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) whe re 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 t he iron-containing series show that both Fe3+ and Fe2+ are present, the maj ority being Fe3+, The isomer shifts show that the coordination number of Fe 3+ is low, probably 4, and of Fe2+ somewhat higher though the measurement i s less accurate. The neutron data show that both the iron and the cobalt ar e 4-coordinated with bond lengths of about 1.90 and 1.95 Angstrom, respecti vely with O-TM-O bond angles approaching 90 degrees. The Fe-O bond length d ecreases and the O-O bond length increases with increasing Fe2O3 content. T he first sharp diffraction peak (FSDP) is split on addition of iron and cob alt, showing intermediate-range order (IRO) with a larger repeat distance. The effective Debye temperatures for Fe2+ and Fe3+ are 268 and 312 K, respe ctively, showing the different Fe-O bond strengths of the two oxidation sta tes arising from their different charges. The shift in the Mossbauer spectr a with temperature was not that expected from the second order Doppler shif t and thermal expansion showing an intrinsic isomer shift dependence on tem perature. An attempt was made to relate this to the possible changes in hyb ridization of the iron with the concentration of Fe2O3. (C) 1999 Published by Elsevier Science B.V. All rights reserved.