The environment of iron in silicate glasses

Iron species in glass are vital to applications such as solar control, and greater
understanding of these species is required. Redox, coordination, distribution and
environment of iron in alkali-alkaline earth-silica glasses containing 0-5 molar %
Fe203 have been studied using a multi-technique approach.
Wet chemical analysis provided values of the Fee+/EFe redox ratio and these are
in agreement with those obtained from optical and Mössbauer techniques over a
wide range of glass compositions and Fe203 contents. The Fee+/EFe ratio is
independent of iron content in the glasses studied, provided that melting
conditions allow sufficient equilibration with the furnace atmosphere.
As iron content increases, Fe-Fe near-neighbour interactions have an increasing
influence on measured properties. Mössbauer parameters indicate a wider range
of site occupancies by Fe 2+ and Fe 3+ at low iron contents. Electron spin resonance
suggests some clustering of Fe ions even at low iron concentrations. The amount
of clustering is proportional to the square of the molar Fe203 content. Increasing
alkali / alkaline earth ionic radius ratio promotes clustering at all Fe203 contents.
The effects of alkali and alkaline earth ions on the redox, distribution,
coordination and environment of Fe ions in these glasses generally fall into two
behavioural categories which have been 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
proportionality with theoretical optical basicity of the glass. Many parameters
associated with Fe 2+ ions fall into this category.
Selective behaviour occurs when alkali and alkaline earth ions have opposing
effects on a property, suggesting competition or selectivity between ion types.
This is characterised by proportionality with the alkali / alkaline earth ionic radius
ratio. The redox ratio and several parameters associated with the clustering,
coordination and environment of Fe 3+ ions fall into this category.
Glasses containing MgO can exhibit behaviour dissimilar to the other alkaline
earth oxides; molar volume, redox, coordination and clustering are affected.
These phenomena may be due to some Mg 2+ ions occupying tetrahedral sites.