Deep-UV Raman spectroscopic analysis of structure and dissolution rates of silica-rich sodium borosilicate glasses

As part of ongoing studies to evaluate relationships between structure and rates of dissolution of silicate glasses in aqueous media, sodium borosilicate glasses of composition Na₂O·xB₂O₃·(3 − x)SiO₂, with x ≤ 1 (Na₂O/B₂O₃ ratio ≥ 1), were analyzed using deep-UV Raman spectroscopy. Results were quantified in terms of the fraction of SiO₄ tetrahedra with one non-bridging oxygen (Q³) and then correlated with Na₂O and B₂O₃ content. The Q³ fraction was found to increase with increasing Na₂O content, in agreement with studies on related glasses, and, as long as the value of x was not too high, this contributed to higher rates of dissolution in single pass flow-through testing. In contrast, dissolution rates were less strongly determined by the Q³ fraction when the value of x was near unity, and appeared to grow larger upon further reduction of the Q³ fraction. Results were interpreted to indicate the increasingly important role of network hydrolysis in the glass dissolution mechanism as the BO₄ tetrahedron replaces the Q³ unit as the charge-compensating structure for Na⁺ ions. Finally, the use of deep-UV Raman spectroscopy was found to be advantageous in studying finely powdered glasses in cases where visible Raman spectroscopy suffered from weak Raman scattering and fluorescence interference.     

Effect of BaO addition on the structural aspects and thermophysical properties of sodium borosilicate glasses

Sodium borosilicate glasses containing different amounts of BaO were prepared by a conventional melt quench method and characterized for their structural aspects by ²⁹Si, ¹¹B NMR and IR spectroscopy. From ²⁹Si MAS NMR studies, it has been inferred that these glasses consist of Q² and Q³ structural units of silicon and that the addition of BaO results in the marginal conversion of Q³ to Q² structural units. There is no direct interaction between Ba²⁺ ions and boron structural units, as revealed by the identical values of the relative concentration of BO₃ and BO₄ structural units and quadrupolar coupling constant values for the BO₃ structural units. The identical values of glass transition temperature and vibrational frequencies corresponding to Si–O–Si/Si–O–B and B–O linkages of all the samples further support this. As the borosilicate network is unaffected, the systematic increase in the values of thermal expansion coefficient with increase in BaO content has been attributed to the increase in the relative concentration of less rigid Ba–O linkages compared to the more rigid Si–O and B–O linkages in the glass. Such studies will be useful for the development of matrices for the management of nuclear waste generated during the reprocessing of the spent fuel from thoria based reactors.     

Raman and NMR study of bioactive Na2O–MgO–CaO–P2O5–SiO2 glasses

The results of a structural study combining NMR and Raman spectroscopy of several melt-derived glasses in the system Na₂O–MgO–CaO–P₂O₅–SiO₂ are presented. The Raman spectra show clear changes in the Si–O–Si vibrational modes (related to the bridging oxygen atoms, BO) and also verify the presence of non-bridging oxygen atoms (NBO), also named terminal oxygens. The intensity of the Si–O–NBO stretching mode depends on the cation concentration. It can be concluded from the NMR studies that the MgO-containing samples have orthophosphate units charge-compensated by Ca²⁺ and Mg²⁺. The silicate matrix also contains both types of two-valent cations and consists of Q² and Q¹ units. Similarly, the Na₂O-containing samples contain isolated orthophosphate units in a silicate matrix (Q² and Q³ units), both charge-compensated by mixed cations Ca²⁺ and Na⁺. These experimental data were compared with theoretical parameters given by the Stevels model, which is a suitable tool for understanding bioactive behavior of these glasses. Furthermore, results of the in vitro tests carried out in simulated body fluids are presented and compared with both Raman and NMR structural data.     

Increased Si NMR sensitivity in glasses with a Carr-Purcell-Meiboom-Gill echotrain

The use of a Carr-Purcell-Meiboom-Gill (CPMG) echotrain to increase the ²⁹Si NMR sensitivity in glasses was investigated. The echo intensity decay follows a stretched exponential behavior M(t) = M₀ exp[−(t/t2)^β] with values for the exponent β in the range of 0.41-0.65. The signal to noise in the spectra can be increased by a factor of up to 4 by taking the weighted sum of the spectra obtained from the individual echoes. However, differential T2 relaxation for the different Qⁿ species is observed, with a shorter relaxation time for Q⁴ than Q³. Thus, summing of the echoes leads to distorted spectral intensities and quantitative information can no longer be obtained from the spectra. To circumvent the problem with differential T2 relaxation, an alternative approach is developed in which the spectral intensity for each chemical shift value is determined from the stretched exponential fit to its echo decay. With this approach, the sensitivity can be increased by a factor of up to 2.4, while quantitative information can still be obtained from the spectra. The increased sensitivity permitted the detection of five-coordinated silicon in a potassium silicate glass with natural ²⁹Si abundance at ambient pressure.     

