Preparation,structural and luminescent properties of YAl3(BO3)4:Dy phosphor for white light-emission under UV excitation

A series of YAl₃(BO₃)₄ phosphors doped with different concentrations of Dy₂O₃ (0.1≤x≤5 mol%) were prepared by solid-state reaction method. The crystallization process of the precursor has been examined by differential thermal analysis (DTA) measurements. The phase purity and surface morphological features were characterized by X-ray diffraction (XRD) and scanning electron microscopic (SEM) investigations. The YAl₃(BO₃)₄ nanocrystals obtained were found to be about 45 nm in size and have the trigonal structure with some agglomeration. Fourier transform infrared (FTIR) and energy dispersive X-ray spectra (EDS) measurements were carried out to understand the compositional and elemental analysis. The characteristics emission peaks of Dy³⁺ ion corresponding to the transitions of ⁴F_9/2→⁶H_15/2 at 485 nm and ⁴F_9/2→⁶H_13/2 at 576 nm were observed in the emission spectra. The luminescence quenching noticed at higher Dy₂O₃ concentrations is due to the exchange interaction among the excited Dy³⁺ ions.     

Investigation of Eu2+ luminescence in barium tetraphosphate Ba3P4O13 polycrystalline ceramics

In this paper, Ba₃P₄O₁₃:Eu²⁺ phosphor was synthesized by a solid-state reaction. The photoluminescence (PL) emission spectrum and luminescence decay kinetics confirm that the doped Eu²⁺ ions can occupy two different Ba²⁺ sites. The PL excitation spectrum shows a broad band matching well with the emission of near-UV chip. Ba₃P₄O₁₃:Eu²⁺ is a promising phosphor for near-UV chip excited white LEDs. The doped Eu³⁺ ions can also be reduced to Eu²⁺ ions in air atmosphere at high temperature. Charge compensation mechanism is applied to explain this kind of abnormal reduction.     

Luminescent properties of Pr doped ZrSiO4 phosphors

Mixtures of ZrO₂, SiO₂, Pr₂O₃, and H₃BO₃ were fired at the temperature of 1150–1350 °C under H₂ atmosphere. A single phase ZrSiO₄:Pr phosphors could be achieved at the firing temperature above 1150 °C. Crystallinity and PL properties strongly depended on the flux amounts, firing temperature, and dopant concentrations. ZrSiO₄:Pr phosphors showed a strong red emission at 615 nm and a weak red emission at 622 nm with an excitation wavelength of 295 and 450 nm, respectively. Two sets of the emission bands and excitation spectra originated from Pr³⁺ ions at Zr and Si sites, respectively.     

Solid state reaction preparation and enhanced red luminescence of S-doped La2Mo2O9:Prphosphors

Red-light-emitting phosphors of La₂Mo₂O₉:Pr³⁺ and S-doped La₂Mo₂O₉:Pr³⁺ were prepared by high temperature solid state reaction. Under the excitation of 450 nm blue light, all samples produced a red emission peak at 650 nm corresponding to the characteristic transition of Pr³⁺ (³P₀→³F₂). The dependence of Pr³⁺ doping content (x) on the luminescent intensity was analyzed, and the optimal doping content of Pr³⁺ was x=0.07. After a small quantity of sulfur was introduced into the system, the luminescence intensity of phosphors was obviously enhanced. The reasons for the enhancement of luminescence are due to improved crystallization after S doping and the relatively large electronegativity difference between S and Mo. Additionally, the coincidence of the excitation wavelength with the emission of GaN chips may recommend this phosphor system as a potential candidate for use in white light-emitting diodes.     

Low temperature sintering and microwave dielectric properties of Ce2(WO4)3 ceramics

Ce₂(WO₄)₃ ceramics have been synthesized by the conventional solid-state ceramic route. Ce₂(WO₄)₃ ceramics sintered at 1000 °C exhibited ?_r = 12.4, Qxf = 10,500 GHz (at 4.8 GHz) and τ_f = −39 ppm/°C. The effects of B₂O₃, ZnO–B₂O₃, BaO–B₂O₃–SiO₂, ZnO–B₂O₃–SiO₂ and PbO–B₂O₃–SiO₂ glasses on the sintering temperature and microwave dielectric properties of Ce₂(WO₄)₃ were investigated. The Ce₂(WO₄)₃ + 0.2 wt% ZBS sintered at 900 °C/4 h has ?_r = 13.7, Qxf = 20,200 GHz and τ_f = −25 ppm/°C.     

