PERFECT CRYSTAL BY J R ANSARI

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FOR B.TECH STUDENTS

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A PERFECT CRYSTAL : 

A PERFECT CRYSTAL J R ANSARI Department of Physics Deep IET E-mail:jransari@indiatimes.com

A PERFECT CRYSTAL : 

ds A PERFECT CRYSTAL + + + + + + + + + + + + + _ _ _ _ _ _ _ _ _ _ _ _

FRENKEL DEFECT : 

ds FRENKEL DEFECT + + + + + + + + + + + + + _ _ _ _ _ _ _ _ _ _ _ _ + CATION VACANCY MISPLACED CATION

SCHOTTKY DEFECT : 

ds SCHOTTKY DEFECT + + + + + + + + + + + + + _ _ _ _ _ _ _ _ _ _ _ _ VACANCIES

Frenkel Defect : 

ds Frenkel Defect + - + - + - + - + - + - + - + - + - + - + - + - + Cation vacancy Interstitial cation Occurs in ionic solids having smaller cations Frenkel defect is also called dislocation defect. For example, ZnS, AgCl, AgBr and AgI due to small size of Zn2+ and Ag+ ions.

Schottky Defect : 

ds Schottky Defect + - + - + - + - + - + - + - + - + - + - + - + - + Cationic Vacancy Anionic Vacancy Number of missing cations and anions are equal Schottky defect decreases the density of the substance. Schottky defect is shown by ionic substances in which the cation and anion are of almost similar sizes. For example, NaCl, KCl, CsCl and AgBr.

COMPARISON : 

ds COMPARISON SCHOTTKY DEFECT FRENKEL DEFECT BY MISSING OF EQUAL NUMBER OF ANIONS AND CATIONS BY THE MISPLACEMENT OF CATIONS DENSITY DECREASES DENSITY REMAIN THE SAME OCCURS IN IONIC SOLIDS HAVING SIMILAR IONIC SIZE OCCURS IN IONIC SOLIDS HAVING SMALLER CATIONS ELECTRICAL NUETRALITY REMAIN THE SAME ELECTRICAL NUETRALITY REMAIN THE SAME BUT THE DIELECTRIC CONSTANT SLIGHTY ALTER Eg: NaCl,KCl,CsCl,AgBr Eg: ZnS,AgCl,AgBr,AgI NOTE THAT AgBr EXHIBIT BOTH THESE TYPE OF DEFECTS

METAL DEFICIENCY DEFECT : 

ds METAL DEFICIENCY DEFECT Fe2+ Fe2+ Fe2+ Fe2+ Fe2+ Fe2+ Fe2+ Fe2+ O2- O2- O2- O2- O2- O2- O2- Fe2+ Fe3+ O2- O2- O2- Fe3+ Fe2+ In crystals of FeO some Fe2+ cations are missing and the loss of positive charge is made up by the presence of required number of Fe3+ ions.

METAL EXCESS DEFECT DUE TO EXTRA CATIONS : 

ds METAL EXCESS DEFECT DUE TO EXTRA CATIONS Zn2+ Zn2+ Zn2+ Zn2+ Zn2+ Zn2+ Zn2+ Zn2+ Zn2+ Zn2+ Zn2+ Zn2+ Zn2+ Zn2+ Zn2+ Zn2+ O2- O2- O2- O2- O2- O2- O2- O2- O2- O2- O2- O2- O2- O2- e- e- Zinc oxide is white in colour at room temperature. On heating it loses oxygen and turns yellow. The excess Zn2+ ions move to interstitial sites and the electrons to neighbouring interstitial sites. HEATING

Slide 10: 

ds F centre- formation- Metal excess defect due to anionic vacancies Cl- Na+ Cl- Na+ Cl- Na+ Cl- Na+ Na+ Cl- Na+ Cl- Cl- Na+ Cl- Na+ Na F centre-the elctron trapped anion vacancy Na+ e- Sodium makes NaCl yellow Potassium makes KCl violet Lithium makes LiCl pink

Introduction of cation vacancy in NaCl by substitution of Na+ by Sr2+ : 

ds Introduction of cation vacancy in NaCl by substitution of Na+ by Sr2+ NaCl Na+ Cl- Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Cl- Cl- Cl- Cl- Cl- Cl- Cl- Cl- Cl- Cl- Cl- Cl- Cl- Cl- Sr2+ Cation vacancy created by impurity

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