Occupation of tungsten site by iron in sodium tungstate glasses

Abstract 57 Fe Mossbauer spectra of x Na 2 O · (99 − x )WO 3 · 57 Fe 2 O 3 glasses (30 ≤ x ≤ 42) comprised of a doublet due to octahedral iron, Fe 3+ (O h ), and a weak doublet due to Fe 2+ (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 WO 4 tetrahedra (T d ) when Na 2 O content was increased. Mossbauer spectra of the tungstate glasses irradiated with 60 Co γ-rays showed an increase in the fraction of Fe 2+ , due to electron transfer (scattering) from WO and FeO bonds to Fe 3+ , as the electron spin resonance spectra of iron-free sodium tungstate glass indicated a simultaneous formation of W 5+ and the ‘hole’ trapped on the oxygen atom. From the linear relationship between glass transition temperature, T g , and quadrupole splitting, Δ, of Fe 3+ (‘ T g − Δ rule’), the slope of the straight line was estimated to be 260°C/(mm s −1 ). Mossbauer spectra of heat-treated tungstate glasses showed a gradual change from Fe 3+ (O h ) to Fe 3+ (T d ), along with the precipitation of the Na 2 W 2 O 7 phase composed of W 6+ (O h ) and W 6+ (T d ). These results indicate that iron occupies W 6+ (O h ) sites in glasses, while it occupies W 6+ (T d ) sites in glass-ceramics.