Preparation and luminescent characteristics of Sr3RE2(BO3)4:Dy (RE = Y, La, Gd) phosphors for white LED

Dysprosium-activated Sr₃RE₂(BO₃)₄ (RE = Y, La, Gd) phosphors were synthesized by a high temperature solid-state reaction method. The phase uniformity of the phosphors was characterized by X-ray powder diffraction (XRD) and the luminescence characteristics were investigated. The excitation spectra at 575 nm emission show strong spectral bands in the region of 300-500 nm. The emission spectra of the phosphors with 365 nm excitation show three bands centered at 484 nm, 575 nm and 680 nm, which originate from the transitions of ⁴F_9/2 → ⁶H_15/2, ⁴F_9/2 → ⁶H_13/2 and ⁴F_9/2 → ⁶H_11/2 of Dy³⁺, respectively. The effect of Dy³⁺ concentration on the emission intensity of the phosphors was investigated. The fluorescence decay curves for ⁴F_9/2 → ⁶H_13/2 excited at 365 nm and monitored at λ_em of 575 nm were measured. The decay times decreased slowly with increasing Dy³⁺ doping concentration due to a trap capturing to resonance fluorescence transfer of the activated ions and due to the exchange interactions between activated ion pairs. In order to determine the type of interaction between activated ions, the concentration dependence curves (lg(I/x) versus lg x) of Sr₃RE₂(BO₃)₄:Dy³⁺ (RE = Y, La, Gd) were plotted. The concentration quenching mechanism of the ⁴F_9/2 → ⁶H_13/2 (575 nm) transition of Dy³⁺ is the d-d interaction. All results indicate these phosphors are promising white-color luminescent materials.     

Spectroscopic analysis of Nd:Ca3Gd2(BO3)4 crystal and laser operating at 1.06 μm

Laser crystal Ca₃Gd₂(BO₃)₄ of high quality activated with Nd³⁺ has been grown successfully by Czochralski technique. The product was characterized by means of X-ray diffraction (XRD) powder investigation. The absorption spectra along a-, b- and c-axes were measured and investigated according to the Judd-Ofelt theory. The fluorescence properties were pursued and the factors influencing the fluorescence quantum efficiency were indicated. Xe lamp-pumped pulsed laser results were also obtained and the highest output power was up to 62 mJ.     

Luminescent properties of Na3Gd1−xEux(PO4)2 and energy transfer in these phosphors

A series of Eu³⁺ activated Na₃Gd_1−xEu_x(PO₄)₂ (0 ≤ x ≤ 1) phosphors were synthesized by solid-state reaction method. The structures and photo-luminescent properties of these phosphors were investigated at room temperature. The results of XRD patterns indicate that these phosphors are isotypic to the orthorhombic Na₃Gd(PO₄)₂. The excitation spectra indicate that these phosphors can be effectively excited by near UV (370-410 nm) light. The intensities of magnetic dipole transition ⁵D₀ → ⁷F₁ and forced electric dipole transition ⁵D₀ → ⁷F₂ are comparable, and the energy ratio (⁵D₀ → ⁷F₁/⁵D₀ → ⁷F₂) is 1.1. The emission spectra exhibit strong reddish orange performance (CIE chromaticity coordinates: x = 0.62, y = 0.38), which is due to the ⁵D₀ → ⁷F_J transitions of Eu³⁺ ions. The correlation between the structure and the photo-luminescent properties of the phosphors was studied. The energy transfer and concentration quenching of the phosphors were discussed. Na₃Gd_1−xEu_x(PO₄)₂ has a potential application for white light-emitting diodes.     

Multiwavelength excited novel red-emitting phosphor Eu-activated Li2Zn2(MoO4)3

Eu³⁺-activated Li₂Zn₂(MoO₄)₃ multiwavelength excited red-emitting phosphors were synthesized via a solid state reaction. The structure and photoluminescence characteristics were investigated by X-ray powder diffraction and fluorescent spectrophotometry, respectively. The excitation spectrum included a strong broadband ranging from 250 to 350 nm and some sharp peaks at 363, 384, 395, 465, and 533 nm, which matchs the radiations of near-UV or blue light-emitting diodes chip well. Upon excitation either of near-UV or blue even green light, the intense red emission with 615 nm peak can be observed, which is ascribed to the ⁵D₀-⁷F₂ transition of Eu³⁺ ions. The chromaticity coordinates (x = 0.65, y = 0.34) of the as-obtained phosphor is very close to the National Television Standard Committee standard values (x = 0.67, y = 0.33). All these characteristics suggest that Eu³⁺-doped Li₂Zn₂(MoO₄)₃ wavelength-conversion material to be suitable candidate red component for phosphor-converted white light-emitting diodes.     

