Up to now, we have described perfectly regular crystal structures, called ideal crystals and obtained by combining a basis with an infinite space lattice. In ideal crystals atoms were arranged in a regular way. In actual crystals, however, imperfections or defects are always present and their nature and effects are very important in understanding the properties of crystals. These imperfections, electrical properties etc, to a great extent.
Natural crystals always contain defects, often in abundance, due to the uncontrolled conditions under which they were formed. The presence of defects which affect the color can make these crystals valuable as gems, as in ruby (chromium replacing a small fraction of the aluminium in aluminium oxide: Al2O3). Crystal prepared in laboratory will also always contain defects.
Imperfections are found in all crystals unless some special means are used to reduce them to a low level. The atoms do not have their full quota of electrons in the lowest energy level. But the atoms vibrate due to thermal effect and the electrons also change their positions. There is much other type of defects found in the structure of the crystals.
The crystallographic defects are classified on the basis of their geometry as follows:
(a) Substitutional impurity
(b) Interstitial impurity
D. stacking fault
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