Frenkel defect

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Two Frenkel defects in a silver chloride grid

Frenkel defects (also: Frenkel disorder or Frenkel pairs ) are certain lattice defects that occur in crystal lattices . Along with Schottky defects, they are among the most important point defects . They are named after the Russian physicist Jakow Iljitsch Frenkel , who first described them in 1926.

description

A Frenkel defect occurs when an ion or atom leaves its regular lattice position and moves to a normally unoccupied position in the crystal lattice. This creates a void and an interstitial atom (or an interstitial ion). This is entropically more favorable due to the lower order, but the energy of the crystal increases. The energy of copper increases by 1.2 eV with each vacancy  , and by 3.4 eV with each atom on an interstitial space. Since it is energetically more favorable if the smaller atom is on an interstitial space, Frenkel defects are usually found in the smaller cations.

Frenkel defects are a natural property of many ion crystals, which means that they are already in chemical equilibrium . The concentration of the vacancies can be formally described using a kind of law of mass action . The number of Frenkel defects increases with increasing temperature.

Occurrence

Frenkel defects are mainly found in structures that have larger vacant spaces such as octahedral holes. These include the sphalerite , spinel and fluorite structure . In contrast, Frenkel defects are rarely found in compounds with a sodium chloride structure , an example of this is silver chloride, while alkali metal halides or alkaline earth metal oxides, for example, have no Frenkel defects.

Fluorite structure

The rarer case in which the mostly larger anions are located in interstitial spaces is often also called an anti-Frenkel defect . It is found in many compounds of the fluorite structure , in which the anions are located in the smaller tetrahedral holes and pass through the defect into the larger octahedral holes.

Emergence

Frenkel defects can arise through various processes. These include crystallization , quenching , forming, heat treatment, radioactive irradiation and cutting through dislocation lines . In addition, defects above absolute zero constantly arise by themselves and disappear again when a vacancy and an ion in the interstitial space meet again through diffusion in the solid. The two processes are balanced in thermodynamic equilibrium.

Individual evidence

  1. ↑ J. Frenkel: About the heat movement in solid and liquid bodies. In: Journal of Physics. 1926, 35, pp. 652-669, doi : 10.1007 / BF01379812 .
  2. ^ Ulrich people: Physics and its applications in technology and the environment. 2nd edition, Hanser Verlag, 2004, ISBN 978-3-4462-2884-9 , p. 379.
  3. ↑ Crystal defects. In: Römpp Chemie Lexikon. Thieme Verlag, as of March 2002.
  4. ↑ a b Ralf Alsfasser, Erwin Riedel, C Janiak, HJ Meyer: Modern inorganic chemistry. 3rd edition, Walter de Gruyter, 2007, ISBN 978-3-1101-9060-1 , p. 380.
  5. ^ AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 102nd edition. Walter de Gruyter, Berlin 2007, ISBN 978-3-11-017770-1 , p. 1764.
  6. ↑ Peter Kurzweil, Paul Scheipers: Chemistry: Fundamentals, structural knowledge, applications and experiments. 8th edition, Vieweg + Teubner Verlag, 2009, ISBN 978-3-8348-0341-2 , p. 98.