Properties and Structure of Glasses of B<sub>2</sub>O<sub>3</sub>-GeO<sub>2</sub> System

Abstract

In glass-forming oxide systems without modifier ions, network structures are not broken anywhere, and these glasses must be different in their properties from usual glasses which contain some modifier components. In the glasses of the GeO2-B2O3 system we measured several properties: the thermal expansion, the deforming temperature, the density, the refractive index, the viscosity, and the infra-red absorption. The results of our measurements are shown in Figs. 1-6 and Fig. 11. We can see, in Figs. 1-4 and Fig. 6a, one or two bending points in every curve showing the relation between the composition and the property. The composition of the first point is about 85 cat.% of B2O3, while that of the second point is about 50 cat.%. The latter point appears similarly in every curve; it is assumed that the reason for the appearance of the latter point is a packing effect of two kinds of balls with different radii.The first point appears clearly in the curve of the expansion coefficient, which resembles that of the SiO2-B2O3 system. Accordingly, it is considered that the appearance of the first point is due to an effect of the 4-coordination structure, which interferes with the thermal expansion effect of the 3-coordination. structure. If it is assumed that the interference of the 4-coordination acts not only on the oxygens connecting with the 4-coordination ion directly (marked _??_ in Fig. 7), but also on the next oxygens beyond B (marked _??_), this effect reaches a maximum at 13.04 cat.% of the 4-coordination component and becomes zero at 42.85 cat.%. We can then represent the expansion coefficient of these systems by the following equations:x=0.0000-0.1304α=αIV⋅x+(1-x)αB-4(γ+2δ)xx=0.1304-0.4285α=αIV⋅x+(1-x)αB-4γx-(1.5-3.5x)δx=0.4285-1.0000α=αIV⋅x+(1-x)(αB-3γ), x=4 cat.% fraction of the 4-coordinate component, α=the expansion coefficient of glass, αIV=the expansion coefficient of the 4-coordinate component, αB=the expansion coefficient of B2O3.B2O3-SiO2 system: γ=2.6, δ=2.9B2O3-GeO2 system: γ=2.53, δ=2.0In connection with above things we considered the problem of boric acid anomaly in the B2O3-R2O system (R=Li, Na, K). According to the above results, a bending point must appear at the constant place (Fig. 8), even though the 4-coordination of B increases continuously with the quantity of R. However, the minimum point of the expansion coefficient of these systems depends on the kind of alkali ion. Therefore, the reason for the increase in the expansion coefficient beyond the minimum point is the decrease in the hole radius of the polygonal ring; this shrinkage is caused by the increase in the 4-coordination structure (cf. Fig. 9 and Table 1). The shrinkage of the hole results from the repulsion of the alkali ion or from its exclusion from an intersticial hole.We then studied the expansion coefficient of the B2O3-GeO2-Na2O system. The results correspond with the calculated values assuming the preferential 4-coordination of B (marked in Fig. 10), and does not correspond with the values assuming that of Ge (marked _??_). The addition of the 4-coordination of Ge causes shrinking of the hole, but it does not cause the repulsion of the alkali ion. Accordingly it is assumed that the increase in the expansion coefficient beyond the minimum point arises from the replusion of the alkali ion.We also studied the