Study of the alkaline environment in mixed alkali compositions by multiple-quantum magic angle nuclear magnetic resonance (MQ–MAS NMR)

45S5 Bioglasses of the composition 46.1 SiO₂–2.6 P₂O₅–26.9 CaO–(24.4 ? x) Na₂O–xMe₂O (Me = Li or K) have been investigated using MAS NMR and MQ–MAS NMR methods. The analysis of the ²⁹Si MAS NMR spectrum revealed two lineshapes whose chemical shift is consistent with two silica Q_n=2,3 species. The ³¹P MAS NMR spectrum reveals the effect of both Na and Ca ions. The chemical shift of the observed ³¹P signal is intermediate between those of Na₃PO₄ (near 10 ppm) and Ca₃(PO₄)₂ (near 3–0 ppm) species. The ²³Na MAS NMR spectra were observed in the alkali oxide composition: 24.4 Na₂O, 12.2 Na₂O–12.2 K₂O and 12.2 Na₂O–12.2 Li₂O. The substitution of Na with Li or K was done to determine the extend of alteration of the glass structure. This goal was best accomplished by ²³Na MQ–MAS NMR. The two-dimensional spectra revealed three sites in the 24.4 mol% Na₂O glass. These sites were not resolved in the 1D MAS NMR spectroscopy. In the mixed glasses, only two sites were obtained.     

Phosphate speciation in sodium borosilicate glasses studied by nuclear magnetic resonance

The structure of RNa₂O · B₂O₃ · KSiO₂ · xP₂O₅ (0.5 < R < 2; 0.86 < K < 3) borosilicate glasses has been studied by nuclear magnetic resonance (NMR). ³¹P magic angle spinning (MAS), double quantum-magic angle spinning (DQ-MAS) and ³¹P–¹¹B transfer of populations under double resonance magic angle spinning (TRAPDOR MAS) NMR were used to determine the phosphate speciation in the glasses and their connectivity with the borosilicate network. The structure of the glass network was characterized with ¹¹B, ²⁹Si and ²³Na MAS NMR. Ab initio calculations of the ³¹P chemical shielding were carried out in order to confirm the connectivity between phosphorus and the structural units of the borosilicate glass network. Na₃PO₄ (monophosphate), Na₄P₂O₇ (diphosphate) and P–O–B species (mono- and diphosphate groups with borate units as the next nearest neighbors) are found all along the compositional range studied. The proportion of the P–O–B groups increases as the glass optical basicity decreases, while the proportions of mono- and diphosphate species decrease. The change in the glass transition temperature of the phospho-borosilicate glasses with respect to that of the borosilicate ones is discussed in terms of the structural characterization. The formation of phosphate species gives rise to the increase in T_g, which is attributed to the re-polymerization of the silicate network, while the formation of P–O–B bonds weakens the glass network and produces a decrease in the glass transition temperature.     

Structure–property correlations in lead borate and borosilicate glasses doped with aluminum oxide

Glass samples from four systems: xPbO–(100?x)B₂O₃ (x = 30, 40, 50 and 60 mol%), 50PbO–yAl₂O₃–(50?y)B₂O₃ (y = 2, 4, 6, 8 mol%), 50PbO–ySiO₂–(50?y)B₂O₃ (y = 5, 10, 20, 30 mol%) and 50PbO–5SiO₂–yAl₂O₃–(45?y)B₂O₃ (y = 2, 4, 6, 8 mol%) were prepared by a melt-quench technique. Characterization of these systems was carried out using density measurements, UV–visible spectroscopy, differential scanning calorimetry (DSC), and ¹¹B and ²⁷Al magic-angle spinning (MAS) solid-state nuclear magnetic resonance (NMR). Our studies reveal an increase in glass density with increasing lead(II) oxide concentration in pure lead borates and also with addition of silica into 50PbO–50B₂O₃ glass. ¹¹B MAS NMR measurements determine that the fraction of tetrahedral borons (N₄) reaches a maximum for the glass containing 50 mol% of PbO in the PbO–B₂O₃ glass series and that N₄ is sharply reduced upon adding small amounts of Al₂O₃ into lead borate and lead borosilicate systems. ²⁷Al MAS NMR experiments performed on glasses doped with aluminum oxide show that the Al³⁺ are tetra-, penta- and hexa-coordinated with oxygen, even without any excess concentration of Al³⁺ over charge-balancing Pb²⁺ cations. ^[5]Al and ^[6]Al concentrations are found to have unusually high values of up to 30%. The results of UV–visible absorption spectroscopy, DSC and density measurements support the conclusions drawn from the NMR studies, providing a consistent picture of structure–property relations in these glass systems.     

