Point defects in crystals of elements

Crystals of the solid elements, such as silicon, contain only one atom type. The simplest localised defect that we can imagine in a crystal is a mistake at a single atom site. These defects are called point 

Such vacancies and interstitials, which occur in even the purest of materials, are called intrinsic defects. 

The fraction of atom sites which contain a defect, n /N, at any temperature, can be calculated if the enthalpy of defect formation, AH, is known. 

To obtain the absolute number of defects in the solid, it is necessary to know the number of atoms, iV, in a unit volume of the crystal. This value is often obtained from the crystal structure of the compound. The crystal structure is described in terms of the unit cell, which is a small representative volume of the crystal (see Chapter 5). We may write  

For example, the unit cell of silicon is cubic, with a side of 0.5431 nm, and contains 8 atoms of silicon, which allows N to be found. 

---