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.     

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.     

Nanocrystallization of fresnoite glass. I. Nucleation and growth kinetics

We determined the internal nucleation, crystal growth and overall crystallization kinetics of fresnoite crystal (2BaO · TiO₂ · 2SiO₂) in an almost stoichiometric fresnoite glass. Due to the extremely high nucleation rates (10¹⁷ m⁻³ s⁻¹) that limit the maximum crystal size to 700 nm the nucleation densities and crystal sizes were estimated by scanning electron microscopy (SEM). The volume fraction crystallized was measured by X-ray diffraction. The nucleation rates obtained directly from SEM measurements reasonably agree with those calculated from the combination of overall crystallization with crystal growth kinetics. The activation enthalpies for viscous flow, transport of structural units across the nucleus/melt interface (nucleation) and crystal growth: ΔH_η, ΔH_τ and ΔH_U respectively, follow a similar trend to that observed for other stoichiometric silicate glasses that nucleate internally: ΔH_η=294>ΔH_τ=87>ΔH_U=61 kJ/mol. Fresnoite glass displays the highest internal nucleation rates so far measured in inorganic glasses. These rates are comparable to some metallic glasses and can lead to nanostructured glass-ceramics.     

Studies of the optical properties of sodium copolyphosphate glasses

A number of samples of glassy sodium polyphosphate and copolyphosphates of sodium — cobalt, sodium — copper, sodium — nickel, sodium-manganese and sodium — calcium were prepared and their optical properties were investigated. The ultraviolet and visible spectra of these glasses were recorded at the room temperature. It was found that the fundamental absorption edge of these glasses usually occurs in the ultraviolet — visible region. The optical absorption edges (E₀) were calculated by using the relation h ν = B (hν − E₀)² where B is constant. The linear variation of (hν)^1/2 with hν where is the absorption coefficient and hν is the incident photon energy, is taken as evidence of non-direct interband transitions. The E₀ values obtained for copolyphosphate glasses appeared to depend on the size of the counter cation. The infrared spectra of all these glasses appeared to be almost the same, indicating the presence of characteristic P --- O --- P linkages of linear polyphosphate chains and two non bridging oxygen atoms bonded to phosphorus atoms O --- P --- O (PO₂) units in the polyphosphate tetrahedra.     

Mössbauer studies on some glasses and crystals in the Na2O---Fe2O3---SiO2 system

Mössbauer absorption measurements have been made at room temperature on ⁵⁷Fe in iron sodium silicate glasses containing 3–15 mol% Fe₂O₃ and various iron alkali silicate crystals in order to study the state of iron in these glasses. The spectra of all the glasses gave one doublet with a quadrupole splitting varying from 0.73–0.78 mm s⁻¹, while those of Na₂O · Fe₂O₃ · 4 SiO₂ and 5 Na₂O · Fe₂O₃ · 8 SiO₂ crystals showed much smaller quadrupole splitting, 0.28 mm s⁻¹ and 0.10 mm s⁻¹, respectively, and an asymmetrical doublet of much narrower linewidth. When sodium was replaced by other alkali metals of larger size, such as K and Cs, in MFeSi₂O₆ and MFeSi₃O₈ crystals, the quadrupole splitting became wider and approached to 0.73 mm s⁻¹. Such a variation was not observed for glasses. These results suggest that a larger number of non-identical sites exist in iron sodium silicate glasses than in the corresponding crystals.     

Alkali diffusion and electrical conductivity in sodium borate glasses

The diffusion coefficient of sodium, ²²Na, and silver, ¹¹⁰Ag, and electrical conductivity for sodium borate glasses (4–24 mol% Na₂O) has been measured from 100°C to slightly below the glass transition temperature, T_g. Unlike silicate glasses where the self-diffusion coefficient is much larger than the impurity diffusion coefficient, D_Na and D_Ag had close to the same magnitude in the sodium borate glasses. In some glasses, D_Ag was slightly larger than D_Na. Na₂O-Ag₂O-B₂O₃ glasses show a relatively small mixed alkali effect, despite the significant mass difference between Ag(≈108) and Na(≈23). It is concluded that the mixed alkali effect is more dependent upon the difference in ionic radii rather than differences in mass.     

