vacancy

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 where is the vacancy concentration, is the energy required for vacancy formation, is the Boltzmann constant, is the absolute temperature, and is the concentration of atomic sites i.e. where is mass, the Avogadro constant, and the molar mass.