Nuclear magnetic resonance studies of the glasses in the system Na2OB2O3SiO2

The ¹¹B NMR spectra have been used to study the structure of glasses in the system Na₂OB₂O₃SiO₂. The fraction of BO₄ units, and the fraction of BO₃ units with one or two nonbridging oxygens, are measured and analyzed according to a structural model. The results indicate that: (1) for a sodium oxide to boron oxide ratio of 0.5 or less, the Na⁺¹ ions are attracted primarily by the borate network; therefore, the ternary glasses can be viewed as binary sodium borate glasses diluted by SiO₂; (2) when the sodium oxide to boron oxide ratio exceeds 0.5, the additional Na₂O results in the formation of [BSi₄O₁₀]^?1 units at the expense of diborate and SiO₄ units. In this process, Na⁺¹ ions are still taken up only by the borate network. After all the available SiO₄ units are consumed to form [BSi₄O₁₀]^?1 units, additional Na⁺¹ ions are proportionally shared between the borate and silicate networks.     

A structural interpretation of B10 and B11 NMR spectra in sodium borate glasses

It is shown that the B¹⁰ spectra for sodium borate glasses are sensitive to the different structural groups in the glasses. Five boron sites are inferred from the data: two 4-coordinated sites and three 3-coordinated sites. The two 4-coordinated boron sites are identified as BO₄ units connected to all BO₃ units, and BO₄ units connected to one BO₄ unit and three BO₃ units. The three 3-coordinated boron sites are identified as BO₃ units connected to: (1) all BO₃ units; (2) a mixture of BO₃ and BO₄ units; and (3) all BO₄ units. These five sites can be interpreted in terms of Krogh-Moe's structural model of alkali borate glasses, wherein the fraction of each structural group can be determined for eight sodium borate glasses spanning the compositional range from 0 to 35 mol% Na₂O. The resulting fractions are consistent with Krogh-Moe's structural model.     

Optical absorption of cobalt (II) in binary thallium borate glasses

The optical absorption spectra of cobalt (II) in Tl₂OB₂O₃ glasses have been studied and compared with those in binary alkali borate glasses. In thallium borate glasses cobalt (II) may be present in octahedral and/or in tetrahedral symmetry depending upon the composition of the glass. In low thallium borate glasses cobalt (II) is octahedral while the concentration of tetrahedral cobalt (II) increases with increasing Tl₂O content of the glass; the formation of tetrahedral cobalt (II) becomes noticeable when the concentration of Tl₂O reaches above the critical concentration of about 19 mol %. The ligand field parameters: 10Dq and B have been calculated from the absorption spectra of cobalt (II) in different glasses and it has been found that the Racah parameter, B, is more in Tl₂OB₂O₃ glasses than those in Na₂OB₂O₃ or K₂OB₂O₃ glasses of corresponding molar composition. This indicates that the donor capacity of the BO₄ group in thallium borate glasses is lower than that in alkali borate glasses; this is consistent with the NMR results in Tl₂OB₂O₃ glasses containing less than 20 mol % Tl₂O where three BO₄ groups have been found to form with each Tl₂O unit added.     

11B and 27Al NMR studies of glasses in the system Na2OB2O3Al2O3 (“NABAL”)

¹¹B and ²⁷Al nuclear magnetic resonances (NMR) in glasses of the NABAL system Na₂OB₂O₃Al₂O₃ have been studied as a function of composition. From the boron data, the fraction of four-coordinated BO₄ units has been determined via computer analysis of the NMR spectra; the method is similar to that employed previously for binary and other ternary borate glasses. The ²⁷Al NMR indicates no abrupt change in the average aluminum environment. Certain linear relationships have been found which yield detailed information on the competing processes of BO₃, BO₄ and AlO₄ formation, and the formation of triclusters consisting of three tetrahedra having one oxygen in common. Furthermore, it is concluded that the oxygen available for the formation of various aluminum-containing species is a function of the soda concentration only and that the conversion to AlO₄ is favored as compared with BO₄.     

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.     

