Physical Properties and Structure of Silica-Alumina-Europium Oxide Glasses

SiO₂–Al₂O3–Eu₂O₃ glasses were prepared for the composition 50siO₂·(50 – x )Al₂O_3·xEu₂O₃, and their density, sound velocity, and elastic modulus were measured. The chemical shift of the AIK a band emission spectra and the isomer shift of ¹⁵¹Eu by Mössbauer effect were obtained to determine the coordination states of Al³⁺ and Eu³⁺ ions in these glasses. It was found that the coordination number of Eu³⁺ ions was 12 and that the average coordination number of A1³⁺ ions was almost 5 in these glasses. By introducing Eu₂O₃, the packing of constituent ions was strongly enhanced and the elastic modulus increased in this system. The compositional dependence of the molar volume and elastic modulus were explained by these states of high coordination number for Eu³⁺ and low coordination number for Al³⁺ ions compared with those in the corresponding M₂O₃ crystals.     

Preparation and Faraday Effect of Fluoroaluminate Glasses Containing Divalent Europium Ions

Fluoroaluminate glasses containing various concentrations of Eu²⁺ were prepared under a reducing atmosphere for the present study, and the wavelength dependence of the Faraday rotation angle was examined. The magnitude of the Verdet constant (V_c) increased as the concentration of Eu²⁺ increased. In addition, the Verdet constant of glasses containing 5 cat.% Eu²⁺ was larger than that of fluoroaluminate glasses containing the same concentration of Tb³⁺ in the wavelength region from 400 to 600 nm. The effective transition wavelength, λ_t, for glasses containing Eu²+ as well as those containing Tb³+ was evaluated based on the Van Vleck and Hebb theory. Factors dominating the Verdet constant of those glasses are discussed in this report.     

Mössbauer Spectroscopy of Borate Glasses Containing Divalent Europium Ions

Borate glasses containing 0.99–30.0 mol% EuO have been prepared under a reducing atmosphere and ¹⁵¹Eu Mössbauer effect measurements have been carried out at room temperature in order to examine the chemical state of Eu²⁺ ions in these glasses. Mössbauer spectra indicate that most of the europium ions are present as the divalent oxidation state. In dilute Eu²⁺-containing borate glasses, spectra are split due to paramagnetic hyperfine interactions in the glasses with low and high sodium contents. In concentrated Eu²⁺-containing borate glasses, line broadening results from the contribution of quadrupole splitting due to asymmetrical oxygen coordination around Eu²⁺ and the contribution of inhomogeneous broadening due to site-to-site variation. The compositional dependence of isomer shift and quadrupole interaction parameter can be related to the structural changes in the borate glass. The variation of the isomer shift has a good correlation with the optical basicity of glass, and the trend can be explained in terms of the covalent admixture with 6 s character. The Eu-O bond in the borate glasses is more ionic than that in EuTiO₃ and EuZrO₃, where the oxygen coordination number for Eu²⁺ is 12. The average coordination number for Eu²⁺ is found to be 12 in all of the present glasses.     

Reduction of Eu3+ to Eu2+ in Aluminoborosilicate Glasses Prepared in Air

Eu₂O₃-doped aluminoborosilicate glasses were prepared in air at high temperature. Luminescence measurements were used to investigate a valence change from Eu³⁺ to Eu²⁺ ions in the aluminoborosilicate glasses. The results showed that the doped Eu³⁺ ions were partially reduced to Eu²⁺ in the Eu₂O₃:RO–Al₂O₃–B₂O₃–SiO₂ (RO=CaO, SrO, BaO, Li₂O) glasses, but not in the Eu₂O₃:RO–Al₂O₃–B₂O₃–SiO₂ (RO=Na₂O, K₂O) glasses. The changes of Eu reduction with different RO components were discussed with the variation of optical basicity of RO and with different valency of R cations. The effects of co-doping BaO and ZnO in aluminoborosilicate glasses on Eu reduction were also investigated and discussed.     

Composition Induced Reducing Effects on Eu Ions in Borophosphate Glasses

Europium-doped borophosphate glasses were prepared by the melt quenching method for application in light-emitting diodes. Emission spectra and electron paramagnetic resonance were studied for the glass samples. The relative intensity ratio of Eu²⁺ to Eu³⁺ emissions is composition dependent, especially the B₂O₃/P₂O₅ (B/P) ratio and the type of alkaline earth cations. The possible mechanisms are discussed.     

