A structural model for phosphate glasses

The structural component of the network-former oxide in phosphate glasses has a decreasing number of links between the PO4 tetrahedra. However, in addition to the common depolymerization process, further structural principles are in effect. In the present paper, it is proposed that all of the terminal oxygen atoms, including those doubly bonded within the PO4 branching groups, tend to coordinate a network-modifier cation. In a first range, that leads to an effect which can be called a re-polymerization based on modifier oxide additions. Due to the surfeit of terminal oxygen atoms in phosphate glasses with small metal oxide content, the modifier cations, Me, occupy places with high Me-O coordination numbers surrounded by terminal oxygen atoms on Me-O-P bridges. The system tends to stabilize at the point at which all of the terminal oxygen atoms occupy Me-O-P positions. For even higher concentrations of metal oxide, the coordination number of modifying cations around terminal oxygens exceeds unity, and a modified random network ensues. The model is used for the explanation of the behaviour of packing densities and Me-O coordination numbers. The phenomenon of drastic changes of glass properties at about 20 mol% metal oxide content has been related to a network reconstruction based on the change in the type of linkage between the metal cations and the PO4 middle groups.