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

Surface defects

          Surface defects are the two dimensional regions in a crystal. The defect which takes place on the surface of a material is known as surface defects. It may be noted that surface defects takes place either due to imperfect packing of atoms during crystallisation or defective orientation of the surface. The types of surface defects are:

1) External defects

2) Internal defects

External defects

          The external surface of a material is an imperfection itself because the atomic bonds do not extend beyond it. The surface atoms have neighbor on one side only while atoms inside the crystal have neighbors on either side of them. Since surface atoms are not entirely surrounded by others, they possess higher energy than that of internal atoms. For most metals, the energy of surface atoms is of the order 1 J/m2.

Internal defects

The internal defects are classified as follows:

1. Grain boundaries
2. Twin boundaries
3. Tilt boundary

Grain boundaries

          Engineering materials may be either polycrystalline or single crystal type. A polycrystalline alloy contains an enormous quantity of fine grains. Grain boundary imperfections are those surface imperfections when ever crystals or grains of different orientation separate the general pattern of atoms and exhibits a boundary during nucleation or crystallization. This type of defect generally takes place during the solidification of the liquid metal.

Twin Boundary

          This is another planar surface imperfection.When the boundaries in which the atomic arrangement on one side of the boundary is a somewhat mirror image of the arrangement of atoms of the other side, as shown in Fig. The defect caused is known as Twin boundary. The region in which a twin boundary defect occurs is between the twinning panes as shown in Fig.  The region of material between these boundaries is approximately termed as "Twin". 

          Twins result from atomic displacements that are produced from applied mechanical shear forces, and also during annealing heat treatments following deformation. Annealing twins are typically found in crystals that have the F.C.C. crystal structure. While mechanical twins are observed in B.C.C. and H.C.P metals.

Tilt Boundary

          This is another type of surface defect called low-angle boundary as the orientation difference between two neighboring crystals is less than 10°. This is why the disruption in the boundary is not so drastic as in the high angle boundary. This type of boundary is associated with relatively little energy and is composed of edge dislocations lying one above the other. In general, one can described low-angle boundaries by suitable arrays of dislocation. The angle or tilt, 

Where b is the magnitude of Burgers vector, and

            D is the average vertical distance between dislocations.