The electronic polarisability of oxygen in glass and the effect of composition

The electronic polarisability of the oxide species, αoxide(-II), extends over a wide range of values depending on chemical bonding. Several hundred silicate glasses containing alkali and/or alkaline earth metal ions (also Mg2+) have been investigated, and the value of αoxide(-II) for each of these was calculated from refractive index and density. The relationship between αoxide(-II) and the acid–base nature of the glass, expressed as optical basicity, Λ, was investigated, and the results were used to calculate basicity moderating parameters of the constituent sodium, calcium, silicon, etc. These parameters allow calculation of Λ from glass composition, and hence it is possible to predict how oxide polarisability in a glass system will be affected by compositional changes. This facility is also useful for exposing deviations in αoxide(-II) arising from abnormal chemical or physical behaviour. Such examples include (i) the identification of the `mixed alkali effectʹ in Li2O–K2O–SiO2 glasses, (ii) abnormal refractivity behaviour in glasses based on BeO (in an assessment of the basicity moderating parameter for beryllium in these glasses), and (iii) the effect of stereochemistry on glass basicity, as found for threefold and fourfold co-ordinated boron in borate glasses (such data providing a guide for identifying the co-ordination number of boron in borosilicate glasses). The relevance of the basicity moderating parameter to the polarising power of cations and to electrovalency, covalency and metallic bonding in oxidic systems generally is also discussed.