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 As
2O
3 and B
2O
3 glasses with the mole percents of the components taken into account (Fig. 7). These facts reveal that the structure of these glasses consists of As
4O
6 molecules and boroxol rings randomly distributed each other without forming As-O-B bonds. The increase in transformation temperature (
Tg) 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 As
4O
6 molecules.
The change in ratio of infrared absorption intensities of the BO
4 band at 930cm
-1 and BO
3 band at about 1300cm
-1, shown in Fig. 10, revealed that the fraction of four-coordinated B
3+(N
4) 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
Tg and Vickers microhardness (
Hv), 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
Tg- and
Hv- 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
Tg and
Hv increased with SbO
3/2 content, dangling in a minimum in α, a maximum in
Tg and a break of
Hv-composition curve at about 30mol% SbO
3/2 (Fig. 4, 5, 6). Introducing SbO
3/2 into B
2O
3 glass weak absorption peaks due to BO
4 groups appeared at 850-1050cm
-1 (Fig. 11). These are obviously due to the change of coordination number of B
3+ from 3 to 4, but N
4 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. As
3+ and Sb
3+ 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 Sb
3+ behaves as NWM, resulting in the coordination number change of a small portion of B
3+. As Bi-O bond is stronger in ionic character than Sb-O bond, Bi
3+ behaves as NWM in the same region, resulting in the coordination number change of B
3+. The coordination number of Bi
3+, however, decreases with increasing BiO
3/2 content because of increase in the covalent character of Bi-O bond.
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