Nuclear magnetic resonance studies of the glasses in the system K2OB2O3P2O5

B¹¹ NMR spectra have been used to study the structure of glasses in the system K₂OB₂O₃P₂O₅. The results indicate that the glasses do not contain an appreciable number of boron atoms in BO₃ units with one or two non-bridging oxygens. The fraction N₄ of boron atoms in BO₄ units is measured and analyzed according to a structural model containing the following elements. (1) If the binary borophosphate system forms glasses, they consist of a borophosphate (BPO₄) network and a borate network for K<1, or a borophosphate (BPO₄) network and a phosphate network for K>1, where K = mol.% P₂O₅/mol.% B₂O₃. (2) The conversion rates of BO₄ units (i.e. the rate of production or destruction by added oxygens) in the borate network and the borophosphate (BPO₄) network are given as (+2) and (?0.38), respectively. (3) K⁺¹ ions are proportionally shared between the two networks; (i.e. between the borate and borophosphate (BPO₄) networks for K<1, and between the phosphate and borophosphate (BPO₄) networks for K>1).     

Structural investigation of SnO-B2O3 glasses by solid-state NMR and X-ray photoelectron spectroscopy

Local structure of the SnO-B₂O₃ glasses was investigated using several spectroscopic techniques. ¹¹B MAS-NMR spectra suggested that BO₄ tetrahedral units maximized at around the composition with 50 mol% SnO. The BO₄ units were still present at compositions with high SnO content (67 mol% SnO), suggesting that SnO acted not only as a network modifier but also as a network former. O1s photoelectron spectra revealed that the addition of small amounts of SnO formed non-bridging oxygens (NBO) (B-O?Sn) and the amounts of NBO increased with an increase in SnO content. ¹¹⁹Sn Mössbauer spectra indicated that Sn was present only as Sn(II) in the glasses. The structure of the SnO-B₂O₃ glasses was compared with that of conventional alkali borate glasses and lead borate glasses. The thermal and viscous properties of these glasses were discussed on the basis of the glass structure revealed in the present study.     

10B NMR studies of lithium borate glasses

¹⁰B NMR studies have been carried out on lithium borate glasses over the entire glass-forming region of the system. Using the ideas of Krogh-Moe, the relative abundances of the various structural groupings are inferred from the data by fitting computer-simulated lineshapes to the experimental spectra. Structural model are proposed which are consistent with the data: for 0.4 R < 1.0, where R = mol% Li₂O/mol% B₂O₃, the model states that diborate and tetraborate units are proportionately destroyed to form metaborate units and loose BO₄ units; for 1.0 < R < 1.86, the model states that metaborate units and loose BO₄ units are destroyed linearly but not proportionately to form pyroborate and orthoborate units. These results are compared with earlier ¹¹B NMR studies from this laboratory.     

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.     

Anomalous physical properties and Raman spectra of glassy B2O3BaTiO3Na2O materials

Anomalous physical properties (refractive index and density) of B₂O₃BaTiO₃Na₂O ternay glasses are determined and discussed on the basis of the structure present in the glasses and evidenced by vibrational Raman spectroscopy.These glasses behave in a manner analogous to the alkali B₂O₃X₂O binary glasses for molar ratio R = basic oxide/B₂O₃ up to 0.3, with oxygen binding by means of bridging bonds while boron coordination changes from the trigonal to tetrahedral type. The phenomenon is indicated by a progressive weakening of the 806 cm^?1 peak (attributable to a breathing vibration of the boroxol unit) and by a concomitant strengthening of the ～775 cm^?1 peak (attributable to a vibrational mode of boroxol units, or derived units, containing at least one 4-coordinate boron atom). For higher R values the Raman spectra bring to light the progressive demolition of the structural units responsible for the 775 cm^?1 Raman peak, which gives rise (the transformation is complete for R ～ 1) to two new main structural units, orthoborate [BTi₄O₁₀]^?1 (peak at 845 cm^?1) and metaborate BO₂^? (peak at 715 cm^?1).     

Solid-state NMR study of metastable immiscibility in alkali borosilicate glasses

In several series of lithium, sodium, and potassium borosilicate glasses whose compositions traverse known regions of liquid-liquid phase separation, we have applied triple-quantum magic-angle spinning (3QMAS) ¹¹B and ¹⁷O NMR to obtain high-resolution information about short-range structure and connections among various network structural units, and their variation with composition and thermal history. Oxygen-17 3QMAS spectra reveal changes in connectivities between silicate and BO₃ (^[3]B) and BO₄ (^[4]B) units, by quantifying populations of bridging oxygens such as B-O-B, Si-O-B and Si-O-Si, and of non-bridging oxygens. ^[3]B-O-Si and ^[4]B-O-Si as well as ^[3]B-O-^[3]B and ^[4]B-O-^[3]B linkages can be distinguished. ¹¹B MAS and 3QMAS at a magnetic field of 14.1 T allow proportions of several borate units to be determined, including ^[3]B in boroxol ring and non-ring sites and ^[4]B with 3 versus 4 Si neighbors. By combining the ¹¹B and ¹⁷O NMR results, detailed information on Si/B mixing in sodium borosilicates can be derived, showing, for example, that ^[4]B and non-ring ^[3]B tend to mix with silicate units, while ring ^[3]B is mainly connected to borate groups. In a preliminary study of the effects of varying alkali cation, potassium-containing glasses are similar to those in the sodium borosilicate system, but a lithium borosilicate seems to exhibit considerably greater chemical heterogeneity. In annealing experiments that converted an optically clear to obviously phase-separated glasses, the ratio of ^[3]B to ^[4]B does not change significantly, but part of the non-ring ^[3]B converts to ring ^[3]B as the degree of unmixing increases.     

