Effect of basicity on chemical bonding of metal ions in glass and its relevance to their stability

An investigation is made concerning the negatively charged environment, which the oxygen atoms of a glass provide for hosted metal ions, and the extent to which this changes on varying the glass composition. For Mn2+, d-d spectra show that the changes in the (average) oxygen electronic density of the glass are faithfully transmitted to the metal ion, and previous results for Pb2+ and Tl+ suggest that this probably operates generally for uni- and dipositive ions. This principle also appears to operate for Fe3+ in glasses where it is sixfold coordinated. However, in silicate glasses, where the coordination is fourfold, the sites occupied by Fe3+ are, surprisingly, of much lower electron density than the average for the oxygen atoms of the glass. The relevance of this study to the stabilisation of oxidation states for redox reactions, such as the Fe2+/Fe3+ equilibrium, is discussed in terms of the extent of electron donation in the Fe3+-O bonding and the charges residing on the Fe3+ species.