SPECTROSCOPIC STUDY OF CERIUM-DOPED SILICA-GEL MONOLITHS AND THEIR DENSIFIED DERIVATIVES

Cerium is exploited as a probe cation for elucidating the structure of an alkoxide-derived silica gel and its progressive evolution to a gla ss network as a function of heat-treatment up to 1000 degrees C. At in termediate temperatures, the host structure exhibits inhomogeneity due to insufficient formation of siloxane bonds, which is reflected by at least two different sites and co-ordination spheres (termed ''high'' and ''low'' water ligation) for cerium. This is proved by the response of the gels heated up to 700 degrees C to rehydration. Further format ion of Si-O-Si network (900 degrees C) leads to the destruction of the ''high water'' sites of cerium and progression towards a glassy struc ture. It is, however, only after heat-treatment at 1000 degrees C that a dense silica glass network, not responding to rehydration, is final ly obtained with cerium ions embedded in it.