Silica gel glasses with a high efficiency of luminescence sensitization in the Ce<Superscript>3+</Superscript>-Tb<Superscript>3+</Superscript> system

Terbium-and (Ce, Tb)-containing glasses prepared using the direct sol-gel-glass transition are studied. It is shown that glasses doped with one activator contain two main types of optical centers, namely, isolated and complex centers, which are characterized by weak and strong cross-relaxation quenching of luminescence from the ⁵D₃ state of Tb³⁺ ions, respectively. The Ce⁴⁺-Tb³⁺ (Tb⁴⁺) complex centers are formed during sintering of coactivated xerogels in oxygen and can be transformed into Ce³⁺-Tb³⁺ centers through saturation of the samples with hydrogen. The Ce³⁺-Tb³⁺ centers exhibit efficient luminescence from the ⁵D₄ state upon excitation into the absorption bands of Ce³⁺ ions.     

Luminescence-spectral properties and structure of optical centers in Eu-and Ce-Eu-containing quartz gel-glasses

Eu-and Ce-Eu-containing quartz glasses obtained by a direct sol-gel-glass transition are investigated. It is discovered that the conversion of a xerogel into a glass leads to lowering of the symmetry of the Eu³⁺ optical centers from hexagonal to orthorhombic and a many-fold increase in the efficiency of the excitation of their luminescence through the charge-transfer band. In the co-activated glasses, the distortion of these centers is enhanced, and compound Ce⁴⁺-Eu³⁺ centers appear, in which the Eu(III) oxo complexes are characterized by cubic symmetry and by sensitization of the luminescence of the Eu³⁺ ions by photoreduced (Ce⁴⁺)⁻ ions. Crystal-field theory shows that the coordination number of the cation in such oxo complexes is equal to 8. It is established that similar structural formations exist in annealed xerogels, but they are manifested spectroscopically only upon low-temperature shortening of the interatomic distances, as a result of which the sensitization indicated becomes possible. Fiz. Tverd. Tela (St. Petersburg) 41, 229–234 (February 1999) Deceased.     

Nanocrystalline nature of high-symmetry Ce<Superscript>4+</Superscript>-Eu<Superscript>3+</Superscript> centers in silica gel glasses

The spectral-luminescent properties of silica gel glasses coactivated by Ce⁴⁺ and Eu³⁺ ions are investigated. The structure of the glasses is studied using x-ray powder diffraction and small-angle neutron scattering. The inference is drawn that Ce⁴⁺-Eu³⁺ centers with high-symmetry Eu(III) oxo complexes, which are formed in the glasses, have nanocrystalline nature. These centers are characterized by a weak vibronic interaction of Eu³⁺ ions with the matrix. The size of nanocrystallites formed under the synthesis conditions and at coactivator concentrations used is approximately equal to 10 nm.     

Growth and the luminescence properties of a lutetium gadolinium garnet doped with Ce3+ and Pr3+ ions

Crystals of lutetium gadolinium garnet solid solutions (Lu_{1 − x} Gd _x )Al₅O₁₂ (0 ≤ x ≤ 0.6) doped with Ce³⁺ and Pr³⁺ ions have been prepared by the horizontal directional crystallization method, and their optical and luminescence properties have been investigated. It has been established that the introduction of gadolinium into the lutetium garnet lattice leads to a decrease in the antisite luminescence (Lu_Al centers) in the UV spectral range and to sensitization of the Ce³⁺ ion luminescence. By contrast, the presence of gadolinium results in the quenching of the Pr³⁺ luminescence due to the nonradiative excitation transfer from Pr³⁺ ions to Gd³⁺ ions.     

Spectral-kinetic studies of SrAlF5 doped by trivalent rare-earth ions

Photochemical properties of Ce³⁺:SrAlF₅ and Ce³⁺,Yb³⁺:SrAlF₅ single crystals together with spectroscopic and kinetic characteristics of several optically nonequivalent impurity centers and energy transfer between them are described. It is shown that co-activation by Yb³⁺ ions effectively suppresses color centers in Ce,Yb:SAF crystals. It was found out that in Ce,Yb:SAF crystals Yb ions exist simultaneously in 2+ and 3+ valent state. Three types of optically nonequivalent luminescent centers corresponding to the doublets in luminescence spectrum centered at 290, 305 and 370 nm (Ce^I, Ce^II, Ce^III, respectively) have been observed. Analysis of luminescence spectra and decays leads to the conclusion that there is no energy transfer between either cerium centers or from Ce³⁺ to Yb²⁺ apart from the Ce^III center which luminescence is slightly quenched by Yb²⁺.     

Laser spectroscopy of optical centers in borosilicophosphate glass, coactivated by erbium and ytterbium ions

Based on analysis of the laser emission spectra of borosilicophosphate glass, coactivated by Er³⁺ and Yb³⁺ ions, we have determined the structure of the Stark splitting of the luminescence band for the erbium ion in the 1.5 μm region (the transition ⁴I_13/2 → ⁴I_15/2). In the wavelength interval 1532–1547 nm, we identified 12 sets of lines belonging to different types of optical centers of predominantly cubic symmetry. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 4, pp. 483–487, July–August, 2006.     

