In crystallography, a vacancy is a type of point defect in a crystal where an atom is missing from one of the lattice sites. Crystals inherently possess imperfections, sometimes referred to as crystallographic defects.

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. This temperature dependence can be modelled by

${\displaystyle N_{\rm {v}}=N\exp \left({\frac {-Q_{\rm {v}}}{k_{\rm {B}}T}}\right)}$

where N_v is the vacancy concentration, Q_v is the energy required for vacancy formation, k_B is the Boltzmann constant, T is the absolute temperature, and N is the concentration of atomic sites i.e.

${\displaystyle N={\frac {mN_{\rm {A}}}{M}}}$

where m is mass, N_A the Avogadro constant, and M the molar mass.

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.