IMPERFECTS (DEFECTS) IN CRYSTALS

          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:

1. Point Defects

A. vacancy defects

B. schottky imperfections

C. interstitial defects

D. Frenkel defect

E. compositional defects

(a) Substitutional impurity

(b) Interstitial impurity

F. electronic defects

2. Line Defects

A. edge dislocation

B. screw dislocation

3. Surface Defects

        a. External Defects

        b. Internal Defetcs

     A. grain boundaries

     B. tilts boundaries

     C. twin boundaries

     D. stacking fault

4. Volume Defects

VOLUME IMPERFECTIONS

          Volume imperfections, e.g. cracks may arise when there is only small electrostatic dissimilarity between the stacking sequences of close packed planes in metals. Moreover, when clusters of atoms are missing, a large vacancy or void is got which is also a volume imperfection. Foreign particle inclusions, large voids or non-crystalline regions which have the dimensions of the order of 0.20 nm are also termed volume imperfections. Bulk or volume defects are normally introduced during processing and fabrication steps.

Volume or Bulk defects includes pores, cracks, foreign inclusion, and other phases.

Note: A color centre is lattice defect that absorbs visible light. The usual place to find color centre is in ionic crystals.