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, whi ch the oxygen atoms of a glass provide for hosted metal ions, and the exten t to which this changes on varying the glass composition. For Mn2+, d-d spe ctra show that the changes in the (average) oxygen electronic density of th e glass are faithfully transmitted to the metal ion, and previous results f or 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 c oordination is fourfold, the sites occupied by Fe3+ are, surprisingly, of m uch lower electron density than the average for the oxygen atoms of the gla ss. The relevance of this study to the stabilisation of oxidation states fo r 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 r esiding on the Fe3+ species.