Occupation Of Tungsten Site by Iron In Sodium Tungstate Glasses

⁵⁷Fe Mössbauer spectra of xNa₂O · (99 − x)WO₃ · ⁵⁷Fe₂O₃ glasses (30 ≤ x ≤ 42) comprised of a doublet due to octahedral iron, Fe³⁺(O_h), and a weak doublet due to Fe²⁺(O_h); a large Debye temperature of 580 K was obtained at low temperatures. Fourier transform infrared spectra of the sodium tungstate glasses showed a gradual increase in the fraction of WO₄tetrahedra (T_d) when Na₂O content was increased. Mössbauer spectra of the tungstate glasses irradiated with ⁶⁰Co γ-rays showed an increase in the fraction of Fe²⁺, due to electron transfer (scattering) from WO and FeO bonds to Fe³⁺, as the electron spin resonance spectra of iron-free sodium tungstate glass indicated a simultaneous formation of W⁵⁺ and the ‘hole’ trapped on the oxygen atom. From the linear relationship between glass transition temperature, T_g, and quadrupole splitting, Δ, of Fe³⁺ (‘T_g − Δ rule’), the slope of the straight line was estimated to be 260°C/(mm s⁻¹). Mössbauer spectra of heat-treated tungstate glasses showed a gradual change from Fe³⁺(O_h) to Fe³⁺(T_d), along with the precipitation of the Na₂W₂O₇ phase composed of W⁶⁺(O_h) and W⁶⁺(T_d). These results indicate that  iron occupies W⁶⁺(O_h) sites in glasses, while it occupies W⁶⁺(T_d) sites in glass-ceramics.

 

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