Crystalline vs. amorphous materials

Fig. 3.1 shows the PDFs of crystalline Quartz and amorphous silica glass, the structures of which are shown in Fig. 3.2.

Fig. 3.1(a) shows that there are some well defined peaks in the PDF for low-r (less than ∼7.5 Å), but above this the PDF becomes featureless. Whereas crystalline materials are defined by a repeating unit that makes the structure periodic over long distances, long-range order, here there is no such order. However, there are short-range correlations as shown by the strong peaks at short distances. In Fig. 3.1(b) there are strong peaks and co-ordinating distances across the r-range due to the crystalline nature of the material.

It is important to note the similarity in the short-range correlations in these two spectra however. Both show strong peaks at r ∼ 1.6 Å and r ∼ 2.7 Å. These correspond to the Si-O and O-O bond distances respectively. Importantly, this demonstrates that both materials are made up of the same structural unit. This is silica tetrahedra- a silicon atom surrounded by four oxygens in a tetrahedral arrangement.
Whilst for Quartz these tetrahedra are joined together to form the regular crystalline structure, for amorphous Silica they are randomly oriented.