Imperfection in Solids

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Imperfection in Solids
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Imperfection in Solids
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Stochiometric Defects
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Non- Stoichiometric
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Schotkky Defects
In in ionic crystal of the type A B-, equal
number of anions and cations are missing from
the lattice sites so that electrical neutrality
is maintained it is called schottky defect.
Types of compounds exhibiting Schottky
Defect  This type of defect is shown by highly
ionic compounds which have  High coordination
number  Cations and anions of similar sizes
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Consequences of Schottky Defect 

• As the number of ions decrease, volume is
• the same, so density decreases.  Crystals
• with Schottky defect conduct electricity to a
• small extent.  Due to the presence of holes
• the stability of the crystal decreases.

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Frenkel Defect

• If an ion is missing from its lattice site
  (causing a
• vacancy or a hole there) and it occupies the
• interstitial site so that electrical neutrality
  as well as
• stoichiometry of the compound are maintained.
• This type of defect is called Frenkel defect.  

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Types of compounds exhibiting Frenkel defect 

• This type of defect is present in those compounds
  which have  Low co-ordination number. 
• Large difference in size of anion and cation. 
• Examples AgCl, AgBr, AgI and ZnS. 

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Consequences

• Solids with this defect conduct electricity to a
  small extent 
• The dielectric constant of the crystal
  increases 
• The density of the solid is unchanged 
• Due to the presence of holes, the stability of
  the crystal decreases

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Interstitial Defects

• Atoms or ions, which occupy normally vacant
• interstitial positions in a crystal, are called
• interstitials. The important factor determining
• the formation of interstitials is the size of the
  
• atom/ion, because they are accomodated in the
• voids. 

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Non-Stoichiometric Defects

• If an imperfection causes the ratio of cations to
  
• anions to become different from that indicated by
• the ideal chemical formula, the defect is called
  
• non-stoichiometric.

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Metal Deficiency due to anion vacancies

• A negative ion may be missing from its lattice
  site, leaving a hole, which is occupied by an
  electron thereby maintaining an electrical
  balance. The trapped electrons are called
  F-centers or color centers because they are
  responsible for imparting color to the crystal. 

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By presence of extra cations in the interstitial
sites

• Extra cations occupying interstitial sites with
  electrons present in another interstitial site to
  maintain electrical
• neutrality can cause metal excess. This defect
  is similar to Frenkel defect and is formed in
  crystals having Frenkel
• defects. Example If ZnO is heated, it loses
  oxygen and
• turns yellow. 

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Metal Deficiency due to cation vacancies

• In case of ionic solids, the impurities are
  introduced by adding
• impurity of ions. If the impurity ions are in a
  different oxidation state from
• that of the host ions, vacancies are created. 
• For e.g., If molten NaCl, containing a little of
  SrCl2 as impurity is
• allowed to cool, in the crystals of NaCl formed,
  at some lattice sites Na
• ions are substituted by Sr2 ion. For every Sr2
  thus introduced two
• Na ions are removed to maintain electrical
  neutrality.