Influence of silver on the Sm<Superscript>3+</Superscript> luminescence in “Aerosil” silica glasses

The introduction of silver into the samarium-containing silica glasses prepared by the original solgel method leads to the formation of complex optical centers involving samarium ions and simple and/or complex silver ions. These centers are characterized by the effective sensitization of Sm³⁺ luminescence by Ag⁺, (Ag₂)⁺, and (Ag⁺)₂ ions according to the exchange mechanism for, at least, Sm³⁺-Ag⁺ centers. The formation of Sm-Ag centers is accompanied by an increase in the concentration of nonbridging oxygen ions, which prevent the reduction of silver ions by hydrogen. Silver nanoparticles formed in small amounts upon this reduction are effective quenchers of luminescence from the corresponding excited states of Sm³⁺ ions.     

Spectral luminescence properties of nanostructured Ce-Nd-Containing silica gel-glasses

Complex Ce⁴⁺-Nd³⁺ centers were formed in silica gel-glasses. These centers were characterized by weak cross-relaxation quenching of luminescence; an increased luminescence branching ratio in the ⁴F_3/2→⁴I_11/2, ⁴I_13/2 transitions; strong structuring of the analogous spectral bands; and effective intracenter sensitization of luminescence. On reducing the Ce⁴⁺ ions to the triply charged state, the structure of the luminescence bands of Nd³⁺ ions became weaker and the ratio of their intensities approached the value typical of an Nd-containing silica gel-glass.     

Irregular Ceand defect-related luminescence in YAlO3 single crystal

Using the methods of time-resolved and steady-state luminescence spectroscopies, the luminescence and defects creation processes were studied at 4.2-300 K under excitation in the 3.0-10.5 eV energy range for an YAlO₃:Ce crystal with very low concentration of Ce³⁺ ions. The results were compared with those obtained at the study of YAlO₃:Ce crystals with large Ce³⁺ content coming from the same technological laboratory. Three irregular Ce³⁺ centers were found and two intrinsic defect luminescence centers related to the cation and oxygen vacancies were evidenced. The origin and structure of luminescence centers are discussed.     

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.     

Luminescence studies on lanthanide ions (Eu, Dy and Tb) doped YAG:Ce nano-phosphors

Yttrium aluminum garnet nanoparticles both undoped and doped with lanthanide ions (Ce³⁺, Eu³⁺, Dy³⁺ and Tb³⁺) having average size around 30 (±3 nm) nm were prepared by glycine nitrate combustion method followed by annealing at a relatively low temperature of 800 °C. Increase in the annealing temperature has been found to improve the luminescence intensity and for 1200 °C heated samples there exists strong energy transfer from Tb³⁺ to Ce³⁺ ions in YAG:Ce(2%),Tb(2%) nanoparticles as revealed by luminescence studies. Co-doping the YAG:Ce nanoparticles with Eu³⁺ results in significant decrease in the emission intensity of both Ce³⁺ and Eu³⁺ ions and this has been attributed to the oxidation of Ce³⁺ to Ce⁴⁺ and reduction of Eu³⁺ to Eu²⁺ ions. Dy³⁺ co-doping did not have any effect on the Ce³⁺ emission as there is no energy transfer between Dy³⁺ and Ce³⁺ ions.     

The dependence of luminescence on reduction of Sm2+ ions doped in lithium barium borate glasses

Sm³⁺-doped Li₂O–BaO–B₂O₃ glass was prepared by the conventional melt quenching method in air atmosphere. Sm²⁺ ions were obtained by two methods, i.e. heating the as-made glass in a reducing atmosphere and irradiating the sample under X-rays. The two obtained samples were investigated by luminescence spectra and lifetime measurements. It was found that the conversion of Sm³⁺→Sm²⁺ after X-ray irradiation is efficient in this borate glass. Photo-stability of Sm²⁺ ions was evaluated by the photo-bleaching method. Furthermore, thermo-luminescence was also measured. The different defects and the reduction mechanism of Sm²⁺ ions in this borate glass were discussed. This would be helpful to understand the reduction mechanism of Sm²⁺ ions in borate glasses.     

Luminescence properties of Eu3+ and Sm3+ coactivated Gd(III) tungstate phosphor for light-emitting diodes

Rare-earth ions coactivated red phosphors Gd_0.2RE_1.8(WO₄)₃ (RE=Eu³⁺ and Sm³⁺) were synthesized by conventional solid-state reaction using boric acid as a flux agent. The samples were characterized by X-ray diffractometer (XRD), energy-dispersive X-ray spectrometer (EDS) and luminescence spectrometer (LS). The results showed that the Eu–Sm system exhibits higher emission intensity than those of the Eu single-doped system and Sm separate-doped system under ultraviolet (UV) radiation. Samarium(III) ions are effective in broadening and strengthened absorptions around 400 nm. Furthermore, it exhibits enhanced luminescence emission. when the mole ratio of boric acid is about 0.16, the luminescence capability is optimum. Two strongest lines at ultraviolet (394 nm) and blue (465 nm) in excitation spectra of these phosphors match well with the output wavelengths of UV and blue GaN-based light-emitting diodes (LEDs) chips.     