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.     

Structure and properties of MoO3-containing zinc borophosphate glasses

ZnO–B₂O₃–P₂O₅ glasses doped with MoO₃ were investigated in the series (100?x)[0.5ZnO–0.1B₂O₃–0.4P₂O₅]–xMoO₃, where bulk glasses were obtained by slow cooling in air within the compositional region of 0 ? x ? 60 mol% MoO₃. The incorporation of MoO₃ into the parent zinc borophosphate glass results in a weakening of bond strength in the structural network, which induces a decrease in chemical durability and glass transition temperature. Raman spectra reflect the incorporation of molybdate groups into the glass network of the studied glasses by the presence of the polarized vibrational band at ≈976 cm^?1 ascribed to the MO_x symmetric stretching vibrations and the depolarized band at ≈878 cm^?1 ascribed to the Mo–O–Mo stretching vibration. The incorporation of molybdate units into the glass network results in the depolymerization of phosphate chains and the formation of P–O–Mo bonds, as reflected in Raman and ³¹P NMR spectra. According to the ¹¹B MAS NMR spectra, tetrahedral B(OP)_4?x(OMo)_x units are formed in the glasses, whereas only a small amount of BO₄ units is converted to BO₃ units in the MoO₃-rich glasses.     

Effect of glass composition on activation energy of viscosity in glass-melting-temperature range

In the high-temperature range, where the viscosity (η) of molten glass is < 10³ Pa s, the activation energy (B) is virtually independent of temperature (T). Moreover, the coefficient A in the Arrhenius relationship, ln(η) = A + B/T, is nearly independent of melt composition. Hence, the viscosity–composition relationship for η < 10³ Pa s is defined by B as a function of composition. Using a database encompassing over 1300 compositions of high-level waste glasses with nearly 7000 viscosity data, we developed mathematical models for B(x), where x is the composition vector in terms of mass fractions of components. In this paper, we present 13 versions of B(x) as first- and second-order polynomials with coefficients for 15 to 39 components, including Others, a component that sums constituents having little effect on viscosity.     

Structure and properties of potassium niobato-borophosphate glasses

Bulk glasses of the series (1 ? x)[0.5K₂O–0.1B₂O₃–0.4P₂O₅]–xNb₂O₅ with x = 0–45.7 mol% Nb₂O₅ were prepared by slow cooling in air and investigated by Raman, ³¹P, and ¹¹B MAS NMR spectroscopy. The incorporation of Nb₂O₅ into the parent borophosphate glass results in a substantial increase in the glass transition temperature and chemical durability of glasses. Raman spectra showed that Nb atoms form distorted NbO₆ octahedra, which are isolated at low Nb₂O₅ content, whereas at higher Nb₂O₅ content they form clusters. ¹¹B NMR spectra of the glasses revealed the interaction between Nb₂O₅ and BO₄ tetrahedral units, which results in a partial transformation of tetrahedral BO₄ units to trigonal BO₃ units and the formation of mixed B(OP)_4?n(ONb)_n units.     

Spectral studies of Sm3+ and Dy3+ doped lithium cesium mixed alkali borate glasses

The effect of host glass composition on the optical absorption and fluorescence spectra of Sm³⁺ and Dy³⁺ has been studied in mixed alkali borate glasses of the type 67B₂O₃ · xLi₂O · (32 − x)Cs₂O (x = 8, 12, 16, 20 and 24). The Judd–Ofelt intensity parameters (Ω₂, Ω₄ and Ω₆) are calculated. The radiative transition probabilities (A), radiative lifetimes (τ_R), branching ratios (β) and integrated absorption cross-sections (Σ) are computed for certain excited states of Sm³⁺ and Dy³⁺ ions for different x values in the glass matrix. Stimulated emission cross-sections (σ_p) are obtained for certain emission transitions of two ions in these mixed alkali borate glasses. These parameters are compared for different x values in the glass matrix. Variation of these parameters with x in the glass matrix has been studied.     

