Defects - CrystalDefects

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Crystal Defects Perfect crystals do not exist; even the best crystals have 1ppb defects. defects are imperfections in the regular repeating pattern and may be  classified in terms of their dimensionality (Point vs. Extended). 1. Point Defects A. Vacancies given a perfect crystal (e.g. of Cu), an atom can be placed on the  outside of the cell to produce a vacancy (   ); remember atom  ≡ □ migration. e.g. TiO has 1:1 stoichiometry and NaCl structure, but has 15%  vacancies on the Ti sites and 15% vacancies on the O sites. Both sets of  vacancies are disordered.
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Crystal Defects driving force? movement of the atom requires breaking (endothermic)  and making (exothermic) of bonds. Because the atom is moving from  an internal site (w/say 6 bonds) to an external site (w/say 3 bonds),  there are more bonds broken than being made, so this is an overall  endothermic process. counteracting this is an obvious large increase in disorder (from perfect  crystal to defect); in addition, atoms around the vacated site can vibrate  more, further increasing the disorder. ΔG(n) = nΔH –nTΔS      n= #defects ΔG(n) n             max min Implications: a)n≠ 0; ΔG = 0, so no driving force. b)there is some min value of n which  is most stable. c)there is some minimum n after  which ΔG becomes positive. d)as T  n min  and n max  will also  increase.
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Crystal Defects A. Ionic Crystal Defects in pure metal compounds, don’t need to  worry about electroneutrality. in an ionic crystal, the interior and surface  must remain   neutral. 1) Shottky Defect take anions and cations and place them on  surface in equal numbers. stoichiometric effect: equal numbers of  anion and cation vacancies. may be randomly distributed, but tend to  cluster because of oppositely charged  vacancies. most important with alkali halides. at room temp, ~1 in 10 15  pairs vacant in  NaCl, so 1mg sample has ~10,000 Shottky 
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Crystal Defects 1) Frenkel Defect take cation out of position and cram it into an  interstitial site (void between normal atomic  position).
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This note was uploaded on 03/20/2010 for the course IMSE tb1073 taught by Professor Musharaf during the Spring '10 term at University of Malaya.

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Defects - CrystalDefects

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