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

2021, Physical Chemistry Chemical Physics

Bismuth-containing borate glasses, xBi 2 O 3-(1 À x)B 2 O 3 , were synthesized in the broad composition range 0.20 r x r 0.80 by melting in Pt crucibles and splat-quenching between two metal blocks. Infrared reflectance spectra, measured in the range 30-5000 cm À1 , 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 À1 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 BØ 4 À , BØ 2 O 2 3À , BØ 3 , BØ 2 O À , BØO 2 2À and BO 3 3À , where Ø and O À denote bridging and non-bridging oxygen atoms. Borate tetrahedral units were found to be present in both the meta-borate, BØ 4 À , and ortho-borate, BØ 2 O 2 3À , forms with BØ 4 À constituting the dominating tetrahedral species for 0.20 r x r 0.70. The BØ 2 O 2 3À units prevail at higher Bi 2 O 3 levels (x 4 0.7), and coexist with their isomeric triangular borate species BO 3 3À (BØ 2 O 2 3À " BO 3 3À). 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 X 3Àn with n = 0, 1, 2 and 3. The IR region below 550 cm À1 was found to be dominated by the Bi-O vibrational activity in coexisting ionic (160-230 cm À1) and distorted BiO 6 sites (330-365 cm À1 and 475-510 cm À1), a result reflecting the dual role of Bi 2 O 3 as glass-modifier and glass-former oxide. The latter role dominates in glasses exceeding 60 mol% Bi 2 O 3 , 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 Bi 2 O 3 content in the three regimes 0.2 r x r 0.4, 0.4 o x r 0.6 and 0.6 o x r 0.83, and contribute collectively to the composition dependence of glass transition temperature.