Structural characterization of ionomer glasses by multinuclear solid state MAS-NMR spectroscopy

In the present study we report the results of ²⁹Si, ²⁷Al, ³¹P and ¹⁹F magic angle spinning nuclear magnetic resonance spectroscopy (MAS-NMR) of 4.5SiO₂-3Al₂O₃-1.5P₂O₅-(5−z)CaO-zCaF₂ glasses with z = 0-3 to elucidate the effect of fluoride content on the glass structure. The ²⁹Si MAS-NMR spectra gave a chemical shift of about −90 ppm corresponding to Q³(3Al) and Q⁴(3Al). The ²⁷Al MAS-NMR showed a large broad central peak around 50 ppm, which is assigned to four-coordinated Al linked via oxygen to P. A shoulder around 30 ppm and a small peak at about 0 to −10 ppm appeared in the ²⁷Al MAS-NMR spectra of the glasses on increasing the fluoride content assigned to five-coordinated and six-coordinated Al species, respectively. The ³¹P MAS-NMR spectra indicated the presence of Al-O-P bonds. The ³¹P chemical shift decreased with increasing fluoride content as a result of calcium being complexed with fluoride. This resulted in a reduction of the number of available cations to charge balance non bridging oxygens in phosphorus and an increase in the number of Al-O-P bonds being formed, instead. The ¹⁹F spectra indicated the presence of Al-F-Ca(n) and F-Ca(n) species in all the glasses containing fluoride as well as an additional Si-F-Ca(n) species in the glasses with higher fluoride content.     

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.     

Effect of P2O5 content in two series of soda lime phosphosilicate glasses on structure and properties - Part I: NMR

The effect of the variation in phosphate (P₂O₅) content on the structure of two series of bioactive glasses in the quaternary system SiO₂-Na₂O-CaO-P₂O₅ was studied. The first series (I) was a simple substitution of P₂O₅ for SiO₂ keeping the Na₂O:CaO ratio fixed (1.00:0.87). The second series was designed to ensure charge neutrality in the orthophosphate , therefore as P₂O₅ was added the Na₂O and CaO content was varied to provide sufficient Na⁺ and Ca²⁺ cations to charge balance the orthophosphate present. The glass network connectivity (NC) was calculated for each glass and a modification for the presence of a separate P₂O₅ phase was included (NC′). ³¹P and ²⁹Si magic-angle-spinning nuclear magnetic resonance (MAS-NMR) spectroscopy was performed on glass series I and II to determine the structural units present and their relation to glass properties. ³¹P MAS-NMR spectra of series I resulted in a broad resonance around 9 ppm corresponding to orthophosphate in an amorphous environment. The 9.25 mol% P₂O₅ glass shown to be partially crystalline by X-ray diffraction was heat treated, and the ³¹P MAS-NMR spectrum showed a sharp peak around 3 ppm corresponding to calcium orthophosphate or sodium pyrophosphate and overlapping broader peaks at 8.5, 10.5 and 14 ppm possibly corresponding to two mixed calcium-sodium orthophosphate phases and amorphous sodium orthophosphate respectively. ³¹P MAS-NMR spectra of series II resulted in a broad resonance around 10.5 ppm corresponding to orthophosphate in an amorphous environment. ²⁹Si MAS-NMR spectra of glasses from series I showed a shift in the resonance peak from around −78 to −86 ppm indicating an increase in Q³ species in the glass and a reduction in Q² with phosphate addition confirming the presence of orthophosphate. The heat treated sample showed a sharp ²⁹Si-NMR resonance at −88 ppm, indicating a crystalline Q² six-membered combeite (Na₂O · 2CaO · 3SiO₂) silicate-type phase, which was confirmed by powder X-ray diffraction. ²⁹Si MAS-NMR spectra of glasses from series II showed no shift in the resonance at around −78 ppm across the series, confirming an orthophosphate environment.     