Synthesis, photoluminescence, thermoluminescence and dosimetry properties of novel phosphor KSr4(BO3)3:Ce

Polycrystalline powder sample of KSr₄(BO₃)₃ was synthesized by high-temperature solid-state reaction. The influence of different rare earth dopants, i.e. Tb³⁺, Tm³⁺ and Ce³⁺, on thermoluminescence (TL) of KSr₄(BO₃)₃ phosphor was discussed. The TL, photoluminescence (PL) and some dosimetric properties of Ce³⁺-activated KSr₄(BO₃)₃ phosphor were studied. The effect of the concentration of Ce³⁺ on TL intensity was investigated and the result showed that the optimum Ce³⁺ concentration was 0.2 mol%. The TL kinetic parameters of KSr₄(BO₃)₃:0.002 Ce³⁺ phosphor were calculated by computer glow curve deconvolution (CGCD) method. Characteristic emission peaking at about 407 and 383 nm due to the 4f⁰5d¹ → ²F_{(5/2, 7/2)} transitions of Ce³⁺ ion were observed both in PL and three-dimensional (3D) TL spectra. The dose–response of KSr₄(BO₃)₃:0.002 Ce³⁺ to γ-ray was linear in the range from 1 to 1000 mGy. In addition, the decay of the TL intensity of KSr₄(BO₃)₃:0.002 Ce³⁺ was also investigated.     

Growth and spectroscopic properties of Nd-doped Na2Gd4(MoO4)7 crystal

This paper reports the growth and spectral properties of Nd³⁺:Na₂Gd₄(MoO₄)₇ crystals. An Nd³⁺:Na₂Gd₄(MoO₄)₇ crystal with dimensions of Ø20 × 25 mm³ has been grown by the Czochralski method. The spectroscopic properties of Nd³⁺:Na₂Gd₄(MoO₄)₇ crystal were investigated. The Judd-Ofelt theory was applied to calculate the spectral parameters. The polarized absorption cross-sections of Nd³⁺:Na₂Gd₄(MoO₄)₇ crystal are 4.25 × 10⁻²⁰ cm² with full width at half maximum (FWHM) of 14.6 nm for the π-polarization and 2.87 × 10⁻²⁰ cm² with FWHM of 16.2 nm for the σ-polarization, respectively. The emission cross-sections are 10.0 × 10⁻²⁰ cm² at 1060 nm for π-polarization and 13.6 × 10⁻²⁰ cm² at 1067 nm for σ-polarization, respectively. The fluorescence quantum efficiency has been estimated to be 90.0%. Nd³⁺:Na₂Gd₄(MoO₄)₇ crystal may be considered as a potential laser gain medium for the diode laser pumping.     

Structural, magnetic and magnetocaloric properties in Pr0.5M0.1Sr0.4MnO3 (M = Eu, Gd and Dy) polycristalline manganites

Structural, magnetic and magnetocaloric properties of Pr_0.5M_0.1Sr_0.4MnO₃ (M = Eu, Gd and Dy) powder samples have been investigated by X-ray diffraction (XRD) and magnetic measurements. Our samples have been synthesized using the solid state reaction method at high temperature. Rietveld refinements of the X-ray diffraction patterns show that all our samples are single phase and crystallize in the distorted orthorhombic system with Pbnm space group. Magnetization measurements versus temperature in a magnetic applied field of 50 mT show that all our samples exhibit a paramagnetic-ferromagnetic transition with decreasing temperature. The Curie temperature T_C is found to be 270 K, 258 K and 248 K for M = Eu, Gd and Dy, respectively. Arrott plots show that all our samples exhibit a second order magnetic phase transition. From the measured magnetization data of Pr_0.5M_0.1Sr_0.4MnO₃ (M = Eu, Gd and Dy) samples as a function of magnetic applied field, the associated magnetic entropy change |ΔS_M| and the relative cooling power RCP have been determined. In the vicinity of T_C, |ΔS_M| reached, in a magnetic applied field of 1 T, maximum values of 1.37 J/kg K, 1.23 J/kg K and 1.18 J/kg K for M = Eu, Gd and Dy, respectively.     

