Structure of ultraphosphate glasses with small rare-earth ions by X-ray diffraction

Rare-earth ultraphosphate glasses with nominal R₂O₃ fractions of 10 and 15 mol% and small ionic radius (large atomic number) R³⁺ ions (R = Tb, Tm, Lu) are measured by X-ray diffraction at a synchrotron with photons of 119 keV (maximum scattering vector 260 nm^− 1). The total correlation functions T(r) show well-resolved R-O and O-O first-neighbor peaks. In contrast to all ultraphosphate RP₅O₁₄ crystals and the ultraphosphate glasses of larger R³⁺ ions, where RO₈ polyhedra (mean R-O coordination numbers of ~ 8 for the glasses) are observed, the R³⁺ ions in glasses with R = Lu, Tm, Tb have mean R-O coordination numbers of ~ 7.5. The R-O first-neighbor peaks extracted from the T(r) functions are compared with those obtained from atomic coordinates of related RP₅O₁₄ and RP₃O₉ crystals. The R-O distances of the ultraphosphate glasses studied are found to fall between those of the two crystals but with tails to the side of longer bonds.     

Structure of binary neodymium borate glasses by infrared spectroscopy

Using fast roller quenching techniques, a new series of binary rare earth oxide borate glasses were synthesized, with general formula xNd₂O₃ + (1 − x)B₂O₃, where x varies up to 35 mol%. The glasses were investigated by using mid infrared reflectance spectroscopy. The results show that the neodymium acts as a modifier, similar to an alkali metal. As x is increased, the borate glass network is shown to change from a three-coordinated to four-coordinated boron system. The results are further investigated by analyzing the spectra in terms of the location of the bands to show how the borate groups change upon neodymium addition.     

Rapid quenching and glass formation in chalcogenide glasses: Preparation and properties of Ge–As–Te glasses over an extended composition ranges

In Ge–As–Te system, the glass forming region determined by normal melt quenching method has two regions (GFR I and GFR II) separated by few compositions gap. With a simple laboratory built twin roller apparatus, we have succeeded in preparing Ge_7.5As_xTe_92.5−x glasses over extended composition ranges. A distinct change in T_g is observed at x = 40, exactly at which the separation of the glass forming regions occur indicating the changes in the connectivity and the rigidity of the structural network. The maximum observed in glass transition (T_g) at x = 55 corresponding to the average coordination number (Z_av) = 2.70 is an evidence for the shift of the rigidity percolation threshold (RPT) from Z_av = 2.40 as predicted by the recent theories. The glass forming tendency (K_gl) and ΔT (=T_c−T_g) is low for the glasses in the GFR I and high for the glasses in the GFR II.     

Phase transitions and macroscopic properties of NaNO3 embedded into porous glasses

The crystal structure and dielectric response of nanocomposite materials on base of porous glasses with average pore diameters of 320, 46 and 7 nm with embedding sodium nitrate have been studied by neutron diffraction and dielectric spectroscopy in low and high temperature phases up to melting. In porous glasses with 46 and 7 nm pores NaNO₃ forms dendrite nanoclusters with "diffraction" sizes of 50(2.5) and 20(2) nm. Decreasing of particle sizes results in decreasing of T_c (temperature of order-disorder orientational transition) and T_melt and in smearing of structure phase transition. The values of critical exponent β for orientational transition are estimated from temperature dependences of intensities of superstructure elastic peaks for these three types of nanocomposite materials.     

Optical and spectroscopic properties of germanotellurite glasses

In this work, new glass compositions in the TeO₂-GeO₂-Nb₂O₅-K₂O system have been prepared and studied. The germanotellurite glasses were prepared by melt-quenching and their density, refractive index and characteristic temperatures have been determined. The structure of these glasses has been studied by infrared and Raman spectroscopies.The progressive replacement of TeO₂ by GeO₂ led to an increase of the glass transition and crystallisation temperatures of the glasses and a simultaneous decrease of their density and refractive index. Typical density and refractive index values of these glasses ranged from 4.98 to 3.85 g cm^− 3 and 2.08 to 1.79, respectively, with increasing GeO₂ content. The infrared spectra are dominated by a band ~ 640 cm^− 1 in the tellurite glass and ~ 800 cm^− 1 in the germanate glass. The Raman spectra of the germanotellurite glasses present an intense boson peak between ~ 34 and 47 cm^− 1, together with high frequency peaks at ~ 670 cm^− 1 and ~ 470 cm^− 1 for high tellurite and high germanate glass compositions, respectively. The vibrational spectra of these germanotellurite glasses indicate that the glass network consists basically of TeO₄ and [TeO₃]/[TeO_3 + 1] units, mixed with GeO₄ and NbO₆ polyhedra.     

