Spectral analysis of a novel Eu2+ activated SiO2–BaO glass ceramics phosphor under UV-LED excitation

A novel Eu²⁺ activated 60SiO₂–40BaO (mol%) glass ceramics phosphor was prepared and the optical properties were investigated. X-ray diffraction (XRD) and Raman spectra confirmed the formation of Ba₂Si₃O₈ nano-crystals in the glass matrix. The Eu²⁺ activated glass ceramics exhibited broad emission band centered at 518 nm due to the 4f⁶5d¹→4f⁷ transition of Eu²⁺. Compared with the glass, the emission intensity of Eu²⁺ activated glass ceramics was much stronger, and the peak wavelength shifted toward shorter wavelength. The photoluminescence excitation (PLE) spectra of the glass ceramics showed an overlap with the main emission region of an ultraviolet light-emitting diode (UV-LED). According to the photoluminescence (PL) spectra, the CIE chromaticity coordinates of the Eu²⁺ activated glass and glass ceramics were calculated. The results indicated that the Eu²⁺ activated glass ceramics containing Ba₂Si₃O₈ nano-crystals can be used as a potential green emitting phosphor under UV-LED excitation.     

Recharging processes of Cr ions in Mg2SiO4 and Y3Al5O12 crystals under influence of annealing and γ ‐ irradiation

Recharging processes of chromium ions were investigated for Mg₂SiO₄:Mg, Cr single crystals using annealing in O₂ and in air and γ‐irradiation, as compare to YAG :Ca, Cr single crystals. The formation of tetravalent Cr ions in the Mg₂SiO₄ :Mg, Cr is related not only to the initial Cr content in the melt, oxygen partial pressure and O₂‐ vacancy existing in the crystal, but also to the external field such as γ‐irradiation. The additional absorption after γ‐irradiation shows the decrease in intensity of the absorption of Cr³⁺ and Cr⁴⁺ ions in some part of the spectrum and increase in the other giving evidence on recharging effects between Cr³⁺ and Cr⁴⁺. There arises also color centers observed between 380 nm and 570 nm that may participate in energy transfer of any excitation to Cr⁴⁺ giving rise to Cr⁴⁺ emission. Opposite to forsterite crystal, absorption spectrum of YAG:Ca, Cr crystal after γ‐irradiation reveals only increase in the absorption of the Cr bands. The observed behavior of the absorption spectrum of YAG:Ca, Cr crystal under influence of γ‐irradiation suggests that γ‐irradiation ionizes only Cr ions. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The effect of PbF2 content on the microstructure and upconversion luminescence of Er-doped SiO2-PbF2-PbO glass ceramics

The Er³⁺ doped transparent oxyfluoride glass ceramics were obtained by appropriate heat treatment of the precursor glasses with composition (mol%) 50SiO₂-xPbF₂-(50 − x)PbO-0.5ErF₃. The microstructure and optical properties of the glasses and glass ceramics were determined by differential scanning calorimetry (DSC), X-ray diffraction (XRD), absorption spectra and luminescence spectra. The intensity of upconversion luminescence significantly increased in glass ceramics compared to that in precursor glass. The emission bands centered around 660 nm (⁴F_9/2 → ⁴I_15/2) and 410 nm (²H_9/2 → ⁴I_15/2) were simultaneously observed in glass ceramics but cannot be seen in the corresponding precursor glass. The influence of different PbF₂ content on the microstructure and upconversion luminescence of the samples was analyzed in detail. The results indicated that with the increase of PbF₂ content, the Ω₂ was almost the same and the ratios of red to green upconversion luminescence decreased in glass ceramics.     

Spectroscopic properties of Cr3+ ions in nanocrystalline glass–ceramic composites

Luminescence properties of Cr³⁺ ions in a silica-based precursor glass, and in fabricated optically transparent glass-based nanocrystalline composites, have been investigated. The luminescence spectra of the precursor glass revealed a wide range of crystal fields and showed the ⁴T₂–⁴A₂ broadband emission of Cr³⁺ ions in a weak crystal field, combined with ²E–⁴A₂ emission characteristic for Cr³⁺ ions in a strong crystal field. Glass–ceramic nanocomposites, with gallium oxide nanocrystals nucleated in a host glass matrix, indicated the prevailing contribution of the crystal-like ²E–⁴A₂ emissions (R-lines) of Cr³⁺ ions in a strong crystal field. The low-temperature studies demonstrated that the fluorescence of Cr³⁺ ions could be altered from sharp R-lines of the ²E–⁴A₂ transition, below 70 K, to a combination of R-lines and their sidebands, above 70 K. Our results indicate that, in the developed glass–ceramic nanocomposites, most of the Cr³⁺ ions have migrated from the host glass matrix to the nucleated gallium oxide nanocrystalline phase.     

