Structural studies on boroaluminosilicate glasses

Two series of boroaluminosilicate glasses having varying mole ratios of B₂O₃/Na₂O (series 1) and B₂O₃/SiO₂ (series II) were prepared by conventional melt-quench method. Based on ²⁹Si and ¹¹B MAS NMR studies, it has been established that for series I glasses up to 15 mol% B₂O₃ content, Na₂O preferentially interacts with B₂O₃ structural units resulting in the conversion of BO₃ to BO₄ structural units. Above 15 mol% B₂O₃ for series I glasses and for all the investigated compositions of the series II glasses, silicon structural units are unaffected whereas boron exist in both trigonal and tetrahedral configurations. Variation of microhardness values of these glasses as a function of composition has been explained based on the change in the relative concentration of BO₄ and BO₃ structural units. These glasses in the powder form can act as efficient room temperature ion exchangers for metal ions like Cu²⁺. It is seen that the ion exchange does not affect the boron and silicon structural units as revealed by IR studies.     

Vibrational spectroscopic studies of some lead silicate glasses

Results are presented of measurements on the infrared and laser Raman spectra of some lead silicate glasses of general formula (PbO)_x(SiO₂)_y.For the infrared spectra, the frequency dependence of the imaginary component of the complex dielectric constant was determined from a Kramers-Krönig analysis of the reflectance spectra of the glasses at normal incidence. Apparently the addition of small amounts of PbO to vitreous silica serves to modify the continuous three-dimensional silica network; whereas in those glasses with a high lead content, the influence of the cation, Pb²⁺, appears to lie between that of network former and network modifier.     

Structural studies on AsSe–AgI glasses

The atomic structure of the binary AsSe, ternary (AsSe)₈₀Ag₂₀, (AsSe)₈₅I₁₅ and quaternary (AsSe)₆₅(AgI)₃₅ glasses has been studied with the X-ray and neutron diffraction. The local order was also probed with the extended X-ray absorption fine structure spectroscopy at Ag, As and Se K-edges. All experimental data were modeled simultaneously by the reverse Monte Carlo technique. Analysis of the partial pair distribution functions and their characteristics enabled to retrace the structural changes from binary AsSe to pseudo-binary (AsSe)_100−x(AgI)_x system and to study the influence of Ag and I incorporation on the local structure.     

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 simulations of calcium aluminate glasses

Molecular dynamics simulations of a series of calcium aluminate glasses have been carried out using empirical potentials with a covalent term. The simulations closely reproduce the total neutron correlation function of glasses with 30 and 38 mol% Al₂O₃ and physical properties such as elastic constants. For compositions close to the eutectic 37 mol% Al₂O₃, aluminum is tetrahedrally coordinated by oxygen, but the proportion of five-fold and six-fold coordination and also edge-sharing tetrahedra gradually increases with increasing Al₂O₃ content. The coordination number for oxygen around calcium atoms is close to 6 and this involves a larger coordination at a shorter, well-defined distance, followed by a smaller coordination with a broader range of distances. When the Al₂O₃ content decreases, the calcium aluminate structure becomes depolymerised and the average ring size increases. Also reported is an X-ray photoelectron spectroscopy measurement on a glass sample with 38 mol% Al₂O₃, which shows that 38.6 ± 0.9% of the oxygens are non-bridging, in excellent agreement with the simulations.     

Spectroscopic studies of lead arsenate glasses doped with nickel oxide

Lead arsenate glasses containing different concentrations of NiO ranging from 0 to 1 mol% (in steps of 0.2 mol%) were prepared. The samples were characterized by X-ray diffraction and differential thermal analysis. A number of studies viz., optical absorption, thermoluminescence, magnetic susceptibilities and IR spectra, have been carried out on these glasses. The bands observed in the optical absorption spectra of the glasses have been analyzed using Tanabe-Sugano diagrams for d⁸ ion; the analysis indicates the presence of Ni²⁺ ions in both tetrahedral and octahedral positions. The increase in the concentration of NiO in the glass matrix shows a gradual transformation of nickel ions from tetrahedral positions to octahedral positions in the glass network. The thermoluminescence light output of the X-ray irradiated glasses has exhibited a glow peak at 353 K (with increasing intensity with an increase in the concentration of NiO) in addition to the conventional peak due to the recombination of electron-hole centers; this glow peak is identified due to ³T₂ → ³A₂ emission transition of octahedral Ni²⁺ ions. The value of the magnetic moment evaluated from the measured magnetic susceptibility show a decreasing trend from 3.84 to 3.10 μ_B with the increase in the concentration of NiO. All these studies indicate an increasing presence of octahedral (lasing) Ni²⁺ ions in these glasses with the increase in the concentration of NiO.     

