Ultraviolet transparency of glass: a chemical approach in terms of band theory, polarisability and electronegativity

The limit of ultraviolet transparency of glass, in the absence of metal ion impurities mch as Fe3+ or Pb2+, is discussed in terms of how the glass composition drastically modifies the properties of the constituent oxygen atoms. T ro distinct approaches are made, one based on band theory and the other on electronic polarisability. Variation Of the band gap, E-g, is related to the electronegativity of oxygen which is shown to decrease with increasing glass basicity This lowers E-g thereby degrading ultraviolet transmission. Simultaneously, increasing basicity causes an increase in the oxygen polarisability which is interpreted in terms of the leas controlled electron charge clouds being unable to oscillate (it higher ultraviolet frequencies. This causes absorption and again ultraviolet degradation results. Absorption by metal ion impurities is also considered in terms of electronegativity and it is pointed out that more study is needed in order: to understand how) increasing glass basicity causes further ultraviolet degradation.