Studies of the Glass-formation Range of Germanate Systems

Abstract

Following the previous reports on borate and silicate systems, we studied the glass-formation range of germanate systems. Germanate glass has a structure similar to that of silicate However, these systems have not been sufficiently studied and practically utilized, because germanium is a rare element. The molecular weight of Ge is nearly three times as large as that of Si. Therefore, germanate glass, compared with silicate glass, has a larger density, refractive index and expansion coefficient, a lower melting point, and a wider range of infrared transmission.This experiment used the same kind of oxides and the same amount of melts as did the previous experiments; the melts were melted at temperatures from 1200 to 1500°C. As germanate systems are melted by gas, it must be remembered that GeO2 sometimes attacks platinum crucibles, especially in systems containing ThO2. Therefore, experiments over a large part of the ThO2-systems were stopped.The glass-formation ranges of binary systems are shown in Table 1. The modifier ions of this system are alkali and alkaliearth, but the Mg-ion, which is a modifier of the silicate system, is not present. This is perhaps because the acidity of germanium oxide is weaker than that of silicon oxide. In the germanate as well as the silicate system, the width of the glass formation range of the binary system is parallel to the ionic radius of the modifier, but the difference between the width of the ranges of the large and small modifiers is greater than in the silicate. This difference may be explained as resulting from the fact that the glass-forming ability of germanium oxide is weak and that, therefore, the effects of the condition for the fittest ionic radius of modifier are large. The glass-formation range of the BaO-system, like that of the tellurite system, is divided into two parts. Concerning the limited composition of the immiscible range, the calculated values from Levin's equation agreed with the experimental values (cf. Table 2).The glass-formation ranges of ternary systems are shown in Fig. 1-35. The number of systems studied reached about 90. The experimental results show that the actual glass-formation rages agree with the ranges (hatched areas in the figures) to be expected from the “Conditions of Glass Formation.” The following systems of germanaes are remarkable: in the GeO2-Li2O-BaO system (Fig. 5) and the GeO2-Al2O3-CaO system (Fig. 7), the glassformation ranges are divided into two parts. In BaO-binary systems, the vitrified range is sometimes divided into two parts, but in BaO-ternary systems it is seldom divided. Moreover, in a SiO2 (or B2O3)-Al2O3-CaO system, the glass-formation range is not divided. The reason for separation in a germanate system is not a difference in tendency to polymerize between the glass-formers of Ge and Al, but, rather, the weak glass-forming ability of GeO2. In a system containing TiO2, like silicate, a limited line of the glass-formation becomes the AC-line; therefore, the coordination number of Ti becomes 4. Moreover, in the germanate system also potassium has the widest vitrified range. (This fact agrees with the rule of the suitable ionic radius of the modifier.) Systems containing WO3, on the contrary, resemble the borate system; the glass-formation range of alkali tungstate has two tails (cf. Fig. 20). We considered in the silicate system the glass-formation range of the left tail, which consists of WO3+alkali-germante, is in an immiscible state, with a large difference in the tendency to polymerize between the two glass-formers (Si and W). The binary systems containing La2O3 or MgO systems have no vitrified ranges; therefore, these ternary systems have C-type glass-formation ranges. The glassformation