Vacancy defect

A blank space (english vacancy ) is in crystallography a place in the regular arrangement of atoms, ions or molecules in the crystal lattice, which is unoccupied. Voids formed in the crystal formation itself, but are caused for example by changes in temperature, radioactivity, or other types of radiation by subsequent interference of the crystal.

Due to the space the perfect translational symmetry is broken in the crystal. Therefore vacancies are among the crystal defects ( lattice defects ). More specifically, it is point defects because only selective changes in the crystal occur (in contrast to dislocations, grain boundaries and stacking faults ).

Since ionic crystals are forced to charge neutrality, it can not come to a single defect. Vacancies in ionic crystals can be caused by Schottky defects (see also Walter Schottky ) or Frenkel defects.

Historical

The existence of vacancies was unconfirmed and disputed until the mid-20th century. Only after Ernest Kirkendalls publication from 1947, which described the now named after Kirkendall effect, the scientific community became aware that this is by diffusion mechanisms without spaces difficult to reconcile. Starting in 1950, then the realization began to enforce that vacancies to explain the diffusion are essential and that they can play an important role in solids.

Proof

The detection of voids and the determination of their concentration is possible, among other things by means of the determination of the lifetime of positrons. These diffuse through the crystal and search for vacancies in which they reside conditionally preferred by the potential. Since in such spaces, the electron density is lower than the surrounding crystal, the life of the positron is extended. Using the measurement of the emitted radiation that is, the location of the space is determined and the size of the space can be estimated by the lifetime measurement. Another very common method is because the Ätzungsmethode. In this case, the micrograph of the crystal is etched, and the dislocations occur at the surface area than typical gaps. Since, depending on the result in differently shaped gaps layer to which the etching is applied, one can determine the orientation of the crystal by this method.