Persistent spectral hole burning studies in europium doped sodium silicate and borosilicate glasses

Spectroscopic studies confirmed the coexistence of Eu³⁺ and Eu²⁺ in sodium silicate and borosilicate glasses doped with europium. Persistent spectral hole burning was observed up to room temperature. Multiple hole burning was possible with moderate laser powers. When the glass composition was modified by adding 4–5% yttrium oxide, hole retention capability, hole burning efficiency and hole density also increased in both silicate and borosilicate glasses. Hole burning is due to a charge exchange between the Eu³⁺ and its environment. Certain defect centers, which were identified by electron spin resonance measurements, contributed to the formation of stable holes in the borosilicate glass.     

Intense red and cyan luminescence in europium doped silicate glasses

The photoluminescence properties of silicate glasses, singly doped with europium ions and co-doped with europium ions and Al powder, have been characterized by the absorption, excitation, and emission spectroscopies. The absorption edge in the absorption spectra shows a large red shift from 320 nm to 405 nm. Under near-ultraviolet (NUV) excitation, sample, doped with Eu³⁺ emit intense red light while those co-doped with Eu³⁺ and Al give intense cyan emission. The influences of the glass compositions on the spectroscopic properties are also discussed. These glasses may be potential candidates for light-emitting diodes (LEDs).     

Sodium-hydrogen interdiffusion in sodium silicate glasses

The hydration of Na₂O · 3 SiO₂ glasses is studied using the sputter-induced photon spectrometry technique (SIPS) to measure the depth-profiles of hydrogen and sodium. The results clearly show a penetration of hydrogen and a leaching of sodium in the glass. The thicknesses of the hydrated layers are determined as functions of time and temperature. The thicknes dependence on the square root of time indicates that a diffusion process is in operation. The data are fitted to he Doremus model of interdiffusing ions and the diffusion coefficients for hydrogen and sodium are calculated.     

Optical hole burning studies in europium doped oxide glasses

Europium doped sodium borate, silicate, borosilicate, germanate and tellurite glasses have been prepared by melt quenching technique. Optical spectroscopy techniques were used to characterize the divalent and trivalent ions. Optical hole burning technique was used to characterize rare-earth ion doped glasses for optical data storage. The hole burning mechanisms, which depend on the glass composition and the preparation methods used, are discussed.     

Large-scale simulations of sodium silicate glasses

Half million and million atom systems of sodium silicate glass have been simulated using classical molecular dynamics, and their structures have been examined. Although the structural features are reasonable for the intended glasses, and have a lower configurational energy than smaller systems, they contain features which are indicative of an effective internal, or configurational, temperature higher than the target temperature. This observation is ascribed to the use of a thermostat which controls only the kinetic energy, or momenta, of the particles, and because of the non-equilibrium nature of the simulations, there is no requirement that the potential, or configurational, energy and kinetic energy be coupled. The use of alternative thermostats is suggested.     

Radiation damage of silicate glasses doped with Tb and Eu

In this work, we studied a set of Tb³⁺ (or Eu³⁺) doped silicate glasses in which some amounts of BaO were added to increase glass density. The irradiation-induced damage was investigated by absorbance measurements performed before and after each irradiation with doses ranging from 3 to 237 Gy. Analysed glasses underwent also light yield measurements investigated in terms of light production. The results showed that radiation damage and light yield depend on glass composition and are very low for the Eu³⁺ containing glass and for the Tb³⁺ activated glass which contains also lead. A possible explanation could be that lead and europium favour in the glass matrix the formation of a higher concentration of defects with respect to Tb³⁺ doping ions.     

Raman spectroscopic study of scandium in sodium silicate glasses

Glasses in the Na₂OSiO₂Sc₂O₃ system have been studied by Raman and difference Raman spectroscopy. Addition of Sc₂O₃ to sodium silicate glasses results in new vibrational bands at 1025 cm^?1 and 360 cm^?1. The high frequency band is interpreted to be due to Sc⁺³ quasi-complexes formed by Sc⁺³ ions coordinated by SiO₄^?4 tetrahedra having non-bridging oxygens. The discrete character of the scandium-produced bands implies incipient separation of Sc⁺³-enriched silicate structures from purely silicate structures.     

XPS and magnetization studies of cobalt sodium silicate glasses

Cobalt sodium silicate glasses with the chemical composition (0.70 - x)SiO₂-(0.30)Na₂O-xCoO, where 0.0 ≤ x ≤ 0.20, have been investigated by means of X-ray photoelectron spectroscopy (XPS) and magnetization techniques. The Co 2p spectra show intense satellite structures 6 eV above the photoelectron peak and the Co separation was 15.9 eV for all the samples studied. These observations indicate the presence of high spin Co²⁺ ions in the glasses. The Co 3p spectra have been fitted with contributions from octahedral and tetrahedral Co²⁺ and the ratio [Co²⁺(oct)]/[Co_Total²⁺] increases with increasing CoO content. The O Is spectra also show composition-dependent changes. The fraction of non-bridging oxygen atoms was determined from these spectra and was found to increase with increasing cobalt oxide. Co(II) ions are found to be incorporated in the glass as network modifiers but the contribution from SiOCo(II) to the non-bridging part of the O Is signal could be separated from that from SiONa by simulating the spectrum. DC magnetic susceptibility measurements were performed on the same samples but these suggest that Co²⁺ exists mainly in octahedral coordination. The magnetic data indicate that the exchange interaction is antiferromagnetic and increases with increasing CoO in the glass.     

