Point Defects

          The defects which take place due to imperfect packing of atom during crystallization are known as point defects. The point defect also takes place due to vibrations of atoms at high temperatures, quenching, by severe deformation of the crystal lattice; e.g., By hammering or rolling. Point imperfections are completely local in effect, e.g., A vacant lattice site. Point defects are always present in crystals and their present results in a decrease in the free energy.

A. Vacancy defect

B. Schottky imperfections

C. Interstitial defects

D. Frenkel defects

E. Compositional defects

F. Electronic defects

Vacancy defect

A vacancy is the simplest point defect and involves one or more atoms are missing from a normally occupying position as shown in the Fig. The defect caused is known as vacancy. Such defects can be a result of imperfect packing during formation of crystals or from thermal vibration of the atoms at high temperature. In the latter case, when the thermal energy due to vibration is increased, there is always an increased probability that individual atoms will jump out of their positions of lowest energy.

Schottky imperfections

          These are closely related to vacancy defect but are found in compounds which must maintain a charge balance. They involve vacancies of pair of icons of opposite charge. This type of imperfection maintains a charge neutrality in crystal. This type is dominent in alkali halides.

          Both vacancies and schottky defects facilitate atomic diffusion.

Interstitial defects

          Whenever an extra atom occupies interstitial position [i.e., voids] in the crystal system without dislodging the parent atoms as shown in Fig. the defect caused is known as interstitial defects. This happens when the atomic packing factor is low. This atom which occupies the interstitial position is generally smaller than the parent atom.

          In close packed structures e.g. F.C.C and H.C.P, the largest size of an atom that can fit in the interstitial void or space have a radius about 22.5% of the radii of parent atoms. Interstitialcies may also be single interstitial, di-interstitials, and tri-interstitials.

Frenkel defects

Whenever a missing atom [responsible for vacancy] occupies interstitial position as shown in Fig then that defect is known as Frenkel defect. Noted that a frenkel defect is a combination of vacancy and interstitial defects. The interstitial and frenkel defects are less in number than vacancy and schottky defects, because additional energy is required to force the atom into the new position.

Compositional defects

          These defects arise from impurity atom during original crystallization. Impurity atoms considered as defects in a perfect lattice are responsible for the functioning of most semiconductor devices.

          A substitutional impurity is created when a foreign atom substitutes for a parent atom in the lattice as shown in Fig.

Ex: In brass, zinc is a substitutional atom in the copper lattice.

          An interstitial impurity is a small sized atom occupying an interstice or space between the regularly positioned atoms; as shown in Fig.

Ex: In steel, carbon atoms occupy the interstitial position in the iron lattice.

Electronic defects

Electronic defects are the errors in charge distribution in solids.

          These so called electronic imperfections are preliminary necessary to explain electrical conductivity and related phenomenon in solids. An important example of this is the creation of p-n junctions and transistors.