Vacancy (chemistry)

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In crystallography, a vacancy is a type of point defect in a crystal. Crystals inherently possess imperfections, often referred to as 'crystalline defects'. A defect wherein an atom, such as silicon, is missing from one of the sites is known as a 'vacancy' defect.

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 0.1%

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.

At any given temperature, the amount of energy needed to create a vacancy is diminished because creating a vacancy disorders the interior of the crystal. The measure of this disorder is called the entropy of the system. Adding vacancies to the material increases the entropy, which tends to reduce the total energy required to create the vacancy. We call this energy the free energy and this is the energy that is required to create an equilibrium concentration of vacancies at a given temperature.