Structure of sodium iron silicate glass - a multi-technique approach

Sodium iron silicate glasses of formula 0.3Na₂O·xFe₂O₃·(0.7-x)SiO₂ (0.0≤x≤0.20) have been prepared by melt-quenching and their structures have been studied using X-ray photoelectron spectroscopy (XPS), neutron diffraction and Moessbauer spectroscopy. XPS of surfaces fractured in ultra-high vacuum showed the presence of both Fe²⁺ and Fe³⁺ in the glasses but the proportion of the smaller oxidation state is <20% for larger x. The O 1 s spectrum can be fitted by bridging and non-bridging oxygen contributions but properties show that this description is inadequate and there are contributions from bridging oxygens, Si-O-Si, from non-bridging oxygens, Si-O^-Na⁺ and also directionally bonded Si-O-Fe. The neutron diffraction and Moessbauer data indicate that iron, irrespective of oxidation state, is 4-coordinated by oxygen. The Fe-O bond length decreases and the O-O bond length increases with increasing Fe₂O₃ content. Moessbauer spectroscopy confirms the predominance of the Fe³⁺ state in these glasses and indicates smaller Fe²⁺ concentrations than does XPS, possibly due to restructuring at the glass fracture surfaces examined in XPS. The effective Debye temperatures for Fe²⁺ and Fe³⁺ are 258 and 312 K, respectively, a consequence of the different M-O bond strengths of the two oxidation states.