Oxygen silicate glasses

The selectivity of glass for alkah metal ions is connected with the presence of oxides of trivalent metals in the glass structure. Zachariasen [450] states that silicate glass has a random cross-linking, where each silicon atom lies in the centre of a tetrahedron formed of oxygen atoms (see planar scheme (6.5.5)). 

The basic structural units of silicate glasses are tetrahedra with Si+4 in the center bonded covalently to O-2 at each corner. In pure silica all comer oxygen ions are shared by two tetrahedra (Figure 15.6). The result is a covalently bonded glass with a very high viscosity. As other oxides are added, not all of the oxygen ions share two comers. This lowers the viscosity. 

Oxynitride glasses are silicate or alumino-silicate glasses in which oxygen atoms in the glass network are partially replaced by nitrogen atoms. As nitrogen increases, glass transition temperature, elastic modulus, viscosity and hardness increase while thermal expansion coefficient decreases. 

Fig. 5.15. The x-ray photoelectron oxygen Is spectrum of a sodium silicate glass (30 Na,O-70 SiO,), showing contributions from bridging oxygens (BO) and nonbridging oxygens (NBO) (after Jen and Kalinowski, 1989 reproduced with the publisher s permission).

Jen, J. S., and M. R. Kalinowski (1989). An ESCA study of the bridging to nonbridging oxygen ratio in sodium silicate glass and the correlations to glass density and refractive index. J. Noncryst. Solids 38-39, 21-26. 

The energy changes observed in the Raman effect have to do with vibrations and rotations of the molecule under study or vibrations of the crystal lattice. Raman spectra can be used to identify particular molecules, or polyhedra in solids or polyhedral linkages (e.g., particular silicon-oxygen units in silicate glasses)... 

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