3.1.The Atomic Structure of Condensed Matter 25 FIGURE 3.1.High-reso- 0.o1m lution electron micro- graph of silicon in which a boundary be- tween a crystalline (lower part)and an amorphous region (upper part)can be observed.(Courtesy of S.W.Feng and A.A. Morrone,University of Florida.) A perfect crystal in which all atoms are equidistantly spaced from each other is,however,seldom found.Instead we know from indirect observations that some atoms in an otherwise or- dered structure are missing.These defects are called vacancies. Their number increases with increasing temperature and may reach a concentration of about 1 per 10,000 atoms close to the melting point.In other cases,some extra atoms are squeezed in between the regularly arranged atoms.They are termed self- interstitials (or sometimes interstitialcies)if they are of the same species as the host crystal.These defects are very rare because of the large distortion which they cause in the surrounding lat- tice.They may be introduced by intense plastic deformation or by irradiation effects,for example,in nuclear reactors.On the other hand,if relatively small atoms are crowded between regu- lar lattice sites which are of a different species(impurity elements such as carbon,hydrogen,nitrogen,etc.),the term interstitial is used instead.Moreover,parts of planes of atoms may be absent, the remaining part ending at a line.These line imperfections are referred to as dislocations.(Dislocations are involved when ma- terials are plastically deformed,as we will discuss in detail later on.) In still other solids,one detects an almost random arrange- ment of atoms,as observed in the upper part of Figure 3.1.This random distribution constitutes an amorphous structure. The mechanical properties of solids depend,to a large extent, on the arrangement of atoms,as just briefly described.Thus,we need to study in this chapter the microstructure and the crystal- lography of solids.The crystal structures,however,depend onA perfect crystal in which all atoms are equidistantly spaced from each other is, however, seldom found. Instead we know from indirect observations that some atoms in an otherwise or￾dered structure are missing. These defects are called vacancies. Their number increases with increasing temperature and may reach a concentration of about 1 per 10,000 atoms close to the melting point. In other cases, some extra atoms are squeezed in between the regularly arranged atoms. They are termed self￾interstitials (or sometimes interstitialcies) if they are of the same species as the host crystal. These defects are very rare because of the large distortion which they cause in the surrounding lat￾tice. They may be introduced by intense plastic deformation or by irradiation effects, for example, in nuclear reactors. On the other hand, if relatively small atoms are crowded between regu￾lar lattice sites which are of a different species (impurity elements such as carbon, hydrogen, nitrogen, etc.), the term interstitial is used instead. Moreover, parts of planes of atoms may be absent, the remaining part ending at a line. These line imperfections are referred to as dislocations. (Dislocations are involved when ma￾terials are plastically deformed, as we will discuss in detail later on.) In still other solids, one detects an almost random arrange￾ment of atoms, as observed in the upper part of Figure 3.1. This random distribution constitutes an amorphous structure. The mechanical properties of solids depend, to a large extent, on the arrangement of atoms, as just briefly described. Thus, we need to study in this chapter the microstructure and the crystal￾lography of solids. The crystal structures, however, depend on 3.1 • The Atomic Structure of Condensed Matter 25 FIGURE 3.1. High-reso￾lution electron micro￾graph of silicon in which a boundary be￾tween a crystalline (lower part) and an amorphous region (upper part) can be observed. (Courtesy of S.W. Feng and A.A. Morrone, University of Florida.)