I am going write the series of brief texts about defects in crystalline solids. Below is the general description of definition and types of various defects.
We define the ideal crystal as a crystal with infinite dimensions, chemically pure, and composed of atoms at rest. Sometimes, we admit in perfect crystals some perturbations of periodicity caused by thermal vibrations. However, in reality, a perfect material does not exist. Real crystals always contain various types of defects. Defect will be defined here as a disruption in the regular arrangement in lattice crystal. The mechanical and electrical properties of solids depend, in large extent, on the presence or absence of defects. In fact, without the attendance of some particular defects, unique phenomena that occur in solids, would never happen. This appendix is not a systematic discussion about defects, but a brief survey over the common imperfections occurred in crystals.
Depending on their dimensionality, one divides the defects into the following classes: a) point defects – appear at single lattice point, and are caused by lack or excess of single atoms, such as vacancies, or substitutional and interstitial impurities; b) line defects – occur along the row of atoms, for example: screw dislocations and edge dislocations; c) planar defects – consisted of planes disturbing the homogeneous regions, for example: grain boundaries, stacking faults, and twinning structures; d) volume defects, associated with the volume, such as voids, pores, or inclusions.
