Transition metal ions in ternary sodium silicate glasses: A Mössbauer 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 and 0.20, have been studied using neutron diffraction and Mössbauer spectroscopy. The Mössbauer 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 Å, respectively with O-TM-O bond angles approaching 90°. 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 Mössbauer 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.