Quantitation of sulfate solubility in borosilicate glasses using Raman spectroscopy

Raman spectroscopy is used here as an innovative technique to investigate sulfate content in borosilicate glasses. Using Raman spectroscopy after having heated the material, the evolution of sulfate amounts can be followed as a function of temperature, time and chemical composition of the starting matrix. The accuracy of this technique was verified using electron probe micro analysis (EPMA), on two systems of glasses (SiO₂–B₂O₃–Na₂O (SBNa) and SiO₂–B₂O₃–BaO (SBBa)) in order to compare the effect of alkaline or alkaline-earth elements on sulfur speciation and incorporation. To quantitate sulfate content with Raman spectroscopy, the integrated intensity of the sulfate band at 990 cm^?1 was scaled to the sum of the integrated bands between 850 and 1250 cm^?1, bands that are assigned to Qⁿ silica units. Calibration curves were then determined for different samples. The determination of sulfate contents with Raman spectroscopy analysis is possible with an accuracy of approximately 0.1 wt% depending on the composition of the glass. It mainly allows us to follow sulfate removal during the elaboration process and to establish kinetic curves of sulfate release as a function of the viscosity of the borosilicate glass.     

Influence of zirconium on the structure of pristine and leached soda-lime borosilicate glasses: Towards a quantitative approach by O MQMAS NMR

¹⁷O MQMAS NMR was used to characterize the influence of zirconium on the structural organization of soda-lime borosilicate glasses. A new method of quantitative analysis of the ¹⁷O MQMAS spectra is presented, by a direct fit of the two-dimensional MQMAS spectrum which provides the resolution of all the structural groups in glasses containing up to five oxides. Additional data were also obtained from the quantitative deconvolution of the ¹¹B MAS NMR spectra, with the help of the direct fit of MQMAS data as well. Excess of non-bridging oxygen is clearly identified in these glasses. Sixfolded zirconium is preferentially compensated rather than the tetrahedral boron and calcium only partially compensate the tetrahedral boron. Alteration gels arising from glass leaching were probed by oxygen-17 supplied by the alteration solution. Most of the zirconium is inserted in the silicate network forming Si-O-Zr bonds with the same configuration in the glass and in the gel. During leaching, calcium clearly remains in the alteration gel, either near non-bridging oxygen or as a zirconium charge compensator. This quantitative approach applied to ¹⁷O MQMAS spectra demonstrates its potential for investigating the structure of increasingly complex glass and gel compositions.     

Cation clustering and formation of free oxide ions in sodium and potassium lanthanum silicate glasses: nuclear magnetic resonance and Raman spectroscopic findings

Alkali silicate glasses containing lanthanum oxide are useful model systems for understanding the structural role of rare earth cations in optical and other types of materials. We report ²⁹Si and Raman spectra of sodium and potassium silicate glasses, both with added La₂O₃ and with La₂O₃ substituted for Na₂O or K₂O on an equal-oxygen basis. In the former series, silicate speciation changes show the formation of more non-bridging oxygens (NBO) as more of the network-modifying La₂O₃ is added. In the latter series, however, in which the nominal ratio of NBO to Si is constant, silicate speciation changes indicate that the actual ratio decreases significantly as La is substituted for 3 Na or K. The simplest explanation of this finding is that up to several percent of the oxygen in the La-rich glasses is not bonded to any Si, but instead forms `free oxide' ions that are part of La-rich domains. Although the size of these domains remains unconstrained, the lack of evidence for phase separation and continuity of trends in structure with composition suggests that the metastable liquid structure at the glass transition contains substantial intermediate-range heterogeneity.     

High resolution X-ray Photoelectron Spectroscopy (XPS) study of K2O–SiO2 glasses: Evidence for three types of O and at least two types of Si

High resolution O 1s, K 2p and Si 2p XPS Spectra were collected for a series of potassium silicate glasses ranging in composition from 10 mol% to 35 mol% K₂O. The mole fraction of bridging oxygen (BO) has been accurately evaluated from the O 1s spectra. BO mole fractions of K-silicate glasses were calculated from Q-species distributions previously reported by ²⁹Si MAS NMR data. The mole fractions of BO are identical for the two techniques (within experimental error) in glasses containing 13 mol% to 25 mol% K₂O but in the compositional range between 25 mol% and 35 mol% BO mole fractions obtained by XPS are slightly greater than values derived from NMR data. The slight discrepancies between the two techniques at higher K₂O content have not been resolved. The experimental data between 13 mol% and 25 mol% K₂O indicate the presence of a third type of oxygen, O^2?, in these glasses. A thermodynamic analysis indicates O^2? is present at a few mol% in the glasses of low K₂O content, but increases monotonically with increased K₂O content.The O 1s XPS line widths for the BO peaks are highly variable. The variation in line widths may result from two types of BO contributing to the BO peak. As in the Na₂O–SiO₂ glass system, one type probably bridges two Si atoms (SiOSi) and the second type is O bonded to two Si atoms and one K atom.The Si 2p XPS spectra are distinctly non-symmetric, with low binding energy shoulders commonly present on the major peak, suggesting two contributions to the Si 2p signal. There is a strong correlation of Si 2p XPS peak and shoulder intensities with the abundances of the Q⁴ and Q³ species in glasses of the same composition suggesting that, with additional resolution, XPS may be capable of resolving individual Q-species in this system.     