Optical properties of fluorine-substituted zinc bismuth phosphate glasses

The effects of the substitution of fluoride ions for oxide ions on the thermal and optical properties of ternary ZnO-Bi₂O₃-P₂O₅ glass with low-P₂O₅ content (20-25 mol%) were investigated. Fluoride ions were introduced into the glass up to about 12 mol% as ZnF₂. Raman spectra indicated that fluoride ions were substituted for oxide ions connected with bismuth ions. Deformation and glass transition temperatures decreased monotonically with fluorine concentration. The absorption edge shifted toward higher energies with increasing fluorine concentration by about 0.3 eV for 12 mol% ZnF₂ substitution. The blue shift of the absorption edge is attributable to two effects. One was a blue shift of an absorption band which was observed as a peak at 4.7 eV in the reflection spectra and was attributed to the spin forbidden 6s-6p interband transition in Bi³⁺ ions. The blue shift originates from a change in electron-donating ability through anions as expected from electronegativity or optical basicity. Another is a disappearance of a shoulder at around 4.3 eV in the reflection spectra. The latter was the major reason for the large blue shift of the absorption edge energy, because the band relating to the 4.3 eV shoulder is close to the absorption edge.     

Structural aspects of the photoelastic response in lead borate glasses

The recent empirical model of photoelasticity found by Zwanziger and co-workers [Chem. Mater. 19, 286-280 (2007)] correlates local structure with stress optic response. The model is tested carefully here on the lead borate system, because this system is amenable to detailed local structural probes around both the boron and lead cations. Nuclear magnetic resonance is used to probe the boron and lead coordination environments, and the data is combined with previous work using X-ray methods. It is found that when only crystal data is used to describe the glass structure the model predicts the zero stress optic composition in fairly poor agreement with experiment, but when the details of the glass structure itself are used, agreement becomes very good. The results are contrasted with predictions of the earlier theory of Mueller, and the utility of the empirical model for practical glass design is discussed.     

Molecular dynamics simulation of radiation damage in glasses

Molecular dynamics simulations of the ballistic effects arising from displacement cascades in glasses have been investigated in silica and in a SiO₂-B₂O₃-Na₂O glass. In both glasses the T-O-T′ angle (where T and T′ are network formers) diminishes, despite radiation causes opposite effects: while the ternary glass swells and silica becomes denser. We show that radiation-induced modifications of macroscopic glass properties result from structural change at medium/range, reflecting an increasing disorder and internal energy of the system. A local thermal quenching model is proposed to account for the effects of ballistic collisions. The core of a displacement cascade is heated by the passage of the projectile, then rapidly quenched, leading to a process that mimics a local thermal quenching. The observed changes in both the mechanical and structural properties of glasses eventually reach saturation at 2 10¹⁸ α/g as the accumulated energy increases. The passage of a single projectile is sufficient to reach the maximum degree of damage, confirming the hypothesis postulated in the swelling model proposed by J.A.C. Marples.     

Growth and characterization of mixed neodymium praseodymium oxalatze decahydrate crystals in silica gel

Mixed single crystals of neodymium praseodymium oxalate decahydrate were grown from silica gel by controlled reaction of rare earth nitrates with oxalic acid. The crystals were characterized using X-ray powder diffraction and optical absorption studies.     

Melt crystallization of zinc alkali phosphate glasses

Melt crystallization of two zinc alkali phosphate glasses was studied with X-ray diffraction (XRD) experiments to accelerate efforts to melt process these glasses with organic polymers. The inorganic glasses differed markedly in chemical durability (water sensitivity) and crystallization rates. They were studied at room temperature prior to and after melt processing with XRD experiments and in situ at melt temperatures without flow in a novel differential scanning calorimeter/XRD apparatus. The glasses were found to be amorphous at room temperature and semi-crystalline above their glass-transition temperatures. Higher temperatures and shear (mixing) rates increased the crystallization rate of the glasses. The non-durable (water-sensitive) glass was observed to contain significant levels of crystalline matter after melt processing at 400°C. This process-induced crystallization of the glasses must be controlled, possibly during processing and/or glass formulation, otherwise it may lead to formation of unwanted phase-separated defects in the glass. If high levels of the crystalline matter are present during melt processing, they may lead to irreversible plugging of the processing equipment.     

