Silicates, binary structural models

Similar calculations of size distribution of ions in silicate melts were made by Hess (213). Structural models of binary silicate melts have also been proposed by Gaskell (214), who discussed experimental evidence for and against the validity of the models. 

Comparing structural information from X-ray and neutron diffraction provides a very valuable way to validate MD simulation results of glasses. In some simple systems, the partial pair distribution function or partial structure factors of all atom pairs can be determined experimentally and they provide excellent validations for simulated structures. However, as the composition becomes more complicated and more elements included, larger number of pair contributions will complicate the comparison and the validation becomes more and more difficult in multicomponent glass systems. For example, for binary oxides, e.g. sodium silicate, there are six partial pair distribution functions, but for a four component systems, for example the bioactive glass composition, there are a total of fifteen partials contributions. The overlap between partial contributions makes it very challenging to assign the peaks and to determine the quality of comparison and hence the validation of the simulated structure models. 

Compared to the silicates, only a few systematic physico-chemical measurements have been carried out on the binary borates. Whereas in silica and silicates the Si04 tetrahedron is the only building unit, both B08 and BCb groups are known to be present in crystalline and glassy borates (52, 60). Structural interpretations similar to those for the silicates are thus more difficult. At present, no satisfactory model exists for the binary borates. Some very interesting results arc, however, available and these will be discussed below. 

Tanabe has reviewed the earlier work with silica-magnesia, silica-zirconia, and other amorphous siliceous materials. In a model for binary siliceous oxide catalysts, only the non-siliceous component was considered in terms of proton affinity and co-ordination number. Tanabe and co-workers " proposed a general model for mixed oxide catalysts in which acidity is caused by an excess of negative or positive charge in a model structure of the binary oxide. The hypothesis is shown to fit 28 of the 31 binary oxides tested. One of these oxides,... 

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