Iron redox equilibrium, structure and properties of zinc iron phosphate glasses

Iron redox equilibrium, structure and properties were investigated for the 10ZnO-30Fe₂O₃-60P₂O₅ (mol%) glasses melted at different temperatures. The structure and valence states of the iron ions in these glasses were investigated using Mössbauer spectroscopy, Raman spectroscopy and differential thermal analysis. Mössbauer spectroscopy indicated that the concentration of Fe²⁺ ions increased in the 10ZnO-30Fe₂O₃-60P₂O₅ (mol%) glass with increasing melting temperature. The Fe²⁺/(Fe²⁺ + Fe³⁺) ratio increased from 0.18 to 0.38 as the melting temperature increased from 1100 to 1300 °C. The measured isomer shifts showed that both Fe²⁺ and Fe³⁺ ions are in octahedral coordination. It was shown that the dc conductivity strongly depended on Fe²⁺/(Fe²⁺ + Fe³⁺) ratio in glasses. The dc conductivity increases with the increasing Fe²⁺ ion content in these glasses. The conductivity arises from the polaron hopping between Fe²⁺ and Fe³⁺ ions which suggests that the conduction is electronic in nature in zinc iron phosphate glasses.     

Influence of P2O5 content on the structure of SiO2-Na2O-CaO-P2O5 bioglasses by Si and P MAS-NMR

²⁹Si and ³¹P MAS-NMR has been used in order to study the influence of P₂O₅ content on the structure of SiO₂-Na₂O-CaO-P₂O₅ bioglasses. The ²⁹Si NMR spectra show many Q_Siⁿ entities co-existing in the same glass. For all the glasses, the Q_Siⁿ species are associated with Ca²⁺ and Na⁺. The glass network becomes more and more polymerized when the P₂O₅ content increases. The ³¹P NMR spectra show that the phosphorus is present as monophosphate with or without diphosphate complexes associated with both Ca²⁺ and Na⁺. The increase in phosphorus content modifies the relative abundance of phosphate complexes and leads to changes in the chemical environment around phosphate indicating that the distribution of cations is not homogeneous. Indeed most phosphate complexes attract calcium. The percentage of diphosphate complexes increases with the P₂O₅ content as the silicate network repolymerizes.     

Density-structure correlations in Na2O-CaO-P2O5-SiO2 bioactive glasses

Relations for Na₂O-CaO-SiO₂ glasses have been developed to calculate the density of Na₂O-CaO-P₂O₅-SiO₂ bioactive glasses. The calculation makes use of NMR results of O’Donnell et al. indicating that P₂O₅ forms a separate phase, containing Na₃PO₄ and Ca₃(PO₄)₂, in the investigated glasses. The volume of the silicate units is the same as that found in Na₂O-CaO-SiO₂, Na₂O-SiO₂, and CaO-SiO₂ glasses. Similarly, the volume of PO₄ units is equivalent to that in Na₃PO₄ and Ca₃(PO₄)₂. Calculated densities are consistent with the experimental data.     

Tin valence and local environments in silicate glasses as determined from X-ray absorption spectroscopy

X-ray absorption spectroscopy (XAS) was used to characterize the tin (Sn) environments in four borosilicate glass nuclear waste formulations, two silicate float glasses, and three potassium aluminosilicate glasses. Sn K-edge XAS data of most glasses investigated indicate Sn⁴⁺O₆ units with average Sn-O distances near 2.03 Å. XAS data for a float glass fabricated under reducing conditions show a mixture of Sn⁴⁺O₆ and Sn²⁺O₄ sites. XAS data for three glasses indicate Sn-Sn distances ranging from 3.43 to 3.53 Å, that suggest Sn⁴⁺O₆ units linking with each other, while the 4.96 Å Sn-Sn distance for one waste glass suggests clustering of unlinked Sn⁴⁺O₆ units.     

The structure of borosilicate glasses studied by Raman scattering

Raman spectra of alkali and alkaline earth borosilicate glasses are reported. These spectra are used to discuss the molecular structure of the glasses. The influence of Al₂O₃ additions on the structure of borosilicate glass is also discussed. It is shown that the same type of groups are present in borosilicate glasses as in borate and silicate glasses. The presence of large borate groups such as tetraborate and metaborate groups is strongly suggested by the Raman spectra. It appears that boron ions are hardly taken up in the silicon-oxygen network. Our results suggest that the region of phase separation is larger than the region presently acknowledged.     