Photoluminescent properties of LiSrxBa1−xPO4:RE (RE = Sm, Eu) f-f transition phosphors

Rare-earth ions (Sm³⁺ or Eu³⁺) doped LiSr_xBa_1−xPO₄ (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) f-f transition phosphor powders were prepared by a high temperature solid-state reaction. The resulted phosphors were characterized by X-ray diffraction (XRD) and photoluminescence (PL) spectroscopy. The results of XRD indicate that the phase structure of the sample changes from LiBaPO₄ to LiSrPO₄ when x changes from 0 to 1.0. The excitation spectra indicate that only direct excitation of rare earth ions (Sm³⁺ or Eu³⁺) can be observed. The doped rare earth ions show their characteristic emission in LiSr_xBa_1−xPO₄, i.e., Eu³⁺⁵D₀-⁷F_J (J = 0, 1, 2, 3, 4), Sm³⁺⁴G_5/2 → ⁶H_J (J = 5/2, 7/2, 9/2, 11/2), respectively. The dependence of the emission intensities of the LiSr_xBa_1−xPO₄:Sm³⁺ and LiSr_xBa_1−xPO₄:Eu³⁺ phosphors on the x value and Ln³⁺ (Ln³⁺ = Sm³⁺, Eu³⁺) concentration is also investigated.     

Substitution effect on metal-insulator transition in Cu(Ir1−xMx)2S4 (M = Sn, Hf)

A spinel CuIr₂S₄ exhibits a temperature-induced metal-insulator (M-I) transition at around 226 K. Non-magnetic substitution effect on the M-I transition, T_M-I, in Cu(Ir_1−xM_x)₂S₄ (M = Sn, Hf) has been studied on the focus of the rather low composition region of x. Magnetic property of Cu(Ir_1−xM_x)₂S₄ (M = Sn, Hf) has been examined experimentally. The T_M-I decreases with increasing x and the temperature hysteresis becomes unclear within the experimental errors. The step anomaly in the magnetic susceptibility smears out and the T_M-I becomes ill defined around x = 0.20 in Cu(Ir_1−xSn_x)₂S₄, and x = 0.10 in Cu(Ir_1−xHf_x)₂S₄, respectively. These substitutions play an important role in decoupling the spin-dimerization of Ir⁴⁺-Ir⁴⁺ in CuIr₂S₄, and lead the destruction of the metal-insulator transition.     

Spectroscopic investigation of Dy-doped Li2Gd4(MoO4)7 crystal for potential application in solid-state yellow laser

Dy³⁺:Li₂Gd₄(MoO₄)₇ crystal with dimensions of ∅20 × 25 mm³ has been grown by the Czochralski method. The spectroscopic properties of Dy³⁺:Li₂Gd₄(MoO₄)₇ crystal have been investigated. Based on the analysis of polarized absorption spectra in the framework of the Judd-Ofelt theory, the main spectroscopic characteristics, including the phenomenological intensity parameters, spontaneous transition probabilities, fluorescence branching ratios and radiative lifetimes of Dy³⁺ in the crystal, have been determined. The emission cross-sections for the ⁴F_9/2 → ⁶H_13/2 transition of special interest for laser application have been calculated using the Füchtbauer-Ladenburg (F-L) equation. The fluorescence and radiative lifetimes of ⁴F_9/2 manifold are equal to 126.5 μs and 201.1 μs, respectively, which mean the quantum efficiency is 62.9%. The results propose the possibility of Dy³⁺:Li₂Gd₄(MoO₄)₇ crystal for solid-state yellow laser pumped by commercially available blue laser diodes.     

High-dielectric-constant and low-loss microwave dielectric in the (1 − x)La(Mg0.5Ti0.5)O3-x(Ca0.8Sr0.2)TiO3 solid solution system

The microwave dielectric properties and microstructures of (1 − x)La(Mg_0.5Ti_0.5)O₃-x(Ca_0.8Sr_0.2)TiO₃ ceramics, prepared by a mixed oxide route, have been investigated. The forming of solid solutions was confirmed by the XRD patterns and the measured lattice parameters for all compositions. A near zero τ_f was achieved for samples with x = 0.5, although the dielectric properties varied with sintering temperature. The Q × f value of 0.5La(Mg_0.5Ti_0.5)O₃-0.5(Ca_0.8Sr_0.2)TiO₃ increased up to 1475 °C, after which it decreased. The decrease in dielectric properties was coincident with the onset of rapid grain growth. The optimum combination of microwave dielectric properties was achieved at 1475 °C for samples where x = 0.5 with a dielectric constant ?_r of 47.12, a Q × f value of 35,000 GHz (measured at 6.2 GHz) and a τ_f value of −4.7 ppm/°C.     

