Novel luminescence behaviors of Ce3+/Tb3+ co-doped phosphate glasses

In the present work, Ce³⁺/Tb³⁺ co-doped 60P₂O₅-30BaO-10B₂O₃ phosphate glasses are prepared and their luminescence properties are presented. Under excitation at 303 nm, the Ce³⁺ ions singly doped sample show a novel red emission besides the UV one. The co-doped samples show enhanced Tb³⁺ ions emission with the increasing of Tb³⁺ ions concentration at the cost of Ce³⁺ emission. The mechanism of this luminescent behavior is discussed with respect to the relevant energy transfer process.     

Ultraviolet to near-infrared spectral modification in Ce3+ and Yb3+ codoped phosphate glasses

The ultraviolet to near-infrared spectral modification in Ce³⁺ and Yb³⁺ codoped phosphate glasses was realized through the energy transfer from Ce³⁺ to Yb³⁺. The absorption spectra, fluorescence excitation and emission spectra, luminescence decay curves, and time-resolved emission spectra were measured and analyzed. The energy transfer efficiency and concentration quenching efficiency were calculated based on the decay curves of Ce³⁺ 340 nm emission and Yb³⁺ 976 nm emission. The calculated and experimental NIR emission intensities on the Yb³⁺ concentrations were compared and discussed.     

Bridgman Growth and Characterization of LuAlO3—Ce3+ Scintillator Crystals

Chemical and structural effects in LuAlO₃-Ce³⁺ single crystals grown by the Bridgman method were studied using spectral emission, x-ray, etching and optical techniques. The Ce concentration distribution was found to exhibit the normal freeze behavior with partition coefficient of 0,17. The expansion of the unit cell volume due to incorporation of Ce was measured. Dislocation etch pits were revealed on (010), (100) and (001) crystal faces. Thermal expansion behavior of LuAlO₃ was studied along the major crystallographic axes in between the room temperature and 1000 °C. The scintillation performance of 5 × 5 × 10 mm³ samples was measured using a 662 keV gamma source with a shaping time of 1.2 μs. The light yield was increasing from 40% BGO to 70% BGO with increasing of the Ce³⁺ content from 0.13 at.% to 0.9 at.%.     

The relationships between crystal structure of alkaline earth metal hexagonal aluminate and 4f-5d transitions of Ce and Tb ions

The Mg-Sr-Ba hexagonal aluminates (MgSrBa)Al₁₂O₁₉ doped with either Ce³⁺ or Tb³⁺ ion have been synthesized by solid-state reaction. Crystal structure variations with the increasing Ba²⁺ concentration have been thoroughly studied. When the concentration of Ba²⁺ reaches 0.39, the crystal structure becomes the ideal magnetoplumbite-type (No. PDF #80-1195), however, it becomes the same structure as BaMgAl₁₀O₁₇ with space group P6₃/mmc when Sr²⁺ is completely replaced by Ba²⁺ ions. In Ce³⁺ doped compounds, the emission peak of the Ce³⁺ ion is gradually red-shifted when the concentration of Mg²⁺ ion increases, while the 4f-5d transition of Ce³⁺ at 260 nm splits into five components. In the Tb³⁺ doped compounds, the spin-allowed and spin-forbidden 4f-5d transitions of Tb³⁺ have been observed. The intensity of the spin-forbidden 4f-5d transition is gradually increased with decreasing Sr²⁺ concentration and becomes dominant when Sr²⁺ is completely replaced by Ba²⁺. These experimental observations are also discussed in the context and match well with the theoretical calculations of 4f-5d transitions of Tb³⁺.     

Growth and downconversion luminescence of Ho3+/Yb3+ codoped α‐NaYF4 single crystals by the Bridgman method using a KF flux