Ultrasound propagation in glasses in the metastable immiscibility region of the sodium borosilicate system

Measurements of ultrasound wave velocity and attenuation have been made between 1.3 K and 400 K in a series of both quenched and heat-treated Na₂OB₂O₃SiO₂ glasses. As in many other inorganic glasses, the ultrasound attenuation of both longitudinal and shear waves below room temperature is dominated by a broad and intense loss peak; the height and temperature of the peak maximum are frequency sensitive. The loss peak characteristics are consistent with a structural relaxation mechanism with a distribution of activation energies and this model is used to analyse the data. The features of the acoustic loss peak and also the absolute value and temperature coefficient of ultrasound velocity are strongly dependent on the total Na₂O network modifier content of the glasses. The ultrasound wave propagation is also affected by phase-separation inducing heat treatment: a steady rise in the height of the acoustic loss peak and an upward shift in the peak temperature takes place with increasing time of heat treatment at 550°C, a finding which suggests that structural rearrangements are still occurring in the individual glassy phases even after long periods of heat treatment. It is proposed that heat treatment causes migration of Na⁺ ions away from BOB bonds in the B₂O₃ rich phase.     

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.     

Structural implications of water and boron dissolution in albite glass

A combined nuclear magnetic resonance, infrared and Raman spectroscopic study on the effect of water dissolution on the structure of B-bearing aluminosilicate glasses is presented. The base composition was albite (NaAlSi₃O₈) to which different amounts of B₂O₃ (4.8, 9.1, 16.7 wt%) were added. Hydrous glasses containing 4.4 ± 0.1 wt% water were synthesized at pressures of 2000 bar. The results show that B dissolves in both dry and hydrous glasses by forming predominantly trigonal BO₃ groups although some tetrahedral BO₄ is also present. In anhydrous glasses prepared at high pressures (above 10 kbar) the fraction of BO₄ increased. The hydrous glasses contain more BO₄ groups compared to the dry counterparts, suggesting that this species is stabilized by water. The Raman and NMR (¹⁷O, ²⁷Al, ²⁹Si) spectra show that B interacts with the aluminosilicate network by formation of Si-O-B and probably Al-O-B units. In the hydrous glasses the water speciation changes significantly towards higher hydroxyl concentrations with increasing B-content. The NIR peaks, which are related to OH groups and molecular H₂O, develop additional shoulders, suggesting that possibly B-OH complexes are formed.     

IR and Raman investigation on the structure of (100-x)[0.33B2O3–0.67ZnO]–xV2O5 glasses

Glasses with molar composition of (100-x)[0.33B₂O₃–0.67ZnO]–xV₂O₅, x = 0, 5, 10, 15 and 20, were prepared, and the effect of V₂O₅ on the structure of the glass was investigated by IR and Raman spectroscopy, TEM and DTA. This investigation shows that [BO₃], [BO₄] and [VO₄] structural units are the predominant coordination polyhedra in the borovanadate glass. From the vanadium-free zinc borate glass, meta-, di-, pyro- and orthoborate groups were observed. Adding V₂O₅ leads to the random substitution of [BO₃] or [BO₄] units in these borate groups by the [VO₄] units, forming corresponding borovanadate groups. With increasing V₂O₅ content, the transition temperature of the glass decreases, that comes from the decrease of the interconnection of the structure units and the substitution of O–B–O by weaker O–V–O linkages.     

