Rietveld refinement of amorphous SiO2 prepared via sol-gel method

Using the Rietveld refinement, we analyze the structural evolution under thermal treatment of silica xerogel samples prepared by the sol-gel method with molar ratio (R) of water to TEOS of R=5 and R=11.66. We refine the structure of compounds using the Maud program and we found the unit cell parameters and atomic positions of the refined silica amorphous atoms for whole samples. The results show us that the amorphous structure is quartz-like or low-cristobalite-like, depending on the molar ratio values and the heat-treatment of the samples. For R=5 different quartz structures are obtained, whereas for R=11.66 we obtain close structures in which a transformation of quartz-like amorphous to lowcristobalite amorphous phase occurs at about 600 degrees C. These results give some scopes to explain the partial crystallization of the silica at relatively low temperatures obtained when metallic species are present. (c) 2006 Elsevier B.V. All rights reserved.