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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 crystalline 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. At the melting point of some metals the ratio can be approximately 1:1000. This temperature dependence can be modelled by
where Nv is the vacancy concentration, Qv is the energy required for vacancy formation, kB is the Boltzmann constant, T is the absolute temperature, and N is the concentration of atomic sites i.e.
where m is mass, NA Avogadro constant, and M the molar mass.