Structural studies of ThO2 containing barium borosilicate glasses

Barium borosilicate glasses containing as much as 18.6 wt% ThO₂ have been prepared by a conventional melt–quench method and characterized by ²⁹Si and ¹¹B magic angle spinning nuclear magnetic resonance (MAS NMR), Infrared (IR) absorption and differential thermal analysis (DTA) techniques for their structural features. Based on ²⁹Si and ¹¹B MAS NMR and IR investigations, it has been established that the borosilicate network is not affected by such a large ThO₂ incorporation. Whereas Si exists as Q³ and Q² structural units in the ratio of ∼0.7, boron exists in both trigonal (BO₃) and tetrahedral (BO₄) configurations and their relative concentrations are not affected by ThO₂ incorporation in these glasses. The higher extent of ThO₂ incorporation in barium borosilicate glasses compared to borosilicate glasses has been attributed to the increased number of non-bridging oxygen atoms present in the barium borosilicate glasses. Based on DTA measurements it has been shown that there is neither any change in the glass transition temperature nor the occurrence of any crystallization up to 1000 °C, for ThO₂ incorporation up to 18.6 wt% in these glasses. For higher concentrations of ThO₂, there is phase separation during glass formation and fine crystallites of ThO₂ are formed as revealed by XRD. These findings are of significance for the nuclear waste management related to thorium fuel cycle.     

Structural studies of solution-made high alkali content borate glasses

The glass forming range of alkali borates has been extended to R = 5.0 (83 mol% alkali oxide) using a solution method. This method involves the reaction between solutions of boric acid (H₃BO₃) and alkali hydroxide (MOH). Physical properties and NMR studies were performed on the intermediate and final glass products of this method. We have obtained results for the entire alkali borate system including lithium, sodium, potassium, rubidium and cesium. The structure of these invert glasses remains enigmatic.     

Structural studies of phosphate glasses with high ionic conductivity

Lithium phosphate glasses with different chemical composition have been studied using infrared absorption spectroscopy (FTIR) and photoelectron spectroscopy (XPS). FTIR and XPS spectra were analyzed to determine the main structural units present in the glass. Absorption bands were identified and assigned to appropriate bond vibrations. It has been shown that analyzed glasses are built up mainly of short chains and rings consisting of metaphosphate structural units - Q². Some pyro- and orthophosphate units Q¹ and Q⁰, respectively are also present. Photoelectron spectroscopy studies have revealed that titanium exists in two valence states: as Ti³⁺ and Ti⁴⁺. Increasing TiO₂ content results in shortening of P-O bonds but leads to elongation of PO bonds in PO₄ tetrahedra.     

Raman spectroscopy of calcium aluminate glasses and crystals

Solar furnace melting and fast-quench techniques have been used to prepare calcium aluminate glasses from 75 mol% CaO to 82 mol% Al₂O₃, which have been studied by Raman spectroscopy. The CaAl₂O₄ glass spectrum may be interpreted in terms of a fully-polymerized network of tetrahedral aluminate units, which is depolymerized on addition of CaO component analogous to binary silicate systems. The spectra of glasses with higher alumina content than CaAl₂O₄ may not be simply interpreted and a structural model is proposed which would be consistent with the glass spectra and with observed crystal structures along the CaAl₂O₄Al₂O₃ join. This model suggests formation of highly condensed aluminate tetrahedral on initial addition of alumina, with the appearance of aluminate polyhedra of higher average coordination at higher alumina content. Similar high coordination polyhedral are also suggested for a limited composition range along the CaOCaAl₂O₄ join. These interpretations are compared with the results of a previous study in the SiO₂Al₂O₃ glass system.     