Bridging, non-bridging and free (O) oxygen in Na2O-SiO2 glasses: An X-ray Photoelectron Spectroscopic (XPS) and Nuclear Magnetic Resonance (NMR) study

High resolution Na 1s, O 1s and Si 2p core level XPS spectra of six Na₂O-SiO₂ glasses ranging in composition from 100 to 45 mol % SiO₂ have been collected using the Kratos Ultra Axis instrument with its unique charge compensation system. The O 1s spectra for the glasses are well resolved so that bridging oxygen (BO, Si-O-Si) and non-bridging oxygen (NBO, Na-O-Si) signals can be accurately fitted and quantified without resorting to constraints or assumptions. The same samples were analysed by ²⁹Si MAS NMR to obtain Q-species abundances from which BO and NBO proportions were calculated. Similar BO:NBO ratios were obtained by both methods over the entire compositional range studied. They are also consistent with most previous XPS and NMR results for glasses containing more than ~ 65 mol % SiO₂. Our XPS and NMR experimental results, however, differ somewhat from previously published XPS and NMR results for glasses containing less than about 65 mol % SiO₂.Na is mobile in the X-ray beam and mobility causes BO:NBO ratios to increase with time of exposure. Na mobility here has been circumvented to yield reliable BO:NBO ratios of the glasses. The ratios are lower than previously reported in XPS studies and are similar to ratios obtained from our ²⁹Si MAS NMR results on the same glasses. The XPS and ²⁹Si MAS NMR results also indicate the presence of a third oxygen species in sodic glasses. As has been proposed for CaSiO₃ glass and for sodic and potassic glasses containing La, we suggest that O²⁻ is present in sodic glasses at small concentrations. The O²⁻ content correlates with increased soda content and may be associated with, and instrumental in development of, three dimensional percolation channels in the glasses. The XPS O 1s line width of the BO peak is broader than the NBO peak, indicating more than one contribution to the BO peak. As observed in crystalline Na metasilicate and Na disilicate, BO of Na-silicate glasses may be of two types, one arising from BO bridging two Si atoms, and the second BO signal arising from BO bonded not only to two Si atoms but also to Na.     

Effect of boron oxide addition on the Nd environment in a Nd-rich soda-lime aluminoborosilicate glass

The environment of Nd³⁺ ions has been studied using optical absorption spectroscopy and EXAFS at the Nd L₃-edge, in a series of soda lime aluminoborosilicate glasses with increasing B₂O₃ content. The proportion of BO₄ units has been determined by ¹¹B MAS NMR in an equivalent glass series with La³⁺ ions replacing the majority of Nd³⁺ ions, and complementary information has been obtained by measuring the Nd³⁺ decay fluorescence times in these latter glasses. In these glasses with low Al₂O₃ content, the R′ ratio, with R′ = [Na₂O^exc] / [B₂O₃] and [Na₂O^exc] = [Na₂O] − [Al₂O₃] − [ZrO₂], plays a key role in controlling the structural organization and crystallization resistance, in a similar way as the R ratio in the Dell and Bray model of sodium borosilicate glasses. At R′ > 0.5, the Nd³⁺ ions are located in a mixed silicate-borate environment and, by slow cooling of the melt, they tend to crystallize within a silicate apatite phase close to the Ca₂Nd₈(SiO₄)₆O₂ composition. At R′ < 0.5, the structural results are compatible with Nd³⁺ ions located in a borate-type environment (not excluding Si neighbors), and, by slow cooling of the melt, they segregate with Ca²⁺ ions within a Si-depleted separated borosilicate phase.     

Quantitative Raman spectroscopy: Principles and application to potassium silicate glasses

A mathematical approach was developed to interpret Raman spectra of binary silicate glasses and melts without the necessity of external calibration, e.g., from NMR spectroscopy. The developed approach is based on Principal Component Analysis (PCA), linear combinations of partial Raman spectra and a linear optimization technique. In order to apply and to test this approach, we developed an experimental method to collect a large number of Raman spectra efficiently. We applied the quantification and the experimental approaches to investigate potassium silicate glasses with compositions from 17.4 to 38 mol% K₂O. The equilibrium constant for the reaction 2Q³ ⇄ Q² + Q⁴ was found log K₃ = −2.37 ± 0.07, in excellent agreement with NMR studies for the same glasses.     

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.