Downconversion (DC) luminescence with emission at about 1000 nm under excitation of 448‐nm light in Ho³⁺/Yb³⁺ codoped α‐NaYF₄ single crystal is realized. The crystal was grown by the Bridgman method using KF as an assisting flux in a NaF‐YF₃ system. The energy‐transfer process and quantum cutting (QC) mechanisms are presented through the analysis of the spectra. The energy‐transfer processes of first‐ and second‐order cooperative DC are responsible for the increase of the emission intensity at 1000 nm, and it is the first‐order cooperative DC that is dominant for the DC process. When the Ho³⁺ concentration is fixed at about 0.8 mol%, the optimal concentration for ∼1000 nm emission is 3.02 mol% Yb³⁺ in the current research. The energy‐transfer efficiency and the total quantum efficiency are analyzed through the luminescence decay curves. The maximum quantum cutting efficiency approaches to 184.4% in α‐NaYF₄ single crystals of 0.799 mol% Ho³⁺ and 15.15 mol% Yb³⁺. However, the emission intensity at 1000 nm decreases while the energy‐transfer efficiency from Ho³⁺ to Yb³⁺ increases, which may result from the fluorescence quenching between Ho³⁺ and Yb³⁺ ions, Yb³⁺ and Yb³⁺ ions.     

Thermal, chemical, optical properties and structure of Er-doped and Er/Yb-codoped P2O5-Al2O3-ZnO glasses

This study was explored in series of the optical, thermal, and structure properties based on 60P₂O₅-10Al₂O₃-30ZnO (PAZ) glasses system that doped with varied rare-earth (RE) elements Yb₂O₃/Er₂O₃. The glass transition temperature, softening temperature and chemical durability were increased with RE-doping concentrations increasing, whereas thermal expansion coefficient was decreased. In the optical properties, the absorption and emission intensities also increase with RE-doping concentrations increasing, When Er₂O₃ and Yb₂O₃ concentrations are over than 3 mol% in the Er³⁺-doped PAZ system and Yb³⁺-doped concentration is over than 3 mol% for Er³⁺/Yb³⁺-codoped PAZ system, the emission intensity significantly decreases presumably due to concentration quenching, formation of the ions clustering, and OH⁻ groups in the glasses network. It is suggested that the maximum emission cross-section (σ_e) is 7.64 × 10^− 21 cm² at 1535 nm is observed for 3 mol% Er³⁺-doped PAZ glasses. Moreover, the maximum σ_e × full-width-at-half-maximum is 327.8 for 5 mol% Er³⁺-doped PAZ glasses.     

Energy transfer studies of Ce:Eu system in phosphate glasses

The preparation of sodium phosphate glasses singly and doubly doped with rare earth ions Ce³⁺ and Eu³⁺ by melt quench method is described. The spectroscopic characterizations of the samples are done using absorption, excitation and emission spectra. The nonradiative energy transfer between trivalent cerium and europium is achieved through the phosphate lattice and the results are incorporated. The main reason of quenching of Ce³⁺ ions and the mechanism of energy transfer is mainly electric dipole-dipole in nature for Ce³⁺:Eu³⁺ system.     

Tb3+-impregnated,non-porous silica glass possessing intense green luminescence under UV and VUV excitation

A colorless transparent luminescence material was successfully prepared by impregnation of leached, porous glass with Tb³⁺ ions followed by reductive sintering in a CO atmosphere. Tb³⁺ emissions under ultraviolet (UV) and vacuum ultraviolet (VUV) excitation clearly showed the most intense emission band to be situated at 543 nm, which corresponds to the ⁵D₄ → ⁷F₅ transition. Sintering of the Tb glass in a reducing atmosphere resulted in a significant enhancement of Tb³⁺ emission intensity in comparison with sintering in air. The presence of traces of cerium ions was verified in Tb glasses, and more Ce³⁺ ions were produced as a result of the reductive sintering. The increase in Ce³⁺ ions was believed to be mainly responsible for the enhancement of ⁵D₃ → ⁷F_j and ⁵D₄ → ⁷F_j transitions from Tb³⁺ ions owing to an energy transfer channel. A clearly defined difference in the spectral energy distribution of Tb³⁺ emissions was found for 231 nm UV and 160 nm VUV excitation of the Tb glass. The phenomenon of cross relaxation was only observed under 231 nm UV excitation. Different excitation mechanisms were taken into account. Direct excitation of Tb³⁺ ions together with Ce³⁺ ions occurred in the Tb glass under the 231 nm UV light, whereas indirect excitation consisting of host absorption of energy and transfer from host to Tb³⁺ ions occurred under the 160 nm VUV light.     

