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.     

Mössbauer effect studies of alkali borate glasses

Mössbauer studies of a large number of glass samples prepared with alkali oxides in the region of glass formation are reported. Some representative samples are studied in the temperature range 85–500 K. The following glass systems are studied.

Room-temperature isomer shift values decrease gradually with the addition of alkali oxide and the values fall sharply for alkali content higher than 22 mol%. We conclude that iron is in the ferric state and that the oxygen polyhedra of iron changes from octahedral to tetrahedral. The structural change results from the fact that the alkali introduces an extra non-bridging oxygen ion. The size of the alkali cation introduced into the glass has considerable influence on the isomer shift values of iron. In fact, the polarizing power decreases in the order LiNaK, hence the s-character of the FeO bond increases in the order LiNaK. Addition of Al₂O₃ has no effect on the isomer shift values of iron, showing that aluminium ions occupy network-forming positions.For some representative samples the second-order Doppler effect was studied as a function of temperature in the range 85–500 K. The thermal red shifts due to second-order Doppler effect are used to estimate the local specific heats of ferric ion in the glass system. The quadrupole splitting has weak temperature dependence, showing that Fe³⁺ is in a high-spin state.     

Mixed-alkali effect in alkali borate glasses

The spectral absorption of Cu²⁺ and Ni²⁺, the density, molar volume, refractive index, molar refraction, thermal expansion coefficient, transformation and dilatometric softening temperatures of mixed alkali borate glasses are reviewed. These results indicate that in borate glasses, mixed alkalis do not produce gross alterations of the glass network, and that the weak deviations observed may be related to the boric oxide anomaly.     

Heat capacities and thermal behavior of alkali borate glasses

The heat capacities of selected glasses in the five alkali borate systems have been measured over a range of high temperatures which includes the respective glass transition regions. The heat capacities per gram atom at 350°K show little variation with composition while those at the low temperature ends of the glass transition, T_g-, show some systematic variations. When compared with the respective 3R values, the heat capacities at T_g- range from about 0.75 for B₂O₃ to values in excess of unity for various alkali borate compositions. The present data on heat capacity have been combined with previous data on the elastic moduli, densities and thermal expansion coefficients to evaluate the Grüneisen constant, γ, for each composition for temperatures around ambient. For B₂O₃, γ has a low value of about 0.25. It decreases with additions of Li₂O and increases with additions of the other alkali oxides.     

Rheological properties of boric oxide and alkali borate glasses

The rheological behaviour of boric oxide and alkali borate glasses has been studied in the region of high viscosity (10¹⁰–10¹³ poise). The change in viscosity of boric oxide glass with the introduction of alkali oxide is in agreement with what is known about the structure of alkali borate glasses.     

Spectral absorption of tellurium in alkali borate glasses

The spectral absorption of tellurium-containing lithium, sodium and potassium borate glasses in the range of 300–700 nm was used to identify the states of tellurium formed in such glasses. Depending on the nature and concentration of the alkali oxide the glasses obtained were either colourless, rose which is attributed to the formation of the polytelluride ion or grey which is attributed to the formation of elemental tellurium particles. The tellurites and/or tellurates are expected to form in glasses of high content of either Na₂O or K₂O which were colourless. The sequence of formation of the different states of tellurium with increasing alkali content is in accordance with acidity-basicity concept.     

The mixed alkali effect in lithium-sodium borate glasses

The electrical conductivity of a series of 0.35 (Li, Na)₂O·B₂O₃ glasses shows a minimum at the composition Na/(Na+Li)～0.6, which becomes stronger as the temperature is decreased; the activation enthalpy for electrical conductivity shows a maximum at this composition. In general, replacing 1% of the total oxygen concentration by chlorine or bromine (keeping the total alkali content fixed) in these glasses increases the conductivity; fluorine doping has an opposite effect. The mixed alkali effect, expressed in terms of the compositional dependence of the activation enthalpy for conductivity, is enhanced when borate glass is doped with fluorine, but is slightly diminished when doped with chlorine or bromine. The results are explained in terms of the structure of halogenated alkali-borate glasses, and discussed in relation to the origin of the mixed alkali effect.     

