Structural Characterization and Luminescent Properties of a Red Phosphor Series: Y2−xEux(MoO4)3 (x=0.4–2.0)

A series of rare earth molybdates, Y_{2− x} Eu _x (MoO₄)₃ for x =0.4, 0.8, 1.2, 1.6 and 2.0 were prepared by solid-state method and their crystal structures, photo luminescent characteristics were investigated. The powders are mainly studied for their red light emission efficiency under near UV excitation. The crystal structures of the powders were found to depend on annealing temperature and the yttrium concentration. Mixtures of monoclinic ( C 2 /c ) and orthorhombic ( Pba 2, Pbna ) structures were formed in varying proportions depending on the value of x and annealing temperatures (700°–800°C). The luminescence behavior depended on the resultant composition of the crystal phase and the Eu³⁺ concentration. The excitation spectra showed the characteristic and broad O→Mo charge transfer (CT) band of the MoO₄ tetrahedra and the sharp intra-configurational 4 f –4 f transitions of Eu³⁺ in the host lattice. The integrated emission ratio (⁵D₀→⁷F₂/⁵D₀→⁷F₁) of Eu³⁺ depends on the annealing temperature and reveals that the local site symmetry of Eu³⁺ ions decreases with increasing concentration of Eu³⁺. The emission spectra obtained by exciting at 396 nm, gave highest red emission intensity for Y_0.4Eu_1.6(MoO₄)₃ annealed at 700°C/6 h among this series of samples.     

A Potential Red-Emitting Phosphor BaGd2(MoO4)4:Eu3+ for Near-UV White LED

Red-emitting phosphor BaGd_{2− x} Eu _x (MoO₄)₄ has been successfully synthesized by a simple sol–gel method. The process of phosphor formation is characterized by thermogravimetric-differential thermal analysis and X-ray diffraction. Field-emission scanning electronic microscopy is used to characterize the size and the shape of the phosphor particles. Photo-luminescent property of the phosphor is also performed at the room temperature. The effects of firing temperature and Eu³⁺ activator concentration on the photoluminescence (PL) properties are elaborated in detail. PL characterization reveals that the sample with the firing temperature at 800°C and the concentration of Eu³⁺ at 0.7 shows the most intense emission, and its intensity is about three times stronger than that of phosphor prepared by solid-state method with the same composition and firing temperature. The new red-emitting phosphor shows an intense absorption at 396 nm, which matches well with commercial near-UV light-emitting diode (LED) chips, therefore, it is a good candidate of red phosphor used for near-UV white LEDs.     

Inclusions of Nanometer-Sized Al2O3 Particles in a Crystalline (Sc,Lu)2(WO4)3 Matrix

Nanometer-sized Al₂O₃ particles were successfully synthesized as crystalline inclusions by mixing both components to form the nanometer-sized particles and the (Sc,Lu)₂(WO₄)₃ matrices in a crystal lattice by preparing a solid solution of (Sc,Lu)₂(WO₄)₃ and Al₂(MoO₄)₃ and then decomposing the solid solution. The particles were dispersed uniformly and without agglomeration, which is commonly observed with conventional preparation techniques. The average particle size of the Al₂O₃ was 3.5 nm, and the standard deviation was estimated to be 1.1 nm.     

Negative Thermal Expansion in (HfMg)(WO4)3

A single-phase material (HfMg)(WO₄)₃ with an orthorhombic structure, A₂ (WO₄)₃-type tungstate, has been successfully prepared for the first time by the calcination of HfO₂, MgO, and WO₃, substituting Hf⁴⁺ and Mg²⁺ for A³⁺ cations in A₂(WO₄)₃. The new material shows a negative thermal expansion coefficient of approximately −2 ppm/°C from room temperature to 800°C. The mechanism of negative thermal expansion is assumed to be the same as that of Sc₂(WO₄)₃.     

