An EXAFS study of silica-titania sol-gels

A series of samples, with up to 41 mol%;, TiO2, have previously been studied using O-17 magic angle spinning nuclear magnetic resonance (MAS NMR) X-ray diffraction and neutron diffraction. We report here the results of an extended X-ray absorption fine structure (EXAFS) experiment an these samples, carried out to probe more directly the coordination state of the titanium site in the dried gel and its evolution with heat treatment. At Ti contents small enough to ensure atomic homogeneity, the data are consistent with 4-fold oxygen coordination around Ti,as indicated by the O-17 MAS NMR spectra. At Ti contents large enough to bring about phase separation, the results show evidence of a structural change around the Ti atoms, with a larger Ti-O distance compared with the results for the glasses with smaller TiO2 contents. (C) 1998 Elsevier Science B.V. All rights reserved.