Structure and electrical conductivity of new Li2O-CeO2-B2O3 glasses

Fourier transformation infrared spectra, density and DC electrical conductivity of 30Li₂O · xCeO₂⋅(70 − x)B₂O₃ glasses, where x ranged between 0 and 15 mol%, have been investigated. The results suggested that CeO₂ plays the role of network modifier up to 7.5 mol%. At higher concentrations it plays a dual role; where most of ceria plays the role of network former. The density was observed to increase with increasing CeO₂ content. The effect on density of the oxides in the glasses investigated is in the succession: B₂O₃ < Li₂O < CeO₂. Most of CeO₂ content was found to be associated with B₂O₃ network to convert BO₃ into B O₄⁻ units. The contribution of Li⁺ ions in the conduction process is much more than that due to small polarons. The conductivity of the glasses is mostly controlled by the Li⁺ ions concentration rather than the activation energy for CeO₂ > 5 mol%. Lower than 5 mol% CeO₂ the conductivity is controlled by both factors. The dependence of W on BO₄⁻ content supports the idea of ionic conduction in these glasses.     

Erbium- and chlorine-doped fluorozirconate-based glasses for up-converted fluorescence

A series of glass ceramics was prepared and the structure–property relationships evaluated. The base composition of the series was comprised of fluorozirconate (FZ) and fluorochlorozirconate (FCZ) glass ceramics. Thermal processing of as-made FCZ-based glass ceramics produced sharp peaks in X-ray diffraction patterns, which was attributed to BaCl₂ nanocrystals formed in the material; this was not observed in the FZ glass. Up-conversion fluorescence experiments were carried out for both the FZ and FCZ samples doped with erbium. The fact that an increase in the up-conversion efficiency was not observed indicates that erbium ions were not embedded in the BaCl₂ nanocrystals. A comparison is made between this series and a similar series that was doped with neodymium.     

d-f and f-f transition bands of praseodymium and samarium ions in silica glasses

d-f and f-f transition bands of Pr and Sm ions in silica glasses have been studied by the absorption, excitation and emission spectra. In particular, Sm-doped silica glasses were treated with different atmospheres around the T_g temperature. The change of the valency state and the transition band shown after the treatments are discussed.     

Properties and structure of binary tin phosphate glasses

The properties and structures of binary xSnO*(100 − x)P₂O₅ (50 ≤ x ≤ 70 mol%) glasses were evaluated. The glass transition temperatures (T_g), determined by differential thermal analysis (DTA), range from 246 to 264 °C, for glasses prepared under identical conditions. The refractive index (n_D) increases from 1.701 to 1.833 as x increases from 50 to 70, and the Abbe number (ν_D) decreases from 29.1 to 20.4 over the same range. Infrared spectroscopy was used to estimate water contents in the glasses, which decreased with an increase in SnO content, from about 1570 ppm OH for x = 50 to about 50 ppm OH for x = 70, for glasses quenched from melts held at 1000 °C for 15 min. Residual water affects thermal properties, like T_g, and variations in water contents due to differences in melt processing explain the wide variety of glass properties reported in the literature. Raman spectroscopy indicates that progressively shorter phosphate chains are present in the structures of the binary Sn-phosphate glasses with increasing SnO contents.     

Modeling oxide glasses with Born–Mayer–Huggins potentials: Effect of composition on structural changes

Using Born–Mayer–Huggins potentials, a molecular dynamics model of a series of 5-oxide (SiO₂, B₂O₃, Na₂O, Al₂O₃, and ZrO₂) glasses of various compositions was developed. The evolution of the glass structure according to the composition provides an overview of the behavior of each species. Some experimental observations were correctly reproduced, e.g., the gradual incorporation of the boron in the silicate network, the attraction of sodium atoms by four-coordinate boron, and the shorter distances between network formers and non-bridging oxygen atoms. Moreover, direct visualization of the structures reveals boron-enriched segregation zones in a composition containing no aluminum, as well as smaller regions comprising only aluminum atoms.     