Spectroscopy and crystal-field analysis of Cr4+-doped transparent silicate glass-ceramics

We report transparent Cr⁴⁺-doped SiO₂–Al₂O₃–ZnO–Li₂O–K₂O glass-ceramics with broadband infrared luminescence. After heart-treatment, Li₂ZnSiO₄ crystallite was precipitated in the glasses, and its average size increased with increasing heat-treatment temperature. Racah parameters of Cr⁴⁺–Li₂ZnSiO₄ glass-ceramics have been calculated, and it was confirmed from absorption spectra that the energy levels of Cr⁴⁺-doped glass-ceramics are close to the cross point ¹E and ³T states. No infrared emission was detected in the as-made glass samples, but the broadband infrared emission centered at 1210 nm with the full width at half maximum (FWHM) of more than 250 nm was observed by exciting the glass-ceramics with excitation of an 808 nm laser diode. In order to analyze the located crystal field of Cr⁴⁺ ions, the emission spectra are fitted by multi-peak Gauss fitting. It is seen that the fluorescence spectra are fitted into two Gaussian bands at around 1195 and 1263 nm with band widths of 208 and 278 nm, respectively. The two Gaussian bands at around 1195 and 1263 nm have about the same decay rate, and hence they would probably originate from the same luminescent centers. The observed infrared emission could be attributed to Cr⁴⁺ ions at low-field sites in Li₂ZnSiO₄ glass-ceramics.     

Stimulated emission of Co2+-doped glass–ceramics

Pump–probe spectroscopy studies on cobalt-doped spinel-based aluminosilicate glass–ceramics of various compositions have been made in the visible part of the spectrum. Stimulated emission from the ⁴T₁(⁴P) state to the ⁴A₂ one of tetrahedral Co²⁺ ion has been observed for all glass–ceramic materials. The peak values of effective stimulated emission cross-sections for Co²⁺ ions are estimated to be 4–5 × 10⁻¹⁹ cm² depending on glass–ceramics composition and are found to be higher than in spinel single crystals.     

Vanadium and chromium redox behavior in borosilicate nuclear waste glasses

X-ray absorption spectroscopy (XAS) was used to characterize vanadium (V) and chromium (Cr) environments in low activity nuclear waste (LAW) glasses synthesized under a variety of redox conditions. V₂O₅ was added to the melt to improve sulfur incorporation from the waste; however, at sufficiently high concentrations, V increased melt foaming, which lowered melt processing rates. Foaming may be reduced by varying the redox conditions of the melt, while small amounts of Cr are added to reduce melter refractory corrosion. Three parent glasses were studied, where CO-CO₂ mixtures were bubbled through the corresponding melt for increasing time intervals so that a series of redox-adjusted-glasses was synthesized from each parent glass. XAS data indicated that V and Cr behaviors are significantly different in these glasses with respect to the cumulative gas bubbling times: V⁴⁺/V_total ranges from 8 to 35%, while Cr³⁺/Cr_total can range from 15 to 100% and even to population distributions including Cr²⁺. As Na-content decreased, V, and especially, Cr became more reduced, when comparing equivalent glasses within a series. The Na-poor glass series show possible redox coupling between V and Cr, where V⁴⁺ populations increase after initial bubbling, but as bubbling time increases, V⁴⁺ populations drop to near the level of the parent glass, while Cr becomes more reduced to the point of having increasing Cr²⁺ populations.     

Optical properties of Cr ion in lithium metasilicate Li2O · SiO2 transparent glass-ceramics

The optical properties of Cr³⁺ ions in lithium metasilicate (Li₂O · SiO₂) transparent glass-ceramics were investigated. The main crystalline phase precipitated was the lithium metasilicate (Li₂O · SiO₂) crystal. The percent crystallinity and crystalline size were ranging 65-75% and 20-35 nm, respectively. The color changes drastically to deep pink from emerald green upon crystallization. New and strong absorption bands appeared and the absorption intensity increases by about 10 times that in glass. These new absorption bands are found to be derived from Cr³⁺ ions in octahedral sites in the lithium metasilicate crystal lattice. Cr³⁺ ions substitute for three Li⁺ ions and occupy the distorted octahedral site between single [SiO₄]_n chains of lithium metasilicate crystal. The ligand field parameters can be estimated: 10Dq = 13 088 cm⁻¹, B = 453 cm⁻¹, Dq/B = 2.89 and C = 2036 cm⁻¹. The near-infrared luminescence centered at 1250 nm was not detected in the deep pink glass-ceramics unlike emerald green glass.     

