The best understood one, called an F-centre (German Farbe, “colour”), results from the absence of a negatively charged ion from a particular point in an ionic solid. This vacancy, which acts like a positively charged particle, attracts and traps an electron, and their combination constitutes an F-centre.
Point defects are lattice defects of zero dimensionality, i.e., they do not possess lattice structure in any dimension. Typical point defects are impurity atoms in a pure metal, vacancies and self-interstitials.
The Solid State. Mention one property which is caused due to the presence of F-centre in a solid. If F-centres are introduced into a transparent crystal lattice then they will induce colour into the materiale for example NaCl heated in sodium vapours the crystal becomes coloured.
Impurity defects are foreign atoms that replace some of the atoms making up the solid or that squeeze into the interstices; they are important in the electrical behaviour of semiconductors, which are materials used in computer chips and other electronic devices.
Definition. Schottky defect is a type of point defect or imperfection in solids which is caused by a vacant position that is generated in a crystal lattice due to the atoms or ions moving out from the interior to the surface of the crystal.
In metal deficiency defect, a cation is missing from its lattice site. To maintain electrical neutrality, one of the nearest metal ions acquires an extra positive charge. This type of defect occurs in compounds where the metal can exhibit variable valency. e.g., Transition metal compounds.
In Frenkel defect, the cation just leaves its position and shifts to another position in the unit cell or lattice but does not leave the crystal. So, no change in mass and volume takes place and hence, there is no change in density.
In Schottky defect, both cation and anion leave the solid crystal. In Frenkel defect, only the smaller ion (cation) leaves its original lattice site; whereas, the anion remains in original lattice sites. The atoms permanently leave the crystal.
A Schottky defect is an excitation of the site occupations in a crystal lattice leading to point defects named after Walter H. In ionic crystals, this defect forms when oppositely charged ions leave their lattice sites and become incorporated for instance at the surface, creating oppositely charged vacancies.
Frenkel defect is found is AgCl because Ag+ is small enough in size so as to fit into interstitial vacant positions. Thus due to large difference in the size of ions Ag+ and Cl-, Ag+ occupies voids. Was this answer helpful?
A metal excess defect is one of the defects seen in the crystal structures. These are the non-stoichiometric inorganic solids that contain constituent elements in non-stoichiometric ration because of the defects in their crystal structures.
: the arrangement of atoms, molecules, or ions of a crystal in the form of a space lattice.
The crystals with metal excess defects are generally coloured due to the presence of free electrons in them. The crystals with metal excess defects conduct electricity due to the presence of free electrons and are semiconductors.
Answer: Non-stoichiometric defect responsible for colour in alkali metal halide is metal excess (anion -vacancy) defect.
AgBr shows both Frenkel as well as Schottky defects.
F-center is a type of crystallographic defect in which an anionic vacancy in a crystal is filled by one or more unpaired electrons. Therefore, F-centers are lattice sites containing electrons.
