When an atom or an ion is missing from its nomal lattice site a lattice vacanecy (Schottky defect) is created. In stoichmeteric ionic crystals, a vacancy of one ion has to be accompanied by the vacancy of the oppositely charge ion in order to maintain electrical neutrality.
In a Frenken defect an ion leaves its position in the lattice and occupies an interstitial void. This id the Frenkel defect commonly found along with the Schottky defects and interstitial. In pure alkali halides. Frenked defects are not found since the ions cannot get into the interstitial sites. Frenkel defects are found in silver halides because of the small size of the `Ag^(+)` ion. Unike Schottky defects, Frenkel defect do not change the density of the solids. in certain ionic solids (e.g., AgBr) both schottky and Frenkel defect occur.
The Defects idiscussed above do not disturb the stoichiometery of the crystalline material. there is large variety of non-stoichiometric inorganic solids which contains an excess or deficienty of one of the elements. Such solids showing deviations from the ideal stoichiometric composition from an important group of solids. For example in the vanadium oxide, `VO_(x),x` can be anywehere between 0.6 and 1.3 there are solids such as difficult to prepare in the soichiometric omposition thus, the ideal composition in compounds such as `FeO` is difficult to obtain (normally we get a compositiion of `Fe(0.95)`O but it may range from `Fe_(0.93)`O to `Fe_(0.96)O`). Non-stoichiometric behavious is most commonly found for transition metal compounds through is also known for some lathanoids and actinoids.
Zinc oxide loses oxygen reversible at high temperature and turns yellow in colour. the excess metal is accomodated interstitial, giving rise to electrons trapped in the neighbourhood, the enchanced electrical conductivity of the non-stoichiometric ZnO arises from these electrons.
Anion vacancies in alkali halides are produced by heating the alkali halid crystals in an atmosphere of the alkali metal vapour. when the metal atoms deposit on the surface they diffuse into the cystal and after ionisation the alkali metal ion occupies cationic vacancy whereas electron occupies anionic vacancy. Electrons trapped i anion vacancies are referred to as F-centers (From Farbe the German word for colouf) that gives rise to interesting colour in alkali halides. Thus, the excess of potassium i KCl makes the crystal appear violet and the excess of lithium in LiCl makes it pink.
Which of the following is most appropritate crystal to show Fremkel defect ?

Options

A. CsCl
B. NaCl
C. AgBr
D. `CaCl_2`

Answer

C

Explanation

Because of small size of `Ag^(+)` ion.