Presentation on theme: "DEFECTS AND THEIR CONCENTRATION 201107115 양 은 목. INDEX An Introduction Intrinsic Defects -Schottky Defects -Frenkel Defects Concentration Of Defects Extrinsic."— Presentation transcript:
INDEX An Introduction Intrinsic Defects -Schottky Defects -Frenkel Defects Concentration Of Defects Extrinsic Defects
AN INTRODUCTION In a perfect crystal, all atoms would be in their correct lattice positions in structure. This situation only exists at the absolute zero of temperature, 0K. Above 0K, defects occur in the structure.
AN INTRODUCTION Perfect Crystal Extended Defects Dislocations Grain Boundaries Point Defects Intrinsic Defects Schottky Defects Frenkel Defects Extrinsic Defects
INTRINSIC DEFECTS Schottky Defects In ionic crystals, the defect forms when oppositely charged ions leave their lattice sites, creating vacancies. These vacancies are formed in stoichiometric units, to maintain an overall neutral charge in the ionic solid. Normally these defects will lead to a decrease in the density of the crystal. NaCl, KCl, KBr, CsCl, AgCl, AgBr.
INTRINSIC DEFECTS Schottky Defects The defect-free NaCl structureSchottky defects within the NaCl structure Na + + Cl - → V Na + V Cl
INTRINSIC DEFECTS Frenkel Defects The defect forms when an atom or ion leaves its place in the lattice, creating a vacancy, and becomes an interstitial by lodging in a nearby location not usually occupied by an atom. These vacancies are formed in stoichiometric units, to maintain an overall neutral charge in the ionic solid. This defect does not have any impact on the density of the solid as it involves only the migration of the ions within the crystal, thus preserving both the volume as well as mass. ZnS, Agcl, AgBr, AgI
INTRINSIC DEFECTS Frenkel Defects The defect-free NaCl structureTwo Frenkel defdcts within the NaCl structure Na + → V Na + Na + interstitial
INTRINSIC DEFECTS Anion Frenkel defect in fluorite Cation Frenkel defects are common because of the typically smaller size of a cation compared to an anion. However, anions in the fluorite structure have a lower electrical charge than the cations and don’t find it as difficult to move nearer each other. The fluorite structure ccp cations with all tetrahedral holes occupied by the anions thus all octahedral holes are unoccupied. CaF 2, SrF 2, PbF 2, ThO 2, UO 2, ZrO 2
INTRINSIC DEFECTS Anion Frenkel defect in fluorite FIGURE 5.3 The crystal structure of fluorite MX2. (a) Unit cell as a ccp array of cations, (b) and (c) The same structure redrawn as a simple cubic array of anions. (d) Cell dimensions.
At room temperature there are very few Schottky defects, even at 1000K there are only about 1 or 2 defects per hundred million sites. Increasing temperature increases defects, in agreement with the endothermic process and Le Chatelier’s principle.
ECTRINSIC DEFECTS Doping with selected ‘impurities’ can introduce vacancies into a crystal. Consider CaCl 2 into NaCl, in which each Ca 2+ replaces two Na + and creates one cation vacancy. An important example that you will meet later in the chapter is that of zirconia, ZrO 2. This structure can be stabilised by doping with CaO, where the Ca 2+ ions replace the Zr(IV) atoms in the lattice. The charge compensation here is achieved by the production of anion vacancies on the oxide sublattice.
REFERENCE SOLID STATE CHEMISTRY: An Introduction Fourth Edition Lesley E.Smart, Elaine A.Moore p201-207 현대고체화학 이규봉, 고원배 p241-253 http://en.wikipedia.org/wiki/Schottky_defect http://en.wikipedia.org/wiki/Frenkel_defect