Tailored photoluminescence of YAG:Ce phosphor through various methods

Trivalent cerium Ce³⁺ (Ce) activated yttrium aluminum garnet Y₃Al₅O₁₂ (YAG) phosphor was synthesized by two methods: solid state reaction (SS), and combustion (CB) with urea, respectively. The crystallization and luminescent properties of the phosphors were studied. Factors influencing on the intensity of luminescence and the location of emission band of YAG:Ce, such as the type of flux used in SS, the reaction atmosphere, the concentration of activator, were investigated in detail. We accomplished red or blue shift of Ce emission band by a number of techniques in order to match with the variable emission wavelength of blue light emitting diodes. The change of emission in color coordinates was illustrated by chromaticity. Co-doping other rare earth ions with Ce³⁺ ions into YAG was attempted to increase the color rendering index.     

EPR study of exchange interactions in the nonmagnetic Kondo system La1−x CexCu6

The EPR of paramagnetic impurities Gd³⁺ and Mn²⁺ was studied in nonmagnetic Kondo system La_1−x Ce_xCu₆ containing in the 1.6–200 K range. The exchange interaction parameters of gadolinium and manganese ions with conduction electrons, of cerium ions with conduction electrons and with one another, the Kondo temperature of cerium ions, and the temperature behavior of cerium-ion spin-fluctuation rate have been determined. A pseudogap in the density of states at the Fermi level has been detected in the CeCu₆ regular system, which is apparently due to s-f hybridization. This pseudogap can be destroyed by introducing an aluminum impurity, which induces strong conduction-electron scattering. It was also found that RKKY interaction among manganese ions in CeCu_6−y Mn_y is considerably stronger than it is in LaCu_6−y Mn_y, which implies enhancement of nonlocal spin susceptibility due to an f band contribution to conduction-electron states. Fiz. Tverd. Tela (St. Petersburg) 40, 593–599 (April 1998)     

Slow components of the emission decay in fluoride crystals doped with Ce3+

Spatially separated defects created by photons with energies 6–8 eV in alkali-earth fluoride crystals doped with cerium are investigated with the help of thermoluminescence. Measuring the spectra of creation of V_k and H peaks of thermostimulated luminescence inBaF ₂:Ce³⁺. we demonstrate that photons with energies higher than 6eV induce H centers (self-trapped holes captured by interstitialF ions), whereas the formation of self-trapped holes begins on exposure to photons with energies greater than 7 eV. The influence of photoionization on theCe ³⁺ luminescence inBaF ₂, SrF₂, CaF₂, andCeF ₃ crystals is investigated in the range of photon energies 4–8 eV. An exponentialCe ³⁺-emisson decay was observed for excitation energy lying in the range 4–6 eV. Slow and fast decay components were observed under excitation by photons with energies higher than 6 eV. We believe that the slow and fast components are due to the tunnel recombination of trapped electrons with hole centers. A. P. Vinogradov Institute of Geochemistry of the Siberian Branch of the Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 43–49, March, 2000.     

Solvothermal synthesis and luminescence properties of the novel aluminum garnet phosphors for WLED applications

The color rendering index (CRI) and structural stability of cerium doped yttrium aluminum garnet (YAG:Ce) based phosphors have been enhanced by replacing Y³⁺ ions by larger radius ions (Tb³⁺, Gd³⁺, Eu³⁺, and Sm³⁺) at the dodecahedral site and replacing Al³⁺ ions by larger ones (Ga³⁺, Y³⁺, Tb³⁺, Gd³⁺, and Sm³⁺) at the octahedral site. These aluminum garnet crystalline powders were prepared by solvothermal reaction method at 300 °C for 48 h. The lattice constant values of synthetic aluminum garnet crystalline powders are larger than that of YAG and the emission wavelength of Ce³⁺ ion of these samples is longer than that of YAG:Ce. FESEM and TEM studies revealed that the Ln₃Ga₂Al₃O₁₂ and Ln₃Al₂Al₃O₁₂ crystalline powders have 3-dimensional star-like morphology with submicron size and good crystallinity, while, Ln₃(LnAl)Al₃O₁₂ garnet crystalline powders were cubic crystalline phases and shaped as cubes with the round edge having an approximate diameter of about 200–400 nm. All the prepared powders were grown along (100) direction and crystallized into single crystal. Also, the effects of treatment time and reaction temperature on the structure of aluminum garnet crystalline powders have been investigated.     

