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(also Schottky defect), a crystal defect owing to the absence of an atom or ion at a crystal-lattice point. Vacancies exist in all crystals, however carefully the crystals are grown. In a real crystal, vacancies arise and vanish as a result of the atoms’ thermal motion. The mechanism of vacancy formation can be represented as the discharge of atoms of the surface layer onto the surface with the subsequent transfer of the surface “holes” developed. Here, in place of bonds with three neighboring atoms, only one bond remains and the other two are broken. Consequently, the work necessary to form a vacancy is equal to the energy of two bonds.
Vacancies migrate randomly in the crystal, changing places with neighboring atoms. The motion of a vacancy is the main cause of the intermixing (self-diffusion) of atoms in a crystal, as well as of the mutual diffusion of contacting crystals. A specific equilibrium concentration of vacancies corresponds to each temperature. The number of vacancies in metal crystals near the melting point reaches 1-2 percent of the number of atoms. For aluminum at room temperature, one vacancy occurs per 1012 atoms, and in metals such as silver and copper, the number of vacancies at room temperature is even smaller. However, despite the small concentration, vacancies substantially influence the physical properties of the crystal: for example, the density decreases and ionic conductance is produced. Vacancies play an important role in heat treatment processes, relaxation of metals, re-crystallization of metals, sintering, and other processes.
REFERENCESWert, C, and R. Thomson. Fizika tverdogo tela. Moscow, 1966. (Translated from English.)
Kitell, C. Vvedenie v fiziku tverdogo tela, 2nd ed. Moscow, 1963. (Translated from English.)