New Red- and Green-Emitting Phosphors, AYP2O7.5:RE3+ (A=Ca and Sr; RE=Eu and Tb) under Near-UV Irradiation

New red- and green-emitting phosphors, AY_{1− x} P₂O_7.5: x RE³⁺ (A=Ca and Sr; x =0.01, 0.03, 0.05, 0.10; RE=Eu and Tb) were synthesized by the conventional ceramic route and their photoluminescence properties under near-ultraviolet (UV) irradiation were investigated. It was found that CaYP₂O_7.5: Eu³⁺ phosphor emits strong red light when excited by a radiation of 394-nm wavelength and SrYP₂O_7.5:Eu³⁺ gives intense orange light when excited by a radiation of 396-nm wavelength. Strong green emission for AY_{1− x} P₂O_7.5:Tb³⁺ is also observed under near-UV irradiation (378 nm). When compared with emission intensity from a standard YPO₄:0.05Tb³⁺, the emission from SrYP₂O_7.5:0.05Tb³⁺ showed greater intensity values under the same excitation wavelength (378 nm). X-ray powder diffraction analysis showed that AYP₂O_7.5 has xenotime-type structure.     

Characterization of Ca2SiO4:Eu2+ Phosphors Synthesized by Polymeric Precursor Process

The green emitting Ca₂SiO₄:Eu²⁺ (C₂S:Eu) phosphors were synthesized by the polymeric precursor process (Pechini-type), and the effects of calcination temperature and europium (Eu) doping concentration on the luminescent properties were investigated. The crystalline β-C₂S was obtained in the calcination temperature of 1100°–1400°C, and Eu was reduced into Eu²⁺ by annealing in 5% H₂/N₂ atmosphere. The obtained C₂S:Eu²⁺ phosphors exhibited a strong emission at 504 nm under the excitation of λ_exc=350 nm. The highest photoluminescence (PL) intensity was observed in the C₂S:Eu²⁺ phosphors either calcined at 1300°C or doped with 3 mol% Eu. The obtained PL properties were discussed in terms of crystal structure, particle size and shape, surface roughness, and effect of concentration quenching.     

Molten Salt Synthesis, Characterization, and Luminescence Properties of Gd2MO6:Eu3+ (M=W, Mo) Phosphors

Large-scale micro- and nano-sized Eu³⁺-doped gadolinium tungstate and molybdate Gd₂MO₆:Eu³⁺ (M=Mo, W) red phosphors with special morphologies have been successfully prepared via an efficient molten salt synthesis (MSS) method at 950°C for 6 h using NaCl and as the reaction medium. The results of X-ray powder diffraction patterns indicate that Gd₂MoO₆ and Gd₂WO₆ have monoclinic structures. The scanning electronic microscope and transmission electron microscope studies revealed that the samples of Gd₂WO₆:Eu³⁺ and Gd₂MoO₆:Eu³⁺ phosphors obtained by the MSS method in NaCl medium have a sphere-like morphology, and the corresponding samples obtained in KCl medium have rod-like morphologies. For the Gd₂MoO₆:Eu³⁺ phosphor obtained in NaCl and KCl media, the sphere-like particles had an average grain size of ∼100 nm, and the size of the rod-like particles ranged from 200 to 400 nm in length, and ∼150 nm in width, respectively. Compared with the same products obtained by the solid-state reaction we had reported, the luminescent properties of Gd₂MO₆:Eu³⁺ (M=Mo, W) are largely dependent on their morphology and crystal size prepared using different fluxes of NaCl and KCl. The reaction time and temperature also play an important role in the crystallization of the samples.     

Synthesis and Luminescence Properties of Orange–Red-Emitting M2Si5N8:Eu2+ (M=Ca, Sr, Ba) Light-Emitting Diode Conversion Phosphors by a Simple Nitridation of MSi2

Eu²⁺-doped M₂Si₅N₈ (M=Ca, Sr, Ba) orange–red phosphors were successfully prepared by a simple, direct, and efficient solid-state reaction using air-stable MSi₂, Eu₂O₃, and α-Si₃N₄ as the starting materials under N₂–H₂ (5%) atmosphere. The influence of the type of the alkaline-earth ion on the phase structure and luminescence properties has been investigated. The results show that the synthesized powders have a single-phase crystal structure of M₂Si₅N₈ for M=Ca, Sr, and a little amount of BaSi₇N₁₀ impurity phase for M=Ba. Under the blue light excitation, M₂Si₅N₈:Eu²⁺ shows a typical broad band emission of Eu²⁺ ranging from orange to red (585–620 nm) depending on the type of M ion. The emission intensity, conversion efficiency, and thermal stability increase with the sequence of Ca2Si₅N₈:Eu²⁺ has the highest application potential as a red conversion phosphor for white light-emitting diodes.     

