Evidence of structural anisotropies in silicate glass fibres by ESR

The ESR spectra of Cr³⁺ and Cr⁵⁺ in soda-lime-silicate glass fibres were investigated with respect to defined parameters of the fibre drawing process from a nozzle. The following results concerning the structure of the fibres were obtained: Although the short-range order of the Cr³⁺ and Cr⁵⁺ ions is identical in the structure of the bulk glass as well as in the fibre glass, changes of the covalent portion of the bonds between the Cr⁵⁺ ion and its oxygen ligands were observed. The fibre glass structure is a frozen-in deformed or orientated (this depends on definition) silicate glass network, which influences the bonds in the square bipyramidal [Cr⁵⁺O₆] ⁷⁻ complexes in the following manner: the σ-bonds between Cr⁵⁺ and the four oxygens in the square plane of the bipyramid are more covalent in the fibre than in the bulk glass. They are also orientation-dependent: more covalent for those of the randomly distributed complexes with their c-axes perpendicular to the fibre axis than parallel to it. The π-bond of the CrO³⁺ oxygen is less covalent in the fibre than in the bulk glass and is also orientation-dependent: more covalent for those complexes with their c-axes parallel to the fibre axis than perpendicular to it.

The ability of the glass network to form [Cr⁵⁺O₆] ⁷⁻ or CrO³⁺ complexes increases with the following increasing parameters of the fibre drawing process: melting temperature, oxygen partial pressure, mean cooling rate, and tensile stress. This fact is primarily a consequence of the shift of the redox-equilibrium and of the difficulty for the structural change from an octahedral symmetry of the Cr³⁺ and Cr⁵⁺ to a tetrahedral symmetry of the Cr⁶⁺ and vice versa during the fibre drawing process. The [Cr⁵⁺O₆] ⁷⁻ complex is highly unstable and is an intermediate state.     

The role of spin-spin interaction in the electrical conductivity of vanadium phosphate glasses

Representative glass series of different vanadium content and valence state have been studied with special emphasis on V⁴⁺−V⁴⁺ spin-spin interaction. From temperature dependent EPR studies paramagnetic Curie temperatures and J values have been determined. We demonstrate that this method is informative in the whole glass formation range. It is shown that there is an antiferromagnetic → ferromagnetic transition at c = V⁴⁺/(V⁴⁺ + V⁵⁺) 0.17 (2.3×10²¹ spin cm⁻³). At this composition the glass is essentially homogeneous and displays maximal conductivity. In glasses of higher V⁴⁺ content, in which the dominating interaction is ferromagnetic, the conductivity gradually decreases. Therefore, ferromagnetically coupled V⁴⁺ − V⁴⁺ pairs seem to contribute to conductivity less effectively.     

Migration of Cu ions in Cu2O---Al2O3---SiO2 glasses on heating in air

The behavior of copper ions in the Cu₂O·Al₂O₃·4SiO₂ (in moles) glass on heating in air at temperatures up to 500°C was studied. When the glass, in which about 90% of Cu was present as Cu⁺ ions, was heated in air above 300°C, a CuO layer was formed on the surface. The amount of CuO was increased with heating temperature and time, corresponding to the decrease in weight of the glass. Furthermore, the fraction of Cu²⁺ ions in the glass increased. These observations suggest that oxygens do not diffuse into the glass, but Cu⁺ ions migrate to the surface from the interior to balance the surplus positive charge produced by the oxidation of Cu⁺ to Cu²⁺ ions inside the glass. The following reaction scheme for the formation of the CuO layer was proposed; 2Cu⁺(interior) + ₂¹O₂(surface) → Cu²⁺(interior) + CuO(surface).     

