Voronoi tessellation analysis of SiO2 systems based on oxygen packing

Packing models have been used as one of powerful analytical tools to investigate atomic structures comprised of single element or metallic alloy. In this study, the structures of SiO2 crystals and glasses were studied by a new packing model. To deal with two-element oxide systems like SiO2, packing structures focused only on oxygen atoms were constructed using the Voronoi tessellation method. First, the method was applied to crystal polymorphs of SiO2. Species of polyhedron, species of polygon and volume of each polyhedron were compared between the polymorphs. The calculated results captured not only structure features inherent in each crystal, but also the indication of structural changes such as α-β transition or corruption of crystal structure. Next, the method was applied to SiO2 glass. The Voronoi analysis distinguished glass from crystals in terms of species of polyhedron. In addition, the same method could distinguish structural changes caused by thermal effects from those by pressure effects. The thermal effects change geometrical symmetry (i.e. shape of polyhedron and polygon) with little change in Voronoi volume, in contrast, the pressure effects change geometrical symmetry less with larger change in Voronoi volume. In addition, a new index, which represents a degree of geometrical similarity between crystals and glasses, was defined in terms of Voronoi polyhedron. In sum, the Voronoi analysis based on oxygen packing is expected to be a promising tool to investigate not only the structure of crystal polymorphs but also that of glass and liquid, taking the complimentary role to the conventional network model.