Investigation of the crystallization features,atomic structure,and microstructure of chromium-doped monticellite

A series of Cr⁴⁺:CaMgSiO₄ single crystals is grown using floating zone melting, and their microstructure, composition, and crystal structure are investigated. It is shown that regions with inclusions of second phases, such as forsterite, akermanite, MgO, and Ca₄Mg₂Si₃O₁₂, can form over the length of the sample. The composition of the single-phase regions of the single crystals varies from the stoichiometric monticellite CaMgSiO₄ to the solid solution Ca_{(1 ? x)}Mg_{(1 + x)}SiO₄(x = 0.22). The Cr:(Ca_0.88Mg_0.12)MgSiO₄ crystal is studied using X-ray diffraction. It is revealed that, in this case, the olivine-like orthorhombic crystal lattice is distorted to the monoclinic lattice with the parameters a = 6.3574(5) Å, b = 4.8164(4) Å, c = 11.0387(8) Å, β = 90.30(1)^o, Z = 4, V = 337.98 ų, and space group P2₁/c. In the monoclinic lattice, the M(1) position of the initial olivine structure is split into two nonequivalent positions with the center of symmetry, which are occupied only by Mg²⁺ cations with the average length of the Mg-O bond R _av = 2.128 Å. The overstoichiometric Mg²⁺ cations partially replace Ca²⁺ cations (in the M(2) position of the orthorhombic prastructure) with the average bond length of 2.347 Å in the [(Ca,Mg)-O₆] octahedron. The average distance in SiO₄ distorted tetrahedra is 1.541 Å.     

Enhanced luminescent properties of Tb3+ ions in transparent glass ceramics containing BaGdF5 nanocrystals

Tb³⁺ doped transparent oxyfluoride glass ceramics containing BaGdF₅ nanocrystals were prepared. The transmission spectra, photoluminescence spectra, decay time and X-ray excited luminescence spectra of the Tb³⁺-doped glass and glass ceramics were investigated. Energy transfer from Gd³⁺ to Tb³⁺ has been observed in the glass and glass ceramics. The emission intensity of green band (⁵D₄ → ⁷F_J) of the Tb³⁺-doped glass ceramics is enhanced compared with that of the glass under ultraviolet and X-rays, which could be attributed to that the generation of BaGdF₅ nanocrystals with low phonon energy reduces the non-radiative transitions.     

Luminescence properties of Tb3+/Sm3+ co-doped 38B2O3―31Al2O3―31SrO glasses and glass ceramics

In this paper, Tb³⁺/Sm³⁺ co-doped 38B₂O₃―31Al₂O₃―31SrO glass was successfully prepared. After heat treatment, single crystal phase SrAl₂B₂O₇ was precipitated from the parent glass. DTA data showed the glass transition temperature at 625 °C and a sharp exothermic peak at 860 °C. XRD patterns demonstrated a regular evolution from glass to glass ceramics with higher treatment temperature and longer treatment time. From the XRD patterns, we supposed that Tb³⁺/Sm³⁺ ions can be most likely contained in the crystal phase. The photoluminescence spectra showed that the crystallization can enhance the emission intensity significantly and there could be an optimum crystallization degree to get the strongest luminescence in glass ceramics. The light scattering of devitrification sample can vary the intensity ratio of Sm³⁺ and Tb³⁺ emission. Therefore, as a potential route, rare earth ions doped glass ceramics could be a further research direction of luminescence glasses for white light emitting diodes application.     

