INTERMEDIATE-RANGE ORDER IN SILICA-GELS

Fresh silica gels prepared by the sol-gel route with acid catalysis di splay typical alterations of the X-ray diffraction (XRD) pattern, in c omparison with vitreous (v) silica. A positive shift of the first peak at k(1) is noticed, which also shows excess broadening. The changes c an be accounted for by the alteration of the intermediate-range order (IRO) in gels; in comparison with v-SiO2 Thermal annealing causes a gr adual decrease in the shift and broadening of the first peak, suggesti ng that the evolution of the vitrification can be followed on the basi s of XRD data. Two types of IRO alteration were modelled, which accoun t quantitatively for the experimental observations using a phe nomenol ogical disorder parameter b. The evolution of the b values with anneal ing temperature reveals well-defined stages in the vitrification proce ss.