Blank space
A blank (English vacancy ) is in the Crystallography a place in the array of atoms , ions or molecules in the crystal lattice , which is unfilled. Vacancies arise during the crystal formation itself, but are also caused by subsequent influences on the crystal, for example by temperature changes , radioactivity or other types of radiation .
The perfect translation symmetry in the crystal is broken by the void. Vacancies therefore belong to the crystal defects ( lattice defects ). More precisely, they are point defects because only point changes occur in the crystal (as opposed to dislocations , grain boundaries and stacking defects ).
Since ion crystals are forced to be charge neutral, a single defect cannot occur. Vacancies in ion crystals can arise from Schottky defects ( see also Walter Schottky ) or Frenkel defects .
Historical
The existence of voids was unconfirmed and controversial until the middle of the 20th century. It was only after Ernest Kirkendall's 1947 publication, which described the effect that is now named after Kirkendall , that the expert world became aware that this is difficult to reconcile with diffusion mechanisms without gaps. From 1950 onwards the realization began that voids are indispensable for explaining diffusion and that they can play an important role in solids.
proof
The detection of vacancies and the determination of their concentration is possible, among other things, by determining the lifetime of positrons . These diffuse through the crystal and look for vacancies in which they prefer to stay due to the potential. Since the electron density in such vacancies is lower than in the surrounding crystal, the life of the positrons is extended. With the help of the measurement of the emitted radiation, the location of the void can be determined and the size of the void can be estimated by measuring the service life. Another very widespread method is the etching method. The cross-section of a crystal is etched and the dislocations appear on the surface as characteristic gaps. Since there are differently shaped gaps depending on the plane on which the etching is applied, the orientation of the crystal can be determined using this method.