The structure of silica glass in dependence on the fictive temperature

Abstract

The structure of vitreous silica was analysed in dependence on the thermal history and the impurity contents, e.g. OH groups, using the X-ray diffraction method. The structural differences were examined in terms of variations in the fictive temperature T f caused by the cooling regime as well as by the impurities. It was shown that the deviation in the fictive temperature strongly influenced the X-ray pattern in the region of the first scattering maximum determined chiefly by the middle-range order of the network. Only small but substantial variations of the short-range order were found in dependence on T f . That is why the experimental results were interpreted from the topological point of view. The statistically dominant structural unit in a silica network was represented by a polyhedron with four sixfold SiO rings. Defect polyhedrons with odd-numbered rings found in a high concentration destroyed the long-range order. The concentration of these defects was dependent on the fictive temperature. There are suggestions that the ordering processes in glass taking place during the cooling of the melt or its relaxation are accompanied by the reduction of the number of the defects. This very self-correction of the defects requires the discovery of a concept of defects and antidefects.