Short-range structure, the role of bismuth and property–structure correlations in bismuth borate glasses

Abstract

Bismuth-containing borate glasses, xBi₂O₃–(1 − x)B₂O₃, were synthesized in the broad composition range 0.20 ≤ x ≤ 0.80 by melting in Pt crucibles and splat-quenching between two metal blocks. Infrared reflectance spectra, measured in the range 30–5000 cm⁻¹, were transformed into absorption coefficient spectra and then deconvoluted into component bands to probe the glass structure as a function of composition. Integrated intensities of bands above 800 cm⁻¹ were used in combination with mass and charge balance equations to quantify the short-range borate structure in terms of the molar fractions X_4m, X_4o, X₃, X₂, X₁ and X₀ for borate units BØ₄ ⁻, BØ₂O₂ ³⁻, BØ₃, BØ₂O⁻, BØO₂ ²⁻ and BO₃ ³⁻, where Ø and O⁻ denote bridging and non-bridging oxygen atoms. Borate tetrahedral units were found to be present in both the meta-borate, BØ₄ ⁻, and ortho-borate, BØ₂O₂ ³⁻, forms with BØ₄ ⁻ constituting the dominating tetrahedral species for 0.20 ≤ x ≤ 0.70. The BØ₂O₂ ³⁻ units prevail at higher Bi₂O₃ levels (x > 0.7), and coexist with their isomeric triangular borate species BO₃ ³⁻ (BØ₂O₂ ³⁻ ⇌ BO₃ ³⁻). The present IR results for the total molar fraction of borate tetrahedral units, X₄ = X_4m + X_4o, are in very good agreement with reported NMR results for the fraction of boron atoms in four-fold coordination, N₄. Besides evaluating X_4m and X_4o, the present work reports also for the first time the fractions of all types of triangular borate species X_3−n with n = 0, 1, 2 and 3. The IR region below 550 cm⁻¹ was found to be dominated by the Bi–O vibrational activity in coexisting ionic (160–230 cm⁻¹) and distorted BiO₆ sites (330–365 cm⁻¹ and 475–510 cm⁻¹), a result reflecting the dual role of Bi₂O₃ as glass-modifier and glass-former oxide. The latter role dominates in glasses exceeding 60 mol% Bi₂O₃, and is consistent with the extended glass formation in the bismuth-borate system. The structural results were used to calculate the average number of bridging B–Ø bonds per boron center, the average Bi–O and B–O single bond energy, and the atomic packing density of the studied glasses. These properties vary approximately linearly with Bi₂O₃ content in the three regimes 0.2 ≤ x ≤ 0.4, 0.4 < x ≤ 0.6 and 0.6 < x ≤ 0.83, and contribute collectively to the composition dependence of glass transition temperature.