Structure and properties of Sb2O3-containing zinc borophosphate glasses

ZnO–B₂O₃–P₂O₅ glasses formulated with Sb₂O₃ were investigated in the series 50ZnO–10B₂O₃–40P₂O₅ + xSb₂O₃ (x = 0–70 mol%). With increasing Sb₂O₃ content, the values of glass transition temperature decrease from 492 °C down to 394 °C. The dissolution rate of the glasses reveals a maximum for the glass with x = 15 mol% Sb₂O₃. Raman spectra with increasing Sb₂O₃ content reflect the depolymerisation of phosphate chains. Antimony at low Sb₂O₃ content forms individual SbO₃ pyramids manifested in the Raman spectra by a broad vibrational band at ∼520–690 cm⁻¹. In the glasses with a higher Sb₂O₃ content SbO₃ units link into chains and clusters with Sb–O–Sb bridges manifested in the Raman spectra by a strong broad band at 380–520 cm⁻¹. The ³¹P MAS NMR spectra with increasing Sb₂O₃ content reflect the depolymerisation of phosphate chains at low Sb₂O₃ content and only small changes in the PO₄ coordination at a high Sb₂O₃ content. ¹¹B MAS NMR spectra reveal a steady transformation of B(OP)₄ units into B(OP)_4−x(OSb)_x units, accompanied by the transformation of BO₄ into BO₃ units with increasing Sb₂O₃ content.     

Structural characteristic of Na2OP2O5GeO2 glass systems

Phosphate glasses based on xNa₂O0.5P₂O₅(0.5−x)GeO₂ (0.0 ⩽ x ⩽ 0.5) mol%, were prepared and their structures were characterized by magic angle spinning (MAS) nuclear magnetic resonance (NMR), Raman and IR spectroscopy techniques. It was found that the phosphate network of these glasses is composed of middle (Q²) and branching (Q³) phosphate tetrahedra, whereas germanium part in the network is composed of three- or four-membered GeO₄ tetrahedral rings. It was also found that the germanium tetrahedral are randomly connected to either Q² or Q³ phosphate units in the network. The glass network, especially the Q² units can be modified by the presence of Na ions. This modification is primarily associated with the phosphate. It is found that these glasses behave as if they are formed from a solution of GeO₂ and sodium–phosphate glass with various GeO₄ units and the Q² and Q³ phosphate units randomly distributed in the network.     

Redox behavior and diffusion of copper in soda-lime-silica melts

Glasses with the base mol% composition xNa₂O · 10CaO · (90 ? x)SiO₂ with x = 10, 16, 20 and 26 were investigated at high temperatures using square-wave voltammetry. The recorded voltammograms exhibit two peaks. That at less negative or (depending on temperature and glass composition) even positive potential is attributed to the reduction of Cu²⁺ to Cu⁺, while that observed at more negative potential is caused by the reduction of Cu⁺ to metallic copper. For both redox steps, the peak potentials decrease linearly with temperature. Those of the composition with 10 mol% Na₂O show the most negative values. The diffusion coefficients can be fitted to Arrhenius equation. If referenced to the same viscosity, the diffusion coefficients decrease with increasing Na₂O-concentration. The effect of composition on the thermodynamics as well as on diffusivities is explained by the incorporation of the copper ions into the melt structure.     

Mixed conductivity in tungstenite–phosphate glasses containing alkali metal ions

The conductivity of glasses in the 50WO₃–(50 ? x)P₂O₅–xA₂O (A = Na, K, Cs) system has been investigated as a function of composition. It is shown that in tungstenite–phosphate glasses containing different alkali metal ions the conductivity decreases with an increase in the alkali metal ion content. A decrease in conductivity is larger for heavier ions and reaches more than seven orders of magnitude in the case of glass containing Cs₂O. This behavior remains in contrast to the literature data on conductivity in transition metal oxide glasses containing alkali metal ions where usually strong conductivity anomalies of several orders of magnitude at certain amount of ions are observed. No necessity of ion–polaron interaction has been pointed out.     