Superhyperfine structure of the EPR spectra of Ce<Superscript>3+</Superscript> ions in Li<Emphasis Type="Italic">R</Emphasis>F<Subscript>4</Subscript> (<Emphasis Type="Italic">R</Emphasis> = Y,Lu, Tm) double fluorides

The EPR spectra of Ce³⁺ impurity ions in LiYF₄, LiLuF₄, and LiTmF₄ double-fluoride single crystals have been investigated at a frequency of ∼9.3 GHz in the temperature range 5–25 K. The effective g factors of the ground Kramers doublet of the cerium ions in three crystals are close to each other (g _‖ = 2.737, g _⊥ = 1.475 for LiYF₄:Ce³⁺). A superhyperfine structure of the EPR spectrum of Ce³⁺ ions in the LiTmF₄ Van Vleck paramagnet has been observed in the external magnetic field B oriented along the crystallographic axis c (B ‖ c). The superhyperfine structure of the EPR soectra of the Ce³⁺ ions in the LiYF₄ and LiLuF₄ diamagnetic matrices is resolved for B ⊥ c. Possible factors responsible for this pronounced difference in the properties of the systems studied have been discussed.     

Solvothermal synthesis and luminescent properties of YAG:Tb nano-sized phosphors

Nano-sized Tb-doped YAG phosphor particles were synthesized by a mixed solvo-thermal method using stoichiometric amounts of inorganic aluminum and yttrium salts. The formation of YAG:Tb was investigated by means of XRD and IR spectra. The pure crystalline-phase YAG was prepared under moderate synthesis conditions (300 °C and 10 MPa), indicating that ethanol partly replaces water as the solvent, thus favoring the formation of YAG. TEM images showed that YAG:Tb phosphor particles sintered at 300 °C were basically of spherical shape, with good dispersion about a particle size of around 80 nm. The crystalline YAG:Tb showed green emission with ⁵D₄−⁷F₆ (544 nm) as the most prominent group. The PL intensity and crystallinity of YAG:Tb phosphors increases with increasing synthesis temperature, and reaches maximum brightness at 300 °C, which is lower than that exhibited by a commercial product.     

Decomposition processes and structural transformations of cerium propionate into nanocrystalline ceria at different oxygen partial pressures

The structural transformations that occur when thermal treatments turn cerium propionate into nanocrystalline ceria have been analysed with thermoanalytical techniques (TG, DTA and MS) and with structural and magnetic characterization (HRTEM, SQUID and XRD) of the final and intermediate products. Attention has been paid to what occurs during the decomposition of propionate and how the process is affected by the furnace atmosphere (oxidizing or inert). In an oxidizing atmosphere, the decomposition of cerium propionate is triggered by the oxidation of Ce³⁺ to Ce⁴⁺. This reaction entails the loss of large unoxidized propionate fragments of the propionate ligands. As decomposition proceeds, the carbonaceous residue makes the oxygen transport inside the material more difficult and decomposition becomes diffusion limited. At this point, extensive oxidation of the residue begins until it is completely removed. Crystallization of CeO₂ occurs simultaneously with decomposition. In these conditions, crystalline nanoparticles (diameter of 3–5 nm) can be obtained at a temperature as low as 300 °C. In an inert atmosphere, decomposition occurs in three steps. During the first step, one of the three propionate ligands is lost, with little oxidation of Ce³⁺, and is substituted by a hydroxyl group. The second step entails the loss of the remaining ligands with a substantial oxidation of Ce³⁺ to Ce⁴⁺. After this step, the intermediate product is, proposed as, a mixture of amorphous Ce(OH)₃ and Ce(OH)₄. Finally, the third step leads to conversion of the Ce hydroxide into crystalline CeO₂. In an inert atmosphere, the process is less reproducible than in air and a carbonaceous residue remains in the product.     

