density is . g cm - . It indicates that there is approximately % Schottky defect in VO crystal. Presence of Schottky defect in the crystal provides a simple way by which atoms or ions can move within the crystal lattice.
. . Frenkel defect: Frenkel defect arises due to the dislocation of ions from its crystal lattice. The ion which is missing from the lattice point occupies an interstitial position.
This defect is shown by ionic solids in which cation and anion differ in size. Unlike Schottky defect, this defect does not affect the density of the crystal. For example AgBr, in this case, small Ag + ion leaves its normal site and occupies an interstitial position as shown in the figure. .
. Metal excess defect: Metal excess defect arises due to the presence of more number of metal ions as compared to anions. Alkali metal halides NaCl, KCl show this type of defect. The electrical neutrality of the crystal can be maintained by the presence of anionic vacancies equal to the excess metal ions (or) by the presence of extra cation and electron present in interstitial position.
For example, when NaCl crystals are heated in the presence of sodium vapour, Na + ions are formed and are deposited on the surface of the crystal. Chloride ions (Cl - ) diffuse to the surface from the lattice point and combines with Na + ion. The electron lost by the sodium vapour diffuse into the crystal lattice and occupies the vacancy created by the Cl - ions. Such anionic vacancies which are occupied by unpaired electrons are called F centers.
Hence, the formula of NaCl which contains excess Na + ions can be written as Na Cl + x Frenkel Defect Ag + Ag + Missing Ag + Missing Br - Ag + in interstitial position Ag + in interstitial position