Spectroscopic evidence for defect structure in irradiated PbWO4 single crystal

PbWO₄ has recently attracted much attention because of its scintillation applications. We have measured its dielectric response subsequent to Ar⁺ and UV irradiation, respectively. Dielectric relaxation was observed, which we attribute to a complex color center in the WO₄⁻² tetrahedron. The complex color center in turn led to the distortion of the WO₄⁻² tetrahedron, which was verified by FTIR spectra showing that the v₃ and v₄ degeneracies were removed.     

B NMR investigations in xV2O5–B2O3 and xV2O5–B2O3–PbO glasses

11B (I=3/2) MAS NMR in the binary glass system xV₂O₅–B₂O₃ (x=0.053, 0.43) and the ternary glass system xV₂O₅–B₂O₃–PbO (0.1x1.5) has been investigated at room temperature. In the xV₂O₅–B₂O₃ glasses, one NMR line due to BO₃ unit was observed. Meanwhile in the xV₂O₅–B₂O₃–PbO, two NMR lines which arise from BO₃ and BO₄ units were detected, where the appearance of BO₄ units is produced by the presence of PbO. From the computer-simulation of the ¹¹B NMR central transition line (m=−1/2↔1/2), the quadrupole parameters (e²qQ/h and η) for BO₃ units in xV₂O₅–B₂O₃, and those for BO₃ and BO₄ units in xV₂O₅–B₂O₃–PbO were obtained as a function of x. As the V₂O₅ content increases in xV₂O₅–B₂O₃–PbO, the e²qQ/h and η values of the BO₃⁻ associated resonance are found to slightly decrease and increase, respectively. Meanwhile, the e²qQ/h and η values of BO₄⁻ associated resonance in xV₂O₅–B₂O₃–PbO are found to slightly increase and decrease, respectively. By comparing the intensities of the total transitions (m=−3/2↔−1/2,m=−1/2↔1/2, and 1/2↔3/2) for the ¹¹B NMR line of BO₃ and BO₄ units contained in xV₂O₅–B₂O₃–PbO with those of respective standard samples of 0.053V₂O₅–B₂O₃ and NaBH₄, the quantitative fractions of BO₃ and BO₄ in xV₂O₅–B₂O₃–PbO were obtained as a function of x.     

Low-frequency internal friction of metallic glasses near Tg A critique of the use of the torsion pendulum

The application of the torsion pendulum to internal friction measurements on metallic glasses near the glass transition is critically analysed. It is pointed out that a difference between the measured damping Q_t⁻¹ and the real internal friction Q_s⁻¹ of the mterial must be taken into account; quantitative expression are given for the inverted and Collette types of torsion pendulum. The Collette pendulum is found particularly useful for measuring the internal friction of metallic glasses during slow heating through the glass transition. The results obtained with this instrument on six different Pd- and Ni-based glasses, as well as theoretical evidence, suggest that the extremely large internal friction “peak” above T_g is generally due to the intervention of crystallization after a large eand monotonous increase of viscoelastic damping in the supercooled liquid. In particular, there is no vidence for a real internal friction peak due to the glass transition. This conclusion is contradictory to some internal friction results found in the literature which have been obtained with a conventional inverted torsion pendulum. For this type of pendulum, the difference between Q_t⁻¹ and Q_s⁻¹ has obviously not been sufficiently tajen into account.     

Viscosity and IR investigations in the Li2O---B2O3 system

The high viscosity in melts of the Li₂O---B₂O₃ system makes it very difficult to grow large crystals of lithium triborate. The viscosity and IR characteristics of molten li₂O---B₂O₃ system are reported in this paper. When the temperature increases the viscosity of li₂O---B₂O₃ system decreases and follows an Arrhenius-type relationship. With an increasing 13203 ratio in Li₂O---B₂O₃ melts, the viscosity rises gradually to a maximum with a composition Li₂O: 3.513203 then it falls rapidly. In order to find active agents to reduce the viscosity, Na₂O, NaCl, LiF, P205, M003, W03 etc oxides were added to Li₂O---B₂O₃ samples respectively and investigated using the orthogonal method. The experimental results show that the addition of acidic oxides can significantly decrease the viscosity in the Li₂O---B₂O₃ system. For Li₂O: 4.513203, an ideal additive agent is 20wt% Li₂O:: 2MoO₃. Near the composition for crystal growth, the percentage reduction of viscosity is 62.2%. The IR spectra of Li₂O---B₂O₃ system revealed that the BO₄⁻/NO₃⁻ ratio is reduced in the melt using Li₂O: 2MoO₃ as an additive. It is proposed that the M003 reduced the concentration of bridging oxygen atoms of BO₄⁻. The change of structure explains the decline in the viscosity. In the crystal structures of lithium triborate, the matrix spaces are so small that larger other cations than Li+ are very difficult to enter the crystal matrix. So the use of additive agents to reduce the viscosity is a possible method if no new phase appears.     

