TOPOLOGY AND TOPOLOGICAL DISORDER IN SILICA

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Abstract

In this contribution, we define silica generically as comprising all compounds of silicon and oxygen with the composition SiO2. These compounds are among the most abundant on the earth’s surface and adopt a large number of possible polymorphic forms, among them the seven compact crystalline polymorphs cristobalite, tridymite, moganite, keatite, quartz, coesite and stishovite (the last two high-pressure forms); a family of porous framework structures (e.g. silicalite); vitreous silica obtained by cooling molten silica without crystallization through a glass transition; aperiodic metamict silicas formed by radiation-induced disordering; and other non-crystalline forms produced by application of pressure, oxidation of silicon, vapor deposition or dehydration of gels [1]. All of these solid forms of silica share a common composition, a common chemistry and even (with the exception of stishovite) a common structural element: a substantially covalent [SiO4] tetrahedral unit; but they are all structurally very different (Table 1).