Sites in oxidic network glasses

Chemically reactive sites in oxidic networks are discussed in terms of the electron densities provided at the sites using the optical basicity model. Ultraviolet/visible/near-infrared spectroscopy has been used to identify sites for metalions. The effect of glass composition on the coexistence and availability of these sites is considered with reference to the structure of the network and sites identified using far-infrared spectroscopy. Ultraviolet spectra of lead(II) ions indicate the existence of two types of site for Pb²⁺ in borate and phosphate networks. The switchover from one site to the other coincides with major changes in the network structure. Remarkably, the natures of the Pb²⁺ sites in all of the binary alkali systems studied are independent of the identify of the alkali. Moreover, data for lead(II) ions in xLi₂O.(1-x)Cs₂O.2B₂O₃ systems indicate that the natures of the Pb²⁺ sites change when more than one alkali is present. The influence of the modifier cation charge on the sites for Pb²⁺ ions has also been investigated. Data for barium, calcium and zinc systems are reported. The possibility of using vanadyl and cobalt(II) ions to quantify the basicities of glasses with poor ultraviolet transparence has been considered. The response of the Co²⁺ ion in borate systems gave the best fit with the optical basicity model. Approximate values have been assigned to the basicities of Tl₂O, PbO, Bi₂O₃ and Ag₂O by comparing the spectra of the Co²⁺ ion in borate glasses containing these oxides to those of Co²⁺ in the equimolar alkali glasses. The results provide an insight into the site selectivity of Tl⁺, Pb²⁺ and Bi²⁺ probe ions. Basicity changes in AgI.Ag₂O.B₂O₃ and NaI.Na₂O.B₂O₃ systems have also been investigated. Ligand field data for Ti³⁺, V³⁺, Cr³⁺, Ni²⁺ and Cu²⁺ ions in oxidic glasses have been collected from the literature with a view to correlating the ligand field parameters Dq and B with optical basicity.