1. Imperfection in Solids

2. Imperfection in Solids

3. Stochiometric Defects

4. Non- Stoichiometric

5. 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

6. 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.

7. 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.  

8. 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. 

9. 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

10. 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. 

11. 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.

12. 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. 

13. 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. 

14. 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.