TABLE OF CONTENTS Vitreous germania has a structure very similar to that of vitreous silica, with the basic building block of germanium-oxygen Q 4 units. Since the germanium ion is somewhat larger in diameter than the silicon ion, the Ge O distance is also somewhat greater, with a bond length of ? 0.173 nm. The Ge O Ge bond angle is smaller than the Si O Si bond angle. Gas diffusion studies suggest that the structure of vitreous germania is more compact than that of vitreous silica and therefore the free, or interstitial, volume of vitreous germania is slightly less than that of vitreous silica. Structural defects are more common in vitreous germania than in vitreous silica, with a measurable concentration of Ge Ge bonds.
The structures of borogermanate and silicogermanate glasses consist of fully linked networks containing mixtures of germanium Q 4 groups with either boron Q 3 or silicon Q 4 groups, as appropriate. The properties of glasses in these systems vary smoothly from those of one pure glassformer oxide to those of the other. Although these glasses are not phase separated, there is some spectral evidence for intermediate range order groups which contain only one or the other of the glassformer oxides. The connectivity number for borogermanate glasses varies between 3 and 4, while that of silicogermanate glasses is always 4.
Two completely different structural models are currently proposed for alkali germanate...
