hole content and the carrier localization due to Zn substitution seem to have significant effect on the coupling properties of the GBs. We have discussed the possible implications of these findings in detail in this article.     

Dielectric properties of valence compensated Ca1−xBixTi1−xCrxO3 perovskite prepared using the sol-gel process

Ca_1−xBi_xNb_1−xCr_xO₃ (x=0.01-0.5) ceramic powders were synthesized using the sol-gel process. The single-phase solids can be presented at x=0.01 and 0.03. The coexistence of orthorhombic perovskite and the secondary phase of BiCrO₃ was verified, as presented for x=0.05-0.5. Grains with a micro-cube topography were obtained for x=0.3-0.5. The average grain size is about 0.4 and 1.1 μm for x=0.3 and 0.5, respectively. The highest dielectric constant peak was measured at around 55 °C for x=0.5 and at 75 °C for x=0.3. The high dielectric constant was caused by the formation of barrier layers at the interface of the bi-phase mixed ceramics. Space charge polarization contributed to the observed behavior.     

A novel yellow phosphor for white light emitting diodes

This paper reports that a novel yellow phosphor, LiSrBO₃:Eusup>2+, was synthesized by the solid-state reaction. The excitation and emission spectra indicate that this phosphor can be effectively excited by ultraviolet (360 and 400~nm) and blue (425 and 460~nm) light, and exhibits a satisfactory yellow performance (565~nm). The role of concentration of Eusup>2+ on the emission intensity in LiSrBO₃ is studied, and it is found that the critical concentration is 3 mol\%, and the concentration self-quenching mechanism is the dipole--dipole interaction according to the Dexter theory. White light emitting diodes were generated by using an InGaN chip (460~nm or 400~nm) with LiSrBO₃:Eusup>2+ phosphor, the CIE chromaticity is (x=0.341, y=0.321) and (x=0.324, y=0.318), respectively. Therefore, LiSrBO₃:Eusup>2+ is a promising yellow phosphor for white light emitting diodes.     

Magnetic and transport properties in Sr1−xLaxFe1−xMnxO3

The magnetic and transport properties in the perovskite Sr_1−xLa_xFe_1−xMn_xO₃ have been explored. As x rises, the systemic ferromagnetism increases gradually and cluster-spin-glass state occurs in the low-temperature region. For 0.3?x?0.7, the ferromagnetic phase separation from the paramagnetic phase was observed from the results of electron-spin-resonance measurement. Although all samples show a semiconducting behavior, their transport properties are dominated by two different mechanisms, namely, the electronic transport of x?0.5 samples is realized by thermal activation but the variable-range hopping is applied in x?0.7 ones. The different transport mechanism can be understood from the Mn/Fe ions interaction.     

Microstructure characterization and magnetic behavior of NiAlxFe2−xO4 and Ni1−yMnyAl0.2Fe1.8O4 ferrites

NiAl_xFe_2−xO₄ and Ni_1−yMn_yAl_0.2Fe_1.8O₄ ferrites were prepared by the conventional ceramic method and were characterized by X-ray diffraction, scanning electron microscopy, and magnetic measurements. The single spinel phase was confirmed for all prepared samples. A proper explanation of data is possible if the Al³⁺ ions are assumed to replace Fe³⁺ ions in the A and B sites simultaneously for NiAl_xFe_2−xO₄ ferrites, and if the Mn²⁺ ions are assumed to replace Ni²⁺ ions in the B sites for Ni_1−yMn_yAl_0.2Fe_1.8O₄ ferrites. Microstructural factors play an important role in the magnetic behavior of Ni_1−yMn_yAl_0.2Fe_1.8O₄ ferrites with large Mn²⁺ content.     

Theoretical investigations on KClxBr1−x, KClxI1−x and KBrxI1−x: A first-principles study

Using first-principles total energy calculations within the full-potential linearized augmented plane wave (FP-LAPW) method, we have investigated the structural, electronic and thermodynamic properties of potassium halides (KCl_xBr_1−x, KCl_xI_1−x and KBr_xI_1−x), with x concentrations varying from 0% up to 100%. The effect of composition on lattice constants, bulk modulus, band gap and dielectric function was investigated. Deviations of the lattice constants from Vegard's law and the bulk modulus from linear concentration dependence (LCD) were observed for the three alloys. The microscopic origins of the gap bowing were explained by using the approach of Zunger and coworkers. On the other hand, the thermodynamic stability of these alloys was investigated by calculating the excess enthalpy of mixing ΔH_m as well as the phase diagram.     

Structural and magnetic properties of ferromagnetic In1−xMnxAs1−yPy layers

The structural and magnetic properties of epitaxial In_1−xMn_xAs_1−yP_y quaternary layers with Mn content ranging from 0.01 to 0.04 and phosphorous content ranging from 0.11 to 0.21 were studied. X-ray diffraction indicated that the films were two phase consisting of an InMnAsP solid solution and hexagonal MnAs nanoprecipitates. Addition of phosphorus promoted precipitate formation. Films were ferromagnetic showing hysteretic behavior in the field dependence of magnetization at 5 and 298 K. From field-cooled magnetization measurements ferromagnetic transitions were observed at 280 and 325 K. The zero field-cooled magnetization versus temperature measurements showed irreversibility for T<300 K that was attributed to the presence of MnAs nanoprecipitates. The calculated coercivity using the Neel model was 1380 G compared to the experimental value of 380 G at 5 K. The difference was attributed to a strong inter-cluster exchange that stabilizes the ferromagnetic state.     

Synthesis and magnetic properties of NiFe2−xAlxO4 nanoparticles

Nanocrystalline Al-doped nickel ferrite powders have been synthesized by sol–gel auto-ignition method and the effect of non-magnetic aluminum content on the structural and magnetic properties has been studied. The X-ray diffraction (XRD) revealed that the powders obtained are single phase with inverse spinel structure. The calculated grain sizes from XRD data have been verified using transmission electron microscopy (TEM). TEM photographs show that the powders consist of nanometer-sized grains. It was observed that the characteristic grain size decreases from 29 to 6 nm as the non-magnetic Al content increases, which was attributed to the influence of non-magnetic Al concentration on the grain size. Magnetic hysteresis loops were measured at room temperature with a maximum applied magnetic field of ≈1 T. As aluminum content increases, the measured magnetic hysteresis curves become more and more narrow and the saturation magnetization and remanent magnetization both decreased. The reduction of magnetization compared to bulk is a consequence of spin non-collinearity. Further reduction of magnetization with increase of aluminum content is caused by non-magnetic Al³⁺ ions and weakened interaction between sublattices. This, as well as the decrease in hysteresis was understood in terms of the decrease in particle size.