Effects of yttrium codoping on fluorescence lifetimes of Er3+ ions in SiO2–Al2O3 sol–gel glasses

In order to find a new glass host and optimize erbium doping for IR glass optical amplifiers in photonic applications, a study on the optimization of the emission of erbium ions in the SiO₂–Al₂O₃ glass by codoping with Y₂O₃ is performed. It is first attempted to make a new sol–gel glass host based on SiO₂, Al₂O₃, and Y₂O₃ doped with Er³⁺ ions of the composition (1−x)SiO₂–xAl₂O₃–yY₂O₃:0.65Er₂O₃ (in mol%), x varies from 0 to 65, and y from 0 to 4. The optimal proportion in mol% of SiO₂ and Al₂O₃ for the Er³⁺ emission (at a fixed optimal concentration of 0.65) was 65 – 35. The effect of Y₂O₃ content on photoluminescence, decay curve profiles and lifetime of the ⁴I_13/2 level of Er³⁺ in SiO₂–Al₂O₃ glass is observed. The largest quantum efficiency and the higher emission intensity are observed in the sample with 65Al₂O₃ and 4Y₂O₃. The emission intensity at 1530 nm is two times higher than in glasses without Y₂O₃. A shift of 3 nm to shorter wavelengths is observed. The emission spectral profiles are flatter and broader for the glasses containing Al and Y (bandwidth of 59.5 nm). The decay curves show strong difference profiles for the different samples. The increase of the lifetime value τ (about ms) of the ⁴I_13/2 level of Er³⁺ in the SiO₂–Al₂O₃ with the Y₂O₃ is discussed.     

Eu2+/Dy3+ co-doped white light emission glass ceramics under UV light excitation

In the present work, transparent Eu²⁺/Dy³⁺ co-doped white light-emitting glass ceramics containing CaF₂ were successfully prepared under reducing atmosphere. Their luminescence properties have been studied by excitation and emission spectra. A combination of blue, yellow and red emissions has emerged in Eu²⁺/Dy³⁺ co-doped glass and glass ceramics, which allows the observation of bright white light when the samples are excited by the ultraviolet light. Energy transfer (ET) from Eu²⁺ to Dy³⁺ is discovered by directly observing the luminescence intensity of Dy³⁺ in the Eu²⁺/Dy³⁺ co-doped glass which is much stronger than that in the Dy³⁺-doped glass. ET from Eu²⁺ to Dy³⁺ in glass is further confirmed by fluorescence studies performed on the samples with various activator (Dy³⁺) concentrations. The color of luminescence could be adjusted by varying the proportions of europium and dysprosium. The optimal composition generates white light with chromaticity coordinates (0.296, 0.311). The results indicate that Eu²⁺/Dy³⁺ co-doped glass ceramics containing CaF₂ nano-crystals is a potential material for white LED.     

High pressure x-ray diffraction measurements on Mg2SiO4 glass

The structure factors of Mg₂SiO₄ glass have been measured using high energy x-ray diffraction up to pressures of 30.2 GPa, and the equation of state measured up to 12.8 GPa. The average Mg-O coordination numbers were extracted from the experimental pair distribution functions assuming two cases (i) there is no change in Si-O coordination number with pressure and (ii) the average Si-O coordination number increases the same as for pure SiO₂ glass. Both analyses give similar results and show a gradual increase in the average Mg-O coordination number from 5.0 at ambient pressure to ~ 6.6(6) at 30.2 GPa. There is good qualitative agreement between the experimental structure and equation of state data for the glass compared to several recent molecular dynamics simulations carried out on liquid Mg₂SiO₄.     

Fabrication of Sm2+-doped macroporous aluminosilicate glasses with high alumina content

Macroporous Al₂O₃–SiO₂ glasses doped with Sm²⁺ have been prepared from a sol–gel system containing aluminum sec-butoxide, tetramethoxysilane, samarium chloride hexahydrate, poly(ethylene oxide), nitric acid, and water. Monolithic gels having interconnected macropores and skeletons are formed by inducing the phase separation parallel to the gelation. The use of aluminum sec-butoxide preheated at 80 °C as the starting material enables the incorporation of Al³⁺ into the gel skeleton up to 20 mol% in cation ratio. The maximum amount of Al³⁺, i.e., 20 mol%, is twice as large as that reported in our previous study, where aluminum sec-butoxide was diluted with sec-butanol prior to the hydrolysis. Heat-treatment of Sm³⁺-doped 20AlO_3/2 · 80SiO₂ macroporous glass under the reducing atmosphere converts Sm³⁺ to Sm²⁺, which is confirmed by the appearance of intense emission peaks attributed to 4f–4f transitions of Sm²⁺.     

