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

Abstract The structure of transition metal (TM) sodium silicate glasses of formulae (Fe 2 O 3 ) x (Na 2 O) 0.3 (SiO 2 ) 0.7− x and (CoO) x (Na 2 O) 0.3 (SiO 2 ) 0.7− x where the nominal x =0, 0.05, 0.10, 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 Fe 3+ and Fe 2+ are present, the majority being Fe 3+ . The isomer shifts show that the coordination number of Fe 3+ is low, probably 4, and of Fe 2+ 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 A, respectively with O–TM–O bond angles approaching 90°. The Fe–O bond length decreases and the O–O bond length increases with increasing Fe 2 O 3 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 Fe 2+ and Fe 3+ 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 Fe 2 O 3 .