Optical and gain properties of series of crystals LiF-YF3-LuF3 doped with Ce and Yb ions

Here, we present first results of systematic studies of host cation variation impact on spectral-kinetic, photochemical and gain properties of Ce³⁺-doped LiYF₄ (YLF), LiLuF₄ (LLF) and LiY_1−xLu_xF₄ family crystals. 5d-4f luminescence decay of Ce³⁺ ions studies, together with pump-probe experiments, indicate that previously reported twice higher luminescence quantum yield in LLF compared with that of YLF crystals is provided by more efficient upper lasing level feeding due to recombination and higher color center destruction rate in LLF against YLF crystals. Namely, it is responsible for higher energetic characteristics of laser based on Ce³⁺:LLF crystals. Strong and wide pump-induced absorption band centered at 310 nm is observed in Ce³⁺:YLF. This band is shifted to blue and its intensity goes down with Lu content. We have evaluated free charges recombination rate, excited state absorption cross-section for Ce³⁺ ions and some other photodynamic processes related microparameters. Fitting results indicate that pump-induced color centers lifetime decreases with Lu-content in LiYF₄-LiLuF₄ mixture and it can be associated with more efficient color center bleaching by Ce³⁺ ions 5d-4f fluorescence.     

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.     

Optical spectra of triply-charged rare-earth ions in polycrystalline corundum

Solid samples of polycrystalline corundum α-Al₂O₃ activated by triply-charged rare-earth ions RE³⁺ (R=Eu³⁺, Er³⁺, Pr³⁺) were synthesized by the sol-gel technology. Characteristic narrow-line optical absorption and luminescence spectra produced by intraconfigurational 4f-4f transitions in RE³⁺ ions have been measured. RE³⁺ ions have been established to form one dominant type of optical centers in the corundum matrix, and the energy diagram of Eu³⁺ and Er³⁺ Stark levels in corundum has been determined. Fiz. Tverd. Tela (St. Petersburg) 40, 1442–1449 (August 1998)     

Spectral kinetics of Ce<Superscript>3+</Superscript> ions in double-fluoride crystals with a scheelite structure

The influence of the cation composition on the spectral kinetics of Ce³⁺ ions in double-fluoride crystals with a scheelite structure is studied. The importance of the photodynamic processes induced in these crystals by the exciting radiation is demonstrated. The difference in luminescence quantum efficiency between Ce³⁺ ions in LiYF₄ and LiLuF₄ crystals is found to be due to the different lifetimes of color centers produced in the samples by the exciting radiation and to the different efficiency of the free-carrier recombination at cerium impurity centers. It is shown that Yb³⁺ ions can increase the carrier recombination rate in the crystals.     

KBaPO4:Ln (Ln=Eu, Tb, Sm) phosphors for UV excitable white light-emitting diodes

This letter reports the novel three emission bands based on phosphate host matrix, KBaPO₄ doped with Eu²⁺, Tb³⁺, and Sm³⁺ for white light-emitting diodes (LEDs). The phosphors were synthesized by solid-state reaction and thermal stability was elucidated by measuring photoluminescence at higher temperatures. Eu²⁺-doped KBaPO₄ phosphor emits blue luminescence with a peak wavelength at 420 nm under maximum near-ultraviolet excitation of 360 nm. Tb³⁺-doped KBaPO₄ phosphor emits green luminescence with a peak wavelength at 540 nm under maximum near-ultraviolet excitation of 370 nm. Sm³⁺-doped KBaPO₄ phosphor emits orange-red luminescence with a peak wavelength at 594 nm under maximum near-ultraviolet excitation of 400 nm. The thermal stabilities of KBaPO₄:Ln (Ln=Eu²⁺, Tb³⁺, Sm³⁺), in comparison to commercially available YAG:Ce³⁺ phosphor were found to be higher in a wide temperature range of 25-300 °C.     

Studies of the structure of Ce3+ centers in SrS phosphors

It is found that the shape of luminescence spectra of SrS−Ce phosphors is independent of both the species and concentration of a cofactor F, Cl, Br, or 1 introduced in the form of ammonium halide at least up to 6.5 at.%. The only exception is F, which, when added as a coactivator with a concentration of 2.4 at.% and above, gives rise to new longwave luminescence bands, part from the luminescence of the Ce³⁺ ion. Analysis has shown that this additional luminescence spectrum is similar to the Ce³⁺ spectrum but is shifted towards the longwave region by about 0.14 eV. The luminescence decay time constant (τ=57 nsec) is very close to Br, I, and small concentrations of F, a model is valid according to which the halogen that substitutes for sulfur and the Ce³⁺ that substitutes for Sr are the closest neighbors not to each other, but to a probable compensator of the charge of Sr vacancies. At high concentrations of fluorine some of the F ions are located at interstitial lattice sites in the immediate neighborhood of Ce³⁺, providing compensation for the charge of Ce³⁺ ions and having a marked effect on them. Tartu University, 18, Ulikooli St., Tartu EE2400, Estonian Republic. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 2, pp. 173–178, March–April, 1997.