Hyperordered structures in silica polymorphs

We have been exploring with the quest hyperordered structures in terms of order within disorder in silica (SiO2) polymorphs. In this article, we review and discuss our recent findings on this topic comprehensively in comparison with those of previous studies. We chose several SiO4 tetrahedral corner-sharing crystalline silica and siliceous zeolites of various density. Furthermore, we attempted to control the intermediate-range ordering of glass by tuning of the density of silica glass under high pressures and temperatures. We extracted the density-driven modification of the topology of tetrahedral silica polymorphs in a wide density range. Our state-of-the-art analyses revealed two descriptors for hyperordered structures in silica polymorphs. The first descriptor of hyperordered structures in silica glass can be expressed by the position and height of diffraction peaks observed in X-ray and neutron diffraction data. This descriptor is not new, but we can systematically understand the density-driven behaviour of diffraction peaks in silica glass. The second descriptor of hyperordered structures in a series of silica polymorphs can be expressed in terms of topological characteristics: ring size distribution, cavity distribution, ring shape, and tetrahedral order. We found an unusually large cavity volume in β-cristobalite which was attributable to the formation of highly symmetrical –Si–O– sixfold rings, and highly symmetrical eightfold and twelvefold rings in coesite even though most of the small rings were significantly buckled, which was due to coesite having the highest density of coesite among the series of silica polymorphs. Moreover, we found a topological similarity between glass and siliceous zeolite (MFI), in which fivefold and sevenfold rings are observed. It is concluded that both diffraction measurement and topological analysis provide us crucial information on hyperordered structures in silica polymorphs.