Phase evolution, crystal structure and dielectric behavior of (1 − x)Nd(Zn0.5Ti0.5)O3 + xBi(Zn0.5Ti0.5)O3 compound ceramics

The phase evolution, crystal structure and dielectric properties of (1 − x)Nd(Zn_0.5Ti_0.5)O₃ + xBi(Zn_0.5Ti_0.5)O₃ compound ceramics (0 ≤ x ≤ 1.0, abbreviated as (1 − x)NZT-xBZT hereafter) were investigated. A pure perovskite phase was formed in the composition range of 0 ≤ x ≤ 0.05. The B-site Zn²⁺/Ti⁴⁺ 1:1 long range ordering (LRO) structure was detected by both XRD and Raman spectra in x ≤ 0.05 samples. However, this LRO structure became gradually degraded with an increase in x. The dielectric behaviors of the compound ceramic at various frequencies were investigated and correlated to its chemical composition and crystal structure. A gradually compensated τ_f value was obtained in (1 − x)NZT-xBZT microwave dielectrics at x = 0.03, which was mainly due to the dilution of dielectric constant in terms of Claussius-Mossotti differential equation.     

Anisotropy compensation and high low-field magnetostriction of epoxy/Tb1−xHox(Fe0.8Co0.2)2 composites (0.60 ≤ x ≤ 1.0)

The anisotropy compensation and magnetostrictive properties of Tb_1−xHo_x(Fe_0.8Co_0.2)₂ (0.60 ≤ x ≤ 1.0) alloys have been investigated. The easy magnetization direction (EMD) at room temperature rotates from the 〈1 1 1〉 axis (x ≤ 0.75) to the 〈1 0 0〉 axis (x ≥ 0.90) through an intermediate state 〈1 1 0〉, subjected to the anisotropy compensation between Tb³⁺ and Ho³⁺ ions. Composition anisotropy compensation is realized near x = 0.75. The Tb_0.25Ho_0.75(Fe_0.8Co_0.2)₂ alloy has a minimum anisotropy and a large spontaneous magnetostriction coefficient λ₁₁₁ (≈740 ppm) at room temperature. The strong 〈1 1 1〉-oriented 1-3 epoxy-bonded composite has been fabricated by curing under a moderate magnetic field. A high low-field magnetostriction of about 400 ppm at 3 kOe is obtained for the 1-3 epoxy/Tb_0.25Ho_0.75(Fe_0.8Co_0.2)₂ composite with 40-vol% alloy particles, which can be attributed to the low magnetic anisotropy, EMD lying along 〈1 1 1〉 direction, the strong 〈1 1 1〉-textured orientation and the chain structure.     

Luminescence properties of a new blue-emitting phosphor Ce-doped CaLaGa3S7

A new series of blue-emitting Ce³⁺-doped CaLaGa₃S₇ thiogallate chalcogenide phosphors were synthesized by a solid-state reaction method. Their luminescence properties were investigated by photoluminescence excitation, emission spectra and lifetime. The critical dopant concentration was found to be 0.15 mol of Ce³⁺ (R_c = 15 Å) and the fluorescence lifetime of Ce³⁺ in CaLaGa₃S₇:0.15 Ce³⁺ was 12.9 ns. The blue-emitting LED was fabricated by combining an InGaN chip (395 nm) with a CaLaGa₃S₇:Ce³⁺ phosphor. The CIE chromaticity coordinates of the blue LED were calculated to be (0.14, 0.23).     

Relationships between Zr substitution for Ti and microwave dielectric properties in Mg(ZrxTi1−x)O3 ceramics

The influence of Zr substitution for Ti on the microwave dielectric properties and microstructures of the Mg(Zr_xTi_1−x)O₃(MZ_xT) (0.01 ≤ x ≤ 0.3) ceramics was investigated. The quality factors of Mg(Zr_xTi_1−x)O₃ ceramics with x = 0.01-0.05 were improved because the solid solution of a small amount of Zr⁴⁺ substitution in the B-site could increase density and grain size. An excess of Zr⁴⁺ resulted in the formation of a great deal of secondary phase that declined the microwave dielectric properties of MZ_xT ceramics. The temperature coefficient of resonant frequency (τ_f) of Mg(Zr_xTi_1−x)O₃ ceramics slightly increased with increasing Zr content, and the variation in τ_f was attributed to the formation of secondary phases.     