Study of the effects of Ga3+ co-doping on the Lu0.8Sc0.2BO3:Ce scintillation crystals

Transparent Lu_0.8Sc_0.2BO₃ crystals doped with 1 at%Ce³⁺ and co-doped with 1 at% and 3 at%Ga³⁺ were grown by the Czochralski method. We applied absorption spectrum, luminescence spectra under UV and X-ray excitation, fluorescence decay curve, three dimensional thermoluminescene and X-ray absorption near edge spectroscopy to study the effect of Ga³⁺ co-doping on the Lu_0.8Sc_0.2BO₃:Ce scintillation crystals. Experimental results indicated that no positive contribution of the Ga³⁺ ion doping on the scintillation efficiency was found. The causes for the deterioration of scintillation efficiency by co-doping Ga³⁺ were revealed. The decrease of practical cerium content and the Ce³⁺/Ce⁴⁺ ratio in crystals, and the increase of the trap concentrations, although the corresponding trap types still maintained the same, played a joint influence on the degrading of scintillation efficiency of Lu_0.8Sc_0.2BO₃:Ce crystals.     

Time-resolved fluorescence measurements on Dy3+ and Sm3+ doped glasses

Various fluoride, phosphate and borosilicate glasses with known properties and global structure have been doped with Dy³⁺ (4f⁹) and Sm³⁺ (4f⁵) between 10¹⁸ and 10²¹ cm^?3 and their time resolved fluorescence in the visible range in combination with characteristic physical properties were studied. Different fit procedures were carried out. Although both ions differ in their intrinsic fluorescence lifetime, with 1.5 ms for Dy³⁺ and 6.5 ms for Sm³⁺, their dependence on glass matrix is remarkable similar. Fluoroaluminate glasses with varying phosphate content between 0 and 20 mol% (FPx), a pure phosphate glass (P100), and two borosilicate glasses with low (DURAN^®-like) and high optical basicity (NBS1) were used for investigations. A strongly ionic surrounding by fluorine ligands, as in fluoroaluminate glass samples, provides the longest fluorescence lifetime. It decreases with increasing phosphate content by increasing oxygen surrounding and with increasing RE³⁺ doping. Large differences were detected in the two borosilicate glasses depending on their optical basicity mainly due to differences in the Na₂O/B₂O₃ ratio. Duran-like samples with low Na₂O content have shown phase separation with higher doping concentration. The RE³⁺ ions are accumulated in the borate-rich droplets. Surprisingly only very low concentration-quenching effects were observed. In the opposite of NBS1 samples with high Na₂O content this generated extremely high quenching effect.     

In situ chemical co-precipitation synthesis of YVO4:RE (RE = Dy3+, Sm3+, Er3+) phosphors by assembling hybrid precursors

YVO₄:RE (RE = Dy³⁺, Sm³⁺, Er³⁺) were prepared via an in situ chemical co-precipitation technology, and the assembly process of hybrid precursors was as follows: using rare earth coordination polymers with salicylic acid as precursors and composing with the polyvinyl alcohol (PVA) as dispersing media. Their microstructure and micromorphology have been analyzed by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM), which indicate that there exist some novel cobblestone-like microcrystalline particles. All the doped rare earth ions showed their characteristic emissions in YVO₄ hosts. The concentration quenching all appeared of the three dopant ions and the optimum concentrations for Dy³⁺, Er³⁺, Sm³⁺ were determined to be 2, 3, 1 mol% in yttrium vanadate particles, respectively.     

Optical and thermoluminescence properties of R2O–RF–B2O3 glass systems doped with MnO

Alkali fluoroborate glass systems containing manganese cations have been thoroughly investigated in order to obtain information about the structural role of manganese in such glass hosts. The amorphous phase of the prepared glass samples R₂O–RF–B₂O₃:MnO (with R = Li and Na) was confirmed from their X-ray diffraction. From the infrared spectra of these glass systems it was concluded that the glass structure contains two group of bands; one due to trigonal BO₃ units and the second due to the tetrahedral BO₄ units. As manganese was introduced, replacing lithium or sodium, it acts as a network modifier and the intensity of the second group of bands increases at the expense of the first group of bands. The optical absorption spectra of R₂O–RF–B₂O₃:MnO exhibited two conventional absorption bands; one due to Mn²⁺ ions and the other due to Mn³⁺ ions. The ESR spectra of these glasses showed a six-line hyper-fine structure centered at g = 2.01 (due to Mn²⁺ ions) and another signal at g = 4.3 (due to Mn³⁺ ions). The intensity of optical absorption bands and the ESR signal due to Mn²⁺ ions decreases with increasing MnO concentration indicating the conversion of Mn²⁺ ions into Mn³⁺ ions in the glass network. The thermoluminescence studies on these glass systems showed a quenching of TL output with increase in the concentration of MnO. All the obtained results were discussed on the basis of the glass structure and the conversion of Mn²⁺ into Mn³⁺ ions with increasing concentration of MnO in the glass systems.     