Compositional dependence of the valency state of Cr ions in oxide glasses

Absorption spectra of Cr ions in multi-component oxide glasses melted in Ar were measured. The integrated extinction coefficient is used as a parameter to analyze the redox reaction of Cr³⁺ and Cr⁶⁺ in glass samples. The relative content of Cr⁶⁺ increased and that of Cr³⁺ decreased with increasing basicity in silicate and borate glasses; this result is consistent with previous studies. On the contrary, only Cr³⁺ exist in phosphate glasses. Cr⁴⁺ ions were found only in aluminate, gallate and alumino-silicate glasses with modifier contents ≥ 60 mol%. The relationship between point defects and Cr⁴⁺ formation is examined on the basis of electron spin resonance measurements. We concluded that the superoxide ion radical (O₂⁻) and the peroxy bonding (-O-O-) oxidize Cr³⁺ to Cr⁴⁺ in aluminate, gallate, and alumino-silicate glasses.     

Spectroscopy of transition metal ions in inorganic glasses

Transition metal (TM) ions have been used as colouring agents in the glass industry for a long time. Recently, great attention has been paid to the TM ion doped glasses for the development of new lasers or luminescence materials. The absorption spectra of TM ions in different kinds of glasses have been studied extensively, but little work has been done for fluorescence and relaxation spectra. In this paper emphasis is laid on analysing the influence of chemical bond characteristics of the base glass on the spectra and the site structure of transition metal ions in glasses. Recent experimental results about the luminescence characteristics of low valence ions (such as Ti³⁺, Cr³⁺, V²⁺, Mn²⁺, Cu⁺, Mo³⁺) in glasses are also reported.     

Dynamical properties of an alkali chloride mixture in the glassy and liquid states

Molecular dynamics calculations have been carried out to reveal the dynamical properties of an alkali chloride mixture (50 mol% LiCl---40KCl---10CsCl) and pure LiCl in the liquid and glassy states. It is found that the mobility of Li⁺ ions in the mixture is significantly lower than that in LiCl, not only in liquid state but also in the glassy state. The partial dynamic structure factors, S_Li---Li (Q, ω) and S_{Cl---Cl (Q, ω)}, are evaluated to investigate the dynamical correlation between the Li⁺ and Cl⁻ ions. In LiCl glass, it is shown that the motions of the Li⁺ and Cl⁻ ions are highly correlated with each other, and both the acoustic and optic modes are spatially delocalized (Q < 1.8 Å⁻¹). On the other hand, in the mixture glass, the motion of the Li⁺ ions is almost independent of that of the Cl⁻ ions, and at small wavelengths there are the intense vibrations localized on single Li⁺ ions similar to an Einstein oscillation.     

Effect of additives in the salt bath on glass strengthening

In this paper, we mainly study the effects of anion or anion group additives (such as the monovalent anions OH⁻, Cl⁻ or NO₃⁻, the divalent anion SO₄⁻, and the trivalent anion PO₄³⁻, respectively) in a salt bath of KNO₃ on glass strengthening. The Na₂O---Al₂O₃---SiO₂ system is selected to be the base glass.     

Structural distribution and rigidity of the glass network determined by EPR of Cu

The linewidth-broadening of the EPR spectra of Cu²⁺ in silicate, borate and phosphate glasses was analyzed in terms of the distribution of g_| and A_| (δ_| and δA_|) and related to the distribution of the rigidity of the network structure. X- and K-band spectra were measured for the glasses doped with ⁶³Cu²⁺ (93% abundance). The linewidth of the HFS shoulders with parallel orientation to H increased linearly with increasing m or microwave frequency. δg_| and δA_| showed a marked dependence on glass composition. For example, in Na₂O---B₂O₃ glasses, on going from x (mol% of Na₂O) being small through intermediate to large, δg_| varied from small through large to negligibly small. In contrast to these glasses δg_| was extremely large for 75PbO · 25B₂O₃ glass. The large δg_| for the Na₂O---B₂O₃ glassesof intermediate x was attributed to the coexistence of various borate groups competitively coordinating to Cu²⁺. Negligibly small δg_| for 70Na₂O · 30B₂O₃ glass and extremely large δg_| for 75PbO ·25B₂O₃ glass, both with a narrower structural distribution, reflect regidity of the glass network. The Pb---O bonding is strong enough to distort the coordination of Cu²⁺-complex. The situation is the reverse in Na₂O---B₂O₃ glasses.     