A reddish orange stoichiometric phosphor LiEu(PO3)4 for white light-emitting diodes

Stoichiometric phosphors LiGd_1−xEu_x(PO₃)₄(x=0, 0.2, 0.4, 0.6, 0.8, 1.0) were synthesized via traditional solid state reactions. The X-ray powder diffraction measurements show that all prepared samples are isostructural with LiNd(PO₃)₄. Eu³⁺ doped phosphors can emit intense reddish orange light under the excitation of near ultraviolet light from 370 to 410 nm. The strongest two at 591 and 613 nm can be attributed to the transitions from excited state ⁵D₀ to ground states ⁷F₁ and ⁷F₂, respectively. The typical chromaticity coordinates (x=0.620, y=0.368) of Eu³⁺ doped phosphors are in red area. The recorded absorbance spectra indicate that there is effective absorbance in the near UV region for all Eu³⁺ doped samples. Present research indicates that LiGd_1–xEu_x(PO₃)₄ is a promising phosphor for white light-emitting diodes.     

Preparation of Ca4Mg5(PO4)6:Eu,Mn phosphor and its photoluminescence properties

Eu²⁺ and Mn²⁺ singly doped and Eu²⁺/Mn²⁺-codoped Ca₄Mg₅(PO₄)₆ phosphors were synthesized via combustion synthesis. Mn²⁺-singly doped Ca₄Mg₅(PO₄)₆ phosphor exhibits a single red emission in the wavelength range of 500–700 nm due to the ⁴T₁(⁴G)→⁶A₁(⁶S) transition of Mn²⁺. Eu²⁺/Mn²⁺ co-doped phosphor emits two distinctive luminescence bands: a blue one centered at 442 nm originating from Eu²⁺ and a broad red-emitting one peaked at 609 nm from Mn²⁺. Energy transfers from Eu²⁺ to Mn²⁺ were discovered by directly observing significant overlap of the excitation spectrum of Mn²⁺ and the emission spectrum of Eu²⁺ as well as the systematic relative decline and growth of emission bands of Eu²⁺ and Mn²⁺, respectively. Based on the principle of energy transfer, the relative intensities of blue and red emission could be tuned by adjusting the contents of Eu²⁺ and Mn²⁺.     

The luminescence spectroscopy and thermal stability of red-emitting phosphor Ca9Eu(VO4)7

Eu-based vanadate Ca₉Eu(VO₄)₇ phosphor was synthesized by the solid state reaction method and was characterized by X-ray powder diffraction (XRD). The photoluminescence excitation and emission spectra, fluorescence decay curves and the dependence of luminescence intensity on temperature were investigated. The phosphor can be efficiently excited by near UV light to realize an intense red luminescence (614 nm) corresponding to the electric dipole transition ⁵D₀ → ⁷F₂ of Eu³⁺ ions. The crystallographic site-occupations of the Eu³⁺ ions in Ca₉Eu(VO₄)₇ were investigated by the site-selective excitation and emission spectra, and the fluorescence decay curves in the ⁵D₀ → ⁷F₀ region using a pulsed, tunable, narrowband dye laser. The red luminescence together with the thermal stability was discussed on the base of the Eu³⁺ site-distribution in Ca₉Eu(VO₄)₇ host.     

Effect of Bi2O3 and B2O3 additives on the sintering temperature, microstructure, and microwave dielectric properties for Sm(Mg0.5Ti0.5)O3 ceramics

