EXAFS and XANES study of (Si, Ge) mullite gels and glasses prepared by slow hydrolysis of alkoxides

Local structures in optically clear gels, glasses and mullite ceramics in the Al ₂O ₃ sbnd SiO ₂ sbnd GeO ₂ system have been studied using X-ray absorption spectroscopy at the aluminium, silicon and germanium edges. In mullite, 3Al ₂O ₃ sbnd 2SiO ₂, and the corresponding germanates, both in gel and thermally treated phases, as well as in 8Al ₂O ₃ sbnd 2SiO ₂ and Al ₂GeO ₅, aluminium is in a hexacoordinate environment, the extent of distortion of which is variable. For the last-mentioned aluminogermanate, GeO ₄ tetrahedra are rapidly formed, and the local environment within the first shell remains stable up to the ceramic state. Longer distance order is observed for the gel and crystal phases, but not for the intermediate glasses. Si sbnd O bonds are longer in mullite gels than the corresponding ceramics, probably due to the existence of bridging hydroxyl groups in the former.