Glasses in the SbPO4–WO3 system

Glasses in the binary system (100 − x)SbPO₄–xWO₃ (20 ⩽ x ⩽ 60, x in mol%) have been prepared and characterized. Differential thermal analysis (DTA) shows that the glass transition temperature, T_g, increases from 412 °C, for samples containing 20 mol% of WO₃, to 481 °C observed for glass containing 60 mol%. Sample containing 40 mol% in WO₃ were observed to be the most stable against devitrification. The structural organization of the glasses has been studied by using Fourier transform infra-red (FTIR), Raman, ³¹P Magic angle spinning (MAS) and spin echo nuclear magnetic resonance (NMR) spectroscopies. Results suggest two distinct networks comprising the glass structure, one with high SbPO₄ content and the other characteristic of the highest WO₃ content samples. The glasses present photochromic properties. Colour changes are observed for samples after exposure to ultraviolet or visible laser light. XANES, at L₁ absorption edge of tungsten, suggests partial reduction from W⁶⁺ to W⁵⁺ species during the laser irradiation. The photochromic effects and the colour changes, promoted by laser excitation, are reversible and easily removed by heat for during 1 h at 150 °C. Subsequent ‘write/erase’ cycles can be done without degradation of the glasses.     

Novel red–blue dichroic glass containing copper nanocrystals

Copper ruby glasses are commonly produced by dissolving copper in glass (also containing a reducing agent) under relatively reducing conditions to stabilize Cu⁺, followed by a heat treatment to generate the characteristic red color attributed to a colloidal dispersion of Cu⁰ particulates. This study produced copper ruby glasses in a one-step synthesis; 0.1–0.2 wt% copper was included in an alkali borosilicate glass that was synthesized at a very reducing atmosphere. The classic red coloration was directly generated by the colloidal dispersion of elemental copper nanoparticles, (Cu⁰)_n, that formed without further heat treatment. Addition of europium to the copper-containing glass melts led to dramatic spectral changes, with the europium addition not only affecting the amount of elemental copper but also the size of these copper particulates. The net result was the production of a dichroic glass that was ruby red in reflected light and blue in transmitted light. This dichroism was stable only over a certain range of copper nanoparticle sizes, unique to particular Cu/Eu ratios in the glass and optimized at 1150 °C in the alkali borosilicate composition.     

Structure and properties of the 70Li2S · (30 − x)P2S5 · xP2O5 oxysulfide glasses and glass–ceramics

The 70Li₂S · (30 ? x)P₂S₅ · xP₂O₅ (mol%) oxysulfide glasses were prepared by the melt quenching method. The glasses were prepared in the composition range 0 ≦ x ≦ 10. The glass–ceramics were prepared by heating the glasses over crystallization temperatures. The PO_nS_3?n (n = 1–3) oxysulfide units were produced in the glasses and glass–ceramics by partial substituting P₂O₅ for P₂S₅. In particular, the P₂OS₆^4? unit would be produced by substituting a small amount of P₂O₅ for P₂S₅. The oxygen atoms were incorporated into the Li₇P₃S₁₁ crystal structure because the diffraction peaks of the oxysulfide glass–ceramic shifted to the higher angle side. The glass–ceramic with 3 mol% of P₂O₅ exhibited the highest conductivity of 3.0 × 10^?3 S cm^?1 and the lowest activation energy for conduction of 16 kJ mol^?1. The P₂OS₆^4? dimer units in the oxygen-incorporated Li₇P₃S₁₁ crystal would improve conductive behavior of the Li₂S–P₂S₅ glass–ceramics.     

Effects of modifier additions on the thermal properties,chemical durability,oxidation state and structure of iron phosphate glasses

Modified iron phosphate glasses have been prepared with nominal molar compositions [(1?x)·(0.6P₂O₅–0.4Fe₂O₃)]·xR_ySO₄, where x = 0–0.5 in increments of 0.1 and R = Li, Na, K, Mg, Ca, Ba, or Pb and y = 1 or 2. In most cases the vast majority or all of the sulfate volatalizes and quarternary P₂O₅–Fe₂O₃–FeO–R_yO_z glasses or partially crystalline materials are formed. Here we have characterized the structure, thermal properties, chemical durability and redox state of these materials. Raman spectroscopy indicates that increasing modifier oxide additions result in depolymerization of the phosphate network such that the average value of i, the number of bridging oxygens per –(PO₄)– tetrahedron, and expressed as Qⁱ, decreases. Differences have been observed between the structural effects of different modifier types but these are secondary to the amount of modifier added. Alkali additions have little effect on density; slightly increasing T_g and T_d; increasing α and T_liq; and promoting bulk crystallization at temperatures of 600–700 °C. Additions of divalent cations increase density, α, T_g, T_d, T_liq and promote bulk crystallization at temperatures of 700–800 °C. Overall the addition of divalent cations has a less deleterious effect on glass stability than alkali additions. ⁵⁷Fe Mössbauer spectroscopy confirms that iron is present as Fe²⁺ and Fe³⁺ ions which primarily occupy distorted octahedral sites. This is consistent with accepted structural models for iron phosphate glasses. The iron redox ratio, Fe²⁺/ΣFe, has a value of 0.13–0.29 for the glasses studied. The base glass exhibits a very low aqueous leach rate when measured by Product Consistency Test B, a standard durability test for nuclear waste glasses. The addition of high quantities of alkali oxide (30–40 mol% R₂O) to the base glass increases leach rates, but only to levels comparable with those measured for a commercial soda-lime-silica glass and for a surrogate nuclear waste-loaded borosilicate glass. Divalent cation additions decrease aqueous leach rates and large additions (30–50 mol% RO) provide exceptionally low leach rates that are 2–3 orders of magnitude lower than have been measured for the surrogate waste-loaded borosilicate glass. The P₂O₅–Fe₂O₃–FeO–BaO glasses reported here show particular promise as they are ultra-durable, thermally stable, low-melting glasses with a large glass-forming compositional range.     