Synthesis and characterization of Cr4+-doped CaO–GeO2–Li2O–B2O3(Al2O3) transparent glass-ceramics

Synthesis and devitrification behavior of Cr-doped CaO–GeO₂–Li₂O–B₂O₃(Al₂O₃) glasses have been studied. A range of glass compositions was found to yield transparent glass-ceramics after devitrification. The size of crystallites is below 1 μm. Glass-ceramic samples exhibit 1050–1600 nm broad-band emission with a maximum around 1260 nm, very similar to the emission of Cr⁴⁺:Ca₂GeO₄ bulk crystals. X-ray diffraction measurements indicate that the structure of crystallites exhibiting near infrared emission in glass-ceramics may be assigned to Cr⁴⁺:Ca₂GeO₄ with increased lattice parameters.     

A square-wave voltammetric study on the diffusivities of polyvalent elements in CaO/BaO/Al2O3/SiO2 glass melts

Diffusion coefficients of various polyvalent ions (Sn²⁺, As³⁺, As⁵⁺, Sb³⁺, Sb⁵⁺, Cr³⁺, Ti⁴⁺, V⁴⁺, V⁵⁺ and Fe³⁺) were measured in melts with the basic compositions of 10CaO·10 BaO·10Al₂O₃·70SiO₂ and 10CaO·10BaO·15Al₂O₃·65SiO₂ by means of square-wave voltammetry. At temperatures in the range of 1300-1600 °C, linear correlations between logD and 1/T were observed. At 1400 °C, the diffusion coefficients obtained are compared with those obtained from other glass melt compositions.     

The effect of composition and temperature on the amount and type of nanoferrite particles inserted in Fe2O3–ZnO–MgO–SiO2 glass–ceramics

Glass–ceramics with the composition 2Fe₂O₃.1ZnO.1MgO.96SiO₂ [4ZnMgFe] and 2Fe₂O₃.2ZnO.3MgO.93SiO₂ [7ZnMgFe] (mol%) were prepared using the sol–gel method. X-ray diffraction (XRD), scanning electron microscopy (SEM), electron diffraction (ED) and Mössbauer spectroscopy (MS) were used to investigate the glass–ceramics structure. The samples contain ferrite nanoparticles embedded in a glass matrix. However, zinc ferrite nanoparticles seems to be the preferential crystalline phase formed. The amount of ferrite particles depends on treatment temperature and sample composition. The Mössbauer spectroscopy measurements show that ferrite nanoparticles can exhibit a ferrimagnetic behaviour combined with superparamagnetism.     

Structure analysis of MgSiO3 glass

The structure of MgSiO₃ glass was investigated by a combination of radial distribution function (RDF) analysis with an intensity comparison method on the basis of X-ray diffraction data. There appeared five peaks at 1.63, 2.08, 2.58, 3.25 and 4.2 Å in the RDF curve. The short range structure was found to be similar to that observed in pyroxenes: single chains composed of SiO₄ tetrahedra are linked by Mg²⁺ ions laterally. The Mg²⁺ ion is surrounded by six oxygen atoms on average, four of which form MgO pairs with an average distance of 2.08 Å and the remainder Mg O pairs of 2.5 Å which contribute to the formation of the third peak at 2.58 Å in the RDF curve together with the OO pairs (2.66 Å) in SiO₄ tetrahedra. The calculated intensity curve based on this model was in good agreement with the observed one.     

Optical and magnetic properties of first-row transition metal ions in lead-silicate glass

First-row transition metal ions in lead-silicate glasses of composition: 37.9% mol PbO, 61.8% mol SiO₂ have been studied with regard to the oxidation state and the coordination by means of optical and magnetic measurements. Optical spectra have been recorded at 300 and 77 K. All the observed bands are assigned as being due to d-d transitions in T_d and/or O_h fields. Magnetic moments are of the order of the spin-only values. By ESR measurements we reveal and quantify titanium, vanadium and manganese oxidation states not detectable by the other techniques.It turns out that the various metal ions have the following oxidation states: Ti³⁺ and Ti⁴⁺, V⁴⁺ and V⁵⁺, Cr³⁺ and Cr⁶⁺, Mn²⁺ and Mn³⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺. The coordination is 4-fold for Ti⁴⁺ and V⁵⁺ and 6-fold for Mn³⁺ and Cu²⁺. Moreover it has been found that Fe³⁺, Co²⁺ and Ni²⁺ are both 4-fold and 6-fold coordinated and that Cr³⁺ and Cr⁶⁺ oxidation states coexist in the same sample.