Occupation of tungsten site by iron in sodium tungstate glasses

Fe-57 Mossbauer spectra of xNa(2)O .(99-x)WO3 .(Fe2O3)-Fe-57 glasses (30 less than or equal to x less than or equal to 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 WO4 tetrahedra (T-d) when Na2O content was increased. Mossbauer spectra of the tungstate glasses irradiated with Co-60 gamma-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, Delta, of Fe3+ ('T-g - Delta rule'), the slope of the straight line was estimated to be 260 degrees C/(mm s(-1)). Mossbauer 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.