Silicate groupings in glassy and crystalline 2PbO·SiO2

The type and the amount of silicate groupings existing in glassy and crystalline 2PbO·SiO₂ have been determined by direct chemical methods: paper chromatography, trimethylsilylation combined with gas-liquid partition chromatography and by the molybdate method. The results obtained by these three different methods are in good agreement and demonstrate, that glassy 2PbO·SiO₂ and each of the three main crystalline polymorphs are characterized by its own specific silicate anion distribution: the distribution in vitreous 2PbO·SiO₂ is of a polyanionic nature; in T---Pb₂SiO₄ dimetic groups [Si₂O₇]⁶⁻ prevail; M₁---Pb₂SiO₄ contains predominantly [Si₄O₁₂]⁸⁻ rings and H---Pb₂SiO₄ is a typical polysilicate with chain anions [SiO₃²⁻]_n. The results fit a structural model according to which glass is a random array of discrete polyatomic groupings; the gradual transition from the glassy state to the stablest crystalline structure is connected with degradation and polymerization of silicate anions.     

Solvent effects on the growth kinetics of subtilisin crystals

The effects of salts on subtilisin crystallization were investigated. Three salts—NaCl, NaNO₃ and NaSCN—were selected to study the effects of different anions on growth kinetics of three subtilisin mutants—Properase^®, Purafect^® and Purafect^®OX. The effectiveness of salts in decreasing the solubility of Properase^® and Purafect^® subtilisin followed the reverse order of the Hofmeister series: SCN⁻>NO₃⁻>Cl⁻. The average length and diameter of crystals were measured during crystallization. The nature of salt changed the length/diameter ratio of crystals, indicating the changes in the relative growth rate of different crystal faces. The required supersaturation, (c−s)/s, for a given growth rate increased in the order of NaCl, NaNO₃ and NaSCN. The observed trend in required supersaturation indicates a kinetic effect and was counter to the trend for the solubility data. A rationale is provided based on the influence of ion binding and kinetics on the energetics of crystal growth and growth rate is correlated to the molar Gibbs free energy of hydration of the anion.     

Effect of Si3N4 on chemical durability of chalcogenide glass

Chemical durability of the Si₃N₄ doped (up to 0.5 wt%) chalconitride glasses was evaluated by weight losses of glass samples after immersion in deionized water, H₂SO₄, HCl and NaOH solutions. IR spectra and SEM were used as an aid to examine the surface structure of corroded glass samples. Weight loss measurements showed a poorer stability of non-oxide glasses in basic solution than in acid solution, and the Si₃N₄ doping was found to improve chemical durability. Comparison of IR spectra of samples exposed to the basic solution with their original ones suggested the higher content of OH⁻ in its reaction layers, indicating that the corrosion of the present non-oxide glasses in the basic solution may result from the breakdown of the bridging Se due to an attack launched by OH⁻. The SEM evidence confirmed that Si₃N₄ doping did hinder the OH⁻ attacks on Ge and/or As, which is most likely related to incorporation of the three-coordinated N into the glass network, as supported by a comparison of the IR transmitting spectrum between uncorroded chalconitride glass and undoped chalcogenide glass.     

An infrared study of crystallization in sodium-disilicate glasses containing iron oxides

The infrared absorption spectra of sodium-disilicate glasses containing various amounts of Fe₂O₃ ([Na₂O · 2 SiO₂]_1−x [Fe₂O₃]_x, where X = 0.05, 0.1 and 0.2) were investigated in the wavenumber range from 200–2000 cm⁻¹. The addition of Fe₂O₃ to the sodium-disilicate glass does not seem to introduce any new absorption band as compared with the spectrum of a pure sodium-disilicate glass; nevertheless, a general shift of the existing absorption bands toward lower wavenumbers is observed. The amount of shift is, in fact, proportional to the content of Fe₂O₃ in the glass. This observation is consistent with the recently proposed structural model for the bonding of Fe³⁺ ions in the iron-sodium-silicate glass system.

