Bond angle distributions in silica polymorphs

Abstract

Bond angle distributions (BADs) in silica crystals, silica glass, and siliceous zeolites were compared to understand the topology of silica polymorphs. It is found that the Si–O–Si bond angle is 180° in β-cristobalite, which is characteristic of highly symmetrical sixfold rings. In contrast, the Si–O–Si BAD of silica glass, obtained from a molecular dynamics–reverse Monte Carlo model, has an average value of 153° with a full width at half maximum (FWHM) of 20°. The Si–Si–Si BAD of β-cristobalite exhibits a peak at 110° demonstrates that SiSi₄ hyper tetrahedra possess high symmetry. This feature is consistent with the presence of symmetric six-membered rings in β-cristobalite. On the other hand, both coesite and silicalite-1 (MFI) shows a variety of bond angles like silica glass, which is consistent with the variation of (Si–O)_n ring size.