Dielectric study and ac conductivity of iron sodium silicate glasses

A series of glasses with formula (SiO₂)_0.7−x(Na₂O)_0.3(Fe₂O₃)_x with ( 0.0 ≤ x ≤ 0.20) were prepared and studied by means of AC measurements in the frequency range 20 kHz to 13 MHz at room temperature. The study of frequency dependence of both dielectric constant ε' and dielectric loss ε" showed a decrease of both quantities with increasing frequency. The results have been explained on the basis of frequency assistance of electron hopping besides electron polarization. From the Cole-Cole diagram the values of the static dielectric constant ε_s, infinity dielectric constant ε_∞, macroscopic time constant τ, and molecular time constant τ_m are calculated for the studied amorphous samples. The frequency dependence of the ac conductivity obeys a power relation, that is σ_ac (ω) = Α ω^s. The obtained values of the constant s lie in the range of 0.7 ≤ s ≤ 1 in agreement with the theoretical value which confirms the simple quantum mechanical tunneling (QMT) model. The increase in ac conductivity with iron concentration is likely to arise due to structural changes occurring in the glass network. The structure of a glass with similar composition was published and showed clustering of Fe²⁺ and Fe³⁺ ions which favor electron hopping and provide pathways for charge transport.     

Crystal-field spectroscopy of Eu3+ doped silica glasses

Fourier transform absorption spectroscopy in the 500–6000 cm^? 1 and 9–300 K ranges is applied to monitor the effects produced by Eu³⁺ incorporation into sol–gel silica samples doped with concentration increasing from 0.001 to 10 mol%. The aim is to investigate the formation of aggregates by exploiting the Eu³⁺ crystal-field transitions. Complementary microreflectance and Raman spectra are also measured in the range of silica intrinsic vibrational modes to confirm the hypothesis of matrix modification induced by increasing doping levels. Evidences of clustering are found for high Eu³⁺ concentrations. Up to 3 mol% the crystal-field line intensities gradually increase and the OH^? content smoothly decreases. A further increase to 10 mol% causes drastic, remarkable changes, i.e. sharp crystal-field lines appear which narrow by lowering the temperature. Furthermore, the OH^? related bands are no longer detectable. For concentrations up to 3 mol% the aggregates are amorphous as the silica matrix, while for the Eu³⁺ 10 mol% sample they show a rather ordered structure.     

Thermal properties of sodium silicate glasses at low temperatures

The heat capacity and the thermal expansion coefficient have been measured at low temperatures for vitreous silica containing 0, 10, 20, 30 and 40% soda. The heat capacity per gram is not much affected at 4 K, but is reduced by up to 30% at 10 K by addition of soda. The effect on the expansion coefficient is much greater; the negative values shown by pure silica below 150 K largely disappear with the addition of {10% soda. Below 10 K the coefficient remains negative until 25% soda is added. The Grüneisen function γ(T) is calculated for the various compositions.     

Thulium environment in a silica doped optical fibre

Thulium-doped optical fibre amplifiers (TDFA) are developed to extend the optical telecommunication wavelength division multiplexing (WDM) bandwidth in the so-called S-band (1460–1530 nm). The radiative transition at 1.47 μm (³H₄ → ³F₄) competes with a non-radiative multi-phonon de-excitation (³H₄ → ³H₅). The quantum efficiency of the transition of interest is then highly affected by the phonon energy (E_p) of the material. For reliability reasons, oxide glasses are preferred but suffer from high phonon energy. In the case of silica glass, E_p is around 1100 cm^?1 and quantum efficiency is as low as 2%. To improve it, phonon energy in the thulium environment must be lowered. For that reason, aluminium is added and we explore three different core compositions: pure silica, and silica slightly modified with germanium or phosphorus. The role of aluminium is studied through fluorescence decay curves, fitted according to the continuous function decay analysis. From this analysis, modification of the thulium local environment due to aluminium is evidenced.     

Formation of fluorescent aggregates in Rhodamine 6G doped silica glasses

The photoluminescence properties of type I sol–gel synthesized Rhodamine 6G doped silica samples were investigated in the 1.5 × 10⁻⁴–1.5 × 10⁻³ mol/l range of concentration by means of steady-state and time resolved photoluminescence measurements. The emission peak red shifts as the concentration increases. For a fixed concentration, the peak position also red shifts as the excitation wavelength decreases. The observed spectroscopic features, including excitation of photoluminescence and decay time, indicate the formation of fluorescent dimers.     

