PARACRYSTALLINE LATTICE STRUCTURE OF SILICA GLASS, ALPHA-CRYSTOBALITE AND BETA-CRYSTOBALITE

The prevailing dualistic concepts of continuous random network structure (CRNS) and assemblies of microcrystals or clusters in silica glass are solved by applying the theory of paracrystallinity and the analysis of the radial distribution functions of X-ray diffraction diagrams. SiO2-glass, as well as the two cristobalite modifications (tetragonal alpha-cristobalite, D4(4)(P4(1)2(1); cubic beta-cristobalite, Fd3m) are built up of microparacrystals (mPC) with different sizes and levels of distortion. The paracrystalline lattice distortion parameter of silica glass is high (g almost-equal-to 12%) while in the two cristobalite types it is much lower (g = 1.3%). The mean number of netplane layers NBAR in the mPCs of silica glass is 3, whereas in alpha- and beta-cristobalite it is much larger; 32 and 45 layers, respectively. A schematic model is given for the topological structure of silica glass, alpha- and beta-cristobalite from which follows, that the structure of silica glass cannot be obtained simply by applying the same paracrystalline distortion g = 12% on either beta- or alpha-cristobalite, but it requires a separate topological model.