Chemical composition of ionically conductive glasses

The glassy systems mentioned in Figs. 4.1(h) and 4.2 show that quite complex chemical compositions have been prepared in the glassy state. Up to three basic constituents are present in all ionically conducting glasses network formers, network modifiers and ionic salts, in different proportions. 

For instance, the reaction between silica and lithium oxide may be expressed schematically as  

The increasing addition of a modifier to a given network former leads to the progressive breaking of all oxygen bridges as in (4.2). 

As the number of non-bridging oxygen or sulphur atoms increases, the average length of the macromolecular chains decreases. The chemical reaction symbolised in (4.2) is strongly exothermic, and the mixing enthalpies are of the order of some hundreds of kilojoules. The magnitude 

The case of boron as a network former cation is somewhat specific in that this element has no available d orbitals. However, a p orbital is available when the boron has a coordination number of 3, which allows stabilisation of an electronic doublet of the oxygen or sulphur introduced by the modifier. This oxygen or sulphur giving up a doublet to another boron atom increases the cross-linking by the formation of two BO4 tetrahedra. In hybridisation terms, the boron is altered from the sp configuration to the sp configuration. The coordination change of boron has been especially well observed by NMR (Bray and O Keefe, 1963 Muller-Warmuth and Eckert, 1982). 

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