Increase in the Fluorine-Ion Conductivity of Single Crystals of Tysonite-type CeF<Subscript>3</Subscript> Superionic Conductor by Substituting Polarized Cd<Superscript>2+</Superscript> Ions for Ce<Superscript>3+</Superscript> Ions

The fluorine-ion conductivity of single crystals with a tysonite (LaF₃) structure with heterovalent isomorphic substitutions of highly polarizable Cd²⁺ cations with a 18-electron shell for rare earth ions Ce³⁺ have been studied for the first time. Ce_0.995Cd_0.005F_2.995 single crystals have been grown from melt by the Bridgman technique in a fluorinating atmosphere. The fluorine-ion conductivity of single crystal is measured by impedance spectroscopy in the temperature range from 153 to 1073 K, where it increases by a factor of 10⁹, approaching the value σ_dc = 5 × 10^–2 S/cm at 1073 K. At T₀ = 450 ± 20 K, the dependence σ_dc(T) is split into two portions with the ion-transport activation enthalpy ΔH_σ = 0.39 ± 0.01 eV (T < T₀) and ΔH_σ = 0.23 ± 0.02 eV (T > T0). It is found that at T = 293 K the conductivity σ_dc = 3 × 10^–5 S/cm of Ce_0.995Cd_0.005F_2.995 crystal is higher by a factor of 10 than the conductivity of the tysonite matrix CeF₃ and close to the σ_dc value for Ce_0.995Sr_0.005F_2.995 crystal. This finding indicates a significant effect of the substitutions of Cd²⁺ ions for Ce³⁺ on the σdc value and the advantage of Cd²⁺ ions over Ca²⁺ and Ba²⁺ from the viewpoint of increasing σ_dc.     

Compositional dependence of ultraviolet fluorescence intensity of Ce in silicate, borate, and phosphate glasses

Fluorescence spectra of Ce³⁺ ions in silicate, borate, and phosphate glasses melted in Ar were measured. The relative fluorescence intensity of Ce³⁺ in the ultraviolet region increased in the order of R = Ba, Ca, Sr, and Mg in the 20Li₂O-20RO-60SiO₂ glass samples and with decreasing BaO content in the BaO-B₂O₃ glass samples, respectively. In contrast, the relative fluorescence intensity of Ce³⁺ did not change with varying the glass composition in phosphate glass samples. The compositional dependence of the relative fluorescence intensity of Ce³⁺ is discussed in terms of redox reaction of Ce³⁺-Ce⁴⁺ in oxide glasses.     

Thermal and optical investigation of EuF3-doped lead fluorogermanate glasses

Europium-doped lead germanate and lead fluorogermanate glasses are studied by using differential thermal analysis, X-ray diffraction, photoluminescence and fluorescence lifetimes measurements of the ⁵D_j, j = 0, 1, 2 levels. PbF₂ addition increases the thermal stability of the lead germanate glass, while Eu³⁺ ions promote the crystallization of β-PbF₂:Eu³⁺ nano-crystals embedded in a glassy matrix. In the lead fluorogermanate glasses, Eu³⁺ ions exhibit a strong affinity for F⁻ ions although oxygen ions are much more numerous. It appears that luminescence concentration quenching is not important, while cross relaxation is very efficient in the glasses. The results allow to propose for these glasses a molecular model in which small fluorine rich island, incorporating the Eu³⁺ ions in low symmetry sites, are separated from each other by chains of germanate (GeO₄)⁴⁻ ions linked together.     

