Metal ions in glass : the nature of the M-O bond and its influence on redox and coordination equilibria

• Main Author: Fong, Shirley Kit-Shan
• Published: University of Aberdeen 2002
• Subjects: 546.3
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.395071

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⁺ (O_h) ions are situated between the phosphate chains cross-linking them together. Consequently, Fe³⁺ (T_d) are favoured in highly cross-linked glasses, whereas Fe³⁺ (O_h) 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>i.e</I>. 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⁺ (O_h) are associated with Fe³⁺ (O_h) are associated with oxygen atoms in Q² groups <I>i.e.</I> chain middle groups whereas Fe³⁺ (T_d) are associated with oxygen atoms in Q¹ groups.