Metal ions in glass : the nature of the M-O bond and its influence on redox and coordination equilibria
Redox and coordination equilibria of the [Mn2+]/[Mn3+] and [Fe2+]/[Fe3+] couples in are studied glasses over a wide structural and basicity range, and discussed in terms of the optical basicity model. Changes in concentration of ionic species were measured using optical spectroscopy. Basicity of the sites occupied by Mn2+ and Fe3+ ions were calculated from spectra and compared to theoretical calculated glass basicity. Behaviour of the [Mn2+]/[Mn3+] couple was consistent with the optical basicity model, with increasing glass basicity the upper oxidation state was increasingly stabilised. Basicity of the Mn2+ hosting site correlated well with calculated glass basicity. Fe3+ ions were present in tetrahedral and octahedral sites. It is proposed that Fe3+ (Oh) ions are situated between the phosphate chains cross-linking them together. Consequently, Fe3+ (Td) are favoured in highly cross-linked glasses, whereas Fe3+ (Oh) are favoured in glasses of high modifier content. The [Fe2+]/[Fe3+] couple did not demonstrate a clear trend as predicated by the optical basicity model. In binary phosphate glasses the [Fe2+]/[Fe3+] ratio was relatively constant. Where changes were observed, it suggested that these arose due to structural effects increasing competition for sites between phosphate chains. Basicity of the Fe3+ site was relatively constant, i.e. average glass basicity was not transmitted to the cation site. This effect appeared to arise due to the constant bond character of the Fe3+ -O bond, when Fe3+ ions are associated with oxygen atoms in similar structural groups. Basicity values indicate that Fe3+ (Oh) are associated with Fe3+ (Oh) are associated with oxygen atoms in Q2 groups i.e. chain middle groups whereas Fe3+ (Td) are associated with oxygen atoms in Q1 groups.