Structural investigation of sodium borate glasses and melts by Raman spectroscopy.: I. Quantitative evaluation of structural units

Short-range and intermediate range structures of the sodium borate glass system were investigated using Raman spectroscopy to quantify their dependence on Na₂O concentration. High-resolution spectra were collected by Raman spectroscopy using the Q-switched, second-harmonic pulse of a Nd:YAG laser as an excitation source. The system was designed for measurement of the spectra of glasses and melts up to temperatures over 1000 °C with high signal to noise ratio. Use of polarized light and the simultaneous analysis of HH and VH spectra allowed deconvolution of Raman spectra into appropriate bands with high reproducibility. The deconvoluted bands in the high-frequency region of 1100-1600 cm⁻¹ could be assigned to the vibration modes due to the short-range structures of BO₃ and BO₂O⁻ units in the glasses. The band intensity ratios showed a simple linear relationship with the molar ratio, symmetric BO₃ triangle unit, N_3s, to asymmetric BO₂O⁻ triangle unit, N_3a, obtained from ¹¹B-NMR results. These results allowed a quantitative measure for normalizing the spectra leading to a direct comparison of the band intensities. The ring-structures of intermediate range order, boroxol, pentaborate, tetraborate and diborate groups, could be quantified from the spectra in the middle-frequency region. Their trends with Na₂O concentration showed a good consistency with ¹⁰B-NMR results and also Krogh-Moe’s model.     

Structure of TeO2 · B2O3 glasses inferred from infrared spectroscopy and DFT calculations

Glasses in the system (1 ? x)TeO₂ · xB₂O₃ glasses (with x = 0.3 and 0.4) have been prepared from melt quenching method. The structural changes were studied by FTIR spectroscopy and DFT calculations. From the analysis of the FTIR spectra it is reasonable to assume that when increasing boron ions content the tetrahedral [BO₄] units are gradually replaced by trigonal [BO₃] units. The increase in the number of non-bridging oxygen atoms would decrease the connectivity of the glass network, would depolymerize of borate chains and would necessite quite a radical rearrangement of the network formed by the [TeO₆] octahedral. This is possible considering that tellurium dioxide brings stoichiometrically two oxygen atoms in [TeO₄] and needs an additional oxygen atom for the formation of [TeO₆] octahedra. This additional oxygen atom is evidently taken off from the boron co-ordination and thus boron atoms transfer their [BO₄] co-ordination into [BO₃] co-ordination. We used the FTIR spectroscopic data in order to compute two possible models of the glasses matrix. We propose two possible structural models of building blocks for the formation of continuous random network glasses used by density functional theory (DFT) calculations.     

X-ray diffraction study of Na2OGeO2 melts

The structure of Na₂OGeO₂ melts in the temperature range from 1100 to 1150°C has been investigated with the high temperature X-ray diffraction technique. Comparing the radial distribution functions obtained for the melts with those for the corresponding glasses, the first peak due to the GeO interatomic distance is invariant upon melting, although it becomes broader due to thermal vibration. The second peak for the GeGe interatomic distance for melts shifts toward the large distance, which is explained by broadening of the GeOGe bond angle, not by the thermal expansion of the GeOGe bond. The composition dependences of GeO distances and coordination numbers of the Ge⁴⁺ ion of the melts are found to be almost the same as the corresponding glasses, indicating that even in melts at such high temperatures 6-fold coordinated Ge⁴⁺ ions are present and their content changes with the Na₂O content as in the case of the corresponding glasses.     

Structural investigation of sodium borate glasses and melts by Raman spectroscopy. II. Conversion between BO4 and BO2O units at high temperature

Raman scattering spectra were recorded for pure boron oxide and sodium borate glasses and their melts at the temperature ranging from room temperature to 1200 °C to investigate the structural changes occurring in the melts. The amounts of short-range order structures (SRO), BO₂O⁻ and BO₃, were estimated from the high frequency bands at 1100-1600 cm⁻¹. The ratio of 4-fold coordinated boron oxide BO₄, N₄, at high temperature could be derived for the glass melts as a function of Na₂O concentration. In Na₂O ?20 mol% region, N₄ showed a slight decrease while the remarkable decrease of N₄ was found in the region of Na₂O ?25 mol% with increasing temperature. The enthalpy of the equilibrium reaction,

     

An application of MNDO calculation to borate polyhedra

MNDO, a semiempirical SCF-MO method, was applied to study the structure of the borate glass. The clusters, B(OH)₃, H₃B₃O₆, B₃O₆^3?, (HO)₂BOB (OH)₂, (HO)₂B₃O₃OB₃O₃ (OH)₂, BO₃[B(OH)₂]₃ and BO₃[B₃O₃(OH)₂]₃ were treated and their geometries, the heats of reactions, the π electron systems and electronic structures were discussed. The geometry and the electronic structures show a good correspondence with the experimental data. The resonance stabilization effect of the π electron system is not so large as to control the geometries and reactions.