Fraction Of Tetrahedral Borons Research Articles

The dual role of bismuth in Li2O–Bi2O3–B2O3 glasses along the orthoborate join

AbstractThe structures of glasses in the lithium–bismuth orthoborate composition range deviate significantly from the short‐range order structure of the two crystalline end‐members. Although binary Li3BO3 and BiBO3 are solely of comprised trigonal orthoborate anions, all glasses formed by their combination contain four‐coordinated borate tetrahedra. We analyze the structure of (75−1.5x)Li2O–xBi2O3–(25+0.5x)B2O3 glasses in increments of x = 5, with 11B magic‐angle spinning nuclear magnetic resonance (NMR), infrared (IR), and Raman spectroscopy. For the full series, the oxygen‐to‐boron ratio remains constant at O/B = 3:1. NMR quantifies an increase in the fraction of tetrahedral boron with increasing bismuth oxide content. Evolution of the mid‐IR profile suggests multiple types of tetrahedral boron sites. Raman spectroscopy reveals that Bi2O3 tends to cluster within the lithium borate matrix when initially introduced and that this behavior transforms into a bismuthate network with increasing bismuth oxide content. In all cases, mixed Bi–O–B linkages are observed. The dual role of bismuth as network modifier and network former is likewise observed in the far IR. The glass transition temperature continuously increases with bismuth oxide content; however, the glass stability displays a maximum in the multicomponent glass of x = 40.

Effects of annealing on density, glass transition temperature and structure of tellurite, silicate and borate glasses

Barium tellurite, lead tellurite, molybdenum tellurite, lead silicate and bismuth borate glasses of the compositions: xBaO-(100-x)TeO2 (x = 10, 15 and 20 mol%), xPbO-(100-x)TeO2 (x = 15 and 20 mol%), 25MoO3-75TeO2, xPbO-(100-x)SiO2 (x = 50, 60 and 65 mol%) and 40Bi2O3-60B2O3 were prepared by melt-quenching. The effects of long duration thermal annealing (below the respective glass transition temperature (Tg) values) on the density, thermal properties and the short-range structure of glasses were studied. Density increases on average by about 0.5% to 1% and Tg values show drastic increase in the range of 8 °C-64 °C after annealing. Raman and FTIR studies were performed on glasses before and after annealing, FTIR studies found that bismuth borate glasses show a small but significant increase in the fraction of tetrahedral borons in the borate network with annealing. Raman studies on lead metasilicate glass showed an increase in the concentration of SiO4 units that contain three bridging oxygens. Raman and FTIR spectra of all annealed barium and lead tellurite glasses were indistinguishable from those of the initial samples, however the FTIR spectra of annealed 25MoO3-75TeO2 glass show changes in the relative intensities of the peaks due to MoO vibrations along with the growth and sharpening of low frequency Raman bands, furthermore this glass also showed large differences in its crystallization and melting properties during thermal analysis.

Mechanisms of boron coordination change between borosilicate glasses and melts

The structure of two borosilicate glasses and melts has been investigated by neutron diffraction coupled with Reverse Monte Carlo (RMC) simulation. For both compositions (10B2O3⋅30Na2O⋅60SiO2 and 10B2O3⋅15Na2O⋅15CaO⋅60SiO2), the fraction of tetrahedral boron ([4]B) drops from 0.7 to 0.3 from the glass to the melt. The change from a modifying to a charge compensating role of cations during quench is related to the boron coordination change. These molecular scale modifications explain the low viscosity of alkali/alkaline-earth borosilicate melts relative to alkali/alkaline-earth silicate melts, as the viscous flow is enhanced with the proportion of [3]B. RMC simulations of soda borosilicate glass and melt show a preferential interbonding between B and Si polyhedra. Such medium range order interbonding is unfavored in the soda-lime borosilicate glass showing a nearly random network. The effect of Ca enhancing the proportion of B–B pairs is here highlighted.

Devitrification properties of lead borate glasses

Lead borate glasses containing 30 to 60 mol% PbO were prepared by melt quenching technique and devitrified by long duration heat treament in the supercooled region. Glasses crystallized on heating above their glass transition temperature, and the crystalline phases produced on devitrification were characterized by XRD and DSC analyses. Glass with 30 mol% PbO slowly formed a solid solution of Pb6B10O21 and Pb5B8O17 crystalline phases, while glasses with 40 and 50 mol% PbO formed a mixture of Pb6B10O21, Pb5B8O17 and the remanent glassy phase. Glasses with higher PbO concentration of 56 to 60 mol% devitrified completely and produced only Pb5B8O17 crystalline phase. Lead borate glasses with PbO concentration of 40 to 50 mol% showed maximum thermal stability against devitrification, the ease of crystallization of glasses was correlated with the fraction of tetrahedral borons in them.