Photoluminescence Enhancement of (La0.95Eu0.05)2Ti2O7 Nanophosphors via Li+ Doping

Li⁺ ions have been successfully doped into the La sites of (La_0.95Eu_0.05)₂Ti₂O₇ nanocrystals through a facile citric acid sol–gel method. The doping concentration of Li⁺ ions can be as high as 15 mol%. Photoluminescence (PL) performances of the obtained samples have been investigated. The results showed that a doping with small number of Li⁺ ions improves the PL intensity of the synthesized La₂Ti₂O₇:Eu³⁺ nanophosphors. The highest emission intensity was observed using the formula of (La_0.92Eu_0.05Li_0.03)₂Ti₂O₇, whose brightness was increased by almost 20% in comparison with that of (La_0.95Eu_0.05)₂Ti₂O₇.     

Peculiar Dielectric Behaviors of (Na1/2Bi1/2)0.87Pb0.13TiO3 Thin Films

PbTiO₃-doped sodium bismuth titanate (Na_1/2Bi_1/2)_{1− x} Pb _x TiO₃ of perovskite structure is one of the best-known piezoelectrics/ferroelectrics. However, it has not been properly investigated in any thin-film forms. In this study, the dielectric properties of (Na_1/2Bi_1/2)_0.87Pb_0.13TiO₃ thin films synthesized via a sol–gel route were investigated. They exhibit a strong frequency dispersion of the dielectric permittivity at relatively high frequencies, which is shifted to lower frequencies with increasing temperature. The electrical behavior can be fitted using Jonscher's universal law for dielectric relaxation. The peculiar dielectric behaviors observed can be ascribed to the coexistence of two different dielectric phases in the films, which is believed to be associated with the growth of the local Pb²⁺TiO₃ nanoclusters upon substitution of Pb²⁺ for Na⁺/Bi³⁺ in the (Na_1/2Bi_1/2)_{1− x} Pb _x TiO₃ films.     

Improved Fluorescence from Tm-Ho- and Tm-Ho-Eu-Codoped Transparent PbF2 Glass-Ceramics for S+-Band Amplifiers

Tm³⁺-Ho³⁺- and Tm³⁺-Ho³⁺-Eu³⁺-ion-codoped oxyfluoride transparent glass-ceramics containing PbF₂ nanocrystals were prepared, and the near-infrared fluorescence properties of the Tm³⁺ ions were investigated for their potential use as a 1.4 μm amplifier. For all samples, the lifetime of the Tm³⁺:³ H ₄ level increased with heat treatment because of the decrease of the phonon energy as PbF₂ crystals were formed. Moreover, it was revealed that codoping with Ho³⁺ or Eu³⁺ was effective in suppressing the lifetime of the Tm³⁺:³ F ₄ level by energy transfer to the Ho³⁺:⁵ I ₇ or Eu³⁺:⁷ F ₆ level. For the codoped samples, the heat treatments decreased the Tm³⁺:³ F ₄ lifetime and increased the Tm³⁺:³ H ₄ lifetime. This was attributed to the concentration of rare-earth ions in the fluoride crystallites. These properties improved the population inversion of the 1.4 μm transition.     

Thermal Expansion Behavior of NaZr2(PO4)3Type Compounds

The thermal expansion of the skeletal framework was essentially zero for NaZr₂(PO₄)₃-type compounds; the interstitialion, e.g., Na⁺, was primarily responsible for the total thermal expansion. The composition dependence of the thermal expansion is discussed in terms of the amounts, crystallographic sites, and ionic radii of the interstitial ions. The mechanism which results in low thermal expansion was clarified, particularly for KZr₂(PO₄)₃, in which a larger ion is substituted for Na⁺, and NbZr(PO₄)₃, which does not contain Na⁺. Polycrystalline ceramics formed from these crystals might be useful as thermal-shock-resistant materials.     

