The Structure of Glasses

2. Bridging and non-bridging oxygens (page 2 of 8)

Zachariasen coined the name network-forming cations for cations which according to his rules (see previous page) in association with oxygen form the random network of glasses. The term network former is now generally adopted for oxides capable of glass formation. Oxygen ions which act as bridges between the structural units are called bridging oxygens (BO). In addition to the network former, glasses may contain oxides which do not participate in forming the network structure and which are called network modifiers. There are three principal actions of network modifiers in glasses, which can be summarized for an A2O3 glass as follows:

  1. breaking of A-O-A bonds and creation of non-bridging oxygens,
  2. increasing the oxygen coordination of cation A,
  3. a combination of both.

[Adopted from A.C. Wright, in Experimental Techniques of Glass Science, C. J. Simmons and O. H. El-Bayoumi (Eds.), The American Ceramic Society, Westerville (1993) p.205]

The breaking of A-O-A bonds leads to the creation of non-bridging oxygens (NBO). These oxygen ions carry a partial negative charge and are connected to the glass network at one end only. The resulting network gets loose and by decreasing the connectivity a larger flexibility of the structure is obtained.

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