Study of PbO films on sodium borate glasses part II: Formation of interdiffusion layers at the glass surface

By selecting carefully controlled conditions for the thermal treatment of sodium borate glasses coated with PbO films, it is possible to prevent Pb²⁺ ions from penetrating deeply into the glass. For low alkali glasses, an interdiffusion layer can be formed, which sub-surface cation profiling (by ion beam induced radiation) shows is a solid solution of Na₂O and B₂O₃ in PbO which acts as solvent. Experiments with 18.0 mol-% Na₂O glass show that it is possible to transform such an interdiffusion layer into a second type in which Na₂O and B₂O₃ together act as solvent and the concentration of PbO solute varies through the layer. For both types of layer the Na₂O : B₂O₃ ratio is different from that of the glass substrate, and for the production of the second type of layer it is shown that an important factor connected with penetration of PbO into the glass surface is a “sweating” process in which thermal treatment of the glass, even in the absence of PbO, results in migration of Na⁺ ions so that the glass surface has a Na₂O content higher than that of the bulk glass. Changes in the UV spectra of the Pb²⁺ ion are correlated with the formation of the interdiffusion layers, and results show that types of layer have optical basicities significantly greater than that of the glass substrate, through either the high PbO or high Na₂O content.     

An estimation of the distribution functions of doped Tb and Nd in glasses by fluorescence measurements

The distribution functions of doped Tb³⁺ and Nd³⁺ in silicate, germanate, borate and phosphate glasses, in which cross-relaxation among active ions plays an important role, have been estimated by fluorescence measurements. In order to analyze both the steady state and the transient fluorescence characteristics, a non-linear model for the resonant energy transfer has recently been developed. The experimental fluorescence intensity and decay curves, measured as a function of acceptor concentration, suggested a modified distribution function, which included the effect of the segregation of donors and acceptors. The number of segregated Tb³⁺ ions at the second nearest neighbor position around a Tb³⁺ ion was estimated to be 0.1–3.4, depending on the host glass.     

The effect of mixed alkaline earths on the diffusivity and the incorporation of iron in 5Na2O · xMgO · (15−x)CaO · yAl2O3 · (80−y)SiO2 melts

Diffusion coefficients of iron were measured in glass melts with the basic compositions 5Na₂O · xMgO · (15−x)CaO · yAl₂O₃ · (80−y)SiO₂ with x=5, 10 and y=0, 5, 7.5, 10 and 15. The melts were doped with 0.25 mol% Fe₂O₃ and studied in the temperature range from 1000 to 1600 °C using square-wave voltammetry. The voltammograms exhibited distinct peaks attributed to the reduction of Fe³⁺ to Fe²⁺, from which peak currents mixed diffusion coefficients of iron were calculated. Diffusion coefficients in all melt compositions which did not show crystallization could be fitted to Arrhenius equation. The diffusivities measured in different melt compositions were related to the same viscosity, i.e. not the same temperature. Increasing the alumina concentration from 5 to 10 mol% resulted in an increase of the viscosity corrected diffusivities. At further increasing alumina concentrations, the diffusivities get smaller again. This can be explained by the stabilizing effect of Na⁺ and Ca²⁺ on FeO⁻₄ and AlO⁻₄-tetrahedra, which strengthens the incorporation of Fe³⁺ into the glass structure.     

The vanadyl ion as a spectroscopic probe:: Measurement of optical basicity in the visible / near-infrared region

Previously optical basicity was always measured using s-p absorption spectra of Tl⁺, Pb²⁺ and Bi³⁺, but since these spectra are in the UV region, media containing oxides of transition metal aons have been excluded from such measurements. The present work shows that for VO²⁺ the d-d transition ²B₂ → ²E red-shifts with increasing optical basicity, and a study in the Na₂O---P₂O₅ glass system allows comparison with results obtained previously with Tl⁺ and other probe ions. It is shown that under controlled conditions determination of optical basicity with VO²⁺ is viable, and since the ²B₂ → ²E spectral band occurs in the visible / near-IR region, UV transparency of the medium is no longer necessary. The VO²⁺ probe therefore offers a means of obtaining optical basicities of glasses containing e.g. PbO, or of metallurgica slags containing e.g. Fe₂O₃ or MnO.     

