Properties and Structure of Glasses in the Systems MO_3/2-BO_3/2(M: As, Sb, Bi)

Abstract

In AsO_3/2-BO_3/2 glass system molar volume (V), molar refractivity (R) and thermal expansion coefficient (α) increased linearly with AsO_3/2 content (Fig. 1, 2, 3). Infrared spectra of these glasses showed good agreement with the summed curves for As₂O₃ and B₂O₃ glasses with the mole percents of the components taken into account (Fig. 7). These facts reveal that the structure of these glasses consists of As₄O₆ molecules and boroxol rings randomly distributed each other without forming As-O-B bonds. The increase in transformation temperature (T_g) of the glasses containing more than 70mol% BO_3/2, shown in Fig. 5, is due to the structural change into a layer network of boroxol rings interlaced with As₄O₆ molecules.
The change in ratio of infrared absorption intensities of the BO₄ band at 930cm^-1 and BO₃ band at about 1300cm^-1, shown in Fig. 10, revealed that the fraction of four-coordinated B³⁺(N₄) increased rapidly up to 25mol% BiO_3/2 and reduced to zero at 65-70mol% BiO_3/2. This change of boron coordination estimated from infrared spectra corresponds fairly to the changes of the other properties, that is, the maxima in T_g and Vickers microhardness (H_v), and the minima in V and α at 20-25mol% BiO_3/2 (Fig. 5, 6, 3, 4), and a maxima in α and breaks of T_g- and H_v- composition curves at 65-70mol% BiO_3/2 (Fig. 4, 5, 6).
In binary SbO_3/2-BO_3/2 glasses, α decreased slightly, and T_g and H_v increased with SbO_3/2 content, dangling in a minimum in α, a maximum in T_g and a break of H_v-composition curve at about 30mol% SbO_3/2 (Fig. 4, 5, 6). Introducing SbO_3/2 into B₂O₃ glass weak absorption peaks due to BO₄ groups appeared at 850-1050cm^-1 (Fig. 11). These are obviously due to the change of coordination number of B³⁺ from 3 to 4, but N₄ in these glasses is much smaller than that in BiO_3/2-BO_3/2 glasses.
The difference in the tendency of the coordination number change of boron in three binary borate glasses was interpreted as follows. As³⁺ and Sb³⁺ are coordinated trigonal-pyramidally because of stronger covalent characters of As-O and Sb-O bonds. In the region rich in BO_3/2 content owing to stronger ionic character of Sb-O bond compared to As-O bond, a small amount of Sb³⁺ behaves as NWM, resulting in the coordination number change of a small portion of B³⁺. As Bi-O bond is stronger in ionic character than Sb-O bond, Bi³⁺ behaves as NWM in the same region, resulting in the coordination number change of B³⁺. The coordination number of Bi³⁺, however, decreases with increasing BiO_3/2 content because of increase in the covalent character of Bi-O bond.