The effect of hydrostatic pressure on the dielectric constants, and their temperature dependences, of phosphate and tellurite glasses

Measurements of the pressure dependence of the static dielectric constant of tellurite (pure TeO₂ and 67%TeO₂ + 33%WO₃) and samarium phosphate (5%Sm₂O₃ + 95%P₂O₅ and 15%Sm₂O₃ + 85%P₂O₅) glasses at elevated pressures (0–70 kbar) for a range of temperatures (77–380 K) are reported. The electrical properties under pressure have been determined from the low-frequency complex plane analysis of glass discs contained within a Bridgman opposed anvil cell. The most notable observation is that the pressure dependence of the static dielectric constant, of all glasses studied, is positive, for example for vitreous TeO₂ ln ε/P is equal to 4.41 × 10⁻¹¹ (Pa⁻¹) at 293 K. Behaviour of this type is common to a number of materials (plastics and chalcogenide glasses) for which it is not possible to define any long-range order. It is in direct contrast with the behaviour of crystalline insulators, for which ε/P is usually negative. The effect of pressure on the dielectric constant has been analysed using two different approaches based on the macroscopic Clausius-Mossotti equation. The effects of high pressure on the dielectric constant have been correlated with the temperature dependence of the dielectric constant at atmospheric pressure.     

Nuclear magnetic resonance studies of alkaline earth phosphosilicate and aluminoborosilicate glasses

The ¹¹B, ²⁷Al, ²⁹Si and ³¹P magic angle spinning (MAS) NMR spectra of MO–P₂O₅, MO–SiO₂–P₂O₅ and MO(M^′₂O)–SiO₂–Al₂O₃–B₂O₃ (M=Mg, Ca, Sr and Ba, M^′=Na) glasses were examined. In binary MO–P₂O₅ (M=Ca and Mg) glasses, the distributions of the phosphate sites, P(Q_n), can be expressed by a theoretical prediction that P₂O₅ reacts quantitatively with MO. In the ternary 0.30MO–0.05SiO₂–0.65P₂O₅ glasses, the 6-coordinated silicon sites were detected, whose population increases in the order of MgOxCaO–0.05SiO₂–(0.95−x)P₂O₅ glasses, its population increases with an increase in f (=([P₂O₅]−[MO]−[B₂O₃]−[Na₂O])/[SiO₂]) and has maximum at f=9. The signal due to the 5-coordinated silicon atoms is also observed when x is smaller than 0.45. When three network-forming oxides such as SiO₂, Al₂O₃ and B₂O₃ coexist, Al₂O₃ reacts preferably with MO. The populations of 4-coordinated boron atoms, N₄, are expressed well with r/(1−r), where r=([Na₂O]−[Al₂O₃])/([Na₂O]−[Al₂O₃]+[B₂O₃]). The correlation of the Raman signal at 1210 and 1350 cm⁻¹ with the NMR signal of Si(Q₆) at −215 ppm is also seen.     

Influence of chlorine-fluorine substitution on the electrical properties of some fluoride glasses

Study of electrical properties of two series of glasses (ZrF₄)₅₄(BaF₂)₃₀(ThF₄)₅(LaF₃)₃(RbF)_8(1−x)(RbCl)_8x (0 x 1) and (ZrF₄)₅₀(BaF₂)₃₀(ThF₄)₅(LaF₃)₃(RbF)_12(1−x)(RbCl)_12x (0 x 1) demonstrates the existence of a mixed mobile anion effect in these anion-conducting glasses. The effective medium approach has been applied to these materials: a satisfactory fitting is obtained as a function of the Cl/(Cl + F) ratio and temperature; the ionic transport in these glasses takes place along preferential pathways.     

Study of electrical properties of MoO3–Fe2O3–P2O5 and SrO–Fe2O3–P2O5 glasses by impedance spectroscopy. II

The electrical and dielectric properties for three series of MoO₃–Fe₂O₃–P₂O₅ and one series of SrO–Fe₂O₃–P₂O₅ glasses were measured by impedance spectroscopy in the frequency range from 0.01 Hz to 3 MHz and over the temperature range from 303 to 473 K. It was shown in Part I that the MoO₃ is incorporated into phosphate network and the structure/properties are strongly influenced by the overall O/P ratio. The Fe₂O₃ content and Fe(II)/Fe_tot ratio in these glasses have significant effects on the electrical conductivity and dielectric permittivity. With decreasing Fe₂O₃ content in MoO₃–Fe₂O₃–P₂O₅ glasses with O/P at 3.5 the dc conductivity, σ_dc(ω) decreases for two orders of magnitude, which indicates that the conductivity for these glasses depends on Fe₂O₃ and is independent of the MoO₃ content. Also, the dielectric properties such as ^′(ω), ^″(ω) and σ_ac(ω) and their variation with frequency and temperature indicates a decrease in relaxation intensity with increase in the concentration of MoO₃. On the other hand, the dc conductivity for MoO₃–Fe₂O₃–P₂O₅ glasses with O/P > 3.5 increases with the substitution of MoO₃ which has been explained by an increase in the number of non-bridging oxygens and formation of Fe–O–P bonds that are responsible for formation of small polarons. The increase in the dielectric permittivity, ^′(ω) with increasing MoO₃ content is attributed to the increase in the deformation of glass network with increasing bonding defects. For SrO–Fe₂O₃–P₂O₅ glasses the conductivity and dielectric permittivity remained constant with increasing SrO.     

