= Structure of Glass
The works of \b[Zachariasen] and Warren (Network Theory and its verification by X-ray diffraction) resulted in big advancements in the study of glass structure. This theory can predict a large number properties of conventional glasses. His theory was later extended by Dietzel.
Zachariasen presented three types of cations that form a glass:
* \b[Network-formers] that have a high oxidation state with a coordination number (number of bonds they form) generally of 3 or 4 (for example: Si, B, P, Ge, As, Be). These are connected together with the help of anions present in the glass.
* \b[Network-modifiers] that are placed in order to disrupt the organized structure of the network. As they are cations they place themselves close to the anions (oxygen in $SiO_2$ for example) that connects the network. They break some bridging covalent bonds and "stick" to the broken ends electrostatically forming ionic bonds of lower energy. This reduces the viscosity so that the glass can be manufactured more efficiently, as it softens in a lower temperature.
* \b[Intermediates] which assist in glass formation by forming intermediate bonds to oxygen. They can't form glasses on their own.
In order for glass to form, the network-former should (although there are exceptions) form polyhedral groups in its simplest form. (For example silica, $SiO_2$ forms $SiO_4^{4-}$ tetrahedrals). Each polyhedron should only connect with its neighbouring once via bonding bridges formed by the anions (in this example an Si-O-Si).
The network-former of silicate glass is $SiO_2$ which is