Permitted and forbidden structural units in vitreous systems

A central cation in oxide glasses may be surrounded as next neighbors by bridging or passive O ions, the no. of which det. the nos. of network-modifiers and O ions that must be accommodated in the second sphere. By comparing these nos. with those calcd. from the space available, the conclusion was drawn that certain structural units may be permitted and that others must be forbidden. [on SciFinder (R)]