Porous Glass-Ceramics with a Skeleton of the Fast-Lithium-Conducting Crystal Li1+xTi2−xAlx(PO4)3

Porous glass-ceramics with a skeleton of the fast-lithium-conducting crystal Li_{1+ x} Ti_{2− x} Al _x (PO₄)₃ (where x = 0.3–0.5) were prepared by crystallization of glasses in the Li₂O─CaO─TiO₂─Al₂O₃–P₂O₅ system and subsequent acid leaching of the resulting dense glass-ceramics composed of the interlocking of Li_{1+ x} Ti_{2− x} Al _x (PO₄)₃ and β-Ca₃(PO₄)₂ phases. The median pore diameter and surface area of the resulting porous Li_{1+ x} Ti_{2− x} Al _x (PO₄)₃ glass-ceramics were approximately 0.2 μm and 50 m²/g, respectively. The electrical conductivity of the porous glass-ceramics after heating in LiNO₃ aqueous solution was 8 × 10⁻⁵ S/cm at 300 K or 2 × 10⁻² S/cm at 600 K.     

Ionic Conductivity and Sinterability of NASICON-Type Ceramics: The Systems NaM2(PO4)3+yNa2o (M = Ge, Ti, Hf, and Zr)

Sodium-rich NASOCON-type ceramics, the NaM₂(PO₄)₃+_yNa₂O (M = Ge, Ti, Hf, Zr) system, were investigated in order to obtain a material having a high Na⁺ conductivity and high density. The ionic conductivity and the sinterability were greately improved by an increase in the valve of y for all of the system examined. Added Na₂O was not souble in teh NASICON-type skeletton, sice the lattice constants and teh X-ray diffraction patterns were not changed by the Na₂O addintion in all of the samples. Na₂O acts as a flux for obtaining highly dense ceramics and highly conductive grain boundaries. Partial A₂ site insertion by Na⁺ ions is effective for the enhancement of conductivity, because the conductivity for Na_1.5M(III)_0.5Zr_1.5(PO₄)₃ (M = In or Y) is about 1 order of magnitude higher than the maximum conductivity of the NaZr₂(PO₄)₃+_yNa₂O system.     

Upconversion Luminescence in γ-AlON:Yb3+,Tm3+ Ceramic Phosphors

Upconversion emission properties of γ-AlON:Yb³⁺,Tm³⁺ phosphors were investigated under single-wavelength diode laser excitation of 980 nm. Blue (479 nm) and red (653 nm) emission bands were observed which correspond to the transitions of ¹G₄→³H₆ and ¹G₄→³F₄ of Tm³⁺ ions, respectively. The upconversion spectra show a concentration-dependent luminescence intensity, reaching its peak at a concentration of 1.2 mol% Yb and 0.5 mol% Tm. Pump power dependence of the upconversion emission intensity ( P – I ) revealed that a two-photon process was involved in the blue and red emissions.     

Enhanced Phase Stability and Photoluminescence of Eu3+ Modified t-ZrO2 Nanoparticles

Tetragonal ( t ) ZrO₂ nanoparticles have been obtained by a partial Eu³⁺→Zr⁴⁺ substitution, synthesized using a simple oxalate method at a moderate temperature of 650°C in air. The Eu³⁺ additive, 2 mol% used according to the optimal photoluminescence (PL), gives small crystallites of the sample. On raising the temperature further, the average crystallite size D grows slowly from 16 nm to a value as big as 49 nm at 1200°C. The Eu³⁺: t -ZrO₂ nanoparticles have a wide PL spectrum at room temperature in the visible to near-IR regions (550–730 nm) in the ⁵D₀→⁷F_J (Eu³⁺), J =1–4, electronic transitions. The intensity of the ⁵D₀→⁷F₄ group is as large as that of the characteristic ⁵D₀→⁷F₂ group of the spectrum in the forced electric-dipole allowed transitions. The enhanced t -ZrO₂ phase stability and wide PL can be attributed to the combined effects of an amorphous Eu³⁺-rich surface and part of the Eu³⁺ doping of ZrO₂ of small crystallites.     

