DEFECTS IN SOLIDS

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DEFECTS IN SOLIDS:.Point defects are due to deviation around a point or an atom in a crystal. Line defects are due to deviation in the entire rows of lattice points. Point defects are mainly three types:
1.Impurity defects
2. Non-stoichiometric and
This defect is due to the impurity in the crystal. For example in molten NaCl if SrCl2 is present then some of the Na+ ions are occupied by Sr+2 .Each Sr+2 replaces two Na+ ions ,one of the site is occupied and the other remains vacant. The vacancies are equal to no.of Sr+2 ions, another example is solid solution of Cl2and AgCl. This defect is due to non-stoichiometric ratio in the crystal structure. These are of two types: 1. Metal excess defect. 2. Metal deficiency defect.
Metal excess defect is due to anionic vacancies: - It is due to the absence of anion from lattice site leaving a hole which is occupied by electron to maintain electrical neutrality. The anionic sites occupied by unpaired electrons are called F-centers. German word -- Farbenzentes means color centre.more no. of F-centers more will be the intensity of color.Ex: - NaCl is an yellow, LiCl is pink and KCl is violet.
Metal excess defect is due to presence of extraction at interstitial sites: - This is due to the movement of metal ions to interstitial sites and the electrons to neighboring interstitial site.Ex: - ZnOis white when cold and yellow when hot. Because it loses oxygen and Zn+2 ions occupy the interstitial sites.