Effect of heating and cooling atmospheres on colors of glaze and glass containing copper

ESR and ESCA analyses of a copper red glaze prepared by firing under a strongly reducing atmosphere followed by oxidation during a cooling period showed that Cu¹⁺ and Cu₂O were major species in it. From the investigation of behavior and chemical states of copper and tin under various firing conditions, mechanisms of the formation of Cu₂O and its protection were assumed as follows: Cu²⁺ and Sn⁴⁺ are reduced to metals during reducing firing and then the metals aggregate in the glaze surface layer, forming alloy. In the next oxidation process, metallic copper and tin are oxidized to Cu₂O or Cu¹⁺ and SnO respectively. Due to the affinity of SnO on Cu₂O for glass, glass containing Sn²⁺ surrounds and consequently protects Cu₂O from excessive oxidation.

A colorless copper glass before striking contained copper mainly as Cu¹⁺. ESCA analysis showed that striking of the glass caused the selective formation of Cu₂O.     

Homogeneous line width in a zinc borate glass activated by Eu

In this work we report on the temperature depedence of the homogeneous line width (Γ_h) in 4ZnO · 3B₂O₃:Eu³⁺ glass in the region between 30 K and room temperature. The time resolved resonant fluorescence line narrowing technique was applied to the ⁷F₀ ↔ ⁵D₀ transition of Eu³⁺. We obtain that Γ_h follows the commonly found behaviour T² in the whole range of temperature (30–300 K). The Γ_h is comparable to that measured in borate glasses with a small amount of network modifier. This result, together with a linear increase of Γ_h with the excitation energy within the inhomogeneous profile, is indicative of a specific local environment around the Eu³⁺ ion in the zinc borate glass.     

Enhanced Eu emission by non-radiative energy transfer from Tb in zinc phosphate glass

Energy transfer studies have been made in terbium-europium-doped zinc phosphate glass at room temperature. Enhancement of the Eu³⁺ (⁵D₀→⁷F manifold) emission and a decrease in the Tb³⁺ emission (⁵D₄→⁷F manifold) have been observed as a result of energy transfer from Tb³⁺ ions to europium ions. The energy transfer mechanisms have been found to be mainly electric dipole-dipole in nature. The energy transfer efficiencies, transfer probabilities and average donor-acceptor distances have been calculated.     

Diluted and non-diluted ferric ions in borate glasses studied by electron paramagnetic resonance

Electron paramagnetic resonance (EPR) spectra of lithium borate glass (1 - x)(0.63B₂O₃ · 0.37Li₂O) · xFe₂O₃, with x varying from 0.001 to 0.1, were measured at different microwave frequencies and temperatures. For low Fe³⁺ concentrations (Fe₂O₃ molar contents from 0.001 to 0.01), X-band EPR spectra, consisting of a g_{ef = 4.3} peak accompanied by a shoulder continuing down to g_{ef = 9.7}, are computer simulated on the basis of a ‘rhombic’ spin-Hamiltonian with Zeeman and fine-structure terms. A good fit to the experimental spectra for various Fe₂O₃ contents is observed with the same values of the spin-Hamiltonian parameters and assuming a Lorentzian lineshape and a linewidth increasing linealry with the concentration of Fe³⁺ ions. It is concluded that the spectrum is due to diluted Fe³⁺ ions in a relatively strong crystal field of orthorhombic symmetry, with largely distributed fine-structure parameters. From the concentration dependence of the line width, by extending to glasses a theoretical EPR linewidth expression derived for polycrystalline systems, the minimum distance between diluted Fe³⁺ ions is estimated as 4.9 Å. A diluted state of Fe³⁺ ions in the glass network in this range is also confirmed by the temperature dependence of the g_ef = 4.3 resonance which follows a Curie law. For intermediate concentrations of Fe³⁺ ions (Fe₂O₃ molar contents from 0.01 to 0.1), the width of the g_ef = 4.3 line is proportional to the square root of concentration, still indicating dipolar interactions. On the other hand, the microwave frequency dependence of a broad g_ef ≈ 2 line, which coexists at these concentrations with the g_ef = 4.3 line, shows that the former line is due to pairs or small clusters of exchange-coupled Fe³⁺ ions. The temperature dependence of the g_ef ≈ 2 line intensity in 0.1 mol Fe₂O₃ glass is consistent with a more antiferromagnetic character by comparison with the 0.05 mol Fe₂O₃ glass, which is attributed to an appearance, at higher Fe₂O₃ contents, of iron-containing microclusters not incorporated in the random glass network, with smaller distances between the paramagnetic ions. These microcluster are probably the origin of a new narrow line superposed with the broad g_ef ≈ 2 line in the low-temperature EPR spectra.     

