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 Al2O3-SiO2-GeO2 system have been studied using X-ray absorption spectroscopy at the aluminium, silicon and germanium edges. In mullite, 3Al2O3-2SiO2, and the corresponding germanates, both in gel and thermally treated phases, as well as in 8Al2O3-2SiO2 and Al2GeO5, aluminium is in a hexacoordinate environment, the extent of distortion of which is variable. For the last-mentioned aluminogermanate, GeO4 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-O bonds are longer in mullite gels than the corresponding ceramics, probably due to the existence of bridging hydroxyl groups in the former.