Internal friction and dielectric losses of mixed alkali borate glasses

The internal friction and dielectric losses of NaK, LiNa, LiCs, LiK, NaCs, KCs, AgLi, AgNa, AgK and AgCs borate glasses were measured as functions of the temperature at various frequencies. In general, the behavior of mixed alkali borate glasses is very similar to the behavior of the comparable phosphate and silicate glasses. The magnitude of the mixed alkali peak was found to vary systematically with the size of the involved alkali ions. The silver-containing glasses also show the mixed alkali effect. The borate glasses are briefly compared with the silicate and the phosphate glasses and their behavior is found to be in agreement with the recent proposal that the mixed alkali peak is caused by an electro-mechanical cross effect.     

Chemical stability of Ni-enriched nanodomains in alkali borate glasses

The stability of Ni nanodomains observed in low alkali borate glasses (10 mol% alkali oxide) has been investigated at higher alkali contents using Ni K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. EXAFS data indicate that the Ni nanodomains disappear progressively when adding alkali oxide, whatever the nature of the alkali. This medium range reorganization around Ni is associated with a change of the local site from 6- to 5-fold coordination at 27-30 mol% alkali oxide and 4-fold coordination at higher alkali oxide content. Ni-Ni connections are still present for ^[5]Ni but no second neighbors are observed for ^[4]Ni in the EXAFS signal. The changes at both the local and medium range around Ni are likely associated to the disappearance of the large, rigid borate groups with the addition of alkali oxide.     

Probing alkali coordination environments in alkali borate glasses by multinuclear magnetic resonance

Five series of binary alkali borate glasses were prepared to compare the alkali dependence of network and modifier short-range order. ¹¹B magic-angle spinning (MAS) NMR reveals that the fraction of four-coordinate boron depends strongly upon alkali type at high-alkali concentrations: heavier alkalis favour the formation of non-bridging oxygens, whereas lithium borates contain a much higher concentration of tetrahedral boron units. The alkali modifiers were observed directly by MAS NMR to measure the change in chemical shift with composition. All alkali peaks shift to higher frequency with increasing loading, indicative of decreasing average coordination numbers. Relative to their known chemical shift ranges, the heavier alkalis exhibit the greatest shifts, whereas the lithium shifts are subtle. This is interpreted in terms of the availability of charged and partially charged coordinating oxygens in the network. Moreover, the ¹³³Cs chemical shifts plateau at 40 mol%, implying that the Cs⁺ coordination number reaches a lower limit at this composition. This work demonstrates that NMR instrumentation and methodology have reached a level where even challenging nuclei like ³⁹K and ⁸⁷Rb can be probed to yield structural information in glasses.     

The structure of borate glasses studied by Raman scattering

Raman spectra of alkali and alkaline-earth borate glasses are reported. These spectra are used to discuss the molecular structure of the glasses. The influence of additions of Al₂O₃ on the structure of alkali borate glasses is also presented. The experimental evidence indicates that the same type of groups are present in borate glasses as in crystalline borates. The presence of tetraborate, metaborate, pyroborate and orthoborate groups in borate glasses is strongly suggested by the Raman spectra.     

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.     

MNDO studies of basicity in borate glasses

The cluster approximation was adopted for the borate glasses. The geometries and basicity of various isomers of H₈B₉O₁₈⁻, H₈B₁₁O₂₁⁻ and H₈B₁₂O₂₃²⁻ were studied by the MNDO semi-empirical molecular orbital method. The optimized geometries of pentaborate, triborate and diborate structure in the clusters showed good agreement with the experimentally determined ones. The changes of bond length and charge rearrangements accompanying the formation of a four-fold coordinated boron atom (⁴B) accorded with the prediction of Gutmann's bond length variation rule. The basicity in the borate glass was defined as the electron donability of oxygens in the glass network. MO interaction between the occupied orbitals of the clusters and the LUMO of the acidic site in (H₂B₉O₁₄)₂O and the proton affinity of the clusters were estimated. The basicities of the clusters which contained ⁴B and that of the clusters which contained non-bridging oxygen atoms (NBO) were compared. The appearance of NBO in high alkali borate glass was interpreted.     