Absorption spectra of GaN: film characterization by Urbach spectral tail and the effect of electric field

The absorption tail of undoped and Si-doped GaN films was investigated at different temperatures and under applied electric field. It was found that the spectral dependence of logarithm of absorption coefficient is combined of two linear functions: ln[(hν)]=C₁+(hν−E_g)/U₁ for hν<3.42 eV and ln[(hν)]=C₂+(hν−E_g)/U₂ for 3.44<hν<3.5 eV with Urbach energies U₁=400–470 meV and U₂=10–20 meV. The influence of an electric field effect on the absorption spectra follows the Dow and Redfield theory. It was shown that the intrinsic electric field about 10⁵ V/cm exists in our samples. The implemented analysis of the absorption spectra gives the qualitative method of film characterization.     

Structure and properties of the pure and Pr-doped Ge25Ga5Se70 and Ge30Ga5Se65 glasses

The Ge₂₅Ga₅Se₇₀ and Ge₃₀Ga₅Se₆₅ pure and Pr³⁺-doped glasses were prepared by direct synthesis from elements and PrCl₃. It was found that up to 1 mol% PrCl₃ can be introduced in the Ge₂₅Ga₅Se₇₀ and Ge₃₀Ga₅Se₆₅ glasses. Both types of glasses with overstoichiometric and substoichiometric content of Se were homogeneous and of black color. The optical energy gap is E^opt_g=2.10 eV, and the glass transition temperature is T_g=543 K for Ge₂₅Ga₅Se₇₀ and T_g=633 K for Ge₃₀Ga₅Se₆₅. The long-wavelength absorption edge is near 14 μm and it corresponds to multiphonon processes. Doping by Pr³⁺ ions creates absorption bands in transmission spectra, which can be assigned to the electron transitions from the ground ³H₄ level to the higher energy levels of Pr³⁺ ions ³H₅, ³H₆, ³F₂, ³F₃ and ³F₄, respectively. By excitation with YAG:Nd laser line (1064 nm), two intense luminescence bands (1343 and 1601 nm) were excited. The first band can be ascribed to electron transitions between ¹G₄ and ³H₅ energy levels of Pr³⁺ ions. Full width at half of maximum (FWHM) of the intensity of luminescence was found to be 70 nm for (Ge₂₅Ga₅Se₇₀)_1 − x(PrCl₃)_x and (Ge₃₀Ga₅Se₆₅)_1 − x(PrCl₃)_x glasses. The FWHM in selenide glasses is lower than in halide and sulphide glasses. The second luminescence band (1601 nm) can be probably ascribed to the transitions between ³F₃ and ³H₄ energy levels of Pr³⁺ ions. The absorption and luminescence spectra of Pr³⁺ ions in studied glasses are slightly influenced by stoichiometry of glassy matrix. The Raman spectra of studied glasses were deconvoluted and assignment of Raman bands to individual vibration modes of basic structural units was suggested. The structure of studied glasses is mainly formed by corner-sharing and edge-sharing GeSe₄ tetrahedra. The vibration modes of Ga-containing structural units were not found, they are apparently overlapping with Ge-containing structural units due to small difference between atomic weights of Ge and Ga. In the glasses with substoichiometry of Se, the Ge–Ge bonds of Ge₂Se₆ structural units were found. In Se-rich glasses the Se–Se vibration modes were found. In all studied glasses also ‘wrong' bonds between like atoms were found in small amounts. Maximum phonon energy of studied glasses is 320 cm⁻¹.     

Synthesis, growth and characterization of a new laser upconversion crystal Ba2ErCl7

A modified method to synthesize the new laser upconversion material Ba₂ErCl₇ using Er₂O₃, BaCl₂ 2H₂O and NH₄CI is reported for the first time in this paper. Single crystals up to 5–8mm in diameter and 10–20 mm in length have been grown by Czochralski method. The transmittance spectra of Ba₂ErCl₇ single crystal has first been measured by using an HITACHI U-3500 spectrophotometer. There are three intense absorption peaks ⁴I_9/2⁴I_11/2 and ⁴I_13/2 in the infrared range which can be excited by 803nm, 980nm and 1.5μm laser diodes (LDs), respectively. The cut-off wavelength of the crystal is 230mn. Intense green luminescence can be observed when the crystal is pumped by a ED at 803nm. The formation of the defects is also discussed.     

