The structure of sodium iron silicate glass - a multi-technique approach

Sodium iron silicate glasses of formula 0.3Na(2)O . xFe(2)O(3).(0.7-x)SiO2 (0.0 less than or equal to x less than or equal to 0.20) have been prepared by melt-quenching and their structures have been studied using X-ray photo electron spectroscopy (XPS), neutron diffraction and Mossbauer spectroscopy . XPS of surfaces fractured in ultra-high vacuum showed the presence of bot h Fe2+ and Fe3+ in the glasses but the proportion of the smaller oxidation state is <20% for larger x. The O 1s spectrum can be fitted by bridging and non-bridging oxygen contributions but properties show that this descriptio n 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 Mossbauer data indicate that iron, irrespectiv e of oxidation state, is 4-coordinated by oxygen. The Fe-O bond length decr eases and the O-O bond length increases with increasing Fe2O3 content. Moss bauer spectroscopy confirms the predominance of the Fe3+ state in these gla sses and indicates smaller Fe2+ concentrations than does XPS. possibly due to restructuring at the glass fracture surfaces examined in XPS, The effect ive 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 state s. (C) 1999 Elsevier Science B.V. All rights reserved.