The microwave dielectric properties of Sm(Mg_0.5Ti_0.5)O₃ incorporated with various amount of Bi₂O₃ and B₂O₃ additives have been investigated systematically. In this study, both Bi₂O₃ and B₂O₃ additives acting as a sintering aid can effectively lower the sintering temperature from 1550 °C to 1300 °C. The ionic radius of Bi³⁺ for a coordination number of 6 is 0.103 nm, whereas the ionic radius of B³⁺ is 0.027 nm. Clearly, the ionic radius of Bi³⁺ is greatly larger than one of B³⁺, which resulted in the specimens incorporated with Bi₂O₃ having larger lattice parameters and cell volume than those incorporated with B₂O₃. The experimental results show that no second phase was observed throughout the entire experiments. Depending on the interfacial tension, the liquid phase may penetrate the grain boundaries completely, in which case the grains will be separated from one another by a thin layer as shown in Sm(Mg_0.5Ti_0.5)O₃ ceramics incorporated with Bi₂O₃. Whereas, in Sm(Mg_0.5Ti_0.5)O₃ ceramics incorporated with B₂O₃, the volume fraction of liquid is high, the grains may dissolve into the liquid phase, and rapidly rearrange, in which case contact points between agglomerates will be dissolved due to their higher solubility in the liquid, leading plate-like shape microstructure.A dielectric constant (?_r) of 29.3, a high Q × f value of 26,335 GHz (at 8.84 GHz), and a τ_f of −32.5 ppm/°C can be obtained for Sm(Mg_0.5Ti_0.5)O₃ ceramics incorporated with 10 mol% Bi₂O₃ sintered at 1300 °C. While Sm(Mg_0.5Ti_0.5)O₃ ceramics incorporated with 5 mol% B₂O₃ can effectively lower temperature coefficient of resonant frequency, which value is −21.6 ppm/°C. The Sm(Mg_0.5Ti_0.5)O₃ ceramic incorporated with heavily Bi₂O₃ and B₂O₃ additives exhibits a substantial reduction in temperature (∼250 °C) and compatible dielectric properties in comparison with that of an un-doped one. This implied that this ceramic is suitable for miniaturization in the application of dielectric resonators and filters by being appropriately incorporated with a sintering aid.     

The doping concentration dependent tunable yellow luminescence of Sr5(PO4)2(SiO4):Eu

Color tunable yellow-emitting phosphors of Sr_5−5xEu_5x(PO₄)₂SiO₄ (x = 0.05-0.15) were prepared by conventional solid-state reaction method. The X-ray powder diffraction patterns, the photoluminescence excitation and emission spectra were measured. The main excitation bands of the phosphors locate at a broad band extending from 300 to 500 nm, which can match the emission of ultraviolet- and blue-emitting diode chips. The tunable luminescence color was realized by the changing Eu²⁺ doping in Sr₅(PO₄)₂SiO₄. The structure and luminescence properties were investigated. Sr_5−5x(PO₄)₂SiO₄:Eu_5x displays two typical luminescence centers, which originate from two different Sr²⁺ (Eu²⁺) sites in the host. The site-occupation, the luminescence intensity and energy transfer between the Eu²⁺ ions occupying two different crystallographic Sr²⁺ sites were discussed on the base of the luminescence spectra and crystal structure. This is helpful to improve this phosphor for a potential application as a white light emitting diode phosphor.     

Synthesis and photoluminescence properties of La1−xAlO3:xTb green phosphors for white LEDs

Tb³⁺-doped La_1−xAlO₃ phosphor powders are successfully synthesized by the solution combustion method, using citric acid as the combustion fuel. The crystal structure and photoluminescence properties of La_1−xAlO₃:xTb³⁺ phosphors are studied, depending on Tb³⁺ content. The strongest emission peak is found at 543 nm, which originates from the ⁵D₄→⁷F₅ transition of Tb³⁺ ions, indicating green emission. Among the fabricated phosphors, the La_0.9AlO₃:0.1Tb³⁺ phosphor emits the strongest green light. The excellent luminescent properties make it a possible candidate for white light-emitting diodes and various photonic applications.     

Preparation of Sr2SiO4:Eu phosphors by microwave-assisted sintering and their luminescent properties

An Eu³⁺ activated strontium silicate phosphor was synthesized using a microwave-assisted sintering with a flux NH₄Cl. X-ray powder diffraction analysis confirmed the formation of pure Sr₂SiO₄ phase without second phase or phases of starting materials as Sr_1.9SiO₄:Eu³⁺_0.1 powders sintered at various temperatures in microwave furnace for 1 h. Scanning electron microscopy showed smaller particle size and more uniform grain size distributions are obtained by microwave-assisted sintering. In the PL studies, the excitation spectrum of Sr_1.9SiO₄:Eu³⁺_0.1 phosphors exhibited a broad band in the UV region centered at about 270 nm which was consistent with the absorption spectra. Both microwave sintered and conventionally sintered powders emitted a maximum luminescence centered at 617 nm under excitation of 395 nm, with similar luminescent intensity. The results showed that microwave processing has the potential to decrease the sintering time and required energy input for the production of Sr_1.9SiO₄:Eu³⁺_0.1 phosphors without degrading photoluminescence.     

