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

LINE DEFECTS

          Line imperfections are called dislocations. A linear disturbance of the atomic arrangement, which can move very easily on the slip plane through the crystal, is known as dislocation. The dislocation may be caused during growth of crystals from a melt or form a vapor or they may occur during a slip.

          The defect which takes place due to dislocation of atoms along a line in same direction is called line defects. The line defects also takes place when a central portion of a crystal lattice slips without affecting the outer portion. These are two types of line defects:

a. Edge Dislocation

b. Screw Dislocation.

          Both these defects are the most striking imperfections and are responsible for the useful property of ductility in metals, ceramics and crystalline polymers.

Edge Dislocation

          An edge dislocation may be described as an extra plane of atoms within a crystal structure. It is accompanied by zones of compression and of tension so that there is a net increase in energy along a dislocation.

          The displacement distance for atoms around the dislocation is called 'burger's vector'. This vector is at right angle to the edge dislocation.

          Whenever a half plane of atom is inserted between the planes of atoms in a perfect crystals then this defect is produced Fig a: Shows a cross section of a crystal where dots represent atoms arranged in an orderly manner. Fig b: Shows the displacement of atoms when an extra half plane is inserted from the top. It may be noted from this Fig that top and bottom of the crystal above and below the line X-Y appears perfect. If the extra half plane is inserted from top the defect so produced is represented by ┴ [inverted T] as shown in Fig. and if the extra half plane is inserted from the bottom, the defect so produced is represented by T.

          The criterion of distortion is what is called the burgers vector. It can be determined if a closed contour is drawn around a zone in an ideal crystal by passing from one site to another as shown in Fig. (a) and then the procedure is repeated a zone in a real crystal containing a dislocation. As may be seen from Fig. (b), the contour described in real crystal turns out to be unclosed. The vector required for the closing the contour is the Burgers vector. The Burger's vector of an edge dislocation is equal to the inter atomic space and perpendicular to the dislocation line.

Screw Dislocation

          Whenever the atoms are displaced in two separate planes perpendicular to each other then the defect so produced is known as screw dislocation. Fig a: represents an isometric view of a perfect crystal. Fig b: shows the displacement of atoms appear like that of a screw or helical surfaces.

         

          

          In this type of dislocation shear stresses are associated with adjacent atoms and extra energy is involved, along the dislocation. The successive atom planes are transformed into the surface of the helix of screw by this dislocation which accounts for its name as screw dislocation. A screw dislocation has its displacement of Burger's vector parallel to the linear defect but there is distortion of the plane.

Comparison between edge and screw dislocation

S. NO	Edge dislocation	Screw dislocation
1	These dislocations arise due to  introduction or elimination of an  extra row of atoms.	A screw dislocation provides for easy  crystal growth because additional  atoms and unit cells can be added to  step of the screw.
2	Tensile, compressive and shear stress  fields may be present.	Only shear stress field exists.
3	Region of lattice disturbance  extends along an dislocation line.	Region of lattice disturbance extends  in two separate planes at right angles  to each other.
4	Burger’s vector is always perpendicular to the dislocation line	Burger’s vector is parallel to the  dislocation line.
5	These dislocation are formed during  deformation and during crystallisation.	These dislocations are also formed  during deformation and during  crystallisation.
6	An edge dislocation (pure) can glide  and climb	A screw dislocation can glide only.