Optical and FTIR studies of CuO-doped lead borate glasses and effect of gamma irradiation

CuO doped lead borate glasses of the composition PbO 70%–B₂O₃ 30% with varying CuO contents were prepared. UV–visible and infrared spectroscopic studies were measured before and after successive gamma irradiation with two different doses namely 2 and 8 Mrad. The experimental results indicate that the undoped sample reveals strong UV–near visible absorption while copper doped samples show additional broad visible band due to (Cu^2 +) ions.FT infrared absorption spectrum reveals vibrational bands due to triangular and tetrahedral borate groups together with the sharing of Pb–O vibrations.CuO-doped glasses have been found to show a shielding behavior towards the effects of progressive gamma irradiation causing the maintenance of the spectral curves. The changes in the UV–visible and infrared spectral data are discussed in relation to the states of copper ions and structural evolution caused by the change in glass composition including the CuO.     

Structural analysis of bioactive glasses

In this study four series of single and mixed alkali glass systems were made and investigated using MAS NMR. Additionally the densities of the glasses were measured experimentally, as well as calculated theoretically using Doweidar’s model. MAS NMR was used to obtain a quantitative structural understanding of glasses by calculating the concentrations of bridging and non-bridging oxygens per silicon oxygen tetrahedron as a function of the alkali oxide concentration expressed as Qⁿ. ²⁹Si MAS NMR spectra exhibited a single resonance corresponding closely with Si in a Q² state. The chemical shift of the ³¹P MAS NMR peak was attributed to phosphate in an orthophosphate environment. The ²⁹Si NMR spectra are in agreement with the density data. Using Doweidar’s model the proportions of Q² and Q³ were calculated, showing that all glasses studied are predominantly Q² in structure, i.e. silica chains which readily dissolve. The changes in the chemical shifts of the Q² and Q³ species with composition have been interpreted as resulting from the preferential association of Na⁺ with Q³ and Ca²⁺ with Q².     

Thermal and mechanical properties of rare earth aluminate and low-silica aluminosilicate optical glasses

Aluminate glasses containing 45–71.5 mol% alumina, 10–40 mol% rare earth oxide, and 0–30 mol% silica were synthesized from precursor oxides. The glass transition and crystallization temperatures were determined by differential scanning calorimetry; the structural and mechanical properties were investigated by Raman and Brillouin spectroscopy. The range of the supercooled liquid region varies from ∼40 °C to 200 °C, providing a useful working range for compositions with 5–30 mol% silica. Raman scattering showed the presence of isolated SiO₄ species that strengthen the network-forming structure, enhance glass formation, and stabilize the glass even when they are present at fairly low concentrations. Sound velocities were measured by Brillouin scattering. From these and other values, various elastic moduli were calculated. The moduli increased with both aluminum and rare earth content, as did the hardness of the glasses. Young’s modulus was in the range 118–169 GPa, 60–130% larger than that for pure silica glass.     

Structural study of magnesium polyphosphate glasses

Magnesium polyphosphate glasses with molar ratios, y=n(MgO)/n(P₂O₅), ranging from 1.0 to 1.9 have been examined by X-ray and neutron scattering and ³¹P magic angle spinning nuclear magnetic resonance spectroscopy to extract information on their short-range, intermediate-range and submicroscopic structure. The depolymerization of the PO₄ chains with rising MgO content is quantitatively described by the increasing concentration of Q¹ and Q⁰ groups determined by NMR. In the pyrophosphate region the Q¹ sites disproportionate to Q⁰ and Q² groups, whereas there is no disproportionation of the Q² sites at metaphosphate composition. The shortening of the real-space distances, r_m, indicates that the structure of the glasses becomes more compact with progressive depolymerization which is due to the increasing connection of the MgO_n polyhedra by sharing the non-bridging oxygen atoms. The Mg–O co-ordination sphere was found to change not significantly in dependence on composition. Heterogeneities about 1 nm in diameter exist in the glasses with MgO content exceeding 46.6 mol% indicated by a weak small angle X-ray scattering.     

