ABSTRACT
In a perfect crystal, all atoms would be in their correct lattice positions in the structure. This situation only exists at the absolute zero of temperature, 0 K. Above 0 K, defects occur in the structure. These defects may be extended defects, such as dislocations. The strength of a material depends very much on the presence (or absence) of extended defects, such as dislocations and grain boundaries; however, discussion of these types of phenomena lie more in the realm of materials science and will not be discussed further. Defects can also occur at isolated atomic positions and are known as point defects, which can be due to the presence of a foreign atom at a particular site, or to a vacancy where normally one would expect an atom to be present. Point defects can have signicant effects on the chemical and physical properties of a solid, for instance, the beautiful colours of many gemstones are due to defect impurity atoms in the crystal structure. Another example is ionic solids, which are able to conduct electricity by a mechanism that is due to the movement of ions through vacant ion sites within the lattice (this is in contrast to the electronic conductivity explored in the previous chapter, which depends on the movement of electrons).
