1.2pointdefectinioniccrystalsandmetals.docVIP

UNIT –I DEFECTS IN CRYSTALS Structure Introduction 1.1 Objectives 1.2 Point Defect in ionic crystals and metals 1.3 Diffusion in solids 1.3.1 Type of Diffusion 1.3.2 Diffusion Mechanisms 1.3.3 Diffusion Coefficient 1.3.4 Applications 1.4 Ionic Conductivity 1.5 Colour Centres 1.5.1 F- Centres 1.5.2 V-Centres 1.6 Excitions 1.7 General Idea of Luminescence 1.8 Dislocations Mechanical Strength of Crystals 1.9 Plastic Bahaviour 1.10 Type of Dislocations 1.11 Stress field of Dislocations 1.12 Grain Boundaries 1.13 Etching- Types of Etching 1.14 Let Us Sum Up 1.15 Check Your Progress: The Key INTRODUCTION 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. However, the structure of real crystals differs from that of ideal ones. Real crystals always have certain defects or imperfections, and therefore, the arrangement of atoms in the volume of a crystal is far from being perfectly regular. 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 colour can make these crystals valuable as gems, as in ruby (chromium replacing a small fraction of the aluminium in aluminium oxide: Al203). Crystal prepared in laboratory will also always contain defects, although considerable control may be exercised over their type, concentration, and distribution. The importance of defects depends upon the material, type of defect, and properties, which are being considered. Some properties, such as density and elastic constants, are proportional to the concentration of defects, and so a small defect concentration will have a very small effect on these. Other properties, e.g. the colour of an insulating crystal or the conductivity of a semiconductor crystal, may be much more sensitive to the presence of small