In periodic crystal structure of Crystalline solids, the positions of constituent atoms or molecules are repeated, as determined by the unit cell and lattice structure.
We know that materials are never 100% pure, which implies presence of defects in crystal structure. Therefore, the regular patterns of crystal lattice are interrupted by crystallographic defects, point defect being one of these.
Point defects are those where an atom is missing or is located at an irregular place in the lattice structure. These include
- self interstitial atom
- interstitial impurity atoms
- substitutional atoms, could be smaller or larger
- vacancy and
- Frenkel pair, a nearby pair of a vacancy and an interstitial
Some of these are shown in the figure below for a mono-atomic crystal.
Illustration of defects in a compound solid, below
As seen from above discussion the point defects occur only at or around a single lattice point. They are not extended in space in any dimension.
As such dimensions of a point defect are determined by lattice structure or the unit cell.