Fabrication of Sn doped Ba(Mg1/3Ta2/3)O3 transparent ceramics by a solid state reaction method

Transparent disordered BMT ceramics were obtained by solid state reaction. Sn⁴⁺ ions were incorporated to make the B site of the perovskite structure disordered. The stoichiometric powder mixture with and without Sn doping was calcinated at 1300 °C, respectively and they were both characterized. After dry pressing, the pellets with Sn doping were sintered at 1600 °C in oxygen atmosphere for 4 h. The grain size of the transparent ceramics is around 12 μm. No pores were detected in or among the grains. The inline transmittance of the material is 66% at 580 nm. The refractive index is 2.09 at 1600 nm.     

Improvement of photoluminescence properties of Ce3+- doped CaSrAl2SiO7 phosphors by charge compensation with Li+ and Na+

In this work, we prepared CaSr_1-xAl₂SiO₇:xCe³⁺ (0.03 ≤ x ≤ 0.12) and CaSr_0.94Al₂SiO₇:0.03Ce³⁺,0.03 M⁺ (M⁺ = Li⁺ and Na⁺) phosphors via solid-state reaction method. Structural and photoluminescence (PL) properties of the phosphors were also investigated. The prepared phosphors formed an orthorhombic crystal structure with the P2₁2₁2₁ space group. CaSr_1-xAl₂SiO₇:xCe³⁺ phosphors were effectively excited by near-ultraviolet (UV) light (345 nm), which is suitable with the emission of near-UV light emitting diode chips. A broad blue emission (402 nm) was detected in CaSr_1-xAl₂SiO₇:xCe³⁺ and CaSr_0.94Al₂SiO₇:0.03Ce³⁺,0.03 M⁺ phosphors; this was attributed to the 4f⁰5d¹ → 4f¹ transition of Ce³⁺. To maintain charge equilibrium, charge compensators, such as monovalent Li⁺ and Na⁺ ions, were doped into the CaSr_0.97Al₂SiO₇:0.03Ce³⁺ phosphor, significantly improving its PL properties. The strongest emission intensity was achieved in CaSr_0.94Al₂SiO₇:0.03Ce³⁺,0.03Li⁺ phosphor. Addition of Li⁺ charge compensator was highly effective in improving PL properties of CaSr_0.97Al₂SiO₇:0.03Ce³⁺ phosphors.     

Luminescent properties of Eu-doped YTaO4 powders

YTaO₄:Eu³⁺ powders were synthesized by a flux method with LiCl. The use of a flux in the synthesis of M′–YTaO₄ facilitated the reaction of raw materials, leading to lowering the heating temperature, but not effective at the high firing temperature. The red emission peaks were observed around 613 nm with an excitation wavelength of 254 nm. Emission peaks were composed of two sets around 613 nm and 590 nm, which originated from ⁵D₀–⁷F₂ and ⁵D₀–⁷F₁, respectively. PL intensity of YTaO₄:Eu³⁺ prepared with a small amount of LiCl (10 wt%) significantly depended on the firing temperatures, while that with a larger amount of LiCl (40 wt%) slightly relied on firing temperatures. The highest PL intensity could be obtained at the firing conditions of 1300 °C and 10 wt% LiCl.     

Citric based sol-gel synthesis and photoluminescence properties of un-doped and Sm doped Ca3Y2Si3O12 phosphors

The photoluminescent properties of un-doped and Sm³⁺ doped Ca₃Y₂Si₃O₁₂ phosphors were prepared by the citrate sol-gel method. Un-doped sample has shown a strong blue emission, which has its maximum intensity at 389 nm. Among all the observed emission transitions ⁴G_5/2→⁶H_J (J = 5/2, 7/2 & 9/2) of Sm³⁺, the reddish-orange (RO) emission transition ⁴G_5/2→⁶H_7/2 is more prominent, which matches well with the emission wavelength of near UV (n-UV) LED. The reasons for the observance of such prominent visible color emissions from these phosphors have been substantiated appropriately. Besides, structural details of these samples have also been analyzed from the measured XRD, TEM, TG-DTA and FT-IR profiles.     

