IMPERFECTIONS IN SOLIDS
Almost all the crystals encountered in practice suffer from imperfections or defects of various kinds. An ideally perfect crystal is one which has the same unit cell and contains the same lattice points throughout the crystal. The term imperfection or defect is generally used to describe any deviation of the ideally perfect crystal from the periodic arrangement of its constituents.
If the deviation occurs due to missing atoms, displaced atoms or extra atoms, the imperfection is named as a point defect. Such defects arise due to imperfect packing during the original crystallisation or they may arise from thermal vibrations of atoms at elevated temperatures. The most common point defects are the Schottky defect and Frenkel defect. Comparatively less common point defects are the metal excess defect and the metal deficiency defect.
This defect is caused if some of the lattice points are unoccupied. The points which are unoccupied are called lattice vacancies. The number of missing positive and negative ions is the same in this case and thus, the crystal remains neutral. The existence of two vacancies, one due to a missing Na+ ion and the other due to a missing Cl- ion in a crystal of NaCl is shown in Fig.
Schottky defects appears generally in ionic crystals in which the positive and negative ions do not differ much in size.
This defect arise when an ion occupies an interstitial position between the lattice points. This defect occurs generally in ionic crystals in which the anion is much larger in size than the cation. AgBr is an example for this type of defect. One of the Ag+ ion occupies a position in the interstitial space rather than its own appropriate site in the lattice is shown in Fig.
The crystal remains neutral since the number of positive ions is the same as the number of negative ions.
Metal excess defects
If a crystal of NaCl is heated in sodium vapour, it acquires a yellow colour. This yellow colour is due to the formation of a non-stoichiometric compound of NaCl in which there is a slight excess of sodium ions. This defect is called the metal excess defect.
Metal deficiency defects
In certain cases, one of the positive ions is missing from its lattice site and the extra negative charge is balanced by some nearby metal ion acquiring additional charges instead of original charge. This type of defect is generally found in compounds of transition metals which can exhibit variable valency. FeO and FeS show this type of defects.