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Defects in Crystals

Defects in Crystals. CONTENTS Introduction Classification of crystal defects Point defects Line defects Effects of defects in crystals. This work is licensed under a  Creative Commons Attribution 4.0 International License. Defects in Crystals. Introduction

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Defects in Crystals

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  1. Defects in Crystals CONTENTS • Introduction • Classification of crystal defects • Point defects • Line defects • Effects of defects in crystals This work is licensed under a Creative Commons Attribution 4.0 International License.

  2. Defects in Crystals • Introduction Ideal solids may be defined as regular arrangement of atoms over the entire volume of the material. Properties such as specific heat, elasticity, magnetic properties etc can be explained on the basis of the above structure. In real crystals some atoms are not found at their regular lattice position but occupy other locations. This leads to defects in crystals.

  3. Defects in Crystals Classification of crystal defects 1.Point defects (Zero dimension defect) • A. Vacancy/Schottky defect • B. Interstitial/Frenkel defect • C. Composition defect i) Substitutional impurity ii) Interstitial impurity • D. Electronic defect

  4. Defects in Crystals Classification of crystal defects 2. Line defects(One dimensional defect) • Edge dislocation • Screw dislocation 3. Surface defects (Two dimensional defect) 4. Volume defect ( Three dimensional defect)

  5. Defects in Crystals • Point defects • Vacancy refers to an atomic site from where the atom is missing. • Substitutional defect refers to a foreign atom that replaces a parent atom in the crystal. • Interstitial defect refers to a small sized atom occupying the void space in a crystal. • Error in charge distribution in solids are called electronic defect.

  6. Point defects “Download for free at http://cnx.org/contents/85abf193-2bd2-4908-8563-90b8a7ac8df6@9.311.”

  7. Point defects • Electronic defect • Errors in charge distribution in solids are called electronic defects. These defects are produced when the composition of an ionic crystal does not correspond to the exact stoichiometric formula.

  8. Schottky and Frenkel defect From :Principles of General Chemistry -licensed under a Creative Commons by-nc-sa 3.0 license

  9. Schottky and Frenkel defect Schottky defect Frenkel defect A cation leaves a regular lattice site and occupies an interstitial site. Found in ionic compounds where anions are bigger in size compared to cations. Density of solid remains same. • Equal number of cation and anions are missing from the lattice site. • Found in ionic compounds where cations and anions are similar in size. • Density of solid decreases.

  10. Edge dislocation Edge dislocation Edge dislocation • Presence of extra plane. • Above the edge atoms are squeezed and below the edge atoms are pulled apart. • Plane containing the dislocation line is called slip plane. • Burger vector is perpendicular to dislocation line. From: Callister, "Materials Science and Engineering," Wiley (1997)

  11. Screw dislocation Screw dislocation Screw dislocation – burger vector • One part of crystal has slipped compared to rest of the crystal. • Atoms form a spiral ramp arrangement around the dislocation line. • Burger vector is parallel to the dislocation line. education.sptech.xyz/Dislocation.html

  12. Burger Vector • The magnitude and the direction of the displacement are defined by a vector called the Burger vector. • The Burgers vector is determined by the Burgers Circuit. • Right hand screw (finish to start) convention is used for determiningthe direction of the Burgers vector.

  13. Burger Vector Perfect crystal Crystal with edge dislocation

  14. Screw dislocation • Bryan Baker chemed.chem.purdue.edu/genchem/ topicreview/bp/materials/defects3.html

  15. Effects of defects in crystals • Properties of materials are affected due to defects. • Electrical property • Optical property • Mechanical property.

  16. Effects of defects in crystals Electrical property • Doping a intrinsic semiconductor is a defect, but this increases the electrical conductivity. • In metals, the presence of impurity decreases the electrical conductivity. • Example: Semiconductor LASER uses a doped material for its functioning.

  17. Effects of defects in crystals Optical property • Colour centres in crystals are due to presence of impurity atoms in a crystal. • Ruby LASER which is aluminium oxide doped with chromium ions, gives characteristic wavelength due to doping.

  18. Effects of defects in crystals Mechanical property • Copper added to gold increases the ductility of gold, hence it can be drawn into wires. • Strength of iron is increased by the presence of carbon atoms as interstitial impurity.

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