New sonochemiluminescence involving solvated electron in Ce(III)/Ce(IV) solutions

The moving single-bubble sonoluminescence of Ce³⁺ in water and ethylene glycol solutions of CeCl₃ and (NH₄)₂Ce(NO₃)₆ was studied. As found, a significant part of intensity of the luminescence (100% with cerium concentration less than 10^–4 M) is due to the sonochemiluminescence. A key reaction of sonochemiluminescence is the Ce⁴⁺ reduction by a solvated (or hydrated in water) electron: Ce⁴⁺ + e_s (e_aq) → *Ce³⁺. Solvated electrons are formed in a solution via electrons ejection from a low-temperature plasma periodically generated in deformable moving bubble at acoustic vibrations. Reactions of heterolytic dissociation of solvents make up the source of electrons in the plasma. In aqueous CeCl₃ solutions, the Ce⁴⁺ ion is formed at the oxidation of Ce³⁺ by OH radical. The latter species originates from homolytic dissociation of water in the plasma of the bubble, also penetrating from the moving bubble into the solution. The sonochemiluminescence in cerium trichloride solutions are quenched by the Br⁻ (acceptor of OH) and H⁺ ions (acceptor of e_aq). In water and ethylene glycol solutions of (NH₄)₂Ce(NO₃)₆, the sonochemiluminescence also quenched by the H⁺ ion. The sonochemiluminescence in CeCl₃ solutions is registered at [Ce³⁺] ≥ 10^–5 M. Then the sonochemiluminescence intensity increases with the cerium ion concentration and reaches the saturation plateau at 10^–2 M. It was shown that sonophotoluminescence (re-emission of light of bubble plasma emitters by cerium ions) also contributes to the luminescence of Ce³⁺ in solutions with [Ce³⁺] ≥ 10^–4 M. If the cerium concentration is more than 10^–2 M, a third source contributes to luminescence, viz., the collisional excitation of Ce³⁺ ions penetrating into the moving bubble.     

Energy transfer between Ce3+ and Eu2+ in doped KSrPO4

Powder samples of KSrPO₄ doped with Eu²⁺ and Ce³⁺ were prepared by combustion-assisted synthesis. Their structures and photoluminescence spectra were systemically studied. Energy transfer from Ce³⁺ to Eu²⁺ was observed by investigating the optical properties from photoluminescence spectra in Eu²⁺ single doped and Ce³⁺–Eu²⁺ co-doped KSrPO₄. The enhancement of UV excitation is attributed to energy transfer from Ce³⁺ to Eu²⁺, and Ce³⁺ plays a role as a sensitizer. Ce³⁺–Eu²⁺ co-doped KrSrPO₄ powders can possibly be applied as blue phosphors in the fields of lighting and display.     

Y3Al5O12∶Ce3+的余辉和热释光特性

高温固相法合成了Ce³⁺掺杂的Y₃Al₅O₁₂ (YAG)样品,研究了样品的结构、光致发光和热释发光性质. X射线衍射分析结果表明合成样品为YAG纯相,稀土离子的少量掺杂不改变基质YAG的结构. 荧光光谱测试表明在高温空气气氛下Y_2.95Al₅O₁₂ ∶Ce⁴⁺_0.05制备过程中存在Ce⁴⁺关键词： 长余辉 高温热释光 缺陷     

Spectral components that form UV absorption spectrum of Ce3+ and Ce(IV) valence states in matrix of photothermorefractive glasses

We have measured the UV absorption spectra of photothermorefractive glasses of the system Na₂O-ZnO-Al₂O₃-NaF-SiO₂ doped by cerium oxide in the range of (2.8–5.0) × 10⁴ cm⁻¹ (360–200 nm). The spectra have been processed by the method of dispersion analysis based on the analytical convolution model for the complex dielectric function of glasses. We show that the absorption band centered at 3.3 × 10⁴ cm⁻¹ (∼303 nm) that is attributed to the transition 2F _5/2 → 5d in the Ce³⁺ ion, is an envelope of three spectral components. The broad absorption range (3.5–4.7) × 10⁴ cm⁻¹ (200–270 nm) that is commonly interpreted as a charge transfer band of the Ce(IV) valence state, is an envelope of at least three spectral components.     

Effect of γ-irradiation on structural,morphological and luminescence properties of cerium-doped Y3Al5O12 nano-material

ABSTRACT

Cerium-doped yttrium aluminum garnet (YAG: Ce³⁺) nanopowder phosphors have been elaborated by sol–gel process and annealed at 900°C for 2?h. The prepared phosphors were exposed to gamma radiation, using ⁶⁰Co source, at different doses ranging from 5 to 100?kGy. The influence of γ-irradiation on the structural, morphological and luminescence properties of YAG: Ce³⁺ phosphors were investigated in detail by X-ray diffraction, ?eld emission scanning electron microscopy (FESEM), Fourier transforms infrared spectroscopy (FTIR) and photoluminescence measurements. The XRD analysis confirmed the presence of single cubic phase for all samples of YAG: Ce³⁺ nanophosphors independent of γ-rays dose. FESEM micrograph results revealed that the particles present flate-like shapes and high density of dislocation for sample irradiated at 100?kGy of γ-ray. The YAG: Ce³⁺ nanophosphors showed broad green–yellow emission band in the range of 450–700?nm with maximum intensity at 538?nm assigned to the 5d → 4f transitions of Ce³⁺ ion. The emission intensity of YAG: Ce³⁺ phosphors vary with the γ-ray irradiation and reach the maximum for sample irradiated to a dose of 25?kGy. The variation of luminescence intensity is related to the crystallite size and Ce⁴⁺ ions content in YAG host nanomaterial.     