Structural investigations of tungsten silver phosphate glasses by solid state NMR, vibrational and X-ray absorption near edge spectroscopies

Glasses were prepared in the pseudo-binary system (1 − x)AgPO₃-xWO₃ (0≤ × ≤ 0.6 mol%). The structural evolution of the vitreous network was studied as a function of composition by thermal analysis, Fourier Transform Infrared spectroscopy (FTIR), Raman scattering, high resolution ³¹P solid state NMR and XANES at the W-L₁ absorption edge. For compositions with x ranging from 0 to 0.5 a pronounced increase in the glass transition temperature is observed, suggesting a significant increase in the glass network connectivity. At the same time Raman spectra indicate that tungsten atoms are linked to non-bridging oxygen atoms (W-O- or W=O bonded species) whereas the ³¹P MAS-NMR spectra indicate the successive formation of new P-O-W linkages. The formation of some anionic tungsten sites (if these are revealed by the presence of W-O terminal bonds) implies an increase in the average degree of polymerization of the phosphate network, which is consistent with the compositional evolution of the ³¹P MAS-NMR spectra at low x values. For higher x-values, the Raman spectra indicate the presence of W-O-W linkages. W-L₁ XANES data indicate that at least 90% of tungsten atoms are octahedrally coordinated.     

Application of the constant stoichiometry grouping concept to the Raman spectra of Pb(PO3)2-TeO2 glasses

Pb(PO₃)₂-TeO₂ glasses in the whole range of glass composition were first obtained and their properties (refractive index, density, T_g and light scattering losses) were determined. Based on the vibrational spectroscopy data a new approach was applied to investigate the interactions of initial oxides in melts resulted in so-called constant stoichiometry groupings (CSGs) formation symbolizing intermediate range order in glasses. Vibrational spectra of glasses are interpreted as a superposition of unchangeable spectral forms (principal spectral components (PSCs)) belonging to CSGs: PbO · P₂O₅, TeO₂ · 2PbO · 2P₂O₅, TeO₂ · PbO · P₂O₅, TeO₂, and possibly 2TeO₂ · PbO · P₂O₅ and 6TeO₂ · PbO · P₂O₅. In this work Multivariate Data Analysis has been applied as the independent mathematical tool to decompose Raman spectra of glasses and reveal the number of PSCs. It is shown that application of factor analysis results in the same five PSCs that confirms our data obtained from the CSG concept. This concept allows also the prediction of the existence of unknown compounds, and correspondingly some crystals (TeO₂⋅ 2PbO⋅2P₂O₅ and others) were revealed by XRPD of the crystallized glasses. The CSG concept opens the way for elaboration of low scattering glasses as candidates for Raman amplifiers. It is shown that Pb(PO₃)₂-TeO₂ glasses with small content of TeO₂ are of interest to photonic technology.     

Effect of fluoride ion incorporation on the structural aspects of barium-sodium borosilicate glasses

Barium-sodium borosilicate glasses containing upto 6 wt% fluoride ions were prepared by conventional melt quench method and characterized by ¹⁹F, ²⁹Si and ¹¹B nuclear magnetic resonance (NMR) techniques.¹⁹F NMR studies have confirmed the presence of mainly linkages like F-Si(n) or F-B(n) along with F-Ba(n). Their relative concentrations are unaffected by F⁻ content in the glass. Incorporation of fluoride ions in the glass is associated with significant reduction in the nonbridging oxygen concentration attached to silicon, as revealed by the increase in the concentration of Q³ structural units of silicon at the expense of Q² structural units. ¹¹B NMR studies have established that the relative concentrations of BO₃ structural units are higher for F⁻ ion containing glasses compared to the one without F⁻ ion incorporation. The observed increase in the relative concentrations of Q³ structural units of silicon and BO₃ structural units with fluoride ion incorporation in the glass has been attributed to the formation of F-Ba(n) type of linkages, thereby reducing the concentration of network modifying cations for breaking the Si-O-Si/B-O-B linkages. Formation of such structural units weakens the glass network thereby decreasing the glass transition temperatures.     

Thermal stability and crystallization kinetics of new As–Ge–Se–Sb glasses

Thermal stability and crystallization kinetics of As₁₄Ge₁₄Se_72−xSb_x (where x = 3, 6, 9, 12 and 15 at.%) glasses are studied by the differential scanning calorimetry. The values of the glass transition temperature (T_g) and the peak temperature of crystallization (T_p) are found to be dependent on heating rate and antimony content. From the heating rate dependence of T_g and T_p the values of the activation energy for glass transition (E_t) and the activation energy for crystallization (E_c) are evaluated and their composition dependence discussed. Crystallization studies have been made under non-isothermal conditions with the samples heated at several uniform rates. Using a recent analysis developed for non-isothermal crystallization studies, information on some aspects of the crystallization process has been obtained. The stability calculations emphasized that the thermal stability decreases with increasing the Sb content.     

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.