The structural state of iron in oxidized vs. reduced glasses at 1 atm: A57Fe Mössbauer study

A general model for the structural state of iron in a variety of silicate and aluminosilicate glass compositions in the systems Na₂O-Al₂O₃-SiO₂-Fe-O, CaO-Al₂O₃-SiO₂-Fe-O, and MgO-Al₂O₃-SiO₂-Fe-O is proposed. Quenched melts with variable Al/Si and NBO/T (average number of nonbridging oxygens per tetrahedrally coordinated cation), synthesized over a range of temperatures and values of oxygen fugacity, are analyzed with⁵⁷Fe Mössbauer spectroscopy. For oxidized glasses with Fe³⁺/∑Fe>0.50, the isomer shift for Fe³⁺ is in the range ～0.22–0.33 mm/s and ～0.36 mm/s at 298 K and 77 K, respectively. These values are indicative of tetrahedrally coordinated Fe^3?. This assignment is in agreement with the interpretation of Raman, luminescence, and X-ray,K-edge absorption spectra. The values of the quadrupole splitting are ～0.90 mm/s (298 K and 77 K) in the Na-aluminosilicate glasses and compare with the values of 1.3 mm/s and 1.5 mm/s for the analogous Ca- and Mg-aluminosilicate compositions. The variations in quadrupole splittings for Fe³⁺ are due to differences in the degree of distortion of the tetrahedrally coordinated site in each of the systems. The values of the isomer shifts for Fe²⁺ ions in glasses irrespective of Fe³⁺/∑Fe are in the range 0.90–1.06 mm/s at 298 K and 1.0–1.15 mm/s at 77 K. The corresponding range of values of the quadrupole splitting is 1.75–2.10 mm/s at 298 K and 2.00–2.35 mm/s at 77 K. The temperature dependence of the hyperfine parameters for Fe²⁺ is indicative of noninteracting ions, but the values of the isomer shift are intermediate between those values normally attributable to tetrahedrally and octahedrally coordinated Fe²⁺. The assignment of the isomer-shift values of Fe²⁺ to octahedral coordination is in agreement with the results of other spectral studies. For reduced glasses (Fe³⁺/∑Fe≈<0.50), the value of the isomer shift for Fe³⁺ at both 298 K and 77 K increases and is linearly correlated with decreasing Fe³⁺/∑Fe in the range of \(f_{O_2 } \) between 10^?3 and 10^?6 atm when a single quadrupole-split doublet is assumed to represent the absorption due to ferric iron. The increase in value of the isomer shift with decreasing \(f_{O_2 } \) is consistent with an increase in the proportion of Fe³⁺ ions that are octahedrally coordinated. The concentration of octahedral Fe³⁺ is dependent on the \(T - f_{O_2 } \) conditions, and in the range of log \(f_{O_2 } \) between 10^?2.0 and 10^?5 a significant proportion of the iron may occur as iron-rich structural units with stoichiometry similar to that of inverse spinels such as Fe₃O₄, in addition to isolated Fe²⁺ and Fe³⁺ ions.