Crystal Defects

Crystals are never perfect with respect to atomic placements. The lattice points are idealized locations where even in high quality crystals represent only an mean positions of the atoms. In reality, all the atoms are constantly in motion due to thermal effects. Besides this, many other defects occur and can be labeled as 0D, 1D, 2D or 3D defects. Defects that are 0D are point defects or imperfections and include:

1. Frenkel Vacancy – lattice vacancy where the atom leaves its structural site but is retained in an interstitial (nonlattice) position

2. Schottky Vacancy – lattice vacancies of neighboring cation and anion.

3. Substitutional – An impurity atom replaces one of the original atoms in the crystal

4. Interstitial – An impurity atom resides in a nonlattice position

Defects that are 1D are line imperfections and include line dislocations, screw dislocations and mixed dislocations (combination of line and screw dislocation). These defects are characterized by a Burgers vector b which is the resulting vector needed to close an atom-to-atom circuit around a dislocation line (see figure below).

Line Dislocation from Gay Fig. 12.3

Screw Dislocation from Gay Fig. 12.4

Other 2D defects exist such as any crystal surface and twinning as well as 3D defects such as voids.