The environment of Fe³⁺/Fe²⁺ cations in a sodium borosilicate glass

The neutron diffraction isotopic substitution technique is employed to investigate the environment of Fe³⁺/Fe²⁺ cations in a sodium borosilicate glass matrix of composition 0·210Na₂O.0·185¹¹B₂O₃.0·605SiO₂. The neutron diffraction data were obtained using the D4c diffractometer at the Institut Laue-Langevin (ILL; Grenoble, France), and were recorded for three samples; the base glass, the base glass incorporating natural Fe₂O₃ (12 mol%) and a similar glass containing Fe₂O₃ enriched in ⁵⁷Fe. The data are Fourier transformed to yield the real space total correlation function, T(r), and the first co-ordination shells of the Fe³⁺/Fe²⁺ cations are investigated via a peak fit to the isotopic difference correlation function ΔT_Fe(r). It is concluded that the iron is mainly present as Fe³⁺ cations, both tetrahedrally and octahedrally co-ordinated by oxygen atoms, plus a small fraction (0·07±0·01) of Fe²⁺ cations in octahedral co-ordination. The Fe³⁺ tetrahedral fraction is 0·45±0·10, and appears to exist as FeØ₄ ^– structural units incorporated into the network of silicate chemical groupings, with their negative charge being balanced by Na⁺ network modifying cations. The remaining Fe³⁺ cations (fraction 0·48±0·10) are thought to be predominantly octahedrally co-ordinated and associated with BO^3– ₃ orthoborate anions in FeBO₃ chemical groupings, which become non-stoichiometric due to the reduction of some of the Fe³⁺ cations to Fe²⁺.