Enhanced luminescent properties of Y3Al5O12:Tb,Ce phosphor prepared by spray pyrolysis

Yttrium aluminum garnet (YAG) particles doped with Tb³⁺ or double doped with Tb³⁺ and Ce³⁺ were prepared by spray pyrolysis and characterized by photo- and cathode-luminescence. It was tried to incorporate a broad band of Ce³⁺ activator into the line peaks of Tb³⁺ in YAG host without the reduction of emission intensity. Ce-codoped YAG:Tb particles showed a broad band emission due to the d-f transition of Ce³⁺ and a reduction in the intensity of emission peaks due to ⁵D₃-⁷F_j (j=3, 4, 5, 6) transition of Tb³⁺ when they were excited by the ultraviolet light of 270 nm. These results supported that an effective energy transfer occurs from Tb³⁺ to Ce³⁺ in YAG host. Codoping Ce³⁺ ions greatly intensified the excitation peak at 270 nm for the emission at 540 nm of Tb³⁺, which means that more lattice defects, involving in the energy absorption and transfer to Tb³⁺, are formed by the Ce³⁺ codoping. The finding gives a promising approach for enhancing the luminescence efficiency.     

Energy transfer in Gd2SiO5-Ce, Y2SiO5-Ce, and Be2La2O5-Ce crystals during selective VUV and core excitation

Luminescence vacuum ultraviolet time-resolved spectroscopy is used to study electronic excitations and energy transfer in Ce³⁺-doped crystals of gadolinium and yttrium oxyorthosilicates excited by synchrotron radiation in the vacuum ultraviolet (4–30 eV) and x-ray (50–200 eV) regions. At T = 10 K, both crystals exhibit intrinsic electronic excitations whose radiative relaxation occurs through fast (τ = 3 ns) and slow (microsecond) channels, which correspond to two possible types of self-trapped excitons. A comparison of the relaxation of above-edge and core electronic excitations in the Ce³⁺-doped crystals of gadolinium oxyorthosilicate and lanthanum beryllate indicates that the nature of the charge carriers involved in the recombination processes of energy transfer to luminescence centers is diverse. __________ Translated from Fizika Tverdogo Tela, Vol. 47, No. 8, 2005, pp. 1435–1439. Original Russian Text Copyright ? 2005 by Ivanov, Pustovarov, Kirm, Shlygin, Shirinskii.     

High-power single-frequency tunable solid-state lasers

Experimental results are presented on the achievement of single-frequency tunable lasing in ruby, Nd-glass, and Nd:YAG lasers with electrooptic Q switching of the cavity by the injection of an external signal. An optimization of the parameters is carried out for lasers on neodymium ions in yttrium aluminum garnet, lanthanum beryllate, chromium-doped gadolinium scandium gallium garnet, and lanthanum hexaaluminate with passive Q switching of the cavity by means of lithium fluoride shutters containing F ₂ ⁻ color centers. High-power single-frequency generation of giant pulses is achieved, with the output wavelength tunable over the half-width of the gain lines of the active media. Zh. Tekh. Fiz. 68, 74–79 (October 1998)     

Luminescence of YAG doped with Eu,Yb, and Mn ions under VUV excitation

Hydrothermal synthesis has been successfully used to obtain fine-crystalline powders of yttrium aluminum garnet (YAG) doped with manganese ions and codoped with cerium and manganese ions. Using the method of high-temperature solid-state synthesis, ceramic specimens of YAG that contain europium and ytterbium ions have been obtained. In synthesized YAG:Eu and YAG:Yb ceramics, no luminescence that can be attributed to 5d-4f transitions in Eu²⁺ or Yb²⁺ ions has been detected, even though the scheme of energy levels of these ions constructed with respect to YAG energy bands indicates that there is a potential possibility of the occurrence of 5d-4f luminescence for Eu²⁺ ions in YAG. At room temperature, the luminescence spectrum of hydrothermally synthesized YAG doped with manganese ions consists of two broad bands with maxima at ～600 and ～750 nm and does not contain any narrow bands in the red or IR range. Therefore, the spectrum contradicts to the properties of the luminescence of Mn²⁺, Mn³⁺, or Mn⁴⁺ ions in YAG described in the literature, even though the obtained hydrothermal specimens can contain noticeable concentrations only of Mn³⁺ ions.     

