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  Imperfections in Solids

Imperfections in Solids:

Mostly all the crystals encountered in practice suffer from imperfections or defects of different types. An ideally perfect crystal is one that has similar unit cell and have the same lattice points all through the crystal. The term imperfection or defect is usually used to explain any deviation of the ideally perfect crystal from the periodic arrangement of its constituents.

Point Defects:

When the deviation takes place due to missing atoms, it displaced atoms or extra atoms; the imperfection is termed as a point defect. These defects occur due to imperfect packing during the original crystallization or they might occur from thermal vibrations of atoms at elevated temperatures. The most general point defects are the Schottky defect and Frenkel defect. Relatively less general point defects are the metal excess defect and the metal deficiency defect.

Metal excess defects:

When a crystal of NaCl is heated in sodium vapor, it obtains a yellow color. This yellow color is due to the formation of a non-stoichiometric compound of NaCl in which there is a slight surplus of sodium ions. This defect is termed as the metal excess defect.

Metal deficiency defects:

In certain conditions, one of the positive ions is missing from its lattice site and the other negative charge is balanced by certain nearby metal ion obtaining additional charges rather than original charge. This kind of defect is usually found in compounds of transition metals that can exhibit variable valency. FeO and FeS show this kind of defects.

 

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