Origin of blue and green colouration in sulphur containing alkali borate glasses

Simple binary sodium and potassium borate glasses containing sulphur were prepared. Depending on the composition of the glass and the quantity of sulphur added the glasses produced were blue, green or yellow. The absorption spectra of these glasses were measured in the range of 300–700 nm.The blue colour is characterized by an absorption band at 585 nm which is attributed to S₃^?.The green colour is characterized by two absorption bands; the band at 585 nm observed in the spectra of the blue glasses and a more intense band at 390 nm. The latter is attributed to S₂^? and the green colouration to the presence of both S₂^? and S₃^? where the S₂^? state predominates.     

Effect of gamma-rays on some mixed alkali borate glasses containing nickel

The change of optical absorption of some irradiated mixed alkali borate glasses containing nickel has been studied by varying the gamma-ray dose or the mixed alkali oxide content. The induced bands increase as the radiation process proceeds until a certain limit after which a constancy is assumed to be reached.The investigation of the observed spectra shows that there is an induced absorption band with its maximum at 550 nm, in addition to the characteristic absorption bands of the divalent nickel ions with peaks in the visible region at 410, 640, 680 and 750 nm.     

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.     

A revised view on the mixed-alkali effect in alkali borate glasses

The mixed-alkali effect (MAE) of the ionic conductivity is traditionally defined as a negative deviation of the dc conductivity from that of an ‘ideal glass’ obtained according a linear mixing rule. An alternative definition is proposed in this paper which relates the conductivity of a mixed-alkali glass to the conductivity-sum of two single-alkali glasses with total alkali concentrations equal to the corresponding partial alkali concentrations of the mixed-alkali glass. A new reference glass denoted as independent component glass (ICG) is introduced. In contrast to the ‘ideal glass’ the conductivity of a real mixed-alkali glass is larger than the conductivity of the respective ICG resulting in a positive MAE. This fact manifests an enhancement of the conductivity when two single-alkali glasses are combined to a mixed-alkali glass. The conductivities and the tracer diffusivities of the mixed-alkali- and the corresponding single-alkali glasses are compared from this point of view. A subnetwork diffusion concept (SDC) is proposed to describe the diffusion dynamics in Na-Rb-borate mixed-alkali glasses.     

Spectroscopic investigations on alkali earth bismuth borate glasses doped with CuO

Transparent glasses of the composition 10RO.20Bi₂O₃.(70 − x) B₂O₃.xCuO [R = Ca, Ba] with x = 0, 0.4, 0.8 (wt.%) were prepared via melt-quenching technique and characterized using X-ray powder diffraction. Spectroscopic measurements, viz., optical absorption, EPR, FTIR and photoluminescence (PL) were studied at room temperature. Analysis of the present investigations indicates that the concentration of luminescence centers of bismuth ions (Bi²⁺ ions in visible region) decreased by the integration of BaO and also by increasing dopent concentration. It is also observed that addition of CuO decreases stability of the glass network in calcium series and strengthens in barium series.     

Electron paramagnetic resonance and thermal behavior of vanadyl-doped mixed alkali bismuth borate glasses

Glasses with composition 15 Li₂O15K₂O10Bi₂O₃(60 ? x) B₂O₃: xV₂O₅ where x = 1, 3, 5, 7, and 9 mol% are prepared by normal melt-quench technique. Characterization of the prepared samples is done using X-ray diffraction, density and differential scanning calorimetry. The density and molar volume are found to increase with increasing x whereas the glass transition temperature decreases. Electron paramagnetic resonance spectra of the prepared samples are recorded using EPR-spectrometer operating in the X-band frequency. The resonance spectra are well resolved for x ≥ 5 mol% and the intensity of the resonance peak is found to increase with increasing x. The values of spin-Hamiltonian parameters (SHP) and molecular orbital coefficients are evaluated. From the values of SHPs it is concluded that V⁴⁺ ions in the present glass system exist as vanadyl ions in octahedral co-ordination with tetragonal compression. It is observed that the SPHs depend slightly on the relative concentration of V₂O₅. Further, the theoretical optical basicity of the glasses has also been evaluated and it is observed that the changes in optical basicity values are in accordance with the changes in SHPs.