Thursday, January 05, 2006

Vacancy Defects in Crystals

Many solids exist as crystals. In crystals, the constituent atoms or ions are arranged in a periodic and regular pattern, or a lattice structure, which extends into three dimensions. Few examples of crystalline substances having such regular lattice structure are:
  • Diamond, with carbon atoms as lattice points.
  • Sodium Chloride, NaCl, with sodium and Chlorine atoms as lattice points.
  • Zinc Sulfide, ZnS, with Zinc and Sulfur atoms as lattice points.

Due to various thermal and mechanical factors, "defects" in this regularity are present. These defects are of different kinds. In this article, we would focus on the type of defects known as Vacancy Defect.

vacancy defect

Consider this diagram showing an otherwise regular lattice, but with a missing lattice point, which could be an atom or an ion. Because of the vacancy, there is a distortion in the regularity of the surrounding lattice points too.

An interesting thing to note about these vacancy defects in crystals is that they cannot be avoided in a crystal, since they are equilibrium defects. The meaning of being an equilibrium defect is explained below. Another interesting point about vacancies being equilibrium defects is that because of this, their number in a crystal at a particular temperature can be calculated.

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