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.