Novel structural behaviour of iron in alkali–alkaline-earth–silica glasses

The combined effect of alkali and alkaline-earth ions on the redox, distribution, co-ordination and environment of Fe ions in alkali–alkaline-earth–silica glasses has been studied using a multi-technique approach. Wet chemical analysis and Mössbauer, electron spin resonance (ESR), optical absorption and photoluminescence spectroscopies were utilised. Behaviour generally falls into two categories, which we have termed ‘collective’ and ‘selective’. Collective behaviour occurs when alkali and alkaline-earth ions have similar effects on a property and the overall effect is cumulative. This is characterised by a linear relationship with optical basicity of the glass. Some parameters associated with the environment of Fe2+ ions fall into this category. Selective behaviour occurs when alkali and alkaline-earth ions have opposing effects on a property, suggesting competition or selectivity. This is characterised by a linear relationship with the alkali/alkaline-earth ionic radius ratio, cation field-strength ratio or oxide-basicity ratio. The Fe2+/ΣFe ratio and several parameters associated with the distribution, coordination and environment of Fe3+ ions fall into this category. These results have implications for the local structure surrounding Fe species. A relationship has been suggested linking coordination and distribution of Fe3+ ions.