Transition metal ions in ternary sodium silicate glasses: a Mossbauer and neutron study

The structure of transition metal (TM) sodium silicate glasses of formulae (Fe2O3)(x)(Na2O)(0.3)(SiO2)(0.7-x) and (CoO)(x)(Na2O)(0.3)(SiO2)(0.7-x) where the nominal x=0, 0.05, 0.10, 0.15,0.15 and 0.20, have been studied using neutron diffraction and Mossbauer spectroscopy. The Mossbauer spectra of the iron-containing series show that both Fe3+ and Fe2+ are present, the majority being Fe3+, The isomer shifts show that the coordination number of Fe3+ is low, probably 4, and of Fe2+ somewhat higher though the measurement is less accurate. The neutron data show that both the iron and the cobalt are 4-coordinated with bond lengths of about 1.90 and 1.95 Angstrom, respectively with O-TM-O bond angles approaching 90 degrees. The Fe-O bond length decreases and the O-O bond length increases with increasing Fe2O3 content. The first sharp diffraction peak (FSDP) is split on addition of iron and cobalt, showing intermediate-range order (IRO) with a larger repeat distance. The effective Debye temperatures for Fe2+ and Fe3+ are 268 and 312 K, respectively, showing the different Fe-O bond strengths of the two oxidation states arising from their different charges. The shift in the Mossbauer spectra with temperature was not that expected from the second order Doppler shift and thermal expansion showing an intrinsic isomer shift dependence on temperature. An attempt was made to relate this to the possible changes in hybridization of the iron with the concentration of Fe2O3. (C) 1999 Published by Elsevier Science B.V. All rights reserved.