A study of optical absorption in some sodium titanium silicate glasses

A series of glass specimens was prepared from mixtures of SiO₂, TiO₂ and Na₂O, and their optical absorption coefficients measured as functions of photon energy in the range 2.1–3.3 eV. From the results, values of the optical energy gap are calculated and found to be dependent on the glass compositions. The results are analysed in terms of a mechanism of optical absorption arising from forbidden indirect transitions.     

Strontium environment transition in tin silicate glasses by neutron and X-ray diffraction

The effect of Sr modifier atoms on the structure of stannosilicate glasses of composition (Sr0)_x(SnO)_0.5−x(SiO₂)_0.5, with 0 ⩽ x ⩽ 0.15, has been studied using Mössbauer spectroscopy and neutron and X-ray diffraction. The tin is mostly in the Sn²⁺ state. The Sr–O bond length undergoes a step decrease from (2.640 ± 0.005) Å to (2.585 ± 0.005) Å as x increases from 0.10 to 0.15, indicating a decrease in co-ordination number from 8 to 7. A Sn–Sn distance of 3.507 ± 0.005 Å is revealed by a first-order difference calculation from the x = 0 sample. This is too short to be consistent with significant edge sharing of [SnO₃] trigonal pyramids.     

Properties and structure of Yb3+ doped zinc aluminum silicate phosphate glasses

This study explores a series of optical, thermal, and structural properties based on 60P₂O₅–30ZnO–10Al₂O₃ (60P) glasses that doped with varied rare earth (RE) elements Yb₂O₃ and P₂O₅ components replaced by SiO₂. It was found that the glasses density decrease with SiO₂ concentration added to replace P₂O₅, whereas they increase with increased concentration of Yb³⁺-doped. Moreover, the glasses transition temperature, softening temperature, and refractive index increase with Yb³⁺ concentrations added, whereas the thermal expansion coefficient decreases. For the 60P glasses, 7 mol% Yb₂O₃ doped has the maximum fluorescence which is suppressed when Yb₂O₃ is doped up to 9 mol%. In addition, maximum lifetime was found to be 2.68 ms at an optimal Yb³⁺-doping at 1 mol% for 53P₂O₅–7SiO₂–30ZnO–10Al₂O₃ glass.     

The local environment of trivalent lanthanide ions in sodium silicate glasses: A neutron diffraction study using isotopic substitution

Neutron diffraction data on lanthanide-bearing sodium silicate glasses were collected using the glass, liquid and amorphous materials diffractometer (GLAD) at the intense pulsed neutron source (IPNS), Argonne. Measurements were made on four glass samples; a sodium silicate base glass with no added lanthanide, a sample with 6 mol% La₂O₃ added to the base composition, and two isotopically-distinct samples containing 6 mol% Dy₂O₃. Of the Dy-bearing samples, one contained natural enrichment Dy, and the other a ‘null scattering’ mixture of natural and ¹⁶²Dy isotopes. The first-order isotope difference of the scattering from the Dy-bearing samples revealed a Dy–O nearest-neighbor distance of 2.3 Å and a mean coordination number of 5.9 oxygen atoms around each dysprosium ion; the latter is in good agreement with the coordination number derived from bond–valence theory. The results for the La-bearing glass were also consistent with a sixfold coordination of oxygen atoms around the rare earth. The diffraction data are used in combination with Reverse Monte Carlo modeling techniques to interpreter the structural role of the rare earth ions. From these models, it is apparent that there is competition between monovalent sodium ions and the rare earth ions for the same non-bridging oxygen atoms. As a result the addition of rare earth ions appears to cause disruption of the Na-rich percolation domains characteristic of the sodium silicate base glass. These neutron results are consistent with the formation of a specific lanthanide Q³ as suggested by NMR and Raman spectroscopy, but offer no direct evidence for the formation of ‘oxide-like’ lanthanide clusters.     

Cation clustering and formation of free oxide ions in sodium and potassium lanthanum silicate glasses: nuclear magnetic resonance and Raman spectroscopic findings

Alkali silicate glasses containing lanthanum oxide are useful model systems for understanding the structural role of rare earth cations in optical and other types of materials. We report ²⁹Si and Raman spectra of sodium and potassium silicate glasses, both with added La₂O₃ and with La₂O₃ substituted for Na₂O or K₂O on an equal-oxygen basis. In the former series, silicate speciation changes show the formation of more non-bridging oxygens (NBO) as more of the network-modifying La₂O₃ is added. In the latter series, however, in which the nominal ratio of NBO to Si is constant, silicate speciation changes indicate that the actual ratio decreases significantly as La is substituted for 3 Na or K. The simplest explanation of this finding is that up to several percent of the oxygen in the La-rich glasses is not bonded to any Si, but instead forms `free oxide' ions that are part of La-rich domains. Although the size of these domains remains unconstrained, the lack of evidence for phase separation and continuity of trends in structure with composition suggests that the metastable liquid structure at the glass transition contains substantial intermediate-range heterogeneity.