Synthesis and photoluminescence properties of CaGd2(MoO4)4:Eu3+ red phosphors

The novel red-emitting Eu³⁺ ions activated CaGd₂(MoO₄)₄ phosphors were prepared by a citrate sol–gel method. The X-ray diffraction patterns confirmed their tetragonal structure when the samples were annealed above 600 °C. The photoluminescence excitation spectra of CaGd₂(MoO₄)₄:Eu³⁺ phosphors exhibited the charge transfer band (CTB) and intense f–f transitions of Eu³⁺ ion. The optimized annealing temperature and Eu³⁺ ion concentration were analyzed for CaGd₂(MoO₄)₄:Eu³⁺ phosphors based on the dominant red (⁵D₀→⁷F₂) emission intensity under NUV (394 nm) excitation. All decay curves were well fitted by the single exponential function. These luminescent powders are expected to find potential applications such as WLEDs and optical display systems.     

VUV photoluminescence properties of Y1−xGdxVO4:Eu phosphors prepared by ultrasonic spray pyrolysis

The red-emitting (Y_1−xGd_x)_0.94Eu_0.06VO₄ (0 ≤ x ≤ 1.0) phosphors were synthesized by ultrasonic spray pyrolysis. The (Y_1−xGd_x)_0.94Eu_0.06VO₄ (0 ≤ x ≤ 1.0) phosphors had the tetragonal xenotime structure with a space group of I4₁/amd (1 4 1). The calculated crystallite sizes of the annealed phosphors ranged from 58 to 68 nm. In this study, we discussed the photoluminescence properties of the (Y_1−xGd_x)_0.94Eu_0.06VO₄ phosphors under VUV excitation, depending on Gd content. The emission intensity of the (Y_1−xGd_x)_0.94Eu_0.06VO₄ phosphors increased with increasing Gd content up to x = 0.5, and then decreased with a further increase in Gd content. The purest red color was obtained for the (Y_0.5Gd_0.5)_0.94Eu_0.06VO₄ phosphors.     

Enhancement of Ca3Co4O9 thermoelectric properties by Cr for Co substitution

Ca₃Co_4−xCr_xO₉ polycristalline thermoelectric ceramics with small amounts of Cr have been synthesized by the classical solid state method. Microstructural characterizations have shown that all the Cr has been incorporated into the Ca₃Co₄O₉ structure and no Cr-containing secondary phases have been produced for Cr contents≤0.05. Apparent density measurements have shown that all samples are very similar, with densities around 75% of the theoretical one. Electrical resistivity decreases and Seebeck coefficient slightly raises when Cr content increases until 0.05 Cr addition. The improvement in both parameters leads to higher power factor values than the usually obtained by conventional solid state routes.     

Synthesis, characterization and optical properties of Mg(OH)2 micro-/nanostructure and its conversion to MgO

Magnesium hydroxide (Mg(OH)₂) micro- and nanostructures have been synthesized by a single step hydrothermal route. Surface morphology analysis reveals the formation of micro- and nanostructures with varying shape and size at different synthesis conditions. Structural investigations by X-ray diffraction (XRD) and transmission electron microscopy (TEM) confirm that the synthesized material is Mg(OH)₂ with hexagonal crystal structure. An optical band gap of 5.7 eV is determined for Mg(OH)₂ nanodisks from the UV–vis absorption spectrum. A broad emission band with maximum intensity at around 400 nm is observed in the photoluminescence (PL) spectra of Mg(OH)₂ nanodisks at room temperature depicting the violet emission, which can be attributed to the ionized oxygen vacancies in the material. Furthermore, Mg(OH)₂ has been converted to MgO by calcination at 450 °C. Optical studies of the MgO nanodisks have shown an optical band gap of 3.43 eV and a broadband PL emission in the UV region. Mg(OH)₂ and MgO nanostructures with wide-band gap and short-wavelength luminescence emission can serve as a better luminescent material for photonic applications.