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what is frenkel defect,schottky defect,f- centred ?
The Schottky Defect

This is a defect where an ion is removed from its lattice site, leaving a vacancy. There are generally equal numbers of vacancies on cation and anion lattice points, so as to preserve charge neutrality, and so the overall stoichiometry is unchanged. The concentration of Schottky defects varies from a concentration of 10-12 molL-1 in alkali metal halides, to about 12 molL-1 in some d-metal oxides. This corresponds to one defect for every 1014 formula units in the alkali metals, up to one defects for every 10 formula units in the d-metal oxides, so there is a huge range of defect concentrations.

In general, Schottky defects are found when the metal ions are able to have more than one oxidation state.
This is a defect where an ion is removed from its lattice site, leaving a vacancy, and moved into an interstitial site. Ionic solids may have cation interstitial, as in silver halides, anion interstitials, as in BaF2, PbCl2, and PbBr2, and a mixture of both cation and anion interstitials, as in PbI2.

Frenkel defect formation is therefore favoured by the ready availability of interstitial sites large enough to hold the displaced ion. Open structures, such as sphalerite and wurtzite, with low coordination numbers are those in which Frenkel defect formation commonly occurs.

F-center defects

These are electrons trapped in anion vacancies, and produced by the exposure of an alkali metal halide crystal to the alkali metal vapour. There is addition of metal to the system.

H-center defects

These are self-trapped holes, and correspond to the removal of metal from the system. The holes are, in fact, a single negative charge spread over two anions, creating an X2- species. The formula is M1-xX, but is more properly written as (M+)1-x(X-)1-2x(X2-)x. The energy levels in the X2- species are well described by the usual molecular orbital diagram for a diatomic species, and electronic transitions between the energy levels in the X2- species may be observed.


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