Pt.1-Basic Concepts and the Constitution of Alkali Silicate Glasses
The Significance of the Co-ordination Requirements of the Cations in the Constitution of Glass. I. Basic Concepts and the Constitution of Alkali Silicate Glasses
JSGT 1951 V35 T421-T447
In its present form the random-network theory of the constitution of glasses fails to account for a considerable number of the properties of glasses established by careful experiment. Examples occur even among the simplest of glasses, namely, those formed from silica and the alkalis, in connection with the effects of the different alkali oxides on lowering the melting point of cristobalite and with the rate of evaporation of alkali from the glasses at high temperature; thermal expansion coefficients; specific volume; electrical resistivity; and chemical durability, in all of which cases there is evidence of the existence of molecules of chemical compounds. The author takes the view that the assumption hitherto made that the O2- ions are rigid spheres with a size which remains unaffected by powerful electrical fields is a handicap to the network theory. The view of K. Fajans that the anions are deformed in the electrical field of the cations is regarded by the author as an important principle in solid-state chemistry., The electrical interactions of ions, leading to mutual deformation and polarisation, are particularly pronounced if the anions are large and the cations have incomplete electronic shells. The author shows that polarisation phenomena are not restricted to non-noble-gas-type ions, but are equally important for the systems of noble-gas-type ions of the alkali silicate glasses. Here each large O2- ion, polarised under the influence of neighbouring cations, has as its nearest neighbours a silicon ion at one side and either a silicon or an alkali ion on the other. In general, taking the deformation of the O2- ions into consideration, the network theory is modified by considering the binding forces between the ions.
W. A. Weyl