Temperature dependence of hyperfine fields in intermediate valent140CeLa and140CeLu

The hyperfine fields for the systems¹⁴⁰CeLa and¹⁴⁰CeLu have been determined by time differential perturbed angular correlations (TDPAC) in the temperature range 4.2 K≤T≤330 K. Magnitude and temperature dependence of the paramagnetic enhancement factor β indicate that cerium in either system is of intermediate valence. This behaviour is discussed in terms of a theory proposed by Ramakrishnan et al. [1–3].     

Mixed conductivity of zircon-type Ce1−xAxVO4±δ (A=Ca, Sr)

Incorporation of alkaline-earth cations into the zircon-type lattice of Ce_1−xA_xVO_4+δ (A=Ca, Sr; x=0−0.2) was found to significantly increase the p-type electronic conductivity and to decrease the Seebeck coefficient, which becomes negative at x≥0.1. The oxygen ionic conductivity is essentially unaffected by doping. The ion transference numbers of Ce_1−xA_xVO_4+δ in air, determined by the faradaic efficiency measurements, are in the range from 2×10⁻⁴ to 6×10⁻³ at 973–1223 K, increasing when temperature increases or alkaline-earth cation content decreases. The results on the partial conductivities and Seebeck coefficient suggest the presence of hyperstoichiometric oxygen, responsible for ionic transport, in the lattice of doped cerium vanadates. The activation energies for the electron-hole and ionic conduction both decrease on doping and vary in the ranges 39–45 kJ/mol and 87–112 kJ/mol, respectively. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15–21, 2002.     

A novel strategy to improve the sensitivity of aerosol phosphor thermometry using co-doped phosphors

A novel strategy is presented to improve the temperature sensitivity and potentially extend the temperature range of aerosol phosphor thermometry (APT) by co-doping host materials with two rare-earth ions. The ratio of the emission bands from each ion are measured and calibrated versus temperature to utilize the high sensitivity of thermographic phosphor absolute signal levels. The potential of the technique is illustrated using trivalent cerium (Ce³⁺) and praseodymium (Pr³⁺) co-doped into yttrium aluminum garnet (Ce,Pr:YAG). The measured fractional sensitivity of this phosphor from 300–450 K was 0.004–0.006 K⁻¹, a factor of 1.5–2 better than previously observed for Eu:BAM. Additionally, the single-shot precision (1σ) was between 9 and 25 K over the range of temperatures measured, illustrating the utility of this co-doping strategy. The level of temperature sensitivity and single-shot precision observed here should be achievable over different temperature ranges by doping Ce³⁺ and Pr³⁺ into different hosts. This new strategy should provide a pathway to ultimately extend the high-temperature single-shot measurement limit for APT to temperatures greater than 1000 K and push forward the state-of-the-art for planar temperature diagnostics in combustion applications.     

Growth and characterization of cerium-substituted yttrium iron garnet single crystals for magneto-optical applications

This paper is concerned with the preparation and characterization of cerium-substituted yttrium iron garnets, which are known to be oxides having a large Faraday rotation effect. Using the improved flux method we successfully grew bulk single crystals of iron garnet doped with Ce³⁺ ions with maximum substitutions up to 0.349. Here we investigate different solution compositions for maximum Ce³⁺ substitution. The Faraday rotation and optical absorption spectra were measured in the near infrared region for different Ce³⁺ ion substituted iron garnets. The specific Faraday rotation of Ce_0.349Eu_0.195Y_2.456Fe₅O₁₂ was found to be 1430 deg/cm at a wavelength of 780 nm and –1280 deg/cm at 1150 nm. The Ce substitution prominently enhances the Faraday rotation effect, and Yb³⁺ and Eu³⁺ ions substituted for Y³⁺ in the dodecahedral sites of YIG can increase the concentration of Ce³⁺ ions, depressing the formation of nonmagnetic Ce⁴⁺ ions by charge compensation. Received: 24 January 2001 / Accepted: 2 March 2001 / Published online: 27 June 2001