Color-conversion and photoluminescence properties of Ba2MgW(Mo)O6:Eu phosphor

Compounds B₂AXO₆:Eu (B = Ba, Sr; A = Ca; and X = W, Mo) have recently been investigated and suggested to be color-conversion phosphor for WLED devices. In this work, we investigated the photoluminescence properties of the analogues Ba₂MgXO₆:Eu (X = W and Mo) and the energy transfer from W(Mo)O₆ groups to Eu³⁺ ions within the phosphors. The phase structure, UV-vis diffuse reflectance spectrum, photoluminescence properties and decay of Ba₂MgW_(1−x)Mo_xO₆:Eu were studied as a function of the W/Mo ratio. It was found that these phosphors showed excellent color-conversion capability from near-UV to orange-red light. The color-conversion process was considered to be performed by energy transferring from MoO₆ groups to doped Eu³⁺ ions. The MoO₆ → Eu³⁺ energy transfer efficiency could be greatly enhanced by partial substitution of Mo by W. The structure and photoluminescence properties of Ba₂AW_0.5Mo_0.5O₆:Eu (A = Ca and Mg, respectively) compounds were also investigated to reveal the effect of the Eu³⁺ ion coordination environment on its photoluminescence properties.     

Preparation of Dy-activated strontium orthosilicate (Sr2SiO4:Dy) phosphors and its photoluminescent properties

Dy³⁺-activated β/α′-Sr₂SiO₄ phosphors were successfully prepared by solid-state reaction method with ammonium chloride (NH₄Cl) as the flux. The influences of calcination temperatures, amounts of NH₄Cl and the concentrations of Dy³⁺ on phase composition, morphology and the photoluminescent properties of as-prepared powders were investigated in detail. The β and α′ phases of Sr₂SiO₄ were obtained with 1 wt% and 2-5 wt% NH₄Cl, respectively, as the sintered condition was at 1000 °C for 4 h. With increasing the amount of NH₄Cl, the morphology of phosphors changed from needlelike to regular polyhedron shape and the colors of the Sr₂SiO₄:Dy³⁺ phosphors changed from blue-green to white. The luminescence intensity of ⁴F_9/2 → ⁶H_15/2 transition was slightly higher than that of ⁴F_9/2 → ⁶H_13/2 (ΔL = 2, ΔJ = 2) transition owing to the low-symmetry around Dy³⁺ ions. The optimum concentration of Dy³⁺ was 2.0 mol% and the concentration quenching were caused by the d-d interaction and a cross relaxation. The yellow-to-blue intensity ratio (Y/B) of Dy³⁺ emission did not to change with varying the Dy³⁺ concentration using Li⁺ ions for charge compensation. These indicate that this phosphor can be used as a potential candidate for the phosphor-converted white LEDs with a UV chip.     

Large magnetic entropy change near room temperature in La0.7(Ca0.27Ag0.03)MnO3 perovskite

In this paper, the magnetic properties and magnetocaloric effect (MCE) of La_0.7(Ca_1−xAg_x)_0.3MnO₃ (x = 0, 0.1, 0.2, 0.7, and 1) powder samples are reported. Our polycrystalline compounds were synthesized using the solid state reaction method at high temperature. Magnetization measurements versus temperature showed that all our samples exhibited a paramagnetic to ferromagnetic transition with decreasing temperature. The Curie temperature, T_C, has been found to increase from ∼250 K for x = 0-270 K for x = 1. Ag doping weakens the first order phase transition, and at higher Ag doping, the phase transition is of second order. For the La_0.7(Ca_0.27Ag_0.03)MnO₃ composition, the maxima of the magnetic entropy changes from the applied magnetic field (ΔS_M) at 2 and 5 T are about 4.5 and 7.75 J/kg K, respectively, at the Curie temperature of ∼263 K. The relative cooling power (RCP) values without hysteresis loss are about 102 and 271 J/kg for the applied fields of 2 and 5 T, respectively. Due to the large ΔS_M, large RCP, and high Curie temperature, La_0.7(Ca_0.27Ag_0.03)MnO₃ is promising for application in potential magnetic refrigeration near room temperature.