Annealing of 20 mol% iron oxide glasses at 550 and 580°C produced an extra sharp infrared absorption peak at about 610 cm⁻¹ wavenumber. This new peak is believed to be related to the crystallized particles of the glass as concluded from both a scanning electron micrograph and an electron diffraction pattern.     

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 Raman spectra of defects in neutron bombarded and Ge-rich vitreous GeO2

This paper reports the polarized Raman spectra of three forms of vitreous GeO₂: the pure glass, neutron irradiated pure glass and unirradiated Ge-rich glass of composition Ge_1.1O₂. The data reveals that the line seen at 520 cm⁻¹ in the pure glass is due to a network defect that is not a Ge---Ge bond and very probably also not an O---O bond. Comparison with spectra of fused silica suggests that the 606 cm⁻¹ defect line seen in v-SiO₂ is not due to Si---Si or O---O bonds.     

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.     

The effect of oxide impurity on the physical properties of fluorozirconate glass

The effect of oxide impurity on the physical properties of 62ZrF₄---8LaF₃---30BaF₂ (mol.%) glass was studied by equimolecular substitution of BaO for BaF₂. It is shown that the oxide impurity decreases the infrared transparency beyond 6 μm, shifts the transmission cut-off wavelength to higher frequencies and causes an additional absorption shoulder at 1350 cm⁻¹. The oxide impurity also increases the glass transition temperature, the crystallization temperature and the viscosity of the melt. The additional infrared absorption of oxide impurity in the fluorozirconate glasses results from the multiphonon process of the vibration of F---Zr---O bonds at 680 cm⁻¹.     

Optical third-harmonic generation from some high-index glasses

Optical third-harmonic generation from some high-index glasses was investigated. The highest χ⁽³⁾ was obtained from As₂S₃ glass 2.2 × 10⁻¹² esu/ This value was 100 times higher than that of pure SiO₂ glass and comparable with that of the polymer with monomer-doping, which are known as organic materials with quite high χ⁽³⁾. From the relationship between χ⁽³⁾ and composition, sulfide glasses were found to have higher χ⁽³⁾ than oxide or semiconductor-doped oxide glasses with similar refractive indices.     

Li conductivity in lithium niobate: silica glasses

Glasses of composition 65 mol% LiNbO₃:: 35 mol% SiO₂ have been shown to be Li⁺ ion conductors with a conductivity at 200°C > 1 × 10⁻⁵ (η cm)⁻¹ and an activation energy of 0.54 eV. The addition of approximately 0.1 mol% Fe₂O₃ leads to an enhancement of conductivity to ≈10⁻³ (η cm)⁻¹ at 200°C and an activation energy of 0.67 eV. The effect of Fe is shown to be in the control of microstructure in the glass, with Fe₂O₃ concentrations < 1 mol% acting as a grain growth inhibitors and larger concentrations acting as a nucleating agents. A model for this process based on the expected stoichiometry of the melt and the effect of Fe²⁺ and Fe³⁺ in charge compensation is in excellent agreement with experimental data from electron spin resonance.     

The effect of MgO on the thermodynamics of the Fe/Fe-redox equilibrium and the incorporation of iron in soda-magnesia-aluminosilicate melts

Melts with the basic compositions 10Na₂O · 10MgO · xAl₂O₃ · (80−x)SiO₂ (x=0, 5, 10, 15 and 20), 10Na₂O · xMgO · 10Al₂O₃ · (80−x)SiO₂ (x=5, 10, 15 and 20) and xNa₂O · 10MgO · 10Al₂O₃ · (80−x)SiO₂ (x=5, 10 and 15) all doped with 0.25 mol% Fe₂O₃ were studied using square-wave voltammetry. The temperatures applied were in the range of 1000–1600 °C. The square-wave voltammograms recorded show peaks caused by the reduction of Fe³⁺ to Fe²⁺. The attributed peak potentials measured decreased linearly with decreasing temperatures. Increasing the MgO-concentration led to more negative peak potentials. Introducing alumina in the melt first resulted in less negative peak potentials. If the molar Al₂O₃-concentration is equal to that of Na₂O (=10 mol%) the peak potentials are least negative. Further increase of the Al₂O₃-concentration led to more negative peak potentials. The variation of the Na₂O-concentration led to a maximum in the peak potentials at an Na₂O-concentration of 10 mol%. An empirical formula which allows the calculation of standard potentials from the chemical composition is proposed. Furthermore, a structural explanation for the effect of the chemical composition is given. Especially, the incorporation of Al₂O₃ as AlO₄⁻-tetrahedra at [Al₂O₃] < [Na₂O] and as network modifier at larger concentrations was structurally explained by the similarities of Fe²⁺ and Mg²⁺, with respect to cation radii and metal–oxygen bond lengths.     

