Iron K-edge EXAFS and XANES have been used to study the local structural environment of dilute quantities (0.2mol% Fe2O3) of Fe3+ in seven representative SiO2–R2O–R’O glasses where R=(Li, Na, K), R’=(Mg, Ca, Sr, Ba). Sample preparation was carried out by adding excess CeO2 as an oxidising agent and it was confirmed by optical absorption spectroscopy that iron was fully oxidised to Fe3+. This methodology has made possible, for the first time, an unambiguous EXAFS study of the coordination and local structural environment of dilute levels of Fe3+ in alkali–alkaline earth–silica glasses without the complications presented by the presence of Fe in multiple oxidation states. Coordination of Fe3+ in these glasses is strongly affected by the nature of the alkali and alkaline earth cations, with Fe3+–O CN increasing from 4.0±0.4 to 6.2±0.9 with decreasing alkali/alkaline earth ionic radius ratio (IRR). Respective FeO bond lengths of 1.87±0.01Å to 1.92±0.01Å were also obtained. Dilute Fe3+ in the studied glasses displays selective behaviour, as demonstrated by the opposing effects of alkali and alkaline earth ions on measured Fe3+ CN and Fe3+O bond length.

It is an established methodology to use crystallographically well-defined standard materials for understanding site geometries in glasses. Here we discuss the benefits and the limitations of this approach for the investigation by Mössbauer and EXAFS of Fe2+ Fe3+ in aluminosilicate glasses. As a case study we specifically consider [5]Fe2+ [5]Fe3+ sites and whether these exist in our glasses; and if so, whether they have defined site geometries or occur simply as a consequence of the site distortions and Fe-O bond length distributions. Results are consistent with the existence of [5]Fe2+ and [5]Fe3+ but do not prove this because site distortion and a mixture of 4- and 6-coordinated sites can produce comparable results. This exemplifies the need for caution when interpreting glass data based on standards.

Modified iron phosphate glasses have been prepared with nominal molar compositions [(1−x)·(0.6P2O5–0.4Fe2O3)]·xRySO4, where x=0–0.5 in increments of 0.1 and R=Li, Na, K, Mg, Ca, Ba, or Pb and y=1 or 2. In most cases the vast majority or all of the sulfate volatalizes and quarternary P2O5–Fe2O3–FeO–RyOz glasses or partially crystalline materials are formed. Here we have characterized the structure, thermal properties, chemical durability and redox state of these materials. Raman spectroscopy indicates that increasing modifier oxide additions result in depolymerization of the phosphate network such that the average value of i, the number of bridging oxygens per –(PO4)– tetrahedron, and expressed as Qi, decreases. Differences have been observed between the structural effects of different modifier types but these are secondary to the amount of modifier added. Alkali additions have little effect on density; slightly increasing Tg and Td; increasing α and Tliq; and promoting bulk crystallization at temperatures of 600–700°C. Additions of divalent cations increase density, α, Tg, Td, Tliq and promote bulk crystallization at temperatures of 700–800°C. Overall the addition of divalent cations has a less deleterious effect on glass stability than alkali additions. 57Fe Mössbauer spectroscopy confirms that iron is present as Fe2+ and Fe3+ ions which primarily occupy distorted octahedral sites. This is consistent with accepted structural models for iron phosphate glasses. The iron redox ratio, Fe2+/ΣFe, has a value of 0.13–0.29 for the glasses studied. The base glass exhibits a very low aqueous leach rate when measured by Product Consistency Test B, a standard durability test for nuclear waste glasses. The addition of high quantities of alkali oxide (30–40mol% R2O) to the base glass increases leach rates, but only to levels comparable with those measured for a commercial soda-lime-silica glass and for a surrogate nuclear waste-loaded borosilicate glass. Divalent cation additions decrease aqueous leach rates and large additions (30–50mol% RO) provide exceptionally low leach rates that are 2–3 orders of magnitude lower than have been measured for the surrogate waste-loaded borosilicate glass. The P2O5–Fe2O3–FeO–BaO glasses reported here show particular promise as they are ultra-durable, thermally stable, low-melting glasses with a large glass-forming compositional range.

Mössbauer spectroscopy and X-ray absorption spectroscopy (XAS) have been used to obtain information on the redox and coordination of iron in glasses made by the vitrification of simulated and actual sewage sludge combustion ashes. Mössbauer spectra have been deconvoluted using three extended Voigt-based profiles to allow quantitative analysis of multiple Fe3 +  coordination environments. XAS Fe pre-K-edge absorption peaks have been analysed and fitted against reference standards to yield complementary analyses and provide validation of the Mössbauer results. Correlations are noted between the iron redox and the Fe3 +  coordination. Tetrahedral Fe3 +  is much more likely to take part in any redox reaction whilst octahedral Fe3 +  appears to be more resistant to reduction to the ferrous state. Results from XAS also suggest that Fe2 +  ions occupy both four- and six-fold coordination environments. Neither technique has provided any clear evidence for the existence of Fe2 +  or Fe3 +  in five-fold symmetry in the glasses studied.

A statistical approach has been utilised to characterise the structural and chemical properties of vitrified model sewage sludge combustion ashes. A d-optimal design has been applied to determine the effects of the compositional variability of vitrified wastes on iron redox and coordination as measured by 57Fe Mossbauer spectroscopy. Several Fe sites have been observed through the deconvolution of Mossbauer spectra, representing the Fe2+ oxidation state and tetrahedrally and octahedrally coordinated Fe3+ ions. The interdependency of iron redox and iron coordination is demonstrated, and suggests that conversion of tetrahedral Fe3+ to octahedral Fe2+ preferentially occurs upon increasing Fe2+/ΣFe. For Fe2+/ΣFe=0·30-0·89, the abundance of octahedrally coordinated Fe3+ remains approximately constant whilst tetrahedrally coordinated Fe3+ exhibits a linear decrease in its abundance with increasing Fe2+/ΣFe ratio.