ABSTRACT
The atomic structure defines all major properties of both crystalline and non-crystalline substances, so it is important to understand the structure. At the XIX International Glass Congress, it was pointed out that “commercial as well as research activity is taking place in the absence of anything like a complete understanding of the atomic structure of glass” (Cormack, Du, and Zeitler, 2001). Thus, above all, it is necessary to answer the following question: which characteristics of the structure are necessary and sufficient for defining non-crystalline state of solids? Let us start with the simplest case. From the viewpoint of a structure, the simplest case is an ideal single crystal. In order to completely describe its structure, it is enough to know the structure of an elementary cell or a short-range order of the arrangement of atoms (Table 2.1) (Popov, 2004). It is necessary to add a defect subsystem for a whole definition of any real single crystal. To describe the structure of polycrystals, it is necessary to consider the morphology and heterogeneity of the material, that is, crystal size distribution, crystal texture, formation of spherulites, and so on in
addition to short-range order and defects. And for non-crystalline solids, four levels of structural characteristics for describing their structure should be taken into consideration (Table 2.1):
1. Short-range order of atomic arrangement
2. Medium-range order of atomic arrangement
3. Morphology and heterogeneity
4. Defect subsystem
One can see in the table that the number of characteristics necessary for describing the structure of a substance increases with growth in its complexity. Where one characteristic for describing the structure of an ideal single crystal is sufficient, it is necessary to use four characteristics to describe the structure of non-crystalline solids.