Ionic transport and defect structure of vitreous beryllium fluoride

Vitreous BeF₂ was prepared by two techniques; (1) remelting of a technical grade material, and (2) vacuum distillation/fluoridation. Infrared spectroscopy studies have established that the first material contains about 0.5 wt.% hydroxyl, predicted to be coherently incorporated into the vitreous network as edge-linked [Be(OH)₄]²⁻ units. The distilled BeF₂ is water-free. The dc electrical conductivity of the remelted BeF₂ was measured as σ = (7.9 × 10³/T) exp(−24500 cal/mol/RT) ω⁻¹ cm⁻¹ and for the distilled BeF₂ as σ = (3.0 × 10⁵/T) exp(−36700 cal/mol/RT ω⁻¹ cm⁻¹ at temperatures to 280°C. Ionic transport studies utilizing a dc electrolysis polarization technique with N₂−F₂ and H₂−HF gas electrodes have demonstrated that the fluorine ion is the transport species. A general model for fluorine transport is proposed based upon a modified anti-Frenkel defect model. The difference in the fluorine transport process for the undistilled grade of BeF₂ is seen as a consequence of the anti-Frenkel defect pair interaction with the [Be(OH)₄[²⁻ groupings.     

Growth and spectral properties of Nd:LaVO4 crystal

This paper reports the growth and spectral properties of 3.5 at% Nd³⁺:LaVO₄ crystal with diameter of 20×15 mm² which has been grown by the Czochralski method. The spectral parameters were calculated based on Judd–Ofelt theory. The intensity parameters Ω_λ are: Ω₂=2.102×10⁻²⁰ cm², Ω₄=3.871×10⁻²⁰ cm², Ω₆=3.235×10⁻²⁰ cm². The radiative lifetime τ_r is 209 μs and calculated fluorescence branch ratios are: β₁(0.88μm)=45.2, β₂(1.06μm)=46.7, β₃(1.34μm)=8.1. The measured fluorescence lifetime τ_f is 137 μm and the quantum efficiency η is 65.6%. The absorption band at 808 nm wavelength has an FWHM of 20 nm. The absorption and emission cross sections are 3×10⁻²⁰ and 6.13×10⁻²⁰ cm², respectively.     

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.     

Monte Carlo simulations of Meyer-Neldel effect on carrier time-of-flight in a-Si:H

We present Monte Carlo simulations of multiple-trapping transport with Meyer-Neldel effect in a-Si:H assuming exponential band tails. The transit time t_T, and the dispersion parameters, ₁ and ₂, before and after the transit time, are extracted from the simulated currents. The simulations show that including the Meyer-Neldel effect improves the agreement of ₁ and ₂ with experimental data, both at low and high temperatures and fits the time-of-flight drift mobility measurements. Best fits to the data yield T₀ = 263 K, T_{MN = 464 K}, v₀₀ = 5 × 10⁹ s⁻¹ and μ₀ = 4 cm² V⁻¹ s⁻¹ for electrons and T₀ = 409 K, T_{MN = 809 K}, v₀₀ = 6.5 × 10¹⁰ s⁻¹ and μ₀ = 0.5 cm² V⁻¹ for holes.     

Viscosity matching of new PbF2–InF3–GaF3 based fluoride glasses and ZBLAN for high NA optical fiber

New multicomponent PbF₂–InF₃–GaF₃ bulk glasses have been investigated. They show lower phonon energy (540 cm⁻¹) in comparison with 580 cm⁻¹ for ZBLAN. Large PbF₂ concentration provided glasses with high refractive index up to 1.582 and the viscosity curves revealed an excellent thermal compatibility with ZBLAYN glass. A multimode fiber with a numerical aperture of 0.51, a loss of 0.85 dB/m at 1.3 μm was fabricated using the rotational casting method.