Spectroscopic Properties and Local Structure of Eu3+ in Ge–Ga–S–CsBr (or CsCl) Glasses

Spectroscopic properties and local structure of Eu³⁺ in Ge–Ga–S–CsBr (or CsCl) glasses were investigated using fluorescence measurements and several spectroscopic methods. Fluorescence from Eu³⁺:⁵D₀→⁷F₂ was observed only from glasses with CsBr/Ga ratios greater than unity and disappeared at temperatures above 140 K. Phonon sideband (PSB) spectra revealed that Eu³⁺ ions are located next to halogen ions, which form part of well-structured complexes such as EuCl₃, tetrahedral [GaS_3/2Cl]⁻, subunits and/or Ga₂Cl₆. These new bonds showed reduced coupling strength compared with Eu³⁺–S bonds in Ge–Ga–S glass. Fluorescence line narrowing experiments showed little site-to-site variation of Eu³⁺ ions.     

Compositional Dependence of Absorption and Fluorescence of b3+ in Oxide Glasses

The integrated absorption cross section, the spontaneous emission probability, and the stimulated emission cross section of Yb³⁺ were determined in silicate, phosphate, borate, germanate, aluminate, gallate, and ZBLAN host glasses. The compositional dependence of the stimulated emission cross section of the ²F_5/2→²F_7/2 transition is determined mainly by the integrated absorption cross section in the glasses. A peak stimulated emission cross section above 1 pm², which is the highest value in glasses, was obtained in a gallate glass with a composition of 40K₂O·20Ta₂O₅. 40Ga₂O₃. The factors affecting the integrated absorption cross section are discussed using the Judd-Ofelt parameters of Er³⁺ calculated in previous studies.     

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.     

Compositional Dependence of Absorption Spectra of Ti3+ in Silicate, Borate, and Phosphate Glasses

Absorption spectra of Ti³⁺ were measured for silicate, borate, and phosphate glasses doped with 0.5 mol% Ti₂O₃.The absorption coefficient at the peak wavelength of the ₂T₂→₂E transition of Ti³⁺ is used as a parameter showing the relative content of Ti³⁺ions in glass samples. The effect of glass composition on Ti³⁺/Ti⁴⁺ redox was studied. For multicomponent glasses, a basicity parameter calculated from glass composition is proposed in terms of coulomb force between the cation and the oxygen ion. The value of the absorption coefficient depends on basicity in silicate and borate glasses; however, it is independent of composition in phosphate glasses.     

Preparation and Faraday Rotation of Divalent Europium Glasses

A large glass-forming region was discovered in the system EUO–Al₂O₃–B₂O₃ and chemically stable glasses of good optical quality were produced. The concentration range extends from about 3 to 43 mole % EuO. Room-temperature Verdet constants as large as –2.55 min/oe-cm were measured on a glass containing 30.5 mole % Eu²⁺. This is somewhat higher than those reported for the trivalent rare-earth ions in similar glass matrices. The effective transition wavelength causing the rotation is 380 ± 20 mμ. Compared to Ce³⁺, Tb³⁺, and Pr³⁺ the large rotation of Eu²⁺ in the visible region is due primarily to a large J (total angular momentum) value as well as to a large effective transition wavelength.     

Compositional Dependence of Judd-Ofelt Parameters in Silicate, Borate, and Phosphate Glasses

Judd-Ofelt parameters Ω _t with t = 2,4, 6 for the rare-earth ions Pr³⁺, Nd³⁺, Sm³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, and Tm³⁺ in alkali and/or alkaline-earth silicate, borate, and phosphate glasses have been determined. The variations of Ω _t with the number of 4 f electrons of the rare-earth ions are demonstrated, and factors affecting the Judd-Ofelt parameter Ω₆are discussed. The intensity parameter Ω₆ depends on the ionic packing ratio of the glass host by changing modifier type in silicate and borate glasses, and it is independent of that in a series of borate glasses as a function of modifier content and phosphate glasses. The peak wavenumbers of the transitions whose intensities are determined mainly by the Ω₆<| U ⁽⁶⁾|>² term—where <| U ⁽⁶⁾|> is one of the reduced matrix elements—shift systematically with the values of Ω₆ for all the rare-earth ions.     

Correlation between Radiative Transition Probabilities of Nd3+ and Composition in Silicate, Borate, and Phosphate Glasses

Compositional dependence of spontaneous emission probabilities between initial ⁴ F _3/2 and terminal ⁴ I _J J = 9/2, 11/2, 13/2, 15/2) levels of Nd³⁺ were studied for about 90 samples of silicate, borate, and phosphate glasses using the Judd–Ofelt theory. The effect of the covalency of the Nd–O bond on the magnitude of intensity parameters was estimated from the variation of spectral profiles of the ⁴ I _9/2→⁴ G _5/2, ² G _7/2 and ⁴ F _7/2, ⁴ S _3/2 transitions. Intensity parameters Ω₄ and Ω₆ and the spontaneous emission probabilities were strongly affected by the ionic packing ratio of the glass host. The results were discussed in terms of the site selectivity of Nd³⁺ ions in glasses.     

