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

Bismuth-containing borate glasses, xBi(2)O(3)-(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, X-3, X-2, X-1 and X-0 for borate units Bo(4)(-), Bo(2)O(2)(3-), Bo(3), Bo(2)O(-), BoO(2)(2-) and BO33-, where o and O- denote bridging and non-bridging oxygen atoms. Borate tetrahedral units were found to be present in both the meta-borate, Bo(4)(-), and ortho-borate, Bo(2)O(2)(3-), forms with Bo(4)(-) constituting the dominating tetrahedral species for 0.20 <= x <= 0.70. The Bo(2)O(2)(3-) units prevail at higher Bi2O3 levels (x > 0.7), and coexist with their isomeric triangular borate species BO33- (Bo(2)O(2)(3-) BO33-). The present IR results for the total molar fraction of borate tetrahedral units, X-4 = X-4m + X-4o, are in very good agreement with reported NMR results for the fraction of boron atoms in four-fold coordination, N-4. 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 X3-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-o 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.