Gel-glass transformation in the SiO 2Fe 2O 3 system

Abstract

Gel-glass transformation has been studied by Mössbauer spectroscopy, DTA-TG analyses and X-ray diffractometry for four compositions in the SiO 2Fe 2O 3 system (A: 5 wt% Fe 2O 3, B: 10 wt% Fe 2O 3, C: 20 wt% Fe 2O 3, D: 40 wt% Fe 2O 3). The gels were prepared by the hydrolysis of silicon tetraethoxide and iron triethoxide and successively dried and heated in oxygen in the temperature range 40–1000°C. Samples A and B gave typical amorphous X-ray patterns up to 700°C; heating at higher temperature yielded the precipitation of quartz, cristobalite and hematite in sample A, cristobalite and hematite in sample B. Crystallization was also detected by DTA in sample A for which X-ray diffraction exhibited a larger effect. In samples C and D crystallization took place starting from 300°C with the precipitation of hematite, which remained the only crystalline phase up to 1000°C. The presence of hematite was confirmed by the obtained Mössbauer spectra which showed the characteristic sextet. The apportion of iron ions in the Fe 3+ and Fe 2+ oxidation states was also determined, together with the attribution of the probable coordination states for Fe 3+ ions. Complex magnetic structure appeared in samples treated above 800°C.