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.     

Glass-forming ability versus stability of silicate glasses. II. Theoretical demonstration

Possible relationships between measures of glass stability (GS) against devitrification on heating (evaluated by the Hrubÿ parameter K_H=(T_c^h−T_g)/(T_m−T_c^h), and the parameter K_w=(T_c^h−T_g)/T_m) and a criterion of glass-forming ability (GFA) – the critical cooling rate – were investigated by computing non-isothermal crystallization for typical values of the main quantities that control crystal nucleation and growth in silicate glasses. We limit these quantities to one thermodynamic parameter – the melting entropy (ΔS_m) and two kinetic parameters that control the viscosity (B and T₀ in the Vogel–Fulcher–Tamman equation or T_g and in Avramov’s equation). The effect of heterogeneous nucleation and, in particular, the possible role of the surface as active substrate is tested. The results presented herein demonstrate that GS and GFA are indeed related concepts.     

Colloidal sol/gel processing of ultra-low expansion TiO2/SiO2 glasses

A series of titania-silica glasses with 0–9% TiO₂ were fabricated using a sol/gel process. The sol was prepared by dispersing colloidal silica fume in an aqueous solution of titania which was synthesized through the acid-catalyzed hydrolysis of titanium isopropoxide. The sols gelled in 2–4 days, and then were dried for 6–8 days. The dry gels were sintered at 1450–1500°C to produce clear, dense, microstructure-free glasses. The gels underwent a total shrinkage of 50% to yield glass rods about 50 mm long and 5 mm in diameter, or glass discs about 4 cm in diameter and 5 mm thick. The drying step was most critical in the production of crack-free specimens.

In the gel, the transmission electron microscope (TEM) revealed the presence of 1–5 nm rutile microcrystallites uniformly distributed within a network of colloidal silica particles. After sintering to 1450–1500°C, though, a dense, transparent, microstructure-free glass was created. Fourier transform infrared spectroscopy (FTIR) verified the formation of an amorphous solid-solution of titania and silica after sintering.

The thermal expansion of the glasses was measured using a differential dilatometer. The average linear coefficients of thermal expansion (CTE @ 25–675°C) varied between +5 × 10⁻⁷ and −0.2 × 10⁻⁷°C⁻¹ in the range 0 to 9% TiO₂. The glass with 7.2% TiO₂ exhibited a zero thermal expansion coefficient at 150–210°C. The hysteresis in CTE on heating and cooling was of the order of 0.01–0.02 ppm.     

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.     

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.     

TiO2-SiO2 monolithic glass formation from sol-gel

This study demonstrates that hydrolysis should be carried out in a step manner in gel synthesis. The key to the increase in the amount of water added is the control of the hydrolysis rate of Ti(OC₄H₉)₄. The hydrolysis of Si(OC₂H₅)₄ can be carried out at about 75°C. The amount of added water (γ_WI), which varied with TiO₂ content (in mol%), was about 64–88% of the total amount of added water. The hydrolysis reaction should be performed at room temperature while Ti(OC₄H₉)₄ is added. The total amount of added water (γ_W) is related to the amount of solvent (R). For example, if TiO₂ is 40mol%, γ_W will vary from 3.2 to 8.0 when R varies from 0.8 to 2.0. The amount of added water was affected by the distribution of solvent in the metal alkoxides. The amount of added water can be increased when R_Si(OC2H5)4 = 1, R_Ti(OC4H9)4 > 1. The rate of rise in temperature of the thermolysis of the dried gel should be less than 10°C per hour, and the heat treatment temperatyre is related to the TiO₂ content (in mol%). Gel glasses without devitrification can only be obtained by thermolysis at 600°C from the gel with no less than 20 mol% TiO₂.     

Raman spectra of rapidly quenched glasses and melts containing large amounts of Li2O

Raman spectra were measured of rapidly quenched glasses and the corresponding liquids in binary and ternary (pseudobinary) systems composed of Li₂O and several glass-forming oxides such as B₂O₃, P₂O₅, and/or SiO₂. The fractions of the various structural units present were determined from the deconvoluted Raman peaks. Glasses with large amounts of Li₂O were composed of discrete structural units such as ortho-oxoanions and pyro-oxoanions. Based on these data the structural units in the glasses and the corresponding liquids were identical, although in some cases those in the corresponding crystals were different. In the pseudobinary systems combining pyro-oxosalts of lithium borate, phosphate and silicate, non-bridging oxygens were preferentially formed at the phosphate structural units rather than at the borate or silicate structural units. This order of preference was consistent with that of the acidity of the glass-forming oxides in the liquids.