Time-resolved fluorescence measurements on Eu3+- and Eu2+-doped glasses

Various fluoride, phosphate and borosilicate glasses with known properties and global structure have been doped with Eu³⁺ (4f⁶) at doping concentrations between 10¹⁸ and 10²¹ cm^?3. By applying reductive melting conditions Eu³⁺ could partially be transformed to Eu²⁺ (4f⁷). Four fluoroaluminate glasses with varying phosphate content between 0 and 20 mol% (FPx), a pure phosphate glass (P100) and two borosilicate glasses with low (DURAN^®-like) and high optical basicity (NBS1) have been used for these investigations. The time-resolved fluorescence in the visible range has been studied for both ions. Although static and dynamic fluorescence of Eu³⁺ and Eu²⁺ are dramatically different (f–f-transitions for Eu³⁺; d–f-transitions for Eu²⁺), glass structure changes have a similar influence on the dynamic fluorescence behavior of both ions. A strongly ionic surrounding due to fluorine ligands as in fluoroaluminate glass samples provides the longest fluorescence lifetime (about 7 ms for Eu³⁺; about 1.3 μs for Eu²⁺). Increasing phosphate content decreases the fluorescence lifetime due to more oxygen ligands. Interesting differences have been found for the two borosilicate glasses due to the difference in their optical basicity (Na₂O/B₂O₃ ratio). Measurements indicate a homogeneous distribution of europium ions in most FP samples. NBS1 measurements suggest that two different local europium sites are formed. For Duran-like samples only one specific europium site was found, although these samples show phase separation at high doping concentrations into a SiO₂-rich phase and borate- and europium-rich droplets. Fluorescence quenching due to energy transfer from Eu²⁺ to Eu³⁺ could be found for co-doped samples; Eu³⁺-doped samples show no fluorescence quenching.     

Cs2LiGdCl6 (Ce): New scintillation material

We investigated the scintillation properties of Cs₂LiGdCl₆:Ce³⁺ as a function of the Ce concentration. X-ray excited luminescence spectra of the scintillation material showed broad emission bands between 360 and 460 nm, with two overlapping peaks, due to the d→f transitions on Ce³⁺ ions. The samples provide good scintillation results. The energy resolution was found to be 5.0% (FWHM) at 662 keV for 10% Ce sample. Under γ-ray excitation, Cs₂LiGdCl₆:Ce³⁺ showed three exponential decay time components of about 130–200 ns decay time constant. The light output of the investigated samples was 20,000 photons/MeV for a 10% Ce concentration. The light output deviation from the linear response is within 7% between the energy range of 31 and 1333 keV. Overall, the scintillation properties confirm that Cs₂LiGdCl₆:Ce³⁺ single crystal is a promising candidate for medical imaging and radiation detection.     

Color centers and charge state change in Ce:YAG crystals grown by temperature gradient techniques

Color centers and impurity defects of Ce:YAG crystals grown in reduction atmosphere by temperature gradient techniques have been investigated by means of gamma irradiation and thermal treatments. Four absorption bands associated with color centers or impurity defects at 235, 255, 294 and 370 nm were observed in as-grown crystals. Changes in optical intensity of the 235 and 370 nm bands after gamma irradiation indicate that they are associated with F⁺-type color center. Charge state change processes of Fe³⁺ impurity and Ce³⁺ ions take place in the irradiation process. The variations of Ce³⁺ ions concentration clearly indicate that Ce⁴⁺ ions exist in Ce:YAG crystals and gamma irradiations could increase the concentration of Ce³⁺ ions. Annealing treatments and the changes in optical density suggest that a heterovalent impurity ion associated with the 294 nm band seems to be present in the crystals.     

Growth and properties of Lu3Al5O12-RE3+ single crystals

Optically transparent garnet single crystals were grown from Lu₃Al₅O₁₂ melts containing different RE³⁺ ions. The distribution coefficient of Nd³⁺ ions is found to be a function of the growth rate. Lu₂O₃–Al₂O₃ system studies have been partially carried out. Measurements of the lattice data, absorption spectra and comparison with some properties of Y₃Al₅O₁₂ single crystals have also been made.     

Synthesis and Luminescence Characteristics of LiMgAlF6:Ln3+ (Ln = Ce,Eu, Tb)

By solid state reaction, LiMgAlF₆ and LiMgAlF₆: Ln³⁺ are synthetized with the ratio 120/100/110 of LiF/MgF₂/AlF₃, at 1008 K, in high-purity Ar stream. Their crystal structure which belongs to hexagonal system are determined by X-Ray-Diffraction (XRD). Luminescence characteristics of Ce³⁺, Eu³⁺, Tb³⁺ and sensitization of Ce³⁺ to Tb³⁺ in LiMgAlF₆ are studied. It is shown that the sensitization of Ce³⁺ to Tb³⁺ is efficient and a bright green emission is observed.