Structural study of bismuth borosilicate, aluminoborate and aluminoborosilicate glasses by 11B and 27Al MAS NMR spectroscopy and thermal analysis

Glasses of bismuth borosilicates, aluminoborates and aluminoborosilicates were prepared by melt quenching and characterized by density measurements, 11B and 27Al Magic Angle Spinning (MAS) Nuclear Magnetic Resonance (NMR) spectroscopy, and Differential Scanning Calorimetry (DSC). Increase in silica concentration increases density without any change in the fraction of tetrahedral borons (N4) in the borosilicate glass network, whereas increase in alumina concentration in aluminoborate and aluminoborosilicate glasses drastically reduces density and N4. 27Al MAS NMR measurements revealed that Al ions are in four, five and six fold coordination with oxygen and about 47% of these are in [5]Al and [6]Al structural units, the concentration of the latter species decreases by adding silica in the glass network. Higher cation field strength of Bi3+ generates very large concentration of non-bridging oxygens, which act as charge balancing centers in the glass network. DSC measurements found that alumina and silica incorporation in bismuth borate glasses do not produce any systematic changes in the glass transition temperature but the tendency towards crystallization decreases steadily.

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

Glass samples from four systems: xPbO–(100− x)B 2O 3 ( x = 30, 40, 50 and 60 mol%), 50PbO– yAl 2O 3–(50− y)B 2O 3 ( y = 2, 4, 6, 8 mol%), 50PbO– ySiO 2–(50− y)B 2O 3 ( y = 5, 10, 20, 30 mol%) and 50PbO–5SiO 2– yAl 2O 3–(45− y)B 2O 3 ( 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–50B 2O 3 glass. 11B MAS NMR measurements determine that the fraction of tetrahedral borons (N 4) reaches a maximum for the glass containing 50 mol% of PbO in the PbO–B 2O 3 glass series and that N 4 is sharply reduced upon adding small amounts of Al 2O 3 into lead borate and lead borosilicate systems. 27Al MAS NMR experiments performed on glasses doped with aluminum oxide show that the Al 3+ are tetra-, penta- and hexa-coordinated with oxygen, even without any excess concentration of Al 3+ over charge-balancing Pb 2+ 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.

Physical properties of barium borate glasses determined over a wide range of compositions

Barium borate glasses are reported over an extended glass-forming range from R=0.2 (16.7 mol% BaO) to R=2.0 (66.7 mol% BaO), where R is the molar ratio of barium oxide to boron oxide. The density, T g (glass transition temperature), T x (glass recrystallization temperature), and optical cutoffs were determined. These data were compared with structural models for the glasses based on nuclear magnetic resonance results of Greenblatt and Bray. The barium borate data were also compared with similar studies of lithium and lead borate glasses. Key results observed include: (1) the density trends are understandable in terms of the abundance of the basic borate groups with the fraction of tetrahedral borons being the most important single factor, and (2) the T gs are anomalously high when compared to either the alkali or lead borate systems.

Infrared spectra of lithium thioborate glasses and polycrystals

Abstract Glasses and polycrystals in the x Li 2 S + (1 − x )B 2 S 3 system have been prepared over the composition range 0 ≤ x Li 2 S ≤ 0.80. The glass-forming range was observed to be ≈0.48 ≤ x ≤ 0.80 for liquids splat-quenched from ≈ 850°C into films ≈ 0.25 mm in thickness. The infrared spectra of the glasses were interpreted as a progressive compositional shift from the glassesith x ≈ 0.5 being based upon a mixture of trigonal (probably six-membered rings) and a high fraction tetrahedral boron groups to glasses with x ≈ 0.80 being based upon isolated trigonal orthothioborate anions. As with nearly all alkali borate and thioborate glasses in the high alkali region, x > 0.5, a decreasing fraction of tetrahedral borons is observed. The fraction of tetrahedral borons for glasses with x ≈ 0.55 is very large, but diminishes to zero for glasses with x = 0.75 and 0.80. Polycrystals were prepared at x = 0.33(≈ c-Li 2 B 4 S 7 ), 0.5 (≈ c-LiBS 2 ) and 0.75 (c-Li 3 BS 3 ). For the first two phases, all borons were found to be tetrahedrally coordinated, whereas for the last phase all borons were found to be in trigonal coordination.

11B NMR studies of the short range order in wide composition range xNa2S + (1 − x)B2S3 glasses

The 11 B NMR spectra of high purity (low oxygen contamination) x Na 2 S+ (1 − x )B 2 S 3 glasses with 0 ≤ x ≤ 0.80 are reported. Glasses were prepared in both the low alkali (0 ≤ x ≤ 0.30) and high alkali (0.60 ≤ x ≤ 0.80) glass-forming ranges. These two glass-forming ranges are similar in composition to the two glass-forming ranges in the analogous x Na 2 O + (1 − x )B 2 O 3 system. As in this latter system, the 11 B NMR spectrum of vitreous B 2 S 3 (v-B 2 S 3 ) consists of a quadrupolar broadened single line and confirms the short range order (SRO) of trigonally coordinated borons. As Na 2 S is added, again in similarity to the oxide system, a single narrow line grows at the resonance frequency and confirms the SRO of the highly symmetric tetrahedral boron group. By contrast with the oxides, however, the fractions of tetrahedral borons that are created by the addition of Na 2 S to B 2 S 3 are a factor of three to four times greater at any given composition. Here, rather than forming two tetrahedral borons for each added S = anion, the rate is likely to be six or eight tetrahedral borons per added S = anion. A structural model incorporating this feature of the sulfide glasses has been developed. After this initial large increase in the fraction of tetrahedral borons in the low alkali glass-forming region, the fraction begins to decrease, and by 75 mol% Na 2 S is approximately zero. This decrease in the tetrahedral boron fraction is associated with the formation of the trigonal, totally depolymerized orthothioborate group, Na 3 BS 3 .