Ca2B5O9Cl:Eu2+, A Suitable Blue-Emitting Phosphor for n-UV Excited Solid-State Lighting

Blue-emitting Ca₂B₅O₉Cl:Eu²⁺ phosphors have been synthesized by solid state reaction. The photoluminescence excitation (PLE) spectra show broad-band absorptions and can match the emission of near ultraviolet (n-UV) chip well. At lower Eu²⁺ concentration, the emission band can be resolved into two bands, which is assigned to the 5 d →4 f transition of Eu²⁺ ions substituting two different Ca²⁺ sites. At higher Eu²⁺ concentration, the energy transfer from Eu(1) to Eu(2) happens and is very efficient. At higher temperature the phosphor exhibits a lower temperature quenching effect. The fluorescence lifetimes are short enough for application in solid-state lighting. The electroluminescence spectrum indicates that the emission of chip can almost be absorbed by phosphor and down-converted into an intensive blue light. The chromaticity coordinates of fabricated light-emitting-diodes (LEDs) is very close to that of BaMgAl₁₀O₁₇:Eu²⁺ (BAM). Ca₂B₅O₉Cl:Eu²⁺ is a good blue component phosphor for n-UV excited solid-state lighting.     

Thermal Expansion of Homogeneous and Gradient Solid Solutions in the System KZr2(PO4)3─KTi2(PO4)3

Low-thermal-expansion ceramics having arbitrary thermal expansion coefficients were synthesized from homogeneous solid solutions in the system KZr₂(PO₄)₃─KTi₂(PO₄)₃ (KZP–KTP). Dense and strong ceramics were fabricated by sintering at 1100° to 1200°C with 2 wt% MgO. The thermal expansion coefficient increased from 0 to +3 × 10⁻⁶/°C with increasing x in KZr_{2 − x}Ti_x (PO₄)₃ (KZTP). In addition, a functionally gradient material with respect to thermal expansion was prepared by forming a series of KZTP solid solutions in a single ceramic body. By heating a pile of KZP and KTP ceramics in contact with each other, KZP and KTP bonded together to form a KZTP gradient solid solution near the interface.     

Photoluminescent Properties of ABaPO4:Eu (A=Na, K) Phosphors Prepared by the Combustion-Assisted Synthesis Method

Blue-emitting phosphors A BaPO₄:Eu ( A =Na, K) are synthesized by the combustion-assisted synthesis method. The formation of single A BaPO₄ ( A =Na, K) phases has been confirmed by X-ray powder diffraction (XRD). X-ray photoelectron spectroscopy (XPS) is used to investigate europium valence in the A BaPO₄:Eu ( A =Na, K) samples. Evidence is found that the A BaPO₄:Eu ( A =Na, K) samples prepared in this paper exhibit a mixed-valence europium state (Eu³⁺ and Eu²⁺). Photoluminescence (PL) results show that the NaBaPO₄:Eu and KaBaPO₄:Eu phosphors emit a strong blue light under UV excitation.     

Effects of M3+ Ions on the Conductivity of Glasses and Glass-Ceramics in the System Li2O—M2O3—GeO2—P2O5 (M = Al, Ga, Y, Dy, Gd, and La)

Glasses with compositions Li_1.2M_0.2Ge_1.8(PO₄)₃ (M = Al, Ga, Y, Gd, Dy, and La) were prepared and converted to glass-ceramics by heat treatment. The effects of the M³⁺ ions on the conductivity of the glasses and glass-ceramics were studied. The main phase present in the glass-ceramics was the conductive phase LiGe₂(PO₄)₃. Al³⁺ and Ga³⁺ ions entered the LiGe₂(PO₄)₃ structure by replacing Ge⁴⁺ ions, but lanthanide ions did not. The glass-ceramics exhibited much higher conductivity than the glasses. With increased ionic radius of the M³⁺ ions, the conductivity remained almost unchanged at ∼3 × 10⁻¹² S/cm for the glasses, but it decreased from 1.5 × 10⁻⁵ to 8 × 10⁻⁹ S/cm for the glass-ceramics at room temperature. The higher conductivity for Al³⁺- and Ga³⁺-containing glass-ceramics was suggested to result from the substitutions of Al³⁺ and Ga³⁺ ions for Ge⁴⁺ ions in the LiGe₂(PO₄)₃ structure.