Paracrystalline Lattice Structure of Silica Glass, α- and β-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 α-cristobalite, D(P4121); cubic β-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 ≈︁ 12%) while in the two cristobalite types it is much lower (g = 1.3%). The mean number of netplane layers N in the mPCs of silica glass is 3. whereas in α- and β-cristobalite it is much larger; 32 and 45 layers, respectively. A schematic model is given for the topological structure of silica glass, α- and β-cristobalite from which follows, that the structure of silica glass cannot be obtained simply by applying the same paracrystalline distortion g = 12% on either β- or α-cristobalite, but it requires a separate topological model.