Changes in the refractive index of fused silica due to implantation of transition-metal ions

The changes in the refractive index of high purity silica glass produced by implantation of Ti⁺, Cr⁺, Mn⁺, Fe⁺ and Cu⁺ ions at 160 keV were measured using ellipsometry at a wavelength of 633 nm. Implantation doses ranged from 10¹⁵ to 6 × 10¹⁶ ions cm⁻². At the highest doses, a relative increase in refractive index of approximately 15% for implanted Cu ions and an increase of about half that for implanted Cr, Mn and Fe ions are observed. These variations in index probably result from colloid formation. For implanted Ti, virtually no change in the measured index of refraction was observed even at the highest doses.     

Chalcogenide glass electrodes sensitive to heavy metal ions

The selective response to Hg²⁺, Cd²⁺ and Cu²⁺ ions was studied for HgS- and CdS---Ge_0.20Te_0.30Se_0.50 and Cu₂Se---As_0.40Se_0.60 glass electrodes. These glass electrodes, containing a suitable amount of the above heavy metals, showed the Nernstian response to the corresponding metal ions. Some metal ions, however, interfered with the selective response. The selectivity of these glass electrodes was found to be connected with the relative position of the Fermi levels of the chalcogenide glasses and redox couples for metal ions.     

EXAFS and XANES joint analyses for semiconducting vanadium phosphate glasses

Vanadium EXAFS spectra of 50V₂O₅-50P₂O₅ glasses with different C = V⁺⁴/V_tot have been measured. The V-O distances increase by ΔR₁ = (0.03 ± 0.02) Å to ΔR₂ = (0.07 ± 0.03) Å going from a glass with C = 0.64 to C = 0.84 and from C = 0.51 to C = 0.84, respectively. The EXAFS data show a basically similar structure of the vanadium sites for both the V⁴⁺ and V⁵⁺ ionic states. The density of the glasses increases with C from the value d₁ = 2.81 g/cm³ for C = 0.51 to d₂ = 2.92 g/cm³ for C = 0.84 indicating a more random packing of glass units.     

Laser precipitation of SnO2 nanocrystals in glass and energy transferred-fluorescence of Eu ions

SnO₂ nanoparticles having an average particle size of 5 nm were precipitated in a transparent glass by irradiation with an 800 nm femtosecond laser. Glass was prepared to co-dope Sn⁴⁺ and Eu³⁺ ions by a sol–gel method, followed by heating in H₂ gas atmosphere to form the Sn⁰₂ point defects with a molecular-like electronic structure in the twofold-coordinated Sn atoms. Upon laser irradiation, the Sn⁰₂ defects react with oxygen, forming SnO₂ nanocrystals. When excited at the energy corresponding to the absorption edge of the SnO₂ nanocrystals, the energies, which are absorbed by the quantum-dot effect in the nanosized crystals, are efficiently transferred to the Eu³⁺ ions, resulting in the large enhanced fluorescence intensities from the Eu³⁺ ions. The observed phenomenon suggests application to high-density memory devices.