In crystallography, a vacancy is a type of point defect in a crystal.  Crystals inherently possess imperfections, sometimes referred to as crystalline defects. A defect in which an atom, such as silicon, is missing from one of the lattice sites is known as a 'vacancy' defect. It is also known as a Schottky defect, although in ionic crystals the concepts are not identical.
Vacancies occur naturally in all crystalline materials. At any given temperature, up to the melting point of the material, there is an equilibrium concentration (ratio of vacant lattice sites to those containing atoms). At the melting point of some metals the ratio can be approximately 1:1000.
It is the simplest point defect. In this system, an atom is missing from its regular atomic site. Vacancies are formed during solidification due to vibration of atoms, local rearrangement of atoms, plastic deformation and ionic bombardments.
The creation of a vacancy can be simply modeled by considering the energy required to break the bonds between an atom inside the crystal and its nearest neighbor atoms. Once that atom is removed from the lattice site, it is put back on the surface of the crystal and some energy is retrieved because new bonds are established with other atoms on the surface. However, there is a net input of energy because there are fewer bonds between surface atoms than between atoms in the interior of the crystal.
- P. Ehrhart, "Properties and interactions of atomic defects in metals and alloys", volume 25 of Landolt-Börnstein, New Series III, chapter 2, page 88, Springer, Berlin, 1991
- R. W. Siegel, Vacancy concentrations in metals, J. Nucl. Mater. 69 & 70, 117 (1978)