1.02.2.1 Point Defects Compared to Defects of Greater Spatial Extent

In crystallography, we learn that the atoms and ions of inorganic materials are, with the exception of glasses, arranged in well-defined planes and rows.3 This is, however, an idealized representation. In real­ity, crystals incorporate many types of imperfections or defects. These can be categorized into three types depending on their dimensional extent in the crystal:

1. Point defects, which include missing atoms (i. e., vacancies), incorrectly positioned atoms (e. g., inter­stitials), and chemically inappropriate atoms (dopants). Point defects may exist as single species or as small clusters consisting of a number of species.

2. Line defects or dislocations, which extend through the crystal in a line or chain. The dislocation line has a central core of atoms, which are located well away from the usual crystallographic sites (in cera­mics, this extends, in cylindrical terms, to a nano­meter or so). Most dislocations are of edge, screw, or mixed type.4

3. Planar defects, which extend in two dimensions and are atomic in only one direction. Many differ­ent types exist, the most common of which is the grain boundary. Other common types include stacking faults, inversion domains, and twins.1,2

The defect types described above are the chemical or simple structural models for the extent of defects. It is critical to bear in mind that all defect types, in all materials, may exert an influence via an elastic strain field that extends well beyond the chemical extent of the defect (i. e., beyond the atoms replaced or removed). This is because the lattice atoms sur­rounding the defect have had their bonds disrupted. Consequently, these atoms will accommodate the existence of the defect by moving slightly from their perfect lattice positions. These movements in the positions of the neighboring atoms are referred to as lattice relaxation.

As a result of the elastic strain and electrostatic potential (if the defect is not charge-neutral), defects can affect the mechanical properties of the lattice. In addition, defects have a chemical effect, changing the
oxidation/reduction properties. Defects also provide mechanisms that support or impede the movement of ions through the lattice. Finally, defects alter the way in which electrons interact with the lattice, as they can alter the potential energy profile ofthe lattice (whether or not the defect is charged). For example, this may lead to the trapping of electrons. Also, because dopant ions will have a different electronic configuration from that ofthe host atom, defects may donate an electron to a conduction band, resulting in n-type conduction, or a defect may introduce a hole into the electronic struc­ture, resulting in p-type conductivity.