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.     

A molecular dynamics study of the role of the cation in modifying the structure of alkali borate glasses

The structures of the full series of alkali borate glasses (M₂O)_x(B₂O₃)_1?x (M = Li, Na, K, Rb and Cs) at two different concentrations, x = 0.14 and x = 0.30, have been investigated by means of molecular dynamics simulations. Additional compositions have also been investigated for the lithium and caesium borate glasses (x = 0.10, 0.20, 0.25, and 0.40). The main experimental trends are well reproduced by the simulations, even if the agreement is not quantitative. Our results indicate that lithium atoms can enter into the matrix of pure vitreous B₂O₃ without inducing large modifications in the B–O network, even at large concentrations. However when the other alkali ions are added to the initial structure, the network opens to accommodate the larger size of the cation. These modifications induce the appearance of a low-Q shoulder or pre-peak, whose intensity increases with increasing alkali concentration as well as with increasing alkali size.     

Spectroscopic properties and ab initio calculations on the structure of erbium–zinc-borate glasses and glass ceramics

Glasses in the (Er₂O₃)_x·(B₂O₃)_(60 ? x)·(ZnO)₄₀ system (0 ≤ x ≤ 15 mol%) have been prepared by the melt quenching technique. X-ray diffraction, FTIR spectroscopy, UV-VIS spectroscopy and ab initio calculations studies have been employed to study the role of Er₂O₃ content on the structure of the investigated glass system.X-ray diffraction and infrared spectra of the glasses reveal that the B–O–B bonds may be broken with the creation of new non-bridging oxygen ions facilitating the formation of Er–O–B linkages. The excess of oxygen can be accommodated in the network by the conversion of sp² planar [BO₃] units to the more stable sp³ [BO₄] tetrahedral structural units. The linkages of the [BO₄] structural units can polymerize in [B₃O₉]^? 9 cyclic trimeric ions which will produce the ErBO₃ crystalline phase. An increase of the efficiency corresponding to the ⁴I_15/2 state to ⁴I_11/2 state (4f–4f) transitions of Er^+ 3 ions was observed for the erbium oxide richest glasses.Ab initio calculations on the structure of the matrix network show the thermodynamic instability of the [BO₄], [ZnO₄] and [Zn₄O] structural units. Formation of three-coordination oxygens was necessary to compensate shortage of oxygens from zinc ions.     

EPR and magnetic susceptibility studies of B2O3–Bi2O3–Gd2O3 glasses

Glasses of the xGd₂O₃ · (100 − x)[B₂O₃ · Bi₂O₃] system with 0.5 ⩽ x ⩽ 10 mol% were studied by electron paramagnetic resonance (EPR) and magnetic susceptibility measurements. Data obtained show that for low gadolinium oxide contents of the samples (x ⩽ 3 mol%) the Gd³⁺ ions are randomly distributed in the host glass matrix and are present as isolated and dipole–dipole coupled species. For higher gadolinium oxide contents of the samples (x > 3 mol%) the Gd³⁺ ions appear as both isolated and antiferromagnetically coupled species. The EPR spectra of the glasses reveal resonance sites with an unexpected high crystalline field in addition to the ‘U’ spectrum, typical for Gd³⁺ ions in disordered systems. This absorption line is due to Gd³⁺ ions that replace Bi³⁺ ions from the host glass matrix and could play the network unconventional former role in the studied glasses.