Structural Similarities and Dissimilarities Between SiO2 and H2O

To date the atomistic structure of a crystalline substance can be determined with high accuracy. Due to lack of long-range order (distances r > 10Å) and the rather low degree of intermediate-range order (5Å ≲ r ≲10Å), the structure of amorphous substances can only be determined with considerably lower precision. For example, in crystalline SiO₂ the bond lengths d(Si-O) and the bond angles ≮ O-Si-0 and ≮ Si-O-Si can be determined with standard deviations of approximately 0.003Å, 0.2° and 0.2° respectively, and the bond topology can be precisely given. In contrast, for amorphous silica corresponding values for the standard deviations of bond lengths and angles are considerably higher and only speculations can be made about the ratio between the numbers of rings containing 3, 4, 5, 6, etc.. Si atoms. Therefore, in order to derive reliable information on the intermediate range order of an amorphous phase this phase should be studied not only by diffraction (X-ray, neutron) and spectroscopic (IR, Raman, NMR etc.) methods but these results should also be compared in light of the structural knowledge of the crystalline phases of the same composition and, if possible, of crystal chemically related compounds. In the case of amorphous SiO₂, the structures of the crystalline silica polymorphs and of the various crystalline phases having H₂O frameworks should be considered.