Synthesis and Fluorescence Properties of Eu2+-Complex-Doped SiO2 Gels

Eu²⁺-complex-doped SiO₂ gels were prepared and their fluorescence properties were studied. SiO₂ gels were obtained by the reactions of silicon tetrachloride, tert -butyl alcohol, and methyl alcohol under nitrogen. Both to protect Eu²⁺ against oxidation and to enhance the emission intensity of Eu²⁺, Eu²⁺ was complexed with 15-crown-5 or its derivative. Samples showed blue emission. Fluorescence lifetimes of samples varied from 690 to 955 ns depending on the Eu²⁺ concentration at 290 K. After drying of samples, 50-70% of Eu²⁺ remained in the divalent state without oxidation in the samples. It was found that Eu²⁺ complexes inside gels were stable against oxidation in air.     

Long-Lasting Phosphorescence of Transparent Surface-Crystallized Glass-Ceramics

Transparent surface-crystallized glass-ceramics with molar compositions of 43SiO₂–2B₂O₃–30SrO–25MgO–0.05Eu₂O₃–0.8Dy₂O₃ (sample GC-A) and 43SiO₂–2B₂O₃–24SrO–6CaO–25MgO–0.05Eu₂O₃–0.8Dy₂O₃ (sample GC-B) were prepared by heat-treating the mother glasses at 850°C for 10 h. The precipitated phases in both glass-ceramics were (Sr_{1− x} Ca _x )₂MgSi₂O₇, in which Eu²⁺ and Dy³⁺ ions were incorporated. Phosphorescence with emission peaks at 470 and 480 nm that is due to the 4 f ⁶5 d –4 f ⁷ transition of Eu²⁺ ions was observed from samples GC-A and GC-B, respectively. The phosphorescence lasted for >5 h at room temperature.     

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.     

Photochemical Hole Burning and Local Structural Change in Sm2+-Doped Borate Glasses

Photochemical hole burning (PHB) not only can be applied for data storage systems but also serves as a powerful method for studying the local structure around optical centers. The present work investigated the effects of aluminum, magnesium, and silicon ions on hole burning and the phonon sideband for borate glasses that exhibit PHB at room temperature. Hole burning was measured for the ⁵ D ₀₋₇ F ₀ transition of Sm²⁺ and the phonon sideband spectrum for the ⁵ D ₀-⁷ F ₀ transition of Eu³⁺. The hole width was closely related to local structural change, especially as it seemed to decrease with decreases in the number of nonbridging oxygens produced around the rare-earth ions. In the case of sodium aluminoborate glasses, the hole width decreased considerably with increasing alumina content. The ratio Γ_ih/Γ_h for 85B₂O3·10Al₂O₃·5Na₂O·1Sm₂O₃ glass, then, was 80 at room temperature, the largest value ever reported.     

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.     

Spectroscopic Analysis of the Structure and Properties of Alkali Tellurite Glasses

Structural development of tellurite glasses with the addition of Li₂O and Na₂O has been studied using infrared, Raman, and X-ray photoelectron spectroscopies. The increase in intensity of the peak at 755 cm⁻¹ in the infrared spectra as compared to the peak at 620 cm ⁻¹ suggests the transformation of TeO₄ building units to TeO₃ pyramids with the addition of alkali oxide. Proposed structural change is further supported by the strong compositional dependence of the 755-cm⁻¹ peak in the Raman spectra as well as by the formation of a shoulder in the O 1 s peak of X-ray photoelectron spectra. In contrast to alkali silicate glasses, formation of nonbridging oxygens with the addition of alkali oxide is not observed.     

Synthesis and Photoluminescence of Eu2+-Doped α-Silicon Nitride Nanowires Coated with Thin BN Film

A feasible doping strategy is introduced to synthesize Eu²⁺-doped α-Si₃N₄ nanowires coated with a thin BN film. The nanowires were characterized by X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, and a fluorescence spectrophotometer. The Eu²⁺-doped α-Si₃N₄ nanowires emitted strong yellow light, which is related to the 4 f ⁶5 d –4 f ⁷ transition of Eu²⁺, upon a broad excitation wavelength range between 250 and 450 nm. The obtained nanowires provided a potential candidate for application in optical nanodevices, as well as in white LEDs.     

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.