Structure and magnetic properties of lead vanadate glasses

X-ray photoelectron spectroscopy (XPS) has been used to obtain structural information on the xPbO · (1−x)V₂O₅ glass system where x=0.22, 0.35, 0.43, and 0.54. The binding energies from the Pb 4f_7/2 and Pb 4f_5/2 core levels decrease with increasing PbO content while the full-width at half-maximum of these peaks increase. The O 1s spectra show an asymmetry for samples having composition x<0.5, which results from oxygen atoms in the V–O–V configuration (bridging oxygens) and from oxygen atoms in the V–O–Pb and Pb–O–Pb configurations (non-bridging oxygens). The number of non-bridging oxygens was found to increase from 81% to 92% with increasing PbO content. For x=0.54, the O 1s spectrum was symmetric indicating that all three oxygen configurations have essentially the same binding energy. This behavior in addition to the decreasing binding energies of the Pb 4f levels with increasing PbO content suggest that the Pb–O bonds are becoming more covalent in nature and that eventually PbO changed its role from a glass modifier to a glass former for x>0.5. The asymmetric V 2p_3/2 peaks for the x<0.4 glasses indicate the presence of a small concentration of V⁴⁺ ions in addition to V⁵⁺ ions, while the symmetric V 2p_3/2 peaks for the more concentrated PbO vanadate glasses indicate only V⁵⁺ being present. The concentration of V⁴⁺ ions (0–4%) from the XPS data is consistent with determinations from magnetic susceptibility measurements on the same glass samples. In addition to the paramagnetic contribution (Curie–Weiss temperature-dependent behavior) from the V⁴⁺ ions, the susceptibility for these oxide glasses consisted of a positive, constant contribution arising from the temperature-independent paramagnetic V₂O₅ exceeding the diamagnetism from the core ions.     

Structure of lead germanate glasses by Raman spectroscopy

Lead germanate glasses of the xPbO(1−x)GeO₂ compositions with x ranging from 0.200 to 0.625 have been synthesised and their crystallisation behaviour and vibrational properties studied by differential thermal analysis (DTA) and Raman spectroscopy. Raman spectra of the Pb₅Ge₃O₁₁ ceramics also have been measured and compared to lead germanate glass spectra and literature data on β-PbO. Ge atoms in low lead glasses are supposed to be surrounded by four to six oxygen atoms, while the high-lead glass (x=0.625) corresponding to stoichiometry of the Pb₅Ge₃O₁₁ ferroelectrics contains predominantly fourfold coordinated germanium. Structural resemblence of high-lead germanate glasses to the Pb₅Ge₃O₁₁ crystal is proposed rather than to β-PbO. The `boson' peak range in the spectra of lead germanate glasses is discussed taking into account the similarities of the spectra of glassy and crystalline Pb₅Ge₃O₁₁.     

Sintering and crystallization of titanium-containing lead borate glasses

The sintering and crystallization of glass powders with chemical composition 47 PbO—38 B₂O₃—15 TiO₂ (mole %) was investigated under non-isothermal conditions by shrinkage measurements, differential thermo-analysis (DTA), X-ray diffraction (XRD), dielectric measurement and positron annihilation (Doppler broadening). The correlation between changes of macroscopic properties and positron annihilation results recommends this method for the detection of structural changes in glasses.     

Optical and electrical properties of lead silicate glasses

The optical and electrical properties of PbO-SiO₂ glasses containing 24.6 to 65 mole % PbO have been determined. While the short-wavelength cutoff increases continuously in wavelength with increasing PbO concentration, an abrupt lightening in color is observed in the vicinity of 38–40% PbO. The darker color of glasses containing 30–38% PbO has been associated with a scattering process. The origin of this scattering is discussed and is tentatively identified with the formation of microcrystals of SiO₂ in the glasses. The alternative process of liquid-liquid phase separation seems less likely, but cannot be ruled out with certainty.The electrical conductivity of all glasses investigated exhibits an Arrhenian temperature dependence. The activation energy decreases from about 1.23 eV for the 30% PbO composition to about 1.11 eV for the 50% PbO composition, and remains about constant for glasses containing higher concentrations of PbO. Addition of NH₄Cl or Sb₂O₃ to the batch or use of Pb(NO₃)₂ or Pb₃O₄ instead of PbO as a raw material does not affect the electrical properties. The dielectric constant at 1kHz increases continuously with increasing PbO concentration, from a value in excess of 20 for the 55% PbO composition. The implications of these results for the conduction mechanism in PbO-SiO₂ glasses are discussed.