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−1were used in combination with mass and charge balance equations to quantify the short-range borate structure in terms of the molar fractionsX4m,X4o,X3,X2,X1andX0for borate units BØ4−, BØ2O23−, BØ3, BØ2O−, BØO22−and BO33−, where Ø and O−denote bridging and non-bridging oxygen atoms. Borate tetrahedral units were found to be present in both themeta-borate, BØ4−, andortho-borate, BØ2O23−, forms with BØ4−constituting the dominating tetrahedral species for 0.20 ≤x≤ 0.70. The BØ2O23−units prevail at higher Bi2O3levels (x> 0.7), and coexist with their isomeric triangular borate species BO33−(BØ2O23−⇌ BO33−). The present IR results for the total molar fraction of borate tetrahedral units,X4=X4m+X4o, are in very good agreement with reported NMR results for the fraction of boron atoms in four-fold coordination,N4. Besides evaluatingX4mandX4o, the present work reports also for the first time the fractions of all types of triangular borate speciesX3−nwithn= 0, 1, 2 and 3. The IR region below 550 cm−1was found to be dominated by the Bi-O vibrational activity in coexisting ionic (160-230 cm−1) and distorted BiO6sites (330-365 cm−1and 475-510 cm−1), a result reflecting the dual role of Bi2O3as 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 Bi2O3content in the three regimes 0.2 ≤x≤ 0.4, 0.4